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

Cr-rich spinels as petrogeneticindicators: MORB-type from the Lamont seamountchain, easternPacific J,trrns F. Ar.r-.c.N* Department of Geological Sciences,Northwestern University, Evanston, Illinois 60201, U.S.A. Rrcn.qno O. Sacr Department of Earth and Atmospheric Sciences,Purdue University, West Lafayette, Indiana 47907, U.S.A. Roonv Bxttzt Department 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 petrogeneticevents such as magma-chamberrecharge, fractionation, and magma mixing. In this paper we examine Cr-rich spinels in MORB-type lavas erupted from the near-ridge Lamont seamountsand from the adjacent East Pacific Rise crest at 10'N. The spinels studied are exclusively from quickly quenchedglassy 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 samplesapproaching the composition of liquids in equilibrium with mantle peridotite. The spinelscover a wide range in Al and Cr contents,with spinel Cr/(Cr + Al) ranging from 0.20 to 0.54 for the entire suite. As in other 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 groundmassspinels strongly correlate with host-glasscomposition, but spinel Cr content shows little correla- tion with host-glassCr content. Castingof spinel compositions in terms of Mg-Fe'z*spinel- host liquid exchangeequilibria and in terms of the compositionally related crystallochem- ical effectson this exchangeshows that for a given suite derived from similar parental lavas, spinel Crl(Cr + Al) increasesand Mg/@g + Fe2+)decreases with the amount of Fe enrichment, Al depletion, and extent of fractionation [as representedby the normative ratio Diop/(Ol + Diop)l of the liquids in which they equilibrated. This changein spinel composition is of much greater magnitude than that of the host liquid; thus spinels are sensitiveindicators of melt composition and crystal-liquid equilibrium and disequilibrium processes.MORB lavas containing strongly zoned spinels or populations of spinels with broad compositional range preservedirect evidenceof host-melt prehistory of interaction with other melts (magma mixing) or other processessuch as wall-rock interaction. The strong compositional covariance of Cr-rich spinel and host silicate liquid makes the for- mulation of a spinel geobarometerdifficult.

INrnonucrroN been made from variations in MORB Cr-spinel compo- Cr-rich spinel within MORBJike lavas has long been sitions. MORB spinel chemistry has also been used to acknowledgedas carrying useful information regarding deduceinformation about magmatic intensive variables, the petrogenesisof its host lavas. Although the crystal including pressure (Sigurdssonand Schilling, 1976; Si- chemistry and thermochemistry of spinels are complex gurdsson,1977;Dlck and Bryan, 1978;Dick and Bullen, (e.9.,Sack, 1982; Irvine, 1976),thecomposition of spinel 198 4), fo" (Fisk and Bence,I 980; Batiza and Vanko, I 984) is a sensitive indicator of magmatic intensive and exten- and temperature (Fisk and Bence, 1980) during crystal- sive variables (Irvine, 1965, 1967). Inferencesregarding lization. We consider here systematic changesin spinel magma mixing (Natland et al., 1983) and initiation of composition among Cr-rich spinels from basaltserupted precipitation ofcoexisting phases(Fisk and Bence, 1980; along the Lamont seamountchain, a group of Foruta and Tokuyama, 1983;Dick and Bullen, 1984)have near the axis of the EastPacific Rise (EPR) occurring west of the EPR at 10'N, just south of the Clippenon Fracture from lavas * Present address:Department of Geological Sciences,Uni- Zone (Fig. 1), and within Cr-rich spinels versity of British Columbia, Vancouver, British Columbia V6T erupted at the EPR axis at l0'N. Sampleswere collected 2B,4, Canada. in situ from sheet and lobated flows, pillows, massive '141 0003404x/88/0708-o741$02.00 742 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

TneLe1. Glassanalyses of spinel-bearinglavas

Cone DTD DTD EPR EPR MIB near Cone near SASHA 't572- Location:* MOK 1572- 1567- 1567- 1560- Sample: F9-1 F2-1 1511 1755 1816 2019 1843 sio, 49.60 50.41 48.40 48.79 48.81 48.68 48.96 50.06 49.87 49.32 1.01 1.14 116 1.20 1.36 Tio, 1.21 1.18 0.86 0.83 0.83 't6.21 Alro3 16.85 15.49 17.38 17.50 17.91 17.64 16.88 16.03 16.99 FerO. 0.84 1.23 081 0.86 1.02 0.89 0.99 0.89 1.02 0.95 FeO 7.83 8.19 7.60 7.45 7.48 7.59 7.94 8.01 8.18 8 09 Mgo 8.97 8.14 9.44 9.50 9.53 9.10 8.51 8 28 8.28 7.84 12.35 13.00 1316 12.87 12.90 12.45 12.28 12.47 12.70 11.95 Naro 2.46 2.41 2.18 2.13 2.08 2.65 2.71 2.46 2.44 3.34 KrO 0.10 0.06 o.02 0.02 0.01 0.03 0.06 0.11 0.09 0.04 DA 0.10 0.11 0.07 0.09 0.09 0.08 0.13 0.14 0.14 0.14 Total 10031 100.22 99.92 100.04 99.94 100j2 99.60 99.79 9995 100.02 Mg/(Mg + F€F-) 0.671 0 639 0.689 0.694 0.694 0.681 0.656 0.648 0.643 0.633 SC 355 ?qo 31.1 31.7 323 28.7 35.4 38.1 38.2 37.0 Cr 430 400 460 440 370 290 330 350 360 280 Co 46.8 44.5 50.0 48.0 48.8 40.3 47.0 43.3 43.6 43.9 Ni 190 120 240 210 200 160 140 160 110 120 2.76 2.04 0.92 0.88 1.20 1.83 2.67 2.71 2.81 Yb 2.24 239 1.81 1.91 1.91 1.64 2.34 2.49 2.54 2.66 Hf 2.06 175 1.28 1.29 1.19 1.46 1.88 2.07 2.06 2.60 (La/Sm)N 0.57 0.47 0.28 0.27 0.34 0.40 0.55 0.57 0.47 Note: Maior elements analyzed by electron microprobe at the Smithsonian Institution (T. O'Hearn, analyst). Trace elements (in ppm) analyzed by INAA at Washington University,St. Louis, Missouri, and reported from Allan et al. (submittedmanuscript; see footnote 1 in text); analyticalprecision discussedin Allan et al. (1987).FeO measuredby LS.E. Carmichael,using wet-chemicalmethods; Fe2O3calculated by differencefrom FeOr,as measured by T. O'Hearn. Mg/(Mg + Fe,*) calculatedfor 1560-1 843 and 1572-151'lassuming Fe2+l(Fe2++ Fe3*): 0.904, the averagevalue for the other analyzed lavas.(La/Sm)N represents La and Sm valuesnormalized to chondriticvalues (Haskin et al., 1968). - See Figure1.

