Intellectual Merit. The abundance and distribution of high field strength elements (HFSE) and rare earth elements (REE) in mafic and ultramafic rocks provide important clues to understanding the origin and evolution of igneous rocks. While the distribution of such trace elements between coexisting minerals and melt in many magmatic processes can be understood in terms of equilibrium fractionation, there are occasions where substantial chemical disequilibria between coexisting phases might provide unique and important insights into the time scales of geological processes. To better understand the various processes that give rise to disequilibrium distributions of HFSE and REE in the mantle and mantle derived rocks, partition coefficients and diffusion coefficients of these trace elements in major rocking-forming minerals are needed. Mineral-melt and mineral-mineral HFSE and REE partition coefficients for important mafic minerals such as clinopyroxene (cpx) and orthopyroxene (opx) are available for a range of temperatures, pressures, and melt and crystal compositions. There exists no HFSE diffusion data for any pyroxene compositions, while REE diffusion data in pyroxene is only available for the near endmember diopside and enstatite. It is not known how strongly REE and HFSE diffusion rates in pyroxenes depend on pyroxene compositions. In order to establish a database for studying kinetic fractionations of HFSE and REE in pyroxene-bearing rocks during magmatic and subsolidus processes, a three-year collaborative research program is proposed that consists of two major components: (1) laboratory studies of HFSE and REE diffusion in pyroxenes; and (2) ion probe and numerical studies of HFSE and REE zoning in coexisting cpx and opx in peridotites from the Trinity ophiolite. A range of pyroxene compositions common in the origin and differentiation of mafic and ultramafic rocks will be explored. Diffusion profiles from most diffusion experiments will be measured with Rutherford Backscattering Spectrometry (RBS). Trace element concentration profiles in coexisting cpx and opx will be measured by ion probe. Together with published partition and diffusion data, diffusion coefficients of HFSE and REE in pyroxenes obtained from this study will be used to develop generalized geospeedometry models that can be used to understand the processes and time scales of melt-rock reaction, via dissolution and reprecipitation, and subsolidus reequilibration experienced by the Trinity peridotites.

Broader Impacts. Diffusion is a fundamental mass transfer process in solid-Earth systems and hence a very general topic. Diffusion coefficients obtained from this work will be widely applicable to pyroxenes and pyroxene bearing rocks from the Earth, Moon, Mars, and various meteorites. The dissolution-reprecipitation model to be developed will be useful to a range of practical applications beyond Earth Sciences. Results from this study will also provide valuable information for a diverse group of petrologists, geochemists, geochronologists, and cosmochemists, promoting cross-discipline integration in Earth Sciences. Results will be disseminated to a broader audience through public lectures and undergraduate and graduate courses. Finally, the proposed project will provide hands-on experience for undergraduates, research opportunities for senior thesis work, and experimental, computational, and educational experience for graduate students.

Project Report

In this project, we have examined the rates of transport (diffusion) of several chemical elements, including titanium, zirconium, and lanthanide (rare-earth) elements, through the lattices of the minerals olivine and pyroxene, magnesium-iron silicate minerals that are dominant mineral phases in the earth's upper mantle. Although the elements studied in this project are often found in relatively low abundance, their concentration and distribution in and among various minerals provide important clues to understanding the origin and evolution of igenous rocks and the timescales of geological processes. Little information on diffusion of these elements had existed prior to this investigation, and in this study, laboratory measurements with experiments under controlled conditions were conducted to quantify diffusion and examine its dependence on mineral type and composition, crystallographic direction, temperature, pressure, and oxidation state of the system. Understanding the effects of these parameters provides additional information for use in modeling the behavior of these elements in a range of geological settings. In this investigation, the laboratory measurements of diffusion were couped with analysis and examination of zoning patterns of titanium, zirconium, and the rare-earth elements in minerals from rocks sampled from natural settings, with numerical models applied to interpret the processes involved in forming these rocks and the effects of subsequent alteration. Becaure the elements Zr, Ti and the rare-earths diffuse relatively slowly in pyroxene and olivine compared with some other elements, including iron and magnesium, modeling their behavior can provide additional insight into the timescales of rock formation and the interactions between minerals and components in melts and fluids. The diffusion data measured in this work are of value to a wide range of applications involving pyroxene and olivine and the rocks that contain them, including those on the earth as well as those found on the moon, Mars, and in various types of meteorites.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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Rensselaer Polytechnic Institute
United States
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