UThRa systematics in Kilauea and Mauna Loa basalts, Hawaii

Abstract

We describe 226Ra 230Th 238U (dis)equilibria and RbBaThULa concentration relationships in historical and prehistoric lavas from Kilauea and Mauna Loa. ( 226 Ra 230 Th ), ( 230 Th 232 Th ), Th/U, Ba/Rb and Ba/La ratios [but not (Rb,Ba,La)/(Th,U) ratios] are essentially identical in both volcanoes, whereas the absolute concentrations (after correction for olivine crystallization) differ by up to a factor of 2, in response to varying melt fractions. This shows that bulk partition coefficients of these elements are significantly smaller than melt fractions. Very small or absent 230Th 238U disequilibrium implies very small or negligible magmatic fractionation between Th and U. 226Ra 230Th disequilibria are significantly larger (∼ 20% excess 226Ra on average) but are also independent of melt fraction. The combination of significant RaTh fractionation together with small or absent ThU fractionation provides constraints on recently proposed models to explain U-series disequilibria during partial melting and melt extraction. Instantaneous melt extraction models are rejected: (a) because they are inconsistent with experimentally determined partition coefficients; and (b) more generally because they would require significant covariation of ( 226 Ra 230 Th ) with melt fraction. On the other hand, dynamic melting models involving slow fractional melting or melt infiltration within the garnet stability region, followed by rapid movement through the lithosphere, are consistent with the results and yield melt porosities between 10 −3 and 10 −2 for plume upwelling velocities of 1 m yr −1. In addition, we tentatively proposed alternative models for creating the Ra excesses in the magma. One such process involves the mobilization of Ra within the volcanic edifice, subsequent advection toward and redeposition within the roof region of the magma chamber, and finally incorporation into the magma itself. Another mechanism for incorporating excess Ra in the magma might be transport of very small amounts of carbonate fluids or carbonatite melts (containing very large excesses of 226Ra) into partially molten regions in the mantle. Given the currently available data and state of knowledge about magma extraction processes, there is no obvious preference for either the purely magmatic models or those involving “extraneous” fluids in the mantle or within the volcanic edifice.