Rates and Timescales of Metamorphism

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.

Relevant Grant Support

NSF Grant EAR-0948308 “Collaborative Research: Testing for Rapid Pulses of Crustal-scale Heat and Mass Transfer by Fluids in Metamorphic "Hot Spots", New Hampshire, USA”, 8/1/10, PI: Ague; Co-PIs: Baxter, Chamberlain

NSF Grant EAR-0911582 “Collaborative Research: Developing a Practical and Quantitative Method for Measurement of Metamorphic Porphyroblast Crystallization Kinetics and Strain Rate”, 7/1/09, PI: Hirsch; Co-PIs: Stowell, Baxter

NSF Grant EAR-0547999 “CAREER: Rates and Timescales of Metamorphic Reactions at Convergent Plate Boundaries”, 1/1/06, PI: Baxter

Selected Publications

Pollington AD and Baxter EF, 2011. High precision microsampling and preparation of zoned garnet porphyroblasts for Sm-Nd geochronology. Chemical Geology, 281, 270-282. (PDF)

Pollington AD and Baxter EF, 2010. High resolution Sm/Nd garnet geochronology reveals the uneven pace of tectonometamorphic processes. EPSL, 293, 63-71. PDF

Lancaster PJ, Baxter EF, Ague JJ, Breeding CM, and Owens TL, 2008. Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA. J Met Geol, 26, 527-538. PDF

Ague JJ, Baxter EF, 2007. Brief Heat Pulses During Mountain Building Recorded by Sr Diffusion in Apatite. EPSL, 261, p. 500-516. PDF

Baxter, E.F. and DePaolo, D.J. 2004. Can metamorphic reactions proceed faster than bulk strain? Contributions to Mineralogy and Petrology, 146, 657-670. PDF

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. PDF

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. PDF

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. PDF

Baxter EF, Ague JJ, and DePaolo DJ, 2002. Prograde Temperature-Time Evolution in the Barrovian Type-Locality Constrained by Precise Sm/Nd Garnet Ages from Glen Clova, Scotland. J Geol Soc Lond, 159, 71-82. PDF

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. PDF