Back Arc Basalts With Subduction Signature?

This study focuses on the geochemistry of submarine basalts from the Fonualei Rifts, a nascent back arc spreading system which lies on the north-eastern corner of the Lau Basin ca. 350km to the west of the trench of the northernmost segment of the Tonga-Kermadec subduction zone, where subduction is very fast (up to 240mm/y). The rift zone extends from the active volcanic island of Fonualei northwards to the Mangatolu Triple Junction (MTJ). The samples were recovered between 1300 and 2500m depth. The glassy rims of the quenched lavas were analysed using SEM, EPMA and LA-ICP-MS, and a thorough geochemical dataset was obtained. The samples range from basalts to basaltic andesites (SiO2 = 49 to 56wt%, Mg# from 25 to 45). Most samples are aphyric to microphyric; if present, the crystalline phases are primarily olivine, orthopyroxene and plagioclase. The water content is generally low (<1wt%); S is low too (100-200ppm) except for the MTJ samples which have an order of magnitude more S than the other rift samples. Cl contents range from 500 to 1000ppm, the exception again being the MTJ samples which have distinctively less Cl at 200ppm. The chondrite-normalised REE patterns of all samples are flat at ~10. No Eu anomaly can be seen, suggesting that no major plagioclase fractionation has taken place. No major differences in the REE patterns can be seen along the spreading centre. The samples originating from the MTJ show an overall REE content slightly higher than the samples from the rift itself. The N-MORB-normalised spidergram shows a clear subduction signature, with typical LILE enrichment, U higher than Th, strong positive Pb anomaly, and strong negative Nb anomaly. Again, the patterns for all samples are comparable, except for the samples from the MTJ which show slightly smoother patterns, with less pronounced anomalies. This suggests that there is a stronger mantle melting component in the source region of the MTJ rocks. We suggest that the presence of the subduction signature in the investigated back arc basalts is related to the unique plate configuration in this part of the Lau Basin. The arc in front of the Fonualei rift is no longer active and it appears that arc activity shut down when the rift started. This leads to the conclusion that all of the subduction zone flux is captured by the back arc.