Zelda and Company: Petrogenesis of sulfide-rich Fremdlinge and constraints on solar nebula processes

Abstract

A detailed mineralogic and chemical study of Zelda, a gigantic (1mm diameter) Fremdling from the Allende CAI, Egg 6, was performed in an attempt to understand the mode of formation of sulfide-rich Fremdlinge and their relation to other types of Fremdlinge. Zelda is composed primarily of pentlandite, pyrrhotite and V-Cr-rich magnetite. Minor phases include NiFePt metal, molybdenite, whitiockite and refractory metal nuggets (RMN) of OsRu and PtIr. The refractory siderophile abundances in Zelda are about 20,000 to 70,000 times chondritic. NiFe grains are highly embayed and are surrounded by a mixture of pentlandite and PtIr RMN. PtIr RMN are found exclusively at the embayed boundaries of NiFe, while OsRu RMN are found distributed throughout the entire Fremdlinge. The texture and mineral and bulk chemistry of Zelda are consistent with closed system sulfidization of a metal-magnetite precursor, with the metal reacting to form a mixture of pentlandite and PtIr, and the magnetite reacting to form pyrrhotite. The evidence suggests that this sulfidization approached equilibrium at a temperature of 800–900°C. Examination of Egg 6 in the vicinity of Zelda indicates that the Fremdling underwent extensive reaction with its surroundings, losing perhaps 40% of its original mass. The reaction involved partial melting, breakdown of NiFe, magnetite and sulfide, formation of V-magnetite-rich veins, and diffusion of Fe, V and Cr into neighboring spinels. Compositional profiles measured in a large spinel near Zelda suggest a diffusion time of approximately 1 hour at a temperature of about 1150°C. Examination of other Fremdlinge indicates that the presence of V-fassaite rims around Fremdlinge helps to protect them from undergoing such types of reaction. The most likely formation of Fremdlinge appears to have involved relatively cold accretion of previously formed grains of magnetite, NiFe and RMN, followed by partial sintering and in some cases sulfidization before incorporation into proto-CAIs. Following incorporation, further sulfidization and reaction of some Fremdlinge with their surroundings occurred. The common coexistence of very high concentrations of refractory aderophiles and low-T metal-magnetite-sulfide assemblages, as well as the evidence that Fremdlinge were incorporated into proto-CAI as solid objects, underscore the complexity of early solar nebula processes and require local mixing of phases formed at considerably different T and fO_2. Many of the phase relations observed in Fremdlinge point to CAIs existing for reasonable periods of time at temperatures of about 850°C and cooler, but only for very short times at temperatures greater than 1000°C, although some of these phase assemblages may have been created by exsolution during lower temperature reequilibration.

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