Geochemical variation of back-arc basin basalt and its genesis

Back-arc basin basalt (BABB) is the product of rifting and spreading ridge magmatism of back-arc basin. The geochemical compositions of BABB record the information of their genesis, which is the key to understand the tectonic evolution of back-arc basin. Back-arc basins are mainly distributed in the Western Pacific. In this study, we summarized the elemental and isotopic characteristics of BABBs from the Western Pacific. In general, the ranges of major elements of BABBs are much larger than those of mid-ocean ridge basalt (MORB). For a given MgO, BABBs have higher Al2O3 content and lower TiO2 content than MORBs, deviating from the elemental trends of MORB on the plots of Al2O3 and TiO2 versus MgO. The trace elemental characteristics of BABB are generally between MORB and arc basalts. On the one hand, BABB has no obvious elemental fractionation between medium and heavy rare earth elements, which is similar to MORB. On the other hand, BABB is rich in Rb, Ba, Th, U, K, with positive Pb anomalies and negative Nb, Ta anomalies, which is similar to arc basalts. Among BABBs, some samples from Lau Basin, the Japan Sea Basin, and the Okinawa Trough have positive anomalies of Nb and Ta, similar to those of E-MORB. The Sr-Nd-Pb isotopic compositions of BABBs in Western Pacific region vary in large ranges. Compared with MORB, the enriched components in mantle sources are more common from BABB. In general, the isotopic compositions of BABB are between the depleted mantle member (DMM), the type-1 enriched mantle (EMI), and the type-2 enriched mantle (EM2). Geochemical difference is also observed for BABB between different basement settings (continental basement versus oceanic basement) and different evolution stages of back-arc basin. The geochemical diversity of BABB is mainly controlled by mantle source (the mantle wedge) heterogeneity, the degree of partial melting, and the magmatic processes during magma transportation. The mantle source heterogeneity is reflected by the difference in chemical properties of mantle wedge itself and material contributions from subduction plate. The abnormally high mantle potential temperature, high 3He/4He ratio, and E-MORB-type trace elemental characteristics in some BABB further indicate that their mantle source could have been affected by mantle plume activities. The higher mantle potential temperature and the more contribution of subduction fluid would cause the greater partial melting degree of the mantle wedge. In addition, crustal assimilation in continental lithosphere, melt-rock interaction in oceanic lithosphere, and the complicated history of fractional crystallization during magma transportation would all modify the geochemical compositions of BABB melts.