Breaking of Henry's law for noble gas and CO2 solubility in silicate melt under pressure

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Abstract

Degassing of the Earth is still poorly understood, as is the large scatter in He/Ar ratios observed in mid-ocean ridge basalts. A possible explanation for such observations is that vesiculation occurs at great depths with noble-gas solubilities different from those measured at 1 bar (ref. 1). Here we develop a hard-sphere model for noble-gas solubility and find that, owing to melt compaction, solubility may decrease by several orders of magnitude when pressure increases, an effect subtly overbalanced by the compression of the fluid phase. Our results satisfactorily explain recent experimental data on argon solubility in silicate melts, where argon concentration increases almost linearly with pressure, then levels off at pressures of 50–100 kbar (refs 2–5). We also model vesiculation during magma ascent at ridges and find that noble-gas partitioning between melt and CO2 vesicles at depth differs significantly from that at low pressure. Starting at 10 kbar (35 km depth), several stages of vesiculation occur followed by vesicle loss, which explains the broad variability of He–Ar concentration data in mid-ocean ridge basalts. ‘Popping rocks’, exceptional samples with high vesicularity, may represent fully vesiculated ridge magma, whereas common samples would simply have lost such vesicles.

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Figure 1: Calculated solubility parameters of pure noble gases in a tholeiitic melt at 1,673 K as a function of pressure.
Figure 2: Calculated argon concentration (weight fraction) for pure argon in contact with haplogranite, tholeiite and olivine melts, as a function of pressure.
Figure 3: Evolution of the vesicularity of a MORB with pressure during magma ascent.
Figure 4: Calculated 4 He/ 40 Ar ratio compared with measurements for MORBs.

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Acknowledgements

We thank P. Burnard for sharing data and discussions, and P. Richet for discussions and encouragements.

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Correspondence to Philippe Sarda.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Notes

A development of the model in some detail, references to the literature, and a legend for the Supplementary Figure S1. (DOC 264 kb)

Supplementary Figure S1

Two diagrams showing pressure (in kbar) vs. density (in g/cm3) for gases and for various silicate liquids, both using the Carnahan Starling equation of state. (PDF 66 kb)

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Sarda, P., Guillot, B. Breaking of Henry's law for noble gas and CO2 solubility in silicate melt under pressure. Nature 436, 95–98 (2005) doi:10.1038/nature03636

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