19. the petrology and ^ar/ ar age of tholeiitic …

Thiede, J., Myhre, A.M., Firth, J.V., Johnson, G.L., and Ruddiman, W.F. (Eds.), 1996Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 151

19. THE PETROLOGY AND ^AR/ AR AGE OF THOLEIITIC BASALT RECOVEREDFROM HOLE 907A, ICELAND PLATEAU1

Linda L. Davis2 and William C. McIntosh3

ABSTRACT

Crystalline rock recovered from the Iceland Plateau at 69°14.989'N, 12°41.894'W in August 1993, is tholeiitic basalt. Pil-low structures are well defined by glassy rims grading into vesicular rinds and aphyric interiors. Some plagioclase and augitemicrophenocrysts are present, and olivine may have been. Pigeonite is found in cores of some augite. Major- and trace-elementabundances vary indicating magma-mixing, but the nature of the magma-mixing is not known. Trace-element ratios also vary,sometimes greatly, indicating that the "mixed magmas" may be magmas derived from different depths in a melting column, ashas been proposed by other workers. The rocks differ from many typical MORB samples in that they are relatively evolved (mg= 46); however, they are strikingly similar to the majority of basalt glasses from the Kolbeinsey Ridge. Iceland Plateau basaltsare T-MORBs, transitional to N-MORB and E-MORB with (La/Sm)n 0.68-0.72, Zr/Nb < 20, and a range of major- and trace-element abundances that also clearly span the gap between average N-MORB and average E-MORB. An 40Ar/39Ar age of 13.2± 0.3 Ma from groundmass concentrates is an accurate but possibly imprecise age.

INTRODUCTION

Site 907 of the Ocean Drilling Program (ODP) Leg 151 wasdrilled on the southern Iceland Plateau, an area of shallow seafloorextending from Iceland to about 70°N, and west to east from Green-land to the Jan Mayen Ridge. Site 907 sits east of the present spread-ing ridge, the Kolbeinsey Ridge, and west of a feature that has beenconsidered by some to be an extinct spreading axis. Vogt et al. (1980)considered this ridge to be the east flank of Kolbeinsey Ridge ratherthan an extinct spreading axis, based on morphology and magneticanomalies. Crystalline rock, or basement, was originally slated to bedrilled only in the Fram Strait and the Yermak Plateau areas; howev-er, basalt was drilled at Site 907, the first site of Leg 151. No othercrystalline rock, besides ice-rafted debris or "dropstones," was recov-ered during Leg 151. Site 907 sits on a shallow part of the Iceland Pla-teau, on ocean crust that was thought to be approximately 22-24 Mabecause of its proximity to magnetic Anomaly 6B. About 8 m of ba-salt was drilled, but only 5 m was recovered. The recovered basaltcores were divided into 12 cooling units, which are individual basaltpillows with tops, bottoms, and sides defined by glassy, vesicle-richchill zones. We have characterized the textures, composition, and ageof the basalts drilled, but it is important to remember that only onehole was drilled so there is little three-dimensional control. Seismicdata show that the basalt came from a very smooth reflector that maynot represent acoustic basement (Shipboard Scientific Party, 1995).This basalt layer may be the same as the layer drilled at Deep SeaDrilling Project (DSDP) Site 348 even though pillow structures arevery well defined at Site 907, and no pillows were found at Site 348(Talwani, Udintsev, et al., 1976). Although many workers have stud-ied the ocean floor in this area, there are few drill sites on the IcelandPlateau where basalt has been recovered (e.g., Talwani, Udintsev, etal., 1976; Wood et al., 1979; Schilling et al., 1983; Elliot et al., 1991;

'Thiede, J., Myhre, A.M., Firth, J.V., Johnson, G.L., and Ruddiman, W.F. (Eds.),1996. Proc. ODP, Sci. Results, 151: College Station, TX (Ocean Drilling Program).

department of Geological Sciences, The University of Texas, Austin, Texas 78712,U.S.A. Present address: Department of Geology, Idaho State University, Pocatello,Idaho 83209-8072, U.S.A. [email protected]

3New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico87801, U.S.A.

Mertz et al., 1991; Fram and Lesher, 1993; Devey et al., 1994). Thechemistry and age of the basalt pillows at Site 907 may be applicableto regional questions regarding the Iceland Plateau basement. We arenot convinced, however, that these pillows are "basement." Becausewe do not know the source of the flow(s) if they are not "basement,"it is necessary to use caution if applying the following data to the Ice-land Plateau as a region.

ANALYTICAL TECHNIQUES

Basalt core was examined, described, sampled, and analyzedaboard the JOIDES Resolution during Leg 151. Polished thin sec-tions were made aboard ship, as were fused disks for X-ray fluores-cence (XRF) analyses. Core and thin section descriptions can befound in Shipboard Scientific Party (1995) along with photographs ofcore and photomicrographs of thin sections.

Microprobe Analysis

Polished thin sections made aboard ship were further analyzedpetrographically at the University of Texas Department of GeologicalSciences. Pyroxene, plagioclase, and ilmenite compositions are re-ported here. A JEOL 733 electron microprobe operated at 15 kv wasused with sample currents of 10-15 nA on brass, beam diameters ofup to 10 µm, and counting times of 10-35 s. Counting was terminatedif a precision of 0.30-0.50 relative percent (lσ) was attained. Naturaland synthetic standards were used for the following elements: GlassK412 (Si), Amelia albite (Na), titanium-doped diopside glass (Ca),fayalite (Fe), orthoclase (K), BaO silica glass (Ba), anorthite (An100,Al), synthetic enstatite (Mg), Mn garnet (Mn), and IM-8 chromite(Cr). Precision estimates based on the standard deviation of replicateanalyses of secondary standards were as follows: Si = 0.1-0.7; Al =0.1-0.3; T = 0.02-0.3; Cr = 0.02-0.04; Fe = 0.1-0.7; Mn = 0.02; Mg= 0.17-0.44; Ca = 0.03-0.3; Na = 0.03-0.09; and K = 0.19-0.24.

X-ray Fluorescence Analysis

Rock powders were ground aboard the JOIDES Resolution fromcore samples that were cut with a steel saw and sonicated in distilled

351

L.L. DAVIS, W.C. MCINTOSH

water. Grinding and crushing methods employed aboard the JOIDESResolution are described in Shipboard Scientific Party (1992). Fusedglass disks were made from powders and analyzed by XRF for majorelements only. Volatile content for each sample was determined byloss on ignition (LOI) aboard ship: no sample had more than 0.70wt% LOI.

International standard BIR-1 was analyzed with the Site 907 ba-salts as a check on precision and accuracy (Shipboard Scientific Par-ty, 1992). Major oxides determined aboard ship for BIR-1 were in ex-cellent agreement with recommended values with the exception ofSiO2. Precision based upon the standard deviation of replicate analy-ses of BIR-1 was: SiO2 = 0.02; TiO2 and A12O3 = 0.01; Fe2O3 = 0.06;MgO and CaO = 0.04 and 0.06; and MnO, K2O, Na2O, and P2O5 =0.00. SiO2 values for the BIR samples analyzed as unknowns wereconsistently 0.4 wt% lower than the recommended value. For thisreason, splits of the same samples were analyzed a second time byXRF in a different laboratory.

