Variable Ti-content and grain size of titanomagnetite as afunction of cooling rate in very young MORB

Weiming Zhou *, Rob Van der Voo, Donald R. Peacor, Youxue ZhangDepartment of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109-1063, USA

Received 12 October 1999; accepted 24 March 2000

Abstract

Transmission electron microscopy observations and rock magnetic measurements on a pillow from the `New Flow',extruded in 1993 at the Juan de Fuca Ridge, demonstrate that Ti-content in large (up to 40 Wm) titanomagnetite grainsvaries as a function of the cooling rate. Large grains in the interior of the pillow have a narrow composition range ofapproximately x = 0.6, where x represents the ulvo«spinel content in the solid solution series with magnetite. In contrast,the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range (averagex =V0.45). Within 0.5 cm of the rim a significant portion of the grains is single domain (SD) to superparamagnetic(SP) and appears to have little or no Ti. This lower Ti-content provides a ready explanation for the higher Curietemperature near the glassy margin of the pillow. Moreover, determination of the oxidation state of the titanomagnetitedoes not show low temperature oxidation anywhere within the pillow, indicating no rapid alteration totitanomaghemite as previously suggested for this very young mid-ocean ridge basalt (MORB) in order to explain theCurie temperatures. Submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variablex values. These grains make contributions to susceptibility that show up as high (up to V580³C) Curie temperatures inthermomagnetic analyses and they appear to be SD to SP. The variation of x values for the titanomagnetite in theMORB is consistent with thermodynamic and kinetic considerations, because crystallization of titanomagnetite in theMORB is controlled by processes depending on cooling rate and crystal-melt fractionation. ß 2000 Elsevier ScienceB.V. All rights reserved.

Keywords: titanomagnetite; titanium; chemical composition; variations; Curie point; mid-ocean ridge basalts; electron microscopy

1. Introduction

It has long been known that pillows of mid-ocean ridge basalt (MORB) show a substantialchange in textural and mineralogical features asa function of cooling rate, which obviously de-

creases rapidly from the pillow rim (`glassy mar-gin') toward the interior. These variations areof great importance to the characteristics ofthe MORB's magnetization. Titanomagnetite,Fe33xTixO4, where x is the ulvo«spinel contentin the magnetite3ulvo«spinel solid solution(09 x9 1), is the major carrier of magnetic rema-nence in MORB. Because its grain size varies rap-idly from rim to interior [1^5], there are inherente¡ects on the magnetization properties that de-pend on the distance from a pillow rim. An addi-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 0 0 - X

* Corresponding author. Tel. : +1-734-764-8322;Fax: +1-734-763-4690; E-mail: zhou@umich.edu

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tional, complicating factor in the granulometricand compositional characterizations of titano-magnetite is that it is known to undergo low tem-perature oxidation as the MORB ages [6^9].However, the time scale and grain-size depen-dence of this maghemitization process are stillcontroversial [10^15], because it has been di¤cultto determine the degree (z, with 09 z9 1) towhich grains are maghemitized.

Although variations of magnetic propertieswithin MORB pillows have been studied for dec-ades [1^5], it is not well understood whether suchvariations are caused by the variations of initialtitanomagnetite mineralogy or by subsequent lowtemperature oxidation. The possible explanationsfor these variations have generally included onlythe e¡ects of one or two of the three main varia-bles, i.e. (1) grain size, d, (2) composition, x, or (3)maghemitization, z. This is because the observa-tions have, in general, not been adequate to eval-uate the e¡ects of all three causes completely andsimultaneously. Previous studies have tended toargue that low temperature oxidation [3,4,12]and rim-to-interior grain-size variations [5] areprimarily responsible for observed variations inmagnetic properties such as Curie temperature(Tc), hysteresis parameters or low temperature be-havior.

