Search

Terrestrial supergene goethites of known ages record information on changes in weathering conditions through time. Here we present a database of (U-Th)/He ages and U and Th contents of goethites from different weathering environments around the globe. By consolidating published data collected at four different laboratories and unpublished data collected at the Noble Gas Laboratory at Caltech, we aim to give an overview of the work carried out by geochronologists and geochemists in the last 20 years. The database contains 2597 (U-Th)/He ages of goethites from 10 countries; most of the ages come from Brazil and Australia. [ABSTRACT FROM AUTHOR]

We present a step-heat method for isolating cosmogenic 3He (3Hec) from mantle He in olivine xenocrysts to date the eruption of very young nephelinites from Volcano Mountain (VM) Yukon, Canada. In these olivines, the standard procedure of powdering grains to <30 µm failed to adequately remove mantle helium prior to fusion analyses. For example, in one powder fusion the concentration of 4He was 2.93 x 106 ± 6.04 x 104 Matoms/g with a 3He/4He ratio of 8.7 ± 0.3 RA (atmospheric ratio; RA = 1.384 x 10-6). Based on the 3He/4He ratio of 8.1 ± 0.2 RA released by crushing of the same sample, the estimated fraction of mantle 3He in the powder fusion is between 87 % and 98 % of the total 3He. The inability to effectively isolate 3Hec from these samples likely arises from the survival of small (<<30 µm) fluid inclusions hosting mantle He through the powdering step. The presence of such unusually small fluid inclusions may relate to the origin of the olivines as disaggregated peridotite xenoliths rather than the more commonly analyzed olivine phenocrysts. Regardless, the high proportion of mantle 3He in the powder fusion yields highly uncertain 3Hec exposure ages. We circumvented this problem by heating powdered olivine in a three-step heating schedule ranging from 700 to 1400 °C. 80–92 % of 3Hec was released in the low temperature step and the rest was released in the middle temperature step. By the highest temperature step, the released He had a mantle-like 3He/4Heratio. Using this technique on two samples from the youngest VM flow, we obtained precise estimates of cosmogenic 3He concentrations, from which we derive an eruption age of 10.9 ka ± 1.1 ka. [ABSTRACT FROM AUTHOR]

The Curiosity rover discovered fine-grained sedimentary rocks, which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy. This aqueous environment was characterized by neutral pH, low salinity, and variable redox states of both iron and sulfur species. Carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus were measured directly as key biogenic elements; by inference, phosphorus is assumed to have been available. The environment probably had a minimum duration of hundreds to tens of thousands of years. These results highlight the biological viability of fluvial-lacustrine environments in the post-Noachian history of Mars.

Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10-20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation. Key Points The north-south limits of Tibet were set by lateral variations in strength Roughly 15 million years ago, deformation of NE Tibet accelerated Since 20-15 million years ago, the orientation of shortening rotated eastward ©2013. American Geophysical Union. All Rights Reserved.

We present a new method for tracing sediment using detrital apatite (U-Th)/He (AHe) thermochronometry, and use this to quantify the spatial distribution of catchment erosion in the eastern Sierra Nevada, California. Well-developed age-elevation relationships permit detrital AHe ages to track the elevations where sediment grains were shed from bedrock. We analyzed sediment exiting nonglaciated Inyo Creek and adjacent (formerly) glaciated Lone Pine Creek. Statistical comparison of measured AHe age probability density functions (PDFs) with predicted PDFs based on catchment hypsometries suggests that Inyo Creek is eroding uniformly, consistent with field observations of weathered hillslopes tightly coupled to the fluvial system. In contrast, significant mismatch between measured and predicted PDFs from Lone Pine Creek reveals that sediment derives primarily from the lower half of the catchment. The dearth of older ages is likely due to sediment storage in cirques and moraines and/or focused erosion at intermediate elevations, both potential consequences of glacial modification. Measured PDFs can also improve cosmogenic nuclide-based erosion rates by more accurately scaling nuclide production rates. Our results demonstrate the utility of detrital AHe thermochronometry for quantifying erosion in fluvially and glacially sculpted catchments. Keywords: detrital thermochronometry, apatite (U-Th)/He, cosmogenic nuclides, erosion, Sierra Nevada.