XRF analyses of sample splits analyzed at the GeoAnalytical Lab-oratory, Department of Geology, Washington State University, werein excellent agreement with the first XRF data set. Estimates of pre-cision of these analyses are very good but variable. They are availablein Hooper et al. (1993), along with analytical procedures. Trace ele-ments were also determined by the GeoAnalytical Laboratory.

The major oxide data presented herein are the average of the anal-yses from the two different laboratories. The trace elements deter-mined by XRF (i.e., V, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, and Pb) arefrom the single analysis done by GeoAnalytical Laboratories.

Instrumental Neutron Activation Analysis

Rock powders ground to 100-200 mesh with a ball mill in a ce-ramic vessel at the University of Texas, Department of GeologicalSciences, were sent to XRAL Activation Services, Inc. in Ann Arbor,Michigan, for instrumental neutron activation analysis (INAA). Priorto grinding, rock cores had been washed and sonicated in distilledwater after having been cut with a steel saw on board. The cut corewas then chipped to pea-size on a hardened steel plate with a rockhammer before grinding in a SPEX ball mill. Rock powders are notsplits of the powders analyzed by XRF; ideally, they would havebeen, but the amount of sample taken at any particular depth in thecore is restricted. These samples were taken as close to the other sam-ples used for thin sections and XRF as possible and certainly withinthe same pillows. One secondary standard previously analyzed byINAA sent as an "unknown" was analyzed with the Site 907 basaltsamples along with a control sample (NIST-278, obsidian). The anal-ysis of NIST-278 compares favorably with the recommended or cer-tified concentrations, and the analysis of the secondary standard sub-mitted as an unknown also compares favorably with a previous INAAanalysis, with the exception of La. La determined by NIST-278 was33.9 ppm whereas the recommended or certified value is 33 ppm;therefore, La values here are probably good. Ce, Nd, and Sm are fis-sion-product corrected based on the amount of uranium in the sam-ple, which is almost negligible here.

40Ar/39Ar Procedures

Groundmass concentrates were prepared by crushing and sievingthe samples to 250-400 µm, followed by ultrasonic cleaning in dilute(7%) hydrochloric acid, followed by hand-picking to remove frag-ments containing visible phenocrysts. Aliquots (30-38 mg) of eachgroundmass concentrate were packaged with alternating flux moni-tors of Fish Canyon Tuff sanidine (27.84 Ma, relative to MMhb-1hornblende at 520.4 Ma; Samson and Alexander, 1987) and irradiat-ed for 5 hours in the L 67 position of the Ford reactor at the Universityof Michigan.

40Ar/39Ar analyses were performed at the New Mexico Geochro-nology Research Laboratory at the New Mexico Institute of Mining

and Technology. This facility includes an MAT215-50 mass spec-trometer attached to a fully automated all-metal argon extraction sys-tem equipped with a 10-watt CO2 laser and a low blank resistance fur-nace. The neutron flux values (J-values) within irradiation packageswere determined to a precision of ±0.25% by averaging laser-fusionresults of four subsamples (each 1-4 crystals, ~l mg) of each sani-dine monitor. Groundmass concentrates were analyzed by incremen-tal step-heating in the resistance furnace, using twelve 10-min heat-ing steps ranging from 500 to 1650°C. Reactive gasses were removedusing SAES AP-10 and GP-50 getters before expansion into the massspectrometer. Extraction line blanks during these analyses rangedfrom 5 × 10~16 to 2 × 10~15 moles 40Ar and 6 × 10"18 to 1 × 10"17 moles36Ar.

MINERALOGY

Composition of microphenocrysts and groundmass phases deter-mined by electron microprobe is discussed here, but is preceded by abrief summary of the petrography of the basalts. The basalts from Site907 are pillow basalts with glassy rinds that grade into a zone rich invesicles and amygdules, and then into aphyric interiors (ShipboardScientific Party, 1995). Plagioclase, pyroxene, and altered olivinemicrophenocrysts are not larger than 2.0 mm and usually less than 1.5mm. Volumetrically, microphenocrysts make up between 5% and20% of the pillows. Detailed thin section and macroscopic core de-scriptions were completed during the cruise and are reported inMyhre, Thiede, Firth, et al. (1995). Textures in the basalts range fromvitrophyric to subophitic, with amygdules, microphenocrysts, andglomerocrysts (Fig. 1; fig. 23 in Shipboard Scientific Party, 1995).Groundmass is typically quenched with skeletal grains and sheavesor blades of plagioclase, pyroxene, and ilmenite (Fig. 1). No spinel orunaltered olivine were found; what may have been olivine is presentin the groundmass as a mixture of brown minerals, but euhedral oli-vine outlines are rare. Glass was possibly present in pillow interiors;however, if so, it has been replaced by a brown fibrous material. Thepillow rims that macroscopically appear glassy are dark brown, tur-bid, and mostly devitrified in thin section (fig. 23B in Shipboard Sci-entific Party, 1995).

Plagioclase

Labradorite with some bytownite (An56_74) is found as euhedralmicrophenocrysts often intergrown with clinopyroxene and pigeo-nite, as skeletal sheaves or blades in the groundmass, and is ophitical-ly intergrown with clinopyroxene (Table 1; Figs. 1, 2). Grains areuniformly poor in the orthoclase component. Although the number ofanalyses are few, they are probably representative of most or all ofthe feldspar in the sampled core.

Compositional differences related to depth or chemical evolutionof the parent magmas are not evident, but this may be related to thenumber of grains analyzed (approximately 55) or to the difficulty inobtaining good analyses of groundmass feldspar, which is related totheir small size. The larger microphenocrysts are slightly more calcicthan the smaller microphenocrysts and groundmass plagioclase (Ta-ble 1). An increase in anorthite content in the plagioclase is not seenwith depth. Microphenocrysts are zoned, but while the core of a givengrain is richer in anorthite content than one rim, the other rim mayhave approximately the same anorthite content as the core (Table 1).Therefore, the zoning does not appear to be concentric and is certain-ly not oscillatory.