In this paper, we will argue that electron micro-scope observations are an essential ingredient, be-cause they can now contribute information aboutx, z and d simultaneously, with z being deter-mined from x and the lattice parameter, a. Thelarger titanomagnetite grains in the interior of aMORB pillow have been relatively well studiedbecause these grains have sizes in the range ofseveral tens of Wm; values of d, x, a or even zcan be determined with an optical microscope,electron microprobe and X-ray di¡ractometer[16^19]. However, rapid cooling of MORB pil-lows results in rims with very ¢ne-grained titano-magnetites, which are beyond the size-limit of thetraditional techniques. As a result, the composi-tion of ¢ne-grained (6 1 Wm) titanomagnetitenear pillow rims has usually been assumed to bethe same as that of the large grains in the interior[2,4,12]. Moreover, in the last few years new stud-ies [13,20] have shown that submicrometer titano-

magnetite grains also occur in pillow interiors,where they are found in interstitial glass. In thepresent study, we carry our electron microscopicobservations one step further, including system-atic observations of d, x and z as a function ofdistance to the pillow margin; such a systematicpro¢ling analysis was not done in our earlierstudy [20]. The sample is from a pillow from`zero-age' MORB (called the New Flow), whichextruded in 1993 at the Juan de Fuca Ridge[20,21]. The sample spans about 7 cm in depthfrom the pillow rim to the interior. The rim, oftencalled `glassy margin', is actually mostly crystal-line and very ¢ne grained, although it containssome glass. Towards the interior, the grain sizeof the silicates increases rapidly. The interior,also, contains glass (`interstitial glass'), but thisglass has a di¡erentiated composition with respectto the original melt. Outside the pillow rim, occa-sional crusty patches (1^5 mm thick) occur of truebasaltic glass; this crust is not included in the 7 cmmentioned above.

2. Titanomagnetite grain size variation within thepillow

In order to describe the granulometry of oursample, we characterize the titanomagnetite grainsin two groups, A and B, beginning at V0.5 cm indepth. These groups can be considered as beingdi¡erent in size, but also in their genesis.

Group A grains crystallized in (metastable)equilibrium with the MORB's melt and have thelarger grain sizes of the two groups. Their max-imum grain size increases from V1 Wm at 0.5 cmdepth (Fig. 1B) to more than 40 Wm at about 6 cmin the interior of the pillow (Fig. 1D). At thesedepths, smaller Group A grains do exist, ofcourse, but in order to portray that the size ofGroup A grains systematically increases withdepth, we have opted to plot the maximum di-mension (longest axis) in Fig. 2A (closed sym-bols). The choice of the maximum dimensionalso avoids the problem that the cutting surfaceoften intersects mineral grains at non-optimal di-mensions. Abundant smaller Group A grains(generallys 0.5 Wm) can be observed together

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with larger grains at intermediate depth (Fig. 1C)(see also [20]), but larger Group A grains becomedominant at greater depth (Fig. 1D), i.e. the rel-ative abundance of the smaller Group A grainsdecreases with depth.

Group B grains are the smaller of the twogroups. In the pillow interior they are embeddedin interstitial glass, which originated as an immis-cible liquid within a highly di¡erentiated residualmelt as has been described in an earlier paper [20].Their maximum sizes vary only slightly withdepth, from less than 0.1 Wm at 1 cm depth tomore than 0.2 Wm at about 6 cm depth (Fig.2A, open circles). Their size distributions corre-spond to the superparamagnetic (SP) and singledomain (SD) range. Such small grains in MORBpillow interiors have previously been recognizedby detailed electron microscopic and rock mag-netic studies [13,20,22,23] and they likely play amore important role in the MORB's magneticproperties than some authors have allowed [5];this will be discussed further in this section, be-low. The interstitial glass commonly occurs inpolygonal areas between large plagioclase and py-roxene crystals. The dimensions of interstitialglass volumes and, therefore, the abundance ofGroup B grains, increase toward the interior ofthe pillow. The submicrometer Group B grainsoften have anhedral shape [20] whereas largerGroup A grains typically show dendritic and cru-ciform quenched morphologies (Fig. 1C,D).

