CO2 content beneath northern Iceland and the variability of mantle carbon

Primitive basalt melt inclusions from Borgarhraun, northern Iceland, display large correlated variations in CO 2 and nonvolatile incompatible trace elements (ITEs) such as Nb, Th, Rb, and Ba. The average CO 2 /ITE ratios of the Borgarhraun melt inclusion population are precisely determined (e.g., CO 2 /Nb = 391 16; 2M [two standard errors of the mean], n = 161). These data, along with published data on five other populations of undegassed mid-oceanic ridge basalt (MORB) glasses and melt inclusions, demonstrate that upper mantle CO 2 /Ba and CO 2 /Rb are nearly homogeneous, while CO 2 /Nb and CO 2 /Th are broadly correlated with long-term indices of mantle heterogeneity reflected in Nd isotopes ( 143 Nd/ 144 Nd) in five of the six regions of the upper mantle examined thus far. Our results suggest that heterogeneous carbon contents of the upper mantle are longlived features, and that average carbon abundances of the mantle sources of Atlantic MORB are higher by a factor of two than those of Pacific MORB. This observation is correlated with a similar distinction in water contents and trace elements characteristic of subduction fluids (Ba, Rb). We suggest that the upper mantle beneath the younger Atlantic Ocean basin contains components of hydrated and carbonated subduction-modified mantle from prior episodes of Iapetus subduction that were entrained and mixed into the upper mantle during opening of the Atlantic Ocean basin.