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Geochimica et Cosmochimica Acta 126 (2014) 284–306

Petrographic, chemical and spectroscopic evidencefor thermal metamorphism in carbonaceous chondrites

I: CI and CM chondrites

Eric Tonui a,b, Mike Zolensky b,⇑, Takahiro Hiroi c, Tomoki Nakamura d,Michael E. Lipschutz e, Ming-Sheng Wang e, Kyoko Okudaira f

a BP Upstream Research and Technology, 501 Westlake Boulevard, Houston, TX 77079, USAb NASA Johnson Space Center, Astromaterials Research and Exploration Science (ARES), Mail Code KT, Houston, TX 77058, USA

c Department of Geological Sciences, Brown University, Providence, RI 02912, USAd Department of Earth and Planetary Materials Science, Faculty of Science, Tohoku University Aramaki, Aoba, Sendai, Miyagi 980-8578, Japan

e Department of Chemistry, Purdue University, West Lafayette, IN 47907-2038, USAf The University of Aizu, Ikki-machi, Aizu-Wakamatsu, Fukushima 965-8580, Japan

Received 20 June 2012; accepted in revised form 31 October 2013; available online 14 November 2013

Abstract

We present a comprehensive description of petrologic, chemical and spectroscopic features of thermally metamorphosedCI-like and CM (and CM-like) chondrites. Only two such CI chondrites have so far been discovered i.e. Y-86029 and Y-82162. Thermal metamorphism in these chondrites is apparent in their low contents of H2O, C and the most thermally labiletrace elements, partial dehydration of matrix phyllosilicates and abundance of thermally decomposed Ca–Mg–Fe–Mn car-bonates, which apparently resulted from heating of Mg–Fe carbonate precursors.

The CM chondrites exhibit a wide range of aqueous and thermal alteration characteristics. This alteration was almost com-plete in Y-86720 and Y-86789, which also escaped alternating episodes of oxidation and sulfidization experienced by the oth-ers. Thermal metamorphism in the CM chondrites is apparent in loss of thermally labile trace elements and also in partial toalmost complete dehydration of matrix phyllosilicates: heating was less uniform in them than in CI chondrites. This dehydra-tion is also evident in strength and shapes of integrated intensities of the 3 lm bands except in PCA 91008, which experiencedextensive terrestrial weathering. Tochilinite is absent in all but Y-793321 probably due to heating. Textural evidence for ther-mal metamorphism is conspicuous in blurring or integration/fusion of chondrules with matrix in the more extensively heated(P600 �C) CM chondrites like PCA 91008 and B-7904. TEM and XRD analyses reveal that phyllosilicate transformation toanhydrous phases proceeds via poorly crystalline, highly desiccated and disordered ‘intermediate’ phases in the least and mod-erately heated (400–600 �C) carbonaceous chondrites like WIS 91600, PCA 91008 and Y-86029. These findings are significantin that they confirm that these phases occur in meteorites as well as terrestrial samples.

Thermal alteration in these meteorites can be used to identify other carbonaceous chondrites that were thermally meta-morphosed in their parent bodies. Combining RNAA trace element data for experimentally heated Murchison CM2 sam-ples with petrographic and spectroscopic data, these thermally metamorphosed carbonaceous chondrites can be ordered byseverity of open system heating as 400 �C 6 Y-793321 < WIS91600 = EET90043 = A881655 < PCA91008 < B-7904 = Y-86029 < Y-82162 < Y-86720 = Y-86789 P 700 �C. Nearly all heated carbonaceous chondrites discovered so far have beenfound in Antarctica, which is known to have sampled the flux of near-Earth material for much longer than exemplifiedby current falls.Published by Elsevier Ltd.

0016-7037/$ - see front matter Published by Elsevier Ltd.

http://dx.doi.org/10.1016/j.gca.2013.10.053

⇑ Corresponding author. Tel.: +1 7138181646.E-mail address: [email protected] (M. Zolensky).


mailto:[email protected]


E. Tonui et al. / Geochimica et Cosmochimica Acta 126 (2014) 284–306 285

1. INTRODUCTION

Thermal metamorphism continues to be one of the mostintriguing of all post-accretionary parent body processes incarbonaceous (C) chondrites. The source, timing and dura-tion of heating is not well established and still poorly under-stood. A proper understanding of this process is essential todetermine the early evolution of protoplanetary bodies, andmap physical and chemical variations across the solar sys-tem where such processing occurred. This is critical indetermining the surface mineralogy and chemistry ofC-type and related asteroids.

Van Schmus (1969) seems to have been among the firstto show that some C chondrites had been significantlymetamorphosed, in his description of Karoonda, followedby McSween (1977) and Kallemeyn et al. (1991). The num-ber of thermally metamorphosed carbonaceous chondritesbeing recovered from Antarctica is particularly significant,and the first realization of the scale of this phenomenoncame from the consortium study of the metamorphosedcarbonaceous chondrites, Belgica 7904, Yamato 86720and Yamato 82162, organized by Ikeda (1992). Most ofthese are CM chondrites: only two CI-like chondrites haveso far been discovered.

Petrologic studies of these meteorites are scattered in theliterature and no one paper has been dedicated to a compre-hensive examination of thermal metamorphic features inpreviously studied and/or new meteorites. We shouldemphasize here that it is not our intention to present a com-prehensive review of the topic of thermal metamorphism;the subject is still evolving and much more still needs tobe done to understand the evolution of thermally metamor-phosed asteroid parent bodies. However, we found it neces-sary to present data and/or results of the key features thatcan be ascribed to this process, based on several years ofour research on the topic. The results of our findings willhopefully act as ‘thermometers’ for identification of thesefeatures in newly discovered meteorites. We also include asummary of salient characteristics of artificially-heated car-bonaceous chondrites, in an effort to better understand thethermal processes operating on carbonaceous chondriteparent asteroids.

In this paper we present data from CI and CM chon-drites that show clear evidence of thermal alteration (weare preparing a similar paper that will cover the metamor-phosed CV chondrites). These meteorites are not CI and/orCM chondrites per se since they now exhibit isotopic andpetrographic characteristics that differ markedly in some in-stances from typical CI and CM chondrites. Nevertheless,the original chondrite affinities can in most instances be in-ferred from original textures that are still preserved. Thealternative is to use names like CI-like and/or CM-like.For the purposes of this paper, we will stick to interpreta-tions of their original genetic interpretations until such atime that the meteorite community comes up with a consen-sus nomenclature for their classification.

Previous and recent studies indicated that these meteor-ites were thermally metamorphosed in their parent bodies(e.g. Paul and Lipschutz, 1989, 1990; Tomeoka et al.,1989a,b; Zolensky et al., 1989a,b; Tomeoka, 1990; Akai,

1990a,b; Ikeda, 1992; Matsuoka et al., 1996; Wang et al.,1998; Wang and Lipschutz, 1998; Lipschutz et al., 1999;Tonui et al., 2002, 2003; Friedrich et al., 2002; Nakamura,2005). Petrographic evidence suggests that this occurredafter the dominant aqueous alteration phase although somemeteorites in this study present evidence of a possible heat-ing event between two aqueous alteration episodes, i.e. pro-and- retrograde aqueous alteration. The age-old notion thatcarbonaceous chondrites are pristine objects no longerholds and there is no doubt that we are dealing with ‘prim-itive’ solar system materials that experienced complicatedhistories.

Perhaps the most intriguing observation to date is thatmost of the thermally metamorphosed carbonaceous chon-drites discovered so far are from Antarctica. Two explana-tions spring to mind: a sampling quirk (e.g. for CI, a 5%probability of random selection); or differences in thenear-Earth meteoroid flux sources sampled by current fallsand by Antarctica 104–106 years ago. Advances in spectro-scopic techniques are urgently required to eliminate onealternative.

2. SAMPLES AND ANALYTICAL TECHNIQUES

All available thin sections of the 5 new meteorites at theJohnson Space Center (JSC) and the National Institute ofPolar Research (NIPR) in Tokyo were examined in thecourse of this study. These include Yamato-86029.51-1 andYamato-793321.91-1 (from NIPR) and WIS 91600.22,PCA 91008.14 and EET 90043 (from the JSC MeteoriteWorking Group). Hereafter the following abbreviations willbe used: “Y” for Yamato, “B” for Belgica, and “A” for Asu-ka. We also revisited the 5 previously studied meteorites, i.e.Y-82162, B-7904, Y-86789, Y-86720 and A-881655, to ascer-tain their heating characteristics and confirm some previ-ously-reported observations.

Mineral grains and matrix in all samples were analyzed formajor elements using a CAMECA SX100 microprobe oper-ated at 15 kV and 20 nA. Carbonates were analyzed at 15 kVand 4 nA. A focused beam (�2 lm) was used for anhydrousmineral grains and defocused beam (10 lm) for matrix phyl-losilicates. We used natural mineral standards, and appliedcorrections for absorption, fluorescence, and atomic numbereffects using the CAMECA on-line PAP program. Theseprobe analyses are accurate to within 1%, relative, for majorelements. Element maps were also made using this instru-ment. Secondary electron and backscattered electron (BSE)images were made using a JEOL JSM-6340F field emissionSEM, operating at 15 kV, a potential that offers optimumvalues of resolution vs. electron penetration (and excitation)of the samples. All electron microprobe (EPMA) and SEManalyses were carried out at JSC.

