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Schlagwörter:
chlorine; assimilation; crust; recycling; high-field-strength elements; Austral Islands
Zusammenfassung:
Chlorine abundance variations in oceanic basalts can provide insights into the degassing and volatile recycling history of the mantle as well as shallow melt/hydrosphere interaction. We have examined major, trace and volatile element abundances in olivine-hosted melt inclusions from the islands of Raivavae and Rapa in the Austral Island chain. The island, of Raivavae sits atop a pre-existing fracture zone and thus provides the opportunity to examine the relationship between melt/hydrosphere interaction and local lithospheric structure. The majority of inclusions from both Raivavae and Rapa have well correlated chlorine and potassium concentrations consistent with a source Cl/K2O ratio of similar to0.04, similar to that of uncontaminated mid-ocean ridge and ocean island basalts. The similarity of chlorine/potassium ratios in mid-ocean ridge basalts, Austral Islands basalts, and basalts from many other ocean islands suggests that chlorine/potassium does not significantly vary in the mantle. Because the plume sources of many ocean island chains contain varying types and quantities of recycled oceanic crust and sediments, this indicates that most of the chlorine added to oceanic crust during seafloor alteration is removed during subduction and is not recycled into the deep mantle. High chlorine contents (up to 0.14 wt%) and chlorine/potassium ratios in melt inclusions from an early-erupted Raivavae lava derive from assimilation of Cl-rich brines or brine-impregnated oceanic crust. A small subset of inclusions from the same lava show more extreme chlorine enrichment (up to 2.5 wt%), are depleted in incompatible trace elements relative to normal inclusions, and show extreme fractionation of high-field-strength elements (HFSEs) relative to large-ion-lithophile or rare-earth elements. These latter inclusions derive from partial melting of the pre-existing oceanic crust under brine-saturated conditions. HFSE depletions in these inclusions reflect the stabilization of a HFSE-bearing phase in the lower crust, probably due to high chlorine fugacity. HFSE anomalies are also associated with high chlorine content in mid-ocean ridge basalts. We suggest that these anomalies are also generated by the stabilization of HFSE-bearing phases in high-chlorine- activity melts or fluids. This process may also provide a means of stabilizing rutile in the sub-arc mantle wedge. (C) 2002 Elsevier Science B.V. All rights reserved.
