American Mineralogist, Volume 73, pages 741-753, 1988

Cr-rich spinels as petrogenetic indicators: MORB-type lavas from theLamont seamount chain, eastern Pacific

J,trrns F. Ar.r-.c.N*Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201, U.S.A.

Rrcn.qno O. SacrDepartment of Earth and Atmospheric Sciences, Purdue University,

West Lafayette, Indiana 47907, U.S.A.

Roonv BxttztDepartment of Geological Sciences, Northwestern University, Evanston, Illinois 60201, U.S.A.

Ansrucr

The composition and morphology of Cr-rich spinels in MORBs reflect and record pre-eruptive petrogenetic events such as magma-chamber recharge, fractionation, and magmamixing. In this paper we examine Cr-rich spinels in MORB-type lavas erupted from thenear-ridge Lamont seamounts and from the adjacent East Pacific Rise crest at 10'N. Thespinels studied are exclusively from quickly quenched glassy to spherulitic flow margins.The host lavas are depleted [(LalSm)" < 0.57] and relatively primitive [Me/(Me + Fe2*)to 0.71, Cr to 460 ppml, with the most primitive samples approaching the compositionof liquids in equilibrium with mantle peridotite. The spinels cover a wide range in Al andCr contents, with spinel Cr/(Cr + Al) ranging from 0.20 to 0.54 for the entire suite. As inother depleted MORBs, TiO, and calculated FerO. contents in the spinels are low (0. 16-0.85 and 5.5-9.2 wto/0, respectively). Contents of Al, Mg, and Fe of groundmass spinelsstrongly correlate with host-glass composition, but spinel Cr content shows little correla-tion with host-glass Cr content. Casting of spinel compositions in terms of Mg-Fe'z* spinel-host liquid exchange equilibria and in terms of the compositionally related crystallochem-ical effects on this exchange shows that for a given lava suite derived from similar parentallavas, spinel Crl(Cr + Al) increases and Mg/@g + Fe2+) decreases with the amount ofFe enrichment, Al depletion, and extent of fractionation [as represented by the normativeratio Diop/(Ol + Diop)l of the liquids in which they equilibrated. This change in spinelcomposition is of much greater magnitude than that of the host liquid; thus spinels aresensitive indicators of melt composition and crystal-liquid equilibrium and disequilibriumprocesses. MORB lavas containing strongly zoned spinels or populations of spinels withbroad compositional range preserve direct evidence of host-melt prehistory of interactionwith other melts (magma mixing) or other processes such as wall-rock interaction. Thestrong compositional covariance of Cr-rich spinel and host silicate liquid makes the for-mulation of a spinel geobarometer difficult.

INrnonucrroN

Cr-rich spinel within MORBJike lavas has long beenacknowledged as carrying useful information regardingthe petrogenesis of its host lavas. Although the crystalchemistry and thermochemistry of spinels are complex(e.9., Sack, 1982; Irvine, 1976),the composition of spinelis a sensitive indicator of magmatic intensive and exten-sive variables (Irvine, 1965, 1967). Inferences regardingmagma mixing (Natland et al., 1983) and initiation ofprecipitation ofcoexisting phases (Fisk and Bence, 1980;Foruta and Tokuyama, 1983; Dick and Bullen, 1984) have

* Present address: Department of Geological Sciences, Uni-versity of British Columbia, Vancouver, British Columbia V6T2B,4, Canada.

0003404x/88/0708-o741$02.00 '141

been made from variations in MORB Cr-spinel compo-sitions. MORB spinel chemistry has also been used todeduce information about magmatic intensive variables,including pressure (Sigurdsson and Schilling, 1976; Si-gurdsson, 1977;Dlck and Bryan, 1978; Dick and Bullen,19 8 4), fo" (Fisk and Bence, I 9 80; Batiza and Vanko, I 9 84)and temperature (Fisk and Bence, 1980) during crystal-lization. We consider here systematic changes in spinelcomposition among Cr-rich spinels from basalts eruptedalong the Lamont seamount chain, a group of seamountsnear the axis of the East Pacific Rise (EPR) occurring westof the EPR at 10'N, just south of the Clippenon FractureZone (Fig. 1), and within Cr-rich spinels from lavaserupted at the EPR axis at l0'N. Samples were collectedin situ from sheet and lobated flows, pillows, massive

742 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

TneLe 1. Glass analyses of spinel-bearing lavas

F2-1

ConenearMOKF9-1

Cone near SASHA DTD1572-1 5 1 1

DTD't572-1755

EPR1 567-181 6

EPR MIB1567- 1560-2019 1843

Location:*Sample:

sio,Tio,Alro3FerO.FeOMgo

NaroKrOD A

Total

Mg/(Mg + F€F-)

SC

CrCoNi

YbHf(La/Sm)N

49.601 .21

16.850.847.838.97

12.352.460 .100 . 1 0

100 31

0.671

3 5 543046.8

1902.762.242.060.57

50.411 . 1 8

15.491 .238 .198 .14

13.002.410.060 . 1 1

100.22

0 639

? q o

40044.5

1202.042 3 91 7 50.47

48.400.86

17.380 8 17.609.44

1 3 1 62 .18o.020.07

99.92

0.689

1 . 8 11 .28

3 1 . 146050.0

240

48.79 48.810.83 0.83

17.50 17 .910.86 1.027.45 7.489.50 9.53

12.87 12.902.13 2.080.02 0.010.09 0.09

100.04 99.94

0.694 0.694

31.7 323440 37048.0 48.8

210 2000.92 0.881 . 9 1 1 . 9 11 . 2 9 1 . 1 90.28 0.27

48.68 48.961 . 0 1 1 . 1 4

17 .64 16.880.89 0.997.59 7.949.10 8 .51

12.45 12.282.65 2.710.03 0.060.08 0 .13

100j2 99.60

0.681 0.656

28.7 35.4290 33040.3 47.0

160 1401.20 1 .831.64 2.341.46 1 .880.34 0.40

50.06 49.87 49.321 1 6 1 . 2 0 1 . 3 6't6.21 16.03 16.990.89 1.02 0.958.01 8 .18 8 098 28 8.28 7 .84

12.47 12.70 11.952.46 2.44 3.340.11 0.09 0.040.14 0 .14 0 .14

99.79 99 95 100.02

0.648 0.643 0.633

38.1 38.2 37.0350 360 28043.3 43.6 43.9

160 110 1202.67 2.71 2.812.49 2.54 2.662.07 2.06 2.600.55 0.57 0.47

Note: Maior elements analyzed by electron microprobe at the Smithsonian Institution (T. O'Hearn, analyst). Trace elements (in ppm) analyzed byINAA at Washington University, St. Louis, Missouri, and reported from Allan et al. (submitted manuscript; see footnote 1 in text); analytical precisiondiscussed in Allan et al. (1987). FeO measured by LS.E. Carmichael, using wet-chemical methods; Fe2O3 calculated by difference from FeOr, as measuredby T. O'Hearn. Mg/(Mg + Fe,*) calculated for 1 560-1 843 and 1572-151'l assuming Fe2+l(Fe2+ + Fe3*) : 0.904, the average value for the other analyzedlavas. (La/Sm)N represents La and Sm values normalized to chondritic values (Haskin et al., 1968).

