Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27s.289s&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 289
Computer Science
2
Scientific paper
Because the fassaite/liquid distribution coefficient for Ti^3+, D^fas/L(sub)Ti^3+, is ~3 and D^fas/L(sub)Ti^4+ is ~1, Ti^tot (Ti^3+ + Ti^4+) contents and Ti^3+/Ti^tot ratios in fassaite should decrease from core to rim in fassaite crystals in Type B CAIs if the residual liquid did not maintain equilibrium with the reducing, solar nebular gas. As reported previously (Simon and Grossman, 1991), such trends have been observed in fassaite crystals in Allende Type B CAIs, as have sharp increases (spikes) in Ti^tot, Ti^3+/Ti^tot, and V near the rims of the same crystals in TS34 (Type B1). We attributed those spikes to changes in liquid composition caused by incoming of anorthite, and to reaction of the Ti^4+ component in fassaite with anorthite (Beckett, 1986). We undertook the present study to test this theory. The Ti^3+-rich fassaite overgrowths are discernible in backscattered electron images because of their higher Ti^tot relative to their Ti^tot-poor fassaite substrates. We found such fassaite in two more CAIs, TS23, and TS33. The boundaries between Ti^3+-poor and Ti^3+-rich fassaite are sharp and irregular, scalloped or embayed. We conducted electron probe traverses (~3 micrometers between analysis spots) and coarser ion probe traverses across the spikes. If the spikes were caused by anorthite crystallization we would expect to see, in the fassaite, sharp increases in elements that are compatible in fassaite and incompatible in anorthite (e.g., Ti^tot, V, Sc). The results do not support the previous conclusions of Simon and Grossman (1991): we do not see evidence of the incoming of anorthite. Calculations that take into account the different fassaite/liquid Ds for Ti^3+ and Ti^4+ show little or no increase in the Ti^tot contents of the liquids in TS33 and TS34. We observe small increases or continued decreases in Sc, and ion probe data across nine spikes revealing none of the changes that would be consistent with anorthite crystallization, e.g., increased Sm/Eu, sharp increases in trivalent REE content. Because the only elements that consistently show spikes, Ti and V, are also the only elements in fassaite with multiple oxidation states at the range of Tfo(sub)2 conditions under which refractory inclusions crystallized (Beckett, 1986), we now believe that the spikes reflect redox reactions that increased the Ti^3+ and V^3+ contents of the residual liquids. Gradual decreases in Ti^3+/Ti^tot from core to rim in fassaite in TS33 and TS34 and before the spikes in TS23 indicate that the residual liquids did not re-equilibrate with the reducing, solar nebular gas throughout most of the interval of fassaite crystallization. Late reequilibration, perhaps by entry of the solar gas via cracks in the cooling CAI, would increase the Ti^3+/Ti^tot of the liquid. The Ti^3+-poor rims of fassaite crystals would then be out of equilibrium with the liquid. Resorption of the rims may have occurred, followed by deposition of relatively Ti^3+-rich fassaite. This is consistent with the observations of sharp, irregular low-Ti/high-Ti boundaries. Of the V species presumed to be present in the CAIs, V^3+ should be favored in fassaite over V^2+, based on its ionic radius. Increasing the V^3+ content of the liquid is a problem, however, because this requires oxidation of V^2+ when Ti is being reduced. Perhaps, prior to reequilibration, the internal fO(sub)2 of the inclusions became high enough to stabilize V^4+, which was reduced to V%3+. REFERENCES: Beckett J.R. (1986) Ph.D. Thesis, University of Chicago, 373 pp. Simon S.B. and Grossman L. (1991) Meteoritics 26, 395.
Davis Aileen M.
Grossman Lawrence
Simon Steven B.
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