Noble Gas Diffusion & 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.

Relevant Grant Support

NSF Grant EAR-0337527 “Partitioning of Noble Gases Between Crustal Minerals: Implications for

Geochronology", 3/1/2004, PI: Baxter

Selected Publications

Baxter EF 2010. Diffusion of Noble Gases in Minerals. Reviews in Mineralogy & Geochemistry, 72, Ch. 11, p. 509-557. (link to GSW)

Clay PL, Baxter EF, Kelley SP, Watson EB, Thomas J, Cherniak DP, 2010. Combined RBS and laser depth profiling of Ar diffusion in quartz: evidence for two diffusion pathways. GCA, 74, 5906–5925. PDF

Watson EB and Baxter EF, 2007. Frontiers: Diffusion in solid-Earth systems. EPSL, 253, 307-327. PDF

Baxter EF, Asimow PD and Farley KA, 2007. Grain boundary partitioning of Ar and He. GCA, 71, 424-451. PDF

Baxter EF, 2003. Quantification of the factors controlling the presence of excess 40Ar and 4He. EPSL, 216, 619-634. PDF

Baxter EF, DePaolo DJ and Renne PR, 2002. Spatially Correlated Anomalous 40Ar/39Ar “Age” Variations About a Lithologic Contact near Simplon Pass, Switzerland: A Mechanistic Explanation for Excess Ar. GCA, 66, 1067-1083. PDF