Fine-grained (matrix) material of some of these meteor-ites was also selected for more detailed TEM characteriza-tion. These observations were made using ultramicrotomedsections prepared from of grains embedded in EMBED-812low-viscosity epoxy. We studied the microtomed sectionsusing a JEOL 2000FX STEM at JSC equipped with aLINK EDX analysis system, operated at 200 kV. Imagingwas also accomplished using ion-milled samples, and a

286 E. Tonui et al. / Geochimica et Cosmochimica Acta 126 (2014) 284–306

JEOL 200CX TEM operated at 200 kV. We used naturalmineral standards and in-house determined k-factors forreduction of compositional data, which we consider to beaccurate only to within ±6%, relative; a Cliff–Lorimerthin-film correction procedure was employed (Goldstein,1979). In the case of phyllosilicates, mineral identificationswere made on the basis of both composition and electrondiffraction data whenever possible (Golden et al., 2001).We are careful to call out situations where both techniquescould not be employed. We also generally lump all of theserpentine minerals into the term “serpentine’, and recog-nize that additional work could be performed to determinethe exact serpentine mineral(s) present in these meteorites(i.e. chrysotile, lizardite, antigorite, etc.).

Approximately 50 mg of samples for RNAA analysiswas sealed in quartz and irradiated with suitable monitorsfor two days at a flux of 8 � 1013 n cm�2 s�1 at the Univer-sity of Missouri Research Reactor (UMRR). Irradiationconditions, monitor preparation, chemical processing, andcounting conditions were essentially identical to those de-scribed by Wang and Lipschutz (1998). Chemical yieldswere satisfactory, ranging up to 92%, and exceeded 40%for all samples. Chemical yields for monitors exceeded50% in all cases. Replicate portions of the Allende Meteor-ite Reference Sample (Wang and Lipschutz, 1998) andMurchison had previously been analyzed to assure thequality of the data.

For X-ray diffraction (XRD) analysis of the naturalmeteorite samples were mounted on a thin glass fiber5 lm in diameter and exposed to Cr Ka X-rays in a Gan-dolfi camera (Gandolfi, 1967). The diffraction data werecollected at beam line 3A at the Photon Factory Instituteof Material Science, High Energy Accelerator ResearchOrganization (Tsukuba, Japan) using a monochromatedbeam at 2.16 A. Using synchrotron radiation, a perfectlyadequate X-ray powder pattern can be obtained from a sin-gle 20-micron piece of matrix with a 12–60 h exposure bythe Gandolfi method. We scanned the X-ray pattern at highresolution to determine precise peak positions and inte-grated intensities. The relative abundances of major constit-uent minerals were thus determined (Nakamura et al.,2001).

We heated 1 g total of chips of Murchison in order to as-sess attendant mineralogical changes with rising tempera-ture. Individually, the chips weighed between 0.1 and0.3 g. This total quantity of Murchison should be composi-tionally representative of the meteorite (Jarosewich, 1971).We heated the samples in sealed quartz glass tubes with apiece of carbon to keep fO2 low, although obviously wecould not measure the fO2 with any accuracy in this mode.We heated samples to a series of temperatures between 400and 1200 �C at 100 �C intervals. For e-beam analysis wemade polished mounts from the heated chips, and per-formed the analyses with the SEM and microprobe men-tioned earlier. We performed XRD of powders of thesesamples using a Rigaku X-ray diffractometer. A drawbackof this particular technique is that layered phases such astochilinite and serpentine usually yield very poor diffractionpatterns, which can result in erroneous conclusions if care isnot taken in data collection and interpretation.

Samples for spectroscopic analyses were ground with amortar and pestle into powders of <100 or <125 lm ingrain size. Laboratory bi-directional reflectance spectra ofthe samples were measured at every 5 nm from 0.3 to2.6 lm in wavelength at 30� incidence and 0� emergence an-gles, using the Relab bi-directional spectrometer at BrownUniversity. The biconal FT-IR reflectance spectra (1.8–2.6 lm) of the samples were measured at 30� incidenceand 30� emergence angles, using a Nicolet 740 spectrome-ter. Samples had been purged in dry air for 24 h beforeFT-IR measurement. These two spectral sets were con-nected at 2.5 lm and were re-sampled at every 5 nm from0.3 to 3.6 lm for this study. Additional information onpreparation of carbonaceous chondrite powders for spec-troscopy is described in Matza and Lipschutz (1977) andHiroi et al. (1993).

3. PREVIOUSLY STUDIED METEORITES, BUT

WITH SOME NEW DATA

Y-86029 and Y-82162 are the only thermally metamor-phosed CI-like chondrites so far discovered. Thermallymetamorphosed CM chondrites are significantly moreabundant. Thermal alteration is mostly manifested in thecompositions of the secondary phases in both CI and CMchondrites. We will only discuss the key features attributedto thermal metamorphism in this section.

3.1. CI-chondrites

3.1.1. Y-86029

The only comprehensive description so far of Y-86029 ispresented in Tonui et al. (2003). It exhibits mineralogicalcharacteristics typical of unheated CI chondrite falls likeOrgueil and Ivuna i.e. it contains no chondrules and abun-dant secondary minerals including coarse- and fine-grained‘phyllosilicates’, Fe–Ni sulfides, carbonates and magnetite.Coarse anhydrous silicates are rare.

Y-86029 is composed of desiccated phyllosilicates, whichconstitute the bulk of Y-86029 (�80%). These ‘phyllosili-cates’ occur as isolated coarse-grained (30–300 lm) clusters,discrete clasts and matrix. Microprobe analysis shows thatthey have high analytical totals (Table 1), one of the majorindicators of thermal metamorphism in these meteorites(see Section 5). Of course in meteorites with low to moder-ate levels of thermal metamorphism phyllosilicates willhave analytical totals that vary from normal to near100% – only thoroughly heated phyllosilicates yield uni-formly high analytical totals. XRD analysis shows that dif-fraction reflections of phyllosilicate minerals are absent(Fig. 1). Instead, the crystalline, well-ordered material inmatrix consists of predominantly of olivine, magnetiteand troilite. TEM analysis reveals the additional presenceof very abundant, thick platy crystals of highly disorderedintermediate phases (Fig. 2).

Ca–Mg–Fe–Mn oxides are also present in Y-86029 asunusually large (300–500 lm) brownish clasts (Fig. 3a)and as smaller (<100 lm) grains within the matrix. Thesegrains appear to be heating products of initially Mg–Fe-rich carbonates. Carbonates are also present in Y-86029

Table 1Microprobe analyses (in wt%) of carbonates, coarse phyllosilicates and Mg–Fe–Mn oxides (periclase) in Y-86029 and Y-82162.

Species Carbonates Coarse phyllosilicates Periclasec

Magnesitec Ankeritec,d Dolomitec,d Breunneritec Y-86029c Y-82162b

SiO2 0.7 0.6 0.5 0.6 50.8 48.4 0.3TiO2 0.0 0.0 0.0 0.6 0.0 0.0 0.0Al2O3 0.2 0.2 0.0 0.0 5.3 4.3 0.0FeO 14.3 35.9 3.9 18.2* 10.9 9.9 41.3MnO 2.2 3.2 1.1 21.1 0.3 0.0 1.5Cr2O3 0.0 0.0 0.0 0.1 1.3 1.6 0.0MgO 37.6 10.5 20.5 34.6 23.6 27.8 51.5CaO 4.0 20.5 30.1 3.2 0.3 0.1 0.1Na2O 0.1 0.1 0.4 0.6 2.2 2.3 0.0K2O 0.0 0.0 0.0 0.0 0.3 0.2 0.0P2O5 0.1 0.1 0.1 0.1 0.0 0.0 0.0S 0.3 0.9 0.8 1.1 0.0 1.4 0.9NiO 0.6 0.1 0.2 0.1 0.1 0.1 0.1

Total 100.0a 100.0a 100.0a 100.0a 95.0 96.6 95.6

* Part of Fe is probably from intergrown Fe oxide or sulfide (Tomeoka et al., 1989a).a Totals calculated assuming all Mg, Fe, Ca and Mn as carbonate (all others as oxides).b From Y-82162 (Tomeoka et al., 1989a,b).c From matrix carbonate in Y-86029 (Tonui et al., 2003).d From rounded to sub-rounded carbonates in Y-86029 (Tonui et al., 2003).


as small (<25 lm) individual grains of magnesite (Fig. 3b)and as rounded to sub-rounded aggregates (Fig. 3c) withhigh Fe and Mn contents (Table 1). The latter have rarelybeen observed in carbonaceous chondrites and consist ofmagnetite enclosed by ankerite and some siderite with inter-stices filled with dolomite (Table 1). Elongate magnetitegrains with sulfide inclusions like those in Fig. 3d are alsopresent; they appear to be heating products of sulfides.

3.1.2. Y-82162

Petrographic characteristics of Y-82162 have been welldescribed (e.g. Tomeoka et al., 1989b; Zolensky et al.,1989a; Ikeda, 1991). Y-82162 and Y-86029 both show sim-ilar petrographic characteristics, with minor exceptions.

Desiccated coarse-grained and matrix phyllosilicates arepresent in Y-82162 (Zolensky et al., 1989a; Tomeoka et al.,1989b) with chemical compositions similar to those of Y-86029 (Table 1). Carbonates are also present in Y-82162,as are small (<25 lm) isolated grains of breunnerite (Ta-ble 1) with traces of dolomite, calcite, ankerite and siderite(Ikeda, 1991; Zolensky et al., 1989a; Tomeoka et al.,1989b). These are similar to the individual carbonate grainsin Y-86029. Periclase grains are also present in Y-82162although in smaller sizes (<100 lm) than in Y-86029.

3.2. CM chondrites

The CM chondrites exhibit varying degrees of thermalalteration ranging from least metamorphosed e.g. Y-793321 to highly metamorphosed e.g. B-7904 and Y-86789.

3.2.1. Y-793321

A comprehensive description of Y-793321 is presentedin Tonui et al. (2003). It is moderately brecciated and con-tains a variety of clasts that exhibit varying degrees ofaqueous alteration and thermal metamorphism. Matrix

phyllosilicates in Y-793321 are mostly serpentine groupminerals but some phyllosilicate clasts exhibit high analyti-cal totals (88–93 wt%; Table 2); these clasts are, however,rare. Some discrete Fe-rich phyllosilicate clusters withtraces of tochilinite are also present in the matrix.