Pyroxene

Resorbed and euhedral clinopyroxene microphenocrysts are gen-erally zoned and are often glomerocrystic with plagioclase (Fig. 1).In the groundmass, pyroxene is either lath-like or plumose and sheaf-

352

PETROLOGY AND AGE OF THOLEIITIC BASALT

Figure 1. Back-scattered electron (BSE) photographs. Scale bar of 100 µm is shown to the left of each photograph. A. Sample 151-907A-25X-1, 108-111 cm.Quenched groundmass (QG) of pyroxene (medium gray), plagioclase (P) (dark gray), and iron oxides, mostly or all ilmenite (bright white specks) withmicrophenocrysts of plagioclase and zoned pyroxene (A). B. Sample 151-907A-26X-2, 6-11 cm. Euhedral plagioclase (P) microphenocryst (dark gray) inter-grown with augite (A) and pigeonite (Pig) (lighter gray). Pigeonite is very difficult to see, but the subtly darker gray within the clinopyroxene grains is thepigeonite. Groundmass is the same as in A. C. Sample 151-907A-26X-1, 85-89 cm. Clinopyroxene (A) and plagioclase (P) coarsely intergrown, skeletalilmenite (I), and a quenched groundmass. D. Sample 151-907A-26X-1, 85-89 cm, enlargement of top left-hand corner of C. Skeletal ilmenite (I) in quenchedgroundmass (QG) near clinopyroxene (A) and plagioclase (P) intergrowth.

like. In places, it is altered to a cryptocrystalline material, perhaps anamphibole, but the composition of the alteration product(s) is unde-termined.

Pyroxene compositions range from augite to subcalcic pyroxeneor pigeonite (Table 2; Fig. 3). These pyroxenes follow a trend char-acteristic of pyroxenes in quenched volcanic rocks as discussed byMuir and Tilley (1964). Pigeonite is not discernible in thin sectionand is only barely distinguishable from augite in back-scattered elec-tron images (Fig. IB). Most of the augites have iron-rich rims as ex-pected, but cores of, or zones central to, some of the grains are actu-ally pigeonite (Table 2; Fig. 1). Subcalcic pyroxene or pigeonite (e.g.,En69Fs23Wo8) was found in two samples (Fig. 3; Table 2) and is prob-ably in many of the basalt pillows at Site 907, but it is perhaps too dif-ficult to distinguish when weighed against the time needed to find thegrains by microprobe analysis. Within each sample analyzed, pyrox-ene grains range mostly in Wo content, but a range in En-Fs is alsoseen (Fig. 3). The compositional ranges of different samples overlapsuch that no pattern is evident.

Structural formulas calculated from microprobe analyses confirmthat the analyses are acceptable and that the pyroxenes are stoichio-metric (Table 2). Deficiencies in the tetrahedral sites are filled byadding about 0.1 Al to Si to make an ideal 2. Cations were normalizedto 4 and Fe3+ was thereby calculated using the summed oxygen dif-ference from 6. If A1VI, Fe3+, Cr3+, Ti, Mg, Fe2+, and Mn are put intothe Ml cation site, then the Ml cation sum is approximately 1.3 forall clinopyroxenes, so all Mn and most Fe2+ must be put into the M2cation site to satisfy criteria outlined by Robinson (1980) for pyrox-ene structure.

Ilmenite

Ilmenite is generally skeletal with a wide variety of shapes (Fig.1) and is only rarely intergrown with pyroxene. There appear to betwo generations present, with most of the grains so fine grained thatanalysis by electron microprobe is not possible, even with a mini-mum beam size. However, it is possible to analyze the flared tip ends

353


Table 1. Selected electron microprobe analyses of feldspars in basalt from Site 907.

151-907 A-:

Interval (cm):

SiO2A12O,Fe2O3CaONa2OK2O

Total

SiAlFe3+

CaNaKAbAnOr

Description

26X-1,

85-89

52.729.4

1.0413.53.650.03

100.3

2.3891.5690.0350.6550.3210.002

32.867.00.20r-

26X-1,

85-89

53.229.0

1.0813.03.910.04

100.2

2.4091.5460.0370.6330.3430.002

35.164.70.24-c-

26X-1,

85-89

54.028.2

1.2612.34.410.03

100.2

2.4431.5070.0430.5960.3870.002

39.360.50.16-r.

25X-1,

108-119

51.429.50.84

13.43.400.06

98.6

2.3691.6030.0290.6600.3040.004

31.468.20.39e ph

25X-1,

108-119

51.729.20.94

13.73.310.10

99.0

2.3761.5810.0320.6760.2950.006

30.269.20.60r-

25X-1,

108-119

50.530.40.80

14.52.850.06

99.1

2.3221.6470.0280.7150.2540.003

26.173.50.36c-

25X-1,

108-119

51.429.50.96

14.03.270.02

99.2

2.3601.5960.0330.6890.2910.001

29.770.20.15-r,

25X-1,

81-84

52.029.5

1.0213.23.450.04

99.2

2.3821.5890.0350.6450.3060.003

32.167.70.28r-

25X-1,

81-84

52.529.00.99

13.13.750.02

99.3

2.3991.5630.0340.6410.3330.001

34.265.80.10-m-

25X-1,

81-84

54.926.9

1.2711.04.670.08

98.8

2.5071.4490.0440.5380.4130.005

43.256.30.50-r,

25X-1,

81-84

51.629.00.76

13.83.320.02

98.5

2.3751.5760.0260.6790.2960

30.469.60.0c-

25X-1,

81-84

50.230.30.9

14.82.850.00

99.1

2.3131.6460.0310.730.2550

25.974.10.0-c.

26X-2,

6-11

52.329.20.93

13.43.290.04

99.2

2.3931.5750.0320.6560.2920.002

30.769.10.3ph-

26X-2,

6-11

53.028.80.97

13.83.510.02

100.1

2.4071.5410.0330.6700.3090.001

31.668.30.1

-sa ph,

26X-2,

6-11

51.829.90.84

14.23.100.03

99.9

2.3581.6050.0290.6950.2740.002

28.271.60.2ph r

Note: Symbols describing the physical nature of the grains are: c = core, r = rim, m = mantle, ph = phenocryst, e = euhedral, and sa = same. Dashes indicate the analysis is one in aseries of the same grain. Cations normalized to 8 oxygens.

151-907A-25X-1, 81-84 cm151-907A-25X-1, 108-119 cm

O 151-907A-26X-1, 85-89 cm× 151-907A-26X-2, 6-11 cm

Ab * Or

Figure 2. Plagioclase compositions determined by electron microprobe.

of some of the largest skeletal grains, which resemble crosses (Fig.1C, D). Two analyses are shown in Table 2: the ilmenites are low inchromium with very low cr' (0.6-1.23), and also have low mg num-bers (4.2-6.0).

