Geochemical systematics of Northeastern Lau basin affected by fertile plume

Abstract:

Lau basin is located in the South Pacific Ocean, forming a triangle shaped oceanic rift basin. It has been made by the collision between the Indian-Australian and Pacific plate, and is producing the specific kind of basalt characterized by intermediate chemistry between the MORB and the IAB. In particular, there is a tear along the northern boundary of the basin, by which the fertile plume can inflow to the mantle wedge beneath the basin. We conducted the petrological study intended to the Back-Arc Basin Basalt in the North Lau basin. There are a total 23 samples from the Fonualei Rift and Spreading Center (FRSC) and 9 samples from the Northeast Lau Spreading Center (NELSC) that were collected. In the spider diagram for trace elements, there is successive change from the Tofua arc through FRSC to NELSC, while the Samoa Island has a different pattern. This tendency presents itself in the diagram for Rare-earth elements as well. The fertility enhances from south to north, having more distinct variations in NELSC compared with FRSC. The degree of melting calculated by fractionation-corrected Na and Fe contents has no significant range in accordance with the assumption that the fertility variations are caused by the melt input, not a degree of melting. Isotope modeling by end-members used in previous studies in this area indicates that NELSC has a 10% more intense influence than FRSC in regards to the fertile plume, itself corresponds with the edifices and previous studies. Influence of subduction-induced fluid indicate that NELSC is less affected by both shallow and deep subduction fluids than FRSC, the subduction input from the northern boundary of the two plates therefore does not contribute to the North Lau basin distinctively. We also plotted the contributions of two main factors, plume (fertile melt input) and fluid (subduction-induced input). The plots show that the plume dominates in the northern part and the fluid dominates in southern part relatively. So, we conclude that NELSC located in the northernmost part of Lau basin is mainly affected by fertile plume, yet further studies are necessary to whether the derivation of this plume is the Samoan Island or not.