Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

A transformative advance in Earth science is the development of low‐temperature thermochronometry to date Earth surface processes or quantify the thermal evolution of rocks through time. Grand challenges and new directions in low‐temperature thermochronometry involve pushing the boundaries of these techniques to decipher thermal histories operative over seconds to hundreds of millions of years, in recent or deep geologic time and from the perspective of atoms to mountain belts. Here we highlight innovation in bedrock and detrital fission track, (U–Th)/He, and trapped charge thermochronometry, as well as thermal history modeling that enable fresh perspectives on Earth science problems. These developments connect low‐temperature thermochronometry tools with new users across Earth science disciplines to enable transdisciplinary research. Method advances include radiation damage and crystal chemistry influences on fission track and (U–Th)/He systematics, atomistic calculations of He diffusion, measurement protocols and numerical modeling routines in trapped charge systematics, development of 4He/3He and new (U–Th)/He thermochronometers, and multimethod approaches. New applications leverage method developments and include quantifying landscape evolution at variable temporal scales, changes to Earth\u27s surface in deep geologic time and connections to mantle processes, the spectrum of fault processes from paleoearthquakes to slow slip and fluid flow, and paleoclimate and past critical zone evolution. These research avenues have societal implications for modern climate change, groundwater flow paths, mineral resource and petroleum systems science, and earthquake hazards

Hematite (U-Th)/He Thermochronometry Detects Asperity Flash Heating During Laboratory Earthquakes

Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an ∼5–30-µm-thick layer of hematite gouge with71% ± 1% (1σ) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities \u3e200–300 µm in diameter, producing temperatures \u3e900 °C for ∼1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the role of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earth’s crust

Nanoscale Evidence for Temperature-Induced Transient Rheology and Postseismic Fault Healing

Friction-generated heat and the subsequent thermal evolution control fault material properties and thus strength during the earthquake cycle. We document evidence for transient, nanoscale fault rheology on a high-gloss, light-reflective hematite fault mirror (FM). The FM cuts specularite with minor quartz from the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning and transmission electron microscopy data reveal that the FM volume comprises a2+ suboxides. Sub–5-nm-thick silica films encase hematite grains and connect to amorphous interstitial silica. Observations imply that coseismic shear heating (temperature \u3e1000 °C) generated transiently amorphous, intermixed but immiscible, and rheologically weak Fe-oxide and silica. Hematite regrowth in a fault-perpendicular thermal gradient, sintering, twinning, and a topographic network of nanometer-scale ridges from crystals interlocking across the FM surface collectively restrengthened fault material. Results reveal how temperature-induced weakening preconditions fault healing. Nanoscale transformations may promote subsequent strain delocalization and development of off-fault damage

Shallow Rupture Propagation of Pleistocene Earthquakes Along the Hurricane Fault, UT, Revealed by Hematite (U-Th)/He Thermochronometry and Textures

The material properties and distribution of faults above the seismogenic zone promote or inhibit earthquake rupture propagation. We document the depths and mechanics of fault slip along the seismically active Hurricane fault, UT, with scanning and transmission electron microscopy and hematite (U-Th)/He thermochronometry. Hematite occurs as mm-scale, striated patches on a \u3e10 m2 thin, mirror-like silica fault surface. Hematite textures include bulbous aggregates and cataclasite, overlain by crystalline Fe-oxide nanorods and an amorphous silica layer at the slip interface. Textures reflect mechanical, fluid, and heat-assisted amorphization of hematite and silica-rich host rock that weaken the fault and promote rupture propagation. Hematite (U-Th)/He dates document episodes of mineralization and fault slip between 0.65 and 0.36 Ma at ∼300 m depth. Data illustrate that some earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust and provide structural and material property constraints for in models of fault slip

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Hematite Fault Rock Thermochronometry and Textures Inform Fault Zone Processes

