Redox evolution of a degassing magma rising to the surface

Burgisser, Alain
Scaillet, Bruno

¹ISTO, UMR 6113 Université d'Orléans-CNRS, 1a rue de la Férollerie, 45071 Orléans cedex 2, France.

doi:10.1038/nature05509

Received 3 April; accepted 28 November 2006.

Supplementary Information is linked to the online version of the paper at www.nature.com/nature.

Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to A.B. ([email protected]).

Nature 445(7124):p 194-197, January 11, 2007.

Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma dynamicsand the composition of the Earth's atmosphere. In particular, the redox state of volcanic gases emanating at the Earth's surface is widely believed to mirror that of the magma source, and is thought to have exerted a first-order control on the secular evolution of atmospheric oxygen. Oxygen fugacity (f_O₂) estimated from lava or related gas chemistry, however, may vary by as much as one log unit, and the reason for such differences remains obscure. Here we use a coupled chemical-physical model of conduit flow to show that the redox state evolution of an ascending magma, and thus of its coexisting gas phase, is strongly dependent on both the composition and the amount of gas in the reservoir. Magmas with no sulphur show a systematic f_O₂increase during ascent, by as much as 2 log units. Magmas with sulphur show also a change of redox state during ascent, but the direction of change depends on the initial f_O₂in the reservoir. Our calculations closely reproduce the H₂S/SO₂ratios of volcanic gases observed at convergent settings, yet the difference between f_O₂in the reservoir and that at the exit of the volcanic conduit may be as much as 1.5 log units. Thus, the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Our findings may require re-evaluation of models aimed at quantifying the role of magmatic volatiles in geological processes.