flow interiors, and talus slopesby the ALVIN srrbmersible 8o/oMgO, average MgO is 8.24o/o),some of which are and by dredging. quite primitive [MgO to 9.74o/o, Mg/(Mg + Fe2*) to 0.70, The seamountsare associatedwith a broad, topograph- Cr to 460 ppm, Ni Io 240 ppm; having mantle ic high at the adjacent EPR, interpreted as representing peridotite compositions of Fon, to Forr; Tables I and 2l recent voluminous ridge volcanism (Gallo et a1., 1986; and could approximate primary mantle melts (Green, Kastenset al., 1986).The EPR at this latitude is appar- l97l; Frey etal.,1978;Sato,1977;Rhodes and Dungan, ently underlain by a magma chamber (Detrick et al., I 979; Presnallet al., I 979).In contrast,the adjacentEPR 1987).All seamountsshow signsof recentvolcanic activ- lavas average7 .57o/oMgO (Allan et al., 1986; Langmuir ity, based on the youthful appearanceof flow surfaces, et al., 1986). All spinel-bearing lavas have Mg/(Mg + lack of sediment cover, and absenceof thick ferro-man- Fe'?+)greater than 0.6 I and contain greaterthan 280 ppm ganesecoatings on surface rocks (Fornari et al., 1988a). Cr. Analysesofglasses from spinel-containinglavas con- This 50-km-longchain consistsof five 1000-to 1400-m- sideredhere are given in Table l, and summariesof min- high seamounts,with volumes ranging from 25 to 120 eral compositional data for theselavas are given in Table km3, and most with summit cratersand calderas(Fornari 2. The glass analysesare considered to represent close et al., 1984). The seamountsin turn are surrounded by compositional approximations of the eruptive host mag- abundant, 100- to 200-m-high lava cones(Fornari et al., mas (Melson el al., 1976;'Byerly et al., 1976).Normative 1988a).The chain has formed on crust of 0.1-0.74 Ma compostions of these glassesare plotted in Figure 2 in in age. The absolute age of the seamountsis unknown, terms of normative Ol, Di, and Neph projecting from but an upper limit is provided by the age of the under- plagioclase,using the algorithms of Sack et al. (1987). lying seafloor. Allan et al. (submitted manuscript)' show that the most primitive glass samples approximate liquids in equilib- Pnrnocnrpuy AND pETRoLocy oF Hosr LAvAS rium with peridotite at pressuresof9-9.5 kbar and soli- The spinels occur within basalts of LREE-depleted dus temperaturesof 1180-1240"C, but Figure 2 shows N-type MORBs (Sun et al., 1979), with NarO varying that most have undergone fractionation or re-equilibra- from 2. I to 3.30/oand (LalSm)N varying from 0.25 to tion at pressuresofless than 9 kbar. 0.57. Other incompatible major elementsand high-field- strength elementsare similarly low (e.g., TiOr, 0.81- 'Allan, Batiza,R., Perfit,M.R., Fornari, D.J., and Sack, 1.43o/o;KrO, <0.llolo;Hf, l.19-2.69 ppm; J.F., seeTable l). R.O. Petrologyof lavas from the Lamont seamountchain, East- The Lamont seamountsand cones have erupted an un- ern Pacific: Seamountlavas as mantle probes for understanding usual percentageof high-MgO lavas (600/oare higher than MORB petrogenesis.Submitted to Journal of Petrology. ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 743

TABLE1-Continued 1100

MIB MIB MOK MOK MOK 1560- 1566- 1564- 1564- I 570- 1922 1642 1857 1949 1949 49.09 50.07 48.94 48.55 49.42 1.43 1.17 0.96 0.94 U.YO 1650 15.14 17.02 16.92 16.60 1.O2 0.91 0.83 no7 0.89 8.07 8.86 8.07 8.23 8.03 7.80 7.81 9.18 9.24 8.78 1181 12.71 12.80 12.88 12.67 2 21 2.60 2.30 2.26 2.43 0.04 0.04 0.04 0.04 0.04 0.17 0.13 0.12 o.'t4 0.08 99,24 99.44 100.26 100.17 99.90 0.633 0.611 0.670 0.667 0.661

38.0 42.9 35.6 JC.J 36.0 300 380 360 360 340 43.4 45.7 50.0 49.0 47.4 100 90 80 90 140 2.87 1.54 1.24 1.59 0.99 1040 50'w /to'w 30'w 104020'W 2.71 2.71 2.08 2.08 2.27 z.oY 1.81 1.40 1.42 1.48 LAMONTSEAMOUNTS 0.47 n2a 0.33 0.43 0.26 10 kn 100 10'N CLIP F

100 00'N fi-2;. \\

875r/865rranges from 0.702218to 0.70267 | within the lavas; the adjacent EPR lavas have a much 9o more restricted range of 875r/865rralios (0.702499- oo'N 0.702780;Perfit er al., 1986;Klein et al., 1986;Fornari Fig. 1. Mapsshowing location and detail of the Lamontsea- et al., 1988b). Calculated eruptive temperatures (based mountchain. General location map afterFig. I of Sempereand on the crystal-liquid geothermometersof Glazner, 1984) Macdonald(1986) shows trace of EastPacific Rise and location just of the Clipperton rangefrom ll80 to 1220"C. Allan et al. (1986)demon- of the seamountchain at 10"N, south FractureZone (F4. Seamountmap derivedfrom SEABEAM strated from sample LREE and major-element contents charts(after Fornari et al., 1988a). that the seamountlavas bypassedsubaxial magma cham- bers thought extant at the adjacent EPR. A fuller treat- ment of the petrology of the Clipperton seamount lavas ivines with Forr*, cores and Forn-norims. Minor (<10/o is given in Allan et al. (submitted manuscript; see foot- Fo) reversezoning is found in a few ofthe olivines within note l). the lava sample 1564-1949 from the MOK seamount;its The spinel-bearingseamount and cone lavas from the plagioclasephenocrysts exhibit oscillatory zoning as well. Lamont seamount chain have a very simple mineralogy Clinopyroxene is typically absent, particularly in the (Allan et al., 1986, and submitted manuscript). Most more primitive lavas (MgO > 8olo).This absenceof cli- samples contain both plagioclaseand as pheno- nopyroxene indicates that it is not generally on the low- crysts. Samplesrange from nearly aphyric to porphyritic pressureliquidus at the pre-eruption temperaturesof the (up to 150/ophenocrysts) with glomeroporphyritic clusters Lamont seamount magmas.Indeed, most lavas from in- of plagioclaseand olivine commonly present.Plagioclase dividual seamountsbroadly define olivine + plagioclase phenocrystsare typically euhedral with sharp, nonscal- fractionation-control lines in MgO variation diagrams, loped outlines and may exceedseveral millimeters in size. consistentwith modeling results using least-squaresmix- Olivine phenocrystsare commonly skeletal and can be ing and with petrographic evidence(Allan et al., submit- equally large. In most cases,both minerals are composi- ted manuscript; see footnote l). Crystals of titanomag- tionally unzoned, with crystals rarely exhibiting any re- netite are absentin most samplesand are always <5 pm verse zoning or normal zoning greater than l-2 mol0/0. in size. Xenoliths are absent. Nevertheless,exceptions do occur. Lava samples from Spinel is often abundant in thesesamples (some 2.3 x one of the cones (F2-l and F2-2) contain two types of 4.6 cm,0.07-mm-thick thin sectionsmay contain over olivine phenocrysts- skeletal,unzoned olivines of Forr_no 50 spinel grains, though this is <<0.10/oby volume) and composition and subhedral-euhedral,reversely zoned ol- forms translucent, light brown crystals up to 200 pm in ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

Cr content (typically much greaterthan99o/o) is contained within glass(representative ofthe host liquid) instead of within the spinels. Rock textures for all spinel-bearing lavas indicate that crystallization of spinel precededand overlappedwith the onset of olivine and plagioclasecrys- tallization. Indeed, the experiments of Fisk and Bence (1980) show that the onset of spinel crystallization in MORBs is only tens of degreescelsius before olivine. The simple mineralogy, the abundanceof spinel, and the primitive, depletednature of the basaltsfrom the La- mont seamount chain suggestthat the melts have had a simple ascentand eruption history. The absenceof xeno- crysts and xenoliths and the unzoned nature of the phe- Fig. 2. Comparison of ratios of nepheline (Neph), olivine nocrysts implies that most of the lavas have undergone (Ol), and high-Ca (Di) normative components of spi- little or no magma mixing or significant crustal assimi- nel-bearing Lamont seamount lavas with those for the l-atm lation. The absenceof clinopyroxene in most of the lavas 66, = QFM) plagioclase-saturatedcotectics and the high-pres- indicates that theselavas spent little time cooling in shal- sure olivine + orthopyroxene + high-Ca pyroxene cotectic as low crustal magma chambers(Walker et al., 1979),but defined by Sacket al. (1987). l.{ormative componentsemployed instead underwent relatively rapid ascent. As a result, in this and subsequentfigures are calculatedfrom the algorithms these seamount and cone lavas are good candidates to givenin Sacket al. (1987,caption to Fig. 5). Most spinel-bearing study how spinel composition is related to hostlava lavas are seento have evolved at pressuresless than 8 kbar by and paragenesis. fractionation or re-equilibration, putting absolutelimits on pres- composition here are suresof spinel precipitation. Dashed line representsan average It is crucial to note that the spinels studied chord emanatingfrom the olivine vertex, representingolivine * exclusively from the glassyto spherulitic and finely mi- plagioclasefractionation. crolitic margins of flows, where the quenchingof the lava by cool seawaterduring eruption was rapid to nearly in- (seconds minutes). Many of the ground- size that are found as groundmass grains, as inclusions in stantaneous to by crystal-free olivine and plagioclase, or as grains accreted to or embed- mass spinels are completely surrounded glass. problems with interpretation of post- ded into olivine and plagioclase. The spinels encompass Therefore, spinel growth or re-equilibration a wide range of shapes and include both euhedral and eruptive or syneruptive lavas represent perhaps na- skeletal shapes, the latter perhaps indicative of rapid are avoided. Indeed, these quench conditions ob- growth (Fig. 3). Curiously, the abundance of spinel in the ture's closest approximation to lavas is independent ofhost-rock Cr content, and Cr-rich tainable in the laboratory. spinel has been found in samples with Cr content varying from 280 to 460 ppm (Table 1). Wide-beam (50 pm) glass SprNrr, cHEMrsrRY microprobe analyses of long counting time (200 s) agree The Cr-rich spinels in this study are classifiedas chro- with mass-balance calculations based on spinel abun- mium spinels or magnesiochromites,using the criteria dance that the great to overwhelming majority of magma and terminology of Sigurdsson(1917). They are plotted

