The mantle is an important, yet poorly understood, part of Earth’s carbon cycle; interacting with Earth’s surface through volcanism and subduction. The CO₂ flux balance in to and out of the mantle regulates the mass of CO₂ in Earth’s crust and hydrosphere, exerting control over the evolution of Earth’s climate and carbon availability for life. However, carbon’s volatility, and therefore tendancy to degas from magmas and emanate at Earth’s surface diffusely, has made identifying the present-day mantle carbon distribution difficult.

Droplets of magma trapped within crystals as they grow deep in the crust offer a chance of observing CO₂ concentrations in magmas prior to degassing. The behaviour of CO₂ during magma evolution is encoded in the covariation of CO₂ and trace element concentrations. In a small number of datasets, a correlation between CO₂ and either Ba or Nb has been reported; consequently identical behaviour, in particular a lack of degassing, has been inferred. These, apparently undegassed, datasets underpin our understanding of carbon distribution in the mantle.

In this paper, we argue that many of the melts supplied from the mantle should be oversaturated in CO₂ vapour at the pressure of magma storage, whilst others will be sufficiently depleted in CO₂ that they should be strongly undersaturated. Such a population of melts will tend to partially degas at the earliest stages of melt evolution, before subsequent mixing and fractionation. We show that positive correlations between CO₂ and both Ba and Nb, are a natural consequence of this process. Furthermore, our new model makes specific predictions about the covariance of CO₂ with a gamut of trace elements, if partial degassing and mixing has taken place.

Since we demonstrate that positive correlations between CO₂ and trace element concentrations are arise from partial degassing and mixing, we cannot use this as a criterion for identifying whether a dataset has been affected by degassing. Mantle carbon contents, derived by assuming such melts preserve primary CO₂ concentrations, are likely to be underestimates. We find the maximum CO₂/Ba ratio in a dataset is the best proxy for mantle carbon content.

Matthews, S., O. Shorttle, J. F. Rudge and J. Maclennan (2017), Constraining mantle carbon: CO₂-trace element systematics in basalts and the roles of magma mixing and degassing, Earth and Planetary Science Letters, 480, 1-14. doi:10.1016/j.epsl.2017.09.047