Melting and source variability in the Tonga-Kermadec-Lau arc- back-arc system
Melting along back-arc spreading centres is dominated by decompression melting typical for ocean floor spreading but with influence by fluids from the subducting slab. Slab derived fluids may enhance the degree of melting exceeding that from decompression melting at mid-ocean ridges. Basaltic lavas occurring along back-arcs are the so-called back-arc basin basalts (BABB) showing elevated water contents and an enrichment in large-ion lithophile elements (LILE) and depleted in high-field-strength elements (HFSE) relative to normal mid-ocean ridge basalts (N-MORB). The elemental and isotopic signature of the subducting slab in lavas from the back-arc is variable in dependence of several processes, amongst which the distance from the volcanic arc and the dip of the subducting slab are particularly important.
Due to the complexity of processes influencing the composition of BABB, backarc systems are not fully understood. In order to make further constraints on the melting and source variability along back-arcs, we selected two areas that are located at the opposite ends of the Lau basin of the Tonga Arc system in the Pacific Ocean. From north to south, the Lau basin displays a decreasing distance from the arc. While the northernmost Lau basin is known as a tectonically highly active region with high mantle temperatures, the spreading rate decreases towards the southernmost Lau spreading centre. In the south, the distance from the volcanic arc is the lowest and previous studies already assumed a high subduction influence.
Major and trace element contents along with water contents and Sr-Nd-Pb isotopic ratios from the North-East Lau Spreading Centre (NELSC; Chapter 3) at the northern end of the basin were analysed to investigate the processes that are dominating the geochemical composition of melts in this area. Sampling was conducted at three different settings that have sparsely been sampled before: the transition to the rear-arc (Diagonal Ridge), the back-arc spreading centre (NELSC) and seamounts from the south. The data show that the area is influenced by three sources variably distributed along the sampled volcanic systems and reflect the heterogeneous mantle of this area. Two endmembers are highly enriched in Pb-isotopes compared to common lavas from the back-arc, and are potentially derived from Ocean Island Basalt (OIB) mantle sources. Although previous studies indicated that the mantle source signature from the subducted Louisville Seamount chain is restricted to the arc, I can show that its reactivation due to slab rollback affects also the rear-arc in this area. The spreading centre is influenced by an OIB source highly similar to the Samoan mantle plume located further north. While the southernmost seamounts are directly connected to the spreading centre and display the continuing spreading towards the south, there is no extensive melt flow between the rear-arc and spreading centre.
Previous melting models have shown that the Lau basin is dominated by two styles of melting: dry melting at the arc averted side of the spreading axis and wet melting on the arc facing side. With increasing distance from the arc the subduction input to the overlying mantle wedge decreases and therefore the effect of wet melting decreases. In the Lau basin, this has been observed as a gradual change from north to south as the distance to the arc decreases. To further investigate the melting model, I used a sample set from an E-W transect (Chapter 4) from the southernmost tip of the Lau Basin in order to disentangle the spatial scales on which changes in fluid input from the slab influence the melts that erupt in back-arcs. Using Uraniumseries and water contents, I can show that the melting zones in the Tonga backarc in <50 km distance from the arc overlap and mix with each other and a gradual decrease of the slab input cannot be seen.