TneLr2. Mineralogicaldata for lavaswith Cr-richspinel

Spinel Sample SampleOl SamplePlag F€F*/(Cr+ analyzed Fo range An range Crl(Cr+ Al) Mg/(Mg+ F€r.) Al + Feo*) No. of analyses F-1-1 86-87 70-74 0.53-0.54 0.67-0.68 0.098-0.106 4 89-90 78-89 0.20-0.50 0.75-0.84 0.056-0.075 29 F2-2 87-90 78-91 0.22-0.51 0.75-0.84 0 058-0.089 29 F3-4 89-90 76-?0 0.27-0.46 0.70-0.81 0.060-0.085 20 F9-1 89-91 0.35-0.41 0.73-0.78 0.069-0.084 12 1560-1 843 87 61-81 0.41-0.43 0.69-0.71 0.096-0.100 4 0.094-0.097 1560-1 922 86-8J 66-81 0.39-0.40 0.71 ,l 1564-1 857 78-91 0.30 0.80 0.078 1564-1 949 88 75-91 0.23-0.36 0.75-0.84 0.062-0.083 1566-1 642 85-86 75-84 0.50-0.54 0.66-0.70 0.100-0.1 1I 21 1567-1 81 6 86-87 69-80 o.46-0.47 0.71-0.72 0.091-0.097 4 1567-201 9 78-88 70-83 0.46-0.47 0.69-0.72 0.094-0.100 5 1570-1 949 88.00 79-89 0.38-0.39 0.74-0.77 0.083-0.101 b 1572-1511 85-89 69-88 0.30-0.32 0.80-0.81 o.071-0.077 A 1572-1755 87-89 65-87 o.37-0.41 0.73-0.75 0.607-0.086 11 Note.'Rangesin plagioclase,olivine, and spinelcomposition observed in spinel-bearingsamples 'Samples lackingeither olivine or plagioclase. ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 745

Fig. 3. Backscatteredimages of selected spinels, with accompanying scale bar rep- resenting100 pm in length. Dark areasrep- resentAl-rich regions of spinel (smaller mean atomic number), and light areas represent Cr-rich regions of spinel. Thin bright lines outlining some of the spinels are causedby edgeeffects. (A) Skeletal,relatively unzoned spinels in groundmass of plagioclase mi- crolites and mesostasisin sample F3-4, rep- resentedby analysisofF3-4-18 in Table 3. (B) Complexly zoned spinels in the glassof sampleF2-2. Spinel compositional banding may be quite complex, with cores either being A1- or Cr-rich. Total compositional variation in Crl(Cr * Al) of cores isO.227- 0.279. Variation in Crl(Cr + Al) zoning in largespinel is 0.227 (core)to 0.277 (5- to l2-pm-wide light band) to 0.242 (darker, 5-pm outer band) to 0.251 (narrow, l- to 3-pm-widerim). Analysesrepresentedare F2- 2-19, -20, -21, -22. (C) Complexly zoned spinelin microlitic groundmassof F2- 1,rep- resentinganalyses F2-l-27, -28, -29. Spinel contains a Cr-rich core, surrounded sharply by a 3- to 5-pm-wide Al-rich band, which is in turn surroundedby a Cr-rich rim. Zoning representedin Crl(Cr + Al) is 0.269,0.220, and0.232, respectively.@) Skeletal spinel attached to rounded olivine and elongate plagioclasecrystals in groundmassof F2-1. Wormy, more aluminous corerepresents last growth phaseofspinel; coreand rim analyses are given in Table 3 (F2-l-3, -4). (E) Rela- tively unzoned euhedral spinel in quenched groundmassof olivine dendrites in glassof sampleF9-1. Representsanalyses F9-1-11, - I 2 in Table 3. (F) Strongly zoned, euhedral spinel and unzoned, skeletal spinel in glasswith plagioclasemi- with sharpcompositional transition to a skeletal,aluminous rim. crolites in sampleF2- I . Cr-rich core of zoned spinel is rounded, Correspondinganalyses are given in Table 3 (F2-l-1, -12). (H) with abrupt transition to aluminous rim. Skeletal spinel is ap- Sharply zoned spinel in glass,with unzoned,Cr-rich core sharply proximately the samecomposition asthe rim ofthe zoned spinel. surroundedby thin (2-15 pm wide) aluminous rim, representing Core and rim analysesofthe zoned spinel are given in Table 3 a zoningin Crl(Cr + Al) of 0.481 to 0.286.Spinel is in sample (F2-l-7, -8). (G) Strongly zoned spinel in glass with feathery F2-2 (analysesF2-2-13, -l4). plagioclasemicrolites in sample F2-1. Cr-rich core is rounded,

in Figure 4, with compositional rangesgiven in Table 2. amongthe most aluminous spinelsreported from MORBs. Fe3+content is low (FerO. varies from 5.5-9.2o/o),similar MgO and AlrO, strongly covary, as do FeO and Cr2O3. to other spinels from MORBs (Dick and Bullen, 1984; TiO, is a minor component,ranging from 0.16 to 0.850/0. BasalticVolcanism Study Project, l98l). The low Fe3+ Other minor components include NiO (<0.02-0.300/o), content of the spinels reflects the low/o, of the host La- MnO (0.05-0.47o/o),CaO (<0.02-0.300/o),and SiO, (typ- mont seamountbasalts, which rangefrom 1.0 to l 8 log,o ically <0.10/o). units below the NNO oxygen buffer (calculatedusing the The MgO, FeO, and AlrO, contents of the groundmass methodsof Sacket al., 1980).Thisf" rangeis similar to spinel rims and of the host glass are strongly correlated that found in other MORBs by Christie et al. (1986). The (Fig. 5). Sigurdssonand Schilling (1976) found a similar compositional rangeexhibited by the spinelsis wide, with correlation between spinel and host-glass Al content. AlrO, varying from 22.3 to 47.3o/o,Cr.O, varying from Strikingly, the CrrO. content of the spinel is independent 18.9 to 39.3o/o,and MgO varying from 14.5 to 21.4o/o. ofhost-glass Cr content (Fig. 5), contrary to the results of The more Al-rich spinels from this seamount group are Irvine (1976).A differencenoted betweenthe ground mass 746 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

d o o I o = o o + o o = & o

o {

g o

o o

Mg/(Mg+Fe2+) G|ass MgO Glass Altog

Fig. 5. Comparisonof the compositionof the rims of + groundmass with thecomposition of thehost glass. Con- ct spinels o compositionalrange found within spi- lt necteddots represent the + nelsof an individuallava. Note the correlationbetween spinel + and glassMgO, FeO, and AlrO3. In contrast,note the poor cor- o relationbetween host-glass Cr and spinelCrrOr. Y o.o8 + et 0.06 spinels and those included within silicate phasesis that the rims of the groundmassspinels, on average,are slight- 0.6 07 0.8 0.9 ly more aluminous and slightly lesschromous than those four Mg(Mg+Fe2+) ofthe included spinels.Figure 6 shows spinels from lavas of the Lamont seamountsand adjacent EPR, each Fig. 4. Composition of Lamont Seamountlaval spinels.Spi- lava with differing MgO and AlrO, contents. Host-lava nels are similar in composition to other spinels reported from MgO and AlrO, contents correlate with the composition MORB-type lavas (e.g., Sigurdsson and Schilling, 1976; Si- result in the sliding of spinel compo- gurdsson,1977; Basaltic Volcanism Study Project, 1981;Dick of the spinels and band and Bullen, 1984). Closed symbols represent spinels from the sitionsup and down the narrow spinel compositional seamountsand cones;open symbolsrepresent spinels from lavas of Figure 6. erupted at the adjacent EPR axis. Most lavas contain only a relatively narrow range of spinel compositions,but in some samples(F2-1, F2-2, F3-4, and 1564-1949)the spinel compositional range is

