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Abstract
Our present understanding of the mantle carbon budget is in part built upon measurements of carbon concentrations in olivine hosted melt inclusions. Only a small number of such datasets are thought to have avoided degassing, having been entrapped prior to CO2 vapour saturation, and are therefore able to constrain primary CO2 concentrations. The absence of degassing in melt inclusion datasets has been inferred from the presence of strong correlations between CO2 and trace elements. In this contribution, we demonstrate that partial degassing followed by magma mixing not only retains such positive correlations, but can enhance them.Simple models of magma mixing and degassing are used to characterise how CO2-trace element systematics respond to CO2 vapour saturation in primary mantle melts entering the crust, followed by magma mixing. Positive correlations are expected between CO2 and most trace elements, and the average CO2/Ba and CO2/Nb ratios are controlled by the pressure of magma storage, rather than the CO2 concentration in the mantle. We find that the best estimates of mantle CO2 are the maximum CO2/Ba ratios observed in melt inclusion datasets, though a large number of analyses are required to adequately characterise the maximum of the CO2/Ba distribution. Using the mixing and degassing models we estimate the number of analyses required to obtain a maximum CO2/Ba observation within 10% of the mantle value.In light of our results, we reassess existing melt inclusion datasets, and find they exhibit systematics associated with partial degassing and mixing. We argue that all the data presently available is consistent with a depleted mantle CO2/Ba ratio of ∼140, and there is as yet no evidence for heterogeneity in the CO2/Ba ratio of the depleted mantle.
Highlights
The mantle is the largest reservoir by mass in the Earth and through volcanism and subduction remains in continual chemical communication with Earth’s atmosphere and oceans
It is this final observational approach that we focus on in this contribution, since undegassed melts provide the most direct constraint on mantle CO2 concentrations and their CO2-trace element systematics can provide a probe of the redox state of mantle carbon (Eguchi and Dasgupta, 2017)
In addition to the systematics in trace element (El)–CO2 space, we show the behaviour in CO2/El–El space (Fig. 4b), and CO2/El–1/El space (Fig. 4c)
Summary
The mantle is the largest reservoir by mass in the Earth and through volcanism and subduction remains in continual chemical communication with Earth’s atmosphere and oceans. Primary CO2 concentrations in basaltic glasses have been inferred by numerous methods: by correcting for degassing using C isotope fractionation (Cartigny et al, 2008); by using Cl concentrations as a proxy for CO2 concentrations (Shimizu et al, 2016); by using maximum values of CO2/Nb ratios in partially degassed datasets (Shaw et al, 2010; Helo et al, 2011; Wanless et al, 2014); and by looking at the covariation of CO2 with Nb or Ba in undegassed suites (Hauri et al, 2002; Saal et al, 2002; Michael and Graham, 2015; Shimizu et al, 2016; Le Voyer et al, 2017) It is this final observational approach that we focus on in this contribution, since undegassed melts provide the most direct constraint on mantle CO2 concentrations and their CO2-trace element systematics can provide a probe of the redox state of mantle carbon (Eguchi and Dasgupta, 2017).
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