- See Figure 1.

flow interiors, and talus slopes by the ALVIN srrbmersibleand by dredging.

The seamounts are associated with a broad, topograph-ic high at the adjacent EPR, interpreted as representingrecent voluminous ridge volcanism (Gallo et a1., 1986;Kastens et al., 1986). The EPR at this latitude is appar-ently underlain by a magma chamber (Detrick et al.,1987). All seamounts show signs of recent volcanic activ-ity, based on the youthful appearance of flow surfaces,lack of sediment cover, and absence of thick ferro-man-ganese coatings on surface rocks (Fornari et al., 1988a).This 50-km-long chain consists of five 1000- to 1400-m-high seamounts, with volumes ranging from 25 to 120km3, and most with summit craters and calderas (Fornariet al., 1984). The seamounts in turn are surrounded byabundant, 100- to 200-m-high lava cones (Fornari et al.,1988a). The chain has formed on crust of 0.1-0.74 Main age. The absolute age of the seamounts is unknown,but an upper limit is provided by the age of the under-lying seafloor.

Pnrnocnrpuy AND pETRoLocy oF Hosr LAvAS

The spinels occur within basalts of LREE-depletedN-type MORBs (Sun et al., 1979), with NarO varyingfrom 2. I to 3.30/o and (LalSm)N varying from 0.25 to0.57. Other incompatible major elements and high-field-strength elements are similarly low (e.g., TiOr, 0.81-1.43o/o; KrO, <0.l lolo; Hf, l.19-2.69 ppm; see Table l).The Lamont seamounts and cones have erupted an un-usual percentage of high-MgO lavas (600/o are higher than

8o/o MgO, average MgO is 8.24o/o), some of which arequite primitive [MgO to 9.7 4o/o, Mg/(Mg + Fe2*) to 0.70,Cr to 460 ppm, Ni Io 240 ppm; olivines having mantleperidotite compositions of Fon, to Forr; Tables I and 2land could approximate primary mantle melts (Green,l97l; Frey etal.,1978;Sato,1977; Rhodes and Dungan,I 979; Presnall et al., I 979). In contrast, the adjacent EPRlavas average 7 .57o/o MgO (Allan et al., 1986; Langmuiret al., 1986). All spinel-bearing lavas have Mg/(Mg +Fe'?+) greater than 0.6 I and contain greater than 280 ppmCr. Analyses ofglasses from spinel-containing lavas con-sidered here are given in Table l, and summaries of min-eral compositional data for these lavas are given in Table2. The glass analyses are considered to represent closecompositional approximations of the eruptive host mag-mas (Melson el al., 1976;' Byerly et al., 1976). Normativecompostions of these glasses are plotted in Figure 2 interms of normative Ol, Di, and Neph projecting fromplagioclase, using the algorithms of Sack et al. (1987).Allan et al. (submitted manuscript)' show that the mostprimitive glass samples approximate liquids in equilib-rium with peridotite at pressures of9-9.5 kbar and soli-dus temperatures of 1180-1240 "C, but Figure 2 showsthat most have undergone fractionation or re-equilibra-tion at pressures ofless than 9 kbar.

'Allan, J.F., Batiza, R., Perfit, M.R., Fornari, D.J., and Sack,R.O. Petrology of lavas from the Lamont seamount chain, East-ern Pacific: Seamount lavas as mantle probes for understandingMORB petrogenesis. Submitted to Journal of Petrology.

ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

1 1 0 0

743

TABLE 1-Continued

M I B1 560-1922

MIB1 566-1642

MOK1 564-1 857

MOK1 564-1 949

MOKI 570-1 949

49.091 .43

1 6 5 01 .O28.077.80

1 1 8 12 2 1

0.040.17

99,24

0.633

38.0300

43.4100

2.872.71z .oY0.47

50.071 . 1 7

15.140.918.867.81

12.712.600.040 .13

99.44

0 .611

42.9380

45.790

1 .542.711 . 8 1n 2 a

48.940.96

17.020.838.079.18

12.802.300.040.12

100.26

0.670

35.636050.0801.242.081.400.33

48.550.94

16.92n o 78.239.24

12.882.260.04o. ' t4

1 00.17

0.667

JC.J

36049.0901.592.081.420.43

49.42U.YO

16.600.898.038.78

12.672.430.040.08

99.90

0.661

36.034047.4

1400.992.271.480.26

30'w 1040 20'W1040 50'w /to'w

100 10 'N

100 00'N

LAMONT SEAMOUNTS10 kn

CLIP F

fi-2;. \\875r/865r ranges from 0.702218 to 0.702 67 | within the

seamount lavas; the adjacent EPR lavas have a muchmore restricted range of 875r/865r ralios (0.702499-0.702780; Perfit er al., 1986;Klein et al., 1986;Fornariet al., 1988b). Calculated eruptive temperatures (basedon the crystal-liquid geothermometers of Glazner, 1984)range from ll80 to 1220"C. Allan et al. (1986) demon-strated from sample LREE and major-element contentsthat the seamount lavas bypassed subaxial magma cham-bers thought extant at the adjacent EPR. A fuller treat-ment of the petrology of the Clipperton seamount lavasis given in Allan et al. (submitted manuscript; see foot-note l).

The spinel-bearing seamount and cone lavas from theLamont seamount chain have a very simple mineralogy(Allan et al., 1986, and submitted manuscript). Mostsamples contain both plagioclase and olivine as pheno-crysts. Samples range from nearly aphyric to porphyritic(up to 150/o phenocrysts) with glomeroporphyritic clustersof plagioclase and olivine commonly present. Plagioclasephenocrysts are typically euhedral with sharp, nonscal-loped outlines and may exceed several millimeters in size.Olivine phenocrysts are commonly skeletal and can beequally large. In most cases, both minerals are composi-tionally unzoned, with crystals rarely exhibiting any re-verse zoning or normal zoning greater than l-2 mol0/0.Nevertheless, exceptions do occur. Lava samples fromone of the cones (F2-l and F2-2) contain two types ofolivine phenocrysts - skeletal, unzoned olivines of Forr_nocomposition and subhedral-euhedral, reversely zoned ol-

9o oo'N

Fig. 1. Maps showing location and detail of the Lamont sea-mount chain. General location map after Fig. I of Sempere andMacdonald (1986) shows trace of East Pacific Rise and locationof the seamount chain at 10"N, just south of the ClippertonFracture Zone (F4. Seamount map derived from SEABEAMcharts (after Fornari et al., 1988a).

ivines with Forr*, cores and Forn-no rims. Minor (<10/oFo) reverse zoning is found in a few ofthe olivines withinthe lava sample 1564-1949 from the MOK seamount; itsplagioclase phenocrysts exhibit oscillatory zoning as well.

