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

Abstract

New REE data, and new Nd, O, Sr, and Pb isotopic data are presented and integrated with previous data for this low-K intra-oceanic arc suite. Geochemically, the arc tholeiites and basaltic andesites range from extremely HFS element depleted (northern Tonga) to near N-MORB-like HFS element abundances in L'Esperance (southern Kermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristic selective enrichment generally recognized in arc magmas, and thus indicate decoupling of the HFS and LIL elements. Modelling suggests a compositionally variable source (mantle wedge) along the arc, ranging from restite after remelting an N-type MORB source (northern end), to progressively less depleted, MORB-like sources southwards. Thus, the low HFS/LIL element ratios are interpreted in terms of HFS depletion followed by LIL element enrichment associated with subduction; broad correlations occur between Zr/Ba and Sr/Nd ratios (fractionation corrected) and 87Sr/86Sr and Nd/Nd ratios.Derivation of the arc magmas from depleted peridotote requires superimposed fractional crystallization, which has been modelled thermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentally produced partial melts from depleted lherzolite (Jaques & Green, 1980). It is shown that the arc tholeiites and basaltic andesites (and also high Mg-andesites) are potentially developed at low pressures ≤ 5 kb), from parental magmas also generated at relatively low pressure (≤ 10 kb). These data further suggest that a southward increasing depth of magma segregation (correlating with Benioff Zone geometry) could account for differences in chemistry 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 involves relatively shallow release, via fluids, of LIL elements into overlying peridotite beneath the fore-arc region. Induced convection in the mantle wedge moves the metasomatized mantle into the zones of magma generation.The development of the inferred, variably depleted mantle wedge source is here related to active back-arc spreading which is slightly older and more rapid behind the northern region of the arc. The lherzolite restite from this spreading process is interpreted to undergo further partial melting in the metasomatized mantle wedge overlying the subduction zone, involving induced convection.The back-arc island of Niua fo'ou is geochemically quite distinct from the arc magmas, being similar to N-MORB in its trace elements, but to OIB in its isotope ratios.