Vestiges of the Ancient: Deep-Time Noble Gas Thermochronology

Elements ◽  
2020 ◽  
Vol 16 (5) ◽  
pp. 325-330 ◽  
Author(s):  
Kalin T. McDannell ◽  
Rebecca M. Flowers
Keyword(s):  
Noble Gas ◽  
Diffusion Kinetics ◽  
Deep Time ◽  
Inverse Simulation ◽  
Simulation Techniques ◽  
Rock Record ◽  
Thermal Histories ◽  
Long Timescales ◽  
Distinct Temperature ◽  
Key Questions

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.

Parameter Study of a Thermal Analysis of a Bead-on-Plate Weld

2018 ◽  
Author(s):  
Spyridon A. Alexandratos ◽  
Lei Shi ◽  
Noel P. O’Dowd
Keyword(s):  
Grain Size ◽  
Steel Plate ◽  
Martensitic Steel ◽  
Sensitivity Study ◽  
Parameter Study ◽  
Stainless Steel 316L ◽  
Material Microstructure ◽  
Simulation Techniques ◽  
Subsequent Effect ◽  
Thermal Histories

Failures in engineering components operating at high temperature often initiate in welded joints, particularly in the heat-affected zone (HAZ) adjacent to a weld. This is due to the inhomogenous microstructure of the weld and adjacent material due to the different thermal histories experienced during the weld cycle. It is therefore important to accurately predict the temperature distributions arising during welding and the subsequent effect on material microstructure. The NET TG1 bead-on-plate weld geometry is examined in this work. This geometry is a single weld bead laid on the surface of an AISI 316L austenite steel plate. Experimental data from the TG1 study are available to validate different weld simulation techniques. Here, a sensitivity study to the thermal properties is carried out and the influence on the HAZ temperatures and grain size is examined. The study shows that the conductivity and the specific heat capacity significantly affect the temperature prediction in the HAZ with a similar influence on predicted grain size following welding. Results are presented for a stainless steel (316L) and a martensitic steel (P91) plate.

scholarly journals Instability of the southern Canadian Shield during the late Proterozoic

2018 ◽  
Author(s):  
Kalin T. McDannell ◽  
Peter K. Zeitler ◽  
David A. Schneider
Keyword(s):  
Elevated Temperatures ◽  
Geothermal Gradient ◽  
Canadian Shield ◽  
Granitic Rocks ◽  
Deep Time ◽  
Magmatic Underplating ◽  
Thermal Histories ◽  
Lower Temperature ◽  
The Stability ◽  
Uplift And Erosion

Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (~150-350°C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth’s Proterozoic “Middle Age.” Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5°C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ~200°C – signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (<< 60°C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible explanation may be found in evidence of magmatic underplating that thickened the crust, driving uplift and erosion. The timing of this underplating coincides with Mid-Continent extension, Grenville orogenesis, and assembly of the supercontinent Rodinia. 40Ar/39Ar MDD data demonstrate that this technique can be successfully applied to older rocks and fill in a large observational gap. These data also raise questions about the evolution of cratons during the Proterozoic and the nature of cratonic stability across deep time.

A southern African perspective on the co-evolution of early life and environments

2021 ◽  
Vol 124 (1) ◽  
pp. 225-252
Author(s):  
K. Hickman-Lewis ◽  
F. Westall
Keyword(s):  
Southern Africa ◽  
Early Life ◽  
Current Knowledge ◽  
Limited Attention ◽  
Deep Time ◽  
Sulphur Cycle ◽  
Rock Record ◽  
Transvaal Supergroup ◽  
African Perspective ◽  
Pilbara Craton

Abstract The Kaapvaal and Zimbabwe cratons host some of the earliest evidence for life. When compared to the contemporaneous East Pilbara craton, cherts and other metasedimentary horizons in southern Africa preserve traces of life with far greater morphological and geochemical fidelity. In spite of this, most fossiliferous horizons of southern Africa have received relatively limited attention. This review summarises current knowledge regarding the nature of early life and its distribution with respect to environments and ecosystems in the Archaean (>2.5 Ga) of the region, correlating stratigraphic, sedimentological, geochemical and palaeontological understanding. There is abundant and compelling evidence for both anoxygenic photosynthetic and chemosynthetic biomes dominating Palaeoarchaean-Mesoarchaean strata dating back to around 3.5 Ga, and the prevalence of each is tied to palaeoenvironmental parameters deducible from the rock record. Well-developed, large stromatolites characteristic of younger Mesoarchaean-Neoarchaean sequences were probably constructed by oxygenic photosynthesisers. Isotopic evidence from the Belingwe greenstone belt and the Transvaal Supergroup indicates that both a full sulphur cycle and complex nitrogen cycling were in operation by the Mesoarchaean-Neoarchaean. The Archaean geological record of southern Africa is thus a rich repository of information regarding the co-evolving geosphere and biosphere in deep time.

