Ethan F. Baxter  

Introduction

Graduate Student Opportunities

Research

Students/Collaborators

Field Work

CV

Publications

Abstracts/Conferences

 

**** Announcing the NEW NSF-Funded Boston University TIMS Facility ****

 

**NEW** EPSL article about Brief Pulses of Metamorphism

 

**NSF CAREER Grant to fund research in metamorphic reaction kinetics**

 

Assistant Professor in Earth Sciences     

Education:

Ph.D. 2000 - University of California, Berkeley
B.S. 1995 - Yale University

Contact Information:

Boston University
Dept. of Earth Science
685 Commonwealth Ave
Boston, MA 02215

Phone: (617) 358-2844
Fax: (617) 353-3290
email: efb@bu.edu

INTRODUCTION

I am a geochemist interested in the processes that govern the transport and exchange of material within the Earth’s crust and mantle ranging from tectonic, to outcrop, to sub-micron scales.  I endeavor to apply that understanding to interpret rock and mineral chemistry in terms of the timescales, rates, and mechanisms of geologic processes.  An important aspect of my research philosophy is that Earth processes can be most fully understood when research is grounded in a breadth of possible scales and methods of observation.  Thus, my research comprises a multi-disciplinary approach including:

The main focus of my research is to study natural rocks and systems, but I have developed complementary experimental techniques in order to augment my assessment of geological processes. While I am an isotope geochemist by training, I hesitate to label myself strictly as such at the risk of diminishing the importance of these other techniques and methodologies that are an integral part of my various research aims.



GRADUATE STUDENT OPPORTUNITIES

Current Opportunity: I recently received a 5-year NSF CAREER Grant for a new graduate student to work on a field-based study of the rates and timescales of metamorphic reaction rates - if you are interested in learning more, please contact me. 

I am always looking for students interested in pursuing a thesis within the general fields of geochemistry, petrology and/or tectonics involving the kind of multi-disciplinary research I describe in this webpage.  In particular, I seek students interested in measuring and understanding the rates of geochemical and geological processes through field work and parallel experimental studies in the laboratory.  Throughout this webpage, I have highlighted in red several particular new research directions which may be developed into a thesis. If you would like any other information, please feel free to send me an email (efb@bu.edu), or check the Boston University Earth Sciences Graduate Studies Webpage.   Also, I have highlighted in green all of the NSF Grants that fund my research; click on the green links to learn more about the grants. 




RESEARCH

Currently, my research may be generalized into three areas (click the links to learn more):

1) kinetics of metamorphic reactions

2) noble gas diffusion and partitioning

3) development of the new BU TIMS Facility

Other past and present research areas of interest are described in the links below:




Measurement and Mechanism of High Temperature Reaction Rates

Motivation: Research in plate boundary processes involves modeling of the production, consumption, and transport of fluids, as well as the evolving physical and chemical properties of the solid earth.  Metamorphic reactions are at the roots of these processes.  For example, model predictions of subduction zone phenomena including devolatilization, magma genesis, and eclogitization require some quantification of the rate and timescale over which metamorphic reactions in the crust and mantle proceed.  However, in light of field-based and lab-based studies which may differ in their kinetic predictions by many orders of magnitude (see figure below), a lack of consensus exists regarding the rates most applicable to natural systems in general and whether or not subduction zones, or regional mountain building processes, should be modeled as equilibrium or disequilibrium systems.  How fast do metamorphic reactions really happen?

Three aspects are fundamental to a full understanding of reaction rates and mechanisms in nature: (#1) laboratory kinetic data from idealized, controlled systems wherein relative effects may be derived, (#2) kinetic theory and knowledge of the governing mechanisms and parameters, (#3) field-based measurement of reaction rates to assess absolute rates applicable in nature.  It is important to develop and conduct systematic field based and laboratory based analysis of high temperature reaction rates and mechanisms in order to amass a useful natural dataset and help bridge the gap between the idealized conditions of the lab and the complexities of nature, and to provide us with relevant and useful reaction rate data for our models and interpretations.  While there is a decent amount of literature on #1 and #2 above, the primary goal of my research in this area is to contribute to #3: field based information on reaction rates.

