The thermal history of the Western Irish onshore

We present here a low-temperature thermochronological study that combines the apatite fission-track and (U + Th)/He dating methods with a pseudo-vertical sampling approach to generate continuous and well-constrained temperature–time histories from the onshore Irish Atlantic margin. The apatite fission-track and (U + Th)/He ages range from the Late Jurassic to Early Cretaceous and the mean track lengths are relatively short. Thermal histories derived from inverse modelling show that following post-orogenic exhumation the sample profiles cooled to c. 75 °C. A rapid cooling event to surface temperatures occurred during the Late Jurassic to Early Cretaceous and was diachronous from north to south. It was most probably caused by c. 2.5 km of rift-shoulder related exhumation and can be temporally linked to the main stage of Mesozoic rifting in the offshore basins. A slow phase of reheating during the Late Cretaceous and Early Cenozoic is attributed to the deposition of a thick sedimentary sequence that resulted in c. 1.5 km of burial. Our data imply a final pulse of exhumation in Neogene times, probably related to compression of the margin. However, it is possible that an Early Cenozoic cooling event, compatible with our data but not seen in our inverse models, accounts for part of the Cenozoic exhumation

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry: Reply

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Tracking the interaction between injected CO₂ and reservoir fluids using noble gas isotopes in an analogue of large-scale carbon capture and storage

Industrial scale carbon capture and storage technology relies on the secure long term storage of CO2 in the subsurface. The engineering and safety of a geological storage site is critically dependent on how and where CO2 will be stored over the lifetime of the site. Hence, there is a need to determine how injected CO2 is stored and identify how injected CO2 interacts with sub-surface fluids. Since July 2008 ∼1 Mt of CO2 has been injected into the Cranfield enhanced oil recovery (EOR) field (MS, USA), sourced from a portion of the natural CO2 produced from the nearby Jackson Dome CO2 reservoir. Monitoring and tracking of the amount of recycled CO2 shows that a portion of the injected CO2 has been retained in the reservoir. Here, we show that the noble gases (20Ne, 36Ar, 84Kr, 132Xe) that are intrinsic to the injected CO2 can be combined with CO2/3He and δ13CCO2 measurements to trace both the dissolution of the CO2 into the formation water, and the interaction of CO2 with the residual oil. Samples collected 18 months after CO2 injection commenced show that the CO2 has stripped the noble gases from the formation water. The isotopic composition of He suggests that ∼0.2%, some 7 kt, of the injected CO2 has dissolved into formation water. The CO2/3He and δ13CCO2 values imply that dissolution is occurring at pH = 5.8, consistent with the previous determinations. δ13CCO2 measurements and geochemical modelling rule out significant carbonate precipitation and we determine that the undissolved CO2 after 18 months of injection (1.5 Mt) is stored by stratigraphic or residual trapping. After 45 months of CO2 injection, the noble gas concentrations appear to be affected by CO2-oil interaction, overprinting the signature of the formation water

The use of in-situ cosmogenic 21Ne in studies on long-term landscape development

Cosmogenic Ne isotopes are stable and are routinely used for constraining the timing of events and the rate of surface change beyond the limit that can be studied with radionuclides 10Be, 26Al, and 36Cl. Cosmogenic Ne analysis can be used in quartz and in a range of other minerals. Analysis typically requires significantly less material than do cosmogenic 10Be and 26Al, opening up the technique for small samples—individual pebbles in river sediments, for example. Analysis is easier and faster than for radionuclides, not least because Ne measurements do not require significant chemical procedures. However, the presence of other sources of Ne in minerals tends to restrict the use of cosmogenic 21Ne to old landscapes and long exposure durations. In this review we briefly outline the background of cosmogenic Ne production in rocks and minerals at the Earth’s surface, then document the key uses of the technique by highlighting some earlier studies, and finish with a short perspective on the future of the technique

