Willy: A prize noble Ur-Fremdling—Its history and implications for the formation of Fremdlinge and CAI

Abstract

A detailed mineralogic and chemical study of Willy, a very large (150 μm diameter) Fremdling from the Allende CAI 5241, was performed and compared to other Fremdlinge from Allende CAI's 5241 and TS-34 in an attempt to understand the nature and mode of formation of these exotic and complex objects. Willy is composed primarily of V-rich magnetite, V-rich fassaite, and Ni-Fe metal containing Co and Pt. Minor phases include an Fe-Ni-sulfide, V-rich spinel, scheelite (the first reported occurrence in a meteorite), and Cl-apatite. Phases found in trace amounts include nuggets of Os-Ru-Re metal, molybdenite, an unidentified Fe-, Mg-molybdate, and diopside. The Fremdling is concentrically zoned and contains a complex porous core of magnetite, metal, sulfide, scheelite, and other minor phases surrounded by a compact mantle of magnetite with minor apatite. The mantle is surrounded by a dense rim composed of fassaite with minor spinel that appears to be a typical occurrence around oxide-containing Fremdlinge. At the boundary between the fassaite-rim and the magnetite-mantle of Willy is a thin zone (<20 μm wide) of an apparent reaction assemblage consisting of V-rich MgAl_2O_4, FeAl_2O_4, and a third V-rich spinel, possibly FeV_2O_4, in intimate intergrowth. From the observed chemistry and texture, a multistage sequence of formation of Willy, possibly occurring in the solar nebula and involving major changes in T and fO_2, can be deduced. The first phases that may have formed in the interior are magnetite and an Fe or Ca tungstate. Refractory metal nuggets and sulfide were introduced after this stage followed by two stages of Ni-Fe formation during which Pt was dissolved in the metal. This was followed by formation of the magnetite mantle, introduction of apatite and possible alteration of ferberite to scheelite. Finally, the V-rich fassaite rim formed, accompanied or followed by reaction forming the complex Fe-, V-rich spinels. All of these steps preceded introduction of the Fremdling into the CAI precursor which in turn appears to have occurred prior to formation of spinel and the major silicate phases in the CAI. The concentric mineral zoning and texture of Willy may indicate that it is one of the few Fremdlinge that was not substantially recrystallized after having been captured in the CAI. It thus may represent the precursor material for many of the other Fremdlinge, since numerous Fremdlinge studied exhibit many of the mineral and chemical features observed in Willy. The co-existence of magnetite, sulfide, and Ni-Fe metal in the interior of Willy and the co-existence of metal, hercynite spinel and magnetite at the rimmantle boundary suggests that the maximum temperature at which Willy could have existed for long periods of time in its present state was about 500–600°C. Similarly, the existence of highly heterogeneous V-, Ti-rich fassaite rims around Willy and other Fremdlinge implies rapid cooling rates for the CAI's.

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