 

Natural constraints on high temperature (>400 C) metamorphic reaction rates. Contact metamorphic constraints are shown as dark shaded boxes (JF (Joesten & Fisher 1988), WL (Waters & Lovegrove 2002)). Un-shaded boxes represent constraints from regional metamorphic systems. Short dashed boxes represent reaction rates derived from direct garnet growth rate measurements (C89 (Christensen et al 1989), C94 (Christensen et al. 1994), VH (Vance & Harris 1999), VO (Vance & O’Nions 1992)). Thick-lined open box represents bulk solid-fluid Sr isotope exchange rate (BD (Baxter & DePaolo 2000, 2002b). Thin-lined dashed boxes represent constraints derived from modeling of crystal size and spatial distributions for diffusion-controlled growth of garnets (CMD, CPM, CWR (Carlson et al. 1995)). The discrepancy between regional and contact metamorphic reaction rates likely relates to differences in fluid content or transient availability. A wide range of lab-based kinetic data are shown in light gray for comparison. What reaction rate is most appropriate to describe the systems YOU are modeling? Figure and calculations taken from Baxter 2003.

 

   

Field Analysis: I have developed and implemented a technique (Baxter & DePaolo 2002a) involving Sr, Nd and major element analysis of samples collected about a lithologic contact to extract bulk reaction rates and chemical diffusivities.  It has been successfully implemented at Simplon Pass, Switzerland (Baxter & DePaolo 2000,2002b).  This work has shown that extrapolation of laboratory based data overestimates the rate of approach to equilibrium by as much as six orders of magnitude (see figure above) in natural regional metamorphic systems.  A compilation of available field-based constraints on reaction rates (Baxter 2003) shows that the situation appears to be less problematic for contact metamorphic environments, when field-based and lab-based reaction rates estimates are similar within an order of magnitude of two at worst (see above).  Application of this and other emerging techniques to geological settings with variable characteristics will help elucidate the mechanisms and parameters that govern natural reaction rates. 

We are developing a new method to constrain reaction rates through the use of precise Sm/Nd geochronology and thermodynamic modeling of zoned garnets (CAREER Grant funded by NSF- EAR, Petrology & Geochemistry).  Field work commenced in summer 2006 in the islands of Greece.  Additionally, we are developing methods to evaluate the duration of metamorphic processes, including whether metamorphism is dominated by short bursts or pulses.  Ongoing research in collaboration with BU MA student Anthony Pollington, former BU undergraduate Penny Lancaster, and Prof. Jay Ague of Yale is examining this issue in three different field areas: the Stillup Tal shear zone, Austria; the Wepawaug Schist, Connecticut; and the Barrovian Type Locality, Scotland.  We are beginning to see evidence for metamorphic pulses [see our abstract for the 2007 Goldschmidt meeting] perhaps related to magmatism and/or fluid flow.  Future research will benefit greatly from the new BU TIMS Facililty which is dedicated to high precision isotopic analysis of the sort required for this work.



Behavior of Noble Gases in Geologic Systems: Physical models for excess Ar (or He) in the crust

Motivation: Noble gases, most notably argon and helium, are important elements for geochemistry on two fronts.  First, they are the basis for widely applicable geochronologic techniques.  Second, they may be used as tracers of the degassing history of the Earth, the quantification of planetary noble gas reservoirs, and the processes that govern their evolution.  Surprisingly, little is understood about the behavior of noble gases within the Earth, in particular the rates and mechanisms by which they are transported, partitioned and stored.  The phenomenon of “excess Ar” in geochronology highlights this well because, despite its common occurrence, there has been no satisfactory physical explanation for its presence (or absence).  The endeavor to understand excess Ar has implications well beyond 40Ar/39Ar geochronology as it will lead us to the fundamental mechanisms and parameters governing the behavior of noble gases in the crust and mantle.

Field Analysis: I have conducted detailed spatial sampling of Ar/Ar ages in biotites about a lithologic contact which reveals a striking relationship between excess Ar and proximity to the contact (Baxter, DePaolo & Renne 2002).  This data highlights the importance of the immediate surroundings of the mineral of interest (in this case the biotite) in controlling excess Ar buildup.  Modeling of this data yields heretofore unavailable quantitative information about the bulk diffusivity of Ar and the local hydrogeologic history, and has also led to the development of a general physical model to explain and quantify the occurrence of  excess Ar (or He) in slowly cooling geological environments (Baxter 2003).  This model includes the effects of open system diffuse loss of argon from the rock system (controlled by the "transmissive timescale" parameter, tT) and the role of argon uptake into other local mineral and fluid phases (controlled by the "total local sink capacity parameter" TLSC).  Baxter (2003) presents an equation that may be used to predict the amount of excess Ar in a mineral above its closure temperature as a function of these parameters.