WHOLE-ROCK GEOCHEMISTRY

Major-Element Compositions

Basalts recovered from Site 907 are tholeiitic, falling well withinMacdonald and Katsura's (1964) tholeiitic field (Shipboard Scientif-ic Party, 1995). The basalts have between 49 and 52 wt% SiO2, yethave a much smaller range in MgO (6.2-7.1 wt% MgO, mg = 44-48)(Table 3; Fig. 4). Because SiO2 variation is greater than that of MgO,variation diagrams are shown with SiO2 as an abscissa rather thanMgO, which is more typically used as a differentiation index to illus-trate variations in basalt compositions (Fig. 5). SiO2 in the basalts ishigh enough for them to be almost andesitic (Fig. 4). In part, norma-tive analyses of the samples also reflect the high SiO2, as most arequartz-normative tholeiites. Three basalt samples are olivine-norma-tive, however, and all are hypersthene-normative (Table 3). Theamount of normative quartz (q) or olivine (ol) present in these analy-ses is related to the amount of Fe2O3 present: here, Fe2O3 is set to beequal to 10% of FeO(Total) as recommended by the Basaltic Volcanism

Study Project (1981). The relative percentages of q and ol will changedepending upon the value of Fe2O3 chosen. So, although the basaltsrange from silica-oversaturated to just slightly silica-undersaturated,the difference between the samples may be more related to the as-sumption that Fe2O3 is 10% of FeO(Total), rather than different lineag-es. Note that Units 5, 9, and 11, which are o/-normative, are also thethree most mafic samples in the group.

Geochemical trends indicative of evolving compositions of agroup of genetically related basalts are not well defined in the IcelandPlateau basalts (Fig. 5). The number of samples analyzed are few,however, and the range in each oxide is small, perhaps lending toscatter at the scales shown. Some of the oxides definitely decreasewith increasing SiO2, including Fe2O3(T), CaO, MnO, and MgO. Oth-er oxides that definitely increase with SiO2 include TiO2 and A12O3.

Major element composition varies more or less consistently withdepth in meters below seafloor (mbsf) (Fig. 6). The trends are in somecases well defined and are quite different above and below about 220mbsf. Samples from near 220 mbsf were not analyzed geochemicallydue to severe alteration of the rocks, leaving a gap in the variation di-agrams just where trends seem to dramatically change direction.With decreasing depth, between 224 and 220 mbsf, basalt pillows in-crease in Fe2O3(T), CaO, and MnO and decrease in A12O3, Na2O, andTiO2. At 220 mbsf, the patterns abruptly change and, from 220 mbsfto the top of the basalt pillows at about 216 mbsf, Fe2O3(T),CaO, andMnO decrease with decreasing depth, and the other oxides, with theexception of P2O5, increase with decreasing depth.

Trace-Element Compositions

Trace-element variation in the basalts is almost as inconsistent asmajor-element variation (Table 4; Fig. 7). Y is perhaps the only ele-ment that varies smoothly with SiO2 as an indicator of evolution ofthe melt(s) (Fig. 7). Some well-defined trends abruptly change direc-tion, as illustrated by V and Eu (Fig. 7). Ga, Sc, and Lu tend to de-crease with increasing SiO2, and Ni, Sr, Zr, La, and Y seem to in-crease with increasing SiO2. Few of these elements show good lineartrends, and most could also be interpreted to be similar to Eu and Vin that trends are different above and below 50% SiO2. The lack ofwell-defined, straight-line trends may not be genetically related, butthis may be due to the small number of samples and some trace-ele-ment abundances that do not greatly vary.

Trace-element variation seems to be considerably more coherentwhen normalized to chondrite or mid-ocean-ridge basalt (MORB)values (database of MORB values from Wheatley and Rock, 1988,comes mainly from Mid-Atlantic Ridge samples). Chondrite-normal-

354


Table 2. Selected electron microprobe analyses of pyroxenes and ilmenites in basalt from Site 907.

151-907A-:

Interval (cm):

SiO2

TiO2A12O,Cr2O3

FeO*MnOMgOCaONa2O

Total

SiTiAlCrFe2+

Fc3+

MnMgCaNa

mgcr'

New FeONew Fe2O3

New total

EnFsWo

26X-1,

85-89

50.50.832.910.00

16.50.29

15.114.20.19

100.4

1.8960.0231.2860.0000.4700.0460.0090.8430.5690.014

64

15.01.65

100.7

44.825.030.2

26X-1,

85-89

51.20.813.300.02

11.80.29

16.317.00.20

100.9

1.8850.0220.1430.00050.3060.0570.0090.8940.6690.014

74

9.932.05

101.1

47.816.335.8

25X-1,

81-84

52.20.422.140.06

0.2317.817.30.19

100.0

1.9190.0120.0930.0020.2400.0580.0070.9740.6820.013

80

7.82.1

100.2

51.412.636.0

25X-1,

81-84

51.80.642.620.07

12.60.32

19.012.80.14

100.0

1.9080.0180.1140.0020.3460.0430.0101.0420.5070.010

75

11.21.55

100.1

55.018.226.8

26X-2,

6-11

53.10.452.090.26

11.00.26

18.515.10.17

100.9

1.9380.0120.0900.0080.3220.0130.0081.0080.5900.012

76

10.50.46

100.9

52.516.830.7

26X-2,

6-11

54.90.180.790.12

15.40.40

24.44.860.05

101.1

1.9780.0050.0340.0030.4640.0010.0121.3120.1880.004

74

66.823.6

9.6

26X-2,

6-11

55.10.190.760.04

14.70.45

25.14.190.05

100.5

1.9870.0050.0320.0010.4440.0000.0141.3510.1620.004

75

69.022.7

8.3

26X-2,

6-11

51.40.653.050.199.510.21

15.519.60.23

100.2

1.9000.0180.1330.0060.2520.0420.0070.8520.7750.017

77

8.151.51

100.5

45.313.441.2

26X-1,

85-89

0.1920.4

2.650.05

70.10.430.430.11

94.4

0.0050.3910.0800.0010.3651.1290.0090.0160.003

4.21.23

17.158.9

100.3

25X-1,

81-84

0.1821.2

2.150.01

68.20.470.630.09

92.9

0.0050.4120.0660.0000.3741.1030.0100.0240.002

6.00.60

17.356.698.6

Note: Cations are normalized to 4. mg = 100 × Mg/(Mg + Fe2+). cr' = 100 × Cr/(Cr + Al).

Λ Λ Λ T

En FsA 151-907A-25X-1, 81-84 cmD 151-907 A-25X-1, 108-111 cm

° 151-907A-26X-1, 85-89 cm× 151-907A-26X-2.6-11 cm

Figure 3. Pyroxene compositions determined by electron microprobe.

ized rare-earth element (REE) patterns are nearly flat, slightly en-riched in REE over chondrite values, with normalized values be-tween 10 and 25. The scale in Figure 8 A is set to emphasize the slightdepletion in the LREEs over the HREEs and to distinguish the Eu de-pletion relative to Sm or Gd for Units 2 and 5. When trace-elementcompositions of the Iceland Plateau basalts are normalized to MORBvalues for incompatibility plots, differences between Site 907 basaltsand typical MORB are accentuated (Fig. 8B). Although some data isscarce in part because some samples were not analyzed by both XRFand INAA (thus, for example, no Rb data is available), compared toMORB all units have depletions in Cr, Ni, and Sr, and some unitshave slight depletions in K. It is difficult to substantiate, but it is like-

ly that the lack of coherency for some of the patterns on the incom-patible element plots is due to analytical error, at least for Ce in Unit9 (Fig. 8B). The spread in K may be real rather than an artifact of al-teration or analytical error and will be discussed later. The incompat-ibility plots normalized to MORB in Figure 8B were normalized asecond time so that K was set equal to 1 (Fig. 8C). Some of the incon-sistencies in the patterns disappear, and the patterns are spread out sothat their shapes can be better contrasted. Units 9 and 11 were not an-alyzed by INAA and therefore are missing some of the elements de-termined for the other units. This makes it difficult to determinewhether Units 9 and 11 are actually different than the other unitsshown. In general, the shapes of all the patterns are the same, exceptfor between Sr and Nb. Units 9 and 11 do seem to be different thanthe other units, particularly with respect to the more incompatible el-ements; they are also two of the three samples that contain ol but no q.