Previous TEM analyses of Y-793321 revealed the pres-ence of incompletely transformed phyllosilicates (Akai,1988), similar to those in Y-86029 and PCA 91008 (de-scribed here). The XRD reflections of serpentine, tochiliniteand mixed layered minerals are absent in Y-793321 andother reflections from olivine and low-Ca pyroxene areweak (Fig. 1b). This is typical of amorphous materialformed by phyllosilicate breakdown. However, secondaryolivine and pyroxene formed at the expense of phyllosili-cates are not discernible. Other secondary phases in Y-793321 particularly the carbonates are highly abundantbut with no clear evidence for thermal alteration.

3.2.2. A-881655

Like Y-793321, evidence for thermal alteration in A-881655 is evident in the composition of secondary phasesnotably phyllosilicates. They range in composition fromsaponite pseudomorphs after olivine around chondrules,and saponite and serpentine - some with high analytical to-tals (Table 2). However, unheated clasts similar to those inY-793321 still persist with analytical totals typical of non-dehydrated phyllosilicates. Carbonates, abundant in someCM2s, are absent in A-881655. Both sulfides (mainly pyr-rhotite) and magnetite are abundant in A-881655 but withno clear evidence for thermal alteration.

3.2.3. B-7904

B-7904 was one of the first thermally metamorphosed CMchondrites to be recognized and hence has been extensivelystudied (e.g. Skirius et al., 1986; Tomeoka, 1990; Ikeda,1992; Kimura and Ikeda, 1992; Ikeda and Prinz, 1993;

Fig. 1. X-ray powder reflections (Cr, Ka) of matrix of (a) Y-86029, (b) Y-793321, (c) Y-86789 and (c) B-7904. O or Ol, T or Tr, En, M orMag, Cl, Ta and J indicate reflections from olivine, troilite, enstatite, magnetite, calcite, taenite and jarosite respectively.


Lipschutz et al., 1999). It also has low bulk H2O and C con-tents (Shimoyama and Harada, 1984). The matrix of B-7904consists largely of desiccated phyllosilicates with high analyt-ical totals (90–96 wt%; Table 2). Akai (1988) and Tomeoka(1990) suggested that these are dehydrated mixtures of ser-pentine and saponite, while Kimura and Ikeda (1992) sug-gested that they are sodian talc and serpentine. Ouranalyses suggest that these phyllosilicates are mixtures ofFe-rich serpentine (octahedral cations averagingFe1.63Mg1.08Al0.19Ti0.04Cr0.06Ni0.04), normal serpentine(octahedral cations averaging Mg1.52Fe0.91Al0.28Cr0.17Ni0.02)and some clinochlore ((Mg2.84Fe1.54Cr0.01Ti0.01)Al1.24

(Si3.51Al0.49)O10 (OH)8).Akai (1992) determined that, like the phyllosilicates in

PCA 91008 and WIS 91600 described here, B-7904 exhibitspoorly-defined lattice fringes of thermally degraded phyllo-silicates and minute grains of olivine. Our XRD analysis ofmatrix (Fig. 1d) shows very strong diffraction peaks forolivine, troilite, taenite, and kamacite, and weak reflectionsfor low-Ca pyroxene. The reflections of olivine and

pyroxene are broad suggesting small grain sizes or poorcrystallinity. The broad olivine and low-Ca pyroxene reflec-tions are characteristic of secondary silicates formed byreplacement of phyllosilicates. Tochilinite is also absent inB-7904.

3.2.4. Y-86720

The dominant secondary phases in Y-86720 are ‘phyllo-silicates’ with consistently high analytical totals (>95 wt%;Table 2) suggesting severe heating (see Section 5). Theyare present within matrix and in chondrules as discrete,rounded to irregularly shaped objects. Our attempts atTEM analysis proved unfruitful because no lattice fringesof any secondary mineral phases were identified, althoughnumerous sub-micron grains of olivine and ferromagnesiansilicates (probably enstatite) are present. Tomeoka et al.(1989a,b) and Zolensky et al. (1989b) suggested fromEPMA analysis that these dehydrated “phyllosilicates” in-clude saponite, serpentine and clinochlore and are moreMgO-rich than those in B-7904 (Table 2). The Mg:Fe ratio

Fig. 2. Electron micrographs of matrix phyllosilicate textures of Y-86029. (a) Thick ‘patchy’ or platy phyllosilicate-like crystals arepredominant with d-spacings between 10 A

0and 13 A

0. The d-spacings are not typical of phyllosilicates. (b) Well-developed lattice fringes with

d-spacings between 9–10 A0

and 4.5–4.8 A0

corresponding to d100 of olivine. These phyllosilicates appear to be ‘intermediate phases’ duringtransformation of phyllosilicates to anhydrous silicates. Figures adapted from Tonui et al. (2003).

Fig. 3. Distribution of heated mineral phases in Y-86029. (a) Large periclase (Pc) grain with minute grains of Fe–S phase (Scale bar = 10 lm).(b) Magnesite (Ms) rimmed by pyrrhotite (Py) and poorly crystalline Fe–Mn phases (Fm). Scale bar = 10 lm. (c) Rounded to sub-roundedcarbonates consisting of inner cores of magnetite (Mt) and outer rims of ankerite (An) and mixtures of carbonate and sulfide (S + C) andpyrrhotite (Py). Dolomite (Dl) occurs in interstices. Scale bar = 10 lm. (d) Elongate magnetite grain (Mt) with small troilite clusters (Tr)within or adjacent to the grain. Similar grains are present in Y-82162. Scale bar = 10 lm. Figures adapted from Tonui et al. (2003).


of these phyllosilicates is 2 to 3 indicating that the corre-sponding aqueous alteration was complete (Lipschutzet al., 1999). Tochilinite is also absent probably as a resultof thermal alteration (see Section 5).

3.2.5. Y-86789

Y-86789 exhibits petrographic and heating characteris-tics similar to Y-86720. The matrix consists of fine-grained,phyllosilicate-like material similar to that in chondrules.

Table 2Microprobe analyses (in wt%) of phyllosilicates and related phases in new and previously studied thermally metamorphosed CM chondrites.

Species A-881655 Y-86720 B-7904 Y-86789 PCA 91008 WIS-91600 Y-793321

1 2 3 4 5 6 7 8 9 10

SiO2 59.6 39.6 34.2 43.9 39.3 36.7 46.5 22.6 36.9 3.9TiO2 0.0 0.0 0.6 nd nd 0.3 0.1 0.8 0.8 0.0Al2O3 1.6 12.4 0.7 4.2 12.3 3.4 3.5 6.3 14.9 0.6FeO 1.3 3.3 24.4 19.0 18.8 24.4 20.4 56.5 20.8 55.3MnO na na 0.5 nd nd 0.1 0.1 0.2 0.7 0.1Cr2O3 0.2 2.4 0.7 2.2 0.4 0.5 0.7 0.1 0.1 2.4MgO 31.1 39.4 3.9 27.0 27.4 19.9 25.9 8.4 18.1 1.9CaO 0.0 0.0 26.7 nd nd 0.4 0.9 0.4 0.0 0.1Na2O 0.6 0.9 0.0 nd nd 1.2 0.6 0.2 1.7 0.2K2O 0.1 0.1 0.0 nd nd 0.1 na 0.0 0.2 0.4P2O5 nd nd nd nd nd 0.0 na 0.0 0.2 0.9S 0.1 0.0 2.2 nd nd 1.8 na 0.9 0.1 16.5NiO 0.5 0.1 2.2 nd 0.0 0.8 na 1.4 0.0 9.6

Total 95.0 98.2 96.2 96.5 98.3 89.7 98.7 97.3 94.4 91.8

1. Heated saponite from matrix (average of 4 analyses; Lipschutz et al., 1999).2. Desiccated high Al, Mg serpentine from matrix (average of 7 analyses; Lipschutz et al., 1999).3. Andradite from a cluster (average of 4 analyses; Lipschutz et al., 1999).4. EDX analysis from TEM of serpentine (Lipschutz et al., 1999).5. EDX analysis from TEM of clinochlore (Lipschutz et al., 1999).6. Selected analysis of matrix (Average of 27 analyses; Tomeoka, 1990).7. Representative analysis of matrix (Matsuoka et al., 1996).8. Fe-rich serpentine rim on spinel in Fig. 7.9. Al-rich serpentine in WIS 91600.10. Tochilinite in Y-793321.nd- Not detected; na- not analyzed.


Like 86720, electron microprobe analyses suggest they aredehydrated mixtures of serpentine and saponite with veryhigh analytical totals (Table 2). XRD analysis (Fig. 1c) re-veals that the matrix is composed primarily of olivine, troi-lite, taenite and kamacite. The diffraction reflections ofolivine show unusual broadening due to small grain size.Contents of volatile trace elements (next section) of Y-86789 are also similar to those of Y-86720; these two mete-orites are by all accounts paired.

4. RESULTS

4.1. Newly-studied meteorites

We will describe the petrographic and mineralogicalcharacteristics of these meteorites in more detail here sincethey have not been described elsewhere before, with theexception of WIS 91600 (Rubin et al., 2007).

4.1.1. PCA 91008

PCA 91008 had an initial recovered mass of 51.66 g andexhibits extensive weathering (weathering category B/C). Itis not as brecciated as the other heated CM chondrites suchas Y-793321. The primary texture of PCA 91008 has beenaltered by recrystallization (Fig. 4), with boundaries be-tween chondrules and matrix being less distinct than inmost CM chondrites (Fig. 4b). The chondrules (100 lm to1 mm in diameter) are mainly olivine-rich (Fo99–90) por-phyritic types (Fig. 6b) although zoned, barred olivine(Fo85 and Fo65) types are also present. Pyroxenes

(En93–99Wo7–1) are also present, while Fe–Ni metal grainsare sparse. Mesostasis in chondrules has been completelyreplaced by phyllosilicates and oxyhydroxide products ofterrestrial weathering but larger olivine crystals are little al-tered. Some chondrules have barely discernible thin(<10 lm) phyllosilicate rims.

