Mantle source variations beneath the Eastern Lau Spreading Center and the nature of subduction components in the Lau basin–Tonga arc system

Abstract

[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.

Chemistry of hot springs along the Eastern Lau Spreading Center

“…. The data are plotted against the two-stage dissolution-precipitation model of which shows one set of contours (thin solid lines) for Sr concentration in solution following early precipitation of anhydrite during recharge, and a second set (dashed lines) for the water/rock mass ratio prevalent during subsequent water/rock interaction in the high-temperature reaction zone with rock having a Sr isotopic ratio of 0.7032, as on the Valu Fa Ridge (Vallier et al, 1991;Escrig et al, 2009). Regression of the vent fluid data (thick solid line) indicates that the majority of ELSC fluids are consistent with approximately 75% Sr removal and water/rock ratios from 10 to 20 (Table 4).…”

“…A plot of the ratio of Ba/La, both highly incompatible elements, from north to south exhibits a pronounced step to higher values near the ABE vent field, reflecting a step-like change to a more andesitic composition and greater subduction influence ( Fig. 3; Escrig et al, 2009). The silica content of lavas recovered in 1989 from near the latitude of the Tui Malila vent field indicates rocks of basaltic andesite composition .…”

Geochemistry of deep-sea hydrothermal vent fluids from the Mid-Cayman Rise, Caribbean Sea

“…However, host rock composition at Lau includes dacite, andesite, basaltic-andesite, and basalt (FOUQUET et al, 1993;ESCRIG et al, 2009). Studies indicate a general increase in the proportion of felsic rocks with increasing proximity of the spreading center to the active Tofua arc to the South (MARTINEZ and TAYLOR, 2002;ESCRIG et al, 2009;MOTTL et al, 2011). More arc-influenced rocks are potentially enriched in 34 S relative to basalt (TAYLOR, 1986), and greater 34 S-enrichment may therefore be anticipated in the southernmost vent fields, closer to the arc.…”

Louisville seamount subduction and its implication on mantle flow beneath the central Tonga–Kermadec arc

“…This increase has been attributed to the north-south reduction in distance between the Valu Fa ridge and the arc front volcanoes, which reaches a minimum of B40 km between the arc front and the southern tip of the Valu Fa Ridge 35 , and whose melts therefore may have influenced the melt genesis in the back-arc mantle (for example, ref. 33). A single analysis from the southernmost Lau Basin sampled at 23.9°S 32 , however, shows relatively low (La/Sm) N of B0.6, also suggesting that local source enrichment beneath the Ata region has taken place.…”