Clinopyroxene is typically absent, particularly in themore primitive lavas (MgO > 8olo). This absence of cli-nopyroxene indicates that it is not generally on the low-pressure liquidus at the pre-eruption temperatures of theLamont seamount magmas. Indeed, most lavas from in-dividual seamounts broadly define olivine + plagioclasefractionation-control lines in MgO variation diagrams,consistent with modeling results using least-squares mix-ing and with petrographic evidence (Allan et al., submit-ted manuscript; see footnote l). Crystals of titanomag-netite are absent in most samples and are always <5 pmin size. Xenoliths are absent.

Spinel is often abundant in these samples (some 2.3 x4.6 cm,0.07-mm-thick thin sections may contain over50 spinel grains, though this is <<0.10/o by volume) andforms translucent, light brown crystals up to 200 pm in

ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

Fig. 2. Comparison of ratios of nepheline (Neph), olivine(Ol), and high-Ca pyroxene (Di) normative components of spi-nel-bearing Lamont seamount lavas with those for the l-atm

66, = QFM) plagioclase-saturated cotectics and the high-pres-sure olivine + orthopyroxene + high-Ca pyroxene cotectic asdefined by Sack et al. (1987). l.{ormative components employedin this and subsequent figures are calculated from the algorithmsgiven in Sack et al. (1987, caption to Fig. 5). Most spinel-bearinglavas are seen to have evolved at pressures less than 8 kbar byfractionation or re-equilibration, putting absolute limits on pres-sures of spinel precipitation. Dashed line represents an averagechord emanating from the olivine vertex, representing olivine *plagioclase fractionation.

size that are found as groundmass grains, as inclusions inolivine and plagioclase, or as grains accreted to or embed-ded into olivine and plagioclase. The spinels encompassa wide range of shapes and include both euhedral andskeletal shapes, the latter perhaps indicative of rapidgrowth (Fig. 3). Curiously, the abundance of spinel in thelavas is independent ofhost-rock Cr content, and Cr-richspinel has been found in samples with Cr content varyingfrom 280 to 460 ppm (Table 1). Wide-beam (50 pm) glass

microprobe analyses of long counting time (200 s) agreewith mass-balance calculations based on spinel abun-dance that the great to overwhelming majority of magma

TneLr 2. Mineralogical data for lavas with Cr-rich spinel

Cr content (typically much greater than99o/o) is containedwithin glass (representative ofthe host liquid) instead ofwithin the spinels. Rock textures for all spinel-bearinglavas indicate that crystallization of spinel preceded andoverlapped with the onset of olivine and plagioclase crys-tallization. Indeed, the experiments of Fisk and Bence(1980) show that the onset of spinel crystallization inMORBs is only tens of degrees celsius before olivine.

The simple mineralogy, the abundance of spinel, andthe primitive, depleted nature of the basalts from the La-mont seamount chain suggest that the melts have had asimple ascent and eruption history. The absence of xeno-crysts and xenoliths and the unzoned nature of the phe-nocrysts implies that most of the lavas have undergonelittle or no magma mixing or significant crustal assimi-lation. The absence of clinopyroxene in most of the lavasindicates that these lavas spent little time cooling in shal-low crustal magma chambers (Walker et al., 1979), butinstead underwent relatively rapid ascent. As a result,these seamount and cone lavas are good candidates tostudy how spinel composition is related to hostlavacomposition and paragenesis.

It is crucial to note that the spinels studied here areexclusively from the glassy to spherulitic and finely mi-crolitic margins of flows, where the quenching of the lavaby cool seawater during eruption was rapid to nearly in-stantaneous (seconds to minutes). Many of the ground-mass spinels are completely surrounded by crystal-freeglass. Therefore, problems with interpretation of post-eruptive or syneruptive spinel growth or re-equilibrationare avoided. Indeed, these lavas represent perhaps na-ture's closest approximation to quench conditions ob-tainable in the laboratory.

SprNrr, cHEMrsrRY

The Cr-rich spinels in this study are classified as chro-mium spinels or magnesiochromites, using the criteriaand terminology of Sigurdsson (1917). They are plotted

Spinel

Sampleanalyzed

Sample OlFo range

Sample PlagAn range

F€F*/(Cr +Al + Feo*) No. of analysesCrl(Cr + Al) Mg/(Mg + F€r.)

F-1 -1

F2-2F3-4F9-11 560-1 8431 560-1 9221 564-1 8571 564-1 9491 566-1 6421 567-1 81 61 567-201 91 570-1 9491572-15111572-1755

86-8789-9087-9089-9089-918786-8J

8885-8686-8778-8888.0085-8987-89

70-7478-8978-9176-?0

61-8166-8178-9175-9175-8469-8070-8379-8969-8865-87

0.53-0.540.20-0.500.22-0.510.27-0.460.35-0.410.41-0.430.39-0.40

0.300.23-0.360.50-0.54o.46-0.470.46-0.470.38-0.390.30-0.32o.37-0.41

0.67-0.680.75-0.840.75-0.840.70-0.810.73-0.780.69-0.71

0.710.80

0.75-0.840.66-0.700.71-0.720.69-0.720.74-0.770.80-0.810.73-0.75

0.098-0.1 060.056-0.0750 058-0.0890.060-0.0850.069-0.0840.096-0.1 000.094-0.097

0.0780.062-0.0830.1 00-0.1 1 I0.091-0.0970.094-0.1 000.083-0.1 01o.071-0.0770.607-0.086

42929201 24

,l

2145bA

1 1

Note.'Ranges in plagioclase, olivine, and spinel composition observed in spinel-bearing samples'Samples lacking either olivine or plagioclase.

ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 745

Fig. 3. Backscattered images of selectedspinels, with accompanying scale bar rep-resenting 100 pm in length. Dark areas rep-resent Al-rich regions of spinel (smaller meanatomic number), and light areas representCr-rich regions of spinel. Thin bright linesoutlining some of the spinels are caused byedge effects. (A) Skeletal, relatively unzonedspinels in groundmass of plagioclase mi-crolites and mesostasis in sample F3-4, rep-resented by analysis ofF3-4-18 in Table 3.(B) Complexly zoned spinels in the glass ofsample F2-2. Spinel compositional bandingmay be quite complex, with cores eitherbeing A1- or Cr-rich. Total compositionalvariation in Crl(Cr * Al) of cores isO.227-0.279. Variation in Crl(Cr + Al) zoning inlarge spinel is 0.227 (core) to 0.277 (5- tol2-pm-wide light band) to 0.242 (darker,5-pm outer band) to 0.251 (narrow, l- to3-pm-wide rim). Analyses representedare F2-2-19, -20, -21, -22. (C) Complexly zonedspinel in microlitic groundmass of F2- 1, rep-resenting analyses F2-l-27, -28, -29. Spinelcontains a Cr-rich core, surrounded sharplyby a 3- to 5-pm-wide Al-rich band, which isin turn surrounded by a Cr-rich rim. Zoningrepresented in Crl(Cr + Al) is 0.269,0.220,and0.232, respectively. @) Skeletal spinelattached to rounded olivine and elongateplagioclase crystals in groundmass of F2-1.Wormy, more aluminous core represents lastgrowth phase ofspinel; core and rim analysesare given in Table 3 (F2-l-3, -4). (E) Rela-tively unzoned euhedral spinel in quenchedgroundmass of olivine dendrites in glass ofsample F9-1. Represents analyses F9-1-11,- I 2 in Table 3. (F) Strongly zoned, euhedralspinel and unzoned, skeletal spinel in glass with plagioclase mi-crolites in sample F2- I . Cr-rich core of zoned spinel is rounded,with abrupt transition to aluminous rim. Skeletal spinel is ap-proximately the same composition as the rim ofthe zoned spinel.Core and rim analyses ofthe zoned spinel are given in Table 3(F2-l-7, -8). (G) Strongly zoned spinel in glass with featheryplagioclase microlites in sample F2-1. Cr-rich core is rounded,

with sharp compositional transition to a skeletal, aluminous rim.Corresponding analyses are given in Table 3 (F2-l-1, -12). (H)Sharply zoned spinel in glass, with unzoned, Cr-rich core sharplysurrounded by thin (2-15 pm wide) aluminous rim, representinga zoning in Crl(Cr + Al) of 0.481 to 0.286. Spinel is in sampleF2-2 (analyses F2-2-13, -l4).

in Figure 4, with compositional ranges given in Table 2.Fe3+ content is low (FerO. varies from 5.5-9.2o/o), similarto other spinels from MORBs (Dick and Bullen, 1984;Basaltic Volcanism Study Project, l98l). The low Fe3+content of the spinels reflects the low/o, of the host La-mont seamount basalts, which range from 1.0 to l 8 log,ounits below the NNO oxygen buffer (calculated using themethods of Sack et al., 1980). Thisf" range is similar tothat found in other MORBs by Christie et al. (1986). Thecompositional range exhibited by the spinels is wide, withAlrO, varying from 22.3 to 47.3o/o, Cr.O, varying from18.9 to 39.3o/o, and MgO varying from 14.5 to 21.4o/o.The more Al-rich spinels from this seamount group are

among the most aluminous spinels reported from MORBs.MgO and AlrO, strongly covary, as do FeO and Cr2O3.TiO, is a minor component, ranging from 0.16 to 0.850/0.Other minor components include NiO (<0.02-0.300/o),MnO (0.05-0.47o/o), CaO (<0.02-0.300/o), and SiO, (typ-ically <0.10/o).

The MgO, FeO, and AlrO, contents of the groundmassspinel rims and of the host glass are strongly correlated(Fig. 5). Sigurdsson and Schilling (1976) found a similarcorrelation between spinel and host-glass Al content.Strikingly, the CrrO. content of the spinel is independentofhost-glass Cr content (Fig. 5), contrary to the results ofIrvine (1976). A difference noted between the ground mass

746 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

=+o=o

o

ooo

o

go

doI

o

{o

o

0.6 07 0.8 0.9

Mg(Mg+Fe2+)

Fig. 4. Composition of Lamont Seamount laval spinels. Spi-nels are similar in composition to other spinels reported fromMORB-type lavas (e.g., Sigurdsson and Schilling, 1976; Si-gurdsson, 1977; Basaltic Volcanism Study Project, 1981; Dickand Bullen, 1984). Closed symbols represent spinels from theseamounts and cones; open symbols represent spinels from lavaserupted at the adjacent EPR axis.

Tnele 3. Representative spinel analyses

Fig. 5. Comparison of the composition of the rims ofgroundmass spinels with the composition of the host glass. Con-nected dots represent the compositional range found within spi-nels of an individual lava. Note the correlation between spineland glass MgO, FeO, and AlrO3. In contrast, note the poor cor-relation between host-glass Cr and spinel CrrOr.

spinels and those included within silicate phases is thatthe rims of the groundmass spinels, on average, are slight-ly more aluminous and slightly less chromous than thoseofthe included spinels. Figure 6 shows spinels from fourlavas of the Lamont seamounts and adjacent EPR, eachlava with differing MgO and AlrO, contents. Host-lavaMgO and AlrO, contents correlate with the compositionof the spinels and result in the sliding of spinel compo-sitions up and down the narrow spinel compositional bandof Figure 6.

Most lavas contain only a relatively narrow range ofspinel compositions, but in some samples (F2-1, F2-2,F3-4, and 1564-1949) the spinel compositional range is

o.o8

0.06

+ctolt+

+oY+et

Mg/(Mg+Fe2+)

&

G|ass MgO Glass Altog

Sample: 1560- 1560- 1564- 1566-1843-1 1843-2 1949-6 1642-1

Descriotion:* PC PR PC PC

1s66- 1566- 1566-1642-2 1642-6 1642-7

PR PC PR

1567- 1570- 1572- 1572- 1572- F1-2019-5 1949-4 1511-6 1511-7 1755-3 1-28 F2-1-11

O O G C G R P R G R G C

sio,Tio,AlroeCrr03FeOMnONioMgoCaO

Total

FeO.-FerO.**

Total--

Crl(Cr + Al)Mg/(Mg +

Fer*)Fe,*/(Cr +

At + Fe+)

0.08 0.100.76 0.76

29.22 30.1532.28 30.8520.06 20.070.23 0.190 . 1 6 0 . 1 0

16 01 15 .900.05 0 .12

98.85 98.24

8 62 8.4512 31 12.4699.71 99.09

0.426 0.4070.699 0.694

0.098 0.096

0.06 0.050.37 0.76

35.76 23.9227.51 37.6416.70 22.020.08 0.220.13 0.06

17.73 14.790.08 0.0s

98.42 99.51

7.10 9 .3210.31 13 .6399.13 100.44

0.340 0.5130.7s4 0.659

0 077 0.108

0.06 0.060.85 0.75

22.28 23.9138.89 37.4121 .95 21.120.28 0.230.10 0.09

14.58 14.910.15 0.07

99.14 98.55

9.44 8.9413.45 13.07

100.09 99.45

0.539 0.5120.659 0.670

0.11 1 0 .104

0.06 0.070.70 0.65

24.63 27.5836.82 34.4620.99 19.300.20 0.260.11 0 .19

14.95 1 6.060.17 0.07

98.63 98.64

8.88 8.4413.00 1' l .7199.52 99.49

0.501 0.4560.672 0.710

0.103 0.096

0.11 0 .060.45 0.37

31.92 37.7629.66 26.1618.36 14.970 . 1 1 0 . 1 40.18 0 .28

17.43 19 .130.19 0 .02

98.41 98.89

9.16 7 .1910j2 8.5099.33 99.61

0.384 0.3170.754 0.800

0.101 0.o77

0.09 0.08 012 0.060.34 0.49 0.69 0.23

39.02 33.96 23.05 32.3025.81 29.72 39.18 34.2814.77 17 .15 20.60 14.050.09 0.00 0.24 0.150.28 0.00 0.07 0.22

