Lunar and Planetary Science XXXI 1081.pdf

PHASE EQUILIBRIA OF THE SHERGOTTY METEORITE: NEW PETROLOGIC CONSTRAINTS 1 2 1 ON THE H2O CONTENTS OF MARTIAN MAGMAS. J. C. Dann , A. H. Holzheid , T. L. Grove and H. Y. McSween, Jr.3, 1Department of Earth, Atmospheric and Planetary Sciences, 54-1218, Massachusetts Institute of Technology, Cambridge, MA 02139 ([email protected], [email protected]). 2Westfaelische Wilhelms-Universitaet Muenster, Institut fuer Mineralogie, Corrensstrasse 24, 48149 Muenster, Germany, ([email protected]), 3Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410 ([email protected]).

Introduction: This abstract provides new con- phases produced in each experiment and an estimate of straints on the conditions of formation of the augite + the starting composition. There has been no gain or pigeonite phenocryst assemblage that is found in the loss of iron in the 0.1 MPa experiments, but there has Shergotty Martian meteorite and on the depths of for- been some Na-loss from the silicate charge to the fur- mation of this assemblage. New experiments on the nace atmosphere (~25 % Na-loss). The major effect of Shergotty liquid composition of Hale et al. [1] have this Na-loss will be to raise slightly the temperature of been performed at 0.1 MPa under anhydrous conditions plagioclase appearance. and at 100 and 200 MPa H2O-saturated. The composi- Results: The experimental results are summarized tions of the most magnesian pyroxene cores in Sher- in Table 1. At 0.1 MPa a high-Cr spinel is the liq- o o gotty are closely approximated in H2O-saturated ex- uidus phase at approximately 1200 C. By 1175 C periments performed at 200 MPa. At 100 MPa H2O- pigeonite has joined the crystallization assemblage. saturated and 0.1 MPa anhydrous, the most magnesian Augite, plagioclase and tridymite join pigeonite as pyroxene is pigeonite. Coexisting augite + pigeonite crystallizing phases by 1075 oC. do not appear until later in the crystallization history at a lower temperature, and are Fe-enriched. Assuming Table 1. Experimental results on Shergotty liquid. that the reconstructed intercumulus liquid composition Expt. P, MPa T¡C products of Hale et al. [1] is correct, the coexisting pyroxenes in S-3 0.1 1230 gl Shergotty represent pre-eruptive conditions of a magma S-5 Ò 1210 gl containing ~ 6 wt. % H2O. S-6 Ò 1190 gl, sp, Experiments: A synthetic analog was prepared S-1 Ò 1175 gl, sp, pig using high purity oxides. The composition is the S-2 Ò 1160 gl, sp, pig average of the estimate of the intercumulus liquid com- S-4 Ò 1145 gl, sp, pig position in samples 321-2 and 321-5 of Shergotty of S-8 Ò 1125 gl, sp, pig Hale et al. [1], who re-evaluated the liquid composi- S-11 Ò 1100 gl, sp, pig tion of the Shergotty meteorite. This starting material S-9 Ò 1075 gl, sp, pig, aug, pl, trd has been used to perform experiments at 0.1 MPa at S-10 Ò 1050 gl, sp, pig, aug, pl, trd the quartz-fayalite-magnetite (QFM) buffer and to carry S-16 100* 1080 gl, sp, pig out experiments at 100 and 200 MPa, H2O saturated at S-17 Ò 1040 gl, sp, pig, aug the nickel Ð nickel oxide (NNO) buffer. S-12 200* 1044 gl, ol, sp, pig, aug

