carbonate in tianshan ultrahigh pressure metamorphic belt

The carbon contents in Earth’s exogenic systems, over the geologic timescale, are primarily controlled by the balance between inputs from Earth’s interior and outputs via burial and subduction. Carbonate-bearing metabasites are common in high-pressure terranes of oceanic origins, suggesting that a basaltic crust underwent extensive hydrothermal alteration prior to subduction. A descending slab at trench sequesters carbon from shallow reservoirs, with a debated fraction recycled via metamorphic devolatilization. Zoned carbonate minerals in high-pressure assemblages could be direct recorders of decarbonation and the P-T-t histories of subduction-zone metamorphism. The long-term objective of this project is to derive metamorphic P-T-t-fluid evolution from carbonate minerals. Petrologic experiments are integrated with field-based and simulation studies to explore the dynamic processes of slab exhumation and long-term carbon cycle in subduction zones. Specifically, the project investigates the diffusion mechanism in carbonate minerals, the P-T-t history of Tianshan UHP slab, thermodynamics of high-P fluids and fluid-carbonate interaction, and carbon flux from the slab.

This project is supported by NSERC Discovery Grant.

gangdese batholith and contact metamorphism

Magmatic, tectonic, and metamorphic processes in continental arcs plays a critical role in Earth’s long-term carbon budget and climate cycle. The Gangdese Arc in southern Tibet is one of the most magnificent continental arcs in the Phanerozoic, of which the role in long-term carbon budget has not been quantitatively investigated. We integrate studies on magmatism and metamorphism to estimate the flux CO2 degassing through country rock assimilation and skarnification. The spatial and temporal distributions of magmatism shed light on the evolution of crustal thickness and elevations, which are critical to the surface erosion rate and accordingly CO2 consumption by silicate weathering. Based on these studies, we estimate the contributions to the atmospheric CO2 by the Gangdese orogeny, which provides broader implications to the impacts of supercontinental dispersion and amalgamation on the long-term evolution of Earth's environments.

This project is supported by State Key Laboratory of Lithospheric Evolution, Chinese Academy of Sciences.

Collaborators: Wenrong Cao, Ming Tang, Weiqiang Ji.

simulation of garnet growth-diffusion-fractionation

The forward modeling of coupled garnet growth and diffusion simulations has led to unprecedented insights into the metamorphic thermal, temporal and chemical histories. The growth zonation is smoothed by intragranular diffusion; the longer a crystal is held at high temperature, the more “blurring” there will be. Thus, the relative timescales can be derived with necessary assumptions. An exciting development is the ability to use diffusion profiles to recognize the geologic records of short metamorphic events that are close to or exceed the resolution of radiometric dating.

I have developed a MATLAB-based code package that combines phase equilibria and garnet diffusion modeling. The phase diagram calculations are largely similar to THERMOCALC, and can also readily incorporate activity models from outside the THERMOCALC family. The forward simulation could adjust the bulk-rock composition stepwise to account for chemical fractionation as garnet grows, and perform intragranular diffusion modeling using equilibrium boundary conditions.

Forward simulation of compositional profiles of garnet along a P-T-t path, for a pelitic gneiss from the Acadian Orogen, Central New Hampshire. See Chu et al. 2018 JPetrol for details.