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NWA 1235: A PHLOGOPITE-BEARING ENSTATITE METEORITE. C. Lorenz , G. Kurat , F. BrandstДt2 11 ter and M. A. Nazarov , Vernadsky Institute of Geochemistry and Analytical Chemistry. Kosygin St. 19, 2 Moscow, Russia, 119991, lorentz@geokhi.ru, Naturhistorisches Museum, Postfach 417, A-1014 Vienna, Austria, gero.kurat@univie.ac.at Introduction: NW A 1235 (found in 2000) is a patchily weathered (W 2-W 3) enstatite rock weighing 80 g. No chondrules or relics thereof are present. It has a coarse-grained hypidiomorphic texture, which is, however, finer relative to that of aubrites. Mineral chemistry and metal-silicate relationships suggest that the rock was formed from an enstatite meteorite source under more oxidized conditions than other enstatite meteorites. A unique features of the meteorite are the unusual composition of troilite and exotic sulfides, presence of a wide set of microinclusions in enstatite and the occurrence of fluorphlogopite. Results: NW A 1235 consists mainly of enstatite En99Wo1 (77 vol%) subhedral prismatic crystals up to 5 mm long (Fig.1). They have a weak mosaicism, undulatory extinction and contain numerous (1-15 µm in size) mineral and glassy inclusions, which are oriented along the long axis of pyroxene crystals. The average composition of the glass inclusions is (wt%): 77.9 SiO2; 13.0 Al2O3; 5.0 Na2O; 2.0 K2O; 0.2 CaO; 1.1 MgO. Some inclusions contain well-shaped crystals of Ti-free, Cr-poor (0.36 Cr, wt.%) troilite, Fe,Ni metal (6.5-40.7 wt% Ni) and niningerite. Minor feldspar Ab74.8-95.6Or0.6-6.8 (8 vol%) occurs interstitial to enstatite grains or forms lathshaped crystals within sulfide. Silica is present as 10-100 µm-sized angular and elongated grains within enstatite or rounded inclusions within metal and sulfide. It usually contains 0.5-1.0 wt% of Na2O and 1.3-1.5 wt% of Al2O3. Rare fluorphlogopite is associated with albitic feldspar. Subhedral or anhedral grains of the phase have 30-60 µm size and show well-developed cleavage planes. The average fluorine content of the phlogopite is about 10 wt%. Fe,Ni metal (4.8 vol%) is present mostly as irregular aggregates (0.1-1 mm), which consist of kamacite: (wt%) 1.8 Si; 8.35 Ni; Ni/Co=12.8, and minor taenite: 2.0 Si; 24.5-25.5 Ni. The kamacite/taenite volume ratio is about 20. Enstatite prisms are included in the metal aggregates. Taenite shows a weak M-shaped Ni profile with a maximum difference in Ni content of about 1.5 wt%. Distribution of Si in taenite is similar to that of Ni. Taenite is always richer in Si than adjacent kamacite that has a uniform Si distribution. Small Fe,Ni metal inclusions (5-10 µm) within enstatite are poorer in Ni (6.67 wt%) than the large aggregates. Schreibersite (19.8 wt% Ni) is usually present within the metal aggregates and forms tiny inclusions within enstatite. The aggregates contain commonly graphite, which occurs as clusters of 5100 µm-sized prismatic and hexagonal crystals. Cr-Ti-bearing troilite (2.5 wt% Cr and 0.13 wt% Ti) is the main sulfide (7 vol%) of NW A 1235. Troilite occurs in the metal aggregates. It fills also interstitial space between enstatite and feldspar and forms numerous inclusions scattered within enstatite. Accessory sulfides are oldhamite, FeS-MnSMgS solid solutions and Fe-rich sphalerite, which occur as rounded (5-15 µm) polymineralic, troilitecontaining inclusions within enstatite (Fig.2). The sulfide solid solutions have compositions outside the alabandite or niningerite compositional fields and have intermediate compositions (Fig.3). There are also rounded inclusions of a Mn-rich oxide phase (up to 40 wt% MnO) in enstatite. The phase could be the result of terrestrial weathering of alabandite. Discussion: NW A 1235 consists of reduced mineral assemblage that is characteristic of E chondrites and achondrites. The texture is achondritic, no chondrules or relics thereof are present. However, in contrast to aubrites, the texture of NW A 1235 is less coarse-grained and demonstrates textural features that are more compatibel with co-crystallyzation of enstatite crystals rather than accumulation. There is also no textural evidence for metal-silicate liquid immiscibility and thermal metamorphism in the rock. It is clear that NW A 1235 belongs to the enstatite meteorite family but nevertheless the rock differs from all representatives of this clan. The metal aggregates containing prismatic enstatite crystals, which are widespread in NW A 1235 and which are a common feature of enstatite chondrites [1]. The modal abundance of Fe,Ni metal (4.5 vol%) in NW A 1235 is higher than that in aubrites (0-0.7 vol%) and close to the metal abundance in EL chondrites. The Si content (wt%) of both kamacite (1.75) and taenite (1.99) in NW A 1235 is lower than that of EH metal (1.9-3.8) and corresponds to that of EL chondrite (0.3-2.1) [2] but not to that of aubrite (0.01-0.9) metals [3]. The

