Hf–Nd evidence for the origin and distribution of mantle domains in the SW Pacific

Abstract

Pb isotope systematics have already been used successfully to demonstrate that the lavas of the arc-basin terrains of the SW Pacific are derived from two mantle domains, one of Pacific-like character and the other of Indian-like character. However, the mobility of Pb during subduction and alteration has mainly restricted the fingerprinting of domains to fresh lavas of MORB composition. We demonstrate that the less alteration-sensitive Hf–Nd isotope projection also discriminates successfully between ‘Pacific’ and ‘Indian’ domains, and thus enables us to extend mantle domain fingerprinting to the back-arc basin basalts and boninites of the Lau and North Fiji Basins and the volcanic arc lavas of the Kermadec, Tonga and Vanuatu arcs. Fingerprinting is facilitated by the observation that the Hf isotope ratio is independent of subduction-input parameters, indicating that Hf has been essentially conservative during the subduction process. Subducted Nd has been added to the mantle source, but subtracting this numerically using the magnitude of negative Hf anomalies filters out the subduction effect. The data show that the ‘Indian’ domain provides the source for magmas erupted at ridges, and arcs near these ridges, that have propagated southwards following the 12 Ma collision of the Ontong-Java Plateau with the Vitiaz Trench. This indicates that the ‘Indian’ domain is actually derived from SOPITA mantle (South Pacific Isotopic and Thermal Anomaly) — mantle modified by the Samoa and other plumes outboard of the trench which only entered the SW Pacific arc-basin system after the Ontong-Java Plateau collision removed the slab barrier at < 12 Ma. In the west, mantle flows beneath the network of south-propagating ridges in the North Fiji and NW Lau Basins, undergoing progressive depletion until the final loss of plume components produces an N-MORB mantle (Indian MORB Mantle) composition in the south North Fiji Basin and Central Lau Spreading Centre. In the east, newly-depleted Samoan plume mantle provides the source for the boninites and depleted arc tholeiites of the northern Tonga arc.