Minor element zoning and trace element geochemistry of pallasites

Abstract

I report here on an ion probe study of minor element spatial distributions and trace element concentrations in six pallasites. Pallasite olivines exhibit ubiquitous minor element zoning that is independent of grain size, morphology, and adjacent phases. Ca, Cr, Ti, V, and Ni concentrations decrease from center to rim by factors of up to 10, while Mn is generally unzoned or increases slightly at the very edge of some olivine grains. The maximum concentrations of these elements at the center of olivine vary from grain to grain within the same meteorite and among the pallasites studied. These zoning profiles are consistent with thermal diffusion during rapid cooling. The inferred cooling rates at high temperature regimes are orders of magnitude faster than the low-temperature metallographic cooling rates (~0.5 to 2 deg degrees C/Ma). This suggests that pallasites, like mesosiderites, have experienced rather complicated thermal histories, i.e., cooling rapidly at high temperatures and slowly at low temperatures. Pallasite olivines are essentially free of REEs. However, the phosphates display a wide range of REE abundances (0.001 to 100 x CI) with distinct patterns. REEs are generally homogeneous within a given grain but vary significantly from grain to grain by a factor of up to 100. Albin and Imilac whitlockite are highly enriched in HREEs (~50 x CI) but are relatively depleted in LREEs (~0.1 to 1 x CI). Eagle Station whitlockite has a very unusual REE pattern: flat LREEs at a 0.1 x CI level, a large positive Eu anomaly, and a sharp increase from Gd (0.1 x CI) to Lu (70 x CI). Eagle Station stanfieldite has a similar REE pattern to that of whitlockite but with much lower REEs by a factor of 10 to 100. Springwater farringtonite has relatively low REE concentrations (0.001 to 1 x CI) with a highly fractionated HREE-enriched pattern (CI-normalized Lu/La ~100). Postulating any igneous processes that could have fractionated REEs in these phosphates is difficult. Possibly, phosphates were incorporated into pallasites during mixing of olivine and IIIAB-like molten Fe. These phosphates preserve characteristics of a previous history. Pallasites have not necessarily formed at the mantle-core boundary of their parent bodies. The pallasite thermal histories suggest that pallasites may have formed at a shallow depth and were subsequently buried deep under a regolith blanket.