U-series Isotope Data on Lau Basin Glasses: the Role of Subduction-related Fluids during Melt Generation in Back-arc Basins

New thermal ionization mass spectrometry U-series disequilibriumdata are presented for 24 basaltic to dacitic glasses from activespreading centres in the back-arc Lau Basin (SW Pacific), togetherwith additional inductively coupled plasma mass spectrometrytrace element analyses and Sr–Nd–Pb isotope data.Valu Fa Ridge samples, adjacent to the arc front, have highU/Th and (²³⁰Th/²³⁸U) <1, implying a recent (<<350ka) addition of a U-rich slab-derived fluid. The Valu Fa datacan be combined with existing ²³⁰Th–²³⁸U data for theCentral Tonga arc to infer a fluid addition event at     

Os Isotopes and the Origin of the Tasmanian Dolerites

New Os isotope data obtained for oxides separated from samplesof the Jurassic dolerites of Tasmania (Australia) are used toconstrain the petrogenesis of the Ferrar continental flood basaltprovince. It is proposed here that the unradiogenic initialOs ratio (¹⁸⁷Os/¹⁸⁸Os = 0·145 ± 0·049 2     

Geochemical and Isotopic Constraints on the Origin of the Jurassic Dolerites of Tasmania

The Jurassic dolerites of Tasmania represent a small part ofthe widespread Mesozoic magmatism of Gondwana, and possess someof the most extreme isotopic and elemental signatures observedin tholeiitic rocks. This study documents the remarkable uniformityof the magma in major, trace element and isotopic compositionat the time of emplacement. Many of the trace element and isotopiccharacteristics compare more closely with those of continentalcrustal rocks than with typical mantle-derived materials, indicatingthe dominance of a crustal component in the petrogenesis ofthese tholeiites. Difficulties in reproducing the trace elementsignatures of the dolerites using calculations involving crustalassimilation, and the unreasonable isotopic compositions requiredof the contaminant, make such models unlikely. We argue that a more likely explanation involves the introductionof a small quantity of sediment (3 wt.%) into a depleted mantlesource by the process of subduction. It is envisaged that thetrace element and isotopic compositions of the mantle sourcewere overprinted by the crustal signatures owing to the lowabundance of incompatible trace elements originally present.These crustal signatures were subsequently inherited by magmasproduced during partial melting.