Research

By studying the vast geological record, in my research group we interpret the chemistry of diverse Earth materials (rocks, minerals, sediments, salts, and more) to elucidate the timescales, rates, and mechanisms of broad geological processes (tectonics, metamorphism, subduction, global geochemical cycles, lithosphere-hydrosphere-atmosphere interactions). An interest in unraveling the history of the Earth, as well as predicting certain aspects of its future, is at the heart of much of the research going on in my group.

Much of the research in the group involves the new TIMS Facility where we are pushing the limits of temporal and spatial resolution in isotopic analysis.

Analysis of carefully collected natural samples provides a crucial field context for research. That fundamental link to natural systems is a key aspect of research going on in my group and has taken us to such field areas as the Greek Isles, Austria, Switzerland, Scotland, California, New England, and more.

If you are interested in discussing any of these existing, or potential new research areas, please feel free to contact me.

Research Areas

**Download Ethan Baxter's "Garnet: Tree Rings of Crustal Processes" powerpoint here

Garnets contain the "tree-rings" of tectonic and metamorphic processes. Each garnet crystal is a history book spanning potentially millions of years waiting to be read and interpreted - the bigger the garnet, the longer the history. In this way, garnets contain otherwise inaccessible information about evolving prograde metamorphic conditions, processes, and their driving forces. We have developed integrated methodologies to chemically map, microsample, and date the growth of zoned garnets using the Sm-Nd isotopic system. New TIMS analytical developments permit precise measurement of very small quantities of Neodymium (Nd) isotopes permitting high resolution geochronology on multiple concentric growth zones within single garnet crystals. Individual age precision better than ±1 million years is now usually achieved. See our recent paper that describes the details of our microsampling and Sm-Nd geochronologic methods.

We are also pushing the limits on sample size and precision even further in developing the geochronology of detrital garnet grains in the ancient sedimentary (and metasedimentary) record. Detrital garnet geochronology would help inform us of the rates and conditions of the planet’s very first tectonic and metamorphic activity, as well as regional patterns of prograde metamorphic timing in more recent orogens.

If you are interested in potential collaborative research involving garnet geochronology in the BU TIMS Facility, please contact me directly.

Garnet Geochronology
Garnet Geochronology
Garnet Geochronology
Garnet Geochronology
Garnet Geochronology

The solid earth plays a major role in the long-term geologic carbon cycle. Atmospheric, oceanic, and mantle derived CO2 or CO2-rich fluids reacts with silicate minerals and/or dissolved cations in the lithosphere to form secondary carbonate minerals in a variety of geological environments (regional metamorphism, contact metamorphism, subduction zone metamorphism, hydrothermal and ore-forming systems in the continental and oceanic crust, sedimentary basins, and weathering). In order to interrogate and quantify matters of rate, timing, and flux of CO2 (and hydrothermal fluid flow in general) within the lithosphere over geologic (i.e. >1 Myrs) timescales, we seek to develop an accurate and precise carbonate geochronometer.

Carbonate geochronology has proven to be a significant challenge due to natural complexities and analytical limitations. Our developmental emphasis will be on the less-frequently tested Sm/Nd system for carbonates, and on the subsequent integration and cross-checking of Sm/Nd and U/Pb data. Preliminary data show that carbonate minerals datable by Sm/Nd do exist, though the context and identity of the datable mineral’s occurrence is not clear.

Carbonate Geochronology
Two people standing outside

Plate boundary processes involve the production, consumption, and transport of fluids, as well as the evolving physical properties of the solid earth. Metamorphic reactions are at the roots of these processes. Model predictions of subduction zone phenomena (including devolatilization, magma genesis, and eclogitization) and regional orogeny require some quantification of the rate and timescale over which metamorphic reactions in the crust and mantle proceed.

