Developing a REE-in-two-pyroxene Thermometer for Pyroxene-bearing Mafic and Ultramafic Rocks

点击翻译按钮获取中文摘要

Temperature of the Earth?s and planetary interior is a key parameter in understanding the thermal and chemical evolution of the Earth and planetary bodies. One standard tool that has been widely used to read temperatures of rocks from the Earth?s interior is geo-thermometer that has been developed on the basis of temperature and pressure dependent distribution of major chemical elements in minerals that form the rocks. Among the most popular are the major element-based pyroxene thermometers that have played an important role in determining thermal structure of the Earth?s upper mantle. Clinopyroxene and orthopyroxene are major rock-forming minerals controlling the abundance and distribution of rare earth elements (REE) in rocks from the Earth?s upper mantle. It has been well documented that ratios of REE abundances between clinopyroxene and orthopyroxene in mantle rocks are sensitive to temperature. A principal goal of the studies proposed is to develop a pyroxene thermometer that is built on the temperature-dependent REE distribution between the two pyroxenes. One of the most important advantages of this new thermometer is that it has a higher closure temperature. This means that one can potentially read high temperature history of rocks now exposed on the surface.In this project, researchers at Brown University will calibrate a REE-in-two-pyroxene thermometer through a combination of laboratory crystal growth and trace element partitioning experiments and chemical analysis of well-equilibrated two-pyroxene bearing ultramafic and mafic samples from the Earth?s upper mantle and lower crust. Together with published high quality pyroxene-melt trace element partitioning data, they will develop internally consistent lattice strain models for REE partitioning in the system clinopyroxene-melt, orthopyroxene-melt, and orthopyroxene-clinopyroxene using nonlinear least squares regression methods. With a laboratory calibrated temperature- and composition-dependent orthopyroxene-clinopyroxene REE partitioning model, they will be able to calculate equilibrium temperatures for ultramafic or mafic samples using measured REE abundances in the two coexisting pyroxenes. Potential applications of the new thermometer to ultramafic and mafic rocks from a range of geological settings are outlined. The broader impacts of this project will derive from the importance of geo-thermometer and pyroxene-melt REE partitioning models to petrologic, geochemical, and geodynamic applications. Results from this study will provide valuable information for a diverse group of petrologists, geochemists, geodynamicists, and planetary scientists, promoting cross-discipline integration in Earth and Planetary Sciences. The internally consistent pyroxene-melt REE partitioning models will provide key parameters to geochemists to infer melting processes in the Earth and planetary interior. The high closure temperature of REE-in-two-pyroxene thermometer may provide a new tool to petrologists, geodynamicists, and planetary scientists to estimate cooling rate and thermal structure of the lithosphere, to better understand the thermal and chemical history of mafic and ultramafic rocks from the Moon, Mars, and other planetary bodies. Results from this study will also be disseminated to a broader audience through public lectures and undergraduate and graduate courses. And 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.