Tracing Continental Weathering Using Lithium and Magnesium Isotopes
Rudnick, Roberta L.
McDonough, William F.
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Chemical weathering is an important mechanism that changes the mass and composition of the continental crust and regulates the global CO<sub>2</sub> cycle over geological time scales. Basalt, in particular, is the main building block of the juvenile continental crust, and it is estimated that basalt weathering currently accounts for more than 30% of global CO<sub>2</sub> consumption due to silicate weathering. Here I explore Li and Mg isotopes as tracers of chemical weathering. The primary objective of this dissertation is to understand Li and Mg isotopic fractionation mechanisms during chemical weathering of basalts by analyzing Li and Mg isotopic compositions in weathering-related reservoirs. In addition, I perform mass balance modeling of Li to place constraints on the amount of continental crust that has been removed by chemical weathering. The samples include different flows of the fresh Columbia River Basalts (CRBs), drill cores through bauxites developed on the basalts, eolian dust, streams and ground waters draining only/mainly the CRBs. The results show that: 1) leaching, secondary mineral formation, and eolian addition are the main processes controlling Li and its isotopic composition in weathered basalt, 2) gibbsite in the basalt regolith generates large Mg isotopic fractionation and δ<super>26</super>Mg is also influenced by eolian addition, 3) the variations of δ<super>7</super>Li in streams are likely to reflect weathering intensity, with lower δ<super>7</super>Li corresponding to higher chemical weathering intensity, and 4) the mass of juvenile continental crust lost due to chemical weathering is at least 15% of the original crustal mass. Collectively, the results imply that: 1) chemical weathering may decrease the magnitude of the Nb anomaly of the bulk continental crust, 2) chemical weathering produces isotopically light Mg in water, leaving behind an isotopically heavy regolith and driving the bulk continental crust composition to heavier values, 3) Li and Mg isotopes have the potential to be useful in tracing continental chemical weathering, 4) chemical weathering played a significant role in changing the bulk continental crustal composition from basaltic to andesitic.