High Field Strength Element Anomalies in Arc Lavas: Source or Process?

An understanding of the origin of depletion in the high fieldstrength elements (HFSE), Nb, Zr and Ti, relative to rare earthelements (REE) in arc lavas is critical to models both for magmagenesisin ares and for the relationship between are magmatism and growthof the continental crust. The presence of HFSE depletion inboth are lavas and in the bulk continental crust constitutessome of the strongest evidence that continental crust is/wasgenerated in subduction zones, especially if the HFSE are retainedrelative to REE in the subducting slab (Saunders et al., 1980;McDonough, 1991). Recently, however, it has been proposed thatHFSE depletion develops during the main are magma melting eventin the mantle wedge (McKenzie & O'Nions, 1991), during meltascent to the surface (Kelemen et al., 1990), or even that aworld-wide shallow mantle reservoir with HFSE depletion exists(Salters & Shimizu, 1988). If so, it is possible that HFSEdepletion may have developed in magmas unrelated to subductionzones during crust-generation processes in the Precambrian.The common presence of high-MgO lavas in the Southern LesserAntilles provides a rare opportunity to test these models, becausetheir chemistry is essentially unmodified since derivation fromthe mantle. We show that depletion (relative to REE) in theHFSE Ti, Zr, and Nb exists in the mantle wedge before melting,and is probably produced by an REE-rich slab flux. In contrastto many other arcs (Woodhead et al., 1993), there is no evidencethat the Lesser Antilles mantle source is more depleted in HFSEthan the source of mid-ocean ridge basalts. Relative to REE,Ti depletion in melts is enhanced during melting, requiringa Ti-rich phase in the residue at low melt fractions. Ti depletionis also enhanced during fractionation of magnetite and amphibole,whereas relative Zr depletion is reduced during fractionation.In most arc magmas (usually <6% MgO), fractionation is probablya major control on the extent of Ti and Zr depletion. In theLesser Antilles, the extent of Nb depletion relative to La islargely unaffected by melting or crystal fractionation processes.