Further study of metamorphic rocks, xenoliths, or degassing igneous intrusions, may ultimately lead to the identification and explanation of lithologic or other trends in the occurrence of excess Ar or potentially excess He.  With Dr. Jennifer Matzel at the BGC, we have also initiated  preliminary field based experiments in Sierra Nevada granites and sediments (funded by NSF - EAR, Petrology & Geochemistry) to test the possibility that quartz is an important sink for Argon in the crust and to test hypothesized correlations of excess Ar in quartz with cooling rate, bulk rock potassium content, and proximity within heterogeneous lithologies.

Biotite single grain Ar/Ar age data (diamonds) from a field site near Simplon Pass, Switzerland.  Note the large difference in apparent age between biotites in the pelite and biotites in the amphibolite rock.  The systemticaly higher amounts of "excess Ar" in the amphibolite are due to the more sluggish ability of Ar to escape from the intergranular medium of the amphibolite rock as compared to the pelite.  Triangles are fine grained multi-grain analyses.  Squares are integrated ages from samples for which no plateau was found - for all other samples, plateau and integrated ages were almost identical.
(see Baxter, DePaolo & Renne, 2002 for more)


Experimental Analysis of Noble Gas Partitioning: Data on the equilibrium partitioning of noble gases between phases is extremely limited and controversial.  Yet, the equilibrium partitioning of noble gases between mineral and fluid phases is crucial to geochronologic interpretations, and indeed any use of the noble gases as a tracer of geological processes.   New experiments (funded by NSF - EAR, Petrology & Geochemistry) have been developed by BU Master's student Patricia Clay (now PhD candidate at Open U) in collaboration with Prof. Bruce Watson, Jay Thomas, and Daniele Cherniak at RPI and Dr. Simon Kelley at The Open University, England to directly measure the partitioning of noble gases between key crustal minerals, such as quartz, mica, and feldspar [see our recent AGU abstract for which Patricia Clay was awarded a "Best Student Presentation Award"].  Preliminary work has indicated complex diffusion and partitioning behavior in at least two key minerals: quartz & kspar.  Currently, we are working to further explore and quantify what appear to be two different diffusion pathways for Ar in these minerals which would have major implications for models of thermochronololgic closure, as well as the design and interpretation of laboratory-based experiments to measure noble gas solubility.

In a separate study in collaboration with Profs. Paul Asimow and Ken Farley at Caltech, I have designed and implemented experiments to measure the partitioning of Ar between diopside and its grain boundaries in a synthetic polycrystalline aggregate (Baxter, Asimow & Farley 2007).  Within the framework of noble gas geochemistry, there has been very little attention devoted to this very important “phase” in all rocks: the grain boundary.  This too often forgotten phase is fundamental to understanding chemical exchange between minerals as well as bulk transport through rocks.  Knowledge of grain boundary partitioning is important because the grain boundaries are the fastest pathways for chemical migration through rocks and may represent a significant noble gas reservoir in the mantle and crust. 

Isotope Geochemistry of Salt Deposits on Antarctic Surfaces

Antarctica's Dry Valleys host abundant salt deposits concentrated in layers within soil profiles or in evaporative lakes, and also on the undersides of cobbles. These cobbles rest on modern and ancient surfaces, some buried and preserved by deposits at least 15 million years old.  Depending on the source of Sr and Nd isotopic tracers in the salts, there exists a potential to exploit the archive of modern and ancient surface salts to produce a 15+ million year record of Sr, Nd from atmospheric precipitation and/or from surface weathering processes in Antarctica.  Preliminary work (funded by NSF - Office of Polar Programs) has shown that, while Sr is affected by local sources and processes, Nd isotopes appear less prone to local effects and may in fact record atmospheric dust input from as far away as South America: the same inferred source as dust in the Antarctic ice cores.  Preliminary samples for this work were kindly provided by Prof. David Marchant of BU, Prof. Huiming Bao of LSU, and Prof. Gregory Retallack of OSU.  See the abstract of our poster from the NE GSA Meeting in Saratoga Springs, NY for early results.  This material is based upon work supported by the National Science Foundation under Grant No. NSF-OPP 0441104.



STUDENTS and COLLABORATORS

Below are listed individuals with whom I have worked in the past, am working with currently, and with whom I hope to continue to collaborate in the future.  I thank them all for their contributions and look forward to further collaboration.

 

Graduate Students

Undergraduate Students

Collaborators





FIELD WORK

Analysis of carefully collected samples from various geologic environments is fundamental to my research.  Some of my field areas include:


I plan future work with samples already collected from several of these sites.  Other field areas with particular characteristics appropriate for the geologic questions at hand are always sought, and will be sampled in the future.  Come join me in the field!