Measuring plume-related exhumation of the British Isles in Early Cenozoic times

Mantle plumes have been proposed to exert a first-order control on the morphology of Earth's surface. However, there is little consensus on the lifespan of the convectively supported topography. Here, we focus on the Cenozoic uplift and exhumation history of the British Isles. While uplift in the absence of major regional tectonic activity has long been documented, the causative mechanism is highly controversial, and direct exhumation estimates are hindered by the near-complete absence of onshore post-Cretaceous sediments (outside Northern Ireland) and the truncated stratigraphic record of many offshore basins. Two main hypotheses have been developed by previous studies: epeirogenic exhumation driven by the proto-Iceland plume, or multiple phases of Cenozoic compression driven by far-field stresses. Here, we present a new thermochronological dataset comprising 43 apatite fission track (AFT) and 102 (U–Th–Sm)/He (AHe) dates from the onshore British Isles. Inverse modelling of vertical sample profiles allows us to define well-constrained regional cooling histories. Crucially, during the Paleocene, the thermal history models show that a rapid exhumation pulse (1–2.5 km) occurred, focused on the Irish Sea. Exhumation is greatest in the north of the Irish Sea region, and decreases in intensity to the south and west. The spatial pattern of Paleocene exhumation is in agreement with the extent of magmatic underplating inferred from geophysical studies, and the timing of uplift and exhumation is synchronous with emplacement of the plume-related British and Irish Paleogene Igneous Province (BIPIP). Prior to the Paleocene exhumation pulse, the Mesozoic onshore exhumation pulse is mainly linked to the uplift and erosion of the hinterland during the complex and long-lived rifting history of the neighbouring offshore basins. The extent of Neogene exhumation is difficult to constrain due to the poor sensitivity of the AHe and AFT systems at low temperatures. We conclude that the Cenozoic topographic evolution of the British Isles is the result of plume-driven uplift and exhumation, with inversion under compressive stress playing a secondary role

Earth-atmosphere evolution based on the new determination of Devonian atmosphere Ar isotopic composition

The isotopic composition of the noble gases, in particular Ar, in samples of ancient atmosphere trapped in rocks and minerals provides the strongest constraints on the timing and rate of Earth atmosphere formation by degassing of the Earth's interior. We have re-measured the isotopic composition of argon in the Rhynie chert from northeast Scotland using a high precision mass spectrometer in an effort to provide constraints on the composition of Devonian atmosphere. Irradiated chert samples yield 40Ar/36Ar ratios that are often below the modern atmosphere value. The data define a 40Ar/36Ar value of 289.5±0.4 at K/36Ar = 0. Similarly low 40Ar/36Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40Ar/36Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40Ar–39Ar–36Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% (5.1±0.4×1016 mol) of the total 40Ar inventory since the Devonian. The average accumulation rate of 1.27±0.09×108 mol40Ar/yr overlaps the rate over the last 800 kyr. This implies that there has been no resolvable temporal change in the outgassing rate of the Earth since the mid-Palaeozoic despite the likely episodicity of Ar degassing from the continental crust. Incorporating the new Devonian atmosphere 40Ar/36Ar into the Earth degassing model of Pujol et al. (2013) provides the most precise constraints on atmosphere formation so far. The atmosphere formed in the first ∼100 Ma after initial accretion during a catastrophic degassing episode. A significant volume of 40Ar did not start to accumulate in the atmosphere until after 4 Ga which implies that stable K-rich continental crust did not develop until this time

Noble gases constrain the origin, age and fate of CO2 in the Vaca Muerta Shale in the Neuquén Basin (Argentina)

