Research Interests

Interplay between equilibration and kinetics during metamorphism

Petrologists often work under the assumption that rocks are never very far from their equilibrium state, and that barriers to chemical equilibrium are negligible. This permits application of thermodynamic approaches to constrain the P–T–composition (X) history of mineral crystallization. However, kinetically-controlled processes (e.g., nucleation, diffusion) are often rate-limiting, and may invalidate assumptions of equilibrium and broader tectonic interpretations reliant upon them.

My research into this topic is focused on understanding the extent of and controls on chemical equilibration (George et al., CMP, 2018) and the kinetic processes controlling mineral nucleation and growth. Essentially I ask: what are the fundamental processes driving rock recrystallization?! To this end, I integrate spatially-resolved geochemical records (election microprobe and laser ablation analyses), with thermodynamic (e.g., see left) and numeric modelling approaches.

Left: Driving force for nucleation of garnet at overstepped and equilibrated conditions. Right: conditions of progressive nucleation and pressure–temperature path of garnet crystallization in the inverted Barrovian Sequence of the Sikkim Himalaya.

How can mineral-scale chemical features inform our understanding of subduction zone processes?

Fluid transfer at high pressure

In subduction zones, the downgoing slab is a messy mélange of disaggregated crust and mantle fragments that experience complex cycling movement, mixing, and dehydration. Fluid mobility and associated chemical transfer in these settings is similarly expected to be spatially and temporally heterogeneous, yet remains enigmatic (cf. Camacho et al. Nature, 2005, and Bebout & Penniston-Dorland, Lithos, 2016). Only by addressing questions such as (i) where in the slab melange does porous flow versus channelized flow regulate mass transfer? (ii) do impermeable systems provide a mechanism to transfer volatiles to the deep Earth? and (iii) how does changing stability of hydrous metamorphic minerals (e.g., lawsonite) modify the availability of fluid mobile elements? can we fully understand the role of subduction zones in past and present chemical cycling and plate tectonic localization.

My ongoing postdoctoral research at Johns Hopkins addresses these questions by utilizing a globally diverse suite of exhumed eclogite and blueschist from Venezuela, California, Oman, Syros, Russia, and the Dominican Republic.

Microstructure Formation

Rock microstructure fundamentally influences the yield strength of a rock during plastic deformation (e.g., Kumamoto et al., 2017) and thus the strength of Earth's lithosphere, the uplift of mountain belts, the geometry of subduction zones (e.g., Buffett and Becker, JGR:SE, 2012), and the development of tectonic plate boundaries (Thielmann and Kaus, EPSL, 2012). As metamorphic reactions proceed, the spatial distribution of mineral nucleation and growth and the consumption of unstable phases directly exerts control on the resulting rock microstructure, such as the example shown left. However, despite its fundamental importance, qualitative and quantitative understanding of the recrystallization of porphyroblastic microstructure in strain fields remains limited.

Recent work awarded the 2020 Early Career JMG Best Research Paper Prize (George & Gaidies, JMG, 2020) took an initial step in this direction by determining a hierarchy of variables that exert control on the microstructure of amphibolite grade garnet populations: element availability, epitaxial crystallization of garnet on micas, and temporally late nucleation and growth in strain-poor domains.

X-ray µ-CT scans of metapelites from the Inverted Barrovian Sequence in the Sikkim Himalaya. Red denotes segmented garnet volume, remaining phases in beige.

Moving forward, questions pertaining to the potentially significant role of interfacial strain in facilitating nucleation, epitaxy during crystallization, and the progressive evolution of microstructure with increasing metamorphic grade are of particular interest to me. These themes can be addressed with an approach that integrates comprehensive microstructural characterization (using petrographic approaches, electron backscatter diffraction [EBSD], and X-ray µ- computed tomography), in situ geochemical constraints, and numerical modelling approaches (e.g., Gaidies & George, Geology, 2021).

Orogenesis and crustal evolution

Plane polarized photomicrograph of coronitic garnet granulite from the Jijal Complex, Pakistan

While much of my research focuses on questions relating to fundamental processes common to metamorphic rocks in general, another aspect of my research aims to better understand regional tectonics and crustal evolution. Recent and current projects focus on:

The evolution of magmatic–metamorphic assemblages and architecture in the lowermost crust in the Kohistan Arc, Pakistan (left; George et al, in review, JMG).
The timing of amphibolite facies metamorphism in the Baltimore Terrane using Sm-Nd garnet geochronology, and implications for diachroneity of Acadian orogenesis in the eastern United States.

A methodology that integrates geochemical, microstructural and modelling approaches:

Field observations and petrographic constraints.

Phase equilibria modelling (Theriak-Domino and Thermocalc) and diffusion speedometetry.

In situ microstructural and textural characterization with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD, above) and X-ray µ-CT scanning.

Major and trace element zonation discerned in metamorphic minerals with laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS, above) and electron probe microanalysis (EPMA).

Isotopic variation with secondary ionized mass spectrometry (SIMS).