Tnele3. Representativespinel analyses

Sample: 1560- 1560- 1564- 1566- 1s66- 1566- 1566- 1567- 1570- 1572- 1572- 1572- F1- 1843-1 1843-2 1949-6 1642-1 1642-2 1642-6 1642-7 2019-5 1949-4 1511-6 1511-7 1755-3 1-28 F2-1-11 Descriotion:* PC PR PC PC PR PC PR OOGCGRPRGRGC sio, 0.08 0.10 0.06 0.05 0.06 0.06 0.06 0.07 0.11 0.06 0.09 0.08 012 0.06 Tio, 0.76 0.76 0.37 0.76 0.85 0.75 0.70 0.65 0.45 0.37 0.34 0.49 0.69 0.23 Alroe 29.22 30.15 35.76 23.92 22.28 23.91 24.63 27.58 31.92 37.76 39.02 33.96 23.05 32.30 Crr03 32.28 30.85 27.51 37.64 38.89 37.41 36.82 34.46 29.66 26.16 25.81 29.72 39.18 34.28 FeO 20.06 20.07 16.70 22.02 21.95 21.12 20.99 19.30 18.36 14.97 14.77 17.15 20.60 14.05 MnO 0.23 0.19 0.08 0.22 0.28 0.23 0.20 0.26 0.11 0.14 0.09 0.00 0.24 0.15 Nio 0.16 0.10 0.13 0.06 0.10 0.09 0.11 0.19 0.18 0.28 0.28 0.00 0.07 0.22 Mgo 1601 15.90 17.73 14.79 14.58 14.91 14.95 16.06 17.43 19.13 19.31 17.68 14.88 18.35 CaO 0.05 0.12 0.08 0.0s 0.15 0.07 0.17 0.07 0.19 0.02 0.09 0.17 0.18 0.03 Total 98.85 98.24 98.42 99.51 99.14 98.55 98.63 98.64 98.41 98.89 99.80 99.25 99.01 99.67 FeO.- 8 62 8.45 7.10 9.32 9.44 8.94 8.88 8.44 9.16 7.19 6.76 7 27 8.42 5.65 FerO.** 1231 12.46 10.31 13.63 13.45 13.07 13.00 1'l.71 10j2 8.50 8.68 10.60 13.02 8.97 Total-- 99.71 99.09 99.13 100.44 100.09 99.45 99.52 99.49 99.33 99.61 100.48 99.98 99.85 100.24 Crl(Cr + Al) 0.426 0.407 0.340 0.513 0.539 0.512 0.501 0.456 0.384 0.317 0.307 0.370 0.533 0.416 Mg/(Mg + 0.699 0.694 0.7s4 0.659 0.659 0.670 0.672 0.710 0.754 0.800 0.798 0.748 0.671 0.78s Fer*) Fe,*/(Cr + 0.098 0.096 0 077 0.108 0.111 0.104 0.103 0.096 0.101 0.o77 0.071 0.079 0.098 0.061 At + Fe+) * C refers to spinelcore, R reters to spinel rim, P refers to spinel includedin plagioclase,O refers to spinel includedin olivine,and G refers to spinel found in the groundmass. .'Recalculation of FeO and FerO3after Carmichael(1967), based on stoichiometry. ALLAN ET AL,: CT-RICH SPINELS IN MORB-TYPE LAVAS 747 much broader (Table 2). Similarly, compositional zoning .r .151 c8'rb.. HOST GLASS in the spinelsof most lavasis minor (for example,1560- o5 za 16.O o Abos I 843-l, -2; | 566- | 642-1,-2; 15 66- | 642-6,-7 ; | 572-l 5| | - 8.3"A 6, -7; and F9 I - 1I , - l2 in Table 3). Nevertheless,zoning tor o 169 o HOST GLASS S^o- of some spinelswithin samplesF2-l and F2-2 is strongly Y SO -1ZG o developed (Figs. 3 and l0), with zoning parallel to the o.3 Mso 9.1 Cr/(Cr + Al) vs. Me/(Mg + Fe2*) compositional trend ofFigures 4 and 6 and typically progressingfrom a core o.2 07 0a with higher Crl(Cr + Al) and lower Mg/(Mg * Fe'z+) to Mg/(Mg+Fe2+) a rim with a lower Cr/(Cr + Al) and a higher Mg/(Mg + Fe'?+).For the strongly zoned spinels,the zoning is typi- Fig. 6. Spinel compositions plotted from four lavas erupted generally how host-Iava cally quite abrupt; most strongly zoned spinels contain a from the Lamont seamounts,showing composition affectsspinel composition. Diferent symbols rep- rounded, Cr-rich core surrounded by an Al-rich rim that resent spinels from different indiwidual lavas, with hostJava MgO may have skeletal The either or euhedral outlines. spinel below and host-lava AlrO. above the corresponding spinels. compositionaltrends are crystal-chemicallycontrolled and Symbolsrepresenting spinel compositionsare the sameas in Fig. reflectthe stronglycoupled substitutions of Mg2* and Al3* 7, with solid diamonds representingspinels from 1566-1642; and of Fe2+and Cr'* into the spinel structure (e.g.,Sack, open triangles,from I 567-20I 9; open diamonds, from F9- I ; and 1982; Hill and Sack, 1987; Engi, 1983).These well-de- upside down, open triangles,from I 572- I 5 11 Note how chang- veloped zoning trends are distinct from other spinel crys- ing hostJava AlrO, and MgO slides the spinel composition up tallization trends predicted (Dick and Bullen, 1984) or and down the narrow spinel composition band. experimentally observed (Fisk and Bence, 1980; Irvine, 1976;Sack, 1982) for spinelscoprecipitating with olivine or plagioclaseor both. exhibit only limited variability in CrlAl and Fe3*/Al ra- Complex compositional zoning occurs in some spinels tios. These display simple Fe2*-Mg zoning patterns that of F2-l andF2-2, but is generallyonly of limited com- may be related to fractionation of the lavas and to inter- positional range (Figs. 3 and l0). Compositional bands lava compositional variations that are largely crystal- may be quite narrow (l prmor larger), and cores may be chemically controlled. Both of these points are readily either relatively Cr- or Al-rich. illustrated with a plot that makes provision for crystal- chemical effects on the Fe'?+-Mgexchange reaction be- DrscussroN tween spinel and host silicate liquid. We assumethat spi- Spinel compositionalsystematics nel behaves as an ideal reciprocal solution (e.g., Wood Despitethe complexity of spinelzoning patternsin some and Nicholls,1978; Sack, 1982)and that silicateliquids of the Lamont seamount lavas, most have spinels that are ideal with respect to mixing of Fe2+ and Mg'z+. In

