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A petrological and laser-ablation inductively coupled plasma mass spectrometry study of some volatile elements in unbrecciated eucrites


Polished thin sections of ten eucrites, meteorites originating from the asteroid Vesta, were examined using polarized light microscopy, photomicrograph mapping, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) to investigate their compositions, with particular emphasis on elements known to be volatile at planetary formation conditions. The samples were assessed for pristinity and grouped into impact melts, granular, and subophitic textures. LA-ICP-MS analysis was performed on selected samples for individual pyroxene and plagioclase grains – the main silicate phases - as well as fusion crust, to measure major and trace element abundances, with an emphasis on moderately volatile elements (MVE’s) such as Cs, Zn, K, Rb, and Pb. Incompatible trace element abundances are in generally good agreement between samples, and both plagioclase and pyroxene grains show depletions in the MVE’s. Modal reconstructions were done using plagioclase and pyroxene and compared to bulk rock data. They fairly accurately recreate the bulk rock, besides the most incompatible elements, which must exist in minor accessory minerals known to occur in eucrites (e.g., apatite, zircon, baddeleyite). Fusion crust measurements, along with ALHA 81001 (a fine-grained impact melt,) are almost identical to the bulk rock data, confirming their usefulness as a proxy for bulk rock composition. Volatile abundances were assessed using Rb/Ba, Rb/Sr, and Zn/Fe ratios, which compare an MVE to a refractory element. These show eucrites having similar levels of volatile depletion to the Moon. Finally, normalized elemental abundances were plotted versus the element’s volatility, in which volatile elements fall along a downward trend, with the slope indicating the level of volatiles present. From this perspective, eucrite volatile abundances fall in between those of the Earth and the Moon.

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