Within 0.3 cm of the pillow rim and within thecrusty, glassy patches outside the pillow rim ofour sample, only tiny titanomagnetite grainshave been observed with a maximum size ofonly tens of nm. The glassy crust above the pillowrim consists of massive basaltic glass as a result ofextremely rapid cooling. Spherical (titano-)mag-netite grains with sizes of tens of nm were previ-

6Fig. 1. TEM and scanning electron microscopy (SEM) im-ages showing variation of titanomagnetite grain size withinthe New Flow pillow. (A) TEM bright-¢eld image within afew mm of the pillow rim. Small spherical grains are (tita-no)magnetite. (B)^(D) SEM backscattered-electron imagesfrom depths of 0.6, 3 and 6.5 cm, respectively. Grains withbright contrast are titanomagnetite. Mt, titanomagnetite; Pl,plagioclase; Cpx, clinopyroxene; Gl, interstitial glass.

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ously observed by Pick and Tauxe [24]. The sizeof tiny titanomagnetite grains in the rim's inter-stitial glass (Fig. 1A) suggests an a¤nity withgroup B grains. On the other hand, the interstitialglass within a few mm of the rim is less di¡er-entiated compared to that in the interior of thepillow and its composition is close to those ofbasaltic melts, implying an origin similar to thatof Group A grains. Although an origin throughcrystallization from the melt (Group A) is mostlikely, we cannot completely rule out that thesegrains are genetically related to Group B; theyhave therefore been given a separate symbol(Fig. 2a, star). Material within 0.3 cm of the pil-low rim (`glassy margin') consists of ¢ne-grainedsilicates, dominantly plagioclase and augite, withminor proportions of small titanomagnetitegrains, the glassy appearance being a result ofsmall crystallite size rather than the presence ofdominant glass. Such tiny titanomagnetites oftenhave spherical shape, similar to those in the ba-saltic glass and are embedded in glass surroundedby silicates (Fig. 1). Chemical analyses on thesesmall spherical grains show high Fe contents andelectron di¡raction patterns indicate a face-cen-tered cubic structure, consistent with (titano)mag-netite. Some grains also contain sulfur, possiblyfrom pyrrhotite. These grains are likely to be SDand SP.

While it is clear from the above that the size ofGroup A titanomagnetite grains indeed varies asa function of cooling rate within the pillow [1^5],it is also increasingly clear that the interior of thepillow contains a very large variation in grain size,from less than 0.1 to tens of Wm, when Group Bgrains are included. Only the size of Group Atitanomagnetite grain can be considered as in-creasing signi¢cantly in size towards the interiorof the pillow, as consistent with decreasing cool-ing rate (Figs. 1 and 2A).

Magnetic hysteresis measurements (Fig. 2B) areconsistent with our electron microscopic observa-tions. Both susceptibility (M) and saturation mag-netization (Ms) increase, as expected, with depthwithin the pillow. The ratio of saturation mag-netic remanence (Mrs) to saturation magnetization(Ms) is V0.6 in sub-samples a and b near thepillow rim (see bottom of Fig. 2 for location)

and decreases gradually to V0.35 for sub-samplesk and l. The ratio of the coercivity of remanence(Hcr) to the bulk coercivity (Hc), meanwhile, ¢rstdecreases and then increases from sub-sample cinward (Fig. 2B, inset). This looping behavior inthe Day plot [25] is identical to the observationsof Gee and Kent [5,12], indicating that the hyste-resis parameters that we observe are in good ac-cord with those from other MORB pillows. The

Fig. 2. Variation of titanomagnetite grain size and magneticproperties with depth from the glassy margin of New Flow,with sub-sample labels as indicated at the bottom. (A) Maxi-mum grain size for Group A and Group B grains (see textfor explanations). The plotted data represent the long axesof the largest titanomagnetite grains observed at a particulardepth. The star symbol represents the (titano)magnetitegrains within 0.3 cm of the pillow rim. (B) Variation of mag-netic properties within the pillow. The saturation magnetiza-tion (Ms) and magnetic susceptibility (M) increase with depth.Inset: Plot of hysteresis parameters [25], showing a clear var-iation with depth.

