Distinguishing melt and fluid subduction components in Umnak Volcanics, Aleutian Arc

Plate processing at convergent margins plays a central role in the distribution of elements among major earth reservoirs. The mechanisms by which this distribution occurs, however, have remained poorly constrained. This paper provides new constraints through a detailed isotope and trace element study of volcanic rocks from Umnak Island, Central Aleutian Arc. The data require the addition of three distinct subduction components to the subarc mantle, which are characterized and quantified: (1) a hydrous fluid from subducted oceanic crust, with mid-ocean ridge basalt (MORB)-like isotopic compositions but high Pb/Nd; (2) a hydrous fluid from subducted sediment, with sediment-like isotopic compositions and an enrichment in fluid-mobile elements; (3) a sediment partial melt, with sediment-like isotopic compositions and high Th/Nd and Th/Nb compared to both regional and global sediments. The sediment melt is depleted in fluid-mobile elements, indicating loss of fluid prior to melting. The high Th/Nb of the sediment melt indicates presence of a Ti-rich residual phase such as rutile during partial melting. The observation that sediment fluid and sediment melt can be distinguished in different volcanic rocks suggests that they arrive separately at the sites of arc magma formation. This indicates release of multiple discrete fluid and melt phases from sediment to the overlying mantle wedge, which can be viewed as a natural consequence of progressive metamorphism of the subducting slab.