U-Th-Ra radioactive disequilibria and magmatic processes

Details U-Th-Ra radioactive disequilibria and magmatic processes U-Th-Ra radioactive disequilibria and magmatic processes

Nov 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988e%26psl..90..243c&link_type=abstract

Earth and Planetary Science Letters, Volume 90, Issue 3, p. 243-262.

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65

Scientific paper

This paper presents the available data on U-Th-Ra disequilibria in recent volcanic rocks, and their significance to the understanding of magmatic processes. It is shown that 238U-230Th and 226Ra-230Th disequilibria are particularly useful in the study of mantle sources, their recent modifications, the transfer time of the magmas towards the surface and magma dynamics in active volcanoes. The Th-Sr isotopic correlation is confirmed by new results on MORB and OIB. These magmas must have short (< 104 years) transfer times. In contrast with MORB and OIB data, which have activity ratios (238U/230Th) < 1, many subduction zone volcanoes display activity ratios (238U/230Th) > 1, together with 226Ra enrichments over 230Th. The latter are largest in continental ultrapotassic and related rocks. This suggests that fluids play an important role in magma genesis processes in subduction zones, before (metasomatism) and/or during partial melting. In the absence of fluids, the enrichment pattern is Ra ⋍ Th > U, and in a ``wet'' environment Ra > U > Th.
The study of (230Th/232Th)0 initial ratios through the volcanic history of a single volcano confirms that basaltic volcanoes on oceanic islands, such as Piton de la Fournaise (Reunion), Mauna Kea (Hawaii), Marion (Prince Edward hot spot have short magma transfer times (< 104 years). Other volcanoes, like Etna, located in continental crust areas, appear to have longer transfer times (> 105 years).
From the few available results on Ra-Th disequilibria, it seems that the time of magma transfer in the volcanoes studied so far is always longer than 100 years. Work in progress on Etna shows that Ra enrichment is a characteristic of the whole historic period and a deep-seated phenomenon.

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