TY - JOUR
T1 - Influence of chemical weathering on the composition of the continental crust
T2 - Insights from Li and Nd isotopes in bauxite profiles developed on Columbia River Basalts
AU - Liu, Xiao Ming
AU - Rudnick, Roberta L.
AU - McDonough, William F.
AU - Cummings, Michael L.
N1 - Funding Information:
We thank Stan Mertzman for performing XRF analysis and Peter Zavalij for helping with XRD analysis. We thank Richard Ash for assistance with the MC–ICP-MS analyses, Igor Puchtel for help in the clean lab, Steve Reidel for discussions and supply of samples and major element data (WC), Terry Tolan for sampling guidance and discussion, Marshall Gannett and Mike Free for help with fieldwork logistics, and Rich Gaschnig for discussion. The manuscript benefited from the comments of three anonymous reviewers. This work was supported by a grant from the National Science Foundation (EAR 0948549).
PY - 2013/8/5
Y1 - 2013/8/5
N2 - Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8-9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including "mobile" and some "immobile" elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and εNd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20-60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and εNd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust.
AB - Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8-9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including "mobile" and some "immobile" elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and εNd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20-60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and εNd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust.
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U2 - 10.1016/j.gca.2013.03.043
DO - 10.1016/j.gca.2013.03.043
M3 - Article
AN - SCOPUS:84877839199
SN - 0016-7037
VL - 115
SP - 73
EP - 91
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
ER -