Intellectual Merit. Significant variations in stable isotope ratios of several non-traditional stable isotope (NTSI) systems have recently been observed in igneous rocks. The origin of these variations remains largely unknown despite the critical observation that the variations correlate with extent of differentiation. At present, it is not possible to evaluate whether traditional processes such as fractional crystallization may account for the observations, because the fractionation factors among magmatic phases have not been determined. The exciting new discovery that large isotopic fractionations occur in melts held within a temperature gradient offers an alternative possible explanation for the observed isotopic variations, but to fully assess this mechanism requires further experimental investigation. The fractionation by thermal diffusion appears to be so large that it may provide the basis for a unique tool to discern the role of thermal-gradient-driven processes for magmatic differentiation. This project represents a synergistic collaboration involving four interrelated and complementary studies: 1) Characterize NTSI fractionation in laboratory silicate Soret (fully molten) experiments. Soret experiments will use both natural compositions and selected simple systems; 2) Characterize major and trace element behavior and NTSI fractionation in thermal migration (partially molten) experiments; these will focus on basalt to rhyolite bulk compositions and also determine thermal diffusion fractionations for Mg, Si and Fe; 3) Use molecular dynamics simulations of isotope fractionation within a temperature gradient in the MgO-SiO2 system to investigate the physical basis for the effects of thermal diffusion on NTSI fractionation; 4) Determine equilibrium fractionation factors between melts, vapor and mineral phases for Mg, Si and Fe using the three-isotope method. The proposed science plan brings together four groups of researchers with complementary expertise. The team connects diverse areas of petrology/geochemistry research from field petrology to experimental petrology to molecular dynamics simulation. The project will provide constraints on a first-order problem in igneous petrology, the origin of NTSI fractionations in igneous rocks. It will provide further understanding of the fundamental process of thermal diffusion and its strong mass dependence and improve the NTSI tool for discerning the role of temperature gradients in magma differentiation.
Broader Impacts: The project will support involvement of three graduate students, a postdoc and early career faculty (UNLV) and a research scientist (UCD) in a complex project utilizing a broad range of conceptual, analytical, experimental and simulation tools. Group meetings and video conferencing will foster the collaboration and involve the students and postdoc in all aspects of the study. Each PI expects to incorporate the results of this study into regular teaching of petrology, geochemistry and numerical modeling.
This project involved laboratory experiments designed to understand isotopic fractionations in high temperature magmatic systems with the goal of identifying the process(es) by which silicic igneous rocks form. This has broad implications for why Earth has a silicic continental crust and, as an offshoot discovery finding of this project, possible implications for understanding of how/why silicic volcanoes erupt. Intellectual merit impacts include determination of measured changes in isotope ratio as isotopes partition between two coexisting phases and determination of the extent of isotope fractionation in a temperature gradient. Measurements of isotope ratios in experiments reveal an unrealized behavior of magmatic water in a temperature gradient facilitating rapid reaction and transport. This work may even have implications for using the isotopes of water (H, O) for understanding climate changes in the Earth's past. This work also provides a new model for how differentiation of magmas to silicic compositions occurs. In addition, a new model for how the isotopic fractionation in melts in a temperature gradient occurs was proposed. Broader impacts include support of undergraduate student research, a graduate student (from the University of Michigan) and a post doctoral fellow as well as visitations and presentations at several elementary school science days.
