Comparison of ^(230)Th-^(238)U disequilibrium systematics in lavas from three hot spot regions: Hawaii, Prince Edward and Samoa

Abstract

^(230)Th-^(238)U disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of Hawaii, Marion Island (Prince Edward hot spot), and Samoa. The (^(230)th/^(232)Th) activity ratios of lavas from these three hot spots (1.06 ± 0.07, 1.04 ± 0.08, and 0.81 ± 0.06, respectively) imply that the source regions are each nearly homogeneous with Th/U weight ratios of 2.9, 3.0, and 3.8. For Marion Island and Mauna Kea, Hawaii, negligible secular variation occurs in the (^(230)Th-^(232)Th) initial ratios. This supports other evidence for very short transfer time between source and surface. Significant residence time at depth prior to eruption cannot be ruled out for the Samoan lavas we have studied; however, the data for one of these flows deviate from the proposed ((^(230)Th-^(232)Th)-^(87)sr/^(86)Sr)- correlation (Condomineset al., 1981a) in the opposite sense from that expected for such residence. If it is assumed that the measured ((^(230)Th-^(232)Th)) ratios of the young lavas reflect Th/U in their mantle sources, then the observed variations among these three hot spots, combined with those reported by other workers for Iceland, the Azores and Tristan de Cunha, suggest that these sources are characterized by Th/U ratios ranging from values similar to that of MORB source (~2.5) to values similar to those of bulk earth (~3.8). Mixing of different proportions of depleted and enriched mantle may be responsible for the observed range.

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