Noble Gas Thermochronology

Thermochronometric data can record the thermal history of rocks as they cool from high temperatures at depth to lower temperatures at the surface. This provides a unique perspective on the tectonic processes that form topography and the erosional processes that destroy it. However, quantitatively interpreting such data is a challenge because multiple models can do an equally good job at reproducing the data. In this article, we describe how inverse modeling can be used to improve quantitative interpretations of noble gas thermochronometric data on a variety of scales, ranging from mountain belts to individual mineral grains.

Rocks from extraterrestrial bodies in the Solar System are influenced by thermal processes occurring within planetary interiors and on their surfaces. These range from the extremely hot and brief, in the case of impact events, to the comparatively cool and protracted, in the case of solar irradiation of rocks residing in regoliths for millions to billions of years. Noble gas thermochronology applied to meteorites and extraterrestrial materials returned by space missions enables us to decipher the histories of these materials and thereby understand fundamental aspects of the evolution of terrestrial planetary bodies, including the Moon, Mars, and asteroids.

Ancient rocks have survived plate tectonic recycling for billions of years, but key questions remain about how and when they were exhumed to the surface. Constraining exhumation histories over long timescales is a challenge because much of the rock record has been lost to erosion. Argon and helium noble gas thermochronology can reconstruct deep-time <350 °C thermal histories by using the distinct temperature sensitivities of minerals such as feldspar, zircon, and apatite, while exploiting grain size and radiation damage effects on diffusion kinetics. Resolution of unique time–temperature paths over long timescales requires multiple chronometers, appropriate kinetic models, and inverse simulation techniques to fully explore and constrain possible solutions. Results suggest that surface histories of ancient continental interiors are far from uninteresting and may merely be misunderstood.

Fault zones record the dynamic motion of Earth’s crust and are sites of heat exchange, fluid–rock interaction, and mineralization. Episodic or long-lived fluid flow, frictional heating, and/or deformation can induce open-system chemical behavior and make dating fault zone processes challenging. Iron oxides are common in a variety of geologic settings, including faults and fractures, and can grow at surface- to magmatic temperatures. Recently, iron oxide (U–Th)/He thermochronology, coupled with microtextural and trace element analyses, has enabled new avenues of research into the timing and nature of fluid–rock interactions and deformation. These constraints are important for understanding fault zone evolution in space and time.

Advances in detrital noble gas thermochronometry by 40Ar/39Ar and (U–Th)/He dating are improving the resolution of sedimentary provenance reconstructions and are providing new insights into the evolution of Earth’s surface. Detrital thermochronometry has the ability to quantify tectonic unroofing or erosion, temporal and dynamic connections between sediment source and sink, sediment lag-times and transfer rates, the timing of deposition, and postdepositional burial heating. Hence, this technique has the unique ability to use the detrital record in sedimentary basins to reconstruct Earth’s dynamic long-term landscape evolution and how basins are coupled to their hinterlands.

Heat transfer in the solid Earth drives processes that modify temperatures, leaving behind a clear signature that we can measure using noble gas thermochronology. This allows us to record the thermal histories of rocks and obtain the timing, rate, and magnitude of phenomena such as erosion, deformation, and fluid flow. This is done by measuring the net balance between the accumulation of noble gas atoms from radioactive decay and their loss by temperature-activated diffusion in mineral grains. Together with knowledge about noble gas diffusion in common minerals, we can then use inverse models of this accumulation–diffusion balance to recover thermal histories. This approach is now a mainstream method by which to study geodynamics and Earth evolution.