Minerals on fault surfaces record fluid-rock interaction, strain, and/or heat during fault slip. Fault-hosted hematite is amenable to (U–Th)/He (He) thermochronometry. Hematite He dates reflect fault-related processes or exhumation, depending on hematite growth above or below the grain-size controlled closure temperature and processes that promote open system behavior. Deconvolving these phenomena requires multi-scale hematite textural characterization and constraints on the ambient thermal history from conventional thermochronometry. Two examples of this approach include investigations of hematite-coated fault surfaces from exhumed damage zones in the seismically active Wasatch fault zone (WFZ), UT, and Painted Canyon fault (PCF) paralleling the southern San Andreas fault, Mecca Hills, CA. WFZ fault mirrors preserve sub-μm-scale polygonal grain textures and hematite He data patterns that, together with host-rock apatite He data, are consistent with localized friction-generated heat, hematite He loss, and hematite recrystallization during damage zone seismicity \u3c4.5 Ma. In contrast, PCF hematite slip surfaces comprise foliated, nm-thick, high-aspect ratio plates, analogous to scaly clay fabrics, and hematite He dates record syn-kinematic mineralization episodes ~0.7–0.4 Ma via creep processes. These case studies highlight the power of integrated hematite textural observations and targeted fault rock thermochronometry to inform the timing and conditions of seismic to aseismic slip processes

Microtextural constraints on the interplay between fluidrockreactions and deformation

Schists from two mylonitic localities in the footwall of a low-angle normal fault in the eastern Alps record different degrees of embrittlement during exhumation, depending on the extent to which fluid–rock reactions proceeded. At one site, graphitic schists preserve textural evidence for two metamorphic reactions that modified XCO2XCO2 and/or fluid volume: (1) reaction between graphite and aqueous fluid that increased XCO2XCO2 without changing the molar amount of fluid, and (2) replacement of titanite by rutile, calcite, and quartz. The latter reaction involved net consumption of increasingly CO2-rich fluid. Areas where the first reaction proceeded are associated with abundant Mode I microcracks. Fluid inclusion arrays within the microcracks show that XCO2XCO2 increased from ∼0.1 to 0.6 during decompression from 4.75 to 2 kbar at a reference temperature of 500°C. Titanite consumption is most pronounced within transgranular Mode I microcracks, but microcracks do not crosscut products of this reaction; fluid consumption during reaction was coeval with the end of microcracking, at least on a local scale. At the other site, graphitic schists lack small-scale Mode I cracks as well as evidence for graphite consumption during decompression. SEM imaging shows that graphite is anhedral and pitted at the first site, but occurs in clusters of euhedral grains at the second site. Mass balance calculations demonstrate that rocks with partially consumed graphite were infiltrated by an externally derived, H2O-rich fluid that drove subsequent graphite-fluid reaction. Evidence for similar fluid infiltration is absent at the second site. Variations in the degree of reaction progress indicate that fluid pathways and deformation style were heterogeneous on the scale of millimeters to kilometers during exhumation from mid-crustal depths

Is apatite U-Th zonation information necessary for accurate interpretation of apatite (U-Th)/He thermochronometry data?

New U, Th and Ce concentration maps were acquired by LA-ICPMS for 70 apatites from 18 cratonic basement samples from the Canadian shield to characterize the nature and variability in apatite U–Th zonation. All apatites are zoned in effective U concentration (eU), with 80% exhibiting modest zoning that varies from a factor of ∼1.2 to ∼2.4. Zonation patterns include those with a general eU decrease from core to rim (∼25%), a core to rim eU increase (∼35%), and patchy or irregularly shaped eU distributions (∼40%). Most samples consist of individual apatites with markedly different and opposing eU profiles. Cathodoluminescence (CL) images were obtained for 258 apatites in 25 samples. Comparison of eU and CL patterns reveals no consistent relationships, suggesting that CL is not a reliable proxy for apatite eU zonation. We explore the implications of eU zonation for the interpretation of apatite (U–Th)/He (AHe) data by comparing the age predictions for representative and endmember apatite eU profiles with those for unzoned apatites. We focus on thermal histories that should magnify eU zonation effects, including slow monotonic cooling, extended He partial retention zone (HePRZ) residence, and protracted reheating and cooling. Application of incorrect α-ejection correction (FT) factors, different He concentration gradients, and variable intracrystalline retentivity due to heterogeneous radiation damage may cause the AHe dates for zoned and unzoned apatites to differ. In our dataset, the magnitude of the FT effect is \u3c1.5% for most samples. The He diffusion gradient and heterogeneous radiation damage effects cause apatites with eU enriched cores to yield AHe dates equal to or older than unzoned grains, with the opposite true for apatites with eU enriched rims. For monotonic cooling rates as slow as 0.1 °C/Myr, apatites of typical eU zonation exhibit a \u3c1 °C difference in effective closure temperature from equivalent unzoned grains. The difference in HePRZ temperature at 50% of an 150 Myr isothermal holding time is \u3c1.4 °C for typically zoned apatites. For most reheating simulations considered here, age deviations between zoned and unzoned apatites are not significant, and reach maximum differences of ∼10% for peak temperatures that induce partial He loss from the apatites. The age dispersion caused by zonation is comparable to that induced by grain size variations, but is considerably less than what can be caused by differences in mean eU and associated radiation damage for certain histories. By simulating zoned apatite suites from representative and endmember samples we find that eU zoning adds modest age dispersion to most samples. The commonly opposing eU profiles for individual apatites in most samples have different effects on the predicted age, and partially cancel each other out to reduce the predicted age difference at the sample level. Higher predicted age differences between zoned and unzoned samples generally correlate with greater sample scatter. The age deviations generally fall within the ∼15% uncertainty range that we commonly apply for sample AHe dates. Except in unusual circumstances, it appears unlikely that the unzoned apatite assumption will lead to misinterpretation of AHe datasets, thus precluding the need to routinely acquire apatite eU zonation information as part of AHe dating studies

