Intellectual Merit: Igneous rocks formed by the accumulation of minerals during fractional crystallization (cumulates) are widely distributed, host economic mineral resources, and provide a record of the physical and chemical processes in magma chambers. Cumulates record chemical and textural evidence of both cumulus (those that build the crystal pile) and postcumulus (those that modify the crystal pile) processes. The proposed study will use in situ trace-element analyses by laser ablation ICP-MS (LA-ICPMS) to document the importance of postcumulus processes and analyses of the mineral apatite to understand the evolution of volatile components in these systems. The research will be conducted on cumulates formed from basaltic liquids and will contrast rocks from the Stillwater complex, Montana that crystallized from a relatively dry liquid with those from the Rymmen and Eriksberg gabbros, Sweden that crystallized from a wet liquid. Preliminary studies of oikocrysts of clinopyroxene from the Stillwater and amphibole from the Eriksberg reveal as much as three orders of magnitude variation in trace elements. By mapping the compositional variation in oikocrysts the compositional evolution and spatial distribution of the interstitial liquid can be documented. The halogen content of interstitial apatite has proven to be valuable for understanding the late-stage evolution and exsolution of volatiles in cumulates. LA-ICPMS analyses of apatite will provide a means of relating the trace-element evolution of these systems to their volatile evolution.
Broader Impacts: This work will provide a fundamental understanding of the processes that operate in a partially molten rock. Knowledge of these processes is essential for understanding the genesis of important ore resources that host precious (platinum and palladium) and base metal (especially chromium and nickel). Understanding these processes is also central to interpreting the chemistry of these rocks and their constituent minerals. Given that nearly all of our samples from the Moon and Mars are similar to the rocks to be studied, the work is likely to advance our understanding of these bodies as well. The work will also help to establish an empirical understanding of how trace elements are partitioned between crystallizing magmas and growing crystals over an extended growth history for a single crystal.
