Role of fO₂ on fluid saturation in oceanic basalt

Abstract

Arising from: A. E. Saal, E. H. Hauri, C. H. Langmuir & M. R. Perfit Nature 419, 451–455 (2002); Saal et al. reply Assessing the conditions under which magmas become fluid-saturated has important bearings on the geochemical modelling of magmas because volatile exsolution may profoundly alter the behaviour of certain trace elements that are strongly partitioned in the coexisting fluid¹. Saal et al.² report primitive melt inclusions from dredged oceanic basalts of the Siqueiros transform fault, from which they derive volatile abundances of the depleted mantle, based on the demonstration that magmas are not fluid-saturated at their eruption depth and so preserve the mantle signature in terms of their volatile contents. However, in their analysis, Saal et al.² consider only fluid–melt equilibria, and do not take into account the homogeneous equilibria between fluid species, which, as we show here, may lead to a significant underestimation of the pressure depth of fluid saturation.

Main

For any basalt melt that is at fixed temperature and pressure in fluid-saturated conditions with known H₂O and CO₂ concentrations, the corresponding volatile fugacities, fH₂O and fCO₂, can be calculated³. The phase rule states that this in turn fixes the fugacities of all other C–O–H fluid species, including fO₂ (ref. 4). Figure 1a shows the covariation of the mole fraction of H₂O and CO₂ (XH₂O and XCO₂) in a C–O–H fluid calculated for various fO₂ at 1,200 °C and 400 bar (fO₂ expressed in log units relative to the solid buffer Ni–NiO, referred to here as NNO). It can be seen that at a very low mole fraction of H₂O (XH₂O < 0.05), reduced fluids are poorer in CO₂ than oxidized ones: for instance, at ΔNNO = −2 the mole fraction of CO₂ is 0.8, whereas at ΔNNO = −0.8, it is 0.95. This is due to the progressive reduction of CO₂ into CO, which becomes significant at fO₂ below ΔNNO = −1 (ref. 4).

Figure 1: Effect of fO₂ on fluid speciation and fluid saturation in basalts.

a, Covariation of XH₂O and XCO₂ (where Xi is the mole fraction of species i) in a C–O–H fluid calculated for various values of fO₂ (numbers along each curve). The calculations were done by fixing fH₂ (either 0.01 bar or 1 bar, corresponding to red and green symbols, respectively) and fH₂O, which allows us to calculate fCO₂ in the C–O–H system⁴. Once fH₂ and fH₂O are fixed, fO₂ can be calculated through the equilibrium H₂ + 0.5 O₂ = H₂O. T, 1,200 °C; P, 400 bar. b, H₂O–CO₂ solubility diagram for a basalt at 1,200 °C and 400 bar and equilibrated with fluid compositions shown in a. For any given fH₂O and fCO₂ set of values, the corresponding H₂O and CO₂ contents of the melt are computed according to ref. 3. The fO₂ is shown along each line in log units calculated relative to the solid buffer Ni–NiO. The Siqueiros bar shows the range of H₂O content of Siqueiros melt determined by Saal et al.². c, Evolution of the pressure of fluid saturation with fO₂ of a basalt melt having 40, 132 and 240 p.p.m. CO₂ and 0.1 wt% H₂O, which are minimum, average and maximum CO₂ contents, respectively, of the Siqueiros melt inclusions². At an fO₂ below ΔNNO = −1, the pressure of saturation in fluid rises because of the continuous increase in CO of the coexisting gas phase. Grey horizontal line corresponds to the average collection pressure of Siqueiros basalts.

Figure 1b shows the H₂O and CO₂ concentrations of basalt melts that coexist with fluids shown in Fig. 1a. Under oxidizing conditions (fO₂ > ΔNNO = −1), the overall shape of the curve resembles the pattern of the curve when it is calculated by considering only fluid–melt equilibria². By contrast, for fO₂ < ΔNNO = −1, the isobaric curve displays an asymmetric bell-shaped pattern characterized by a strong lowering of the melt CO₂ content at low H₂O. As already stated, this is the result of the reduction of CO₂ to CO at low fO₂, CO being an insoluble species in silicate melts at low pressures⁵. The two curves merge at melt H₂O contents higher than 1 wt%, which shows that, for basalt melts with a higher meltwater content, the calculation of pressure for fluid saturation in the C–O–H system does not require an accurate knowledge of their redox state — unlike H₂O-poor basalts, such as oceanic basalts².

The fO₂ of primitive melt inclusions at Siqueiros is at present not well constrained but is estimated to be around ΔNNO = −2 (ref. 2), which would fall at the upper end of the range of fO₂ estimated for mid-ocean-ridge basalt⁶ (MORB). However, given the general inverse correlation between fO₂ and MgO of MORB documented worldwide⁶, the Siqueiros magmas would be expected near the lower end of the range (that is, ΔNNO = −3.5; ref. 6). The CO₂ contents of Siqueiros melt inclusions average at 132 ± 34 p.p.m. but range from 43 p.p.m. up to 243 p.p.m. (ref. 2).

Figure 1c shows the evolution of the pressure of fluid saturation with fO₂ of basalt melts having 40, 132 and 240 p.p.m. CO₂ and 0.1 wt% H₂O. It can be seen that, except for the lowest CO₂ contents, most melts would be fluid-saturated at their collection pressure for an fO₂ < ΔNNO = −2.5. Considering the uncertainties associated with the determination of dissolved CO₂ in MORB glasses (± 15 p.p.m.) and with the redox state of Siqueiros magmas, we contend that the condition of fluid saturation before eruption cannot be disregarded for at least the most CO₂-rich Siqueiros melt inclusions.

We note that this condition is in agreement with earlier findings showing that the redox state of oceanic basalts is compatible with mantle melting under fluid-present or graphite-saturated conditions^7,8. Therefore, although the variable CO₂ content of quenched oceanic basaltic glasses results from syneruptive degassing³, part of this variability may also reflect regional-to-local variations in fO₂. In general, a quantitative modelling of volatiles' behaviour in MORB magmas will require explicit consideration of the role of fO₂ (ref. 9).