U-Th-Pa-Ra study of the Kamchatka arc:New constraints on the genesis of arc lavas

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Dosseto, Anthony, Bourdon, Bernard, Joron, Jean Louis, Dupré, Bernard
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2003
• Subjects: Kamchatka
• Online Access: https://researchers.mq.edu.au/en/publications/daf5776a-b9d2-4faa-99d0-26d392552f39; https://doi.org/10.1016/S0016-7037(03)00086-3; http://www.scopus.com/inward/record.url?scp=0041564012&partnerID=8YFLogxK

The 238 U- 230 Th- 226 Ra and 235 U- 231 Pa disequilibria have been measured by mass spectrometry in historic lavas from the Kamchatka arc. The samples come from three closely located volcanoes in the Central Kamchatka Depression (CKD), the most active region of subducted-related volcanism in the world. The large excesses of 226 Ra over 230 Th found in the CKD lavas are believed to be linked to slab dehydration. Moreover, the samples show the uncommon feature of ( 230 Th/ 238 U) activity ratios both lower and higher than 1. The U-series disequilibria are characterized by binary trends between activity ratios, with ( 231 Pa/ 235 U) ratios all >1. It is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting. We suggest that they are more likely to reflect mixing between two end-members: a high-magnesia basalt (HMB) end-member with a clear slab fluid signature and a high-alumina andesite (HAA) end-member reflecting the contribution of a slab-derived melt. The U-Th-Ra characteristics of the HMB end-member can be explained either by a two-step fluid addition with a time lag of 150 ka between each event or by continuous dehydration. The inferred composition for the dehydrating slab is a phengite-bearing eclogite. Equilibrium transport or dynamic melting can both account for 231 Pa excess over 235 U in HMB end-member. Nevertheless, dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid-derived 226 Ra/ 230 Th. A continuous flux melting model is also tested. In this model, 231 Pa- 235 U is quickly dominated by fluid addition and, for realistic extents of melting, this process cannot account for ( 231 Pa/ 235 U) ratios as high as 1.6, as observed in the HMB end-member. The involvement of a melt derived from the subducted oceanic crust is more likely for explaining the HAA end-member compositions than crustal assimilation. Melting of the oceanic crust is believed to occur in ...