A Geochemical Investigation of the Dynamics of Mantle Melting within the Reunion Plume

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Pietruszka/HauriEAR-0003359A fundamental goal of geochemical research at mid-ocean ridge and ocean-island volcanoes is to infer the process of basaltic melt generation and transport. The U-series isotope abundances of young lavas (e.g., 226Ra and 230Th) are particularly well suited for this purpose because the half-lives of 226Ra and 230Th (1600 yr and ~75 kyr, respectively) are thought to be similar to the time scale of magmatic processes. This unique feature of U-series isotopes offers the potential to study both the nature and timing of magma genesis. The goal of this project is to investigate the dynamics of mantle melting within the Reunion plume using the 226Ra-230Th-238U disequilibria of a detailed time series of historical lavas from Piton de la Fournaise Volcano (1931-1998). This volcano is ideally suited for a U-series isotope study of mantle melting because (1) its historical lavas display systematic temporal fluctuations of incompatible trace element ratios that are thought to result from rapid changes in the degree of partial melting and (2) it is known to tap a homogeneous mantle source region. Thus, we expect to be able to isolate the U-series isotope signatures of mantle melting at Piton de la Fournaise from the complicated effects of source heterogeneity, which is difficult at most mid-ocean ridge and ocean-island volcanoes. We will evaluate a range of melting models with this data set. These model results will be used to explore the basic links between the geochemical and geophysical parameters of mantle melting within the Reunion plume, such as the melt fraction, the fertility of the mantle source region, the rates of mantle melting and upwelling, the melt-zone porosity, and the mechanism of melt generation and transport. Additionally, we will compare our new data and model results for Piton de la Fournaise with previous U-series isotope and other geochemical work from Kilauea and Mauna Loa to test the common assumption that Hawaiian volcanoes serve as models for other ocean-island volcanoes.