Taete3-Continued

F2- F2- F2- F2- F2- F2- F2- az- F2- F2- F2- F3- F3- F9- F9- 1-12 1-2 1-3 1-4 1-7 1-8 2-10 2-11 2-19 2-24 2-25 4-12 4-18 1-11 1-12 uhuuu GR GC GR oc OR GC OC OR PC GGCGR 0.10 0 06 0.09 0 15 0.12 0 10 0.08 0.09 0.09 0.08 0.06 0.08 0.10 0.10 0 14 0.21 0.37 0.15 0 22 0.28 0.26 0.39 0.31 0.16 0.73 0.56 0.38 o.28 0.46 0.44 45 85 26.42 47.21 42.90 5I .CI 43.48 38.70 42.79 45.35 25.73 29.21 27.91 38.07 35.33 34.08 1894 39.15 17.88 21.84 28.30 20.57 25.82 21.57 19.81 39.30 35.32 35.98 26.72 30.02 30.12 12.76 14.56 1206 13.15 13.50 13.65 13.91 13.48 12.',t1 15.67 14.83 18.36 14.23 14.81 16.51 o.17 0.22 0.08 0.16 0.17 0.16 0.18 0.15 0.10 0.15 0.18 0.28 0.14 0.06 0.15 0.24 0.10 0.33 0.24 0.22 0.28 0.29 0.19 0.31 0.26 0.13 0.12 0.27 0.24 0.24 20.70 17.09 20.80 1995 19.27 20.14 19.62 20.29 20.75 17.39 17.68 15.74 18.92 18.60 17.97 0.10 0.16 0.05 0.13 007 0.13 0.03 0.08 0.00 0.04 0.01 0.09 0.06 0.04 0.10 99.07 98.13 98.65 98.74 99.50 9877 99.02 98.95 98.68 99.35 97.98 98.94 98.79 99.66 99.75 6.14 5.63 5.47 589 5.68 6.71 6.67 6.63 5.b/ o,cY 6.09 6.94 6.09 6.03 7.16 7.23 9 49 7.14 7.85 8.39 7.61 7.91 7.52 7.01 974 9.35 12.11 8.75 9.38 10.07 99.69 98 69 99.20 99.33 100.07 99.44 99.69 99.61 99.25 100.01 98.59 99.64 99.40 100.26 100.47 0.217 0 498 0.203 0.254 0.336 0.241 0.309 0.253 0.227 0.506 0.448 0.464 0.320 0.363 0.372 0.836 0.762 0.838 0.819 0.804 0.82s 0.815 0.828 0.841 0.761 0.771 0.698 0.794 0.779 0.761 0.063 0.064 0.056 0.061 0.060 0.070 0.071 0.069 0.058 0.075 0.068 0.078 0.078 748 ALLAN ET AL.: CT.RICH SPINELS IN MORB.TYPE LAVAS addition, we assumethat the pressureeffect over the spi- ratio Dil(Ol + Di) of the host glasses(Fig. 8). The inde- nel crystallization range on spinel composition is rela- pendenceofspinel and glassCr contents may reflect the tively small compared to the effect of varying the host- paucity of Cr in the melt. lava composition, and so may be neglected.For these Finally, we may estimate an approximate value for assumptions we may write the following expressionfor -AG!- from the FeO/MgO ratios of the glass chemical the Fe-Mg exchangereaction between spinel and silicate analyses,from Equation2, and from the compositions of liquid: the rims of groundmassspinels (to insure approximation of crystal-liquid equilibria). To do so we assume(l) that phase (e.g., - ad' o"l'(t1 spinel is the first to crystallize from the lavas In Kgu'o: + a _!&.6;+L$tx,),\--4, . .. (l) Ri Ri;...,, R. Rf Sack et al., 1987, and from petrographic observations); (2) that spinelsdo not exhibit retrogradeFe-Mg exchange where during crystallization (unlikely because of quick mag- aG& : (Gfu,o- Gg"o)uo* (Gk^,oo - GRn"o,,oJ.o, matic quench after eruption on the ocean floor); (3) and an averagetemperature of initial crystallization of 1200 apgz: (G$.rr.oon - (Gg...roo * GB"o,roo)"o * GRnr^roJ"o, "C (derived from olivine-liquid geothermometry based Lt{o : (t/zGf;a,"r.1i6o * Gg"o,roJ"o on Glazner, 1984).From the values of ln K3-tiocalculated by employing these assumptions,we obtain an estimate - (r/zGoe,rlioo* G$,*roJ'o, for -AG!. of -3.40 + 0.18 kcal/gfu. This value for : - -AG!* is slightly smaller than that (-3.16 t 0.1I kcal/ LrSs (Gg,r..roo+ G9"^,ro)"o (Gg"roo* GR*,roJ.o, gfw) which may be calculatedfom the calibration of Hill : t/zn"f,t*, r/zn{.t*, X, Xo: n"fie*,Xr: and Sack (1987) for ln Kg*o, using the averagevalues of XRI*,r,onand (Xfi1"5/Xgi3") (Xgl,sto4/XRl,,r'oo)of 0.878 and and the r?,terms are the number of the i cations in a 0.267 ! 0.008, respectively,as determinedfor the La- formula unit basedon threecations (e.g., Sack, 1982; Hill mont seamountlavas by microprobe analysis.Neverthe- and Sack, 1987). less, these estimates are within analytical error of each In a first approximation suitable for illustration, sev- other, indicating that the calibration of Hill and Sack eral simplifications of Equation I may be made. First, (1987) is essentiallycorrect. We use the former estimate becauseTia+ is negligible in the Lamont seamount spi- of - 3.40 t 0. I 8 kcal/gfw to calibrate the correlation for nels, we will ignore the third term on the right-hand side Equation 2 plotted in Figure 7 for illustrative purposes. of Equation l. Second, we approximate that N.4, : % The compositions of spinels within lavas exhibiting Aplr, where Apgr:4.80 kcal/gfw and Ap!, : 6.39 kcaU limited spinel compositional range essentiallyagree with gfw, as establishedby Hill and Sack(1987). Making these the crystal-chemicaltrend as defined by Equation 2. Spi- modifications reducesEquation I to nels in these lavas exhibit some variation in ln[(MgO/ FeO)e'*"(pe:+ /Mg2+ which perhaps could be attrib- lnKS-ric: -AGl^/RT + (4.80/RQ(y), (2) )"oi'a1, uted to limited coprecipitation with olivine. The earlier- where Y: Xj + %(X). We note that land Crl(Cr + Al) crystallizing spinels that have not fully re-equilibrated with are essentiallyequivalent for spinels within a given sam- the evolving host liquid will define more negative ln Ko ple, becauseof the minor variation of Fe3*. values at a given Y. Spinel compositions from the I I lava samples exhib- iting limited spinel compositional range and little spinel Spinel zoning and its petrogeneticimplications zoning are plotted in Figure 7 using these parameters. We now use these observations to examine more closely These spinels define a crystal-chemical trend with posi- the Lamont seamount spinels exhibiting strong zoning, tive slope, with spinels from more primitive, MgO- and complex zoning, or extensive compositional variation AlrOr-rich lavas plotting at lower values of ln Ko and,Y within an individual sample. Spinels from the four La- and spinels from more evolved lavas plotting at the higher mont seamountlavas (F2-1,F2-2, F3-4, and 1564-1949) ln Ko and I values in the trend. These results show that that exhibit wide compositional variation are plotted in the Cr-rich spinels in this study become progressively Figure 9 in terms of the parametersdefined in Equation more Cr-rich as the host lava becomesmore fractionated. 2. Selectedexamples of spinel zoning are presented in This trend is due to the reciprocal exchangesubstitution Figure 10. of Cr and Fe2+for Mg and Al in the spinel. Olivine and Lava samplesF2-1 and F2-2 contain two types of oliv- plagioclasefractionation will causemagmatic Fe2+to in- ine phenocrysts-skeletal, unzoned olivines of Forn-no creaseand magmatic Al and Mg to decrease,resulting in composition, and subhedral-euhedral,reversely zoned ol- the crystallization of spinels higher in Cr/(Cr + Al) and ivines with Forr-r, cores and Forn-norims. As illustrated lower in M/(Mg + Fe'z+).This correlation between spi- in Figure 9, the spinels in these samples span a broad nel composition and degreeof fractionation of the host compositional range,containing spinelsstrongly zoned in lava is strikingly shown by comparing the Crl(Cr + AD Cr/(Cr + AD F2-l-7, -8; F2-l-l l, -12; andF2-2-13,-14 ratio or the quantity I in the spinels with the normative in Figs. 3 and I 0). The rounded, resorbed-appearingcores ALLAN ET AL.: CT-RICH SPINELS IN MORB.TYPE LAVAS 749

oo LAMONT SEAMOUNT LAVAS -o.2 a-- th -o.4

$o^ gls-(}6 . Fl-1 ztE o ru-1 g r 15@{Si(t E J .156O1922 r 664181i7 . 1s@1642 -o8 , 1567-160 ^ 1567-2019 o lsTO{919 e 152-t511 .1572'1755 .1.O o'3 0.4 o'5 06 Y=%n!L.f%np!o