Unconventional hydrocarbon resources such as shale oil/gas and coal-bed methane have become an increasingly important source of energy over the past decade. The Vaca Muerta Shale (Neuquén Basin, Argentina) contains the second largest technically recoverable quantity of shale gas in the world. Exploitation of the play has been complicated by elevated concentrations of CO2 in several fields, the origin of which is currently poorly understood. Elevated CO2 levels are consistently encountered when deep-rooted faults in the Auquilco Evaporite Formation, present below the Vaca Muerta Shale, overlap with shallower faults that propagate from the top of evaporites into the shale, indicating a sub-evaporate origin of the CO2. Here we report new isotopic analysis of CO2-rich gases from two producing fields. CO2 concentrations increase with C1/(C2 + C3) values (4.8–33.5) and fractionation of δ13CCO2 (−0.9 to −7.7‰), suggest that CH4 have been displaced by CO2 which entered the shale after hydrocarbon maturation. The noble gas composition (3He/4He of 3.43–3.95 RA, where RA is the atmospheric ratio of 1.399 × 10−6, 21Ne/22Ne of 0.0310–0.0455, 20Ne/22Ne of 9.89–10.52, 40Ar/36Ar of 2432–3674 and CO2/3He 6.8–20.2 × 107) of the gases is consistent with mixing of magmatic CO2 with crustal hydrocarbon-rich gases and provides evidence for the loss of significant CO2. Using inverse modelling techniques, we determine that the magmatic gas has a 3He/4He of 3.95–4.08 RA, CO2/3He of 8.8–16 × 108 and 20Ne/22Ne of 12.13−0.10+0.08, 21Ne/22Ne of 0.074−0.003+0.004. Based on the radiogenic He and Ne this is consistent with a depleted asthenosphere mantle source, which has been trapped in the crust since 6.0–22.8 Ma. This is significantly younger than Late Cretaceous maturation of the hydrocarbon source rocks. Mantle melting as a result of asthenosphere upwelling induced by the collision of the South Chile Ridge and the Chile Trench at ~14 Ma is the most likely source of the CO2. Gases from below the shale contain two air saturated water-derived noble gas components, distinguished on the basis of 20Ne†/36Ar, 84Kr/36Ar, 132Xe/36Ar ratios. These are consistent with early and late stage open system Rayleigh fractionation of groundwater-derived noble gases. We find evidence that these mix with the magmatic component prior to entering the Vaca Muerta and mixing with an adsorption derived gas retained in the source kerogen

The geology and geochronology of Al Wahbah maar crater, Harrat Kishb, Saudi Arabia

Al Wahbah is a large (∼2.2 km diameter, ∼250 m deep) maar crater in the Harrat Kishb volcanic field in western Saudi Arabia. It cuts Proterozoic basement rocks and two Quaternary basanite lava flows, and is rimmed with an eroded tuff ring of debris from the phreatomagmatic explosion that generated the crater. A scoria cone on the northern wall of the crater was dissected by the explosion and exposes a dolerite plug that was intruded immediately prior to crater formation. The dolerite plug yields a <sup>40</sup>Ar/<sup>39</sup>Ar age of 1.147 ± 0.004 Ma. This is the best possible estimate of the time Al Wahbah crater formed. It is a few tens of thousand years younger than the age of the lower and upper basalt flows, 1.261 ± 0.021 Ma and 1.178 ± 0.007 Ma respectively. A dolerite dyke exposed within the basement in the wall of the crater is dated at 1.886 ± 0.008 Ma. This is the most precise age so far determined for the initiation of basaltic volcanism of Harrat Kishb, and confirms that it is significantly younger than the other post-rift volcanic provinces in the region. This study provides constrains the timing of humid climatic conditions in the region and suggests that the Quaternary basaltic volcanism that stretches the length of the western side of the Arabian peninsula may prove to be useful for establishing palaeoclimatic conditions

Neogene fluvial landscape evolution in the hyperarid core of the Atacama Desert

Dating of extensive alluvial fan surfaces and fluvial features in the hyperarid core of the Atacama Desert, Chile, using cosmogenic nuclides provides unrivalled insights about the onset and variability of aridity. The predominantly hyperarid conditions help to preserve the traces of episodic climatic and/or slow tectonic change. Utilizing single clast exposure dating with cosmogenic 10Be and 21Ne, we determine the termination of episodes of enhanced fluvial erosion and deposition occurring at ~19, ~14, ~9.5 Ma; large scale fluvial modification of the landscape had ceased by ~2–3 Ma. The presence of clasts that record pre-Miocene exposure ages (~28 Ma and ~34 Ma) require stagnant landscape development during the Oligocene. Our data implies an early onset of (hyper-) aridity in the core region of the Atacama Desert, interrupted by wetter but probably still arid periods. The apparent conflict with interpretation that favour a later onset of (hyper-) aridity can be reconciled when the climatic gradients within the Atacama Desert are considered

From source to sink in central Gondwana: Exhumation of the Precambrian basement rocks of Tanzania and sediment accumulation in the adjacent Congo basin

Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000 m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200 m below surface. Surface samples were also analyzed. AFT dates range between 317 ± 33 Ma and 188 ± 44 Ma. Alpha (Ft)-corrected AHe dates are between 433 ± 24 Ma and 154 ± 20 Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340–220 Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70 Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9 ± 2 km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes