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

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 813C = -4.6 to -0.9%o, while alkali basalts range from -7.2 to -2.1%o. Calculated total (vesicle+glass) 813C for the majority of the basalts ranges from -6 to -2%o assuming the vapor-melt fractionation factor A (= 8vapor - 8melt) is +2 to +4%o as measured in basaltic systems. This relatively narrow range of 813C 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 813C for Loihi undegassed parental magmas is -1.5%o and the 813C range is -4 to +1%o. Estimates of this 813C range are only weakly dependent on the choice of A and initial [CO2] in closed-system degassing scenarios. The Loihi mantle plume source is therefore characterized by 813C values that are higher than the range of -6 to -4%o 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.

Automated summary

We analyzed the carbon isotope composition of vesicle CO2, He isotopes and concentrations in basalts from Loihi Seamount, and found that alkali basalts have a wider range of carbon isotope composition compared to tholeiites and transitional basalts. The restricted range of carbon isotope values for Loihi basalts suggests a degassing process, but the similarity to other mantle-derived basalts presents a conundrum. The high carbon isotope values in Loihi basalts may be due to primordial carbon isotope heterogeneity, exchange of carbon at the core-mantle boundary, or the presence of recycled carbonate in the Hawaiian plume source region.