PHONOLITES, S-SOLUBILITY, AND SODALITE-HAÜYNE STABILITY

Sulfur solubility in magmas is known to strongly depend on such factors as melt composition, magma oxidation state (fO₂), P, and T. Unlike the more common magmatic volatiles (H₂O, CO₂), sulfur can occur in a variety of forms, mainly controlled by magma oxidation state; the most important S-species in natural systems appear to be H₂S, S₂, and SO₂ as gas/fluid phase components, and S^2-, HS^-, or SO₄^2-, as species dissolved in silicate melts. As a result of this complexity (and possible S-rich crystals), it is not always easy to interpret the significance of observed variations in S-emissions in active volcanic systems in terms of what is happening at depth. Furthermore, estimation of S-yields to the atmosphere in older volcanic eruptions requires accurate knowledge of not only how much S was in the pre-eruptive melt (e.g., via melt inclusions), but also an estimate of the mass and fugacities of sulfur species in any pre-eruptive magmatic fluid phase.

We have conducted experiments on S-solubility in a sodic phonolitic melt composition (with, in wt% ~60 SiO₂, 18.8 Al₂O₃, 10 Na₂O, 5.5 K₂O), at T= 930°C, P = 50-390 MPa, using mainly IHPV with controlled H2 fugacity and S fugacity roughly controlled by using samples with different amounts of S added as elemental S. Oxidation states range from approximately -3 to +3 log fO2 units relative to the NNO buffer. Depending on the amount of S added, the experiments range from nearly water saturated (low S added) to significantly H₂O undersaturated (P(H₂O) down to ~0.5*P_total), with significant fugacities of H₂S or SO₂ in the fluid phase, depending on experimental oxidation state. Sulfur solubilities in these alkali-rich phonolites are generally higher than those for dacitic to rhyolitic melts reported in other studies at similar fO₂ and fS₂ conditions. While previous studies have reported anhydrite stable in oxidized, S-rich andesitic to rhyolitic melts, in the studied phonolite we observe the stabilization of S-rich Sodalite (Haüyne) with up to ~12 wt% SO₃ at pressures up to 150 MPa, whereas at higher pressures the phonolitic melts coexist with an oxygen-rich Fe-S-O immiscible liquid, but no anhydrite was observed. Additional work will help understand if Sodalite-Haüyne S (and Cl) contents in natural samples may be useful for estimating pre-eruptive S and Cl activities in magmas.