Crystal chemical effects on the partitioning of trace elements between mineral and melt: An experimental study of melilite with applications to refractory inclusions from carbonaceous chondrites

Abstract

The partitioning behavior of the trace elements Be, Sc, Ba, La, Ce, and Tm between melilite and liquid has been determined using stepwise integration of a Rayleigh fractionation equation for ion microprobe analyses of synthetic zoned melilite crystals. Distribution coefficients between melilite and liquid (D^(Mel/L)_i) were determined over the entire range of melilite + spinel crystallization for one bulk composition corresponding to that of an average Type B inclusion from the Allende C3V carbonaceous chondrite. Beryllium is incompatible in gehlenitic meliliters (e.g.,D^(Mel/L)B_e = 0.5 for X_(ak) = 0.3) but compatible in åkermanitic melilites (e.g., D^(Mel/L)B_e = 1.9 for X_(Ak) = 0.75). Barium (D^(Mel/L)B_a = 0.04-0.05), Sc (D^(Mel/L)S_c = 0.01-0.02), La (D^(Mel/L)L_α ≤ 0.07-0.29), Ce (D^(Mel/L)C_e ≤ 0.05-0.21), and Tm (D^(Mel/L)T_m ≤ 0.03-0.14) are all incompatible in melilites in the range Ak25-Ak75. Variations in D^(Mel/L)i as a function of X_(Ak) are explicable in terms of a simple thermodynamic treatment of trace element partitioning based on crystal chemical considerations. Characteristic trace element zoning patterns are predicted for early crystallizing melilite from meliliterich Type B inclusions. For example, concentrations of the REEs in melilite should continually decrease with increasing degrees of crystallization, opposite the behavior normally expected of an incompatible element. Concentrations of Be should rise with increasing degrees of crystallization even when the element is compatible, again opposite the behavior normally expected of compatible elements. In general, zoning patterns of trace elements in melilite from Type B inclusions are consistent with those predicted for fractional crystallization from a melt in a closed system. Trace element zoning patterns in meteoritic melilite crystals constrain the origin and thermal histories of Ca-, Al-rich inclusions from carbonaceous chondrites.

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