Petrologic Phase Equilibria

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, and calculate phase diagrams for special purposes, including externally-controlled oxygen fugacities. The MATLAB codes are available for download [here].

We applied this code set in a study of phase equilibria relevant for graphitic metapelite (Chu and Ague, 2013). We calibrated a solubility model for CO2 in the felsic melt, discussed the graphite precipitation/dissolution processes in closed systems, and illustrated the effects of graphite-buffering on phase assemblages and fluid compositions. The results suggest that partial melts derived from graphite-bearing metapelitic rocks can contain substantial CO2 that could be transported from depth to shallow crustal reservoirs by magma ascent, thus contributing to the long-term carbon cycle.

Garnet Diffusion Kinetics

The chemical-zoning patterns of garnet are extremely useful in reconstructing the pressure-temperature-time histories of metamorphic rocks. The cation diffusion is not too fast, so the garnet growth zonation is not totally erased; the diffusion is neither too slow, so the degree of chemical modification reflects timescales of orogenic events. Timescale estimation is critically dependent on knowledge of diffusion rates in garnet. A new set of diffusion parameters has been calibrated by incorporating the results of published diffusion experiments into a Bayesian statistical framework implemented using the Markov-Chain Monte Carlo approach. These coefficients are significantly different than previously-published ones (particularly for Ca and Mn), and thus facilitate new petrotectonic interpretations.

I modified my code set so that it can adjust the bulk-rock composition stepwise to account for chemical fractionation as garnet grows, and perform intragranular diffusion modeling using equilibrium boundary conditions. An example is presented here:

Hot Spot Terrane, New Hampshire

Field-Based Petrology

My field-based research involves integrated petrology, geochronology, geochemistry and modeling studies to understand the tectonic processes in various settings. My thesis research areas are in southern New England, including the Taconian retrograde eclogites from Northwest Connecticut, and the Acadian metamorphic "hot spot" terrane in New Hampshire. For my senior research, I was working on the Tianshan UHP belt in western China.

Northwest Connecticut

The Taconic orogenic belt records the collision between the Laurentian passive margin and Taconic arcs. In spite of the large-scale subduction, high pressure (HP) eclogite and blueschist are scarce in this mountain belt. A persistent question is whether the rarity of HP rocks reflects widespread overprinting, or the uncommon attainment of HP facies conditions. Field mapping by Harwood (1979) revealed mafic lenses and deformed dikes in a Taconic thrust slice in Northwest Connecticut. The mafic rocks contain relict mineral assemblages and decompression textures suggesting a high-P precursor (Harwood, 1979). Whether the rocks were HP mafic granulites or reached the eclogite facies remained uncertain. We are conducting comprehensive petrological study which demonstrates that the rocks did in fact reach eclogite facies conditions. The 456 Ma metamorphic age (zircon U-Pb dating), together with the 454 Ma intrusions that cut the Taconic lithologic lineation (Sevigny and Hanson, 1995), provides a tight constraint for the timing of Taconic subduction polarity reversal from east- to west-directed.

New Hampshire Metamorphic 'Hot Spot' Terrane

The metamorphic "hot spot" terrane in New Hampshire is an ideal locality to test the role of COH fluids in lithospheric heat transfer. Steep metamorphic field temperature and geochemical gradients are found centered on networks of graphite-bearing quartz and pegmatitic veins near Bristol, New Hampshire. Chamberlain and Rumble (1988) proposed that the hot spots were zones where large quantities of hot fluids ascended; timescales of flow were inferred to have been extremely short. We are applying the new garnet growth-diffusion models to derive the crustal P-T-t histories, and to map the P-T schemes to the spatial distribution of isograds in the field. The preliminary results indicate that a short-lived thermal pulse (>100 °C, < 100 kyr) was superimposed on the regional high-T/low-P metamorphism; this heated the rocks to ultrahigh-temperature (UHT) conditions of ~ 900 °C. Consequently, orogenic heat transfer events can be extremely brief.

Western Tianshan UHPM belt

The Western Tianshan Orogen extends between the Tarim Plate to the south and the Yili-Kazak Plate to the north. The Metamorphic belt consists of blueschist, eclogite, and phengite schist, and is recognized as an ultra-high-pressure metamorphosed oceanic slab.  This study was my senior research project in Peking University, China. I investigated a spectacular metasomatic aureole which formed around a deerite-bearing metaquartzite inclusion embedded in the mafic rocks. From the core of the inclusion outward to the blueschist matrix, three zones of different lithologies were developed: 1) clinopyroxenite, 2) eclogite and 3) zoisite-zone. I applied the phase equilibria tools to model the compositional zoning of garnet and cpx in both major and trace elements. The phase assemblages in different lithological zones record different sections of the P-T history. The composite P-T path suggests that the rock was subducted along a cold geotherm (< 7 °C/km), reached the peak pressure (>25 kbar), and was exhumed via a nearly-isothermal decompression process.

Alpine Corsica

Major questions surround the role of COH fluids in high-pressure metamorphism, mass transfer and decarbonation in subduction zones. The problems are exemplified by the high-pressure metamorphic belt of Alphine Corsica. Through collaboration with Alberto Vitale-Brovarone and Olivier Beyssac from IMPMC, Paris, we apply thermodynamic tools to decipher the petrography of the metasomatized metabasalt breccia (right figure), and to quantify the fluid composition and the time-integrated fluid flux through the slab surface. This study is supported by the Deep Carbon Observatory. The preliminary results suggest that the high-pressure fluid carries a significant quantity of COthat is the product of devolatilization reactions. The calculated CO2 content in the fluid, if in equilibrium with the peak assemblage, is consistent with the carbonate dissolution experiments of Caciagli and Manning (2003).

Olivine Rheology and Grain Growth in Lithospheric Mantle

The study was my minor research project in geodynamics, developed on the basis of a term project in Prof. Korenaga's course. We used the unique geologic setting of the Kaapvaal craton to exploit its continental lithosphere as a natural laboratory. The spatial distribution of the mantle xenolith ages provides a tight constraint on the lithospheric-scale deformation history. Our modeling results indicate that the shear stress has been increasing in the past 3.5 billion years, owing to the cooling of the ambient mantle. The shear stress is about two orders magnitude lower than that predicted by traditional grain-size-based piezometers, suggesting that olivine grain growth is considerably suppressed by some mechanisms such as orthopyroxene pinning.