19.31 1 7.68 14.88 1 8.350.09 0.17 0.18 0.03

99.80 99.25 99.01 99.67

6.76 7 27 8.42 5.658.68 10.60 13.02 8.97

100.48 99.98 99.85 100.24

0.307 0.370 0.533 0.4160.798 0.748 0.671 0.78s

0.071 0.079 0.098 0.061

* C refers to spinel core, R reters to spinel rim, P refers to spinel included in plagioclase, O refers to spinel included in olivine, and G refers to spinelfound in the groundmass.

.'Recalculation of FeO and FerO3 after Carmichael (1967), based on stoichiometry.

ALLAN ET AL,: CT-RICH SPINELS IN MORB-TYPE LAVAS 747

much broader (Table 2). Similarly, compositional zoningin the spinels of most lavas is minor (for example, 1560-I 843- l, -2; | 5 66- | 642-1, -2; 1 5 66- | 642-6, -7 ; | 57 2-l 5 | | -6, -7; and F9 I - 1 I , - l2 in Table 3). Nevertheless, zoningof some spinels within samples F2-l and F2-2 is stronglydeveloped (Figs. 3 and l0), with zoning parallel to theCr/(Cr + Al) vs. Me/(Mg + Fe2*) compositional trendofFigures 4 and 6 and typically progressing from a corewith higher Crl(Cr + Al) and lower Mg/(Mg * Fe'z+ ) toa rim with a lower Cr/(Cr + Al) and a higher Mg/(Mg +Fe'?+). For the strongly zoned spinels, the zoning is typi-cally quite abrupt; most strongly zoned spinels contain arounded, Cr-rich core surrounded by an Al-rich rim thatmay have either skeletal or euhedral outlines. The spinelcompositional trends are crystal-chemically controlled andreflect the strongly coupled substitutions of Mg2* and Al3*and of Fe2+ and Cr'* into the spinel structure (e.g., Sack,1982; Hill and Sack, 1987; Engi, 1983). These well-de-veloped zoning trends are distinct from other spinel crys-tallization trends predicted (Dick and Bullen, 1984) orexperimentally observed (Fisk and Bence, 1980; Irvine,1976; Sack, 1982) for spinels coprecipitating with olivineor plagioclase or both.

Complex compositional zoning occurs in some spinelsof F2-l andF2-2, but is generally only of limited com-positional range (Figs. 3 and l0). Compositional bandsmay be quite narrow (l prm or larger), and cores may beeither relatively Cr- or Al-rich.

DrscussroN

Spinel compositional systematics

Despite the complexity of spinel zoning patterns in someof the Lamont seamount lavas, most have spinels that

Taete 3-Continued

. r .151c8 ' rb . .za

HOST GLASSMso

o.2 07 0aMg/(Mg+Fe2+)

Fig. 6. Spinel compositions plotted from four lavas eruptedfrom the Lamont seamounts, showing generally how host-Iavacomposition affects spinel composition. Diferent symbols rep-resent spinels from different indiwidual lavas, with hostJava MgObelow and host-lava AlrO. above the corresponding spinels.Symbols representing spinel compositions are the same as in Fig.7, with solid diamonds representing spinels from 1566-1642;open triangles, from I 567-20 I 9; open diamonds, from F9- I ; andupside down, open triangles, from I 572- I 5 1 1 Note how chang-ing hostJava AlrO, and MgO slides the spinel composition upand down the narrow spinel composition band.

exhibit only limited variability in CrlAl and Fe3*/Al ra-tios. These display simple Fe2*-Mg zoning patterns thatmay be related to fractionation of the lavas and to inter-lava compositional variations that are largely crystal-chemically controlled. Both of these points are readilyillustrated with a plot that makes provision for crystal-chemical effects on the Fe'?+-Mg exchange reaction be-tween spinel and host silicate liquid. We assume that spi-nel behaves as an ideal reciprocal solution (e.g., Woodand Nicholls,1978; Sack, 1982) and that silicate liquidsare ideal with respect to mixing of Fe2+ and Mg'z+. In

o5

t o roY

o o.3

16.O"A o8.3

HOST GLASSAbos

o 169S^o

SO -

-1ZG9.1

F3- F9- F9-4-18 1-11 1-12

G G C G R

F3-4-12PC

F2-2-25OR

F2- F2-2-19 2-24GC OC

az-

2-11OR

F2- F2- F2-1-4 1-7 1-8GR GC GR

F2-2-10oc

F2- F2- F2-1-12 1-2 1-3u h u u u

0.10 0 060.21 0.37

45 85 26.4218 94 39 .1512.76 14.56o.17 0 .220.24 0.10

20.70 17.090 . 1 0 0 . 1 6

99.07 98.13

6.147.23

99.69

0.2170.836

0.063

0.09 0 150.15 0 22

47.21 42.9017.88 21.8412 06 13 .150.08 0.160.33 0.24

20.80 19 950.05 0 .13

98.65 98.74

0 10 0.08 0.090.26 0.39 0.31

43.48 38.70 42.7920.57 25.82 21.5713.65 13.91 1 3.480 . 1 6 0 . 1 8 0 . 1 50.28 0.29 0.19

20.14 19.62 20.290.13 0.03 0.08

9877 99.02 98.95

0.09 0.080.16 0 .73

45.35 25.7319.81 39.3012.',t1 15.670.10 0 .150.31 0.26

20.75 17.390.00 0.04

98.68 99.35

5.b/ o,cY

7.01 97499.25 100.01

0.227 0.5060.841 0.761

0.058 0.075

0.06 0.080.56 0.38

29.21 27.9135.32 35.9814.83 1 8.360.18 0.280.13 0 .12

17.68 15.740.01 0.09

97.98 98.94

6.09 6.949.35 12.11

98.59 99.64

0.10 0 .10 0 14o.28 0.46 0.44

38.07 35.33 34.0826.72 30.02 30.1214.23 14.81 16.510.14 0 .06 0 .150.27 0.24 0.24

1 8.92 18.60 17 .970.06 0.04 0.10

98.79 99.66 99.75

6.09 6.03 7.168.75 9.38 10.07

99.40 100.26 100.47

0.320 0.3630.794 0.779

0.120.28

5 I . C I

28.3013.500.170.22

19.27007

99.50

0.2530.828

5.63 5.479 49 7.14

98 69 99.20

0 498 0.2030.762 0.838

5 8 97.85

99.33

0.2540.819

5.68 6.71 6.67 6.638.39 7 .61 7.91 7 .52

100.07 99.44 99.69 99.61

0.336 0.241 0.3090.804 0.82s 0.815

0.3720.761

0.078

0.448 0.4640.771 0.698

0.068 0.0780.064 0.056 0.061 0.060 0.070 0.071 0.069

748 ALLAN ET AL.: CT.RICH SPINELS IN MORB.TYPE LAVAS

addition, we assume that the pressure effect over the spi-nel crystallization range on spinel composition is rela-tively small compared to the effect of varying the host-lava composition, and so may be neglected. For theseassumptions we may write the following expression forthe Fe-Mg exchange reaction between spinel and silicateliquid:

ad' o"l '( t1 a _!&.6;+L$tx,), ( l)In Kgu'o : - Ri

+ Ri;...,, R. \--4, . Rf ..

where

aG& : (Gfu,o - Gg"o)uo * (Gk^,oo - GRn"o,,oJ.o,

apgz: (G$.rr.oon * GB"o,roo)"o - (Gg...roo * GRnr^roJ"o,

Lt{o : (t/z Gf;a,"r.1i6o * Gg"o,roJ"o

- (r/zGoe,rlioo * G$,*roJ'o,

LrSs : (Gg,r..roo + G9"^,ro)"o - (Gg"roo * GR *,roJ.o,

X, : t/zn"f,t*, Xo: n"fie*, Xr: r/zn{.t*,

and the r?, terms are the number of the i cations in aformula unit based on three cations (e.g., Sack, 1982; Hilland Sack, 1987).

In a first approximation suitable for illustration, sev-eral simplifications of Equation I may be made. First,because Tia+ is negligible in the Lamont seamount spi-nels, we will ignore the third term on the right-hand sideof Equation l. Second, we approximate that N.4, : %Aplr, where Apgr:4.80 kcal/gfw and Ap!, : 6.39 kcaUgfw, as established by Hill and Sack (1987). Making thesemodifications reduces Equation I to

lnKS-ric: -AGl^/RT + (4.80/RQ(y), (2)

where Y: Xj + %(X). We note that land Crl(Cr + Al)are essentially equivalent for spinels within a given sam-ple, because of the minor variation of Fe3*.

Spinel compositions from the I I lava samples exhib-iting limited spinel compositional range and little spinelzoning are plotted in Figure 7 using these parameters.These spinels define a crystal-chemical trend with posi-tive slope, with spinels from more primitive, MgO- andAlrOr-rich lavas plotting at lower values of ln Ko and, Yand spinels from more evolved lavas plotting at the higherln Ko and I values in the trend. These results show thatthe Cr-rich spinels in this study become progressivelymore Cr-rich as the host lava becomes more fractionated.This trend is due to the reciprocal exchange substitutionof Cr and Fe2+ for Mg and Al in the spinel. Olivine andplagioclase fractionation will cause magmatic Fe2+ to in-crease and magmatic Al and Mg to decrease, resulting inthe crystallization of spinels higher in Cr/(Cr + Al) andlower in M/(Mg + Fe'z+). This correlation between spi-nel composition and degree of fractionation of the hostlava is strikingly shown by comparing the Crl(Cr + ADratio or the quantity I in the spinels with the normative

ratio Dil(Ol + Di) of the host glasses (Fig. 8). The inde-pendence ofspinel and glass Cr contents may reflect thepaucity of Cr in the melt.

Finally, we may estimate an approximate value for-AG!- from the FeO/MgO ratios of the glass chemicalanalyses, from Equation2, and from the compositions ofthe rims of groundmass spinels (to insure approximationof crystal-liquid equilibria). To do so we assume (l) thatspinel is the first phase to crystallize from the lavas (e.g.,Sack et al., 1987, and from petrographic observations);(2) that spinels do not exhibit retrograde Fe-Mg exchangeduring crystallization (unlikely because of quick mag-matic quench after eruption on the ocean floor); (3) andan average temperature of initial crystallization of 1200"C (derived from olivine-liquid geothermometry basedon Glazner, 1984). From the values of ln K3-tio calculatedby employing these assumptions, we obtain an estimatefor -AG!. of -3.40 + 0.18 kcal/gfu. This value for-AG!* is slightly smaller than that (-3.16 t 0.1I kcal/gfw) which may be calculated fom the calibration of Hilland Sack (1987) for ln Kg*o, using the average values ofXRI*,r,on and (Xfi1"5/Xgi3" ) (Xgl,sto4/XRl,,r'oo) of 0.878 and0.267 ! 0.008, respectively, as determined for the La-mont seamount lavas by microprobe analysis. Neverthe-less, these estimates are within analytical error of eachother, indicating that the calibration of Hill and Sack(1987) is essentially correct. We use the former estimateof - 3.40 t 0. I 8 kcal/gfw to calibrate the correlation forEquation 2 plotted in Figure 7 for illustrative purposes.

The compositions of spinels within lavas exhibitinglimited spinel compositional range essentially agree withthe crystal-chemical trend as defined by Equation 2. Spi-nels in these lavas exhibit some variation in ln[(MgO/FeO)e'*"(pe: + /Mg2 + )"oi'a1, which perhaps could be attrib-uted to limited coprecipitation with olivine. The earlier-crystallizing spinels that have not fully re-equilibrated withthe evolving host liquid will define more negative ln Kovalues at a given Y.

Spinel zoning and its petrogenetic implications

We now use these observations to examine more closelythe Lamont seamount spinels exhibiting strong zoning,complex zoning, or extensive compositional variationwithin an individual sample. Spinels from the four La-mont seamount lavas (F2-1,F2-2, F3-4, and 1564-1949)that exhibit wide compositional variation are plotted inFigure 9 in terms of the parameters defined in Equation2. Selected examples of spinel zoning are presented inFigure 10.

