Th isotope and U-series studies of subduction-related volcanic rocks

Details Th isotope and U-series studies of subduction-related volcanic rocks Th isotope and U-series studies of subduction-related volcanic rocks

May 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.1427g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 54, Issue 5, pp.1427-1442

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39

Scientific paper

The activities of 6-7 radionuclides measured by alpha spectrometry are reported for 35 samples of recent volcanic rocks from 24 volcanoes in 9 arcs. These include continental margins (Cascades, Alaska, Costa Rica), island arcs (the Sunda, Banda, and Sangihe arcs of Indonesia, Marianas, Japan, Aleutians, Antilles, Tonga), and a backarc basin (Lau Basin). Enrichment of 238 U over 230 Th is more common (over 2/5 of the samples) and greater (up to 59%) in these subduction-related volcanics than in those from other tectonic environments. 230 Th / 232 Th ratios also extend to higher values, both in absolute numbers and relative to other isotope ratios. Enrichment of 210 Po and 226 Ra over 230 Th is widespread and is more common and greater in island arcs than continental margins. The level of Po or Ra enrichment (50-300% in island arcs, 10-50% at continental margins) is similar to that in ocean island and ridge basalts and decreases with differentiation. The differences in Th-U systematics between subduction-related and other volcanic rocks are attributed to variations in the process of melt extraction, changes in bulk partition coefficients within the mantle wedge, or preferential addition of U from subducted lithosphere. Ra enrichments are attributed to partial melting processes which are similar to those at ocean ridges. Smaller excesses at continental margins are attributed either to slower ascent or to differences in the process of melt formation beneath continents.

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