Research

Overview

The Earth system comprises numerous interacting components, ranging in spatial scale from microscopic to macroscopic, and in temporal scale from seconds to billions of years. Geological field studies document the interacting processes currently in operation or reveal the outcomes of the interactions from Earth’s past. Laboratory experiments extend our vision to directly observe these interactions in a manageable environment. Theory, and the intimately related numerical modelling, aim to capture all the observables into a consistent framework to make quantitative scientific predictions.

The central theme of my research is to characterize and quantify the volatile (e.g., H2O and CO2) and heat communication between Earth’s surface and interior. As life on our planet depends critically on its surface conditions (e.g., temperature, oxygen, nutrients), to better understand the habitability of our planet and beyond, it is fundamental to study how Earth’s internal dynamics (e.g., mantle convection) modulates the planet surface over geological times. The first-order phenomenon for the surface expression of Earth’s mantle convection is plate tectonics. At convergent plate boundaries, subduction takes place and causes considerable volatile and energy transport via metamorphism and volcanism. Continuing subduction eventually leads to orogenesis (e.g. Himalaya), and the concomitant degassing or weathering regulates on planetary scale the long-term atmosphere evolution.

Deep Carbon Transport at Subduction Zones

I and collaborators have developed models to quantify the CO2 and H2O fluxes that can emanate from downgoing subducting slabs. There is one significant challenge in modelling this: owing to the migration of CO2 and H2O, the bulk volatile content of the slab is spatially heterogeneous and temporally varying. This is a chemically open-system behavior, and cannot be treated using conventional thermodynamic software that assumes a closed system and thus unchanging bulk composition. We addressed this challenge by performing a thermodynamic parameterization of the coupled (de)carbonation-(de)hydration reactions that typically occur in subducting plates, and wrote a C-language code to implement it (Tian et al., 2019a). This customizable thermodynamic module can be interfaced with fluid dynamical simulations and allows the calculation of CO2 and H2O fluxes from the surface of subducting slabs (Tian et al., 2019b).

Thermal Pulses Caused by Channelized Fluid Flows

Metamorphic field studies suggest that localized, kilometer-scale "hot spots" can be caused by focused fluid flow over time sales of 1e5 years or less (Chamberlain and Rumble III, 1989). These field-documented "hot spots" are thus hypotheizes to be fossil thermal pulses induced by focused fluid flows, and the thermal transients are superimposed on regional-scale metamorphis belts that develop over time scales of tens of millions of years. Based on two-phase fluid flow dynamics, I and collaborators constructed a fluid dynamical model to provide the first detailed physical mechanism for metamorphic "hot spots" (Tian et al., 2018).

Carbon Sequestration Associated with Porosity Waves

A critical question for porosity wave transport is whether it has left recognizable footprints---physically and chemically---in the field. We constructed a one-dimensional model to compare and contrast the potential of driving chemical reactions for porosity wave model and traditional Darcian flow model, taking a typical CO₂-involving reaction 7CO₂ + 3Calcite + Tremolite = 5Dolomite + 8Quartz + H₂O as an example (Tian and Ague, 2014). The example reaction here sequesters CO2 if proceeding forward.

Fluid Inclusions in Eclogites

Geochemical work on the abiotic hydrocarbons trapped in fluid inclusions in eclogites from Tianshan Mountain, China, further corroborated by experimental studies. Check out Tao et al., 2018.

Compaction versus Convection in Porous Media

Driving forces for pore fluid migration come from two sources: the density contrast between fluid and solid, and the thermal and/or chemical buoyancy in pore fluid itself. The former causes compaction-driven fluid flow that is uni-directionally upward (upper panel), whereas the latter causes fluid circulation which in essence is convection in porous media (lower panel). Theoretical efforts have been devoted to this fundamental problem of competition between the aforementioned two driving forces, with application to deep crustal fluids.