17:15 〜 18:45
[PPS08-P05] Beardsleyコンドライト (H5)の岩石成因とアルカリ物質の起源
キーワード：Hコンドライト、カリウム、高アルカリ物質、衝撃溶融、マルチインパクト
Introduction: Alkali-rich materials with igneous texture (igneous clasts) have been identified in some brecciated LL chondrites, such as Yamato (Y)-74442, Bhola, and Krähenberg [1,2]. Formational process of these materials is explained as condensation from heterogeneous solar nebula followed by impact melting [2,3]. Alkaline materials are also identified in H chondrites, Zag and Monahans (1998), as halite and sylvite [4]. The Cs/Ba abundances of Zag and Beardsley chondrites are similar with that of CAI in carbonaceous meteorites, suggesting that primitive aqueous alteration on their parental bodies [5] and are important for elucidating the elemental distribution and/or fractionation process in early solar system. Beardsley chondrite contains high-K2O materials (up to 8 wt.%) in its matrix. However, petrogenesis and mixing process of alkaline rich materials in Beardsley parent body is still unclear [6]. In this study, we conduct petrological and mineralogical studies for Beardsley chondrite to clarify origin of alkaline element fractionation in Beardsley.
Sample and method: A fragment of Beardsley is allocated from Arizona State University, and we made a polished thin section (PTS) from it for petrological and mineralogical works. The PTS was observed under an optical microscope and a scanning electron microscope (SEM: JEOL JSM-6490). Elemental distribution maps of major elements were obtained using an EDS (OXFORD INSTRUMENTS INCA x-sight) attached to the SEM. Compositions of minerals were analyzed using an electron probe microanalyzer (EPMA: JEOL JXA-8230). All analyses were conducted at Okayama University of Science.
Results: Beardsley chondrite consists of two different lithologies of brown and grey by macroscopic observation. The major constituent phases of Beardsley are coarse-grained pyroxene, olivine, Fe-Ni metal and troilite set in fine-grained glassy matrix. Abundant submicron sized Fe-Ni metal droplets were observed in the coarse-grained fragments. Overgrowth of pyroxene with a Ca-rich composition (En47.39-51.68Fs5.63-6.87Wo41.45-46.39) occurs at the rim of coarse-grained pyroxene fragments surrounded by feldspathic glass. A dark lithic clast is found in grey lithology, which is irregularly rounded in shape and consist of fine-grained euhedral to subhedral pyroxene and olivine embedded in glassy matrix, showing igneous texture. Fe-Ni metal and K-bearing material are merely observed in the clast. Chemical compositions of pyroxene were slightly Mg- and Ca-rich (En80.20-82.43Fs16.19-18.41Wo0.90-1.63; En49.15-50.26Fs6.08-6.29Wo43.45-44.77) relative to pyroxene grains in host Beardsley chondrite.
Discussion and summary: The brown lithology contains coarse Fe-Ni metal grains with rusty rim showing brown color due to alteration, whereas the gray lithology has abundant sub-micrometer sized metal grains occurring in coarse lithic fragments of olivine and pyroxene and are causing weak darkening. The composition of the overgrowth of pyroxene and glassy matrix indicates that the Beardsley chondrite experienced an incomplete shock-melting like that of Y-791088 [7]. An igneous clast is found in grey lithology, consists of mostly fine-grained euhedral pyroxene with high abundance of Ca without Fe-Ni metals and high-K materials. These signatures are not consistent with Beardsley host, indicating exogeneous melt droplets. Precursor materials of K-rich glass of Beardsley could have derived from multiple exogenous materials supplied during impact(s) or heterogeneously distributed in Beardsley host prior to impact event(s).
References: [1] Ruzicka A. M. et al. (2019) GCA 266, 497–528. [2] Yokoyama T. et al. (2013) EPSL 366, 38-48. [3] Niihara T. et al. (2021) MAPS 56, 8, 1619-1625. [4] Zolensky M. E. et al. (1999) Science 285, 1377-1379. [5] Hidaka H. et al. (2001) EPSL. 193, 459-466. [6] Niihara T. et al. (2023) LPSC LIV, Abstract#2052. [7] Niihara T. et al. (2011) Pol. Sci. 4, 558-573.
Sample and method: A fragment of Beardsley is allocated from Arizona State University, and we made a polished thin section (PTS) from it for petrological and mineralogical works. The PTS was observed under an optical microscope and a scanning electron microscope (SEM: JEOL JSM-6490). Elemental distribution maps of major elements were obtained using an EDS (OXFORD INSTRUMENTS INCA x-sight) attached to the SEM. Compositions of minerals were analyzed using an electron probe microanalyzer (EPMA: JEOL JXA-8230). All analyses were conducted at Okayama University of Science.
Results: Beardsley chondrite consists of two different lithologies of brown and grey by macroscopic observation. The major constituent phases of Beardsley are coarse-grained pyroxene, olivine, Fe-Ni metal and troilite set in fine-grained glassy matrix. Abundant submicron sized Fe-Ni metal droplets were observed in the coarse-grained fragments. Overgrowth of pyroxene with a Ca-rich composition (En47.39-51.68Fs5.63-6.87Wo41.45-46.39) occurs at the rim of coarse-grained pyroxene fragments surrounded by feldspathic glass. A dark lithic clast is found in grey lithology, which is irregularly rounded in shape and consist of fine-grained euhedral to subhedral pyroxene and olivine embedded in glassy matrix, showing igneous texture. Fe-Ni metal and K-bearing material are merely observed in the clast. Chemical compositions of pyroxene were slightly Mg- and Ca-rich (En80.20-82.43Fs16.19-18.41Wo0.90-1.63; En49.15-50.26Fs6.08-6.29Wo43.45-44.77) relative to pyroxene grains in host Beardsley chondrite.
Discussion and summary: The brown lithology contains coarse Fe-Ni metal grains with rusty rim showing brown color due to alteration, whereas the gray lithology has abundant sub-micrometer sized metal grains occurring in coarse lithic fragments of olivine and pyroxene and are causing weak darkening. The composition of the overgrowth of pyroxene and glassy matrix indicates that the Beardsley chondrite experienced an incomplete shock-melting like that of Y-791088 [7]. An igneous clast is found in grey lithology, consists of mostly fine-grained euhedral pyroxene with high abundance of Ca without Fe-Ni metals and high-K materials. These signatures are not consistent with Beardsley host, indicating exogeneous melt droplets. Precursor materials of K-rich glass of Beardsley could have derived from multiple exogenous materials supplied during impact(s) or heterogeneously distributed in Beardsley host prior to impact event(s).
References: [1] Ruzicka A. M. et al. (2019) GCA 266, 497–528. [2] Yokoyama T. et al. (2013) EPSL 366, 38-48. [3] Niihara T. et al. (2021) MAPS 56, 8, 1619-1625. [4] Zolensky M. E. et al. (1999) Science 285, 1377-1379. [5] Hidaka H. et al. (2001) EPSL. 193, 459-466. [6] Niihara T. et al. (2023) LPSC LIV, Abstract#2052. [7] Niihara T. et al. (2011) Pol. Sci. 4, 558-573.