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.
The greater McArthur Basin of northern Australia is a vast frontier exploration province for basin-hosted resources, both hydrocarbons (oil and natural gas) and metals (critical metals [e.g. rare earth elements, Co], Cu, Pb, Zn and Au). This basin system covers much of northern Australia and may have included much of North China that lay off northern Australia when the basin formed—ca. 1820–1325 Ma. Hydrocarbon and metal deposits in the basin are largely controlled by host sediment composition and ‘redox traps’ related to ancient water chemistry, which, in-turn, are modulated by biological activity, tectonism and relative sea level change. None of these controls are fully understood or constrained throughout the basin.
Abstract The McArthur Basin of the North Australian Craton is one of the very few places on Earth where extensive hydrocarbons are preserved that were generated from Mesoproterozoic source rocks, prior to the development of extensive multicellular life. It is, however, unclear precisely when hydrocarbons from these source rocks matured, and if this occurred as a singular event or multiple phases. In this study, we present new apatite fission track data from a combination of outcrop and sub‐surface samples from the McArthur Basin to investigate the post‐depositional thermal history of the basin, and to explore the timing of hydrocarbon maturation. Apatite fission track data and thermal modelling suggest that the McArthur Basin experienced a basin‐wide reheating event contemporaneous with the eruption of the Cambrian Kalkarindji Large Igneous Province in the North and West Australian cratons, during which thick (>500 m) basaltic flows blanketed the basin surface. Reheating at ca. 510 Ma coinciding with Kalkarindji volcanism is consistent with a proposed timing of elevated hydrocarbon maturation, particularly in the Beetaloo Sub‐basin, and provides a mechanism for petroleum generation throughout the basin. Subsequent regional cooling was slow and gradual, most likely facilitated by gentle erosion (ca. 0.01–0.006 km/Ma) of overlying Georgina Basin sediments in the Devonian–Carboniferous with little structural reactivation. This model provides a framework in which hydrocarbons, sourced from Mesoproterozoic carbon‐rich rocks, may have experienced thermal maturation much later in the Cambrian. Preservation of these hydrocarbons was aided by a lack of widespread structural exhumation following this event.
Sedimentary rocks provide important insight regarding the evolution of Earth’s surface environments through deep time. Such sequences are pervasive through the geological record and currently cover more than 70% of the planet’s surface. They are also a key repository for energy and mineral resources. However, absolute chronometry of sedimentary rocks can be difficult to do using conventional methods due to their low abundances of radiogenic elements. Establishing chronology is particularly challenging for Precambrian sedimentary rocks, where the lack of a diverse fossil record makes biostratigraphic correlations ambiguous. In this study, we use shale and carbonate samples from the Proterozoic greater McArthur Basin in northern Australia as a case study to demonstrate two emerging in-situ laser-based methods that have the potential to quickly and accurately resolve the depositional age of a sedimentary package. The first method aims to date the formation of authigenic clay minerals in shales using in situ laser ablation Rb–Sr geochronology. A gas in a reaction-cell equipped laser ablation–inductively coupled plasma–tandem mass spectrometer is used to remove the isobaric interference between 87Rb and 87Sr. The second method looks to date carbonate sedimentation using U–Pb geochronology via a laser isotopic mapping approach. Laser rasters are compiled into isotopic maps, and this spatial and geochemical information is used to target subdomains within the sample. Detrital or altered regions can be avoided by monitoring chemical signatures and pixels corresponding to the most authigenic domains are then subdivided that give the best spread of data on an isochron. Both approaches provide the key advantage of preserving, and through the mapping approach further resolving, sample petrographic context, which together with complementary geochemical data can be triaged to yield a more appropriate age and interpretation.
Abstract The Altai is an enigmatic, relatively young mountain belt with sharp relief (up to 4,500 m high) that developed thousands of kilometers away from the nearest current plate margins. The Fuyun area, at the interface between the southern margin of the Chinese Altai and the Junggar Basin, represents a key locality for understanding the Meso‐Cenozoic deformation and exhumation history of the Altai. The complex structural architecture of the Fuyun area suggests that multiple deformation events affected the area, which ultimately led to the exhumation of the Altai. Furthermore, the area hosts orogenic‐type mineralization, suggesting a history of fluid alteration. However, in contrast to the well‐constrained Palaeozoic history, the timing of Meso‐Cenozoic deformation, metasomatism and exhumation has not been comprehensively studied. This study presents new apatite U‐Pb, trace element and fission track data for the Fuyun area and integrates these with previous studies for the Altai to shed more light on the Meso‐Cenozoic tectonic history. The apatite U‐Pb dates, associated with LREE‐depleted trace element profiles, suggest that a phase of Middle–Late Jurassic (∼170–160 Ma) metasomatism affected the Keketuohai area, which is potentially linked to the timing of rare‐metal mineralization. The apatite fission track results and thermal history models reveal rapid early Late Cretaceous (∼100–75 Ma) cooling linked to tectonic exhumation throughout the Chinese Altai, associated with distant plate‐margin processes. In addition, samples taken in vicinity to the frontal thrusts of the Altai record evidence for Cenozoic partial resetting of the apatite fission track system.
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