Chromite and platinum group elements mineralization in the Santa Elena Ultramafic Nappe (Costa Rica) : Geodynamic implications

• Chromitites associated with strongly altered peridotite from six distinct localities in the Santa Elena ultramaficnappe (Costa Rica) have been investigated for the first time. Santa Elena chromitites commonly display acompositional variation from extremely chromiferous (Cr/(Cr+Al)=0.81) to intermediate and aluminous(Cr/(Cr+Al)=0.54). This composition varies along a continuous trend, corresponding to calculated parental liquidswhich may have been derived from the differentiation of a single batch of boninitic magma with Cr-rich and(Al, Ti)-poor initial composition. Fractional precipitation of chromite probably occurred during differentiation ofthe boninitic melt and progressive metasomatic reaction with mantle peridotite. The distribution of platinum groupelements (PGE) displays the high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio typical of ophiolitic chromitites and, consistently,the platinum group minerals (PGM) encountered are mainly Ru-Os-Ir sulfides and arsenides. Textural relations ofmost of the platinum group elements suggest crystallization at magmatic temperatures, possibly under relativelyhigh sulfur fugacity as indicated by the apparent lack of primary Os-Ir-Ru alloys.The chemical and mineralogical characteristics of chromitites from the Santa Elena ultramafic nappe have astrong affinity to podiform chromitites in the mantle section of supra-subduction-zone ophiolites. Calculatedparental melts of the chromitites are consistent with the differentiation of arc-related magmas, and do not supportthe oceanic spreading center geodynamic setting previously proposed by some authors.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

Nyckelord

Applied Geochemistry

Tillämpad geokemi

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