Mineral Composition in the Mantle

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Science  21 Jan 2000:
Vol. 287, Issue 5452, pp. 393
DOI: 10.1126/science.287.5452.393a

Earth's lower mantle—which makes up most of the Earth, from a depth of about 670 to 2700 km—is believed to be composed primarily of magnesium silicate (MgSiO3) in the perovskite structure and magnesiowustite (Mg,Fe)O, and several less abundant phases. Knowing the proportions of these minerals is important for understanding Earth's dynamics, such as whether the lower mantle evolved separately from the upper mantle. Determining the mineralogy requires matching high-pressure experimental results, most of which have been on the end-member compositions, with seismic and other geophysical measurements. Recent work has shown that minor amounts of ferric iron (Fe), aluminum (Al), and calcium (Ca) greatly affect the match to geophysical parameters, as well as to the geochemistry of the mantle, because they are fractionated differently between perovskite and magnesiowustite.

Wood presents a series of experiments designed to evaluate further the effect of Ca, Al, and Fe in the reactions at the top of the lower mantle that produce Mg perovskite and magnesiowustite. The results show that formation of Mg-rich, Al-free perovskite at the top of the lower mantle can explain the sharp seismic signal; at deeper levels, the perovskite becomes more aluminum-rich as garnet breaks down. The data are consistent with a lower mantle that is about 80% Mg-rich perovskite containing Al and Fe, 15% magnesiowustite, and 5% Ca-perovskite.—BH

Earth Planet. Sci. Lett.174, 341 (2000).

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