Carbon isotope composition of basalts from Loihi Seamount: Primordial or recycled carbon in the Hawaiian mantle plume?

Abstract

We analyzed the carbon isotope composition of vesicle CO2, plus He isotopes and He and CO2 concentrations in the vesicle (vapor) and glass (melt) phases of 37 submarine basalts from the summit, north and south rifts, and east flank of Loihi Seamount. Tholeiites and transitional basalts lie in a narrow range of vesicle δ13C = −4.6 to −0.9‰, while alkali basalts range from −7.2 to −2.1‰. Calculated total (vesicle+glass) δ13C for the majority of the basalts ranges from −6 to −2‰ assuming the vapor-melt fractionation factor Δ (= δvapor - δmelt) is +2 to +4‰ as measured in basaltic systems. This relatively narrow range of δ13C resembles mantle source values deduced from gas-rich mid-ocean ridge basalts and basalts from Iceland, and for Kilauea volcano deduced from its fumarole gas. However, this similarity presents a conundrum because Loihi basalts have degassed >97% of their initial CO2 as deduced from CO2 - Ba systematics and crystal fractionation modeling.Loihi parental magma (MgO=18 wt.%) had initial CO2 concentrations of 0.6 to 1.9 wt.%. Most tholeiitic and transitional basalts appear to have followed a quasi closed-system degassing history. Correcting for this degassing indicates the median δ13C for Loihi undegassed parental magmas is −1.5‰ and the δ13C range is −4 to +1‰. Estimates of this δ13C range are only weakly dependent on the choice of Δ and initial [CO2] in closed-system degassing scenarios. The Loihi mantle plume source is therefore characterized by δ13C values that are higher than the range of −6 to −4‰ prevalent in mantle-derived basalts and many diamonds. This could be due to primordial carbon isotope heterogeneity in Earth's mantle, exchange of carbon at the core-mantle boundary between ultra-low velocity zone silicates and the metallic liquid outer core, or to the presence of a small fraction (<1% by mass) of surficial carbonate that was tectonically recycled to the Hawaiian plume source region. Currently, an origin from recycled carbonate has the most supporting evidence.