Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.579s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 579
Other
1
Hibonite, Inclusions, Refractory, Meteorites, Murchison
Scientific paper
Spinel-hibonite-rich (SH) spherules are a major type of refractory inclusion in the Murchison (CM2) carbonaceous chondrite. They typically consist of radially oriented, intersecting hibonite laths 5-10 micrometers wide and 10-30 micrometers long, partially enclosed in Mg-Al spinel +/- perovskite (pv), and are thought to have been at least partially molten [e.g.,1]. Among a suite of ~ 20 SH spherule fragments recovered by freeze-thaw disaggregation of Murchison, we have discovered several in which the hibonite exhibits wider ranges in composition than previously observed within single inclusions and has complex zoning patterns not consistent with crystallization from a melt in a single-stage cooling event. For example, in H2-3, a spherule fragment with ~ 1 % pv, most of which is enclosed in spinel, hibonite has from 0.8 to 7.4 wt % TiO2, and one crystal with ~ 2 wt % TiO2 has thin, linear zones, mostly parallel to cleavage, with ~ 7 wt % TiO2. In H2-18, an SH inclusion with a trace of pv, small, contiguous grains of hibonite with ~1.3 wt % TiO2 are completely rimmed with relatively Ti-rich (~ 6.5 wt % TiO2) hibonite, so that the latter appears to define angular, Ti-poor islands in backscattered electron images (BEI). This inclusion also contains a hibonite crystal that is nearly TiO2-free (< 0.3 wt %) in which Mg (up to 1 wt % MgO) is largely balanced by Si (up to 1.4 wt % SiO2) instead of Ti. This grain has an ~ 1 micrometers-wide rim of Ti-bearing (~ 3 wt % TiO2) hibonite, which is possibly related to the other Ti-rich overgrowths. We also found several SH spherule fragments in which the hibonite has patchy zoning, with, for example, regions with ~ 7 and 0.7 wt % TiO2 separated by diffuse and irregular contacts within a single crystal. Aureoles of Ti-rich hibonite around perovskite inclusions are not apparent in BEIs. Virtually Ti-free hibonite is clearly not in equilibrium with overgrowths of Ti-rich hibonite, and in these inclusions the Ti-rich material is texturally late relative to the Ti-poor hibonite. This relationship is not expected from zoning trends observed in SH spherules such as H2-5 and B6 [2], which contain hibonite laths which appear to have nucleated in sprays or on the edges of the spherules. In these grains, TiO2 contents decrease with increasing distance from the nucleation points, consistent with experiments on Al2O3-rich, SiO2-bearing liquids [3], which give crystal/liquid distribution coefficients for Ti in hibonite > 1. Unless early hibonite crystallized metastably and was followed by high degrees of spinel fractionation, SH melts should not yield the late, Ti-rich hibonite observed in the overgrowths described here, nor should early hibonite be Ti-poor relative to late hibonite. The Ti-poor hibonite found in the present samples probably represents an earlier generation of hibonite. In the Si-rich hibonite in H2-18, dominance of Si + Mg over Ti + Mg substitution, despite DSi << DTi [3], indicates that it formed in an environment in which very little Ti was available. Like the Ti-poor hibonite in the other inclusions considered here, it either had Ti-rich hibonite deposited on it or reacted with Ti-bearing liquid or vapor, or possibly with perovskite. Further investigation, such as determination of systematic variations of hibonite Ti content with distance from perovskite inclusions, or with inclusion bulk composition, may help distinguish between these possibilities. References: [1] MacPherson G. J. et al. (1983) GCA, 47, 823-839. C312. [2] Simon S. B. et al. (1994) GCA, 58, 1937-1949. [3] Kennedy A. K. et al. (1994) Chem. Geol., 117, 379-390. _
Grossman Lawrence
Simon Steven B.
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