WILLIAM MARSH RICE UNIVERSITY
Many of the mineral resources necessary for modern society are found in the continental crust, particularly in the form of ores. Ore formation requires trace metals from the crust to be scavenged, redistributed and then re-deposited, processes which in some cases are controlled by physical and chemical interactions (weathering) between the crust, atmosphere and hydrosphere. The purpose of this project is to explore isotopic proxies that could eventually be used to assess the role of chemical and physical weathering on the composition of the continents in order to better understand the pathways of chemical transport. The proposed study also has indirect implications for understanding feedbacks between the deep Earth and climate.
In detail, the Early Grant for Exploratory Research (EaGER) is focused on finding ways to quantify the extent to which chemical weathering modifies the major element composition of the continental crust from its basaltic parentage to its defining felsic (Mg-poor, Si-rich) character. In particular, this proposal seeks to test the hypothesis that some fraction of Mg lost from continents is accommodated by the leaching out of Mg during weathering, followed by sequestration in oceanic crust during hydrothermal circulation, and finally subduction back into the mantle. The other lost component is through the generation of mafic cumulates or residues at arcs, which founder back into the mantle. The approach will be to use Mg isotope ratios as a tracer of chemical weathering because weathering processes leach out light Mg and leave behind heavy Mg in the soils and sedimentary residues. High temperature processes should not fractionate Mg isotopes significantly and thus Mg isotopes should be most sensitive to the weathering process. Mg isotopes will be measured on mafic minerals in granitic plutons from the Cretaceous Peninsular Ranges Batholith in southern California and will be combined with existing oxygen, strontium, and lead isotope data on the same samples. The Peninsular Ranges Batholith represents a mixture of juvenile magmas and preexisting metamorphic basement, the latter of which has a Paleozoic sedimentary protolith. Preliminary results show correlations between Mg isotopes and O, Sr, and Pb isotopes, allowing an estimate of the extent of chemical weathering experienced by metamorphic basement earlier in its history. To further complement this work, the Mg isotopic compositions of hydrothermally altered oceanic crust and their metamorphic equivalents (greenschists and amphibolites) will also be examined. Collectively, this work should shed more quantitative light on the rock cycle.