Geochemistry

Mantle-Derived Magmas High in Silica

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Science  11 May 2001:
Vol. 292, Issue 5519, pp. 1023-1025
DOI: 10.1126/science.292.5519.1023e

Most rock samples are derived from Earth's crust. Information about the rest of the solid Earth is limited to the less abundant macrocrystalline rocks called peridotites, which are derived more directly from the mantle and are rich in olivine and pryoxene. Pin et al. have found samples of a chemically unusual mantle-derived rock in the Pyrenee Mountains of France. The rock occurs as silicon-, aluminum-, and sodium-rich dikes intruded into a peridotite massif in the Cretaceous composed of lherzolite. The dikes are also enriched in strontium, barium, niobium, tantalum, and the light rare earth elements. Their chemical characteristics indicate that they evolved from small degrees of partial melting of a peridotite rich in harzburgite in the uppermost mantle to produce magmas enriched in silica. The high sodium content and enrichments in some other trace elements suggest that the melts were infiltrated by carbon dioxide and water-rich fluids brought in by carbon-rich magmas. The fluids reduced the viscosity of the melts and allowed them to aggregate into the larger dikes. The dikes are a distinctive example of silica-rich melts derived by low degrees of partial melting in the mantle; previously such melts were only produced in laboratory experiments or found as small glassy inclusions in rocks. — LR

Geology29, 451 (2001).

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