NanoSIMS measurements of trace elements at the micron scale interface between zircon and silicate glass

Abstract

The partition coefficient of zircon/melt (D^zircon/melt) is a crucial parameter to obtain the chemical compositions of the parent melts from which the zircons crystallized, especially when we have no rock record and only relict zircons left (such as Hadean Earth, lunar breccia, and regolithic martian meteorites). However, the partition coefficients in the literature exhibit a large range, up to several orders of magnitude. A main cause for this discrepancy is heterogeneity of zircon's chemical composition at the submicron to micron scale, which requires an analytical method with a very high spatial resolution. Here we report trace element variations of P, Ti, Y, Ce, Sm, Ho and Lu across the interface between synthetic zircon and co-existing silicate melt measured at the submicron to micron scale with a NanoSIMS. The trace element composition profiles were calculated using SIMS image processing software to improve the spatial resolution close to the probe size scale (∼800 nm, 80 pA O⁻). The analytical precision varied from 5 to 30% (1SD) depending on the concentrations of the elements. The calculated partition coefficients show relatively small variations (e.g., Ce: 0.18–0.23, Ho: 9.2–9.9, Lu: 16–18) compared to those reported in the literature. Our study suggests that the large variation of zircon/melt partition coefficients reported in the literature may result from trace element variations in zircon zonation instead of reflecting two-phase equilibrium partitioning. Thus, zircon/melt partition coefficients must be measured by the high spatial resolution method. The general methodology could be applied to the determination of elemental distribution in other zoned minerals.