GEOCHEMISTRY: Mantle Melting Mechanisms

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Science  08 Jun 2007:
Vol. 316, Issue 5830, pp. 1395d
DOI: 10.1126/science.316.5830.1395d

During Earth's history, heavy metals gradually decoupled from silicate rocks and sank toward the core, which became differentiated from the overlying mantle. Signatures of this process can be traced in the distribution of highly siderophile elements (Re and the platinum group elements Os, Ir, Ru, Pt, and Pd) that are chemically associated with iron. These elements tend to concentrate in base metal sulfides rather than in silicate-based minerals. Over time, they then fractionate and particular elements become sequestered in different types of sulfides, depending on the ways in which they melt. In low-sulfur rocks, their distributions are less well understood.

Luguet et al. have analyzed the distribution of highly siderophile elements in four rock samples from the low-sulfur harzburgite lenses in the Lherz massif in the French Pyrenees. The platinum group elements were found to concentrate in rare, micrometer-sized minerals (sulfides and alloys) in the intergranular spaces. These are thought to be the residues of base metal sulfides that were lost from these rocks after a high degree (up to 25%) of partial melting. Because of their high melting temperatures and high Os content, these minerals may preserve Os isotope compositions after several rounds of mantle melting, possibly explaining the patchy Os distribution of the convecting upper mantle. — JB

Geochim. Cosmochim. Acta 10.1016/j.gca.2007.04.011 (2007).

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