HIGH-FIELD STRENGTH ELEMENT ANOMALIES IN ARC LAVAS - SOURCE OR PROCESS

An understanding of the origin of depletion in the high field strength elements (HFSE), Nb, Zr and Ti, relative to rare earth elements (REE) in arc lavas is critical to models both for magmagenesis in arcs and for the relationship between arc magmatism and growth of the continent al crust. The presence of HFSE depletion in both arc lavas and in the bulk continental crust constitutes some of the strongest evidence that continental crust is/was generated in subduction zones, especially if the HFSE are retained relative to REE in the subducting slab (Saunder s et al., 1980; McDonough, 1991). Recently, however, it has been propo sed that HFSE depletion develops during the main arc magma melting eve nt in the mantle wedge (McKenzie & O'Nions, 1991), during melt ascent to the surface (Kelemen et al., 19901), or even that a world-wide shal low mantle reservoir with HFSE depletion exists (Salters & Shimizu, 19 88). If so, it is possible that HFSE depletion may have developed in m agmas unrelated to subduction zones during crust-generation processes in the Precambrian. The common presence of high-MgO lavas in the South ern Lesser Antilles provides a rare opportunity to test these models, because their chemistry is essentially unmodified since derivation fro m the mantle. We show that depletion (relative to REE) in the HFSE Ti, Zr. and Nb exists in the mantle wedge before melting, and is probably produced by an REE-rich slab flux. In contrast to many other arcs (Woo dhead et al., 1993), there is no evidence that the Lesser Antilles man tle source is more depleted in HFSE than the source of mid-ocean ridge basalts. Relative to REE, Ti depletion in melts is enhanced during me lting, requiring a Ti-rich phase in the residue at low melt fractions. Ti depletion is also enhanced during fractionation of magnetite and a mphibole, whereas relative Zr depletion is reduced during fractionatio n. In most arc magmas (usually < 6% MgO), fractionation is probably a major control on the extent of Ti and Zr depletion. In the Lesser Anti lles, the extent of Nb depletion relative to La is largely unaffected by melting or crystal fractionation processes.