Sulfur in the Apollo Lunar Basalts and Implications for Future Sample-Return Missions

Between 1969 and 1972, Apollo mission astronauts explored the lunar surface, collecting geologic materials and returning them to Earth for careful study. After consideration of many lines of evidence, one of the many major results of studying the Apollo rocks is the broad scientific consensus that the Moon formed from the debris of a giant impact of a large body with the proto-Earth (e.g., Stevenson 1987). This left the Moon depleted in highly volatile elements such as hydrogen, relative to Earth. So it was thought.

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Measuring Noble Gases for Thermochronology

The articles in this issue show how applications of noble gas thermochronology can help answer fundamental questions about Earth and planetary processes. Here, we discuss how noble gas measurements are actually made. We review the different methods used to extract and isolate noble gases from natural materials and to measure those gas concentrations and isotopic compositions using mass spectrometry.

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Sailing the Sea of Open Access: Celestial Navigation or Dead Reckoning?

The notion of open access (OA) began to gain traction in the mid–late 1990s (Laakso et al. 2011). The Bethesda Statement (2003) followed a year later with the definition of ‘open access’ as: “free, irrevocable, worldwide, perpetual right of access to, and a license to copy, use, distribute, transmit, and display the work publicly and to make and distribute derivative works, in any digital medium for any responsible purpose, subject to proper attribution of authorship.”

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v16n5 From the Editors

Marissa Tremblay, Emily Cooperdock, and Peter Zeitler, guest editors of this issue of Elements, introduce us to another application of noble gases: thermochronology. In addition to editing the six thematic articles on the utility of noble gas thermochronology to fundamental geological questions (e.g., What are the rates of exhumation? How does a fault zone evolve?), these guest editors also wrote this issue’s Toolkit, which introduces the different methods used to extract, isolate, and measure the concentration of noble gases (and their isotopes) derived from natural materials. We hope you enjoy reading about this fascinating topic!

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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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