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segment a^b of the loop is interpreted [5] as dueto a mixture of SD and SP grains and we concur,as it agrees well with our measured granulometry(Fig. 2A). The segment c^l is interpreted as amixture of MD+SD+SP grains, which di¡ersfrom the MD+SD interpretation of Gee andKent [5], in that they reason that SP grains arelargely absent in the pillow interior. However, wehave seen that our Group B grains, also describedearlier [13,20], range in size from a maximum ofabout 0.2 Wm to tens of nm. This suggests thatsome of them are likely to be SP.

In passing we note that interpretations of hys-teresis parameters in a Day diagram are mademore complicated if the magnetic grains are notmonomineralic. Not only can compositional var-iations (x) and oxidation (z) play a role, but acombination of titanomagnetite and its alterationto goethite [13], can lead to high Hcr/Hc ratios,which can be misidenti¢ed as SD+SP titanomag-netite mixtures [5].

3. Ti-contents variation within the pillow

Compositions and lattice parameters of titano-magnetite grains have been obtained using trans-mission electron microscopy and analytical elec-tron microscopy (TEM/AEM), which allows adetermination of ulvo«spinel content (x) and de-gree of oxidation (z) even for some submicrometergrains. Rock chips from the rim to the interior ofthe New Flow pillow were ion-milled for TEM/AEM analysis, overlapping in depth with (but notidentical to) the sub-samples shown at the bottomof Fig. 2. We tried to analyze as many titanomag-netite grains (group A) as possible. We also usedan electron microprobe to verify the compositionof large titanomagnetite grains in the interior ofthe pillow. Fe3O4 and Fe2TiO4 components to-

Fig. 3. Histograms of x values in group A titanomagnetite atdi¡erent depths of the New Flow pillow. All x values arecalculated from the Ti/Fe ratio (x = 3Ti/(Ti+Fe)) obtained inchemical analyses using TEM/AEM, except that the histo-gram at 5.0^7.0 cm depth also includes 47 electron micro-probe analyses. All histograms have the same vertical scaleexcept for the one at 5.0^7.0 cm.6

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gether make up 90^94% of titanomagnetite, withminor Al2O3, V2O3, MnO, CaO and MgO. Fig. 3shows histograms of the titanomagnetite compo-sitions (x values) at di¡erent depths of the pillow.Titanomagnetite in the interior of the pillow atdepthss 3 cm shows a narrow compositional var-iation with x values between 0.55 and 0.69. Themajority of x values is close to 0.6, which is sim-ilar to those commonly reported [16,17]. The com-positional variation increases and a signi¢cantfraction of x values is 6 0.6 toward the pillowrim. Not only does the range of x values broadentowards the rim, the average value of x also de-creases (Fig. 3).

The tiny grains in the basaltic glass and thosevery close to the rim, with dimensions of tens ofnm (Figs. 1A and 2), are too small for accuratechemical analysis. Fig. 3, therefore, does not in-clude data from these grains. We will see in Sec-tion 4, however, that Tcs as high as 580³C implymuch lower x values, even approaching those forpure magnetite (x = 0). When x values at the rimapproach zero, it is not necessary to invoke rapidlow temperature oxidation to explain the high Tcs[12].

Fig. 4 shows the histogram of x values of groupB titanomagnetite grains, which is based on datafrom our previous study [20]. It shows a widerange of x values between 0 and 0.8. The compo-sitional variation of group B grains, therefore, is

much more variable than that of Group A. As wewill see in a later section, thermomagnetic experi-ments support the co-existence of low-Ti andhigh-Ti magnetite in the pillow interior.

Magma composition is an important factor af-fecting the composition of titanomagnetite in theMORB. Therefore, we analyzed the compositionof basaltic glass from the rim of the New Flowpillow with an electron microprobe. The analyses(by weight) yield 50.32% SiO2, 1.61% TiO2,13.56% Al2O3, 12.63% FeO (total iron), 0.29%MnO, 6.66% MgO, 11.39% CaO, 2.56% Na2O,0.12% K2O and 0.16% P2O5, with a total of99.31%. The glass has an atomic Ti/Fe ratio of0.11; such a ratio is typical for MORB melt andwould result in titanomagnetite with x = 0.30 if allTi and Fe crystallized exclusively in these grains.We will return to this possibility in our discussionbelow of the thermodynamic and kinetic controlson titanomagnetite crystallization, but note that

Fig. 4. Histogram of x values of Group B titanomagnetitesin the interior of the New Flow pillow (data from [20]).

