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Title:
Mbosi: an anomalous iron with unique silicate inclusions
Authors:
Olsen, Edward J.; Clayton, Robert N.; Mayeda, Toshiko K.; Davis, Andrew M.; Clarke, Roy S., Jr.; Wasson, John T.
Publication:
Meteoritics, vol. 31, pages 633-639.
Publication Date:
08/1996
Origin:
METIC
Bibliographic Code:
1996M&PS...31..633O

Abstract

The Mbosi iron meteorite contains millimeter size silicate inclusions. Mbosi is an ungrouped iron with a Ge/Ga ratio >10, an anomalous property shared with the five-member IIF iron group, the Eagle Station pallasites and four other ungrouped irons. Neither the IIF group nor the four other ungrouped irons are known to have silicate inclusions. Chips from three Mbosi inclusions were studied, but most of the work concentrated on a whole 3.1 mm circular inclusion. This inclusion consists of a mantle and a central core of different mineralogies. The mantle is partially devitrified quartz-normative glass, consisting of microscopic crystallites of two pyroxenes and plagioclase, which are crystalline enough to give an x-ray powder diffraction pattern but not coarse enough to permit analyses of individual minerals. The core consists of silica. The bulk composition does not match any known meteorite type, although there is a similarity in mode of occurrence to quartz-normative silicate inclusions in some IIE irons. Mbosi silicate appears to be unique. The bulk REE pattern of the mantle is flat at A7XC1; the core is depleted in REE but shows a small positive europium anomaly. The oxygen isotope composition of bulk silicate lies on a unit slope mixing line (parallel and close to the C3 mixing line) that includes the Eagle Station pallasites and the iron Bocaiuva (related to the IIF irons): all of these share the property of having Ge/Ga ratios >10. It is concluded that Mbosi silicate represents a silica-bearing source rock that was melted and injected into metal. Melting occurred early in the history of the parent body because the metal now shows a normal Widmanstatten structure with only minor distortion that was caused when the parent body broke up and released meteorites into interplanetary space. The cause of Ge/Ga ratios >10 in these irons is unknown. The fact that silicates in Mbosi, Bocaiuva (related to IIFs) and the Eagle Station trio of pallasites, all characterized by a Ge/Ga ratio >10, lie on a unit slope mixing line in the oxygen isotope diagram suggests that their origins are closely related. C3 chondrites appear to be likely precursors for silicates in Mbosi, Bocaiuva and the Eagle Station pallasites.

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