Variance From MORB

Tholeiitic basalts from Site 907 are similar to N-MORBs, but avariety of trace- and major-element abundances differ composition-ally from N-MORBs. The tholeiites have major element concentra-tions similar to N-MORB with the exception of K2O, A12O3, andTiO2. The extremely low K2O contents for six of the eight samples(Table 3) are most similar to N-MORBs; however, Units 9 and 11have much higher K2O, similar to P-MORBs/E-MORBs (Schilling etal., 1983). TiO2 contents are higher than is typical of MORB compo-sitions and A12O3 is lower; both oxides are more similar in abundanceto those of ocean island basalts (OIBs) (BVSP, 1981). K2O/TiO2 val-ues (0.04-0.16, Table 3), are most similar to N-MORB K2O/TiO2

( 0.06), but Units 9 and 11 still have higher K2O and therefore higherK2O/TiO2 values, approaching those for E-MORB (0.25) (Floyd,1991).

Trace-element abundances vary, but many are closer to E-MORBcompositions than N-MORB. For example, Nb values range from 6.6to 7.8 ppm for the Site 907 basalts; average N-MORB contains 2.33ppm and average E-MORB contains 8.3 ppm (values from Floyd,


Table 3. Major-element compositions of Leg 151 basalt samples, Site 907.

151-907A-:

Interval (cm):

Unit 2

25X-1,

13-16

Unit 3

25X-1,

81-84

Unit 5

25X-2,

22-26

Unit 6

25X-2,

73-77

Unit 9

26X-1,

35-38

Unit 10

26X-1,

85-89

Unit 11

26X-2,

6-11

Unit 12

26X-2,

56-59

SiO2TiO2A12O3FeO*MnOMgOCaONa2OK2OP2O5K2O/TiO2Orig. summg

qorabandihyolmtilap

51.81.94

14.213.30.186.259.592.470.120.190.06

100.546

50.81.92

13.913.70.197.099.772.320.070.180.04

100.448

CIPW normati

3.550.71

20.927.315.825.7

1.933.680.44

1.840.41

19.627.316.628.1

1.993.650.42

49.41.88

13.814.70.237.07

10.42.260.070.120.04

100.146

51.21.86

13.614.30.216.37

10.22.070.090.190.05

99.846

49.51.78

13.115.40.266.89

10.62.080.210.180.12

99.844

49.91.85

13.314.90.227.13

10.22.220.050.180.03

99.946

ve analyses (Fe2O3 as 0.10 × FeOTotal)

0.4119.127.319.625.5

2.032.133.570.28

3.700.53

17.527.618.226.6

2.073.530.44

1.2417.625.821.427.3

0.682.233.380.42

0.390.30

18.826.219.328.9

2.163.510.42

49.31.80

13.216.10.236.989.712.230.280.200.16

99.944

1.6518.925.218.127.0

3.042.333.420.46

51.41.86

13.813.80.196.909.672.180.100.170.05

99.446

3.350.59

18.427.616.028.2

2.003.530.39

Note: mg = 100 × Mg/(Mg + Fe). Analyses have been normalized to 100 wt% (volatile-free basis) and then rounded off for standard presentation. Original sums (Orig. sum) are pre-sented.

1 0 -

8 -

6 -

4 -

2 -

Foidite

-

/

/T

Basε

Picro-

basalt

/

/

\

sphrite

nite /

/

/ \ Tephriphonolite /

Phonotephrite )/

\ / \yf Basaltic \/ \ trachy- \

/ \ andesite \

Trachy- \ ^ ^ ^

basalt \ ^ - * ^ '

BasaltBasaltic

andesite

\ Trachy-

\ dacite

\

Trachy- \

andesite \

\

Andesite

40 45 50

SiO2 wt%55 60

Figure 4. Total-alkali silica diagram using the I.U.G.S. classification scheme

(Le Bas et al., 1986). Solid squares are averaged XRF analyses from Site 907,

normalized to 100% without volatiles.

1991). For Cr, Cs, Nb, Ni, and the LREE, where a significant differ-ence exists between N-MORB and E-MORB, the Site 907 basalts areclosest to E-MORB compositions (Table 4; compared to values fromSun and McDonough, 1989). For Rb, Sc, Sr, and the HREE, the Site907 basalts are closest to N-MORB compositions (Table 4). Therange in each of the ratios can vary a great deal. For example, Hf/Th

ranges from 5.6 to 11.3, P/Ce ranges from 19 to 32, K/Rb ranges from145 to 249, and Hf/Lu from 4.0 to 5.2. K2O/TiO2 variance is similarand was discussed above. Trace element ratios also vary between be-ing more similar to N-MORB or to E-MORB. Some examples of ra-tios varying from typical E-MORB to N-MORB follow. La/Nb of 0.6to 0.7 is closer to E-MORB (0.8) than N-MORB (1.1); K/Rb of 145to 249 is much closer to E-MORB (415 vs. 1067 of N-MORB); Hf/Th of 11-6 bridges the gap (N-MORB has 17.08 and E-MORB has3.4); La/Nb of 0.6 to 0.7 is closer to 0.76 of E-MORB than 1.1 of N-MORB. So far, no real pattern has emerged.

40AR/39AR DATING

The area of the Iceland Plateau on which Site 907 is located has acomplicated tectonic history. The plateau is in an area where seafloorspreading is asymmetric, where ridge-jumping has occurred morethan once, and where most magnetic anomalies are poorly identified(Talwani and Eldholm, 1977). Anomalies 5A to 6C on the IcelandPlateau are the most recognizable anywhere in the Atlantic and al-lowed Vogt et al. (1980) to pinpoint a late Tertiary plate accelerationat 12-14 Ma. Site 907 sits east of the present-day spreading ridge butwest of the ridge some have considered to be an extinct spreading ax-is. The nearest drill hole where basalt has been recovered is Site 348,Leg 38 (Talwani, Udintsev, et al., 1976), which sits on the oppositeside of the extinct ridge and south of Site 907. The basalt from Site348 has a K-Ar age of about 19 m.y. (Kharin et al., 1976). The age ofthe basalts at Site 907 could be a very valuable tool in unraveling thetectonic and magmatic history of the area.

