Radiogenic Decay Research Articles

Numerical Analysis of Low-Enthalpy Deep Geothermal Energy Extraction Using a Novel Gravity Heat Pipe Design

Geothermal energy, derived from the Earth’s internal heat, can be harnessed due to the geothermal gradient between the Earth’s interior and its surface. This heat, sustained by radiogenic decay, varies across regions, and is highest near volcanic areas. In 2020, 108 countries utilised geothermal energy, with an installed capacity of 15,950 MWe for electricity and 107,727 MWt for direct use in 2019. Low-enthalpy sources require binary systems for power production. Open-loop systems face issues like scaling, difficult water treatment, and potential seismicity, while closed-loop systems, using abandoned petroleum or gas wells, reduce costs and environmental impacts greatly. The novel geothermal gravity heat pipe (GGHP) design eliminates parasitic power consumption by using hydrostatic pressure for fluid circulation. Implemented in an abandoned well in north-east (NE) Slovenia, the GGHP uses a numerical finite difference method to model heat flow. The system vaporises the working fluid in the borehole, condenses it at the surface, and uses gravitational flow for circulation, maintaining efficient heat extraction. The model predicts that continuous maximum capacity extraction depletes usable heat rapidly. Future work will explore sustainable heat extraction and potential discontinuous operation for improved efficiency.

Attack and Decay

This essay theorizes about nuclear militarism in the Pacific—specifically, US nuclear militarism in the Marshall Islands—by tracing the critical musicological term resonance, or the deep, full, reverberant vibrational connections that disrupt modern constructs of spatiotemporal presence. Resonance refers to the amplification of a (sound) system through the matching of a frequency with an external system, shifting perceptual magnitude through connection of two separate systems that share some vibrational quality. I use the term resonance in these literal and metaphorical ways to examine how, in the aftermath of the United States’ many attacks on the Marshall Islands—nuclear, experimental, sociocultural—that created radiogenic decay as well as broader systemic decay for the Indigenous communities, the United States’ culture of silence and secrecy around nuclear testing disabled the resonance of Marshallese appeals for nuclear justice, thus promulgating such decay. Focusing on what I call “radiation songs” and nuclear commemoration, I explore how vocal resonance moves across generations as Marshallese navigate militarized currents and remember other forms of navigation or wayfinding, such as oceanic outrigger canoe–based navigation that, like singing, brings more-than-human communities together.

Helium - Relationships to other reservoir gases and some implications for exploration: The New Mexico Example

Helium (He) is the second most abundant element in the universe after hydrogen but is relatively rare on earth. He occurs as two stable isotopes, 3He and 4He. 4He is the dominant isotope in crustal gases and is a radiogenic decay product of uranium and thorium mainly in granitic basement rocks. 3He is dominantly primordial and primarily originates from the earth’s mantle. 3He may also be formed by radiogenic decay of 6Li (Lithium) which may be found in argillaceous sediments deposited in evaporitic settings. Although He occurs in most natural gases, it almost always occurs in extremely low, subeconomic concentrations, less than 0.1%. It is rare in concentrations more than 1%. A very few small reservoirs have gases with more than 7% He. Other gases that constitute the dominant components of helium-bearing natural gases are nitrogen (N2), carbon dioxide (CO2), and methane (CH4). The highest He concentrations occur where the dominant gas is N2 but most He has historically been produced as a byproduct of gases that are hydrocarbons. Hydrocarbons are generated from petroleum source rocks. Their presence in a reservoir is dependent upon the presence of a mature source rock in the basin and a migration path between the source rock and the reservoir. Large accumulations of CO2 in the southwestern U.S. resulted from the degassing of rising Tertiary magmas and subsequent migration of the gases into crustal reservoirs. N2 appears to originate mostly from degassing of the mantle but may also be formed in some strata by the thermal maturation of kerogens or by diagenetic alteration of clays or organic compounds in red bed sequences. The presence of economic concentrations of He in reservoir gases is dependent not only on an adequate source of 4He generated from granitic basement rocks but also on accommodating flux rates of N2, CO2, and CH4. These gases differ in their origins, places of generation and rates of generation, migration and emplacement. While basement-derived 4He and N2 enter reservoirs at slow rates over long periods of geologic time, hydrocarbons and CO2 enter the reservoir over much shorter time periods and dilute the 4He and N2. Basement-derived gases may be characterized by differing N2:He ratios which may indicate greater rates of He production within the crust in some areas. Exploratory drilling for He on Chupadera Mesa in the late 1990’s and early 2000’s encountered He-rich gases in Lower Permian strata. Isotopic analyses suggest that 93% of Chupadera Mesa He originated from radiogenic decay in crustal rocks while 7% is derived from the mantle or with a possible contribution by evaporitic Permian shales. Marked differences in the CO2 concentrations in different strata indicate that some strata acted as carrier beds for magmatically-derived CO2 while strata with N2-rich and CO2-poor gases were isolated from CO2 sources.

