Buried Deep

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Science  14 Jan 2000:
Vol. 287, Issue 5451, pp. 189
DOI: 10.1126/science.287.5451.189n

The continental crust was formed from Earth's upper mantle; thus elements that are enriched in the crust should be depleted in the mantle, and vice versa. This mass balance, along with knowledge of the starting composition of Earth's mantle, can be used to evaluate crustal growth through time and the degree of homogeneity in the mantle. This mass balance seems to hold for many elements and element ratios, for an assumption that the Earth's mantle had an initial composition similar to that of chondrites. However, a few elements, notably niobium, titantium, and tantalum, seem to be depleted in all samples of both crust and mantle. Rudnick et al. (p. 278) propose a solution by identifying that rutile, a trace phase in eclogite that forms during subduction of oceanic crust, may provide the missing reservoir for these elements. Sequestration of rutile-bearing eclogite near the base of the mantle would provide a solution to the mass balance.

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