Complex Zoning in Fassaite: a Recorder of Growth and Resorption in Type B1 Cais

Introduction: Type B1 CAIs are a major type of refractory inclusion in CV3 carbonaceous chondrites. An intriguing feature of fassaite in many Type B1 inclusions is complex zoning within single crystals. Seen in backscattered electron images (BEIs), the zoning reflects irregular, sometimes patchy, distributions of TiO2 and correlated and anticorrelated oxides; this zoning is not synonymous with sector zoning. Several interpretations of complex zoning have been proposed. Paque [1] suggested that the patchy core of a grain she studied was relict and that the texture resulted from metamorphic recrystallization or metasomatism. Kennedy et al. [2], however, found no isotopic or trace element contrasts between the core and the normally zoned rim of the grain and concluded that the patchy core was not relict. These authors later suggested [3] that the patches represent trapped melt pockets related to the incorporation of spinel by pyroxene, but patchy zoning can be found in spinel-free fassaite. In his study of Allende Type B1 CAI 5241, Meeker [4] suggested that patchy fassaite formed from assemblages of small crystals + melt that cooled quickly. Another unsupported explanation, also presented in [4], is that patchy fassaite represents annealed cracks through which fluid could have flowed. Another possibility is partial melting of inclusions, leading to leaking of melt into relict fassaite grains. We have undertaken a study via SEM, EMP and IMP of patchy fassaite in several Allende Type B1 CAIs because a better understanding of this feature could yield important insights into the crystallization histories of Type B CAIs. Observations: Normally-zoned fassaite has Ti, V, and Sc contents that decrease and Mg contents that increase smoothly with distance from core to rim [5]. For this study we have focussed on complexly zoned fassaite grains in TS23, TS34, and 5241 that have relatively low-Ti cores enclosed in optically continuous fassaite of higher Ti content. The contacts between lowand high-Ti material, seen in BEIs, are sharp whether they are straight or irregular. TS23 PF1 is an anhedral grain 1.1 mm long and up to 1.0 mm wide. It encloses many euhedral, 10-50 μm spinel grains and is dominated by an inner zone with 5.7 wt % TiO2 (all Ti as TiO2) enclosed in a normally-zoned, ~50100 μm-wide outer zone whose TiO2 contents decrease from ~6-7 wt % at the contact with the inner zone to 3-4 wt % at the edge of the grain. Both zones also enclose several angular, ~50 μm, Ti-rich (6.1-7.1 wt %) patches. Results show that the relatively Ti-rich patches in this grain neither have the same composition nor plot along the same Mg-Ti trend as either the relatively Ti-poor host or the Ti-rich fassaite near the outside of the grain. Ion probe analysis of an interior Ti-rich patch shows that it is slightly richer in refractory elements than the host fassaite, whether they are compatible (Sc, Zr) or incompatible (REE) in fassaite. The Ti-rich patches near the outside of the grain appear to be related to the outer, Ti-rich layer, but Ti-rich patches in the interior of the grain are not, and must be from a different generation of melt. TS23 PF3 (Fig. 1) is 1 x 0.4 mm and has a two-zone core: one light (in electron albedo), with low TiO2 contents; and one dark, with lower TiO2 contents. The core and several nearby, irregularly-shaped islands, also with light and dark zones, are enclosed in relatively Ti-rich (~7 wt % TiO2) fassaite that is normally zoned, with Ti contents decreasing from the contact with the core to the edge of the crystal. Grains PF6 and PF9 in TS34 also have isolated Ti-poor regions enclosed in Ti-rich ones.