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

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• 作者：Bernard Bourdon ; James A. Van Orman
• 关键词：U-series ; melting ; lithosphere ; seamount ; Pitcairn ; phlogopite
• 刊名：Earth and Planetary Science Letters
• 出版年：2009
• 出版时间：30 January 2009
• 年：2009
• 卷：277
• 期：3-4
• 页码：474-481
• 全文大小：650 K

文摘

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 ⁸⁷Sr/⁸⁶Sr and relatively low ²³⁰Th/²³⁸U and a ‘lithosphere’ endmember with less radiogenic ⁸⁷Sr/⁸⁶Sr and relatively larger ²³⁰Th/²³⁸U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by ²²⁶Ra deficits relative to ²³⁰Th. 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 ²²⁶Ra–²³⁰Th 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.