Magma Differentiation Time Scales in Carbonatite -Silicate Systems

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Intellectual Merit. Phase separations between crystals, liquids, and vapor are the most common processes for magma diversification and also set the stage for how magmas solidify or erupt, yet the exact mechanisms of how this occurs remain poorly understood. The goal of this study is to constrain the origins of spatially associated carbonatite and silicate magmas and to quantify the time scales during which they interacted in a shallow magma system. Laacher See volcano is an ideal place to investigate these processes: it is a well studied example of a compositionally and thermally zoned magma chamber, and it erupted a remarkable suite of carbonatite - silicate xenoliths that provide insights into the crystal-dominated portions of the magma system. Because of its youthful age (eruption age 12,900 a BP), U-series geochronology can be applied which will provide unrivaled temporal resolution in the order of 10E3 -10E4 years. The proposed research will employ secondary ionization mass spectrometry (SIMS, ion microprobe) U-Th isotopic analysis of zircon from carbonatite - syenite xenoliths to determine their crystallization ages and their relation to the main phonolite magma. REE and oxygen isotopic compositions in zircon will also be used to trace crystal origins (e.g., formed in-situ or entrained xenocrysts). Finally, bulk sample U-Th isotope data will be used to model differentiation times for carbonatite and silicate magmas and to detect assimilation of older wall rocks. These data will contribute to understanding fluid-dynamic time scale estimates for magma differentiation. The results from Laacher See will ultimately contribute to an improved understanding of rates of magma transfer and storage, and mass and heat transport in crustal magma chambers.Broader Impact. Laacher See is a potentially still active volcanic system, located in a densely populated and industrialized region. This research will improve understanding of the behavior and hazards of such differentiated intraplate volcanic systems. PI Schmitt is the primary host of researchers that visit the NSF National Ion Microprobe Facility at UCLA for geochronological or stable isotope analyses. These visiting scientists and graduate students will directly benefit in their research projects from methodological advances related to this project. The international collaboration in this project (with Universities of Gottingen and Geneva) offers foreign students experience with universities in the US.