Splitting of the 520-Kilometer Seismic Discontinuity and Chemical Heterogeneity in the Mantle

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Science  14 Mar 2008:
Vol. 319, Issue 5869, pp. 1515-1518
DOI: 10.1126/science.1152818

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Seismic studies indicate that beneath some regions the 520-kilometer seismic discontinuity in Earth's mantle splits into two separate discontinuities (at ∼500 kilometers and ∼560 kilometers). The discontinuity near 500 kilometers is most likely caused by the (Mg,Fe)2SiO4 β-to-γ phase transformation. We show that the formation of CaSiO3 perovskite from garnet can cause the deeper discontinuity, and by determining the temperature dependence for this reaction we demonstrate that regional variations in splitting of the discontinuity arise from variability in the calcium concentration of the mantle rather than from temperature changes. This discontinuity therefore is sensitive to large-scale chemical heterogeneity. Its occurrence and variability yield regional information on the fertility of the mantle or the proportion of recycled oceanic crust.

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