Sample Description

The specimens were chosen from the interior of Units (pillows) 3,6, and 10, as close as possible to sections of the core that had beenanalyzed microscopically in thin section and chemically (Table 3); ineach case, samples for 40Ar/39Ar analysis were taken from the samepillow as the nearby previously analyzed samples. The samples cho-

356


14.4

• •• •

O 0.15Q_

01049

50 sio, 51

0.27

0.155 0 SiO2

5 1 52 49 50 SiO2 5 1

Figure 5. Whole-rock major-element compositions of Site 907 basalts.

357


12.6216 218 220 222 224 216 218 220 222 224

216 218 220 222 224 216 218 220 222 224

Sü

10.8

10.2

216 218 220 222 224 216 218 220 222 224

Qi -

2.00

1.92

1.84

1.76216 218 220 222 224 216 218 220 222 224

0.20

9, 0.15

0.10216 218 220 222 224

0.27

0.15216 218 222 224

Depth (mbsf)

Figure 6. Change in basalt composition with depth of Hole 907.

358


Table 4. Trace-element compositions of Leg 151 basalt samples, Site 907.

Interval (cm):

ScVCrCoNiCuZnGaAsBrRbSrYZrNbCsLaCeNdSmEuTbYbLuHfPbTiiUNd/SmZr/HfHf/ThLa/TbLa/CeHf/LuLa/Nb

tents determined

Unit 2

8-13

54.5

7378

14.4

4.513103.881.281.14.430.663.4

0.30.42.6

28.511.34.10.355.20.63

by INA^

13-16

505

4811713718

27346977.2

. include:

Unit 3

81-84

521

43112147

17

06748986.6

85-89

53.3

6760

14.3

0.54.6

13113.941.651.14.650.723.3

0.3

2.829.711.04.20.354.60.70

Sc, Cr, Co, As, Br, Cs,

Unit 5

22-26 26-32

484

5410814219

0684996

53.1

7265

14.4

6.6

3

REEs, Th,

4.414103.951.231.25.060.763.3

0.42.5

29.1

3.70.314.30.67

Unit 6

73-77 76-82

53.6

7070

14.4

4.513113.93t.551.24.840.693.4

0.52.8

3.80.354.9

Unit 9

35-38

480

4510612621

666

937.5

and U. Elements determined by XRF include: V, N:

Unit 10

72-77

52.3

7061

43

4.213103.781.541.24.510.692.8

O.5

2.633.2

5.63.50.324.00.56

85-89

493

4011013820

0644995

6.5

Unit 11

6-11

55482

66

3696

12820

8664793

7.8

i, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, and PIin ppm.

sen from the recovered core were those that showed the least alter-ation. Complete thin section descriptions are in Myhre, Thiede, Firth,et al. (1995). Brief sample descriptions follow here so that the readermay fully evaluate the degree of alteration of the basalts and thereforeevaluate the age of the basalt presented. All are of tholeiitic olivinebasalts, sampled in the center of pillows away from glassy rims andvesicular rinds. Sample 151-907A-25X-1, 85-89 cm, Piece 11, alsosampled for thin section and chemical analysis at 81-84 cm, rangesfrom glassy to 2.0 mm in grain size. The textures are subophitic tomicroporphyritic and quenched. Plagioclase and pyroxene pheno-crysts or microphenocrysts to 2.0 mm in size are present, and thegroundmass contains plagioclase, pyroxene, and opaque minerals.Olivine and glass were perhaps initially present but are now altered.Secondary mineralogy includes pyrite, unknown brown minerals,sericite, chalcedony and an unknown mineral(s) that replaces cli-nopyroxene. Sample 151-907A-25X-2, 76-82 cm, Piece 7B, over-lapping with a piece of the core sampled for thin section and XRFanalysis at 73-77 cm, is also glassy to 2.0 mm in grain size. Texturesare microglomeroporphyritic, subophitic, quenched, and variolitic.Plagioclase and pyroxene microphenocrysts are present, and thegroundmass contains plagioclase, clinopyroxene, and opaque miner-als. Olivine and glass may have been present as indicated by alter-ation products and "ghosts." Alteration products or secondary miner-als include pyrite, chalcedony, a brown clay mineral, and an un-known mineral(s) replacing clinopyroxene. Sample 151-907 A-26X-1, 72-77 cm, Piece 10B, as close a piece as possible to Piece 11 A,85-89 cm, which was analyzed previously, is the only sample with a40Ar/39Ar analysis that is probably accurate and that is reported here.This sample has a grain size that ranges from glassy to 1.0 mm. Tex-tures are microporphyritic, subophitic, and variolitic. Clinopyroxene

and plagioclase are the microphenocrysts present, and it appears thatolivine may also have been a phenocryst phase. What may have beenolivine is now replaced by brown clay minerals or mineraloids. Thegroundmass contains plagioclase, clinopyroxene, and opaque miner-als. Glass and olivine may have been present in the groundmass ini-tially. Secondary minerals include pyrite, brown clay minerals, seric-ite, and chalcedony.

Results

Results from Sample 151-907A-25X-2, 76-82 cm, Piece 7B,were discounted because of a very disturbed spectrum and negativeradiogenic yields. Results from Sample 151-907A-25X-1, 85-89 cm,Piece 11, were discounted mostly because of relatively high radio-genic Ar yield above 900°C suggestive of zeolites or chert releasingtrapped Ar, and because of climbing spectra above the same temper-ature.

40Ar/39Ar age spectra and data (along with % radiogenic argon, K/Ca, and Cl/K for each temperature increment), for Sample 151-907A-26X-1, 72-77 cm, Piece 10B, are presented in Figure 9 and in Table5. We regard this 40Ar/39Ar data as tentative and we are in the processof trying to duplicate the determined age. We have presented data foreach of the heating increments to show the consistency of the low-temperature steps. The lower temperature spectra give younger ages,while the higher temperature ages range up to 24 Ma. The integratedage of all heating steps, equivalent to a conventional K-Ar age, is19.7 Ma. The best estimate of the eruption age of this sample is rep-resented by the isochron of the low-temperature heating steps (Table5; Fig. 10). This isochron, which is well correlated and has an atmo-spheric intercept, gives an age of 13.2 ± 0.3 Ma. This age is consid-

359


50 51SiO2

50 51SiO5

50 51SiO2

120

CO

52 49 50 SiO2 51 5 2

Figure 7. Whole-rock trace-element compositions of Site 907 basalts.

