Magma genesis and mantle heterogeneity in the Manus back-Arc Basin, Papua New Guinea

Geochemical data from back-arc volcanic zones in the Manus Basin are used to define five magma types. Closest to the New Britain arc are medium-K lavas of the island arc. association and back-arc basin basalts (BABB). Mid-ocean ridge basalts (MORB), BABB and mildly enriched T-MORB (transitional, MORB) occur along the Manus Spreading Center (MSC) and Extensional Transform Zone (ETZ). The MSC also erupted extreme back-arc basin basalts (XBABB), enriched in 1ight rare earth elements, P, and Zr. Compared with normal MORB, Manus MORB are even more depleted in high field strength elements and slightly enriched in fluid-mobile elements, indicating slight, prior enrichment of their source. with subduction-related components. Chemical variations and modeling suggest systematic, coupled relationships between extent of mantle melting, prior depletion of the mantle source, and enrichment in subduction-related components. Closest to the arc, the greatest addition of subduction-related components has occurred in the mantle with the greatest amount of prior depletion, which has melted the most. Variations in K₂O/H₂O indicate that the subduction-related component is best described as a phlogopite and/or K-amphibole-bearing hyhridized peridotite. Magmas.from the East Manus Rifts are enriched in Na and Zr with radiogenic ⁸⁷Sr/⁸⁶Sr, possibly indicating crustal interaction in a zone of incipient rifling. The source for XBABB and lavas from the Witu Islands requires a mantle component similar to carbonatite melt.