James Brenan
From
In residence at
Earth sciences institute of Orleans (ISTO) - CNRS, BRGM, OSUC / University of Orléans - FR
Host scientist
Kenneth Koga
PROJECT
Experiments and observations bearing on the origin of highly-fractionated granites and their critical metal endowment
Mineral deposits of some of the critical metals essential to society are associated with the end stages of crystallization of the felsic magmas produced by melting of Earth’s sedimentary crust. A unique attribute of these felsic magmatic systems is their extreme level of chemical fractionation. Key to determining how these deposits form, and where to find new ones, lies in understanding the processes by which such high levels of element enrichment can be achieved. To address this issue, here we propose to conduct laboratory experiments at controlled pressure, temperature and oxygen fugacity to simulate the crystallization process, in which felsic melts are segregated from their crystalline residue, and their compositions measured for major and trace elements. We will employ methods of melt “capture” previously applied to basaltic systems to assess the composition of the small melt fractions produced at the end stages of crystallization. We will incorporate rhenium sponge into experiments, which is expected to resist compaction and provide a migration pathway into available pore space for melt collection and subsequent microbeam analysis. Experiments will involve a natural starting composition comprised of “unfractionated” granodiorite from the peraluminous South Mountain batholith (Nova Scotia, Canada), with or without additional fluxing components. Results will be used to directly test crystallization and melting models, which are subject to uncertainties of important input parameters, such as mineral-melt partition coefficients, the variation in melt fraction with temperature, and the volatile element evolution of the melt. Such information would serve to establish the geochemical linkages between highly fractionated granites, their less evolved counterparts, and potential source rocks.