Melting of enriched mantle beneath Pitcairn seamounts: Unusual U-Th-Ra systematics provide insights into melt extraction processes

摘要

U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical 'plume' endmember with radiogenic Sr-87/Sr-86 and relatively low (230)Trh/U-238 and a 'lithosphere' end member with less radiogenic Sr-87/Sr-86 and relatively larger Th-230/U-238. Remarkably, all the lavas, except for a few arguably older samples, are characterized by Ra-226 deficits relative to Th-230. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual Ra-226-Th-230 signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates.