Redox control of sulfur degassing in silicic magmas

Abstract

Abstract. Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in arc settings, which invariably have displayed an excess of sulfur relative to that calculated from melt degassing. If this excess sulfur is stored in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partition coefficients varying between 50 and 2612, depending on melt composition. Experimentally determined sulfur partition coefficients for a dacite bulk composition confirm this trend and show that in volcanic eruptions displaying excess gaseous sulfur, the magmas were probably fluid-saturated at depth. The experiments show that in more reduced silicic magmas, those coexisting only with pyrrhotite, the partition coefficient decreases dramatically to values around 1, because pyrrhotite locks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these results shows that for oxidized magmas, the presence of 1-5 wt % fluid may indeed account for the differences observed between the petrologic estimate of the sulfur yield and that constrained from ice core data. Explosive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only minor amounts of sulfur and could have consequently negligible long-term (years to centuries) atmospherical effects. This redox control on sulfur release diminishes as the melt composition becomes less silicic and as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.

Ice core and palaeoclimatic evidence for the timing and nature of the great mid‐13th century volcanic eruption

“…If we consider only the correlations between estimated sulphur yield and eruption magnitude for known events, for different ice cores (three such calibrations are shown in Figure 3), then the estimated sulphur yields of the mystery eruption correspond to total erupted masses in the range of ∼5 × 10 14 -2 × 10 15 kg. The dense rock equivalent (DRE) volume (at 2500 kg m −3 ) corresponding to this range is 200-800 km 3 , which coincides with the 'super-eruption' class (the term super-eruption is not well defined in the literature, but it has been (Bluth et al, 1993(Bluth et al, , 1997, diverse petrologically based estimates (Devine et al, 1984;Scaillet et al, 1998;Wallace, 2001), and calibrations of sulphate anomalies in Arctic and Antarctic cores (Hammer et al, 1980;Legrand and Delmas, 1987;Langway et al, 1988;Clausen and Hammer, 1988;Delmas et al, 1992;Zielinski, 1995;Clausen et al, 1997;de Silva and Zielinski, 1998;Oppenheimer, 2002). Eruption magnitudes are mostly taken from Carey and Sigurdsson (1989).…”

“…This is generally believed to occur for intermediate to silicic composition eruptions when a vapour phase enriched in sulphur has accumulated at the top of a magma reservoir prior to its eruption (e.g. Luhr, 1990;Wallace et al, 1995;Scaillet et al, 1998), though other mechanisms are possible (Oppenheimer, 1996). The source of this vapour could be mafic composition magma intruded into the lower parts of the chamber, which is not subsequently erupted but which, over centuries or millennia, has contributed its complement of volatiles to the overlying magma (see Wallace (2001) for a penetrating account of the origins, extent and time scales of pre-eruptive exsolved volatiles in magmas).…”

Apatite: A new redox proxy for silicic magmas?

“…However, there is also significant intra-sample scatter, with 506 interquartile ranges up to ~ 10 log units in Zone 4, similar to those reported by Trail et climatically important elements such as sulphur (Scaillet et al, 1998;Robock, 2000). In the 578 presence of only 1-5 wt % fluid, there is a marked preference for sulphur to partition into the 579 fluid rather than the melt under oxidising conditions (fO 2 >NNO+1, Scaillet et al, 1998). 580…”

C–O–H–S fluids and granitic magma: how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200 MPa

“…6B). Sulfur partitioning does vary with ƒ O2 as reported previously for C-free granitic systems (Scaillet et al 1998;Keppler 1999;Scaillet and Pichavant 2003;Clemente et al 2004;Keppler 2010), but it does not vary with the (CO 2 /CO 2 +H 2 O) fluid . Our data are compared in Figure 6B with the partitioning behavior of S for O-H-S fluids and haplogranitic melts at 200 MPa and 850°C with ƒ O2 equal to that of the Co-CoO buffer and also in the range of ca.…”

“…Interpretation of processes of magmatic fluid exsolution, fluid evolution, and subsequent surficial degassing through geochemical research calls for experimental and thermodynamic constraints on the solubilities of H 2 O, CO 2 , and S in a variety of silicate melts over a range of mantle and crustal pressures and temperatures. At present, systematic experimental studies of solubilities and partitioning behavior of volatile components in fluid-saturated felsic melts are limited to individual and binary volatile pseudosystems involving melt plus H 2 O±CO 2 (Holloway and Blank 1984;Tamic et al 2001;Moore 2008), H 2 O±S (Scaillet et al 1998;Keppler 1999;Clemente et al 2004;Burgisser et al 2008;Keppler 2010), or H 2 O-Cl (Webster 1992 a,b). Some recent studies, however, offer promise because they involve experimental investigation of O-H±C±S±Cl±F-bearing fluids and a variety of aluminosilicate melts (Botcharnikov et al 2004;Spilliaert et al 2006;Nicholis and Rutherford 2006;Botcharnikov et al 2007;Webster et al 2006;2009;Parat et al 2008;Teague et al 2008;Beermann 2010), but most of them have involved basaltic to intermediate-silica melts.…”