Glass-bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub-igneous temperatures

Details Glass-bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub-igneous temperatures Glass-bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub-igneous temperatures

Jan 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000m%26ps...35...39v&link_type=abstract

Meteoritics & Planetary Science, vol. 35, no. 1, pp. 39-52 (2000).

Computer Science

14

Scientific paper

Three types of glass-bearing inclusions are present in olivine and chromite of the Chassigny achondrite: Pure glass, monocrystal (glass plus a single mineral grain) and multiphase (glass plus a variety of minerals) inclusions. The occurrence, texture and mineralogy of these inclusions and the chemical composition of the glass suggest an origin by heterogeneous trapping of these phases. The glass is rich in SiO2, Al2O3, Na2O, K2O and poor in MgO, FeO and CaO and contains appreciable amounts of Cl. The compositional variability of the glass is independent of the mineral content of the inclusions. Heating experiments with final temperatures of 900 deg C, 1000 deg C and 1200 deg C were performed with Chassigny inclusions for the first time. The glass of the heated inclusions has a chemical composition similar to that of unheated inclusions. This situation suggests that the glass cannot be a residual melt but rather is an independent component which was trapped with or without mineral phases. The extreme heterogeneity in alkali contents and in particular Rb and Sr contents also suggests precipitation and mixing of solid precursors. The most Rb-rich glasses have near-chondritic Rb/Sr ratios possibly indicating a chondritic source for their precursor(s). None of the inclusions contains a bubble like those of typical melt inclusions in terrestrial igneous minerals. Furthermore, many inclusions are the center of radial cracks in the host olivine indicating development of an overpressure within the inclusions at some time. A volume increase of the inclusions could have been achieved by differential thermal expansion of the content of the inclusion during a heating event. That mechanism requires bubble-free and solid pre-heating inclusion contents. These features are incompatible with an origin of the inclusions by trapping of a silicate melt and point toward heterogeneous trapping of solid phases. The first nitrogen analyses performed in Chassigny glass-bearing inclusions by Nuclear Reaction Analysis (NRA) revealed high and variable N contents of the glass suggesting trapping of a solid precursor presumably at relative low temperatures from a fluid rather than a melt. In conclusion, the glass-bearing inclusions in Chassigny olivine are not a residual after a closed-system evolution of a trapped melt but rather heterogeneously trapped precipitates of a fluid that existed during formation of Chassigny constituents. Consequently, it is very unlikely that the host olivine has an igneous origin.

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