BU students Leah Mehl & Julie Barkman on Sifnos, Greece, summer 2006


Field Site in Simplon Pass, Switzerland


     Fun with lava in Hawaii

 

BU Undergraduate Penny Lancaster in Connecticut, summer 2003

 

BU Graduate Student Patricia Clay in the High Sierra, summer 2005

 

BU Undergraduate Field Trip in Boston Area, Spring 2005

 

BU Undergraduate Field Trip in Nahant, MA, Spring 2004



PUBLICATIONS

·  Ague, J.J. and Baxter, E.F., (in press). Brief Heat Pulses During Mountain Building Recorded by Sr Diffusion in Apatite. Submitted to Earth and Planetary Science Letters. (Full PDF version)

·  Watson, E.B. and Baxter, E.F., 2007. Frontiers: Diffusion in solid-Earth systems. EPSL, 253, 307-327. (Full PDF version)

·  Baxter, E.F., Asimow, P.D and Farley, K.A. 2007. Grain boundary partitioning of Ar and He. Geochimica et Cosmochimica Acta, 71, 424-451. (Full PDF version)

·  Baxter, E.F. and DePaolo, D.J.  2004. Can metamorphic reactions proceed faster than bulk strain? Contributions to Mineralogy and Petrology, 146, 657-670. copyright Springer-Verlag. (Full PDF version)

·  Baxter, E.F.  2003. Quantification of the factors controlling the presence of excess 40Ar and 4He.  Earth and Planetary Science Letters, 216, 619-634. (Full PDF version)

· Baxter, E.F. 2003. Natural Constraints on Metamorphic Reaction Rates. in Geochronology - linking the isotopic record with petrology and textures. eds. Vance, Muller & Villa. Geological Society of London, Special Publication, 220, 183-202. (full PDF version)

· Baxter, E.F., and DePaolo, D.J., 2002a.  Field Measurement of High Temperature Bulk Metamorphic Reaction Rates I: Theory and Technique. American Journal of Science, 302, p. 442-464. (full PDF version)

· Baxter, E.F., and DePaolo, D.J., 2002b.  Field Measurement of High Temperature Bulk Metamorphic Reaction Rates II: Interpretation of Results from a Field Site near Simplon Pass, Switzerland.  American Journal of Science, 302, 465-516. (full PDF version)

· Baxter, E.F., DePaolo, D.J., and Renne, P.R., 2002.  Spatially Correlated Anomalous 40Ar/39Ar “Age” Variations About a Lithologic Contact near Simplon Pass, Switzerland: A Mechanistic Explanation for Excess Ar. Geochimica et Cosmochimica Acta, 66, p. 1067-1083. (full PDF version)

· Baxter, E.F., Ague, J.J., and DePaolo, D.J., 2002.  Prograde Temperature-Time Evolution in the Barrovian Type-Locality Constrained by Precise Sm/Nd Garnet Ages from Glen Clova, Scotland.  Journal of the Geological Society, London, 159, p. 71-82. (full PDF version)

· Ague, J.J., Baxter, E.F., and J.O. Eckert, 2001.  High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland. Journal of Petrology, 42, p. 1301-1320. (full PDF version)

· Baxter, E.F., and DePaolo, D.J., 2000.  Field Measurement of Slow Metamorphic Reaction Rates at Temperatures of 500-600°C. Science, 288, p. 1411-1414. (full PDF version)




ABSTRACTS/CONFERENCES [*denotes Baxter-advised student]

·    Baxter E.F. 2007. KEYNOTE: Disequilibrium and Excess Argon: Teaching some bad dogs new tricks. Goldschmidt Conference, Cologne Germany [2007 Clarke Award Lecture].

·    Baxter E.F., Ague, J.J., *Lancaster, P.J. 2007. Focused pulses of regional metamorphism. Goldschmidt Conference, Cologne Germany.

·    *Mehl, L.Y., *Barkman, J., Baxter, E.F., 2006. Constraining the Rate of Water Releasing Metamorphic Reactions in Subduction Zones. AGU Fall Annual Meeting.

· †*Clay, P.L., Baxter E.F., Kelley, S.P., Watson, E.B., Thomas, J., Cherniak, D.P., 2006. Multi-path diffusion: Implications for the measurement of Ar solubility and partitioning between quartz and feldspar. AGU Fall Annual Meeting [†Clay received AGU Fall Meeting Student Presentation Award from the VGP Section].

·  Ague, J.J., and Baxter, E.F. 2006. Extremely short-duration peak metamorphism in the Barrovian zones, Scotland. AGU Fall Annual Meeting.

