Complex Al and P zoning in pallasite olivine: Constraints on high-T history

Pallasites are mixtures of olivine and Fe-Ni metal,possibly formed near the core-mantle boundary of adifferentiated asteroid [1]. The distribution of trace elementsin pallasite olivine appears to reflect diffusion profiles [2]although complex structure has also been reported [3]. It hasrecently been shown experimentally [4] and in various naturalsamples [5] that Al and P diffuse very slowly in olivine andthese elements can preserve early distributions which areresistant to later thermal modification.To search for evidence of the early history of pallasiteolivine, we have used Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) to produce traceelementmaps of olivine from two pallasites: Brahin (withfragmental olivine) and Brenham (rounded olivine).Heterogeneous distributions are found over scales of ~100microns for Al and elements likely to associate with it incoupled substitutions. P is also heterogeneously distributed butis negatively correlated with or unrelated to Al. In Brahin,these distributions bear no relationship to the olivinemorphology; for Brenham, Al concentrations decrease at grainmargins but the interiors have complex distributions.Heterogeneous distributions of Al and P in pallasiteolivine provides some constraints on its residence time in ahigh-T environment. Taking the diffusion parameters for Si[6] as a proxy for the diffusion behaviour of these elements(since they reside in the same crystallographic site), at 1650 K(~OPX out for a fertile peridotite composition; [7]) thecharacteristic timescale for diffusion over 100 microns is ~1Ma. This represents an upper limit on the time between theestablishment of trace element systematics in olivine andformation of pallasites by mixing with metal.[1] Scott, E.R.D. (1977) GCA 41, 693. [2] Miyamoto, M.(1997) JGR 102, 21613. [3] Tomiyama T. & Huss G.R. (2006)LPSC 37, 2132. [4] Spandler C. & O’Neill H.St.C. (2010)CM&P 159, 791. [5] Milman-Barris M. et al. (2008) CM&P155, 739. [6] Dohmen R. et al. (2002) GRL 29, 2030.[7] Walter M. J. (2003) in Treatise on Geochemistry 2.08,363–394.