Table 1Determination of oxidation state for selected titanomagnetitegrains in New Flow

Depth (cm) z a x ax�0:6

Group A0.5 V0 8.452 0.50 8.4710.5 V0 8.463 0.56 8.4711.0 V0 8.453 0.49 8.4741.0 V0 8.455 0.51 8.4721.5 V0 8.455 0.52 8.4711.5 V0 8.471 0.60 8.4712.0 V0 8.471 0.60 8.4712.0 V0 8.452 0.50 8.4712.5 V0 8.453 0.50 8.4722.5 V0 8.467 0.56 8.4753.0 V0 8.478 0.64 8.4703.5 V0 8.473 0.61 8.4714.0 V0 8.473 0.61 8.4714.5 V0 8.477 0.62 8.473s 5.0 V0 8.473 0.60 8.473s 5.0 V0 8.475 0.61 8.473s 5.0 V0 8.477 0.63 8.472Group B4.5 V0 8.473 0.57 8.4795 V0 8.461 0.55 8.4716 V0 8.470 0.58 8.474

a = lattice parameter calculated from TEM HOLZ patterns,x = x value from AEM analyses, z = estimated oxidation statebased on a and x values, ax�0:6 = a values adjusted to x = 0.6.

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some x values as low as 0.3 are indeed observednear the pillow rim (Fig. 3).

4. Determination of oxidation state (z)

The degree of low temperature oxidation hasbeen investigated using TEM. Because the degreeof oxidation (z) is a function of lattice parameter(a) and composition (x) [26,27], z can be esti-mated if the values of a and x are known. Todetermine a we have employed higher-orderLaue zone lines in convergent-beam electron dif-fraction [28] of individual titanomagnetite grains.The values of x have been obtained for the samegrains by AEM, as described in Section 3. Tita-nomagnetite in MORB often contains minor ele-ments other than Fe and Ti, including Al, Mg andMn, which may a¡ect a values and Curie temper-atures [19,29] of titanomagnetite. To solve theproblem, we established the relationships betweena, z and x values for naturally occurring titano-

magnetite in MORB [28]. These relationshipswere used to determine z values of titanomagne-tite in this study. At any rate, because variationsin contents of minor elements in titanomagnetiteare small owing to a relatively uniform chemicalcomposition of MORB, they have insigni¢cante¡ects on the determination of z values. For atypical range of values of the content of the minorelements, the corresponding uncertainty in z is ofthe order of 0.05 [28].

Table 1 lists the measured x values and latticeparameters (a), obtained from representativeGroup A titanomagnetite grains at di¡erentdepths in the pillow, together with the z valuesestimated from these determinations. The z valuesfor all grains are V0, indicating no oxidation.Only a few determinations were possible of aand x for Group B grains, but they similarlylead to estimates of z for these grains that areclose to zero (Table 1). The near-zero z valuesfor all studied grains support our contentionthat the observed higher Tcs are not caused bylow temperature oxidation, but instead by thelower Ti-contents shown in Figs. 3 and 4.

These electron microscopic data are consistentwith the magnetic properties. Magnetic suscepti-bility versus temperature curves obtained fromrock chips are fully reversible (e.g. sub-samples cand f in Fig. 5), implying that low temperatureoxidation is unlikely.

