Behavior of Niobium, Tantalum and other high field strength elements in adakites and related lavas from The Philippines

Pliocene-Quaternary adakites and spatially and temporally associated niobiu m-enriched basalts (the latter thought to be derived by melting of slab mel t-metasomatized mantle) from the Philippine island arcs have been selected for analysis of high field strength elements (HFSE). All these samples have nearly constant and chondritic Zr/Hf ratio (36.5) and slightly infrachondr itic Nb/Ta ratio (14.7). We interpret adakitic magmas to be derived from th e partial melting at approximately 900 degreesC of subducted mid-ocean ridg e basalts (MORB) crust, with rutile and/or ilmenite as residual minerals. M elting calculations show that, under these conditions, slab melts should ha ve suprachondritic Nb/Ta ratios. The obvious discrepancy with our data is a ttributed to insufficient knowledge of rutile-melt partition coefficients f or HFSE. Consequently, abnormal Nb/Ta or Zr/Hf ratios cannot be considered as potential markers of slab melting processes in island arcs.