Exploring the 36Cl/Cl input in arid environments: New insights gained by 81Kr groundwater dating in the Negev Desert, Israel

2021 ◽  
Author(s):  
Roi Ram ◽  
Roland Purtschert ◽  
Christof Vockenhuber ◽  
Reika Yokochi ◽  
Eilon M. Adar ◽  
...  
Keyword(s):  
Land Surface ◽  
Noble Gas ◽  
Climatic Conditions ◽  
Arid Environments ◽  
Near Surface ◽  
Spatial Differences ◽  
Practical Tool ◽  
Groundwater Dating ◽  
Long Timescales

<p>   <sup>36</sup>Cl and <sup>81</sup>Kr (half-lives of 301 and 229 kyr, respectively) are among a very few age tracers with dating capabilities in the 10<sup>4</sup>–10<sup>6</sup> yr timescale. Although widely applied since the 1980s in various hydrological studies, the <sup>36</sup>Cl/Cl system has been found complex as an effective dating tool. In contrast, <sup>81</sup>Kr has become a practical tool only recently and is considered to be an ideal dating tool due to the inert properties of the noble gas. In the present study, simultaneous measurements of both radioisotopes were used to assess the <sup>36</sup>Cl/Cl input ratios and the Cl<sup>-</sup> content for paleorecharge into the deep, transboundary Nubian Sandstone Aquifer (NSA) which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel).</p><p>   By means of <sup>81</sup>Kr data, reconstructed Cl<sup>-</sup> content of recharge that occurred during the late Pleistocene was found to be 300–400 mg/L with an initial <sup>36</sup>Cl/Cl ratio of 50 × 10<sup>-15</sup>. This latter value is in agreement with the <sup>36</sup>Cl/Cl ratio in recent local rainwater, indicating constancy over prolonged periods with possible variable climatic conditions. This similarity in values suggests a process that is rather insensitive to atmospheric <sup>36</sup>Cl fallout rates. Erosion and weathering of near-surface materials in the desert environment could dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. This near-surface Cl<sup>-</sup> reservoir integrates various sources and processes, including marine and terrestrial Cl<sup>-</sup>, cosmogenic <sup>36</sup>Cl fallout, and cosmogenic <sup>36</sup>Cl production in the shallow unsaturated zone, all of which are active over long timescales and accumulate on the land surface and in the epigene zone.  Spatial differences in the reconstructed initial <sup>36</sup>Cl/Cl ratio are attributed to differences in the mineral aerosol sources for specific recharge areas of the NSA. The results of this study highlight the potential of integrating <sup>81</sup>Kr age information in evaluating the initial <sup>36</sup>Cl/Cl and Cl<sup>-</sup> input, which is essential for the calibration of <sup>36</sup>Cl radioisotope as a long-term dating tool for a given basin.</p>

Noble Gases Deliver Cool Dates from Hot Rocks

Elements ◽  
2020 ◽  
Vol 16 (5) ◽  
pp. 303-309 ◽  
Author(s):  
Cécile Gautheron ◽  
Peter K. Zeitler
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Solid Earth ◽  
Noble Gases ◽  
Noble Gas ◽  
Radioactive Decay ◽  
Gas Diffusion ◽  
Clear Signature ◽  
Inverse Models ◽  
Thermal Histories

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.

scholarly journals Development of an inverse simulation method for the analysis of train performance

2017 ◽  
Vol 232 (5) ◽  
pp. 1295-1308 ◽  
Author(s):  
DJ Murray-Smith
Keyword(s):  
Initial Conditions ◽  
Simulation Models ◽  
Simulation Method ◽  
Time Instant ◽  
Tractive Force ◽  
Inverse Simulation ◽  
Simulation Techniques ◽  
Train Speed ◽  
Route Information ◽  
Input Variables

Conventional methods of computer-based simulation allow prediction of output variables, often as a function of time, for a given model of a physical system for a given set of initial conditions and input variables. In the case of train performance simulation models, the possible output variables include train speed or distance travelled, both expressed as functions of time. The corresponding input variables, also expressed as functions of time, are the tractive force or power levels for given train characteristics and route information such as gradients, track curvature and speed restrictions. Inverse simulation methods, on the other hand, allow selected model variables (such as the tractive force at any time instant) to be found from other specified model variables applied as input (such as the train speed or distance travelled versus time) for a given set of route conditions and train characteristics. The specific inverse simulation method presented in the paper is based on feedback principles. Illustrative results are used to verify this inverse simulation approach for train performance applications, and further cases are used to show that the inverse formulation provides an insight that is different from that obtained using more conventional forward simulation techniques.

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