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Possible fluid-rock interactions on differentiated asteroids recorded in eucritic meteorites |
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| Authors: | JA Barrat A Yamaguchi TE Bunch C Bollinger G Ceuleneer |
| Institution: | a Université Européenne de Bretagne, France b CNRS UMR 6538 (Domaines Océaniques), UBO-IUEM, Place Nicolas Copernic, 29280 Plouzané Cedex, France c National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan d Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tokyo 190-8518, Japan e Department of Geology, Northern Arizona University, Flagstaff, Arizona, United States f CNRS, Université Paul Sabatier - Observatoire Midi-Pyrénées, 14 Av. Édouard Belin, 31400 Toulouse, France |
| Abstract: | The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: (1) Fe-enrichment along the cracks that cross cut the pyroxene crystals (“Fe-metasomatism”); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; (2) deposits of Fe-rich olivine (Fa64-86) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An97-98) is associated with the fayalitic olivine; (3) at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites. |
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