Fission tracks in apatite must be etched for counting and measuring with an optical microscope. Track etching is described with the aid of two velocities: the track etch rate vT and the apatite etch rate vR. The efficiencies of track counts, the selection of confined tracks, and the distinctive shapes of etched tracks depend on the values of vT and vR; vR is also an indicator of the apatite composition and annealing kinetics. Despite their importance, measurements of vT and vR have been lacking. We here report measurements of vT and vR and investigate factors controlling them. The results support a constant core model, i.e., a constant vT over most of the track length. The host-confined-track intersections, c axis angles, and track-etch rates have near-identical distributions across all experiments. Neither lab annealing of induced tracks nor geological ageing of fossil tracks has a significant effect on vT. Overall, for a one-step etch protocol, the lengths of fission tracks exhibit a weak positive correlation with their track etch rates and effective etch times. The effect is however too small to be significant for geological applications. When using a step-etch protocol, the etchant advances at a finite rate along tracks etched during preceding steps. This broadens the effective-etch-time and track-length distributions, while the summed immersion times overestimate the aggregate effect of the procedure. In contrast to vT, vR exhibits clear variation related to damage from self-irradiation in the case of fossil tracks and to a minor extent from neutron irradiation in the case of induced tracks. Greater vR shorten the access times to confined tracks, increasing their effective etch times. This implies a need for caution when using the track openings for assessing annealing rates.
A noble-gas analysis line capable of accurate and precise measurements of small absolute amounts of 4He released from crystals is a key analytical step in the production of (U-Th)/He chronologic data. He analysis lines that are custom-built in-house can be optimized for specific lab needs and facilitate continued maintenance, repair, and upgrades. However, there is little information in the published literature about the methods and approaches for building a He line. Here, we describe the design, construction, automation, and metrological calibration of a custom 4He extraction and analysis line as part of establishing laser-ablation (U-Th)/He methods in the University of Colorado Thermochronology Research and Instrumentation Lab (CU TRaIL). The line, called the Jimbochron, is designed to precisely measure very small (∼ fmol) amounts of 4He while being fully automated and easily modifiable in the future. These goals are achieved by minimizing the line volume, adopting a unique double-hexagonal manifold configuration, installing a high-sensitivity quadrupole mass spectrometer, and developing editable LabView code for instrument communication and automation that is straightforward to update. We also explain the steps used to calibrate the Jimbochron metrologically from first principles with a new in-house calibration volume to ensure high-accuracy He measurements. The Jimbochron accurately and precisely measures small gas amounts (< 0.1 fmol) and now routinely generates accurate and precise He data for laser-ablation (U-Th)/He applications.
Advances in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have largely focused on improving spatial resolution through progressively smaller laser spot sizes. Here, we explore the opposite end of the analytical limits by investigating the lower concentration limits of in-situ U–Pb geochronology in garnet. Using a Neptune Plus multi-collector ICP-MS equipped with seven ion counters, we developed an analytical workflow specifically designed for metamorphic garnet with ultra-low U concentrations (< 10 ng g−1). The method was used to date garnet from a wide range of ages and geological settings, including granulites, eclogites and hydrothermal demantoids. Uranium concentrations were exceptionally low, even below 1 ng g−1 in some cases. At these concentrations, the total amount of U ablated during a single analysis is at femtogram levels, more than three orders of magnitude lower than that of a typical in-situ analysis of zircon. Despite these extremely low signal intensities, geologically meaningful ages were obtained for the analysed samples. Analytical precision depends on the U and radiogenic Pb concentrations, but we have obtained precisions of ca. 5 %–6 % for garnet containing less than 1 ng g−1 U. In garnet with higher U concentrations or ages as old as the Archean, internal precision of ca. 1 % may be achieved. These results expand the applicability of in-situ garnet U-Pb geochronology to the vast majority of metamorphic garnet, providing a powerful new tool for constraining garnet growth, prograde metamorphism, and deep crustal evolution.
Cosmogenic-nuclide surface exposure ages provide empirical data for validating models simulating the timing and pace of ice-sheet response to a warming climate. Increasing emphasis is being placed on obtaining exposure ages that both accurately constrain Holocene deglaciation and are precise enough to capture ice sheet change at the sub-millennial scale. However, longer-lived nuclides such as 10 Be are susceptible to cosmogenic nuclide inheritance often persisting through multiple periods of exposure and burial, which can impact the accuracy of the most recent Holocene exposure history. Shorter-lived in situ cosmogenic 14 C (in situ 14 C) is largely insensitive to nuclide inheritance pre-dating the last glacial maximum (LGM), and when combined with longer-lived nuclides can be used to constrain complex ice sheet histories over Holocene timescales. Here, we present new in situ 14 C exposure ages from nine erratic cobbles from Mount Murphy, West Antarctica. Six of these suggest Mt Murphy deglaciated from 5–3 ka; this is inconsistent with previously measured 10 Be ages of the same samples that place deglaciation from 8–6 ka. We investigate potential explanations for the conflicting exposure histories by analysing paired 14 C– 10 Be data of Holocene age presently archived in the informal cosmogenic-nuclide exposure-age database (ICE-D, https://version2.ice-d.org/, last access: 29 March 2024). Our analysis reveals that neither variations in geologic setting nor modelled scenarios of subsurface nuclide production can explain the conflicting Mt Murphy ages. However, replicate in situ 14 C measurements indicate that initial in situ 14 C concentrations used to calculate the youngest exposure ages (5–3 ka) do not reproduce within stated 2 σ uncertainty, whereas measurements used to calculate the older ages (8–6 ka) are reproducible. Furthermore, we observe that in situ 14 C concentrations measured in 15 of 31 samples taken from ICE-D do not replicate within their nominal 2 σ analytical uncertainty. Together, these results suggest that analytical uncertainty for in situ 14 C measurements may currently be underestimated. We provide recommendations for improving measurement precision that will benefit future Holocene deglaciation studies, including analysis and publication of more replicate measurements and the continuation of efforts to quantify and minimise sources of scatter in blank measurements.
Thermochronological data are commonly used to study the activity of geological processes over timescales of millions of years. Ages produced by thermochronological measurements, however, are non-unique and do not directly record rates of processes, which has led to the development of a variety of software tools for interpreting age data in the context of geological processes. Most of the widely used software packages focus on determining thermal histories, which are easy to use but do not provide direct quantitative estimates of geological process rates. In contrast, more sophisticated and complex thermo-kinematic modeling software can link ages to process rates but may require greater computational expertise and resources for use. Here we introduce Tc1D, a 1D thermal and thermochronometer age prediction software package designed to provide users with the opportunity to explore geological processes from thermochronology data in a computationally efficient and accessible framework. The software is open source and written in the Python programming language, and provides functionality for forward and inverse modeling of thermochronometer data, visualization using built-in plotting, a variety of options for defining exhumation histories, and more. This work presents an overview of how Tc1D was designed, several illustrative examples of how the code can be applied, instructions for how to get started using Tc1D, and some plans for future development.