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NW A 1235: A PHLOGOPITE-BEARING ENSTATITE METEORITE: C.A. Lorenz et al.

Ni/Co ratio of NW A 1235 metal (12.8) is lower than that of E chondritic and achondritic metals (21.1-25.4 [3]). Only the Mayo Belwa aubrite metal has a similar Ni/Co ratio (14.2) [3]. The intermediate compositions of FeS-MnS-MgS solid solutions of NW A 1235 are not common for either E chondrites or achondrites and were reported only from "EH-melt rocks", QUE 94204 [4] and Yamato 793225 [5] and from Itqiy, an unusual Eachondrite [6]. The Cr-content of troilite (2.48 wt%) falls into the E chondrite and aubrite ranges. Based on the Ti content of troilite, redox conditions of the NW A 1235 melt can be constrained. As it was shown by [7], the Ti content of a sulfide melt is increasing when fO2 is decreasing. Troilite of NW A 1235 contains 0.13 wt% Ti, which is distinctly less than the Ti content of troilites in all enstatite meteorites analyzed so far. Troilite of aubrites, E6 and E4-5 chondrites contains 0.5-5.7, 0.65 and 0.4 wt% Ti, respectively [8]. Therefore, NW A 1235 must have formed under more oxidized conditions compared to all other enstatite meteorites. The phlogopite occurrence is a special characteristic of NW A 1235. Phlogopite has previously been reported only from the EH melt rock Yamato 82189 [5]. In NW A 1235, phlogopite could have crystallize from a residual melt because there is no evidence for a K-F- bearing fluid activity in the rock. Apatite, the main F-bearing phase is unstable under reducing conditions and, therefore, F, which is very abundant in E chondrites, needs to enter another phase. Conclusion. The texture of NW A 1235 is that of an achondrite. Mineral modes, metal-silicate relationships and sulfide chemistry could indicate a close relationship to EL chondrites. The very low Ti content of NW A 1235 troilite indicates that it crystallized under higher fO2 conditions than the common E chondrites and aubrites. Acknowledgements: This work was supported of Austrian Academy of Sciences and FW F. References: [1] Okada A. et al. (1988) Meteoritics, 23, 59-74. [2] Zhang Y. Et al. (1995) J. Geophys. Res. 100, 9417-9438. [3] Easton A. J. (1986) Meteoritics, 21, 79-93.. [4] Rubin A. E. et al. (1997) GCA 61, 847-858. [5] Lin Y., Kimura M. (1988) Meteoritics and Planet. Sci. 33, 501-511. [6] Patzer D.H. et al. (2001) Meteoritics and Planet. Sci. 36, 1495-1505. [7] Fogel R. A. (1997) Meteoritics and Planet. Sci. 32, 577-591. [8] W atters T. R. and Prinz M. (1979) Proc. LPSC X, 1073-1093. [9] Brearley A. J. and Jones R. H. (1988) Rev. Min. 36, 3-01-3-370

Fig 1. Reflected light microphotograph of NW A 1235 (picture width is 3.5 mm). Enstatite with interstitial albite is dark grey, metal is white, troilite is light gray, iron hydroxide is gray.

Fig 2. BSE image of a polymineralic sulfide inclusion within enstatite. Troilite is white, FeS-MgSMnS sulfide is gray, oldhamite is dark-grey.

FeS

nini nge rite (EH )

ala ba nd ite

(E L)

MgS

MnS

Fig 3. The composition of FeS-MgS-MnS solid solutions of NW A 1235 (EL and EH data from [9]).