Ternary plots (Fig. 5a) suggest that the PCA 91008‘phyllosilicates’ are serpentines with consistently high ana-lytical totals (85.6–99.7; average 94.2 wt%). These phyllosil-icates have variable FeO (30.1–65.7 wt%) contents and thedata extend towards the Fe apex (Fig. 5a). The Mg# is alsolarge and ranges from 0.20 to 0.77 (average 0.32). TEMimages of matrix show the presence of thick, platy, sheet-like crystals with poorly-developed basal lattice fringes hav-ing spacings ranging from 10 to 12 A (Fig. 6). Latticeimages of phyllosilicates are absent but crystals with spac-ings of 5 A are common, which represent (020) latticeplanes of the olivine structure. Rare tiny olivine grainsare interspersed within these poorly crystalline phases.

XRD analysis of matrix confirms extensive terrestrialweathering of PCA 91008 as shown by the presence of verystrong jarosite ðKFeþ3

3 ðOHÞ6ðSO4Þ2Þ reflections (Fig. 7a),derived from weathering of sulfides and Fe oxides in an acidenvironment. Sharp olivine peaks are also present, whilephyllosilicate peaks are absent.

Tochilinite is also absent in PCA 91008. Other second-ary phases in PCA 91008 include those within CAIs, whichare present as irregular aggregates (10–50 lm) of Fe-richphyllosilicates (Table 2). Fe–Ni sulfides (mainly pyrrhotite)also occur within matrix as small (<50 lm), irregular,

Fig. 4. BSE image of (a) a section of PCA 91008 and, (b) close up view showing blurring and fusion of chondrules and other aggregates withmatrix. Scale bar measures 1 mm.

Fig. 5. Compositions of phyllosilicates in newly discovered meteorites (WDS data) plotted onto Fe–Si–Mg atom% ternary diagrams. (a)Phyllosilicates within chondrule and matrix in PCA 91008. (b) Phyllosilicates within chondrules and their rims and matrix in WIS 91600. (c)Phyllosilicates within chondrules and their rims and matrix in EET 90043.


Fig. 6. High-resolution electron micrographs of matrix textures of PCA 91008. Poorly developed, phyllosilicate-like crystals are predominant(a and b). Lattice fringes of normal phyllosilicates are absent. As in Y-86029, lattice d-spacing of about 10–12 A

0are predominant in PCA

91008, while those �5 A0

appear to correspond to residually formed olivine.


porous aggregates. Magnetite occurs mainly within chond-rules although a few scattered grains are present within ma-trix. Neither magnetite nor sulfides are as abundant in PCA91008 as in other CM chondrites we have examined.

4.1.2. WIS 91600

WIS 91600 had a total recovered mass of 184.1 g andexhibits mild weathering (category A/B), and has previ-ously been described in some detail by Rubin et al.(2007). The abundance of chondrules in WIS 91600 proba-bly precludes a CI classification (see more about this in thesection on O isotopes). However, the matrix of WIS 91600is very fine-grained and contains abundant magnetite,which is anomalous for a CM.

WIS 91600 consists of a variety of olivine-rich chond-rules (porphyritic, granular, barred and radial) ranging insize from 100 lm to 1 mm. Olivines are forsteritic exceptin barred chondrules where olivine exhibits normal zoning,which ranges in composition between Fo90 and Fo70. High-Ca pyroxenes (En49–62Wo32–50) are present mainly withinthe radial chondrules; some low-Ca pyroxenes (En95–

99Wo4–1) are also present within the porphyritic and barredchondrules but most have completely altered to secondaryproducts. Small (<10 lm) unaltered spinel (MgAl2O4)grains are also present in some of the chondrules.

The mesostasis and some phenocrysts in chondrules andaggregates have altered to high-Al (9.1–18.3 wt% Al2O3)and low-Al (5.9–9.7 wt% Al2O3) phyllosilicates (Table 2),which often appear as discrete irregular lumps or ‘balls’within the chondrules (Fig. 8a). These high-Al phases alsohave slightly higher Mg and lower Fe contents than dothe low-Al phases. Microprobe analyses of these phyllosili-cates (Fig. 5b) fall between the serpentine and smectite solidsolution lines suggesting that they are intergrowths of thetwo secondary phases. Many chondrules and aggregateshave phyllosilicate and Fe-sulfide rims ranging in widthfrom 10 to 50 lm.

Like chondrule phyllosilicates, matrix phyllosilicates inWIS 91600 plot between the serpentine and smectite solidsolution lines (Fig. 5b). As in other CM chondrites, thesephyllosilicates exhibit very variable FeO compositions

(8.7–33.9 wt%; average 19.6 wt%) and Mg# between 0.40and 0.83 (average 0.64) indicating variable degrees of alter-ation. These phyllosilicates have slightly high analytical to-tals (84.2–95.9; Table 2).

TEM analysis of WIS 91600 matrix shows the presenceof poorly crystalline intermediate phases with lattice spac-ings between 8 and 9.5 A (Fig. 9). Unlike Y-86029, thesestructures have better-developed lattice fringes. The latticespacings of olivine (�4.8 A) observed in more highly heatedY-86029 are absent in WIS 91600. Degraded flakes withring-like diffraction in ED patterns are also present, somehaving lattice spacings of about 18 A (Fig. 9c and d). Theseappear to be flakes of partially dehydrated saponite. Smallgrains of magnetite are also interspersed within these heatedphyllosilicates (Fig. 9c).

XRD analysis of matrix shows the absence of 100 reflec-tions of either serpentine or saponite, although prism reflec-tions of these phases are still present (Fig. 7a). This istypical of very weakly dehydrated phyllosilicates, i.e. theyare present but not in well-crystalline anhydrous phases.This appears to explain the absence of olivine grains fromthe TEM examination.

Magnetite is the second most abundant secondary phasein WIS 91600 and occurs as framboidal aggregates, plac-quettes and euhedral grains often intergrown with othermineral phases, mainly Fe–Ni sulfides (Fig. 8b). A highabundance of magnetite is also confirmed by large peaksin the XRD spectrum (Fig. 7a). The order of precipitationbetween magnetite and sulfides is unclear but magnetiterims on sulfides in some locations, and well-developedpseudomorphs of framboidal magnetite after sulfides(Fig. 8c) indicate a late-stage oxidation event. Sulfidesoccur mainly as pentlandite, and less commonly, as pyrrho-tite. Most of the Fe–Ni sulfides are very fine-grained andwidely dispersed in the matrix although some occur asirregular or elongate crystals. Unusual rounded to sub-rounded sulfide (pyrrhotite) aggregates (Fig. 8d and e) arealso present with phyllosilicates in intervening spaces sug-gesting multiple precipitation episodes. We observed similarsulfides in the ungrouped Tagish Lake meteorite. Some cal-cium carbonate is also present within matrix in WIS 91600

Fig. 7. X-ray powder reflections (Cr, Ka) of matrix of (a) WIS 91600, (b) PCA 91008 and, (c) EET 90043. M, O and J indicate reflections frommagnetite, olivine and troilite respectively.


as small irregular grains (<10 lm) or patches within thematrix.

4.1.3. EET 90043

EET 90043 is moderately weathered (weathering cate-gory B) and had a total recovered mass of 12.537 g. Chond-rules within EET 90043 are small (100 to 600 lm), althougha few (�20 vol%) are > 600 lm, i.e. within the dimensionsfor CM chondrites. Its volatile trace element chemistry(next section) is also typical of CM chondrites.

Virtually all chondrules and anhydrous or opaquephases in EET 90043 contain very thick (20–50 lm) ‘phyl-losilicate’ rims, which are almost indistinguishable frommatrix in most parts. Chondrules in EET 90043 are

mainly porphyritic and radial types consisting primarilyof olivine. Pyroxenes (En94–99Wo5–1 to En55–65Wo35–45)are also present together with magnetite and sulfides(mainly pentlandite). Texturally, the chondrules and sur-rounding rims and matrix in EET 90043 are blurred(Fig. 10a), similar to those for PCA 91008. The mesostasiswithin chondrules is altered to ‘phyllosilicates’ (Fig. 10b).Anhydrous olivine-rich (forsteritic) aggregates exhibitingvarying degrees of alteration are also abundant in theEET 90043 matrix.

The ‘phyllosilicates’ in EET 90043 plot between the ser-pentine and smectite solid solution lines in the ternary plot(Fig. 5c). The thick ‘phyllosilicate’ rims on most of themineral phases make it difficult to distinguish them from

Fig. 8. BSE images of WIS 91600 showing: (a) Porphyritic chondrule replaced by discrete ‘phyllosilicate’ phenocrysts. Scale bar = 100 lm. (b)Magnetite framboids, placquettes and spheroids and carbonate (C) within a very-fine-grained matrix. Scale bar = 10 lm. (c) Magnetiteframboid pseudomorphs after sulfides probably pyrrhotite. Scale bar = 10 lm. (d) and (e) Pyrrhotite aggregates with phyllosilicate inintervening spaces suggesting multiple precipitation episodes. Scale bars = 10 lm.