5. Curie temperature variation within the pillow

Curie temperatures (Tc) have been determinedfrom the in£ection point of the susceptibility (M)versus temperature curves by heating in noble gasusing a KappaBridge [30]. The Tcs obtained forsub-samples a through l (i.e. at increasing depthfrom the rim) correlate with the x values of thetitanomagnetite grains to a signi¢cant degree(Figs. 3 and 5). Tcs are dominated by the volu-metrically dominant Group A grains in the pillowinterior and are generally between 130 and 190³C(sub-samples d^l at depths of 1.5 to 6.2 cm fromthe rim; see Fig. 2 for location). These temper-atures correspond to those of unoxidized titano-magnetite with x values of about 0.55 to 0.62

Fig. 5. Susceptibility (M) versus temperature curves obtainedfrom rock chips at di¡erent depths of the New Flow pillow,showing an increase in Curie temperature from the interior(sub-sample l, see Fig. 2 for location) toward the rim of thepillow (sub-sample a) and for a chip of basaltic glass outsidethe pillow rim. The two dotted lines are cooling curves forsub-samples c and f, respectively.

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[16,17,29] and this matches well the observed xvalues of Group A grains at these depths (Fig.3). Group B grains are volumetrically less impor-tant than Group A grains, but they neverthelessappear to have a minor in£uence on the Tc prop-erties. About 5% of the susceptibility signal sur-vives up to temperatures of about 575³C, whichagrees with the lower x values for Group B grains[20].

Sub-sample a and a chip of the basaltic glassycrust are characterized by much higher Tcs thanthe sub-samples in the pillow's interior. The sus-ceptibility of these chips shows a gradual dropbetween 200 and 550³C, followed by a sharp de-crease between 550 and about 580³C (Fig. 5). Thisbehavior matches the temperatures expected onthe basis of intermediate to low x values forthe tiny grains very close to the pillow rim andhas previously been observed in rim glass [24,31^33].

In Fig. 5, the curves for di¡erent sub-samplesare normalized to Mmax = 1. However, when theabsolute values of the susceptibility are takeninto account, the shoulders below about 550³Cin the curves for the glass and sub-sample a areseen to be of about the same magnitude as thesusceptibility values for sub-samples c^l at tem-peratures between 300 and 580³C. This impliesthat the volume of tiny (titano)magnetite in theglassy crust and pillow rim and the volume ofGroup B grains with very low x values in thepillow interior are approximately equal.

6. Discussion and conclusions

6.1. Thermodynamic and kinetic control ontitanomagnetite composition

A broad range of cooling rates occurs in aMORB pillow. The interior cools relativelyslowly, with a rate of about 0.07³C/s at 6 cmdepth, as estimated from heat conduction. In con-trast, the pillow rim probably cooled at a rapidrate of some 50^100³C/s at 2^3 mm depth, where-as the glassy crust outside the pillow rim mayhave cooled at V1000³C/s [34]. The trends inTi-content and grain sizes of Group A titanomag-

netites at di¡erent depths are consistent with thesecooling rates.

In the interior of the pillow, the crystallizationof titanomagnetite may roughly approach (meta-stable) equilibrium owing to a relatively low cool-ing rate. This means that the crystallization ofoxides and silicates proceeds following thermody-namic principles. Accordingly, the Ti/Fe ratio in atypical titanomagnetite crystal will be about0.26 þ 0.03 (corresponding to x = 0.62 þ 0.08) andthis ratio is related to the Ti/Fe ratio in the melt(0.11, x = 0.30) through surface equilibrium, i.e.the crystal composition at the surface is in equi-librium with, and therefore di¡erent from, that ofthe melt at the time of crystallization. As a result,the scatter of Ti/Fe ratios and x values in titano-magnetite is small in the pillow interior (Fig. 3).The Ti/Fe ratio is higher in the titanomagnetitethan in the melt as a result of fractionation be-tween the melt and the crystal in equilibrium withthe melt. This is not surprising, even though thephase equilibrium between titanomagnetite andmelt is not well understood. However, studieson equilibrium between silicates and melt showsimilar behavior and are better understood. Asan example, one can examine the equilibriumcomposition of olivine, which has much higherMg/Fe and Mg/Si ratios than the melt.

