My research investigates the geochemistry behavior of volatiles (S, H2O, CO2, and halogens) and other trace elements from the planetary interior to the surface, and the potential impacts of magmatic volatiles on the evolution of the surface environment. My research approach involves high-pressure experiments and analytical geochemistry.
Modeling sulfur degassing during magma ascent
(a) Sulfur speciation in the melt (light and dark orange) and in the vapor (green) as a function of fO2 and the gas equilibrium reaction. All the calculations are carried out at 200 MPa, with a basaltic melt composition and 60% H2O in the vapor. (b) Experimentally determined partition coefficients (KdS) of sulfur between fluid and basaltic-andesitic melt at pressure > 1atm as a function of fO2 (∆FMQ). The experiments are grouped into three categories based on the sulfur speciation in the melt: S2- only (black circles), S6+ only (open diamonds), and a mixture of S2- and S6+ (grey triangles).
Modeling equilibrium degassing of CO2 and S is commonly employed to explain the CO2/SO2 ratio in volcanic gases, CO2-S contents in mineral-hosted melt inclusions, and to infer the degassing depths. The CO2-S co-degassing trend revealed by the rehomogenized melt inclusions from Fuego and Seguam by Rasmussen et al. (2020) challenges the conventional understanding that S only degasses at low pressure after CO2 is almost entirely degassed from the ascending magma and questions the interpretation of the increase of CO2/S in the volcanic gas as a surge of deep, CO2-rich magmas. None of the existing forward degassing models can reproduce the CO2-S co-variation shown by the rehomogenized Fuego melt inclusions. Inspired by the new melt inclusion data, we develop a new degassing model to predict the evolution of S, CO2, and H2O in basaltic magma and co-existing vapor by combining existing COH degassing models with experimentally constrained sulfur partition coefficients. This model also tracks the redox evolution during closed-system S degassing by redox budget conservation among S and Fe. The biggest challenge modeling S degassing is its multiple valent states in the melt (S2- and S6+) and vapor (SO2 and H2S). To tackle this challenge, we considered three sulfur degassing reactions based on sulfur speciation in the melt and vapor and developed kd(S)fl/m for each reaction guided by previous experiments.
(a) Molar CO2/SO2 in melt inclusions and calculated fluid composition vs. calculated degassing pressures (Scaillet and Pichavant, 2003). (b) Correlations between bulk rock Ba/La, a tracer of slab-derived fluids, and magmatic gas CO2/SO2 for Central American volcanoes. The 2012-2013 gas data are from Aiuppa et al. (2014), while the 2014-2015 gas data are from de Moor et al. (2017). (c) Time series of gas CO2/ST at Turrialba Volcano from June 2014 to May 2015 (de Moor et al., 2016a).
What controls the CO2/SO2 ratio in arc volcanic gas?
The degassing of CO2 and S from arc volcanoes is fundamentally important to global climate, eruption forecasting, and cycling of volatiles through subduction zones. Measurements of CO2/SO2 ratios in high-temperature volcanic gases have become the main approach to quantifying the arc volcanic CO2 and S outflux to the atmosphere. New high-rate, long-time series measurements of gas emissions from several volcanoes have now shown precursory signals in the rise of CO2/SO2 weeks before the eruption, which is typically considered to reflect the higher pressure of degassing. On the other hand, recent work has shown that volcanoes with intrinsically high CO2/SO2 gas ratios seem to correlate globally with regions of carbonate subduction. Such correlations have led to the suggestions that CO2/SO2 ratios reflect the source of the parental magma. The source of variation in the CO2/SO2 ratio of magmatic gas is thus debated. My current project at LDEO aims at resolving the pressure vs. source effects on CO2/SO2 ratios by providing novel CO2-S data of Ol-hosted melt inclusions from Central American volcanoes with published gas records. This project bears directly on the interpretation of CO2/S gas precursors to volcanic eruptions, of apparent relevancy to developing timely forecasts and warnings to local populations.
S degassing in the 1257 Samalas eruption
Ice core record of sulfur release during different volcanic eruptions in the past 2000 years.
Volcanic degassing of S could have significant impact on the short-term climate at local-global scale. I am currently working with colleagues at CUNY and Southern Methodist University to understand what process(es) had led the greatest volcanic sulfur aerosole release (~160Tg SO2) to the stratosphere of the common era during the 1257 eruption of Mt. Samalas in Indonesia. We employed state-of-art geochemistry analyses, including XANES, SIMS and EMPA, on the plagioclase-hosted melt inclusions and apatites from this VEI 7 eruption to track the S and Fe redox states evolution and S isotopes in the Samalas system.
Deep Sulfur cycle on Earth
Sulfur, one of the major magmatic volatiles on Earth, has played an important role in planetary differentiation, volcanic degassing, surface environment, and ore deposit formation. Central to the question of distribution of sulfur among various reservoirs on Earth from the core to the atmosphere through time is the sulfur extraction from the Earth mantle through partial melting, which is very diverse due to the plate tectonics and mantle heterogeneity. Two main pathways of sulfur outflux from deep mantle to the surface reservoirs are magmatism at mid-ocean ridges and oceanic islands. I developed a framework to describe the fate of sulfide and Cu during decompression melting in MORBs (Ding and Dasgupta, 2017, EPSL) and OIBs (Ding and Dasgupta, 2018, J. Petrology) by using the pMELTS thermodynamic model/previous partial melting experiments, empirical SCSS (an important parameter describing the sulfur carrying capacity of the silicate melt) models, and available S and Cu data. Results from these studies suggest that sulfur extraction from the mantle is an interplay between the SCSS, decompression melting, and S abundance in the mantle source.
Decompression melting and the fate of sulfide beneath the intraplate ocean islands (modified from Ding and Dasgupta, 2018).
Deep sulfur inventory of other planetary bodies
Sulfur is also present in high abundances in Martian and Lunar meteorites, mare basalts, and Martian crust.
Experimental constraint on the sulfur concentration at sulfide saturation in Martian basalt.
In particular, the sulfur exchange between Martian interior and the exosphere might have been key in early Martian evolution, when volcanic activity was likely much more vigorous. To understand the deep sulfur cycle on Mars, I experimentally constrained SCSS of Martian basalts (Ding et al., 2014, GCA). I also measured the bulk S concentration of seven Martian meteorites to investigate the fate of sulfide during cooling and crystallization (Ding et al., 2015, EPSL). Understanding of S abundance and distribution in the interior of the Moon is crucial for any comprehensive model of Earth-Moon formation and lunar differentiation. Ding et al. (2018, GCA) provides the first SCSS model applicable but not limited to lunar basalts, and its estimate on S abundances of lunar mantle reveals the heterogeneous sulfur inventory in the lunar mantle.
Backscatter images of experimental constraints on the solidus temperature of Martian mantle at high pressure.
Partial melting of a model Martian mantle
Understanding of the melting process is essential to any geochemical fractionation linked to it, which appears to be particularly important when studying non-Earth planets where samples, observational and experimental data are limited. Therefore, another research focus during my doctoral study is to experimentally investigate partial melting of model Martian mantle compositions at nominally anhydrous conditions. In this study, I constrained the location of the Martian mantle solidus as a function of pressure, temperature, and bulk composition. This study provides important parameters such as solidus temperature, its evolution with bulk composition, and melt productivities necessary for any numerical modeling on differentiation and thermal history of Mars (Ding et al., JGR: Planet, 2020).