Abstract
We have measured δ 13C values and CO 2/ 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 CO 2/ 3He ratio increases and the δ 13C value decreases with K 2O content. Since the variety of K 2O content of North Fiji Basin basalt is the result of two-component mixing, the variety of the CO 2/ 3He ratio and δ 13C value of the North Fiji Basin basalt should be mainly a result of mixing between the mantle component (low-CO 2/ 3He, high- δ 13C and low-K 2O) and the subducted component (high-CO 2/ 3He, low- δ 13C and high-K 2O). 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.
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