Orthopyroxene as a recorder of primitive achondrite petrogenesis: Major-, minor-, and trace-element systematics of orthopyroxene in Lodran

Details Orthopyroxene as a recorder of primitive achondrite petrogenesis: Major-, minor-, and trace-element systematics of orthopyroxene in Lodran Orthopyroxene as a recorder of primitive achondrite petrogenesis: Major-, minor-, and trace-element systematics of orthopyroxene in Lodran

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.517p&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p.517

Computer Science

Achondrites, Meteoritic Composition, Petrology, Pyroxenes, Trace Elements, Abundance, Mass Spectroscopy, Melts (Crystal Growth), Rims

Scientific paper

Considerable attention has been paid recently to the primitive achondrites because they may form a link between chondrites and more differentiated achondrite meteorites. A recent paper by Miyamoto and Takeda addresses the thermal history of lodranites Yamato 74357 and MAC 88177 as inferred from chemical zoning of pyroxene and olivine determined by electron microprobe analyses. Their results suggested that interstitial melt was present and then extracted. We have taken the analysis of Lodran-type meteorites one step further by incorporating the techniques of Electromagnetic Pulse/Wavelength Dispersive Spectroscopy (EMP/WDS) compositional imaging and scanning ion mass spectroscopy (SIMS) analysis. Orthopyroxene in Lodran is strongly zoned in CaO, Al2O3, TiO2, and Cr2O3 within the last 10-30 microns from the grain boundaries. The rims are reversely zoned in Mg-Fe, exhibiting Mg enrichment, and compositions change from a fairly uniform Wo3En94 within the grains to Wo1En96 at the rims. CaO drops from 1.6 to 0.6 wt% and Al2O3, TiO2, and Cr2O3 exhibit similar depletions. MnO is fairly uniform throughout the grains at around 0.5 wt%. Olivine is also reversely zoned with respect to not only grain boundaries but also to fractures within the grains, giving many olivine grains a complex, patchy zoning pattern. Some of the core-rim trace-element systematics for orthopyroxene are illustrated. Because of the rather narrow zoned rims in Lodran orthopyroxene and the low trace-element abundances, it is difficult to clearly resolve the trace-element systematics. Nevertheless it is evident that the cores are enriched in the incompatible trace elements Ce, Nd, Dy, Er, Yb, Y, and Zr relative to the rims.

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