Lazed and Diffused: Untangling Noble Gas Thermochronometry Data for Exhumation Rates

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.

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Noble Gas Thermochronology of Extraterrestrial Materials

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.

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Vestiges of the Ancient: Deep-Time Noble Gas Thermochronology

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.

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Iron Oxide (U–Th)/He Thermochronology: New Perspectives on Faults, Fluids, and Heat

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.

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Earth’s Dynamic Past Revealed by Detrital Thermochronometry

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.

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Noble Gases Deliver Cool Dates from Hot Rocks

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.

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From Mine to Mind and Mobiles: Society’s Increasing Dependence on Lithium

Lithium is everywhere. If you have a mobile phone or a laptop, you are taking advantage of one of the technological revolutions of the last 30 years: lithium-ion batteries. Lithium has long been used in pharmaceuticals and in the manufacture of grease, ceramics, and glass, but has now become the symbolic element of the current energy revolution. Lithium is ubiquitous in our society and plays a role in our lives that could not have been previously imagined. From its mining to its applications in advanced battery materials and pharmaceuticals, welcome to the lithium decade. Electric mobility will become the new normal.

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Classification and Characteristics of Natural Lithium Resources

There are three broad types of economic lithium deposit: 1) peralkaline and peraluminous pegmatite deposits and their associated metasomatic rocks; 2) Li-rich hectorite clays derived from volcanic deposits; 3) salar evaporites and geothermal deposits. Spodumene-bearing pegmatites are the most important and easily exploitable Li deposits, typically containing 0.5 Mt Li. Salar deposits hold the largest Li reserves, can reach up to 7 Mt Li, but are more difficult to exploit. Allowing for recycling, the current predicted demand up to the year 2100 is 20 Mt Li; world resources are currently estimated at more than 62 Mt Li. Thus, abundant resources exist, and no long-term shortage is predicted.

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Lithium and Lithium Isotopes in Earth’s Surface Cycles

Lithium and its isotopes can provide information on continental silicate weathering, which is the primary natural drawdown process of atmospheric CO2 and a major control on climate. Lithium isotopes themselves can help our understanding of weathering, via globally important processes such as clay formation and cation retention. Both these processes occur as part of weathering in modern surface environments, such as rivers, soil pore waters, and groundwaters, but Li isotopes can also be used to track weathering changes across major climate-change events. Lithium isotope evidence from several past climatic warming and cooling episodes shows that weathering processes respond rapidly to changes in temperature, meaning that weathering is capable of bringing climate back under control within a few tens of thousands of years.

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High-Temperature Processes: Is it Time for Lithium Isotopes?

The field of high-temperature Li isotope geochemistry has been rattled by major paradigm changes. The idea that Li isotopes could be used to trace the sources of fluids, rocks, and magmas had to be largely abandoned, because Li diffusion causes its isotopes to fractionate at metamorphic and magmatic temperatures. However, diffusive fractionation of Li isotopes can be used to determine timescales of geologic processes using arrested diffusion profiles. High diffusivity and strong kinetic isotope fractionation favors Li isotopes as a tool to constrain the durations of fast processes in the crust and mantle, where other geochronometers fall short. Time may be the parameter that high-temperature Li isotope studies will be able to shed much light on.

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