THE MANTLE SOURCES OF OCEAN RIDGES, ISLANDS AND ARCS - THE HF-ISOTOPE CONNECTION

This paper presents Nd- and Hf-isotope data for oceanic volcanics and island arc basalts in order to (1) further examine the correlation between the two isotopic systems, (2) determine Hf-176/Hf-177 of the mantle components as described by Zindler and Hart (1986) [1], and (3) assess the origin of HFSE depletions in calc-alkaline volcanics. Ocean island basalts (OIB) form an array on a Nd-143/Nd-144 versus Hf-176/Hf-177 diagram, whereby the mantle endmembers as defined in Sr-Nd-Pb isotopic space can be extended to the Hf system. For MORB, however, the decoupling of Hf and Nd isotopes requires a variable MORB endmember. Melting models in which Lu/Hf and Sm/Nd are fractionated during the formation of the MORB reservoir can explain the complete range in Nd and Hf isotopes in MORB as long as variable amounts of both diopside and garnet are present during melting. A model in which a MORB-type mantle is contaminated with high Hf-176/Hf-177, high Nd-143/Nd-144 material is also consistent with the isotope data. This high Hf-176/Hf-177 component may be generated by high Lu/Hf caused by metasomatism with a melt/fluid which was in equilibrium with phases with a high D for Hf like perovskite. The existence of high-field-strength-element (HFSE) depletions has often been thought to be a widespread characteristic of subduction-related volcanism (Gill, 1981) [2]. However, a literature survey shows that Ti, Zr and Hf depletions are not a ubiquitous a feature in calc-alkaline volcanics (see also White and Patchett, 1984 [3] and Woodhead, 1989 [4]), and that, except for Zr and Hf, the HFSE (Nb, Ta, Ti, Zr, and Hf) behave independently from each other. Hf-176/Hf-177 ratios in island arc lavas are not related to the degree of Hf/REE variation, strongly suggesting that the Hf-Zr depletion is less than 250 m.y. old. The HFSE variations are not compatible with generation during melting nor are the HFSE variations solely due to the addition of a slab-derived component. However, the available data are compatible with the variations being a reflection of a HFSE-depleted mantle source of calc-alkaline volcanics. The HFSE depletions in this sub-arc mantle have to be less than 250 m.y. old.