Experimental Study On The Solubility And Speciation Of Sulfur In Melts And Fluids

Sulfur is one of the most abundant volatiles in magmas and hydrothermal fluids,and its solubility and speciation in silicate melts as well as aqueous fluids are critical for understanding sulfur cycling in the Earth’s interior,oxidation mechanism of mantle wedge,and the cooling history of magma.Sulfur is present in silicate melts in two forms,S2-and SO42-,and these two sulfur species are associated with different solubilities in silicate melts:(1)sulfur content in silicate melt at sulfide saturation(SCSS)and(2)sulfur content in silicate melt at sulfate saturation(SCAS),respectively.Sulfur speciation in aqueous fluids is more variable and unquenchable.Previous studies have made significant progress with sulfur solubility and speciation in silicate melts and aqueous fluids under the action of oxygen fugacity,melt composition,temperature,pressure,melt water content and fluid acidity.However,some important issues remain to be solved:(1)previous SCSS models did not take into account the two important factors of sulfide composition and melt water content simultaneously,which limited their application to natural magma systems;(2)while the promotive role of melt CaO in anhydrite dissolution in silicate melt was noted before,the dependence of SCAS on melt CaO content and the underlying dissolution mechanism have not been well constrained;(3)previous in situ experimental studies of sulfur-bearing fluids mainly focused on oxidized conditions or low P-T conditions(1 kb,≤500℃);the sulfur speciation in reduced fluids under high P-T conditions has not been well constrained;(4)there is a lack of comprehensive understanding regarding the kinetics of the Fe-S redox exchange reaction in silicate melt.This dissertation therefore carried out experimental studies on a series of sulfur-bearing silicate melts and aqueous fluids with different compositions at high temperature and high pressure conditions.(1)Eighteen basaltic melt-sulfide melt phase equilibrium experiments were carried out at 1-2.5 GPa and 1250-1400℃ in piston-cylinder apparatus to investigate the effects of melt water content and sulfide composition on basaltic melt SCSS,respectively.Experimental results show that the effects of melt H2O content and sulfide composition on SCSS in basaltic melts are independent and additive.The SCSS is found to be proportional to the mole fraction of FeS in sulfide,and to increase by～104 ppm per wt%H2O added to the melt.A simple SCSS model for natural basaltic melts is developed to account for the effects of melt H2O,sulfide composition,temperature and pressure together.This model can be applied to a range of geological conditions relevant to basaltic magmatism(<2.5 GPa,1523-1673 K,and<6 wt%H2O).With the new model,we find that the observed S contents in MORBs are generally consistent with-15%melting of a subridge mantle,whereas S-rich MORBs with evidence of sustained sulfide saturation(such as those from the Siqeuiros Fault Zone and ODP Hole 896A)probably derive from a mantle with highNiS sulfide(XNiS≈0.4).Comparison between calculated SCSS and S contents in melt inclusions of primitive arc basalts from various global arcs indicates that,with few exceptions,sulfate is generally not required to be present in its mantle source.(2)A series of experiments of phase equilibrium between anhydrite and felsic melt were carried out at 1-3 GPa and 1050-1350℃ in piston-cylinder apparatus to obtain constraints on the effect of melt CaO concentration,melt H2O content,temperature(T)and Ca/(Ca+Mg)ratios on SCAS.Electron microprobe analyses of quenched products confirmed that the dissolved sulfur was predominantly as SO42-.The experimental results show that SCAS increased significantly with increasing melt CaO.At fixed silicate polymerisation(NBO/T=0.43),SCAS increased with increasing Ca/(Ca+Mg)ratios,indicating that Ca2+have the stronger affinity with SO42-than Mg2+.The SCAS also increased with increasing T and dissolved water content.By combining new experimental data with previous literature data,we developed a new empirical quantitative model of SCAS.Based on the model calculation results,we find that slab melt with high CaO content could efficiently transport S from the slab to the mantle wedge in the Archean.In modern hot subduction zones,slab melt was also able to carry with more than 1800 ppm sulfur and elevate S concentration in the mantle wedge to the lower end of observed level by metasomatism.In modern intermediate subduction zones,however,slab melt is not an effective agent of sulfur transfer even at CaO content>10 wt%.(3)S speciation in S+H2O+HCl/NaOH/Na3AlSi5O13 and Na2S2O3+H2O±Na3AlSi5O13 system were determined in hydrothermal diamond anvil cell(HDAC)at up to 800℃ and 2 GPa combined using Raman spectroscopy.The effects of fluid acidity,dissolved silicate materials and temperature on sulfur speciation were investigated,respectively.Experimental results show that in the S+H2O system,the dominant sulfur species were S3-,H2S and SO2,and the proportion of S3-decreases with rising temperature.Increasing fluid alkalinity jeopardize the stability of Sbearing species.With addition of silicate in the S+H2O system,the H2S or SO2 disappeared and most of sulfur was present in the form of S3-.In the Na2S2O3+H2O system,S3-is the major sulfur species with minor HS-and SO42-,and S3-partially disproportionated into HS-and SO42-with the addition of silicate.However,no silicate-associated sulfur species was detected in fluids,indicating that the presence of silicates may change fluid acidity and affect the sulfur speciation indirectly.Combined with DEW model calculations,we find that under typical redox conditions(from the fayalite-magnetite-quartz buffer to FMQ+2)and pH(ApHn=1.5-2)of subduction zone fluids,H2S and S3-are the dominant sulfur species,and HSO4-/SO42-or SO2 cannot exist stably in abundance.Sulfur in subduction zone fluids may therefore not be an effective oxidant for the mantle wedge.(4)The valence states of Fe and S,two major multi-valence elements,changed during cooling of magma,but the kinetics of Fe-S redox exchange reaction is not well understood.Fe-S redox exchange reaction experiments in basaltic melt with controlled cooling rates of 60℃/s and 10℃/s were carried out in piston-cylinder apparatus at 0.2 GPa and 1300℃.The experimental quenched products were analyzed by Synchrotron X-ray absorption near-edge structure spectroscopy(XANES).Preliminary experimental results show that the reaction S2-+8Fe3+=S6++8Fe2+proceeds toward the right during the cooling of magma,i.e.,the S2-appear to may be oxidized to sulfate,contrary to previous thermodynamic prediction.