Person:

Escrig, S.

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 degree S and 22.3 degree 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.

Basalts from the Mid-Atlantic Ridge change progressively in composition with increasing distance from the Azores platform. Study of the Lucky Strike and Menez Gwen segments reveals much complexity in the gradient. Both segments contain only basalts enriched relative to normal mid-oceanic ridge basalt, but in two distinct groups. Moderately enriched basalts occur throughout the segments, with proximal Menez Gwen enriched relative to Lucky Strike. Highly enriched basalts occur at segment centers. Incompatible element ratios of the highly enriched basalts exceed those of the Azores platform, while isotopic compositions are less enriched. These observations can be explained by a low-degree melt of garnet-bearing Azores mantle added to mantle depleted by previous melt extraction. Melting this “metasomatized” mantle produces lavas that match the enriched samples. The Azores gradient cannot be explained by simple two-component mixing; rather, it reflects recent melt extraction and addition processes related to southward flow of the Azores plume. The Azores gradient also permits tests of segmentation models. Central supply models predict step functions in chemical compositions between segments. Within-segment gradients require vertical supply. Central supply is supported by robust central volcanoes, thicker crust at segment centers, and a step function in isotopes between the segments. The lava diversity at segment centers, however, requires batches of distinct magma that are preserved through melting and melt delivery. Within-segment gradients in moderately incompatible element ratios support a component of multiple supply. The data suggest partial homogenization of magma within a segment and preferential melt focusing to segment centers with some vertical transport.

Detailed major element, trace element and isotopic study of the FAMOUS and North Famous segments within the geochemical gradient south of the Azores platform provides new constraints on controls on chemical variations at the segment scale and the origin of plume geochemical gradients. A comprehensive investigation of 110 samples along the entire length of the FAMOUS segment, coupled with a recent extensive melt inclusion study by Laubier et al. (2012), shows large trace element diversity within a single segment and substantial isotopic variability that largely correlates with trace element variations. Substantial variations are also present along the short (18 km) North Famous segment despite the presence of an axial volcanic ridge. These results confirm multiple supply of magmas along the length of these segments, the lack of a centrally supplied magma chamber, and the ability of melting processes to deliver highly diverse melts over short distances and times. With the exception of one group of high Al2O3, low SiO2 magmas (HiAl–LoSi) largely recovered in the original small FAMOUS area, the data can be simply explained by a two-component mixing model coupled with melting variations. The HiAl–LoSi magmas reflect assimilation and mixing in the crust, an interpretation supported by the diverse melt inclusions in these lavas.Since the mantle heterogeneity reflects two-component mixing, the end members can be constrained. Surprisingly, source mixing between the Azores plume and depleted mantle cannot produce the observations. This is evident regionally from the fact that nearly all basalts have highly incompatible trace element ratios (e.g., Th/La, Nb/La) as high or higher than the most plume-influenced MORB near the Azores hotspot, despite being over 300 km farther south and much less enriched isotopically. To account for the elevated highly incompatible trace element ratios, a metasomatic component formed by adding deep, low-degree melts of Azores plume material to a depleted mantle is required. The regional gradient south of the Azores then requires different processes along its length. Close to the Azores, plume material mixes with depleted mantle. The pure plume influence is spatially restricted, and enrichment farther to the south is caused by shallow mantle metasomatized by low-degree melts from deep plume flow. North Famous lavas are spatially closer to the Azores and yet are more depleted in trace elements and isotopes than FAMOUS lavas, suggesting delivery of the enriched component to individual segments is influenced by additional factors such as segment size and offset. The extent to which these processes operate in other regions of plume–ridge interaction remains to be investigated.