Lava samples F2-1 and F2-2 contain two types of oliv-ine phenocrysts-skeletal, unzoned olivines of Forn-nocomposition, and subhedral-euhedral, reversely zoned ol-ivines with Forr-r, cores and Forn-no rims. As illustratedin Figure 9, the spinels in these samples span a broadcompositional range, containing spinels strongly zoned inCr/(Cr + AD F2-l-7, -8; F2-l-l l , -12; andF2-2-13, -14

in Figs. 3 and I 0). The rounded, resorbed-appearing cores

ALLAN ET AL.: CT-RICH SPINELS IN MORB.TYPE LAVAS 749

a--th

$o^glsz tEg

E

oo

-o.4

-(}6

.1.O

-o.2

-o8J

. F l -1o ru-1r 15@{Si(t.156O1922r 664181i7. 1s@1642, 1567-160^ 1567-2019o lsTO{919e 152-t511.1572'1755

o'3 0.4 o'5Y=%n!L.f %np!o

Fig. 7. Compositions of spinels from lavas with spinels ex-hibiting restricted compositional ranges, plotted in terms of ln'(K3iiq) vs. I, where Y : lrn"!t- -l ln!.* : X3 + %Xt (n, indi-cates the ,1h cation calculated on a three-cation basis). Note thatMgO and FeO in the axis label refers to the molecular equiva-lents ofthese oxides. The solid line represents the calibration ofEquation 2 given in the text by the rim compositions ofground-mass spinels. The spinels from these l l lavas define a simple,straightforward crystal-chemical trend, reflecting equilibria be-tween the spinels and their host silicate liquid. Spinels with highervalues of Y(those richer in Cr) are associated with more evolvedmagmas richer in Fe. See text for further discussion.

ofthe zoned spinels are Cr-rich and sharply bounded byskeletal Al-rich rims. The Al-rich rims are similar incomposition to that of the bulk of the spinels found with-in these lavas and are in apparent equilibrium with thehost glass. In contrast, the Cr-rich spinel compositionslikely crystallized from a liquid considerably more evolvedand Fe-rich than the host glass. As shown in Figure 10,the Cr-rich spinels (high in Y-) included within olivinesare found within Fo' olivine cores, whereas the Al-richspinels included in olivine are found within skeletalForn-no olivines. These more Cr-rich spinels in the Fortolivines are similar in corriposition to other Lamont sea-mount groundmass spinels found within other lavas thatcrystallized Fo" olivine.

Other Cr-rich spinels from F2-l and F3-4 are foundwithin plagioclase phenocrysts that presumably armoredthem from interaction with the host liquid. They are con-siderably less aluminous than other spinels found in thegroundmass and in apparent equilibrium with the hostglass. The Cr-rich spinels in F3-4 are found as inclusionswithin the core of a single large plagioclase phenocrystthat shows simple reverse zoning from Anru to Anno mid-way from core to rim. The core of this crystal also con-tains an inclusion of Fo' olivine, whereas the other oliv-ines of this sample are Fose-e. in composition. Again, theCr-rich spinels in this sample are similar in compositionto spinels crystallizing from other lavas in this suite thatcrystallized Fo' olivines. Sample 1564-1949 contains a

Y=14n8f* +2/snT*,.Fig. 8. Comparison of the range in value of I for spinels in

individual lavas with the normative Ol/(Ol + Di) ratio of thehost glass calculated from the algorithms ofSack et al. (1987).Dots represent the range in spinels of the 1 I lavas where thespinel compositional range is small, as shown in Fig. 7. Opensquares represent the range in composition of the rims ofgroundmass spinels in the four lavas where the range in spinelcomposition is large, as shown in Fig. 9. Diagonal line representsan approximate fit to the data.

compositionally diverse population of spinels as well, yet

its silicate mineralogy is more diverse than that of othersamples with complex spinel populations. Besides con-taining slight reverse zoning in many of its olivine phe-

nocrysts, its plagioclase phenocrysts are reversely andcomplexly zoned (up to An,r).

The Cr-rich spinels in F2-1, F2-2, F3-4, and 1564-1949 were likely introduced externally into the risingmagma, having crystallized from melts more evolved andFe-rich than the lavas in which they are found' Never-theless, the lack of xenoliths, the primitiveness of the hostglasses, the lack ofclinopyroxene, and the lack ofsignif-icant zoning in most olivines and in the feldspars of alllavas except 1564-1949 argue that processes such as mag-ma mixing or crustal assimilation were of minor signifi-cance in the evolution ofthese lavas. The range in spinelcomposition probably reflects magma mixing during theperiodic refilling of small crustal or edifice magma cham-bers just prior to eruption, where the magma residencetime is short and the fractionation minimal. The skeletalnature of some of the Al-rich spinels may represent rapidgrowth triggered by magma mixing, on the basis of theconcave curvature of the experimental basalt liquidussurfaces away from the spinel field (Dick and Bullen, 1984;Irvine, 1977). Viewed this way, the composition of theCr-rich spinels provide qualitative limits for determiningthe compositions of the end-member liquids being mixed.

Several of the groundmass spinels within samples F2-l

06

o.5

LAMONT SEAMOUNT LAVAS

-1.O

-o.2

-o.4

-o.6

-o.8

-1.O

F2-2

1564-1949

t

F3-4

A GmasCA Gmass Ra O l l n CO O l n RI Plag In CEl Plag ln R

Y =/ztfJ'",*2Atfr"'

750 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

;-3t2^I | l .i t DE l =

6 ^ l9lo5tEc

Fig. 9. Spinels from the lavas of the Lamont seamount groupexhibiting wide variation in spinel composition, plotted as inFig. 8. Closed symbols refer to spinel cores and open symbolsrefer to spinel rims; triangles represent groundmass spinels; cir-cles represent spinels included in olivine; and squares representspinels included in plagioclase. Reference lines representing the

calibration of Equation 2 and shown in Fig. 8 are also givenhere. Note that the F2-2 spinels high in Yand included in olivineare found within Fo' cores of olivine reversely zoned to Fonocomposition, whereas the low-I spinels included in olivine arefound within skeletal olivine phenocrysts of Foro composition.See text for further discussion.

and F2-2 contain minor complex zoning (F2-l-27, -28,-29 andF2-2-19, -20, -21, -22;Figs.3 and l0). The com-positional range represented by this zoning falls withinthe range outlined by the rims of other groundmass spi-nels found in these lavas. Therefore, it is not necessaryto invoke magma mixing or assimilation to account forthis zoning; rather, it may be related to local melt deple-tion in Cr from rapid spinel crystallization (Thy, 1983;Allan et al., 1987) with subsequent transportation of thespinel to more Cr-rich portions of the magma body. Onecurious spinel within F2-l (F2-l-3, -4; Fig. 3, Table 3)consists of a skeletal, Al-rich im (42.9o/o AlrO3) with ananhedral, worrny core representing not only the most alu-minous composition (47.2o/o Alroj) in the entire suite but

presumably the last portion of the spinel to crystallize.The compositional zoning represented within this spinelseems rather large to be related to local melt depletion inCr, and the origin of its zoning remains somewhat enig-matic.

Comments on spinel as a possible indicator ofpressure

Although all of the strongly zoned spinels examinedhave Cr-rich cores, it is likely that Al-rich cores of strong-ly zoned spinels may also result from magma mixing orassimilation. Sigurdsson ( I 977) has reported "reversely"zoned spinels from DSDP Hole 3328 plagioclase-phyricbasalts that may represent just such a case. These spinels

ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 751

contain highly aluminous cores with sharp increases ofCrl(Cr + Al) and decreases of Mg/(Mg + Fe'?+) from coreto rim. Other authors (Sigurdsson and Schilling, 1976;Dick and Bryan, I 978; Dick and Bullen, I 984) have foundunzoned high-Al spinels in picritic lavas. Both zoned andunzoned high-Al spinels, similar in Al content to the morealuminous of the Lamont seamount spinels, were previ-ously interpreted as representing high-pressure relicts, onthe basis of (l) scant experimental evidence that the AIcontent of spinels increases with pressure (Green et al.,197 l;Jaques and Green, 1979;'P. Roeder, pers. comm.,1987) and (2) the observation that spinels in lherzolitenodules are typically aluminous. Dick and Bullen (1984)suggested that pressure may affect spinel composition bychanging the melt/spinel partitioning coefficient for Cr,because of increased octahedral melt sites with pressureresulting in increased Cr solubility in the melt.

