American Mineralogist, Volume 73, pages 741-753, 1988 Cr-rich spinels as petrogeneticindicators: MORB-type lavas 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 lava 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 seamounts 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; olivines 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.