Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone

We have measured δ13C values and CO2/3He ratios of vesicle-gas, and chemical compositions of North Fiji Basin basalt glasses, to estimate the contribution of subducted carbon in back-arc basin basalt quantitatively. It has made clear that the CO2/3He ratio increases and the δ13C value decreases with K2O content. Since the variety of K2O content of North Fiji Basin basalt is the result of two-component mixing, the variety of the CO2/3He ratio and δ13C value of the North Fiji Basin basalt should be mainly a result of mixing between the mantle component (low-CO2/3He, high-δ13C and low-K2O) and the subducted component (high-CO2/3He, low-δ13C and high-K2O). From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most (∼90%) carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc, since the subducting carbonate and organic matter throughout the North Fiji subduction zone are estimated in a proportion of 20:1. This may be one clue that carbonate can be transported into the mantle through the subduction zones.