Composition of continental crust altered by the emergence of land plants

The evolution of land plants during the Palaeozoic era transformed Earth’s biosphere. Because the Earth’s surface and interior are linked by tectonic processes, the linked evolution of the biosphere and sedimentary rocks should be recorded as a near-contemporary shift in the composition of the continental crust. To test this hypothesis, we assessed the isotopic signatures of zircon formed at subduction zones where marine sediments are transported into the mantle, thereby recording interactions between surface environments and the deep Earth. Using oxygen and lutetium–hafnium isotopes of magmatic zircon that respectively track surface weathering (time independent) and radiogenic decay (time dependent), we find a correlation in the composition of continental crust after 430 Myr ago, which is coeval with the onset of enhanced complexity and stability in sedimentary systems related to the evolution of vascular plants. The expansion of terrestrial vegetation brought channelled sand-bed and meandering rivers, muddy floodplains and thicker soils, lengthening the duration of weathering before final marine deposition. Collectively, our results suggest that the evolution of vascular plants coupled the degree of weathering and timescales of sediment routing to depositional basins where they were subsequently subducted and melted. The late Palaeozoic isotopic shift of zircon indicates that the greening of the continents was recorded in the deep Earth. Colonization of continents by plants some 430 Myr ago enhanced the complexity of weathering and sedimentary systems, and altered the composition of continental crust, according to statistical assessment of zircon compositions.

Constraints on the formation of basaltic magmas. Comment on “Lithosphere thickness controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth – a review and new perspectives” – by Niu Y. (2001)

Niu (2021) reviews fundamental topics of basaltic magma generation in the oceans and continents, including passive and active margins as well as intraplate regions. In his approach, Niu (2021) focuses on the role exerted by lithospheric (“LID”) thickness in controlling basalt compositions and volumes, proposing only a minor role, if any, for the thermal state of the mantle. A consequence of this is rejection of mantle plumes in controlling the compositions of MORB and OIB, albeit not excluding completely their existence to explain basalt petrogenesis.Although the fundamental role of the lithospheric mantle on continental breakup, deformation and volcanism was recognized over 50 years ago, this region has only rarely been considered a candidate source for basaltic rocks, being thought to be too cold, stiff and sterile to produce significant amounts of partial melt. However, the stability of C-H-bearing mineral phases such as carbonates, phlogopite and pargasitic amphibole at lithospheric mantle depths indicates relatively low solidus temperature, and the possibility to produce partial melts even under relatively cold thermal regimes.We contribute to this debate by proposing that a wide range of variables explain igneous activity on Earth and, likely, the other rocky planets. The “lid effect” is certainly important but there is evidence that other factors and variables also influence the compositions and volumes of melts produced in the upper mantle. Among these, bulk- and trace-element chemical heterogeneities, the distribution and speciation of volatiles, and large-wavelength variations in temperature caused by radiogenic decay, are the most relevant.

Deformation recorded in polyhalite from evaporite detachments revealed by <sup>40</sup>Ar ∕ <sup>39</sup>Ar dating

Abstract. The Salt Range Formation is an extensive evaporite sequence in northern Pakistan that has acted as the primary detachment accommodating Himalayan orogenic deformation from the north. This rheologically weak formation forms a mylonite in the Khewra Mine, where it accommodates approximately 40 km displacement and is comprised of intercalated halite and potash salts and gypsiferous marls. Polyhalite [K2Ca2Mg(SO4)4⚫2H2O] grains taken from potash marl and crystalline halite samples are used as geochronometers to date the formation and identify the closure temperature of the mineral polyhalite using the 40Ar/39Ar step-heating laser and furnace methods. The diffusion characteristics measured for two samples of polyhalite are diffusivity (D0), activation energy (Ea), and %39Ar. These values correspond to a closure temperature of ca. 254 and 277 ∘C for a cooling rate of 10 ∘C Myr−1. 40Ar/39Ar age results for both samples did not return any reliable crystallisation age. This is not unexpected as polyhalite is prone to 40Ar* diffusion loss and the evaporites have experienced numerous phases of deformation resetting the closed K/Ar system. An oldest minimum heating step age of ∼514 Ma from sample 06-3.1 corresponds relatively well to the established early Cambrian age of the formation. Samples 05-P2 and 05-W2 have measured step ages and represent a deformation event that partially reset the K/Ar system based on oldest significant ages between ca. 381 and 415 Ma. We interpret the youngest measured step ages, between ca. 286 and 292 Ma, to represent the maximum age of deformation-induced recrystallisation. Both the youngest and oldest measured step ages for samples 05-P2 and 05-W2 occur within the time of a major unconformity in the area. These dates may reflect partial resetting of the K/Ar system from meteoric water infiltration and recrystallisation during this non-depositional time. Otherwise, they may result from mixing of Ar derived by radiogenic decay after Cambrian precipitation with partially reset Ar from pervasive Cenozoic deformation and physical recrystallisation.