·  Baxter E.F., *Clay, P.L., Kelley, S.P., Watson, E.B., Thomas, J., Cherniak, D.P., 2006. Two diffusive pathways for quartz and feldspar. Goldschmidt Conference, Melbourne Australia.

· *Clay, P.L., Baxter E.F., Kelley, S.P., Watson, E.B., Thomas, J., Cherniak, D.P., 2005. Experimental Study of Noble Gas Partitioning and Diffusion in Common Crustal Minerals. AGU Fall Annual Meeting.

· Baxter, E.F. 2005. INVITED: Importance of Ar, He Transport and Partitioning in Grain Boundaries.  Goldschmidt Conference, Moscow Idaho.

· Baxter, E.F., 2005. INVITED: Comparing natural reaction kinetics for isotopic exchange and net-transfer reactions. Goldschmidt Conference, Moscow Idaho.

· *Levine, J.A. and Baxter, E.F. 2005. A Sr and Nd isotopic study of the sources and age of salts from the Dry Valleys of Antarctica. NE Section GSA Annual Meeting.

· *Lancaster, P.J., Baxter, E.F., and Ague, J.J. 2005. Temperature-time development in the Wepawaug Schist. NE Section GSA Annual Meeting.

· Baxter, E.F. 2004. Field based constraints on reaction rates in the crust. AGU Fall Annual Meeting.

· Baxter, E.F., Asimow, P.D., & Farley, K.A. 2003. INVITED: Measurement of Grain Boundary Partitioning of Ar and He. AGU Annual Fall Meeting.

· Baxter, E.F. 2003. Deciding What Reaction Rate to Use in Your Model. GSA Annual Meeting.

· Baxter, E.F. 2003. Accommodation of crustal excess Ar by mineral and fluid sinks. AGU Annual Spring Meeting, Nice, France.

· Baxter, E.F. & DePaolo, D.J., 2002. INVITED: Can metamorphic reactions proceed faster than strain?.  Goldschmidt Conference, Davos, Switzerland.

· Baxter, E.F., Asimow, P.D., and Farley, K.A., 2001. Experimental Study of Grain Boundary Partitioning of Ar.  AGU Annual Fall Meeting.

· Baxter, E.F., 2001. The Transmissive Timescale: A System Parameter that Controls the Presence or Absence of Excess Ar.  GSA Annual Meeting.

· Baxter, E.F., 2001.  INVITED: Field Measurement of Slow Metamorphic Reaction Rates and the Implications for Local Equilibrium-Based Geochemical Methods. Goldschmidt Conference, Hot Springs, Virginia.

· Baxter, E.F., DePaolo, D.J., and Renne, P.R., 2001.  Importance of the “Transmissive Timescale” for Ar in the Crust and a Hypothesis for Local Non-K Bearing Mineral Sinks for Ar.  Goldschmidt Conference, Hot Springs, Virginia.

· Baxter, E.F., DePaolo, D.J., and Renne, P.R., 2000.  Ar Isotopic Variations in Biotites About a  Lithologic  Contact near Simplon Pass, Switzerland: Implications for Ar Bulk Diffusivity, Excess Ar, and Geochronology.  AGU Annual Fall Meeting.

· Baxter, E.F., Ague, J.J., and DePaolo, D.J., 2000.  Tectonometamorphic History of the Barrovian Type-Locality Constrained by Precise Sm/Nd Garnet Ages from Glen Clova, Scotland.  GSA Annual Meeting.

· Baxter, E.F, DePaolo, D.J., and Renne, P.R., 2000.  Ar and Sr Isotopic Variations About a Lithologic Contact near Simplon Pass, Switzerland: Implications for Diffusional Exchange and Geochronology. Goldschmidt Conference, Oxford, England.

· Presented research at 1999 Gordon Research Conference on Rock Deformation

· Baxter, E.F., and DePaolo, D.J., 1999. Similar Rates of Reaction and Strain in the Central Alps: Evidence for a Mechanistic Link? EOS, Transactions of the American Geophysical Union, v. 80, p. 1021.

· Baxter, E.F., and DePaolo, D.J., 1999.  Field Measurement of Very Slow Metamorphic Reaction Rates at Simplon Pass, Switzerland. Abstracts with Programs, GSA Annual Meeting, v.31, p. 103.

· Baxter, E.F., and DePaolo, D.J., 1998.  Field Constraints On Syn-Metamorphic Exchange Rates, Diffusivities, and Disequilibrium from Garnet and Whole Rock Rb-Sr Isotope Systematics. Abstracts with Programs, GSA Annual Meeting, v.30, p. 381.