Fig. 9. High-resolution electron micrographs of matrix phyllosilicates in WIS 91600. (a) and (b) Lattice images of serpentine-likephyllosilicates with d-spacings between 8 and 9.5 A

0. (c) and (d) Degraded flakes of thermally affected saponite with ring-like diffraction in ED

patterns and d-spacings of up to 18 A0. Magnetite (Mt) grains are also interspersed within these poorly crystalline phases.


matrix. These ‘phyllosilicates’ have variable compositionswith Mg# between 0.25 and 0.65 (average 0.43) and highanalytical totals (85.2–99.5 wt%; average 90.8 wt%) sug-gesting that they are thermally decomposed. Indeed,

XRD analysis of matrix confirms absence of phyllosilicatesand presence of re-crystallized olivine defined by weakreflection peaks (Fig. 7c). Magnetite is very abundant, thesharp reflections being typical of framboidal types.

Fig. 10. BSE images showing: (a) Blurring of chondrules as a result of thermal alteration in EET 90043. Scale bar = 100 lm. (b) Magnetitepseudomorph after sulfide in EET 90043. Scale bar = 10 lm. (c) Fe–Ni metal altered to sulfide (pentlandite) in EET 90043. Scale bar = 10 lm.

Table 3Oxygen isotopic compositions of thermally metamorphosed CI,CM and related chondrites.

Class Meteorite Whole rock

d18O d17O D17O

CI-like Y-86029a 21.89 11.59 0.21Y-82162 21.56 11.59 0.38

CM Y-793321a 10.62 2.58 �2.94PCA 91008a 5.26 �0.69 �3.42A-881655 7.05 1.30 �2.37

CM-like B-7904 21.07 10.91 �0.05Y-86720 22.29 11.58 �0.01Y-86789 21.16 10.94 �0.06WIS 91600a 17.32 8.82 �0.18

a This study (analyzed by Mayeda, T and Clayton, R, personalcommunication, 2011); all others from Clayton and Mayeda(1999).


Magnetite is visually the most abundant secondaryphase after phyllosilicates and occurs as a component ofthe chondrules, isolated grains and as replacement and/orrims on sulfides (Fig. 10c and d). The dominant sulfide ispentlandite but pyrrhotite is also present.

4.2. Oxygen isotopes

The oxygen isotope results reported here include thosefor previous and newly studied meteorites (WIS 91600,PCA 91008 and EET 90043). Whole-rock samples of new-ly-identified heated chondrites were analyzed for oxygenisotopes by R.N. Clayton using the methods described inClayton and Mayeda (1963, 1983). The isotopic composi-tions are reported as & deviations from the SMOW (Stan-dard Mean Ocean Water) standard for both 18O/16O and17O/16O ratios. This is based on data presented as valuesof D 17O = d17O � 0.52 d18O, where each d is defined asusual: ((d18 = 18O of sample/ 18O of SMOW) � 1) � 1000.The value of D 17O is a convenient measure of departureof a sample from the terrestrial fractionation line (Claytonand Mayeda, 1983). The values for these heated chondrites,plus data from several other CI and CM chondrites, areshown in Table 3 and illustrated in Fig. 11.

The oxygen isotopic data span the entire range of theCM (EET 90043 is at the junction of CO and CM fields),CI and even beyond the CI range. The data suggests exis-

tence of three isotopic groups i.e. the first two having isoto-pic compositions near the main CM and CI trends and thethird above the CI trend. Y-86029, Y-82162, B-7904, Y-86789 and Y-86720 have very high d18O and d17Osuggesting CI-like precursor compositions. WIS 91600 lieswell beyond the range of typical CMs indicating a twofoldhigher water–rock ratio (Clayton and Mayeda, 1999).Its bulk oxygen isotopic composition falls within the

Fig. 11. Oxygen isotopic compositions of new thermally metamorphosed CI and CM (and CM-like) chondrites compared to their non-heatedcounterparts. Data for previously studied thermally metamorphosed carbonaceous chondrites extracted from Clayton and Mayeda (1999),and data for Y86029, Y793321, PCA 91008, and WIS 91600 are from R. Clayton, personal communication, 2011. For B-7904, the quoteddelta-values are the average of two fluorinations: first d18O = 20.85, d17O = 10.80, oxygen yield was 9.76 micromoles O2/mg of sample, secondd18O = 21.28, d17O = 11.01, oxygen yield was 10.35 micromole O2/mg; for Y-86720, there was one preparation: d18O = 22.29, d17O = 11.58,oxygen yield was 9.58 micromole O2/mg; for Y-86789, there were two fluorinations: first, d18O = 21.47, d17O = 11.10, yield 8.76 micromoleO2/mg, second d18O = 20.84, d17O = 10.77, yield 9.82 micromole O2/mg; for Y-793321, two fluorinations: d18O = 10.33, d17O = 2.49, yield9.79 micromole O2/mg, second d18O = 10.90, d17O = 2.67, yield 9.62 micromole O2/mg; for y-82162, from Yanai, one fluorination:d18O = 21.56, d17O = 11.59, yield 9.19 micromole O2/mg (all R. Clayton, personal communication, 2011). Also shown are results oflaboratory heating of Murchison (CM2) and Ivuna (CI1) chips (both as filled six-sided stars) in closed system experiments. The temperatureranges indicated are 400–100 �C and 100–700 �C for, respectively, Murchison and Ivuna. The direction of compositional change for Ivuna isindicated by a arrow, revealing progressive loss of heavy isotopes with increasing temperature.


non-heated CI chondrite field (Fig. 11) although its petro-logic and chemical characteristics suggest it is a CM1 chon-drite (see Section 5).

Clayton et al. (1997) reported O isotope measurementsfor Murchison (CM2) chips artificially heated to 100 �Csteps between 400 and 1000 �C. These experiments usedsealed glass tubes, and resulted in very small changes inoxygen isotopes, as losses in oxygen were small, and frac-tionation factors are generally small at high temperatures(Clayton et al., 1997). The results of these experiments areplotted in Fig. 11. The change in oxygen isotopic composi-tions of Murchison upon artificial heating are significantlysmaller than is observed in the naturally heated CMs.Clayton et al. (1997) also reported results of identicallaboratory heating of Ivuna (CI1) chips to between 100and 700 �C. The results are also plotted in Fig. 11. Ivunaoxygen isotopic composition changes significantly in thistemperature interval, but in the opposite direction fromwhat is observed in the naturally-heated CI meteorites,which is to say the residues exhibit heavy isotope depletion,especially in the temperature range 100–500 �C. This resultindicates progressive removal of a component enriched ind18O and d17O in these closed system heating experiments.

4.3. Trace element chemistry

The trace element data for the newly- and previously-studied meteorites are shown in Table 4. Detailed descrip-tion for Y-86029 is included here for comparison with otherseverely heated chondrites. We have ordered these elements

by increasing degree and ease of their volatization and lossduring extended (one week) heating of primitive meteorites-primarily Murchison (Ikramuddin and Lipschutz, 1975;Matza and Lipschutz, 1977; Bart et al., 1980; Ngo andLipschutz, 1980) under ambient atmospheric conditions(initially 10�5 atm H2) reasonable for early system objects.

Based on these experiments, there is no evidence for anytrace element loss for heated Murchison at 400�C but Cd islowered by at least one order of magnitude, with hints of asmall amount of loss of the next one to three most mobileelements at 500 �C (Fig. 12). At 600�C, there is significantloss of at least the four most mobile elements (In! Cd).By 700 �C, significant loss of five elements is evident asZn joins In! Cd. These observations constitute a contex-tual format for identifying carbonaceous chondrites ther-mally metamorphosed on their parent bodies (Paul andLipschutz, 1989, 1990; Xiao and Lipschutz, 1992).

CI-normalized data for the nine most mobile elements(Cs! Cd) in Y-86029, PCA 91008 and EET 90043 areshown in Fig. 12. The C1-normalized pattern for newlystudied PCA 91008 is quite similar to that of the 500 �CMurchison sample. Both Tl and Cd are significantly de-pleted below the mean value of 0.46 ± 0.13 exhibited bythe other seven mobile trace elements, Cs! Bi (Table 4).Cadmium is depleted below the mean value of 0.39 ± 0.08exhibited by the other eight mobile trace elements, Cs! Tl(Table 4) in EET 90043. WIS 91600 shows moderate loss ofCd below the mean of 0.58 ± 0.09 exhibited by the eightother mobile trace elements (Table 4). The Cd depletion sug-gests that WIS 91600 experienced slightly lower-temperature

Table 4Atomic abundances (Cl-normalized) for mobile trace elements in thermally metamorphosed CI and CM chondrites.

Meteorite Type CI1-normalized atomic abundances

�x� r � Elements (No.) Cd Ref.*

Y-82162 CI 1.36 ± 0.18 Cs! Zn (5) 60.0021 (4)d

Y-86029 CI 1.20 ± 0.19 Cs! Zn (5) 0.168 (2)b

B-7904 CM 0.57 ± 0.08 Cs! Zn (5) 0.0022 (4)c

Y � 86720Y � 86789

�CM 0.65 ± 0.16 Cs! Te (4) 0.0010 (4)e

CM 0.69 ± 0.08 Cs! Te (4) 0.0031 (4)f

A-881655 CM 0.39 ± 0.17 Cs! Tl (8) 0.056 (3)Y-793321 CM 0.51 ± 0.07 Cs! Cd (9) 0.50 (3)PCA 91008 CM 0.46 ± 0.13 Cs! Bi (7) 0.0092 (3)a

WIS 91600 CM 0.58 ± 0.09 Cs! Tl (8) 0.32 (1)EET 90043 CM 0.39 ± 0.08 Cs! Tl (8) 0.13 (1)

� Mean ± r depletion (or enrichment) factors for elements in next column unaffected by thermal metamorphism.* References: (1) This work; (2) Tonui et al. (2003); (3) Wang and Lipschutz (1998); (4) Paul and Lipschutz (1989).

a Also Tl = 0.16.b Also In = 0.29, Bi = 0.20, Tl = 0.091.c Also In = 0.36, Bi = 0.13, Tl = 0.032.d Also In = 0.51, Bi = 0.21, Tl = 0.024.e Also Zn = 0.076, In = 0.16, Bi = 0.028, Tl = 0.024.f Also Zn = 0.082, In = 0.12, Bi = 0.080, Tl = 0.033.