The role of Ediacaran synkinematic anatectic rocks and the late-orogenic charnockitic rocks in the development of the hot Araçuaí belt

The orogenic evolution of the hot Araçuaí belt is characterized by continuous magmatic activity, with magmas of different compositions recording a long-lived (∼630 to ∼530 Ma) tectono-thermal evolution in response to convergence between the Congo and São Francisco continents during the West Gondwana amalgamation. Geochemical data from the Carlos Chagas domain (CCD) and the Nova Venécia Complex (NVC), in the hinterland of this belt, show that these rocks contain high amounts of heat producing elements – HPEs – (Th, U, and K) and have a dominant peraluminous signature, suggesting that they result from partial melting of continental crust. The CCD is intruded by post-collisional charnockites that have a dominant shoshonitic signature, interpreted as representing magmas from an enriched mantle reservoir with a variable amount of crustal contamination. Detailed morphological investigations at the macro- and micro-scales show that the CCD contains remnants of residuum material from metamorphic reactions associated with textures that attest to melt crystallization. This reinforces the interpretation that the CCD is locally derived, i.e., it represents an in-source subhorizontal rheologically weak layer of migmatites and parauthoctonous granites that formed during the orogenic thickening. The CCD likely triggered the formation of an orogenic plateau and a geological setting in which high temperature conditions could be sustained for tens of millions of years. In such a setting, continuous heat supply from radiogenic decay might have been the main heat source for the compositionally diverse magmatism during most of the orogenic evolution of the Araçuaí belt.

Powering the Galilean Satellites with Moon‐Moon Tides

AbstractThere is compelling evidence for subsurface water oceans among the three outer Galilean satellites and evidence for an internal magma ocean in the innermost moon, Io. Tidal forces from Jupiter periodically deform these bodies, causing heating and deformation that, if measured, can probe their interior structures. In addition to Jupiter‐raised tides, each moon also raises tides on the others. We investigate moon‐moon tides for the first time in the Galilean moons and show that they can cause significant heating through the excitation of high‐frequency resonant tidal waves in their subsurface oceans. The heating occurs both in the crust and ocean and can exceed that of other tidal sources and radiogenic decay if the ocean is inviscid enough. The resulting tidal deformation can be used to constrain subsurface ocean thickness. Our understanding of the thermal‐orbital evolution and habitability of the Jovian system may be fundamentally altered as a result.

Primary or secondary? A dichotomy of the strontium isotope anomalies in the Ediacaran carbonates of Saudi Arabia

Secular variation of 87Sr/86Sr in carbonate strata has been widely used in regional and global chemostratigraphic correlations. Typically, diagenesis results in higher 87Sr/86Sr signals relative to their primary composition due to the alteration by Rb-rich fluids and radiogenic decay of 87Rb to 87Sr. Surprisingly, 87Sr/86Sr values in the Ediacaran limestones from Saudi Arabia (from 0.7029 to 0.7059) are significantly lower than typical Ediacaran seawater values (mostly from 0.7080 to 0.7090) based on a global compilation. Understanding the origin of these anomalies is important insofar as early macrofossils are preserved in these strata. Two hypotheses have been independently evaluated in this study. The first hypothesis shows a low temperature scenario with isolated oceans or lakes in proximity to a mafic source. The second hypothesis is characterized by a high temperature scenario with profound overprints by juvenile hydrothermal fluids. Integrated Sr and Nd isotope data reveal that the 87Sr/86Sr anomalies are closely coupled with positive εNd(t = 560 Ma) values (up to +4.1). Clear covariations between 87Sr/86Sr, εNd, TOC, δ13Ccarb, δ13Corg, and δ18Ocarb were found. Based on multiple lines of petrographic, field, and geochemical evidence, the second hypothesis (i.e., hydrothermal alteration by juvenile fluids) is preferred in this study. Thermogenic breakdown of organic matter in host sediments may have caused the increasingly low TOC abundance and high δ13Corg signals of the residual organic matter. We argue that the concept that the Ediacaran biotic radiation took place in an isolated lake environment should be treated with caution. These remarkably low 87Sr/86Sr signals have neither temporal nor biogeochemical significance. Sr isotope chemostratigraphy in this particular region may not be a reliable tool for stratigraphic correlations.

