Abstract Lu Hf geochronology is a powerful method to constrain the temporal evolution of geological systems. Traditional application of this dating method requires time-consuming chemical separation of the parent (176Lu) and daughter (176Hf) isotopes that is commonly accompanied by loss of textural context of the analysed minerals. In contrast, In-situ (laser-ablation based) Lu Hf geochronology offers a number of advantages including rapid analysis with high spatial resolution, as well as control on textural relationships of the analysed mineral. However, laser-ablation based Lu Hf geochronology has been hindered by isobaric interferences of 176Yb and 176Lu on 176Hf that have effectively masked reliable determination of 176Lu and 176Hf. We present a methodology that resolves these interferences using LA-ICP-MS/MS (laser ablation tandem inductively coupled mass spectrometry) and NH3 gas to separate Hf from Lu. Both Lu, Yb, and Hf react with NH3 to form a variety of product ions. By measuring high order reaction products (e.g. Hf(NH)(NH2)(NH3)3+), we demonstrate that 176Hf can be measured interference-free from 176Lu and 176Yb with sufficient sensitivity to yield useful geochronological age data. The novel in-situ Lu Hf technique has been successfully applied to a variety of Palaeozoic and Precambrian-aged garnet, apatite and xenotime samples, including published reference materials. The resulting age uncertainties are as low as ~0.5% (95% conf. interval). The technique has the potential to obtain spatially-resolved Lu Hf ages in garnet-bearing samples that would be difficult to obtain by conventional techniques. The method also offers the opportunity for rapid “campaign style” geochronology in complex terrains that record poly-metamorphic histories. In apatite, the expected higher closure temperature of the Lu Hf system compared to the commonly used U Pb system allows high-temperature thermal history reconstructions. In addition, Lu Hf dating of apatite allows dating of samples with low U and high common Pb (e.g. mafic and low-grade metamorphic rocks and ore deposits). Furthermore, apatite tends to incorporate little to no common Hf, allowing single grain ages to be calculated, which opens new doors for detrital provenance studies. In situ Lu Hf dating of xenotime offers an additional avenue to U Pb dating, and may be particularly beneficial to dating of rare earth element ore deposits that often have complex temporal records of development.
<p>Central Asia is one of the most tectonically active and orographically diverse regions in the world and is the location of the highest topography on Earth resulting from major plate tectonic collisional events. Yet the role of tectonics versus climate on erosion remains one of the greatest debates of our time. We present the first regional scale analysis of 2526 published low-temperature thermochronometric dates from Central Asia spanning the Altai-Sayan, Tian Shan, Tibet, Pamir, and Himalaya. We compare these dates to tectonic processes (proximity to tectonic boundaries, crustal thickness, seismicity) and state-of-the-art paleoclimate simulations in order to constrain the relative influences of climate and tectonics on the topographic architecture and erosion of Central Asia. Predominance of pre-Cenozoic ages in much of the interior of central Asia suggests that significant topography was created prior to the India-Eurasia collision and implies limited subsequent erosion. Increasingly young cooling ages are associated with increasing proximity to active tectonic boundaries, suggesting a first-order control of tectonics on erosion. However, areas that have been sheltered from significant precipitation for extensive periods of time retain old cooling ages. This suggests that ultimately climate is the great equalizer of erosion. Climate plays a key role by enhancing erosion in areas with developed topography and high precipitation such as the Tian Shan and Altai-Sayan during the Mesozoic and the Himalaya during the Cenozoic. Older thermochronometric dates are associated with sustained aridity following more humid periods.</p>
Abstract The Tianshan and Altai mountain belts dominate the present‐day topography of central Asia and are major constituents of the Central Asian Orogenic Belt. These mountain belts have been extensively studied using low‐temperature thermochronology to understand their uplift and exhumation history. In comparison, few studies have focused on the spatial extent and timing of intracontinental deformation in the intervening ranges, such as the West Junggar Mountains. Apatite fission track (AFT) and apatite U‐Pb dating of igneous samples from the West Junggar Mountains reveals AFT central ages ranging from latest Carboniferous to Middle Jurassic and mean confined track lengths between 14.1 and 12.0 µm, which are negatively correlating to AFT central ages. Apatite U‐Pb dating produced mid‐Carboniferous‐early Permian ages within uncertainty of zircon U‐Pb crystallization ages, indicating cooling to below ∼ 450–550 °C immediately following emplacement. Thermal history modeling of our AFT data combined with published (U‐Th)/He data predicts rapid cooling to <60 °C during the early‐middle Permian (285–260 Ma), followed by slower cooling rates during the Mesozoic‐Cenozoic. The rapid cooling event coincides with (1) the timing of major basinward thrusting in the Junggar Basin adjacent to the study area and (2) exhumation in the Chinese Altai to the north. In contrast to previous studies, our results imply that the West Junggar Mountains do not record significant exhumation during the Mesozoic‐Cenozoic. This suggests that Mesozoic deformation of the Central Asian Orogenic Belt was less pervasive and widespread than previously assumed.