Fig. 12. Contents of 9 thermally mobile trace elements in Y-86029,EET 90043 and PCA 91008 compared with data for MurchisonCM2 chondrite heated for 1 week at 500, 600, 700 �C underambient conditions simulating those during metamorphism ofprimitive parent object. Elements ordered from left by increasingdegree and ease of loss at 600–700 �C during artificial heating.Comparison of data for these meteorites with those of artificiallyheated Murchison suggests metamorphic temperatures of 500–600 �C for Y-86029 and PCA 91008 and 400–500 �C for EET90043.


heating than did A-881655 (Table 4). Means for all nine ele-ments and all but Cd in WIS 91600 are consistent withthose in CM/C2 chondrites (Xiao and Lipschutz, 1992).

We note that the Cd values shown in Fig. 12 are higherthan Tl for the 600�C heated Murchison samples, and Cd inthe 600�C run is higher than in the 500�run. The data for Y-86029 seems to follow this same trend. We do not knowwhether this is the result of a problem in the Cd analyses,or to unusual differences in the initial composition of the

chips used in the study. Nakamura (2005) also found thatthe behavior of Cd was not completely predictable at lowto moderate heating levels. This is an important point sincethe Cd values should be key in interpreting the tempera-tures for thermal alteration of the meteorites. We furthernote that for the 700 �C run and EET 90043 Zn is lowerthan In and Bi. This may be due to vagarities in the miner-alogical site of the X/Zn (usually sulfides and spinels). Werecommend that future workers reexamine these issues.

Two of the three most refractory elements (Co! Sb) inY-86029 accord well with data for Y-82162 but contents ofthe next seven elements (Ga! Zn) in Y-86029 are lowerthan those in Y-82162. Contents of the four most mobiletrace elements, In! Cd, in Y-86029 and Y-82162 areundoubtedly depleted relative to Orgueil: those of Tl andCd are even lower in Y-82162 than in Y-86029 (Table 4).The data for Y-82162 accord with those of Murchisonheated at 600–700 �C, while the Y-86029 results suggestlower temperatures, 500–600 �C.

The data for other CM chondrites are described in moredetail in Paul and Lipschutz (1989, 1990) and Xiao andLipschutz (1992) and are summarized here. Y-793321shows no significant loss of any mobile trace element con-sistent with the observation that heating was not pervasivein this meteorite. A-881655 on the other hand shows signif-icant depletion of Cd suggesting heating temperature of�500 �C. B-7904 exhibits a pattern similar to that of600 �C Murchison sample. The data for Y-86789 and Y-86720 is consistent with heating at 700 �C suggesting thatthey are the most substantially heated of the CMchondrites.

4.4. UV–visible-near infra-red spectroscopy

As with the trace element data, interpretations ofdiffuse reflectance spectra of thermally metamorphosed

Fig. 14. Reflectance spectra of heated CI and CM chondritepowders (<100 or <125 lm). All spectra are scaled to 1.0 at1.95 lm and offset for clarity.


chondrites are based on artificially-heated Murchison.Reflectance spectra of Murchison meteorite powder(<63 lm) and the laboratory-heated ones (Fig. 13) showsthat the 3 lm hydration band gradually weakens up to500 �C and essentially vanishes at 600–700 �C. The com-posite UV absorption strength (obtained by connectingthe UV–Vis-NIR and FT-IR spectra) continues to de-crease until 700 and 800 �C and then again thereafter.The phyllosilicate bands around 0.7 lm, which are evidentin the unheated sample, vanish on heating even at 400 �C,perhaps even lower, mainly reflecting dehydration ofserpentine.

Data for the CI and CM chondrites described here areshown in Fig. 14. Thin sections of A-881344, a heatedCM2 chondrite (T. Nakamura, Pers. Comm., 2002), wereunavailable for this study but we have nevertheless addedit here for reference. Spectra of the CI and CM meteoritesall have a characteristic 3 lm hydration band of variousstrengths (Fig. 14). Most of them have weak 0.7, 0.9 and1.1 lm bands, which represent ferric/ferrous Fe in phyllosil-icates. The shape of the 3 lm band in B-7904, Y-86720, Y-86789 and A-881334 is more rounded and smoother thanthose of Y-793321, WIS 91600 and EET 90043, comparableto those of Murchison heated at 500–600 �C and 400–500 �C, respectively. The 3 lm band for PCA 91008 is quitesteep suggesting very weak dehydration or effects of terres-trial weathering (see Section 5).

4.5. XRD and electron beam analyses of artificially-heated

Murchison

The results of the XRD of artificially-heated Murchisonshould help guide the mineralogical interpretation of the

Fig. 13. Laboratory reflectance of Murchison powders (<63 and63–125 lm) unheated and heated at seven different temperatures.Reflectances are scaled to 1.0 at wavelength 0.55 lm and shifted by0.5 or 1.0 from one another.

naturally-heated CM chondrites. Of course our heatingexperiment was rather crude, our control of fO2 was lim-ited, and results for the naturally-heated materials wouldvary with changing pH. Nevertheless, our results in thisparticular experiment were in general accord with previoussuch work. Our results are given in Fig. 15. Under ourexperimental conditions, which were reducing, tochilinitedisappears at �300 �C, serpentine at �400 �C, while olivineand low-Ca pyroxene persist. At 1000 �C kamacite becomesevident in the XRD pattern which, since it is actually pres-ent at all temperatures in small amounts, indicates that it isincreasing dramatically in quantity and/or crystallinity. Inthe SEM images (Fig. 16) of the melted Murchison(1200�) it is clear that the quantity of kamacite here in-creases still further, at the expense of troilite, which disap-pears as sulfur is lost. The only strange result is an increasein the quantity of magnetite at about the point where tochil-inite breaks down. One would think that the tochilinite

Fig. 15. Stability diagram of minerals in the artificially-heatedMurchison experiments.

Fig. 16. Backscattered electron images (BSE) of the artificially-heated Murchison samples. Each photo pair shows a view at a lower andhigher magnification (note scale bars).


would break down to form troilite, as has been observed.Possibly the dehydration of serpentine is affecting the fO2.

Microprobe analyses of olivine in the heated Murchisonsamples (Fig. 17) reveal what has already been known forsome time – as temperatures climb olivine compositionsequilibrate at �Fo65. However, following melting the newgeneration of olivine has a considerably lower iron concen-tration, �Fo93, as new olivine competes with kamacite un-der the reducing conditions.

5. DISCUSSION

5.1. Aqueous alteration

The carbonaceous chondrites described here exhibit awide range of aqueous alteration and thermal metamorphicfeatures. Because the latter process appeared to haveoccurred after the dominant aqueous alteration episode(s),it often presents considerable challenges in disentanglingthe nature and chronology of their alteration. The newmeteorites PCA 91008, WIS91600 and EET 90043 are noexception in this regard.

PCA 91008 and EET 90043 exhibit partial aqueousalteration of chondrule mesostasis and low Ca-pyroxenes.Fe–Ni metal grains are still present in matrix in PCA91008 surrounded by rims of magnetite, while the matrixis mainly composed of Fe-rich phyllosilicates, primarilyserpentine. The occurrence of magnetite as rims and

replacement for sulfides in EET 90043, and the dominanceof pentlandite rather than pyrrhotite, suggests that thismeteorite records extensive oxidation. Other indicators ofpartial alteration in both PCA 91008 and EET 90043include the presence of tochilinite. In essence, PCA91008 shows similar alteration features as Y-793321(Tonui et al., 2003).

WIS 91600 experienced a slightly different alteration his-tory. Chondrules in WIS 91600 exhibit partial alterationsimilar to that in B-7904. However, the matrix of WIS91600 is very fine-grained and consists of magnetite andpentlandite, indicating that it experienced late stage oxida-tion common to most CM1s. The presence of Ca-carbonatealso suggests CM1 affinities. The matrix of WIS 91600 anddistribution of secondary phases within it is similar to thatin CI chondrites.

The oxygen isotopic composition of WIS 91600 is closerto that of unmetamorphosed CI’s and Tagish Lake(Zolensky et al., 2002) than B-7904. Its D17O isotopic value(Table 3) is much more positive than are most CMs andsince WIS 91600 was not extensively heated (next section),it is unlikely that the isotopic composition has changedmuch since the original aqueous alteration (R. Clayton,Pers. Comm., 2002). There is no doubt that the petrologicand chemical characteristics of WIS 91600 are CM1-like,but the isotopic composition suggests otherwise; this couldbe an indication that it may not be have come from thesame asteroid as CM1s.

Fig. 17. Results of microprobe analyses of olivine in artificially-heated Murchison samples.


5.2. Thermal metamorphism

5.2.1. Isotopic evidence

Y-86029, Y-82162, Y-86720, Y-86789 and B-7904 exhi-bit unique d18O and d17O isotopic ratios (Fig. 11), the high-est recorded so far in meteorites. Fundamental tointerpretation of the whole-rock oxygen isotopic composi-tions are two well established systematic patterns: (1) thecompositions of the main population of inferred thermallyunmetamorphosed CM chondrites form an approximatelylinear array on the oxygen three-isotope graph, reflectingchemical interaction between anhydrous silicates and aque-ous solutions, resulting in progressive increases in d18O andd17O with increasing water–rock ratio (Clayton andMayeda, 1999), and (2) thermal dehydration is accompa-nied by a kinetic isotopic fractionation, in which the resid-

ual silicates are further enriched in the heavy isotopes ofoxygen (Clayton and Mayeda, 2009).

