Modern Earth Processes

We explore how plate motion transfers chemicals into and out of the modern Earth.

 

The Earth is unique among the rocky planets in our Solar System in having an ocean-atmosphere comprising water (H2O), carbon dioxide (CO2), nitrogen (N2) 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 surface. The Earth is also unique in having a surface that is constantly evolving. New crusts are being formed beneath oceans at mid-oceanic ridges as well as in subduction zones where oceanic plate enters back into the Earth’s mantle and continents are being slowly drifted riding on their often old mantle roots. An overarching theme of our group’s research is to probe various aspects of ongoing Earth processes that are well below our feet that serve as engines of the evolving Earth – its interior dynamics and surface conditions.

Some of the current sub-themes (see more details below) are:

  • Deep Carbon Cycle
  • Deep Sulfur Cycle
  • Oceanic Mantle Melting and Crust Formation Beneath Oceans
  • Continental Lithospheric Mantle Processes
  • Subduction Zone Processes

Estimated carbon contents and fluxes in and between reservoirs in present-day Earth. ( Dasgupta and Hirschmann, 2010 — EPSL )

Estimated carbon contents and fluxes in and between reservoirs in present-day Earth. (Dasgupta and Hirschmann, 2010 — EPSL)

Deep Carbon Cycle

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; Dasgupta, 2018 – AJS). 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. We have also been looking into dissolution capacity of carbon-bearing gases such as CO2 (Duncan and Dasgupta, 2015 – CMP; Eguchi and Dasgupta, 2018 – Chem Geol) in various types of silicate melts that are key to understand root causes of volcanic eruption styles and fluxes of fluids and gases slowly coming out of the Earth’s interior. Furthermore, we also seek to constrain how carbon may be stored in the deep interior of our planet (e.g., Tsuno and Dasgupta, 2015 – EPSL; Eguchi and Dasgupta, in review).

Deep Sulfur Cycle

We are interested in the abundance and storage of S in mantle source regions such as those below ocean islands and how S in extracted by partial melting. (See  Ding and Dasgupta, 2018 – JPet )

We are interested in the abundance and storage of S in mantle source regions such as those below ocean islands and how S in extracted by partial melting. (See Ding and Dasgupta, 2018 – JPet)

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 direction that we have started to work on is the petrology of global sulfur cycle. Some of the key questions are the efficiency of sulfur transfer from subducting slab to the mantle wedge, 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. Thus far we have looked into the fate of sulfide or sulfate during fluid-present melting of subducting ocean crust (Jégo and Dasgupta, 2013 – GCA, 2014 – JPet) and into the fate of sulfide during melting beneath mid-ocean ridges and ocean islands (Ding and Dasgupta, 2017 – EPSL; Ding and Dasgupta, 2018 – JPet).

Oceanic Mantle Melting And Crust Formation

Cartoon showing the locus of generation of kimberlitic magma at a redox front, deep beneath a mid-ocean ridge ( Dasgupta et al., 2013 — Nature ).

Cartoon showing the locus of generation of kimberlitic magma at a redox front, deep beneath a mid-ocean ridge (Dasgupta et al., 2013 — Nature).

From birth and till today Earth remained a magmatically and volcanically active planet. Partial melting processes in the Earth’s mantle is responsible for generating basaltic primary magmas that segregate from the mantle, arrive at shallow depths or at the surface and cool and crystallize to form the crust. This happens in a number of tectonic settings on Earth such as below mid-ocean ridges, below intraplate ocean island, and in subduction zones. Basalts from oceanic settings also provide a window to the Earth's convecting mantle. Over the years we have worked 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 (e.g., Jackson and Dasgupta, 2008 — EPSL; Dasgupta et al., 2010 — EPSL; Mallik et al., 2016 - GCA). Recent work in this sub-theme included partial melting behavior of various mantle lithologies (with or without volatiles) aimed at petrogenesis of various flavors of basalts, mantle hybridization via melt-mantle reaction and the role of melt-rock reaction and other reactive processes on the generation of erupted basalts (e.g., Mallik and Dasgupta, 2012 — EPSL; Mallik and Dasgupta, 2013 — JPet).

Continental Mantle Processes

Sub continental lithospheric mantle (SCLM), especially those beneath old continental domains is thought to be largely isolated from the convecting mantle below. Yet episodic basaltic magmatic events and eruptions of compositionally extreme, volatile-rich magmas such as carbonatite and kimberlite specifically take place through SCLM. Therefore, SCLM may serve as a unique long-term storage location for volatiles. One line of investigation is how continental lithospheric mantle might have originally formed and how it continues to evolve through episodes of melt and fluid interactions. New seismological studies are extracting more details on the structure of SCLMs and the lithosphere-asthenosphere boundary (LAB) regions, which may be linked to processes of volatile-induced partial melt generation (e.g., Dasgupta, 2018 – AJS) or melt-mantle interactions involving fluids (e.g., Saha et al., 2018 – G3). At the moment we are collaborating with seismologists and mineral physicists to better constrain the plausible causes of seismic features such as mid-lithospheric discontinuity.

Subduction Zone Processes

Carbon goes through changes as it is subducted, melted, and erupted onto the surface, affecting the atmosphere and climate. ( Duncan & Dasgupta, 2017 — Nat Geo )

Carbon goes through changes as it is subducted, melted, and erupted onto the surface, affecting the atmosphere and climate. (Duncan & Dasgupta, 2017 — Nat Geo)

Other rocky planets in our Solar System (e.g., Mars, Mercury, Venus) have been volcanically active in the geologic past or some continues to be volcanically active even today (e.g., Jupiter’s moon Io) but what truly sets Earth apart is having ongoing plate recycling process at subduction zones. We actively work on various mass exchange processes between the Earth’s surface and the mantle that take place via subduction of sediments, ocean crust, and oceanic lithospheric mantle. Questions of interest are how efficiently the life-essential ingredients such as carbon, water, nitrogen, and sulfur are subducted back to the deep mantle or what fraction of these volatiles makes their way back through arc magmatism. We are also interested in the influence of subduction zone volatile cycling on magma production in the mantle wedge and possible redox evolution of the sub-arc mantle. We have written extensively on deep subduction of carbon (e.g., Hirschmann and Dasgupta, 2009 – Chem Geol; Dasgupta, 2013 – RiMG; Tsuno and Dasgupta, 2012 – EPSL) and sulfur (e.g., Jégo and Dasgupta, 2014 – JPet), mobilization of CO2 via crustal melts (e.g., Duncan and Dasgupta, 2015 – CMP), and crustal melt-mantle wedge interactions and generation of arc magmas (e.g., Mallik et al., 2016 – GCA).