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. Despite recent advances in theoretical and field studies that quantify glacial erosion processes, few studies have documented glacial erosion rates over long (>[10.sup.6] yr) or large (more than tens of kilometers) scales. We use apatite (U-Th)/He (AHe) and apatite fission track (AFT) cooling ages to address the late Miocene to Holocene erosion history across two 60-km-long transects of the heavily glaciated southern Coast Mountains, British Columbia. Observed AHe cooling ages from equal elevation samples range between 1.5 and 8 Ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1.5-4.0 m.y. We evaluated temporal and spatial variations in erosion rates using a three-dimensional thermal-kinematic model that predicted AHe and AFT ages at the surface for different erosion histories. Comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300% over the past 1.5-7 m.y., coincident with the onset of glaciation of this range. Keywords: glacial erosion, apatite (U-Th)/He, numerical modeling, Coast Mountains, paleotopography.

(U-Th)/He dating of late-stage authigenic goethite, combined with corrections for diffusive loss of [sup.4]He by the [sup.4]He/[sup.3]He methodology, reveals strong correlation between a sample's age and its depth in ferruginized channel sediments from the Yandicoogina deposit, Western Australia. Corrected ages, ranging from ca. 18 Ma near the surface to ca. 5 Ma at the bottom of the profile, indicate that ferruginization of the aggraded channels becomes progressively younger with depth. This trend is consistent with goethite precipitation at the groundwater-atmosphere interface during water table drawdown driven by the aridification of Western Australia during the Neogene. The results demonstrate that the (U-Th)/He system is ideal for dating goethite if diffusive loss corrections are applied. The approach is suitable for dating weathering reactions on Earth and should also be suitable for dating Fe oxyhydroxides in the Martian regolith. Keywords: (U-Th)/He dating, goethite, weathering geochronology, channel iron deposits, iron ore.

The end-Permian mass extinction (251 Ma) was the largest in Earth's history, and the great extent of biospheric perturbation is recorded as dramatic shifts in carbon isotope ratios of sedimentary materials. Both terrestrial and extraterrestrial events are commonly invoked as causative mechanisms for the crisis, and the primary reason for the event remains the subject of controversy. Geochemical indicators sensitive to the influence of extraterrestrial material involve platinum group elements and osmium and helium isotope ratios. Analyses of extinction levels in two sections from Austria and Italy reveal no evidence of an extraterrestrial impact. The end-Permian crisis, it appears, was a homegrown catastrophe. Keywords: Permian-Triassic boundary, impacts, mass extinctions, Os isotopes, He isotopes.

Uranium, thorium and helium age profiles are presented for Late Cretaceous transverse river drainages in the Sierra Nevada Mountains of California. The profiles place certain elevations at slightly older ages than previous studies. Calculations from these findings indicate a Cordilleran plateau lay at a 3,000 m elevation during the Cretaceous era.

A technique to estimate the age of topographic relief using the ages of apatites determined from uranium, thorium and helium content is described. The results from the sierra Nevada, California, suggest that the sierra developed an Andean size topography between 185 Myr and 70-80 Myr. This coincided with crustal thickening. A gradual reduction of mean crestal elevation is inferred.

