Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: Petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

Ultramafic, mafic and sedimentary xenoliths have been recovered from a recently erupted, shoshonitic submarine cinder cone (Tubaf and Edison volcanoes) from the Tabar-Lihir-Tanga-Feni island arc, located in the New Ireland basin of Papua New Guinea. These samples represent a proxy drill hole that can be re-assembled into an 'ophiolite-type' model of oceanic lithosphere. Petrographic and geochemical examination of the gabbroic and depleted mantle xenoliths indicates that the New Ireland fore-arc lithosphere is a fragment of ancient Pacific Plate generated at a mid-ocean ridge spreading centre and transported to the Pacific-Australian Plate margin. Convergent margin processes subjected the harzburgitic mantle wedge to hydrofracturing and hydration metasomatism at T=790-1030°C as a consequence of dewatering of a subducted slab. Advection of a high-density, H₂O-rich fluid containing a substantial dissolved component (alkali aluminosilicate melt and aqueous carbon and sulphur species) through these mantle fractures caused a net transfer of soluble elements from the lower to upper mantle wedge and created a network of oxidised (ΔFMQ ≈ 1.8-2.0) metasomatised peridotite enriched in orthopyroxene, clinopyroxene, phlogopite, amphibole, magnetite, and Fe-Ni sulphides. The vein mineral assemblage magnetite+sulphide indicates precipitation from a hydrous fluid with high SO₂/H₂S, consistent with the hydrous fluid being derived from dehydration of subducted, altered oceanic crust. Preferential partial melting of these metasomatically enriched mantle wedge regions could account for the highly oxidised, sulphur- and alkali-rich nature of the high-K calc-alkaline volcanoes of the Tabar-Lihir- Tanga-Feni island chain.