Manam Island, Papua New Guinea: Petrology and Geochemistry of a Low-TiO2 Basaltic Island-Arc Volcano

Manam volcano consists of relatively mafic and compositionallysimilar tholeiitic basalts and low-SiO₂ andesites that are characterizedby notably low (mainly 0?3–0?35 weight per cent) TiO₂contents. These rocks provide an ideal opportunity to investigateboth the extent of depletion in their peridotite magma-sourceregions (which are evidently similar in many respects to thehighly depleted sources of boninitic magmas), and the interplayof the high-level processes of magma mixing, crystal fractionation,and upper crustal contamination, in an island-are volcano. Manamrocks have pronounced enrichments in Rb, Ba, K, and Sr relativeto the light rare-earth elements and, especially, to the high-field-strengthelements (Sr/Ti values are exceptionally high). However, thereis no compelling evidence that these enrichments were causedby addition of a hydrous, slab-derived component to the peridotitesource region. Nd and Sr-isotope ratios plot within the oceanicmantle array; ²⁰⁷Pb/²⁰⁴Pb values are only slightly higher thanthose for oceanic rocks; and the absence of hydrous minerals,the early crystallization and modal preponderance of plagioclaseover pyroxene, high estimated quenching temperatures, and lowwater contents in the Manam rocks, are all evidence that themagmas crystallized under markedly water-undersaturated conditions.Unusually anorthite-rich plagioclase phenoerysts in the morediffrentiated rocks may correspond to crystallization underhigher water-vapour pressures, possibly caused by influxes ofgroundwater, or they may be accidental xenocrysts. Fractionationof olivine, clinopyroxene, and spinel (early chromite followedby magnetite) has dominated the evolution of the magma series.However, clear correlations between incompatible trace-elementratios, ⁸⁷Sr/⁸⁶Sr, and 100 Mg/(Mg ? Fe^2?) values are convincingevidence for an accompanying mixing process—either of(1) two basaltic magma types (one more fractionated and lowerin ⁸⁷Sr/⁸⁶Sr than the other), or (2) pristine magmas and contaminantfrom basaltic conduit and reservoir wall rocks. Wall-rock contaminationis the less likely process, and is the more difficult one toidentify, particularly if it accompanied magma mixing.