Mineralogical Comparison of Olivine Xenocrysts in Angrites LEW 87051 and Asuka 881371

Details Mineralogical Comparison of Olivine Xenocrysts in Angrites LEW 87051 and Asuka 881371 Mineralogical Comparison of Olivine Xenocrysts in Angrites LEW 87051 and Asuka 881371

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.549m&link_type=abstract

Meteoritics, vol. 30, no. 5, page 549

Mathematics

Logic

Angrites, Grains, Relict, Impact Melting, Meteorites, Asuka 881371, Lewis Cliff 87051, Olivine, Xenocrysts

Scientific paper

Antarctic angrites LEW87051 (L7) and Asuka-881371 (A8) have distinctive textural and mineralogical similarities. L7 has porphyritic zoned olivines up to 0.5 mm in size distributed in a fine-grained groundmass [e.g. 1]. Some L7 olivines have Mg-rich cores which are out of equilibrium with the melt and are considered to be xenocrysts [e.g. 2]. A8 has an ophitic groundmass that contains olivine xenocrysts larger than 2 mm across [e.g. 3]. These reach up to Fo(sub)89, and are rich in Cr and poor in Ca, like L7 olivine cores. Despite these similarities, two major differences exist between L7 and A8. One is the range of compositions of the olivine xenocrysts. Although olivine cores of L7 are almost homogeneous in composition from one grain to another, A8 shows significant variations. Each large xenocryst in A8 is nearly homogeneous (except for an Fe-rich rim). However, core compositions of A8 olivine xenocrysts vary: Fo(sub)70-89, Cr2O3 0.3-0.7 wt%, CaO 0.0-0.6 wt%, Al2O3 0.05-0.25 wt%, while olivine cores of L7 are nearly uniform: Fo(sub)88-90, Cr2O3 0.2 wt%, CaO 0.3 wt%, Al2O3 0.1 wt% [3,4]. The other difference is grain size. L7 olivines are at most 0.5 mm across, while A8 olivines are generally larger than 2 mm. Thus L7 and A8 show significant differences. One possible interpretation is that both samples contained xenocrysts that were resorbed to differing degrees in a heating event. We previously suggested that L7 xenocrysts less Mg-rich than Fo(sub)85 somehow melted and only Mg-rich olivine survived [5]. The core of L7 olivine is more clearly zoned than A8 xenocrysts, possibly due to a higher degree of atomic diffusion. In the case of A8, xenocrysts with Mg as low as Fo(sub)70 remain, and the xenocrysts are quite large. This is consistent with the hypothesis that A8 experienced less resorption of xenocrysts than L7. It is possible that L7 olivine core and A8 olivine xenocrysts originate from the same source. Their FeO/MnO ratio (wt%) is almost same (ca. 80-90). However, the A8 xenocrysts show a wider compositional range. We previously calculated the cooling rate of L7 olivine and estimated that burial depth was less than 2 m [5]. Compositions of groundmass fassaite in L7 and A8 are nearly consistent [e.g. 6]. This suggests formation from similar melts. Our model of L7 and A8 formation is as follows. Olivine crystals of various compositions were incorporated in a melt and somehow reheated. In the case of L7, all but the most Mg-rich olivines were totally melted, while even Fo(sub)70 olivines did not melt for A8. After the melting stage, the outer part of the L7 porphyritic olivines and the groundmass of both L7 and A8 crystallized. The grain size of the groundmass of each sample reflects its burial depth. L7 crystallized near surface (~2 m) where the degree of melting was larger than for A8, which crystallized at a deeper burial depth but experienced less heating and melting. To realize such heating, impact would be appropriate. Hence, we conclude that L7 and A8 were formed under similar conditions and differences in xenocrysts were caused by differing degrees of melting. The coarser-grained groundmass of A8 was due to crystallization at greater depth than L7. However, some difficulty remains in relating the bulk compositions of A8 and L7 by simple olivine fractionation. This difficulty might be due to sample heterogeneity. Further bulk analysis and investigation of different PTS is required. We thank the MWG and NIPR for the samples. References: [1] McKay G. et al. (1990) LPS XXI, 771-772. [2] Mikouchi T. et al. (1994) LPS XXV, 907-908. [3] Yanai K. (1994) Proc. NIPR Symp. Antarc. Meteorites, 7, 30-41. [4] Mikouchi T. et al. (1995) 20th NIPR Symp. Antarc. Meteorites, 159-162. [5] Mikouchi T. et al. (1995) LPS XXVI, 973-974. [6] McKay G. et al. (1995) 20th NIPR Symp. Antarc. Meteorites, 155-158.

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