Volatile element isotopic systematics of the Rodrigues Triple Junction Indian Ocean MORB: implications for mantle heterogeneity

Mid-ocean ridge basalts (MORBs) from the Indian Ocean have been well-known to differ in Sr–Nd–Pb isotopic composition from those of the North Atlantic and the East Pacific. To understand if such heterogeneity exists also in volatile elements, we have measured elemental and isotopic composition of carbon, nitrogen, helium, and argon in trapped-gas-vesicles of the Rodrigues Triple Junction (RTJ) Indian Ocean MORBs (IOMs). The observed C–N–He–Ar isotopic ratios of the IOMs generally agree with average values of North Atlantic MORBs (NAMs) and East Pacific MORBs (EPMs). In contrast, the N2/40Ar, C/N, and CO2/3He ratios in fluid inclusions of the IOMs are quite unusual: current estimates of the N2/40Ar, C/N, and CO2/3He trapped in vesicles from the NAMs and EPMs are 91±20, 215∼4600, and (1.5±0.5)×109, respectively, whereas those of the IOMs are 120±36, 26∼190, and (0.3±0.1)×109, respectively. The 40Ar/36Ar and the 4He/40Ar* ratios indicate that the unusual N2/40Ar, C/N, and CO2/3He ratios of IOMs in this study have not been produced by atmospheric contamination or progressive fractional degassing, respectively, but more likely by mantle heterogeneity. The IOMs analyzed in this study seem to be depleted in the recycled carbon than the previous reported NAMs and EPMs. Assuming that values of IOMs in this study as those of the primitive (juvenile) mantle carbon, it is derived that most (65∼95%) carbon in the previously reported NAMs and EPMs is the recycled carbon (carbonate and organic matter) in origin, from a simple mass balance calculation. This may be one clue of the carbon recycling in the mantle.