The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presentedand integrated with previous data for this low-K intra-oceanicarc suite. Geochemically, the arc tholeiites and basaltic andesitesrange from extremely HFS element depleted (northern Tonga) tonear N-MORB-like HFS element abundances in L'Esperance (southernKermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristicselective enrichment generally recognized in arc magmas, andthus indicate decoupling of the HFS and LIL elements. Modellingsuggests a compositionally variable source (mantle wedge) alongthe arc, ranging from restite after remelting an N-type MORBsource (northern end), to progressively less depleted, MORB-likesources southwards. Thus, the low HFS/LIL element ratios areinterpreted in terms of HFS depletion followed by LIL elementenrichment associated with subduction; broad correlations occurbetween Zr/Ba and Sr/Nd ratios (fractionation corrected) and87Sr/86Sr and ¹⁴³Nd/¹⁴⁴Nd ratios. Derivation of the arc magmas from depleted peridotote requiressuperimposed fractional crystallization, which has been modelledthermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentallyproduced partial melts from depleted lherzolite (Jaques &Green, 1980). It is shown that the arc tholeiites and basalticandesites (and also high Mg-andesites) are potentially developedat low pressures 5 kb), from parental magmas also generatedat relatively low pressure ( 10 kb). These data further suggestthat a southward increasing depth of magma segregation (correlatingwith Benioff Zone geometry) could account for differences inchemistry between the Tonga and Kermadec arc segments. The mechanism of LILE enrichment is still highly problematic,but it is suggested that the model of Tatsumi et al. (1986)may account for much of the geochemical data; this involvesrelatively shallow release, via fluids, of LIL elements intooverlying peridotite beneath the fore-arc region. Induced convectionin the mantle wedge moves the metasomatized mantle into thezones of magma generation. The development of the inferred, variably depleted mantle wedgesource is here related to active back-arc spreading which isslightly older and more rapid behind the northern region ofthe arc. The lherzolite restite from this spreading processis interpreted to undergo further partial melting in the metasomatizedmantle wedge overlying the subduction zone, involving inducedconvection. The back-arc island of Niua fo'ou is geochemically quite distinctfrom the arc magmas, being similar to N-MORB in its trace elements,but to OIB in its isotope ratios.