For the 0.1 MPa experiments, 0.6-0.7 mg of the * H2O-saturated. Abbreviations: gl = glass, ol = oli- starting material was pressed into pellets along with vine, sp = spinel, pig = pigeonite, aug = augite, pl = polyvinyl alcohol. This pellet was then sintered onto plagioclase, trd = tridymite. a platinum wire loop (0.008 inch diameter). The loop was pre-annealed with 7-9% Fe to reduce iron loss At 100 MPa pigeonite is the only pyroxene present during the experiment. Experimental durations varied at the highest temperature investigated (1080 oC). By from 2 to 240 hours. The 100 and 200 MPa experi- 1040 oC augite has joined pigeonite as a crystallizing ments were carried out in a rapid-quench ZHM (zirco- phase. At 200 MPa the single experiment at 1044 oC nium Ð hafnium carbide Ð molybdenum) cold seal gas contains olivine, spinel, pigeonite and augite. pressure vessel. The experiments used Au or Au80Pd20 The compositional variations in the experimental inner and outer capsules. Temperature measurement, pyroxenes are shown in Fig. 1 along with the average calibration, pressure application, and gas mixtures are compositions of the most magnesian pyroxenes found as described in Sisson and Grove [2]. Experiment in Shergotty. In both the 0.1 MPa anhydrous and 100 duration varied from 6 to 48 hours. The experimentally MPa H2O-saturated experiments, pigeonite is the only produced phases have been analyzed with the JEOL pyroxene to crystallize at near-liquidus conditions. 733 Superprobes at MIT. Crystalline phases were ana- Augite appears as a crystallizing phase at lower tem- lyzed using a spot beam of ~2 microns diameter. peratures, and both pigeonite and augite have lower Glasses were analyzed with a 10 micron beam size. Mg#s than the most-magnesian Shergotty augite and All experiments were checked for element gain or loss pigeonite pair. Also, the experimentally produced during the experiment by a materials balance tech- pigeonite has a higher Wo content and augite has a nique. The mass balance was performed using the lower Wo content reflecting the closure of the two- Lunar and Planetary Science XXXI 1081.pdf

SHERGOTTY PYROXENES: HIGH H2O IN MARTIAN MAGMAS: J. C Dann et al.

pyroxene miscibility gap at lower Mg#s. pre-eruptive H2O contents necessary to reproduce the In contrast the 200 MPa augite and pigeonite are a two-pyroxene phase assemblage found in Shergotty. close match to the most magnesian pigeonite + augite The phase relations are consistent with a two-stage that are found in Shergotty. The 200 MPa experiment evolutionary history for Shergotty. We propose that was chosen based on the temperatures recorded in the the olivine + two-pyroxene assemblage represents con- pigeonite Ð augite assemblage in Shergotty. The ditions in a magma reservoir. As the magma was de- QUILF pyroxene thermometer of Andersen et al. [3] livered to near-surface conditions the liquidus olivine gives a temperature of 1000 oC +/- 100 oC for the py- became unstable and the two-pyroxene assemblage was roxene core compositions reported in Hale et al. [1]. preserved. The pre-eruptive H2O contents of shergot- Although this temperature has a large uncertainty the tite magmas implied by our results have important pyroxenes in the near-liquidus assemblage represented implications for the water inventory of the Martian in the 200 MPa experiment lie in the middle of the mantle and its outgassing history [7]. two-pyroxene estimate and are near-liquidus phases. In contrast the pigeonite at 0.1 MPa crystallizes at 1175 References: [1]ÊVS Hale et al. (1999) Geochim oC, which lies well outside the two-pyroxene estimate Cosmochim Acta 63, 1459-1470. [2]ÊTW Sisson and from the Shergotty pyroxenes. TL Grove (1993) Contrib Mineral Petrol 113, 143- Implications: The experimental investigation of 166. [3]ÊDJ Andersen et al. (1993) Computers Geosci Johnson et al. [4] determined the minimum H2O con- 19, 1333-1350. [4]ÊMC Johnson et al. (1991) Geochim tents necessary to lead to the crystallization of kaersu- Cosmochim Acta 55, 349-366. [5] LL Watson et al. tite amphibole in Chassigny. Although this estimate (1994) Science 265 86-90. [6] PL King et al. (1999) has been criticized as too high based on low H in am- Geochim Cosmochim Acta 63, 3635-3651. [7] HY phibole [5], the possibility of dehydrogenation allows McSween and RP Harvey (1993) Science 259, 1890- higher H2O contents [6]. This study supports the con- 1892. clusion of [4] and provides an independent estimate of Fig. 1. Average compositions of pyroxenes Di Hd from the 0.1, anhydrous 100 and 200 MPa H O-saturated experiments on Shergotty 0.4 Shergotty 0.6 2 intercumulus liquid. Also shown are the experiments average compositions of the most magnesian 0.1 MPa pigeonite and augite reported by Hale et al. 0.2 0.8 [1]. 100 MPa

200 MPa

En1 0.8 0.6 0.4 0.2 0 Fs Wo Shergotty0.5 200 MPa expt. H2O-saturated

augite cores augite

pigeonite cores pigeonite

Fe0.6 En0.9 0.8 0.6 0.4

Fig. 2 Shergotty pyroxenes from Hale et al. [1] and experimental pyroxenes from 1044 oC and 200 MPa.