Garnet frequently grows during dehydration, thus preserving a record of these reactions during metamorphism. In this research, we are integrating high precision zoned garnet geochronology with thermodynamic analysis of garnet forming dehydration reactions to directly measure dehydration fluxes in subduction zones and regional orogeny. Ongoing research has focused on well preserved garnet-bearing blueschists on the islands of Sifnos and Syros, Greece as well as large rotated garnets from Townshend Dam, Vermont.

Garnet Geochronology
Garnet Geochronology
Garnet Geochronology
Garnet Geochronology
Garnet Geochronology

The noble gases, in particular radiogenic Ar-40 and He-4, have been used within the context of geo- and thermo-chronology for decades with applications to planetary, igneous, metamorphic, tectonic and Earth surface processes. Like any geochronometer, the well developed K-Ar (or Ar-Ar) and U/Th-He geochronometers rest upon several key assumptions and tenets that do not always hold true, and in any case must constantly be evaluated and justified. It has been shown, for example, that the rate and mechanisms governing the transport of noble gases within rocks and minerals, as well as the equilibrium partitioning of noble gases between minerals and fluids in geologic systems, controls the accumulation of radiogenic noble gases, and thus the apparent age recorded. "Excess argon" results when rock scale argon diffusion is too slow for it to escape, or, when argon is partitioned strongly into minerals we might wish to date.

In field based, experimental, and theoretical research, my students, collaborators, and I have sought to quantify and model these effects on geo- and thermo-chronology. Apparent biotite Ar/Ar ages were found to be spatially and lithologically correlated in an outcrop of interlayed pelite and amphibolite in Switzerland. This observation has led to a general model for excess argon in slowly cooling systems (see corrected governing equation in Baxter 2003). Partitioning of argon and helium in grain boundaries has been shown to be quite significant in nominally dry systems. We have also recognized complex multiple diffusion pathways for argon in both quartz and feldspar by means of high resolution depth profiling analysis of "in-diffusion" experimental samples. All of this information helps us refine our understanding noble gas behavior and its geochronologic significance.

Direct quantification of the rates and timescales over which metamorphic processes occur have challenged geochemists, petrologists, and geochronologists for years. With recent technologies, methods, and collaborations, we have been able to provide unprecedented field-based quantification of ancient metamorphic and tectonic processes. With these capabilities and quantifications, we are positioned now to address topics and questions within metamorphism and tectonics that were innaccessible in the past.

Rapid Pulses

Evidence is mounting that tectonometamorphic processes may be dominated by brief bursts or "pulses" of activity, rather than slow and steady as the classical paradigm would suggest. Recently, we have shown with zoned garnet geochronology that the rate of garnet growth at a site in the Austria Alps (and the tectonic processes driving it) was punctuated by at least two brief "pulses" of activity. In other work, we have integrated garnet growth ages and zircon geochonology to demonstrate a temporal link between metamorphic mineral growth and igneous activity. Peak metamorphic heating in the Barrovian type-locailty in Scotland occurring 465 million years ago was very brief in duration as evidenced by diffusion modeling of zoned apatite and garnet: a quick pulse lasting just a few hundred thousand years.

Slow Time-Averaged Reaction Rates

Field-based quantification of paleo-metamorphic reaction rates is very difficult to reconstruct. However, existing field-based data consistently indicate that natural reaction rates are significantly slower than one would predict from direct extrapolation of laboratory based experimental data [similar observations have also been made for low-temperature weathering reaction kinetics]. At Simplon Pass, Switzerland, modeling of Sr-isotopic diffusion across a lithologic contact was used to show that time-averaged bulk reaction rates attending metamorphism are extremely slow. In another study, we discuss a genetic relationship between strain rates and reaction rates during dynamic metamorphism.

Ongoing field-based research in New Hampshire and Vermont involves zoned garnet geochronolgy in concert with several other geochemical approches to test models of mineral growth, nucleation, fluid and heat flow during regional metamorphism. The hypothesis of "pulsed" metamorphism (where the pulses could be thermal, fluid, or mineral growth) are also being tested in these field contexts.

A mountain range
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