Search for: All records

Abstract Cosmogenic nuclide dating is an essential component of studying Earth surface processes, but it requires knowledge of how nuclide production rates vary in time and space. Typically, production rates are calibrated at sites with independently well‐constrained exposure histories and then scaled to other sites of interest using scaling frameworks that account for spatial and temporal variations in the secondary cosmic‐ray flux at Earth's surface. To date, scaling schemes for terrestrial cosmogenic nuclide production rates have been developed for the Quaternary, yet cosmogenic nuclide applications that extend beyond the Quaternary are becoming more prevalent. For these deeper time applications, production rate calculations using scaling models optimized for the latest Quaternary neglect longer term spatiotemporal variations in geomagnetic field intensity, paleogeography, and paleoatmospheric depth. We present a production rate scaling scheme for the past 70 million years, SPRITE (Scaling Production Rates In deep TimE). This framework extends existing scaling schemes into deeper time by (a) accounting for site‐specific changes in paleolatitude, (b) integrating a geomagnetic field intensity model rooted in data from a global paleomagnetic database, and (c) incorporating climate‐driven, time‐varying atmospheric depths. We evaluate the efficacy of our model by applying it to existing data sets from paleoexposure sites, and from sites with apparent continuous million‐year exposure histories. This scaling model can be applied with measurements of stable cosmogenic nuclides to research questions such as constraining hiatus durations between ancient lava flows and calculating the formation timescales of stable landforms in arid environments over millions of years. 

Cosmic ray exposure (CRE) ages are used to constrain the orbital and impact history of meteorites and identify their parent body or source region. CRE ages of enstatite (E) chondrites obtained from measurements of 3He are often much younger than 21Ne CRE ages measured in the same meteorite, which is often attributed to diffusive loss of 3He via solar heating during orbit. With knowledge of the diffusion kinetics of 3He in the major minerals making up E chondrites, we can leverage this discrepancy in CRE ages to infer a meteorite’s recent thermal history. To this end, we performed stepwise degassing experiments on fragments of albite, enstatite and kamacite, the dominant minerals in E chondrites, that were irradiated with protons to produce 3He. We find albite displays simple, Arrhenius-dependent 3He diffusion behavior, whereas enstatite and kamacite exhibit somewhat more complex diffusion behavior. We find that cosmogenic 3He will be readily lost from albite in the space environment, enstatite can exhibit significant 3He loss if exposed to high temperatures characteristic of low perihelion on million year time scales, and kamacite is highly retentive of 3He and unlikely to experience direct diffusive loss. These diffusion kinetics parameters can also be used to understand the exposure and thermal histories of other meteorite classes, terrestrial cosmogenic 3He applications, and mantle noble gas systematics. 

This is an update to the SPRITE framework presented in Mijjum et al. (2025) (up to date as of 01/22/2025). This update fixes a typo in the example inputs spreadsheet. PREVIOUS DESCRIPTION: This is latest (as of 10/28/2024) of the production rate calculator (SPRITE) described in Mijjum et al. (2024) (in review with Geochemistry, Geophysics and Geosystems). Model will calculate scaling factors and exposure ages from 0-70 Ma using a time-varying magnetic field (paleointensity), paleoclimate, and latitude (paleolatitude). Code contributions were made by Moshammat Mijjum (@mmijjum), Katie Bristol (@katiebristol), and Richard Bono (@rkbono), manuscript contributions from Marissa Tremblay (@mtrems), Nat Lifton, and Courtney Sprain.