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      Dehydration melting at the top of the lower mantle

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      Science
      American Association for the Advancement of Science (AAAS)

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          Abstract

          The high water storage capacity of minerals in Earth's mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with high-pressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.

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          Bootstrap Methods for Standard Errors, Confidence Intervals, and Other Measures of Statistical Accuracy

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            S40RTS: a degree-40 shear-velocity model for the mantle from new Rayleigh wave dispersion, teleseismic traveltime and normal-mode splitting function measurements

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              Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle.

              The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth's mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO(2)/Nb and CO(2)/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.
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                Author and article information

                Journal
                Science
                Science
                American Association for the Advancement of Science (AAAS)
                0036-8075
                1095-9203
                June 12 2014
                June 13 2014
                June 12 2014
                June 13 2014
                : 344
                : 6189
                : 1265-1268
                Article
                10.1126/science.1253358
                24926016
                4561370a-658f-4497-abcb-ea74fc99d5c6
                © 2014
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