The presence of Al-rich, skeletal rims of groundmassspinels surrounding Cr-rich spinel cores in the Lamontseamount lavas clearly shows that Cr-spinel with Cr/(Cr +Al) as low as 0.2 can crystallize at low pressures. Wetherefore believe that the presence of Al-rich spinels inMORBs may not represent high-pressure processes butmay instead reflect low-pressure equilibrium crystalliza-tion, magma mixing, or wallrock assimilation. Althoughmore experimental work is needed to determine whethersolubility of Cr-rich minerals in melt appreciably changesover the pressures of interest, we wish to note that thedifference in the l-bar molar volumes between FeCrrOo(chromite) and MgAl,Oo (spinel) is on the order of l0o/o(4.4010 vs. 3.9710 J/bar, respectively; Robie et al., 1978).The compositional effect of the AZof the exchange equa-tron

(MgAlrOo)"o + (FeCr,Oo)'io = (FeCrzOo)"o+ (MgAlroo)rio (3)

is likely small, when compared to the very large difer-ences in spinel composition [i.e., Cr/(Cr + Al) variationof 0.2-0.541 caused by the minor variations in host-lavacomposition as seen in the Lamont seamount lavas. Thisobservation implies that a quantitative geobarometerbased on spinel-liquid exchange may be a difficult prob-lem to implement, because of the difrculty of separatingpressure efects from compositional effects.

Coxcr-usroNs

1. Cr-rich spinels from the near-ridge Lamont sea-mounts and from the adjacent EPR crest at l0'N occurin lavas that contain greater than 260 ppm Cr and thathave M/(Mg + Fe'z+) greater than 0.61.

2. These Cr-rich spinels vary systematically in com-position with host-lava composition and "primitive-ness," as measured either by normative Ol/(Ol + Diop)ratios or Mg/(Mg * Fe2+), with spinels richer in Cr- andFe'?+ crystallizing from the more evolved lavas.

3. Recasting of spinel compositions in terms of ther-modynamically relevant components representing ex-change reactions allows the determination of whether

Y=16.n31g,I2/snl!",Fig. 10. Compositional zoning in selected strongly zoned or

complexly zoned spinels from the groundmass of F2-l (stars)and F2-2 (circles); interiors of spinels are shown by solid sym-bols, and spinel rims are shown by open symbols. Arrows indi-cate direction of core to rim, and the analyses numbers are alsogiven for reference.

Cr-rich spinels are in equilibrium with the host liquid'Specific examples given here show that large ranges inspinel Crl(Cr * Al) content within a given lava reflectincorporation of xenocrystal spinels that crystallized froma silicate liquid of different Mg/(Mg * Fe'z+) content thanthe host magma and likely reflect magma mixing in ashallow magma chamber just prior to eruption.

4. Al-rich (to over 47o/o Al'O' by weight) groundmassspinels and skeletal groundmass rims in "primitive" la-vas clearly show that Al-rich spinels crystallize at lowpressure and that Al-rich cores ofreversely zoned spinelsmay in some cases represent magma mixing or wallrockassimilation rather than represent high-pressure relicts(e.g., Sigurdsson, 1977).

5. The Cr content of spinels within a given lava haslittle relationship to host-liquid Cr content. Instead, spi-nel Cr is indirectly controlled by the hostJava Al, Fe2*,and Mg contents through reciprocal exchange reactionsin a manner that is qualitatively but not yet quantita-tively predictable. This observation, coupled with the ob-servation that these elements in spinel (particularly Al)strongly covary with small changes in host-liquid com-position, indicates that quantitative use of Cr-rich spinelsas geobarometers will be a difficult problem to imple-ment. Nevertheless, these same observations show thatCr-rich spinels are powerful petrogenetic tools for pro-viding information about interaction between silicate liq-uids or silicate liquids and solids.

AcxNowr,nocMENTS

Host lavas for the spinels were collected during the Clipperton (1985)

expedition; we are indebted to chiefscientist D. Fomari, the crew ofthe

**F2-1.oF2 -2

752

Atlantis II, and to the ALVIN pllols and crew We are grateful to FBishop, P. Roeder, H. Sigurdsson, and J A. Speer for reviewing the manu-script and to F Bishop and B. Wood for providing numerous suggestionsregarding the petrological and geochemical significance of variations inspinel chemistry. J F.A. also thanks G. Adams, I. Carmichael, p. Castillo,H Dick, M. Fisk, R. Loucks, G. Martioli, J. Natland, J. Pasteris, and M.Perfit for compelling discussions concerning this research and D. Kremserfor analytic assistance on the electron microprobe. We thank W. Melsonand T O'Hearn for providing the glass major-elemental analyses and I.Carmichael and D. Christie for the glass FeO analyses. This work wassupported by NSF $anrs OCE-8308980, OCE-84 I 5270, and OCE-8508042(R. Batiza), ONR grant l0-14-80-C-0856 (R. Batiza), NSF grant EAR-8419158 (R O. Sack), and NSF granr OCE-8414616 (D. Fornari), wirhsupport to J F.A. during revision of the paper from NSERC StrategicOperating Grant 5-55847 (R. Chase).

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Basaltic Volcanism Study Project (1981) Basaltic volcanism on the ter-restrial planets. Pergamon Press, New York.

Batiza, R., and Vanko, D (1984) Petrology ofyoung Pacific seamounts.Journal ofGeophysical Research, 89, 1 1235-l 1260.

Byerly, G.R., Melson, W.G., Nelen, J.A., and Jarosewich, E. (1976) Abys-sal basaltic glasses as indicators of magma compositions. In B. Mason,Ed., Mineral science investigations, I 974-l 975. Smithsonian Contri-butions to the Earth Sciences, 19,22-30.

Carmichael, I.S E. (1967) The iron-titanium oxides ofsalic volcanic rocksand their associated ferromagnesian silicates Contributions to Miner-alogy and Petrology, 14,36-64

Christie, D M , Carmichael, I.S.E., and l,angmuir, C.H. (t986) Oxidationstates of mid-ocean ridge basalt glasses. Eanh and Planetary ScienceLetters, 79, 397411.

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MeNuscnrsr RECEIvED JuNe 29, 1987MANUscRTFT ACCEPTED Mencu 9, 1988

ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS