MINERAL CHEMISTRY OF OLIVINE FROM MARTIAN, VESTAN AND CHONDRITIC METEORITES – IMPLICATIONS FOR THE EVOLUTION OF EXTRA-TERRESTRIAL ROCKY BODIES

Although olivine is an important constituent of many mafic and ultramafic rocks, its trace element composition in terrestrial and extra-terrestrial systems remains understudied. Here we present the preliminary results of a study that investigates the trace element chemistry of olivine in a series of meteorites, i.e., the Martian meteorites ALHA 77005 and NWA 7397, the Vestan meteorite NWA 5480 and the chondrite Ausson. The Martian meteorites were previously suggested to be lherzolitic shergottites with poikilitic textures implying that these rocks underwent partial melting. The Vestan meteorite is classified as a harzburgitic diogenite that underwent multiple deformation events. Ausson is an ordinary chondrite.

Petrographic observations show that all samples are composed of olivine, orthopyroxene, clinopyroxene with locally abundant plagioclase and chromium-rich spinel; rare pyrrhotite was found in NWA 7397. Electron microprobe analysis of olivine show that the forsterite content (Fo = molar [Mg/(Mg+Fe)]*100) is generally between Fo = 70-73, although grains from the Martian meteorite NWA 7397 have lower values of Fo ~60. Nickel contents are <100 ppm Ni in olivine from Ausson and Vesta (NWA 5480), whereas the Martian olivines contain between 200 and 600 ppm Ni. It is noted that Ni does not correlate with Fo in the presented data. The Manganese content displays a distinct negative correlation with Fo ranging from 4500 to 7500 ppm Mn. Aluminum contents are largely independent of Fo and between <100-500 ppm Al; grains from NWA 7397 have notably higher values of ~1000-1500 ppm Al. Laser ablation ICP-MS analysis of olivine from ALHA 77005 and Ausson yield similar trace element patterns when normalized to the composition of the Bulk Silicate Earth (BSE). All grains display concentrations of Ti, Ga, V, Sc, Ge, Mn, Cr, Zn similar to the BSE values, with significant negative anomalies in Cu, and Cr and positive peaks in Mn. The implications of these observations will be discussed in this presentation.