JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Poster

P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS10] [JJ] Formation and evolution of planetary materials in the solar system

Mon. May 22, 2017 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL7)

[PPS10-P13] Discovery of heavily shocked type 3 ordinary chondrites

*Masaaki Miyahara1, Akira Yamaguchi2, Eiji Ohtani3 (1.Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 2.NIPR, 3.Department of Earth Sciences, Graduate School of Science, Tohoku University)

Keywords:ordinary chondrite, shock, high-pressure polymorph

Based on the onion cell model, a parent-body of an ordinary chondrite consists of petrographic type 6, 5, 4 and 3 from inward to outward. A high-pressure polymorph occurring in a shocked ordinary chondrite gives shock pressure, temperature, impact velocity and impactor size, which become clues for understanding a destruction process of an ordinary chondrite parent-body. We have to clarify the inventories of a high-pressure polymorph included in all petrologic types to elucidate the destruction process of an ordinary chondrite parent-body. Accordingly, we will describe the petrologic and mineralogical features of the shock-induced textures and high-pressure polymorphs therein in heavily shocked type 3 ordinary chondrites.
We observed about three hundreds Antarctica type 3 ordinary chondrite (H-, L- and LL-type) petrographic thin sections stored in the NIPR under an optical microscope. We found eight type 3 ordinary chondrites with a distinct melting texture; Y-981139 (H3), A-87170 (L3), A-87220 (L3), Y-000886 (L3), Y-86706 (L3), Y-981327 (L3) A-881199 (LL3.4) and A-881981 (LL3). We also selected thirty three type 3 ordinary chondrite petrographic thin sections without a melting texture as a reference. All these samples were scanned with a field-emission scanning electron microscope (FE-SEM) to observe the fine-textures of melt-pockets and the morphologies of chondrules. Mineralogy was determined by a laser micro-Raman spectroscopy.
FE-SEM observations revealed that the melting textures (melt-pocket) in type 3 always occur around a boundary between a chondrule and matrix. Fine-grained quench silicate crystals and the spherules of metallic iron-nickel + iron sulfide with a eutectic texture filled the melt-pockets. Several interstitial glass fragments were entrained in the melt-pockets of A-881199 (LL3). Their bulk-chemical compositions are similar to that of plagioclase. Based on a Raman spectroscopy analysis, most of them are amorphous. On the other hand, back-scattered electron (BSE) image depicted that one of the interstitial glasses had a granular texture. A strong Raman shifts appeared at 372, 693 and 1032 cm-1 from the interstitial glass with a granular texture, which appear to be those of jadeite (Considering its chemical composition, probably, jadeite-diopside solid-solution) or tissintite. This is a first discovery of a high-pressure polymorph from type 3 ordinary chondrite. The ellipticity of chondrules (1 – (short axis/long axis)) in type 3 chondrites with and without a melting texture was measured. The ellipticity of chondrules in chondrites with a melting texture is ~0.31, which appears to be a bit bigger than those of chondrules in chondrites without a melting texture. The orientation of the long axis of chondrules was also measured. The long axis of chondrules in chondrites with a melting texture appears to be oriented along a specific orientation. The ellipticity and orientation degree of chondrules besides a high-pressure polymorph would be available for estimating shock pressure condition recorded in an ordinary chondrite.