Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle

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The analysis of volatiles in magmatic systems can be used to constrain the volatile content of the Earth's mantle and the influence that magmatic degassing has on the chemistry of the oceans and the atmosphere. But most volatile elements have very low solubilities in magmas at atmospheric pressure, and therefore virtually all erupted lavas are degassed and do not retain their primary volatile signatures. Here we report the undersaturated pre-eruptive volatile content for a suite of mid-ocean-ridge basalts from the Siqueiros intra-transform spreading centre. The undersaturation leads to correlations between volatiles and refractory trace elements that provide new constraints on volatile abundances and their behaviour in the upper mantle. Our data generate improved limits on the abundances of carbon dioxide, water, fluorine, sulphur and chlorine in the source of normal mid-ocean-ridge basalt. The incompatible behaviour of carbon dioxide, together with the CO2/Nb and CO2/Cl ratios, permit estimates of primitive carbon dioxide and chlorine to be made for degassed and chlorine-contaminated mid-ocean-ridge basalt magmas, and hence constrain degassing and contamination histories of mid-ocean ridges.

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Figure 1: Pressure for CO2–H2O saturation, and comparison with pressure of sample collection.
Figure 2: CO2 contents versus Nb abundances in Siqueiros and other MORB samples.
Figure 3: Evaluation of primary Cl content in Siqueiros and other MORB samples.
Figure 4: Relationship between sulphur fugacity fs2 and dissolved sulphur in basaltic liquids along the sulphide saturation surface.


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We thank J. Wang for assistance with the ion probe; C. Hadidiacos and C. Mandeville for help with electron probe analyses; J. Dixon for discussions and access to unpublished MORB volatile data; K. Donnelly, K. Simon, J. Y. Su and P. Asimow for discussions; and W. White and P. Michael for comments and suggestions that considerably improved the manuscript. This work was supported by the US NSF.

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Correspondence to Alberto E. Saal.

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Saal, A., Hauri, E., Langmuir, C. et al. Vapour undersaturation in primitive mid-ocean-ridge basalt and the volatile content of Earth's upper mantle. Nature 419, 451–455 (2002) doi:10.1038/nature01073

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