Fig. 7. Compositionsof spinelsfrom lavaswith spinelsex- o.5 hibitingrestricted compositional ranges, plotted in termsof ln' Y=14n8f*+2/snT*,. (K3iiq)vs. I, whereY : lrn"!t- -l ln!.* : X3 + %Xt (n, indi- catesthe ,1hcation calculated on a three-cationbasis). Note that Fig. 8. Comparisonof the rangein valueof I for spinelsin MgO and FeOin the axislabel refers to the molecularequiva- individuallavas with the normativeOl/(Ol + Di) ratio of the lentsofthese oxides. The solidline representsthe calibrationof hostglass calculated from thealgorithms ofSack et al. (1987). Equation2 givenin thetext by the rim compositionsofground- Dots representthe rangein spinelsof the 1I lavaswhere the massspinels. The spinelsfrom thesel l lavasdefine a simple, spinelcompositional range is small,as shownin Fig. 7. Open straightforwardcrystal-chemical trend, reflecting equilibria be- squaresrepresent the range in compositionof the rims of tweenthe spinelsand their hostsilicate liquid. Spinelswith higher groundmassspinels in the four lavaswhere the rangein spinel valuesof Y(thosericher in Cr) areassociated with moreevolved compositionis large,as shown in Fig.9. Diagonalline represents magmasricher in Fe.See text for furtherdiscussion. an approximatefit to the data. ofthe zoned spinels are Cr-rich and sharply bounded by compositionally diverse population of spinelsas well, yet skeletal Al-rich rims. The Al-rich rims are similar in its silicate mineralogy is more diverse than that of other composition to that of the bulk of the spinelsfound with- samples with complex spinel populations. Besidescon- in these lavas and are in apparent equilibrium with the taining slight reverse zoning in many of its olivine phe- host glass. In contrast, the Cr-rich spinel compositions nocrysts, its plagioclase phenocrysts are reversely and likely crystallized from a liquid considerably more evolved complexlyzoned (up to An,r). and Fe-rich than the host glass.As shown in Figure 10, The Cr-rich spinels in F2-1, F2-2, F3-4, and 1564- the Cr-rich spinels (high in Y-)included within olivines 1949 were likely introduced externally into the rising are found within Fo' olivine cores, whereasthe Al-rich magma,having crystallizedfrom melts more evolved and spinels included in olivine are found within skeletal Fe-rich than the lavas in which they are found' Never- Forn-noolivines. These more Cr-rich spinels in the Fort theless,the lack of xenoliths, the primitiveness of the host olivines are similar in corriposition to other Lamont sea- glasses,the lack ofclinopyroxene, and the lack ofsignif- mount groundmassspinels found within other lavas that icant zoning in most olivines and in the feldspars of all crystallized Fo" olivine. lavas except 1564-1949argue that processessuch as mag- Other Cr-rich spinels from F2-l and F3-4 are found ma mixing or crustal assimilation were of minor signifi- within plagioclasephenocrysts that presumably armored cancein the evolution ofthese lavas. The range in spinel them from interaction with the host liquid. They are con- composition probably reflects magma mixing during the siderably less aluminous than other spinels found in the periodic refilling of small crustal or edifice magma cham- groundmassand in apparent equilibrium with the host bers just prior to eruption, where the magma residence glass.The Cr-rich spinelsin F3-4 are found as inclusions time is short and the fractionation minimal. The skeletal within the core of a single large plagioclasephenocryst nature of some of the Al-rich spinelsmay representrapid that shows simple reversezoning from Anru to Annomid- growth triggered by magma mixing, on the basis of the way from core to rim. The core of this crystal also con- concave curvature of the experimental basalt liquidus tains an inclusion of Fo' olivine, whereasthe other oliv- surfacesaway from the spinel field (Dick and Bullen, 1984; ines of this sample are Fose-e.in composition. Again, the Irvine, 1977). Viewed this way, the composition of the Cr-rich spinelsin this sample are similar in composition Cr-rich spinelsprovide qualitative limits for determining to spinels crystallizing from other lavas in this suite that the compositionsof the end-memberliquids being mixed. crystallized Fo' olivines. Sample 1564-1949 contains a Severalof the groundmassspinels within samplesF2-l 750 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

F2-2

;- 3t2^ -1.O I|l. El=itD 6^l 1564-1949 F3-4 9lo 5tE c -o.2

-o.4

-o.6 A GmasC A Gmass R a OllnC O OlnR -o.8 I Plag In C El Plag ln R

t -1.O Y =/ztfJ'",*2Atfr"'

Fig. 9. Spinelsfrom the lavas of the Lamont seamountgroup calibration of Equation 2 and shown in Fig. 8 are also given exhibiting wide variation in spinel composition, plotted as in here.Note that the F2-2 spinelshigh in Yand included in olivine Fig. 8. Closed symbols refer to spinel cores and open symbols are found within Fo' cores of olivine reversely zoned to Fono refer to spinel rims; triangles representgroundmass spinels; cir- composition, whereasthe low-I spinels included in olivine are cles representspinels included in olivine; and squaresrepresent found within skeletal olivine phenocrystsof Foro composition. spinelsincluded in plagioclase.Reference lines representingthe Seetext for further discussion.