Quartz shielding of sub-10 um zirconsfrom radiation damage-enhanced Pb loss: an example from a metamorphosed maficdike, northwestern Wyoming craton

The coupling of two new approaches enabled acquisition of in situ U–Pb data from zircons as small as 5 μm in a metamorphosed mafic dike from the Northern Madison Range, Montana. Despite negligible zircon yield by mineral separation, automated mineralogy rapidly identified \u3e375 sub-20 μm zircons in a single thin section. Subsequently, zircon crystals were dated using modifications to the conventional SIMS technique to preferentially collect secondary ions emitted from a domain a few microns in size within the ∼20 μm diameter analysis pit. This approach allowed analysis of zircons too small to be dated by standard SIMS and TIMS methods. U–Pb data define a discordia array with upper and lower intercepts of 1753±9 Ma and 63±8 Ma, respectively (1σ error, MSWD=1.5). We interpret the upper intercept to reflect zircon growth during high-temperature and high-pressure metamorphism (800 °C, 1.2 GPa) based on textural relationships between the dated zircons and the peak metamorphic assemblage. The lower intercept is attributable to the thermal pulse associated with emplacement of the nearby ca. 75 Ma Tobacco Root batholith. Percent discordance is linked with both textural setting and U concentration. Zircons located along grain boundaries or within fractured host grains display a positive correlation between U (1049–2817 ppm) and percent discordance (12–82%) that is consistent with radiation damage-enhanced Pb loss. In contrast, three zircons housed completely within unfractured quartz yield the most concordant analyses of the dataset (≤7% discordant), despite U concentrations comparable to highly discordant matrix grains. This relationship suggests that the included zircons were shielded from Pb loss by the encapsulating quartz crystals. The results imply that targeting zircons located completely within unfractured host phases may aid in isolating earlier portions of geologic histories. The primary geological implication of this dataset is to increase the documented extent of Paleoproterozoic high-grade metamorphism in the northwestern Wyoming craton

Synchroneity of cratonic burial phasesand gaps in the kimberlite record: episodic magmatism or preservational bias?

A variety of models are used to explain an apparent episodicity in kimberlite emplacement. Implicit in these models is the assumption that the preserved kimberlite record is largely complete. However, some cratons now mostly devoid of Phanerozoic cover underwent substantial Phanerozoic burial and erosion episodes that should be considered when evaluating models for global kimberlite distributions. Here we show a broad temporal coincidence between regional burial phases inferred from thermochronology and gaps in the kimberlite record in the Slave craton, Superior craton, and cratonic western Australia. A similar pattern exists in the Kaapvaal craton, although its magmatic, deposition, and erosion history differs in key ways from the other localities. One explanation for these observations is that there is a common cause of cratonic subsidence and suppression of kimberlite magmatism. Another possibility is that some apparent gaps in kimberlite magmatism are preservational artifacts. Even if kimberlites occurred during cratonic burial phases, the largest uppermost portions of the pipes would have been subsequently eroded along with the sedimentary rocks into which they were emplaced. In this model, kimberlite magmatism was more continuous than the preserved record suggests, implying that evidence for episodicity in kimberlite genesis should be carefully evaluated in light of potential preservational bias effects. Either way, the correlation between burial and kimberlite gaps suggests that cratonic surface histories are important for understanding global kimberlite patterns