360


CDt »

oü 20

üom

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.1

C 10

5

i i i—i—i—i—i—i—i—i—i—i—i—i—i—i—r

Sr K RbBaTh TaNbCe P Zr HfSmTi Y YbScCr Ni

o Unit 2• Unit 3π Unit 5• UnitθΔ Unit 10

o Unit 2• Unit 3π Unit 5• Unitθ* Unit 9Δ Unit 10v Unit 11T Unit 12

o Unit 2• Unit 3π Unit 5• Unit 6* Unit 9Δ Unit 10v Unit 11

i i i i i i i i—i—i—i—i—i—i—i—i—r

Sr K RbBaTh TaNbCe P Zr HfSmTi YYbScCrNi

Figure 8. Spider diagrams for (A) chondrite-normalized REE, (B) MORB-normalized incompatible element plot, and (C) MORB-normalized incompatible ele-

ment plot double-normalized to K = 1. Diagrams plotted using Spider (Wheatley and Rock, 1988). Chondrite values (Cl) are from Thompson et al. (1984) and

MORB values are those used by Wheatley and Rock (1988) in their plotting program.

361


coüx

0

(Ma)

App

aren

t ag

e

U. 1

0.01 -

.001 -

30-

20-

-10-

~l

B600

i i

i

i i

C700

i

i

1 'X O _i_I O.c. ±

D800

i

i i

0.5 l\/lano.

E900

Integrated age =

_J

1

i

1

F1000

14±3Ma

_ |

i

i

15.2

G1100

±1.5

H120C

i

!Ma

J

I1300

i

1400 K1650

n

-1

-0

-

13m O )idiogenic

irgon

oI/K

0 10 20 80 90 100

Fig

40 50 60 70

Cumulative %39Ar released

ure 9. 40Ar/39Ar age spectra for Sample 151-907A-26X-1, 72-77 cm, Piece 10B. Incremental heating steps (B-K) are shown with temperatures (°C).

Table 5. Summary of 40Ar/39Ar data.

Summary of spectra for Sample 1

Run T (°C)

1832-1A 5001832-1B 6001832-1C 7001832-1D 8001832-1E 9001832-1F 10001832-1G 11001832-1H 12001832-11 13001832-lJ 14001832-lK 1650Total gas age

40Ar/39Ar

2.26E+37.61E+22.90E+21.55E+21.72E+28.71E+11.93E+12.00E+12.36E+12.45E+14.18E+1

Mean of low-temperature stepsMean of high-temperature stepsIsochron age (low-temperature steps)Isochron age (all steps)J = 0.0007683 ± 0.0000002

151-907A-26X-1, 72-77 cm, Piece37Ar/39Ar

5.35E+05.73E+07.16E+01.33E+13.71E+17.00E+16.11E+13.62E+11.88E+26.51E+12.77E+1

36Ar/39Ar

7.63E+02.56E+09.53E-14.98E-15.60E-12.80E-15.04E-23.70E-29.33E-24.17E-29.49E-2

39K (moles)

1.7E-162.8E-167.6E-169.3E-167.2E-165.1E-163.4E-163.0E-163.1E-164.2E-162.1E-164.9E-15

10B

K/Ca

9.5E-28.9E-27.1E-23.8E-21.4E-27.3E-38.4E-31.4E-22.7E-37.8E-31.8E-2

Cl/K

4.0E-22.3E-21.7E-21.3E-22.1E-23.2E-26.1E-28.5E-21.3E-11.9E-24.3E-3

Summary of 40Ar/39Ar results for Sample 151-907A-26X-1,72-77 cm, Piece 10B

Groupings

Low-temperature stepsHigh-temperature stepsAll steps

Spectrum age (m.y.)

n

73

—

Age

13.215.2—

Error

0.51.3—

n

6—11

Age

13.2—19.7

%40Ar*

0.40.63.15.75.6

11.147.259.244.170.038.0

%39Ar*

3.408.97

24.2943.1757.8268.1375.0581.1387.3095.73

100.00n = l ln = 7n = 3n = 6

n= 11

Age (m.y.)

11.26.4

12.512.313.714.113.216.816.5

22.314.113.215.213.219.7

Isochron age (m.y.)

Error

0.3—0.1

MSWD

0.4—

191.4

40A r /36A r

295.2—

288.5

± Error (m.y.)

34.210.13.01.51.91.20.50.60.70.51.03.10.51.30.30.1

Error

1.2—1.1

Note: — = not determined.

362


0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100

39Ar/40Ar

Figure 10. Isochron for Sample 151-907A-26X-1, 72-77 cm, Piece 10B.

2.00 -

1.50 -

1.00 •

0.50 -

0.00 -

••P-type

•

jr"

Iceland Plateau, Leg 151

\ N-type

•

20 40Zr/Nb

60 80

Figure 11. Chondrite-normalized La/Sm compared to Zr/Nb for Iceland Pla-teau basalts at Site 907 and MORB from the Atlantic, Pacific, and IndianOceans. Modified after Wilson (1988).

ered to be relatively accurate, although the very low MSWD value of0.4 reflects imprecision of the analysis due to low potassium contentand low radiogenic yields.

DISCUSSION

Tholeiitic basalts from the Iceland Plateau at Site 907 are notprimitive, and the parent magmas most likely suffered crystal frac-tionation. Pillows are plagioclase and augite phyric, accompanied byrare olivine that is now completely altered. The basalts are evolved asindicated by the rarity of olivine, the absence of spinel, and the abun-dance of augite and ilmenite accompanied by pigeonite. Microphe-nocrysts are not embayed, but they are frequently euhedral and wereprobably in equilibrium with the equivalent melt. Chondrite-normal-ized REE patterns with small Eu-depletions are indicative of plagio-clase fractionation. Trace-element patterns normalized to typical N-MORB values with relative depletions in Cr, Ni, and Sr, could reflectsource characteristics or more likely be indicative of fractionation ofplagioclase and clinopyroxene (Sr), spinel, or olivine (Cr and Ni).

The evolved nature of the basalts is also indicated by whole-rockmajor element composition. They range in composition almost to ba-saltic andesites and have very low Mg numbers ( 46). MORB withmg numbers this low are generally uncommon in the Atlantic Oceanand are also uncommon with respect to all MORB (Wilkinson, 1982).However, the evolved nature of these basalts and their quartz-norma-tive character is in agreement with the earlier findings of unusuallyevolved basalt glasses from the Kolbeinsey Ridge (Sigurdsson,1981). In addition, the Iceland Plateau basalts are strikingly similarto some of the Group A basalts from the Reykjanes Ridge, the west-ern zone of Iceland, and the Kolbeinsey Ridge described by Sigurds-son (1981) in two ways. First, they contain pigeonite or low-Ca py-roxene, and that although in some cases entire grains are Ca-poor,usually a Ca-poor or pigeonite core is enclosed in a Ca-rich rim. Sec-ond, the range in pyroxene composition is also from augite to Ca-poor pyroxene or pigeonite and spans an area of the pyroxene quad-rilateral that is generally referred to as the "forbidden zone"; howev-er, this is characteristic of quenched pyroxenes in quenched basalts(Muir and Tilley, 1964).