Zircons from the oldest dated felsic crust, the Acasta Gneiss Complex, Canada, provide key information that may help understand the generation of crust on our nascent planet. When screened to eliminate grains with secondary alteration by measuring relative hydration (Δ16O1H/16O), primary ≥ 3.99 Ga zircon cores show δ18O of 5.88 ± 0.15 ‰, at the extreme upper (heavy) range for mantle values. Another early (≥3.96 Ga) zircon component indicates distinctly different, primary light δ18O values (δ18O ≤ 4.5 ‰). This bimodality in ancient zircon oxygen isotopes implies partial melting of both deep (lower crustal) and shallower (near surface) source rocks, responsible for felsic crust production on the early Earth. A similar bimodality in zircon δ18O is recognised in data from other ancient cratons, albeit at different times. Although alternative (uniformitarian) interpretations may also satisfy the data, the tempo of this bimodality matches models of planetary high-energy impact flux, consistent with a fundamental role for bolide impacts in the formation of crustal nuclei on the early Earth.
The Junggar Alatau forms the northern extent of the Tian Shan within the Central Asian Orogenic Belt (CAOB) at the border of SE Kazakhstan and NW China. This study presents the Palaeozoic–Mesozoic post-collisional thermo-tectonic history of this frontier locality using an integrated approach based on three apatite geo-/thermochronometers: apatite U–Pb, fission track and (U–Th)/He. The apatite U–Pb dates record Carboniferous–Permian post-magmatic cooling ages for the sampled granitoids, reflecting the progressive closure of the Palaeo-Asian Ocean. The apatite fission track (AFT) data record (partial) preservation of the late Palaeozoic cooling ages, supplemented by limited evidence for Late Triassic (∼230–210 Ma) cooling and a more prominent record of (late) Early Cretaceous (∼150–110 Ma) cooling. The apatite (U–Th)/He age results are consistent with the (late) Early Cretaceous AFT data, revealing a period of fast cooling at that time in resulting thermal history models. This Cretaceous rapid cooling signal is only observed for samples taken along the major NW–SE orientated shear zone that dissects the study area (the Central Kazakhstan Fault Zone), while Permian and Triassic cooling signals are preserved in low-relief areas, distal to this structure. This distinct geographical trend with respect to the shear zone, suggests that fault reactivation triggered the Cretaceous rapid cooling, which can be linked to a phase of slab-rollback and associated extension in the distant Tethys Ocean. Similar conclusions were drawn for thermochronology studies along other major NW–SE orientated shear zones in the Central Asian Orogenic Belt, suggesting a regional phase of Cretaceous exhumation in response to fault reactivation at that time.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Other. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v2]Meso-Cenozoic deformation history of Thailand; insights from calcite U-Pb geochronologyAuthorsAlexander DavidSimpsoniDStijnGlorieChris K.MorleyiDNickRobertsJackGillespieiDJack KleeiDSee all authors Alexander David SimpsoniDCorresponding Author• Submitting AuthorUniversity of AdelaideiDhttps://orcid.org/0000-0001-6029-0116view email addressThe email was not providedcopy email addressStijn GlorieUniversity of Adelaideview email addressThe email was not providedcopy email addressChris K. MorleyiDPTT Exploration and ProductioniDhttps://orcid.org/0000-0002-6075-9022view email addressThe email was not providedcopy email addressNick RobertsNERC Isotope Geosciences Laboratory British Geological Surveyview email addressThe email was not providedcopy email addressJack GillespieiDUniversity of AdelaideiDhttps://orcid.org/0000-0002-3061-6223view email addressThe email was not providedcopy email addressJack K leeiDDepartment of Earth Sciences, University of DurhamiDhttps://orcid.org/0000-0001-8435-3484view email addressThe email was not providedcopy email address
The Idiwhaa gneiss, part of the Acasta Gneiss Complex, Canada, is a key source of information concerning formation of continental crust on the early Earth.However, zircon crystals from this oldest dated felsic crust were affected by multiple stages of alteration and metamorphism, leading to difficulties in disentangling primary from secondary processes.These grains provide an opportunity to understand the alteration processes that affect ancient zircon crystals.Ion imaging reveals pervasive recrystallisation fronts extending inwards from the margins of grains.Ahead of these recrystallisation fronts, grain cores contain isolated pockets of amorphous, but concordant, 3.99 Ga zircon that evidently escaped post-magmatic modification of U and Pb.The transport of these elements, involving the decoupling of parent and daughter isotopes, is highly heterogeneous over space and time within metamict zircon, yet localised domains still retain primary age information.Our data indicate that metamictisation of zircon alone does not lead to radiogenic Pb loss, which requires interaction with fluid.
The primary objective of International Ocean Discovery Program Expedition 381 was to retrieve a record of early continental rifting and basin evolution from the Corinth rift, central Greece.Continental rifting is fundamental for the formation of ocean basins, and active rift zones are dynamic regions of high geohazard
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