Evidence for Local Shock Melting in Seymchan Meteorite
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ence for Local Shock Melting in Seymchan Meteorite N. R. Khisinaa, *, D. D. Badyukova, **, V. G. Senina, and A. A. Burmistrova, b a
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia b Moscow Energy Institute Russia, ul. Krasnokazarmennaya 14, Moscow, 111250 Russia *e-mail: [email protected] **e-mail: [email protected] Received February 10, 2020; revised February 18, 2020; accepted February 25, 2020
Abstract—Pallasite Seymchan contains three morphological types of olivine grains, which occur in a FeNi matrix and have rounded, angular, and fragmental shape. Sometimes the rounded grains form clusters. The texture and mineralogical features of the olivine cluster in pallasite Seymchan were studied by optical microscopy, EMPA, and SEM. The data obtained indicate an imposed impact event in the Seymchan history, which led to local melting and rounding of initially angular olivine grains, as well as to melting of adjacent regions of the host FeNi metal. Local impact melting reasonably explains the observed textures and the coexistence of three morphological types of olivine in the pallasite Seymchan. Configuration of the intergranular boundaries in the olivine cluster indicates the phenomenon of coalescence. The phosphate–metal–sulfide films decorating the intergranular boundaries in the cluster represent the binding medium necessary for the coalescence of the olivine melt drops in the FeNi melt. The texture of the films demonstrates a liquid immiscibility in phosphate–metal–troilite melt. The metal–troilite assemblage in the films differs in texture and chemical composition from that of ordinary chondrites. Keywords: meteorites, pallasite Seymchan, pallasite texture, impact melting, shock-produced local melts, olivine clusters, morphology of olivine grains, rounded olivine grains, coalescence, metal–troilite aggregates DOI: 10.1134/S0016702920090049
INTRODUCTION Pallasites are a class of stony–iron meteorites, which are made up of large olivine grains embedded in a metallic FeNi matrix. Accessory minerals in pallasites are represented by schreibersite, troilite, phosphates, chromite, ortho- and clinopyroxene. Based on the chemical composition of the metal and oxygenbearing phases, pallasites are classified into following groups: main group pallasites (PMG); anomalous pallasites; Eagle Station group; and pyroxene pallasites (Wasson and Choi, 2003). The metallographic and geochemical features of the FeNi metal in pallasites are similar to those of IIIAB iron meteorites (Wasson and Choi, 2003; Clayton and Mayeda, 1978; Yang et al., 2010; Scott, 1977a). At the same time, the mineralogical and chemical composition of oxygen-bearing minerals in pallasites differs from those of iron meteorites with silicate inclusions (group IIE), in which major minerals are pyroxenes, olivine, and phosphates, while the inclusions contain the great amount of pyroxene–plagioclase glass. In spite of the simple mineralogical composition, pallasites show some peculiari
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