The isotopic values of the thermally metamorphosedcarbonaceous chondrites are situated at the critical pointconnecting the terrestrial (and nearly CI chondrite) frac-tionation lines with the CM mixing line. We attribute thisto severe thermal metamorphism although the process thatgoverns the mass-dependent fractionation is poorly under-stood to date (e.g. Clayton et al., 1997). Nevertheless, thedirection of fractionation is always the same whether theprocess is governed by equilibrium or kinetic effects, i.e.preferential loss of isotopically light water accompaniedby concomitant heavy isotope enrichment in the solid resi-due (Valley, 1986; Clayton and Mayeda, 1999). Ikeda(1991) offered an alternative suggestion that most phyllosil-icates in CM chondrites were produced by reactions with a


nebula gas, resulting in the new CM mixing line, whereasmost phyllosilicates in CI chondrites were produced byreactions with liquid water in their parent asteroids. Thus,the heated meteorites may have been produced under inter-mediate conditions connecting the two groups.

The results of the oxygen isotopic compositions of arti-ficially-heated Murchison (CM2) and Ivuna (CI1) reportedby Clayton et al. (1997) continue to be perplexing. Over atemperature interval between 400–1000 �C Murchison com-position only slightly changes. Meanwhile Ivuna over theinterval 100–700 �C changed dramatically, but in the oppo-site direction of the naturally-heated CI chondrites, exhibit-ing heavy isotope depletion. These experiments wereperformed in closed glass tubes, so it is possible that this de-sign was not sufficiently accurate – that the natural sampleswere heated in a significantly open system. Probably onlyfurther experiments can resolve this issue.

5.2.2. Petrographic, trace element and spectroscopic evidence

The most obvious petrographic evidence for thermalmetamorphism in CI and CM chondrites is the dehydrationof matrix phyllosilicates. This is most apparent in high(EPMA wt%) analytical totals as also previously observedby others (e.g. Tomeoka et al., 1989a,b; Zolensky et al.,1989a,b; Ikeda, 1991; Tonui et al., 2002, 2003). Our at-tempts to characterize these heated phyllosilicates byTEM proved to be quite frustrating because they exhibithighly disordered structures and poorly-developed latticefringes, which could otherwise indicate their initialcompositions. However, a combination of TEM andXRD analyses allowed the determination of these phasetransformations during dehydration through their diffrac-tion patterns. XRD is a more potent technique in this casethan TEM because it also enables estimation of heatinglevel based on amounts and degree of crystallinity of new-ly-crystallized phases. This can then be checked with mobiletrace element chemistry for amount of hydrous mineralsand level of heating, respectively.

Spectroscopic data can also be quite revealing in this re-gard. The method relies on the amount of molecular waterin minerals, which shows a broad absorption band near2.94 lm, while that of hydroxyl is near 2.74 lm (e.g.Ryskin, 1974; Salisbury et al., 1987). Integrated intensitiesof 2.94 lm bands of thermally metamorphosed carbona-ceous chondrites are expected to be smaller and smootherthan those of non-heated ones and hence should serve asan independent measure of degree of aqueous alterationand thermal metamorphism (Miyamoto, 1991, 1992; Hiroiet al., 1996).

5.2.2.1. CI-chondrites. Both coarse- and matrix-phyllosili-cates in Y-86029 and Y-82162 show partial dehydrationof phyllosilicates evident in high analytical totals (�87–98 wt%). TEM analyses of these phyllosilicates in Y-86029 show highly disordered and defect structures withlattice fringes between 9 A and 13 A, and localized areaswith 4.7 to 4.8 A, which represent mixtures of residual ser-pentine and neoformed olivine. Phyllosilicates in Y-82162also show lattice fringes between 9 A and 12 A and flakyelongated grains with spacings of � 9.5 A, which Akai

(1990a,b) interpreted as corresponding to dehydrated sapo-nite. XRD analysis of Y-86029 matrix shows general broad-ening of olivine typical of incomplete (fine-grained)recrystallization of olivine after thermal decomposition ofphyllosilicates.

Such minerals with highly-disordered and defect struc-tures have been described in terrestrial samples (e.g.Brindley and Zussman, 1957; Ball and Taylor, 1963;Brindley and Hayashi, 1965) as being derived from thetransformation of serpentine- or saponite-type phyllosili-cates. They had previously been described only in Y-793321 (Akai, 1988) prior to this study. Experimental datashow that thermal transformation of saponite occurs at900 �C, while basal reflections of saponite before structuraldecomposition, have been reported to be 9.7 A at 500 �Cand 9.5 A at 600–750 �C (Midgley and Gross, 1956; Akai,1990a,b): these values also correspond to the interlayerspacing in talc. Transformation of serpentine to forsteritecommences at temperatures between 500 and 600 �C(Brindley and Brown, 1984).

Thermal metamorphism in the CI chondrites is alsoapparent in the compositions of secondary phases such ascarbonates. Thermally decomposed Ca–Mg–Fe–Mn car-bonates are abundant in Y-86029 and Y-82162, rather thancalcite or dolomite as in other CI chondrites. The thermaldecomposition temperatures (Td) for these carbonates havebeen interpreted as �470 �C TdFeCO3 < TdMgCO3 << Tdanke-

rite << Tddolomite << Tdcalcite �900 �C (Tonui et al., 2003)based on their compositions and previously establishedthermodynamic models, e.g. Ikornikova and Sheptunov(1973). Ca–Mg–Fe–Mn oxides are also present in thesetwo meteorites, most certainly as heating products of Mg-rich carbonate precursors (Ikeda, 1991; Tonui et al.,2003). TEM analysis also shows the presence of submicrongrains of pentlandite and pyrrhotite within phyllosilicatesand residual olivine in both meteorites. Such grains are ab-sent in unheated CI chondrites Tomeoka and Buseck (1988)suggesting that they result from thermal metamorphism.Tomeoka et al. (1989a) suggested that these sulfides proba-bly form from heating of S–Ni-bearing ferrihydrite but it isimportant to note that no studies to date have establishedthe extraterrestrial origin of ferrihydrite. The elongate mag-netite crystals with sulfide inclusions observed in Y-86029and Y-82162 also appear to be heating/oxidation productsof sulfides. Other indicators of thermal metamorphism inY-82162 are also suggested by results of wet chemical anal-ysis, which show distinctly lower H2O contents in it than inother CI chondrites (Tomeoka et al., 1989a).

Spectroscopic data show a weak, rounded and smooth3 lm band in Y-82162 typical of desiccated phyllosilicates.Trace element data suggest metamorphic temperatures of600–700 �C for Y-82162, and 500–600 �C for Y-86029, con-sistent with petrographic observations.

5.2.2.2. CM chondrites. The most obvious textural evidencefor heating in CM chondrites involves blurring or integra-tion of chondrules with matrix (incipient recrystallization).This is particularly obvious in meteorites that experiencedmetamorphic temperatures exceeding 600 �C, i.e. PCA91008, B-7904, Y-86720 and Y-86789. The matrix phyllosil-


icates in CM chondrites also record this heating as evi-denced by high analytical totals. This is, however, limitedto a few clasts or breccias in the least heated ones like Y-793321 and WIS 91600 but consistently higher (aver-age > 90 wt%) in more heavily heated ones like PCA91008, Y-86720 and Y-86789. The absence of tochilinitein most of these meteorites is almost certainly due to heat-ing as this mineral decomposes to troilite �245 �C (Fuchset al., 1973; Tomeoka et al., 1989b; McKinnon and Zolens-ky, 1984), before coexisting serpentine has begun to signif-icantly dehydrate.

Akai (1988) described incompletely transformed inter-mediate phases consisting of desiccated phyllosilicates andresidual olivine in Y-793321 similar to those observed inY-86029. Our petrographic observations, however, revealthat heating was limited to a few clasts with no evidenceof pervasive heating. Interestingly, our XRD spectra showvery weak peaks for olivine and pyroxene, while phyllosili-cates are absent. The reflectance spectrum of Y-793321 isalso comparable to that of 400–500 �C Murchison, buttrace element data shows no obvious loss of even Cd. Whilethis may reflect differences in samples utilized in these stud-ies, it is conceivable that Y-793321 may have experienced alate stage retrograde aqueous alteration episode that ob-scured an earlier thermal metamorphic event (Tonuiet al., 2002). However, since Y-793321 has not lost Cd,heating must have been 6400 �C.

TEM observations of PCA 91008 reveal the presence ofolivine with morphological outlines of serpentine-likephases, i.e. flakes and cylinders. Lattice fringes of phyllo-silicates are absent. This is also true of Y-86720 althougha few lattice fringes similar to those encountered in Y-793321 and Y-86029 are present (Akai, 1990a,b). The ma-trix of B-7904 also consists of olivine with lesser amountsof poorly crystallized saponite and a phase intermediatebetween serpentine and enstatite (Zolensky et al., 1989b).WIS 91600 shows poorly-crystallized phyllosilicate phasesas in Y-86029 and Y-793321. This study has more thanconfirmed the presence of these phases in meteorites,which had hitherto been well-described only in terrestrialsamples.

XRD reflections also indicate the level of heating basedon amounts of newly-formed anhydrous phases. The reflec-tions from olivine and low-Ca pyroxene are weak in Y-793321 suggesting that it consists mainly of amorphousmaterial formed by phyllosilicate breakdown as discussedearlier. The degree of decomposition of phyllosilicates(and increase in crystallinity of olivine) is as follows:Y-793321 < WIS 91600 < EET 90043 < PCA 91008 <B-7904 = Y-86789. The well-developed olivine and lowCa-pyroxene peaks in PCA 91008, B-7904 and Y-86789indicate that they were heated to temperature higher than600 �C based on the T–T–T diagram of experimentallyheated Murchison (Akai, 1992).

