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

• Published in: Meteoritics & Planetary Science
• Main Author: FOGEL, ROBERTA.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1997
• Subjects: Antarctic; Elephant Moraine; The Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1111/j.1945-5100.1997.tb01302.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1945-5100.1997.tb01302.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.1997.tb01302.x

Abstract— Enstatite is the primary silicate phase of equilibrated enstatite chondrites (EECs). 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 EECs should commonly be found. When S fugacity is sufficiently high ( e.g. , Fe‐FeS buffer), low O 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: CaMgSi 2 O 6 + MgS = CaS + Mg 2 Si 2 O 6 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 EECs that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET 90102. Elephant Moraine 90102 has a 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 EET 90102 because its bulk composition lies within either the enstatite‐diopside‐oldhamite‐alabandite or diopside‐alabandite‐enstatite stability fields. In contrast, all other EECs analyzed to date have bulk compositions lying in the enstatite‐oldhamite‐alabandite stability field. The discovery of diopside in EET 90102 helps confirm the ...