Researchers have characterized extraterrestrial (ET) helium, likely carried by interplanetary dust particles (IDPs), in deep‐sea sediments spanning more than the last 100 Myr. Here we complement those low resolution and deep time studies by measuring He in modern Antarctic air and recent ice. We analyzed 180 air filter samples collected in 2017 and 2018 at the South Pole and detected 3He above blank levels in 178. The filters collected during the austral springs had elevated 3He in multiple subsamples indicating the presence of many individual IDPs and potentially, a temporal variation in the ET small particle flux. Our calculated mean 3He flux of 1.4 ± 1.2 × 10−12 cc STP cm−2 ka−1 is the first such measurement from air samples. We also melted, filtered, and analyzed one hundred and forty‐one 1 m‐long ice sections from a ˜2000‐yr‐old South Pole core. We detected 3He above blank levels in 139 of the 141 ice core samples and calculated an average flux of 1.2 ± 0.3 × 10−12 cc STP cm−2 ka−1. Our two flux values are within a factor of two of those calculated from stratospheric IDP concentrations, those previously measured for sections of the GISP2 and Vostok ice cores, and from sediment cores from different locations and ages. The similarity of these flux values over disparate time scales (1–108 yr) and geographic locations (90° S to equator) indicates modest temporal variability and remarkable agreement among diverse IDP archives. These data provide a compelling link from IDPs collected in the stratosphere to those recorded in deep time sedimentary archives. [ABSTRACT FROM AUTHOR]

A study of oxygen isotopes suggests that sediment does not survive subduction zone conditions. An oxygen isotope equilibrium exists between olivine phenocrysts and plagioclase as revealed by the studies of phenocrysts from basalts from Pitcairn Island, in the Southeast Pacific Ocean. Possible causes for the differences in the results of the study and analysis of Pitcairn seamount glasses are discussed.

We test whether X-ray micro-computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic ³He exposure dating. We extracted detrital magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of the expected spallation production. We present Li concentrations, major and trace element analyses, and estimated magnetite (U_Th) = He cooling ages of samples in order to model the contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron production of ³He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to 4 times that of the spallation component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30% of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400-800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured ³He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5±2.2 ka, we calibrate a production rate for magnetite 3He at sea level and high latitude (SLHL) of 116±13 at g-1 a-1.We suggest that this microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies. [ABSTRACT FROM AUTHOR]

Alpine glaciation and river incision control the topography of mountain ranges, but their relative contributions have been debated for years. Apatite [sup.4]He/³He thermochronometry tightly constrains the timing and rate of glacial erosion within one of the largest valleys in the southern Coast Mountains of British Columbia, Canada. Five proximate samples require accelerated denudation of the Klinaklini Valley initiating 1.8 ± 0.2 million years ago (Ma). At [east 2 kilometers of overlying rock were removed from the valley at ≥ 5 millimeters per year, indicating that glacial valley deepening proceeded ≥ 6 times as fast as erosion rates before ~1.8 Ma. This intense erosion may be related to a global transition to enhanced climate instability ~1.9 Ma., The hypotheses that alpine glaciers erode at higher rates than rivers (1, 2) and control mountain topographic relief (3, 4) can be tested if pre- and syn-glacial erosion rates can [...]

This study provides an empirical calibration of the apatite (U-Th)/He thermochronometer using the thermal structure derived from an extensive apatite fission-track study of an exhumed, normal-fault--bounded crustal block in the White Mountains in the western Basin and Range province. This fault block has been tilted ~25 [degrees] to the east during extension, exposing a continuous section of rocks previously buried to ~7 km. Apatites yield (U-Th)/He apparent ages of ca. 50-55 Ma at shallow pre-extensional crustal levels that decrease systematically to ca. 12 Ma at [is greater than] 4.5 km paleodepth. The ages exhibit a well-defined exhumed apatite He partial retention zone over a pre-extensional temperature range of ~40-80 [degrees] C and are completely reset above 80 [degrees] C, as calibrated from the apatite fission-track data. This pattern is in good agreement with He diffusion behavior predicted by laboratory experiments. The (U-Th)/He and fission-track methods yield concordant estimates for the timing of the onset of extensional faulting in the White Mountains ca. 12 Ma. Given the partially overlapping temperature-sensitivity windows, the (U-Th)/He and fission-track thermochronometers are highly complementary and may be used together to reconstruct thermal histories over the temperature window of ~40-110 [degrees] C. Keywords: (U-Th)/He dating, fission-track dating, thermochronology, extensional faulting, White Mountains.