XRD reflections also confirm that strongly heated B-7904 and Y-86789 contain abundant kamacite and taenite;this is not obvious in PCA 91008, perhaps because of terres-trial weathering. On the contrary, the less heated meteoritescontain abundant magnetite indicating that oxygen fugacityin the portion of asteroids where these meteorites were

located decreased with increasing temperatures (Nakamuraet al., 2000). The co-existing carbonaceous material proba-bly experienced lower oxygen fugacities at high tempera-tures (Kitajima et al., 2002). The carbonaceous materialacted as a reducing agent, facilitating dehydration reactionsduring and after phyllosilicate dehydration through thereaction: 2C + 2H2O = CO2 + CH4 (Ohmoto and Kerrick,1977). The lowering of oxygen fugacity allowed formationof Fe metal from Fe oxide.

Spectroscopic data confirm the phyllosilicate dehydra-tion in the CM chondrites. The shapes of the 3 lm bandsin B-7904, Y-86720, Y-86789 are more rounded thanthose of Y-793321, WIS 91600 and EET 90043 and com-parable to those in 500–600 �C and 400–500 �C artifi-cially-heated Murchison, respectively. It is important tonote, however, that terrestrial weathering significantly af-fects the shapes of the 3 lm bands, which may explainthe apparent lack of these features in PCA 91008. Traceelement data suggest the following order of heating forthe CM chondrites: Y-793321 < WIS 91600 = EET90043 = A881655 < PCA 91008 < B-7904 = Y-86720 = Y-86789. This is generally consistent with petrographicand spectroscopic data. Of course the duration of heatingcan be an important factor in thermal metamorphicphase changes, and thus study has not attempted to ad-dress this issue. We have assumed that the heating eventwas driven by an internal heating mechanism, and soheating duration would have been approximately equalfor all meteorites. However, if the thermal metamorphicevent was very short-lived for some of these meteorites(due to a large impact, for example) then the metamor-phic order we have suggested above could be in error.We note that Quirico et al. (2009) used the organiccontent of PCA 91008 and WIS 91600 to conclude thatthe duration of thermal metamorphism was “short”,but obviously this result requires confirmation andquantification.

5.2.3. Other indicators of thermal metamorphism

Other indicators of heating especially in CM chondritesinclude lower water and C (as CO2) contents (Ikeda, 1992).Nakamura et al. (2000) and Kitajima et al. (2002) also char-acterized the dehydration of hydrous minerals in CM chon-drites using thermal loss of trapped noble gases andgraphitization of carbonaceous macromolecular matter.Their results indicate that the least heated CM chondrites,e.g. Y-793321, are solar-gas rich, while the most heatedones, like Y-86789, are solar-gas poor. The concentrationof trapped 22Ne (solar + Ne-A) is much higher inY-793321 than in Y-86789, due to a large contribution ofsolar gases. Solar gases are usually released at low temper-ature demonstrating their shallow implantation depth intominerals. Indeed, isotopic ratios suggest that Ne in thelow-temperature steps during heating of these meteoritesconsists mostly of solar gases. The release pattern ofY-793321 shows a lowered solar-gas concentration ex-tracted at low temperatures of 250 and 400 �C, which canbe ascribed to heating at approximately 400 �C (Nakamuraet al., 2000), consistent with petrographic and chemicaldata of our study.


The degree of graphitization of macromolecular matter(e.g. kerogen or poorly-crystalline graphite) has also beenused to determine heating in carbonaceous chondrites(Kitajima et al., 2002). When chondritic carbonaceousmatter was heated in the parent asteroids, it becamegraphitic due to carbonization and ‘graphitization’ reac-tions. The (002) interlayer spacing of the graphitic materialdecreases to a constant value of 3.35 A with an increasingdegree of ordering of carbonaceous matter (Rietmeijerand Mackinnon, 1985). The results showed that pyrolyzatesfrom strongly-heated CM chondrites are lower inconcentration compared with unheated ones. Theweakly-heated ones like Y-793321 showed intermediate lev-els of pyrolyzates. This indicates that graphitization of thecarbonaceous matter is most extreme in the stronglyheated chondrites and that during graphitization, mostvolatile portions are lost, which can generate pyrolyzatessuch as naphthalene (Kitajima et al., 2002). Thus,graphitizaton at high temperatures occurred at oxygenfugacities below the graphite stability limit (Ohmoto andKerrick, 1977) and IW buffer (Ernst, 1976). The reducingconditions may explain why SiC, probably a neoncarrier, in CM chondrites is resistant to heating as isSiC in enstatite chondrites (Huss and Lewis, 1995;Nakamura et al., 2000).

Table 5Classification of thermally metamorphosed CI and CM chondritesin this study.

Meteorite Classification

Y-82162 CI1TII/IIIY-86029 CI1TIIIB-7904 CM2TIVY86720 CM2TIVY86789 CM2TIVA-881655 CM2TIIIY-793321 CM2TIIPCA 91008 CM2TIIIWIS 91600 CM2TIIEET 90043 CM2TII/III

6. CLASSIFICATION

We can propose a simple classification scheme for ther-mally-metamorphosed carbonaceous chondrites that buildson all earlier work. However, the scheme assumes that theoriginal nature of the meteorite is recognizable, and that theworked has access to TEM or XRD instrumentation, andthus may not be widely applicable. Nakamura (2005) hasalready subdivided thermally metamorphosed CI, andCM chondrites into Stages I–IV according to matrix miner-alogical characteristics. His scheme utilizes measurements,variously, of reflectance spectra, trace elements, noblegases, oxygen isotopes, labile organics, petrography, micro-structures, and mineralogy. However, his thermal meta-morphic classes can be defined to a large degree on thebasis of matrix mineralogy. Stage I (heating < 250 �C) hasexperienced the lowest heating, and the sensitive matrix lay-ered, hydrous minerals saponite, serpentine and tochiliniteare largely unaffected. TEM observations of phyllosilicateslattice fringes reveal minor differences from ideal dimen-sions, and increased stacking faults. Reflectance spectrabest reveal the heating. At Stage II (roughly 250–500 �C)serpentine is amorphous, but not recrystallized to olivine.No tochilinite is present in CMs, which has been replacedby low-crystallinity, neoformed troilite. At Stage III(roughly 500–750 �C) low crystallinity, fine-grained olivineis present among still amorphous matrix phases, which alsoinclude low-crystallinity neoformed troilite. By Stage IV(roughly > 750 �C) matrix is completely anhydrous. matrixolivine is well-crystalline, and intergrown with neoformedtroilite. The stage temperatures are approximate becausethey depend on many complex factors, including heatingduration, sample porosity and grain size, presence of organ-ics, etc.

We can combine Nakamura’s heating Stages with themeteorites original classification, to yield a more completeclassification. We add the letter “T” to indicate the activityof thermal metamorphism. Thus a CM2 chondrite that hasbeen metamorphosed to Stage III would be designatedCM2TIII. A CI chondrite metamorphosed to Stage IIwould be designated CITII.

A basic question one should ask is, is a new classificationscheme for metamorphosed carbonaceous chondrites advis-able at this time? In general classification schemes are mostuseful when they do not assume knowledge of the occur-rence of some hypothetical processes, in this case aqueousalteration followed by thermal metamorphism. It is alwayspossible that future work will show that some other pro-cesses were at work for these meteorites, leaving us with aclassification scheme no longer grounded in reality. Forexample the chondrite petrological types 1–5 were origi-nally defined with 1 being most primitive, and 5 being themost thermally processed (Wiik, 1956; Van Schmus andWood, 1967). It was later realized that type 3 was the mostprimitive, and types 2–1 had been increasingly aqueouslyaltered (McSween, 1979), resulting in some years of classi-fication confusion. However the new interpretation of thechondrite types has easily replaced the original one. Onemight also ask whether the number of known thermally-metamorphosed carbonaceous chondrites warrants a newclassification scheme. That might have been true in themid 1980s when the first three of these meteorites were rec-ognized, however now there are at least 2 dozen knownmetamorphosed carbonaceous chondrites, so a basic classi-fication scheme will be useful. In Table 5 we present theproposed classifications for the meteorites examined in thisstudy.

7. SUMMARY

Thermal metamorphism in the CI and CM chondritesdescribed here is apparent in their textural, petrologic,chemical, isotopic and spectroscopic characteristics.Micro-scale examination reveals the presence of poorly-crystalline, highly-disordered phases during transformationof phyllosilicates to anhydrous phases, described previouslyin terrestrial samples. Thermal dehydration is probablyresponsible for the unique isotopic compositions of some


of these meteorites associated in part with release of isoto-pically light water and heavy-isotope enrichment in the so-lid residue. This study underscores the importance ofutilizing a wide range of complementary analytical tech-niques to disentangle the macro- and micro-scale processesassociated with thermal metamorphism and their relativemagnitudes.

ACKNOWLEDGEMENTS

This research has been carried out as part of the NationalResearch Council (NRC) associateship award to E.K.T. M.E.Zwas supported by grants by NASA’s Origins of Solar Systemsand Cosmochemistry Programs. We are grateful to Robert Clay-ton and Toshiko Mayeda for oxygen isotopic analysis of thenewly-analyzed meteorites, and for a detailed review of the man-uscript. An anonymous GCA reviewer significantly improved anearlier version of this paper. M.E.L also acknowledges with greatappreciation the staff at the University of Missouri ResearchReactor for aid in neutron irradiations, which were supportedby DOE grant DE-FG07-01ID14146. Additional support forM.E.L was provided by NASA grant NAGW-3396. We aregrateful to Hideyasu Kojima and NIPR for providing thin sec-tions of some of the new carbonaceous chondrites. We are alsograteful to Craig Schwandt for his assistance with technical as-pects of electron microprobe and SEM work. We acknowledgeNIPR and U.S. National Science Foundation for supportingthe collection of Antarctic meteorites (via the Japanese AntarcticResearch Expeditions, JARE, and the Antarctic Search forMeteorites, ANSMET) which continue to provide us withinvaluable samples for these studies.

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