Fluid- and melt-related enrichment in the subarc mantle: evidence from Nb/Ta variations in island-arc basalts

The single most distinctive feature of volcanic rocks from convergent-margin settings is a marked depletion of the high field strength elements (HFSE) Nb, Ta, and Ti relative to large ion lithophile and light rare earth elements when compared with basalts from mid-oceanic ridges (MORB) and the oceanic islands. A major impediment to a better understanding of this problem has been a lack of high-quality data for the HFSE (particularly Nb and Ta) that occur in very low concentrations in most volcanic rocks from convergent-margin settings. We report new analyses of Nb and Ta for a suite of island-arc volcanic rocks as well as some sea-floor sediments. Our data show that Nb/Ta values for relatively depleted island-arc volcanic rocks are similar to MORB and essentially chondritic (Nb/Ta ∼ 17), whereas more potassic arc volcanics have substantially higher Nb/Ta values (up to 33). We interpret these high values as due to modification of the subarc mantle source by silicic melts derived from the subducting slab, whereas enrichment of the source regions of the less potassic arc rocks involved a slab-derived fluid.