The pillow basalts are almost uniformly fine grained and glassy,indicating rapidly quenched volcanic liquids, thus, variation dia-grams based on rock analyses may truly characterize the magmaticlineage. It is very difficult to interpret the "trends" illustrated byHarker variation diagrams (Figs. 5-7). As discussed, the small num-

ber of samples may be reflected in the scatter; if a larger data base ex-isted, better straight-line trends might emerge. Some degree of alter-ation may explain some of the major-element variation, but it cannotexplain the trace-element variation. We cannot discount more thanone magma source which could lead to the patterns seen. This wouldnot be unusual given the possibility that a large number of small mag-ma chambers may exist below the ridge or the extinct ridge (c.f.Smith and Cann, 1993), and given the distinct break in the layer(s) ofpillow basalt at 220 m (Fig. 6).

The variation in major- and trace-element compositions and trace-element ratios provide strong evidence for magma mixing. The na-ture of the proposed magma mixing is unknown. Variation in trace-element ratios is indicative of different sources and the lack of goodlinear trends in the Harker variation diagrams supports a theory ofmore than one source for the basalt. It may be that different mantlesources are or were present at the time of magma generation, butequally possible is that magma was tapped from different depths inthe magma column as discussed by Devey et al. (1994) and Elliot etal. (1991). Even if different magma chambers containing melts withthe same or similar sources were tapped, the melts in these chambersbeing more or less evolved still would not have different trace-ele-ment ratios, unless the melts came from different depths of a meltingcolumn. The large range in trace-element ratios such as K2O/TiO2

(0.04-0.16) or Hf/Th (5.6-11.3) in the Site 907 basalts is similar toalthough sometimes greater than or less than, ranges in ratios dis-cussed by Devey et al. (1994).

The tholeiitic basalts on the Iceland Plateau at Site 907 are transi-tional MORBs (T-MORB), but just barely. Major- and trace-elementsignatures fall most often between N-MORB and E-MORB/P-MORB. Trace-element ratios also show this dual signature (Fig. 11).MORB compositions from the Atlantic, Pacific, and Indian Oceansform a mixing curve from plume-type or enriched MORB to normalMORB. Schilling et al. (1983) subdivided the Mid-Atlantic Ridgeinto three segments: "normal ridge segments" characterized by (La/Sm)n less than 0.7; "plume segments" characterized by (La/Sm)n

maxima; and "transitional ridge segments" characterized by largescale gradients in (La/Sm)n (e.g., 0.7-1.8) adjacent to plume seg-ments. Samples from Site 907 plot at neither end-point of the mixingcurve but almost directly between the two, with low Zr/Nb and (La/Sm)n of 0.68-0.72. It is important to note that the "plume segment"that these transitional basalts are near is not the Iceland plume, butmay be the hypothesized plume at Jan Mayen. Mertz et al. (1991)have presented convincing isotopic evidence that an Iceland compo-nent is not an end-member in the source(s) of the southern Kolbein-

363


sey Ridge basalts. However, the portion of the ridge north of the Sparfracture zone (at about 69°N) appears to be from a different geochem-ical province showing higher Pb and Sr isotope ratios and higher La/Sm ratios than samples from the southern Kolbeinsey Ridge (Mertzet al., 1991). Site 907 is north of the Spar fracture zone. The data ofSchilling et al. (1983) also show increasing (La/Sm)n from the Sparfracture zone to Jan May en.

The 40Ar/39Ar data may be in agreement with paleontological andpaleomagnetic interpretations and data. There is no firm evidence toprovide disagreement. Sediments near the bottom of Hole 907A(187.85 mbsf) are estimated to be -14.164 Ma from paleomagneticdata; however, several significant hiatuses below 150 mbsf make cor-relation of the magnetozones to the geomagnetic polarity time scaleof Cande and Kent (1992) rather uncertain (Shipboard Scientific Par-ty, 1995). Paleontological data is in general agreement about the ageof the sediments in the bottom of Hole 907A being Middle Miocene,with the possible exception of diatoms. Diatoms may indicate an agein the early part of the Middle Miocene; however, diatom ages arecalibrated to the magnetostratigraphy from Leg 104 (Eldholm,Thiede, Taylor, et al., 1987), which is somewhat controversial.

All three samples analyzed yielded climbing age spectra withlow-temperature, low radiogenic yield initial steps, followed by high-er yield steps of increasing age. The low radiogenic yields (0.6%-47%), unusually low for basalts of about 10 Ma, probably reflect at-mospheric Ar trapped in alteration phases, possibly zeolite or clayminerals. The discordant ages of higher temperature steps (10-25m.y. older than low-temperature steps) probably reflect excess Ar inretentive phases, possibly plagioclase microphenocrysts. An alterna-tive explanation is that the rocks are older than 13.2 Ma (i.e., 24 Ma),but were altered 10 m.y. after emplacement. This does not seem to fitwith what we know about hydrothermal alteration of the seafloor. Ifthe basalts were emplaced at 24 Ma, it is unlikely that hydrothermalalteration would occur 10 m.y. later to produce the low-temperaturespectra we see, unless a second heat source were present. We have noevidence to suggest a second event, and it is more likely that theyounger age of 13.2 Ma is accurate.

If the age presented here is indeed accurate, it is still difficult todetermine the significance of the age with respect to plate tectonic ar-guments. Although the basalts dated herein are pillow basalts and un-doubtedly erupted into water, it is in no way obvious that these ba-salts sampled represent acoustic basement. As discussed in Ship-board Scientific Party (1995), an opaque, extremely smooth acousticbasement reflector is characteristic of the Iceland Plateau, with short,indistinct reflector elements lying below this smooth reflector. Thebasalts drilled at Site 907 came from the upper smooth reflector. Itwas suggested by Eldholm and Windish (1974) that the true oceanicbasement of the Iceland Plateau is below the smooth reflector. Basaltdrilled at Site 348, Leg 38 (Talwani, Udintsev, et al., 1976) near Site907, appears to be from the same smooth reflector; it was sampledand dated by K-Ar analysis at 18.8 ± 1.7 m.y. (Kharin et al., 1976).Neither pillow structures nor glassy rims were found in the basalts atSite 348, and it may be that the basalt is from a major sill-like body;however, Kharin et al. (1976) argue that the basalts at Site 348 are notnecessarily intrusive. Note that the integrated 40Ar/39Ar age of 19.7Ma reported here is within experimental error of Kharin et al.'s(1976) K-Ar age. As discussed in Shipboard Scientific Party (1995),Site 907 was drilled on either Anomaly 6B crust, between 22 and 24Ma (Talwani and Eldholm, 1977), or on crust of Anomaly 5B age, be-tween 14 and 15 Ma (Vogt et al., 1980). The ^Ar/^Ar age presentedhere would seem to indicate that Site 907 sits on crust closer toAnomaly 5B. What is interesting to note here is that the age we haveobtained, 13.2 Ma, correlates well with the time of plate accelerationbetween 12 and 14 Ma ago, discussed by Vogt et al. (1980) that they

have suggested may be connected to a magmatic pulse of global pro-portions.

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