Françoise Capmas

Age of the Deccan traps using 187Re–187Os systematics

A suite of basaltic rocks sampled over a vast exposure and stratigraphic thickness in the Deccan traps has been investigated for Os isotopic systematics. The results plot on a very well defined Re‐Os isochron corresponding to an age of 65:6 0: 3M a (2 uncertainty). This age is in excellent agreement with previous K‐Ar and Ar‐Ar data. Os data also imply a short duration of volcanism, which should have important implications on mantle geodynamics. The 187 Os= 188 Os initial ratio is typically chondritic: 0:12843 0:00047 (2 ) and indicates that metasomatism and crustal contamination played only a very minor role in the Re‐Os budget during formation of the Deccan traps.

Sources of rhenium and osmium enrichment in fumaroles, sulphide sublimates and volcanic rocks from the Kudriavy volcano

Rhenium loss through magma degassing could be partly balanced by rhenium enrichment in fumarolic magmatic gases and Re-bearing precipitates, as may be the case for the Kudriavy volcano associated with an active subduction zone. The relatively unradiogenic 187Os/188Os isotope ratios (0.122 up to 0.152) and high Os contents (averaging 0.6 ppb) of fumarolic gas condensates imply that significant Re and Os are remobilised from depleted MORB mantle. Involvement of a Re-rich component is evident from high Re concentrations in high-temperature gas condensates, ranging from 7 to 200 ppb. Indeed, Re-rich Os-poor components such as organic-rich subducted sediments and volcanic rocks do not significantly shift the isotopic composition of fumarolic products. The relatively radiogenic composition of the dacite-andesite-basaltic arc volcanics (187Os/188Os ratio up to 0.58), however, could result from significant Os (and Re) input from subducted sediments.