Abstract
New high‐density sampling of the Eastern Lau Spreading Center provides constraints on the processes that affect the mantle wedge beneath a back‐arc environment, including the effect of the subduction input on basalt petrogenesis and the change in subduction input with distance from the Tonga arc. We obtained trace element and Pb‐Sr‐Nd isotopic compositions of 64 samples distributed between 20.2°S and 22.3°S with an average spacing of ∼3.6 km. The trace element and isotope variations do not vary simply with distance from the arc and reflect variations in the mantle wedge composition and the presence of multiple components in the subduction input. The mantle wedge composition varies form north to south, owing to the southward migration of Indian‐like mantle, progressively replacing the initially Pacific‐like mantle wedge. The mantle wedge compositions also require an enriched mid‐ocean ridge basalt–like trace element enrichment that has little effect on isotope ratios, suggesting recent low‐degree melt enrichment events. The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas. The combination of the back‐arc and arc data allows identification of several components contributing to the subduction input. These are a fluid derived from the altered oceanic crust with a possible sedimentary contribution, a pelagic sediment partial melt, and, in the southern Lau basin, a volcaniclastic sediment partial melt. While on a regional scale, there is a rough decrease in subduction influence with the distance from the arc, on smaller scales, the distribution of the subduction input reflects different mechanisms of the addition of the subduction input to a variable mantle wedge.
Highlights
Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin–Tonga arc system
The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas
In this paper we report new isotopic and trace element data from the Eastern Lau Spreading Center (ELSC), as well as new Pb isotope data for the Tonga arc, which we compile with published data
Summary
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Lamont-Doherty Earth Observatory, Earth Institute at Columbia University, 61 Route 9W, Palisades, New York 10964, USA Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W, Palisades, New York 10964, USA Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Department of Geosciences, University of Tulsa, 800 South Tucker Avenue, Tulsa, Oklahoma 74104, USA [1] New high-density sampling of the Eastern Lau Spreading Center provides constraints on the processes that affect the mantle wedge beneath a back-arc environment, including the effect of the subduction input on basalt petrogenesis and the change in subduction input with distance from the Tonga arc. We obtained trace element and Pb-Sr-Nd isotopic compositions of 64 samples distributed between 20.2°S and 22.3°S with an average spacing of $3.6 km. The trace element and isotope variations do not vary simply with distance from the arc and reflect variations in the mantle wedge composition and the presence of multiple components in the subduction input. The mantle wedge composition varies form north to south, owing to the southward migration of Indian-like mantle, progressively replacing the initially Pacific-like mantle wedge. The mantle wedge compositions also require an enriched mid-ocean ridge basalt–like trace element enrichment that has little effect on isotope ratios, suggesting recent low-degree melt enrichment events. The composition of the subduction input added to the mantle wedge is geographically variable and mirrors the changes observed in the Tonga arc island lavas. The combination of the back-arc and arc data allows identification of several components contributing to the subduction input. These are a fluid derived from the altered oceanic crust with a possible sedimentary contribution, a pelagic sediment partial melt, and, in the southern Lau basin, a volcaniclastic sediment partial melt. While on a regional scale, there is a rough decrease in subduction influence with the distance from the arc, on smaller scales, the distribution of the subduction input reflects different mechanisms of the addition of the subduction input to a variable mantle wedge. Components: 18,974 words, 17 figures, 3 tables. Keywords: back-arc basin; back-arc basalt; subduction component; mantle wedge; enrichment processes.
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