Close to the pillow rim, the Ti/Fe ratio in tita-nomagnetite grains has been observed to be morevariable, ranging from 0.3 to 0.11 (i.e. becomingthe same as that in the melt). The formation oftitanomagnetite with a Ti/Fe ratio approachingthat of the melt at increasing cooling rate is againnot unexpected on the basis of experimental stud-ies and thermodynamic and kinetic theories.Quantitative theoretical prediction of how the xvalues of titanomagnetite crystallizing from a meltdepend on cooling rates is not possible at present,but again comparisons with silicate systems pro-vide insights. Experimental data for a water-richplagioclase system, for instance, show that, as thecooling rate increases, the composition of the pla-gioclase approaches that of the melt [35].

Qualitatively, the variation in composition oftitanomagnetites depends on the following fac-tors: (i) the temperature and local melt composi-tion (including oxygen fugacity) at the time of

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titanomagnetite formation; (ii) metastable ther-modynamic control and (iii) kinetic control. Allthese factors lead to lower x values at increasedcooling rates. When the cooling rate increases,titanomagnetite would nucleate and grow at alower temperature. Calculation using the MELTSprogram [36] shows that the x value of titanomag-netite in equilibrium with the melt and other min-erals decreases with temperature from V0.6 at950³C to V0.2 at 700³C. Although perfect equi-librium is not expected and titanomagnetite thatformed at a higher temperature is expected tozone toward a lower x value, the calculated trendis nevertheless consistent with observations. Theconcept of metastable surface equilibrium is illus-trated by Zhang et al. [37] for the case of crystaldissolution. For crystallization from a super-cooled melt, applying a similar approach leadsto smaller fractionation between the crystal andthe melt. The kinetic control depends on the rel-ative rate of crystal growth and di¡usion. Whencrystal growth is slow (at small supercooling) rel-ative to di¡usion, di¡usion can supply `nutrients'at the equilibrium proportion to the growing crys-tal. This corresponds to the case of titanomagne-tite in the pillow interior (3^7 cm depth, Fig. 3).However, as the crystal growth rate increases rap-idly relative to the di¡usion rate, di¡usion cannotsupply nutrients at such proportions. Hence thecrystal takes whatever it can get from the melt,leading to increased deviation from the equilibri-um partitioning. At some high growth rates, asteady state may be reached, leading to similarTi/Fe ratios in the titanomagnetite and the melt,as indeed observed at 0.3^0.7 cm depth (Fig. 3).

Rock magnetic data imply that the Ti/Fe ratioin titanomagnetite in the glassy margin and basal-tic glass is 6 0.11 (x6 0.3) (Fig. 5). We infer thatthe composition of titanomagnetite in the basalticglassy crust is almost entirely determined by dif-fusion (di¡usive fractionation) so that elementswith smaller di¡usivity are incorporated in thecrystal to a lesser extent. In this scenario, Ti/Feratio in titanomagnetite would be smaller thanthat in the melt because Ti di¡usivity is muchsmaller than Fe di¡usivity [37^39].

Finally, the variable Ti-content of the tinygrains (group B) within interstitial glass in the

pillow's interior (Fig. 4) remains to be discussed.Details of the oxides and liquid from which theycrystallized were presented earlier [20]. The tinytitanomagnetites are found in globules (up to 0.2Wm in diameter) exsolved from the residual glassowing to immiscibility. The interstitial glass doesnot have the same composition as the basalticglass in the pillow rim. It consists dominantly ofSiO2 and Al2O3 (s 90% by weight) and is de-pleted in other cations, representing a highly dif-ferentiated residual material. In contrast, the ex-solved small globules are highly enriched in totalFeO (up to 40% or more by weight), consistentwith experiments on immiscible silicate liquids(with one phase containing 30^50% total FeOand the other phase depleted in FeO [40,41]).The x values of group B titanomagnetite withinthe immiscible high-FeO globules vary widely(from 0 to 0.8) (Fig. 4) although the size of groupB titanomagnetite does not vary signi¢cantly (Fig.2a). The wide range of x values probably re£ectsthe compositional variation of the immiscible meltitself and fractionation (both equilibrium and ki-netic) within each globule. The crystallization rateof titanomagnetite in the high-FeO immisciblemelt is expected to be much greater than that inthe original melt, causing more e¤cient fraction-ation and hence more compositional variation.

The above discussion concentrates on the com-position of titanomagnetite because of its rele-vance for our magnetic study. The crystal mor-phology, grain sizes and textures of the silicatesin di¡erent parts of the New Flow pillow are alsoin general agreement with previous experimentalstudies with variable rates of crystallization[35,42^44]. The large range of cooling rates expe-rienced by di¡erent parts of a MORB pillow pro-vides a natural laboratory for investigating thee¡ect of cooling rates on the crystallization se-quence, crystal morphology, grain sizes and com-positions.

6.2. No rapid alteration in very young MORB

Whereas Group A grains are likely to have adominating in£uence in the thermomagnetic runs,the SD fraction of the Group B grains is likely tocontribute signi¢cantly to the natural remanent

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magnetization (NRM) of the bulk rock [23]. Un-blocking temperatures of the NRM are higherthan the bulk Tc obtained from rock chips atthe same depth of the very young pillow[4,11,12]. This implies that ¢ne grains eitherhave a lower Ti-content or a higher oxidationstate than large grains.

The decrease in intensity of marine magneticanomalies with increasing age of ocean-£oor hastraditionally been attributed to low temperatureoxidation of titanomagnetite to titanomaghemite[6^9]. Titanomaghemite has indeed been widelyobserved in MORB of ages of the order of 106

years or older. To explain the blocking temper-ature variations at di¡erent depths of `New Flow',Kent and Gee [12] also adopted the oxidationhypothesis and proposed that ¢ne-grained titano-magnetites have been preferentially oxidized inthe New Flow. This is important for many rea-sons, not the least of which is that if MORB onlya few years old could already be oxidized, thenthis would have signi¢cant implications for ma-rine magnetic anomaly studies and paleointensitydeterminations [31^33]. However, the composi-tional variation (lower Ti-content) of group B ti-tanomagnetite that we observe within interstitialglass provides an alternative explanation for thehigher unblocking temperatures. We argue that itis not necessary to invoke a rapid alteration hy-pothesis to explain the Tc variation within NewFlow pillow. Instead, our data imply that the lowtemperature oxidation of titanomagnetite to tita-nomaghemite is a gradual process [13,45].

We also conclude that Tc measurements maygive an overestimation of oxidation state whendealing with materials near the pillow margins,if Ti-content is erroneously assumed to be moreor less constant. As a result, the e¡ects of lowtemperature oxidation on marine magnetizationmay occasionally have been exaggerated.

6.3. Curie temperatures and grain sizes

Increases in Tc near the glassy margin of pil-lows have long been observed [1^4,12]. Becausethe composition of titanomagnetites near theglassy margin is di¤cult to determine owing totheir small grain sizes, it has commonly been as-

sumed that it is the same as that of large grains inthe pillow interior [2,3,12]. The present studyshows that this can result in erroneous interpreta-tions.

Grain sizes of silicates and Group A (s 1 Wm)titanomagnetites are known to decrease regularlywith increasing cooling rate and this is also ob-served in this study. However, tiny, submicrome-ter titanomagnetites are also observed in the pil-low interior, where they occur embedded ininterstitial glass. Thermomagnetic runs con¢rmtheir importance. Hysteresis parameters of pillowrim sub-samples show a mixture of SD and SPminerals, whereas those for pillow interiors areinterpreted as mixtures of SD+SP+MD grains,which is di¡erent from recent publications thatdo not take the tiny SP grains in pillow interiorsinto account.

Acknowledgements

We thank H.P. Johnson for providing the NewFlow sample. We also appreciate helpful com-ments by Je¡ Alt, Eric Essene and journal re-viewers David Dunlop and Michel Prevot. Thisresearch was supported by National ScienceFoundation grant EAR 98-04765 and a visitingfellowship from the Institute for Rock Magnetism(IRM). The IRM is funded by grants from theKeck Foundation, the National Science Founda-tion and the University of Minnesota.[EB]

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