The following is a summary of different research themes that we are actively pursuing at the moment —
The Earth is unique among the terrestrial planets in our solar system in having a fluid envelope comprising water (H2O), carbon dioxide (CO2), and other volatiles that fosters life. Over millions to billion years this is maintained by exchange of volatiles and fluids between the Earth's interior and the exosphere. Our research, over the last few years, provided new insight into partial melting processes of the Earth's upper mantle lithologies in the presence of CO2 and the impact of CO2-induced incipient melting on the geophysical and geochemical properties of the (e.g., Dasgupta and Hirschmann, 2010 — EPSL Frontiers; Dasgupta et al., 2013 — Nature). But still a lot of work remains to be done to constrain the role of mixed C-O-H volatiles on magma genesis, magma-mantle interactions, and to understand the interplay of redox processes and volatile speciation on the conditions and extent of mantle melting. Our most recent efforts, led by post-doctoral researcher Kyusei Tsuno, are constraining the efficiency of carbon and water recycling through subduction, devolatilization, and melting of ocean-floor sediments (Tsuno and Dasgupta, 2011 — CMP, 2012 — EPSL; Tsuno et al., 2012 — GRL). To further constrain the agent of carbon transfer in subduction zones, PhD student Megan Duncan is constraining the solubility and speciation of CO2 in hydrous sediment partial melts at sub-arc depths (Duncan and Dasgupta, 2014 — GCA).
While a lot of attentions are given to fully understand the global carbon and water cycle, deep storage and cycles of sulfur have received far less concerted efforts. Thus another new direction that we have started to work on is the petrology of global sulfur cycle. A key question is the efficiency of sulfur transfer from subducting slab to the mantle wedge and the partitioning of sulfur between various phases such as fluids, silicate melts, and mineral sulfides or sulfates and the redox state of sulfur at different tectono-magmatic settings. At present, in a project spearheaded by post-doctoral researcher Sebastien Jégo, we are investigating the behavior of basalt hosted sulfides during subduction, dehydration, and melting at a range of oxygen fugacity relevant for the shallow upper mantle (Jégo and Dasgupta, 2013 — GCA). We are demonstrating that mineral sulfide remains abundantly stable during fluid-present melting of subducting ocean crust and hydrous fluid is the key vector to mobilize sulfur from the sulfide-saturated crust to the mantle wedge. Moreover, our first study is also showing that transfer via slab-derived reduced fluid (e.g., H2S) is likely sufficient to enrich mantle wedge in sulfur and calling for sulfate species (hence more oxidized) may not be necessary.
A key question related to the volatile-budget of the modern Earth is how it changed through time. Did the Earth acquire the volatiles and achieve their present distribution between the exosphere and the interior at the time of birth, or is the present day budget (including ocean and atmosphere) shaped by later processes, such as late addition of materials (e.g., meteorites, comets)? Our recent efforts are constraining the volatile element partitioning and solubility in Earth materials during the early differentiation such as at the 'Magma Ocean' stage (raining metal droplets in largely molten silicates). We are quantifying the importance of Earth's metallic core as a reservoir to sequester various volatiles (Dasgupta and Walker, 2008 — GCA; Dasgupta et al., 2009 — GCA; Hirschmann and Dasgupta, 2009 — Chem Geol; Dasgupta and Hirschmann, 2010 — EPSL; Dasgupta et al., 2013 — GCA) and how the young Earth might have observed very different inventory of volatiles (Dasgupta, 2013 — RiMG). This new research brings together laboratory experiments and geodynamic models to help constrain the contributions of ongoing versus early differentiation on the volatile and fluid inventory of the Earth.
As far as understanding the early evolution of terrestrial volatiles goes, there are lessons to be learnt from similar evolution in other terrestrial planetary bodies, such as Mars, the Moon, Venus, and Mercury. We do not know why these planets have drastically different atmosphere. Is this owing to accretion from vastly different compositions, or is this owing to the difference in conditions of early evolution (e.g., thermal and oxidation state and depth of core-mantle separation) that caused very different fractionation of fluids between the interior and the exosphere? Our goal is to compare and contrast the volatiles and fluids evolution of various terrestrial planets through time.
In relation to this, we recently constrained the thermal vigor of magma generation through the geologic history of the red planet (Filiberto and Dasgupta, 2011 — EPSL). Former post-doctoral researcher Justin Filiberto's research demonstrated that the thermal state of Martian mantle is hotter than previously thought and hence mantle melting commences at deeper depths. Motivated by the possibility that Martian basalts may be rich in halogens, we also constrained the effect of fluorine and chlorine on the stability of model Martian magmas (Filiberto et al., 2012 — Chem Geol; Filiberto et al., in prep). To build on these, graduate student Shuo (Echo) Ding is now looking at the efficiency of sulfur degassing aided by eruption of anhydrous or hydrous basalts relevant for Mars. The aim is to constrain the carrying capacity of sulfur in model Martian basalts and to test whether sulfur-bearing species could have been responsible for creating Martian greenhouse in the early history of Mars (Ding et al., in press — GCA).
Basalts from intra plate ocean islands provide a window to the Earth's convecting mantle. One of our long-standing interests is to constrain the mineralogic, lithologic, and volatile heterogeneities present in the Earth's mantle by reproducing the chemistry of the primary basalts through laboratory experiments. We combine both experimental and natural observations to decipher the possible nature of intraplate basalt source regions in general and those for ocean island basalts in particular (e.g., Jackson and Dasgupta, 2008 — EPSL; Dasgupta et al., 2010 — EPSL). Active research topics in this theme include partial melting behavior of various mantle lithologies (with or without volatiles) aimed at petrogenesis of various flavors of basalts (e.g., Gerbode and Dasgupta, 2010 — JPet), mantle hybridization via melt-rock reaction and the role of melt-rock reaction and other reactive processes on the generation of erupted basalts. This latest research topic, currently pursued by graduate student Ananya Mallik, is beginning to shed light on to the complex processes of magma-rock interactions in a heterogeneous mantle (Mallik and Dasgupta, 2012 — EPSL; Mallik and Dasgupta, 2013 — JPetrol ).