and F2-2 contain minor complex zoning (F2-l-27, -28, presumably the last portion of the spinel to crystallize. -29andF2-2-19, -20, -21, -22;Figs.3 and l0). The com- The compositional zoning representedwithin this spinel positional range representedby this zoning falls within seemsrather large to be related to local melt depletion in the range outlined by the rims of other groundmassspi- Cr, and the origin of its zoning remains somewhat enig- nels found in these lavas. Therefore, it is not necessary matic. to invoke magma mixing or assimilation to account for this zoning; rather, it may be related to local melt deple- Commentson spinel as a possibleindicator of tion in Cr from rapid spinel crystallization (Thy, 1983; pressure Allan et al., 1987) with subsequenttransportation of the Although all of the strongly zoned spinels examined spinel to more Cr-rich portions of the magma body. One have Cr-rich cores,it is likely that Al-rich coresof strong- curious spinel within F2-l (F2-l-3, -4; Fig. 3, Table 3) ly zoned spinels may also result from magma mixing or consistsof a skeletal,Al-rich im (42.9o/oAlrO3) with an assimilation. Sigurdsson( I 977) has reported "reversely" anhedral,worrny core representingnot only the most alu- zoned spinels from DSDP Hole 3328 plagioclase-phyric minous composition (47.2o/oAlroj) in the entire suite but basaltsthat may representjust such a case.These spinels ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 751 contain highly aluminous cores with sharp increasesof Crl(Cr + Al) and decreasesof Mg/(Mg + Fe'?+)from core to rim. Other authors (Sigurdssonand Schilling, 1976; Dick and Bryan, I 978; Dick and Bullen, I 984) have found unzoned high-Al spinelsin picritic lavas. Both zoned and unzonedhigh-Al spinels,similar in Al content to the more aluminous of the Lamont seamount spinels,were previ- ously interpreted as representinghigh-pressure relicts, on the basis of (l) scant experimental evidence that the AI content of spinels increaseswith pressure(Green et al., 197l;Jaques and Green,1979;'P. Roeder, pers. comm., 1987) and (2) the observation that spinels in lherzolite nodules are typically aluminous. Dick and Bullen (1984) **F2-1 .oF2-2 suggestedthat pressuremay affect spinel composition by changing the melt/spinel partitioning coefficient for Cr, becauseof increasedoctahedral melt sites with pressure resulting in increasedCr solubility in the melt. Y=16.n31g,I2/snl!", The presenceof Al-rich, skeletal rims of groundmass spinels surrounding Cr-rich spinel cores in the Lamont Fig. 10. Compositionalzoning in selectedstrongly zoned or seamountlavas clearly shows that Cr-spinelwith Cr/(Cr + complexlyzoned spinels from the groundmassof F2-l (stars) Al) as low as 0.2 can crystallize at low pressures.We andF2-2 (circles); interiors of spinelsare shownby solid sym- therefore believe that the presenceof Al-rich spinels in bols,and spinelrims areshown by opensymbols. Arrows indi- MORBs may not represent high-pressureprocesses but catedirection of coreto rim, andthe analysesnumbers are also may instead reflect low-pressureequilibrium crystalliza- givenfor reference. tion, magma mixing, or wallrock assimilation. Although more experimental work is neededto determine whether solubility of Cr-rich minerals in melt appreciablychanges Cr-rich spinels are in equilibrium with the host liquid' over the pressuresof interest, we wish to note that the Specific examples given here show that large ranges in differencein the l-bar molar volumes between FeCrrOo spinel Crl(Cr * Al) content within a given lava reflect (chromite) and MgAl,Oo (spinel) is on the order of l0o/o incorporation of xenocrystal spinels that crystallized from (4.4010vs. 3.9710J/bar, respectively; Robie et al., 1978). a silicate liquid of different Mg/(Mg * Fe'z+)content than The compositional effectof the AZof the exchangeequa- the host magma and likely reflect magma mixing in a tron shallow magma chamber just prior to eruption. 4. Al-rich (to over 47o/oAl'O' by weight) groundmass (MgAlrOo)"o+ (FeCr,Oo)'io= (FeCrzOo)"o spinels and skeletal groundmassrims in "primitive" la- + (MgAlroo)rio (3) vas clearly show that Al-rich spinels crystallize at low is likely small, when compared to the very large difer- pressureand that Al-rich coresofreversely zoned spinels encesin spinel composition [i.e., Cr/(Cr + Al) variation may in some casesrepresent magma mixing or wallrock of 0.2-0.541caused by the minor variationsin host-lava assimilation rather than represent high-pressure relicts composition as seenin the Lamont seamountlavas. This (e.g.,Sigurdsson, 1977). observation implies that a quantitative geobarometer 5. The Cr content of spinels within a given lava has basedon spinel-liquid exchangemay be a difficult prob- little relationship to host-liquid Cr content. Instead, spi- lem to implement, becauseof the difrculty of separating nel Cr is indirectly controlled by the hostJava Al, Fe2*, pressureefects from compositional effects. and Mg contents through reciprocal exchangereactions in a manner that is qualitatively but not yet quantita- Coxcr-usroNs tively predictable.This observation,coupled with the ob- 1. Cr-rich spinels from the near-ridge Lamont sea- servation that these elements in spinel (particularly Al) mounts and from the adjacent EPR crest at l0'N occur strongly covary with small changesin host-liquid com- in lavas that contain greater than 260 ppm Cr and that position, indicates that quantitative use of Cr-rich spinels have M/(Mg + Fe'z+)greater than 0.61. as geobarometerswill be a difficult problem to imple- 2. These Cr-rich spinels vary systematically in com- ment. Nevertheless,these same observations show that position with host-lava composition and "primitive- Cr-rich spinels are powerful petrogenetic tools for pro- ness," as measuredeither by normative Ol/(Ol + Diop) viding information about interaction betweensilicate liq- ratios or Mg/(Mg * Fe2+),with spinels richer in Cr- and uids or silicate liquids and solids. Fe'?+crystallizing from the more evolved lavas. 3. Recasting of spinel compositions in terms of ther- AcxNowr,nocMENTS modynamically relevant components representing ex- Host lavas for the spinels were collectedduring the Clipperton (1985) change reactions allows the determination of whether expedition; we are indebted to chiefscientist D. Fomari, the crew ofthe 752 ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS

Atlantis II, and to the ALVIN pllols and crew We are grateful to F basalts, Deep Sea Drilling Project Site 505-4 preliminary interpre- Bishop,P. Roeder,H. Sigurdsson,and J A. Speerfor reviewingthe manu- tation ofelectron microprobe analyses.Initial Reports ofthe Deep Sea script and to F Bishop and B. Wood for providing numerous suggestions Drilling Project,69, 805-810. regardingthe petrological and geochemicalsignificance of variations in Frey,F.A., Green,D.H , and Roy, S.D.(1978) Integrated models of basalt spinelchemistry. J F.A. also thanks G. Adams, I. Carmichael,p. Castillo, petrogenesis:A study of quartz tholeiites to olivine melilites from H Dick, M. Fisk, R. Loucks, G. Martioli, J. Natland, J. Pasteris,and M. southeastAustralia utilizing geochemicaland experimental petrologic Perfit for compellingdiscussions concerning this researchand D. Kremser data. Joumal of Petrology, 19, 463-5 13. for analytic assistanceon the electron microprobe. We thank W. Melson Gallo, D.G., Fox, J., and Macdonald,K.C (1986)A seabeam investi- and T O'Hearn for providing the glassmajor-elemental analyses and I. gation ofthe Clipperton transform fault. The morphotectonic expres- Carmichael and D. Christie for the glassFeO analyses.This work was sion of a fast slipping transform boundary. Journal of Geophysical supportedby NSF $anrs OCE-8308980,OCE-84 I 5270,and OCE-8508042 Research.9 l. 3455-3467. (R. Batiza),ONR grant l0-14-80-C-0856(R. Batiza),NSF grant EAR- Glazner,A.F. (1984) Activities ofolivine and plagioclasecomponents in 8419158(R O. Sack),and NSF granrOCE-8414616 (D. Fornari),wirh silicate melts and their application to geothermometry.Contributions support to J F.A. during revision of the paper from NSERC Strategic to Mineralogy and Petrology,88, 260-268. OperatingGrant 5-55847 (R. Chase). Green, D H. (1971) Compositions of basaltic magmas as indicators of conditions of origin: Application to oceanic volcanism. Philisophical RnrrnpucBs crrno Transactionsofthe Royal SocietyofLondon, ser.A, 268,707-725 Green,D.H., Ringwood,A.E., Ware,N G, Hibberson,W.O, Major, A., Allan, J.F., Batiza,R, Perfit,M., and Fornari, D. (1986)Extremely de- and Kiss, E. (1971) Experimentalpetrology and petrogenesisofApollo pleted basalts from seamountsnear the EPR (9.53'N: Evidence for 12 basalts. Proceedingsof the Second Lunar ScienceConference, l, continued melting during outward lateral transport of sub-axial upper 60l-6 l 5. mantle(abs.). EOS, 67, I 184 Haskin, L.A., Wildeman, T.R., and Haskin, M.A. (1968)An accurate Allan, J.F., Batiza, R, and Lonsdale, P. (1987) Petrology oflavas from procedurefor the determination of the rare earths by neutron activa- seamountsflanking the East Pacific Rise axis, 21.N: Implications con- tion. Joumal of Radioanalytic Chemistry, l, 337-348. cerningthe mantle sourcecomposition for both seamountand adjacent Hill, R.L., and Sack,R.O. (1987)Thermodynamic properties of Fe-Mg EPR lavas. In B. Keating, I Fryer, and R. Batiza, Eds., Seamounts, titanomagnetitespinels. Canadian Mineralogist, 25, 443464. islands, and atolls American GeophysicalUnion GeophysicalMono- Irvine, T.N. (1965) Chrome spinel as a petrogeneticindicator, 1, Theory. gnph 43,255-282- CanadianJoumal ofthe Earth Sciences,2,648-672 BasalticVolcanism Study Project(1981) Basaltic volcanism on the ter- -(1967) Chromium spinel as a petrogeneticindicator: Parl 2, Pe- restrial planets.Pergamon Press, New York. trological applications.Canadian Journal ofthe Earth Sciences,4, 7l- Batiza,R., and Vanko, D (1984)Petrology ofyoung Pacificseamounts. l 03. JournalofGeophysical Research, 89, 11235-l 1260. -(1976) Chromite crystallization in the join MgrSiOn-CaMgSirOu- Byerly,G.R., Melson, W.G., Nelen,J.A., and Jarosewich,E. (1976)Abys- CaAlrSiOr-MgCrrOo-SiOr. CarnegieInstitution of Washington Year sal basalticglasses as indicators of magma compositions.In B. Mason, Book 76, 4654'72. Ed., Mineral scienceinvestigations, I 974-l 975. Smithsonian Contri- -(1977) Origin of chromitite layers in the Muskox intrusion and butions to the Earth Sciences,19,22-30. other stratiform intrusions: A new interpretation. Geology,5,273-277. Carmichael,I.S E. (1967)The iron-titanium oxidesofsalic volcanic rocks Jaques,A L, and Green,D.H. (1979)Anhydrous melting ofperidotite at and their associatedferromagnesian silicates Contributions to Miner- 0-15 kb pressureand the genesisoftholeiitic basalts Contributions to alogy and Petrology, 14,36-64 Mineralogy and Petrology,7 3, 287-3 10. Christie, D M , Carmichael,I.S.E., and l,angmuir, C.H. (t986) Oxidation Kastens,K.A., Ryan,W.B.F., and Fox, J (1986)Structural and volcanic statesof mid-ocean ridge basalt glasses.Eanh and Planetary Science expressionof a fast slipping ridge-transform boundary: Sea MARC I Letters,79, 397411. and photographicsurveys at the Clipperton transform fault. Journal of Detrick, P.S., Buhl, P., Vera, E., Mutter, J , Madsen, I., and Brocher, T. GeophysicalResearch, 9 l, 3469-3488 (1987) Multi-channel seismic imaging of a crustal magma chamber Klein, E., Langmuir,C., Bender,J.,Batiza, R., and Zindler,A. (1986)Nd alongthe EastPacific Rise. Nature, 326,3541 and Sr isotopic composition of volcanic glassesfrom the East Pacific Dick, H.J.B , and Bryan,W.B. (1978)Variation of basaltphenocryst min- Rise(8'N-12"N). EOS, 67, 1255 eralogyand rock compositions in DSDP hole 3968 Initial Reports of Langmuir,C.H., Bender,J.F., and Batiza,R. (1986)Petrological and tec- the Deep SeaDrilling Project, 46, 215-225. tonic segmentationof the East Pacific Rise, 5'30'-14"30'N. Nature, Dick, H.J B, and Bullen,T. (1984)Chromium spinelas a petrogenetic 322,422429. indicator in abyssaland alpine-typeperidotites and spatiallyassociated Melson,W G., Vallier, T.L , Wright, T L., Byerly, G , and Nelen, J. (1976) lavas Contributions to Mineralogy and Petrology, 86,54-76. Chemical diversity ofabyssal volcanic glasserupted along Pacific, At- Engi, M. (1983) Equilibria involving Al-Cr spinel: Mg-Fe exchangewith lantic, and Indian Ocean sea-floorspreading centers. In G.H. Sutton, olivine Experiments,thermodynamic analysis,and consequencesfor M H. Manghnani, and R. Moberly, Eds., The geophysicsof the Pacific geothermometry American Journal of Science,283- A, 29-7 l. Oceanbasin and its margin, American GeophysicalUnion Geophysical Fisk, M R, and Bence,A.E (1980)Expenmental crystallization of chrome Monograph19,351-367. spinel in FAMOUS basalt 527-l-1. Ea(h and Planetary Sciencekt- Natland,J.H , Adamson,A.C , Laverne,C., Melson,W.G., and O'Heam, ters.48.lll-123. T. (1983)A compositionally nearly steady-statemagma chamber at the Fomari,D J., Ryan,W.B.F., and Fox, P.J (1984)The evolutionof craters Costa Rica Rift: Evidence from basalt glass and mineral data. Deep and calderason young seamounts Insights from SEA MARC I and SeaDrilling Project Sites50 l, 504, and 505, Initial Reportsofthe Deep SEA BEAM sonar surveysof a small seamountgroup near the axis of SeaDrilling Project,69, 8 I l-858. the East Pacific Rise at E 10'N. Journal ofGeophysical Research,89, Perfit,M.R., Shuster,R.D., Allan, J., Batiza,R., and Fomari, D. (1986) l 1069-l1083 Trace element and isotopic characteristicsof seamountsnear the East Fornari, D., Perfit, M., Allan, J., Batiza,R, Haymon, R., Barone,A., PacificRise. Probes to MORB sources:EOS.67. 1184. Ryan, W.B F., Smith, T., and Simkin, T. (1988a)Seamounrs as win- Presnall,D.C., Dixon, J.R., O'Donnell, T.H., and Hoover, J.D. (1979) dows into mantle processes:Structural and geochemicalstudies of the Compositions and depth of origin of primary mid-ocean ridge basalts. Lamont seamounts Earth and Planetary ScienceLelters, in press. Joumalof Petrology,20, 3-35. Fornari,D., Perfit,M., Allan, J., and Batiza,R. (1988b)Small-scale het- Rhodes,J.M., and Dungan,M.A. (1979)The evolution of ocean-floor erogeneitiesin depletedmantle sources:Near-ridge seamountlava geo- basallic magmas:Deep drilling resultsin the Atlantic Ocean.American chemistry and implications for ocean ridge magmatic processes.Na- GeophysicalUnion MauriceEwing Series,2,262-272. ture. 33l. 5I l-5 I 3. Robie,R.A., Hemingway,B.S., and Fisher,J.R. (1978)Thermodynamic Foruta, T., and Tokuyama, H. (1983) Chromium spinels in Costa Rica properties of minerals and related substancesat 298.15 K and 1 bar ALLAN ET AL.: CT-RICH SPINELS IN MORB-TYPE LAVAS 753

( I 05pascals) pressure and at higher temperatures.U.S GeologicalSur- chemistry and zoning. Initial Reports ofthe Deep SeaDrilling Project' vey Bulletin 1452. 37,883-89 l Sack, R.O. (1982) Spinels as petrogeneticindicators: Activity-composi- Sigurdsson,H., and Schilling, J.-G. (1976) Spinelsin Mid-Atlantic Ridge tion relations at low pressures.Contributions to Mineralogy and Pe- basalts:Chemistry and occurrence.Earth and PlanetaryScience lrtters, trology,79, 169-186. 29.7-20. Sack,R.O, Carmichael,I S.E, Rivers,M.L., and Ghiorso,M.S (1980) Sun, S.S.,Nesbitt, R.W., and Sharaskin,A.Y. (1979) Geochemicalchar- Ferric-ferrousequilibria in natural silicate liquids at I bar. Contribu- acteristicsof mid-oceanridge basalts.Earth and PlanetaryScience I-et- tions to Mineralogy and Petrology,7 5, 369-376 ters,44, I 19-138. Sack,R.O, Walker, D., and Carmichael,I.S.E. (1987) Experimental pe- Thy, P (1983) Spinel minerals in transitional and alkalic basaltic glasses' trology ofalkaline lavas:Constraints on cotecticsofmultiple saturation Contributionsto Mineralogyand Petrology,83, l4l-149. in natural basic liquids. Contributions to Mineralogy and Petrology, Walker, D., Shibata,T., and Del'ong, S.E.(1979) Abvssaltholeiites from qK t_r? the OceanographerFracture Zone, lI, Phase-equilibriaand mixing. Sato, H. (1977) Nickel content ofbasaltic magmas:Identification ofpri- Contributions to Mineralogy and Petrology,70, Ill-125. mary magmas and a measureof the degreeof olivine fractionation. Wood, B.J, and Nicholls, J (1978) The thermodynamic properties of Lirhos,10,113-120. reciprocal solid solutions. Contributions to Mineralogy and Petrology, Sempere,J.-C., and Macdonald,K.C. (1986) Deep-tow studiesof the 66, 389-400. overlapping spreadingcenters at 9'03'N on the East Pacific Rise. Tec- tonics,5, 88l-900. MeNuscnrsr RECEIvEDJuNe 29, 1987 Sigurdsson,H. (19'17) Spinels in leg 37 basalts and peridotites: Phase MANUscRTFTACCEPTED Mencu 9, 1988