Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea

Abstract

The SiO2‐undersaturated lavas from Lihir island, Papua New Guinea, like most arc lavas are highly enriched in Sr, Ba, K, Rb, and Cs and depleted in Hf, Ta, Nb, and Ti relative to ocean floor basalts and oceanic island basalts. These alkali‐rich lavas have arc trace element signatures and Nd, Sr, and Pb isotopic systematics. However, they are not a product of present‐day subduction, as this volcanism has tapped mantle which was enriched by prior subduction episodes. The narrow range of Pb (206pb/204pb, 18.74–18.76) and Nd (143Nd/144Nd, 0.51297–0.51304) isotopic compositions suggest a cogenetic origin for these lavas. During the fractionation of the primitive Lihir lavas, elements normally considered incompatible (i.e., the light rare earth elements (LREE), Rb, Th, and P) have high bulk solid/melt partition coefficients (0.15–1.5). Relatively higher partition coefficients during forma‐tion of the evolved lavas produced crossing rare earth element (REE) patterns, and primitive lavas have higher incompatible elements abundances than evolved lavas. This results from (1) changes in the amount of apatite in the fractionating assemblage and (2) the abnormal partitioning of trace elements into apatite microphenocrysts which nucleate at the crystal‐liquid interface of rapidly growing clinopyroxene phenocrysts. The Lihir lavas have lower alkali, Sr, Ba, K, Rb, Cs, and LREE abundances than other Tabar‐Feni lavas. They are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands. The similarity of Sm/Nd ratios of these undersaturated arc lavas to those of tholeiitic and calc‐alkaline arc lavas and the moderate chondrite‐normalized La/Yb (La/Ybcn = 3–7) indicates that there has been limited enrichment of the LREE relative to the heavy REE during generation of the arc‐modified source mantle. The alkaline nature of these lavas reflects their generation, in a tensional tectonic environment, from a “fossil” arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes.