New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate
M. Chernonozhkin, Seann J. McKibbin, Steven Goderis, Stijn J.M.Van Malderen, Philippe Claeys, Frank Vanhaecke
Chemical Geology
Available online 20 December 2020
“Pallasites are stony‑iron meteorites consisting mainly of olivine and Fe-Ni metal with a formation history that remains widely debated. Despite their simple mineralogy, relatively limited data are available on their geochemistry and the lateral elemental distribution in individual pallasite olivine crystals. In this work, laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) was used for the elemental analysis of both pallasite olivine and phosphate phases, including 2D trace element mapping of olivine crystals. While the results obtained are in good agreement with literature values, important differences are observed for Al and Ni concentrations compared with bulk analytical methods. The element distribution maps reveal complex zoning in pallasite olivine, which can be explained based on i) diffusion gradients formed during olivine cooling, ii) the crystal chemistry of element substitution due to charge-balancing, and iii) inherited features of olivine before the metal-olivine mixing. Oscillatory zoning in an olivine crystal from Imilac provides strong evidence for the olivine not being a restite of partial melting, but rather having crystallized from a melt. Concentrations of Cr and Al are correlated both within single olivine crystals and between olivine crystals of different pallasites, forming a 1:1 linear trend. This likely results from a spinel-type charge-balancing substitution mechanism in pallasite olivine. Additionally, principle component analysis of laterally resolved multi-element concentration data of pallasite olivine provides an independent measure of the genetic relationships between the different main group pallasites within their parent body (−ies). Rare earth element (REE) contents of pallasite phosphate grains range from concentrations typically measured for chondritic or primitive achondrite-like phosphates to more (light) REE-depleted signatures, revealing the overall primitive nature of the melts from which the phosphate minerals crystallized. Phosphate in pallasites may thus have formed through the consumption of apatites during pro-grade metamorphism, melting, and melt extraction. Combined, the trace element signatures of olivine and phosphate in pallasites of the main group (PMG) suggest that these meteorites are common products of planetary processes on chondritic or primitive achondrite-like precursor bodies, with the different subgroups highlighting distinct formation and evolution histories.”
