Why are so many arc magmas close to 238 U- 230 Th radioactive equilibrium?

Details Why are so many arc magmas close to 238 U- 230 Th radioactive equilibrium? Why are so many arc magmas close to 238 U- 230 Th radioactive equilibrium?

Sep 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.4491c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 18, pp.4491-4497

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Scientific paper

New analyses of 238 U- 230 Th disequilibria are reported for four active volcanoes: Merapi and Krakatoa in the Sunda arc (Indonesia), Masaya in Central America (Nicaragua), and Ambrym in the New Hebrides island arc. Despite a large range in ( 230 Th / 232 Th ) ratios (from 0.65 in Merapi andesites to 2.5 in Masaya basalts), 238 U and 230 Th are close to radioactive equilibrium, as in many other arc magmas. In several mantle sources, Th/U ratios have clearly been modified by metasomatic processes associated with subduction. This is demonstrated in Central America by the correlation between ( 230 Th / 232 Th ) and 10 Be / 9 Be ratios for several active volcanoes along the arc. It is proposed that the 238 U- 230 Th radioactive equilibrium found in many arc magmas is the result of disequilibrium melting involving an easily melted, slab-derived, metasomatic component which dominates the uranium and thorium budget of the mantle sources. The departure from equilibrium may be either due to mixing with 230 Th enriched melts derived from unmetasomatized mantle sources or to a late stage uranium addition by fluids. This latter process, producing uranium enriched magmas, has a greater influence in uranium and thorium poor magmas.

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