On the significance of diopside and oldhamite in enstatite chondrites and aubrites

Details On the significance of diopside and oldhamite in enstatite chondrites and aubrites On the significance of diopside and oldhamite in enstatite chondrites and aubrites

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997m%26ps...32..577f&link_type=abstract

Meteoritics, vol. 32, pages 577-591

Mathematics

Logic

5

Scientific paper

Enstatite is the primary silicate phase of Equilibrated Enstatite Chondrites (EEC's). The CaO contents of these enstatites lie close to or on the enstatite-diopside phase boundary, yet curiously, diopside has always been absent from EEC assemblages. In contrast, aubrites contain abundant diopside even though they are thought to be derived from an E chondrite-like protolith. A phase equilibrium analysis of the Ca-Mg-Fe-Mn-Si-O-S system under reducing conditions solves this enigma and shows that diopside-bearing EEC's should commonly be found. When sulfur fugacity is sufficiently high (e.g., Fe-FeS buffer), low oxygen fugacity limits the stability of diopside in favor of oldhamite. Under such conditions, the relative stability of diopside and oldhamite is described by the reaction: CaMgSi2O6 + MgS = CaS + Mg2Si2O6. A large bulk compositional field exists where diopside and oldhamite are simultaneously stable. The existence of oldhamite does not preclude the stability of diopside. Phase diagram topology demonstrates that bulk compositions lying in the ENSTATITE-OLDHAMITE field and ENSTATITE-OLDHAMITE-ALABANDITE field have enstatite CaO contents nearly identical to that of enstatite in equilibrium with diopside alone. This explains the high enstatite CaO contents of all EEC's that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET90102. EET90102 has typical EL6 texture and contains the assemblage: enstatite, diopside, albite, kamacite, troilite, sinoite, and graphite. Trace quantities of alabandite, oldhamite and daubreelite are also present. Diopside is stable in EET90102 because its bulk composition lies within either the ENSTATITE-DIOPSIDE-OLDHAMITE-ALABANDITE or DIOPSIDE-ALABANDITE-ENSTATITE stability fields. In contrast, all other EEC's analyzed to date have bulk compositions lying in the ENSTATITE-OLDHAMITE-ALABANDITE stability field. The discovery of diopside in EET90102 helps confirm the predictions of the phase equilibrium analysis. EET90102 experienced a high temperature metamorphic equilibration from which it was quenched. The enstatite-diopside, CaS in alabandite and Fe in alabandite geothermometers yield temperatures of last equilibration of 900 oC. The absence of daubreelite and schreibersite along with high troilite Cr contents and high kamacite P contents confirm a high T metamorphic quench. EET90102 experienced a somewhat different cooling history, and has a slightly different bulk composition, than all other EECs studied to date; however, the close mineralogic, petrologic and textural similarities between EET90102 and nominal EL6 chondrites signify that it should be classified as a diopside- and sinoite-bearing EL6 chondrites. Assuming that the aubrites formed from an E chondrite-like protolith, a source rock similar to that of a diopside-bearing EEC offers a clear advantage for aubrite formation. Melting of a diopside-saturated EEC protolith would not require conversion of CaS to diopside in order to become diopside-saturated upon cooling.

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