Geochemical anomalies in the Lower Silurian shale gas from the Sichuan Basin, China: Insights from a Rayleigh-type fractionation model

To verify recent single-well data from the Sichuan Basin, we analyzed the molecular and isotopic composition of late-mature Longmaxi shale gases from the southwestern portion of the Fuling block. The gases are highly enriched in methane (>96.1%) and have low C2+ hydrocarbon contents (<0.51%). The gases exhibit a reversal in carbon isotopes (δ13C3 < δ13C2 < δ13C1), and their δ13C1 values are abnormally enriched (i.e., equal to, or in some cases, more enriched than the associated kerogen). The helium component is typical of crustal genesis from the radiogenic decay of U and Th within the shale, which ranges from 145 ppm to 253 ppm, and its R/Ra ranges from 0.01 to 0.05. The δ15NN2 values range from −5.2‰ to −1.1‰, suggesting that the nitrogen was produced from the thermal de-ammoniation of organic matter. At extremely high thermal maturities and geothermal temperatures, shale gases with abnormally enriched δ13C1 values can best be explained using the model of Burruss and Laughrey (2010), which invokes a Rayleigh-type isotopic fractionation of ethane as it undergoes redox reactions to form methane and residual carbon in the late-stage (post-oil cracking) generation of methane. The variations in the distribution of δ13C1 and δ13C2 within the Fuling block imply that fluid-rock interactions related to the CO2 may have occurred, but not to a significant degree, and the cumulative effects of diffusion were also observed. Unraveling the geological processes that create the geochemical anomalies found in these Lower Silurian shale gas samples can provide valuable insight into the late-stage gas generation.

Serpentinite and the search for life beyond Earth.

Hydrogen from serpentinization is a source of chemical energy for some life forms on Earth. It is a potential fuel for life in the subsurface of Mars and in the icy ocean worlds in the outer solar system. Serpentinization is also implicated in life's origin. Planetary exploration offers a way to investigate such theories by characterizing and ultimately searching for life in geochemical settings that no longer exist on Earth. At present, much of the current context of serpentinization on other worlds relies on inference from modelling and studies on Earth. While there is evidence from orbital spectral imaging and martian meteorites that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, ongoing serpentinization might explain hydrogen found in the ocean of Saturn's tiny moon Enceladus, but this raises questions about how long such activity has persisted. Titan's hydrocarbon-rich atmosphere may derive from ancient or present-day serpentinization at the bottom of its ocean. In Europa, volcanism or serpentinization may provide hydrogen as a redox couple to oxygen generated at the moon's surface. We assess the potential extent of serpentinization in the solar system's wet and rocky worlds, assuming that microfracturing from thermal expansion anisotropy sets an upper limit on the percolation depth of surface water into the rocky interiors. In this bulk geophysical model, planetary cooling from radiogenic decay implies the infiltration of water to greater depths through time, continuing to the present. The serpentinization of this newly exposed rock is assessed as a significant source of global hydrogen. Comparing the computed hydrogen and surface-generated oxygen delivered to Europa's ocean reveals redox fluxes similar to Earth's. Planned robotic exploration missions to other worlds can aid in understanding the planetary context of serpentinization, testing the predictions herein. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.

Subsurface granites in the Franconian Basin as the source of enhanced geothermal gradients: a key study from gravity and thermal modeling of the Bayreuth Granite

The Franconian Basin in NE Bavaria is a region of gravity low between the Bohemian Massif in the east and the Kraichgau Terrane in the west. Borehole measurements have identified the northern part of the Franconian Basin as a regional geothermal anomaly and new heat flow calculations give values of > 100 mW/m2. Distinct negative Bouguer anomalies observed in this basin are modeled as granitic intrusions in the Saxothuringian basement that underlies the Permo-Mesozoic units. The interpretation of gravity gradients, in combination with the filtering of gravity data, gives the possible depth constraints of the intrusive bodies. The resulting depths were cross-checked using microstructural studies of quartz veins in the basement rocks (at > 1341 m depth). The quartz shows structures typical for low-temperature plasticity and we infer a deformation temperature of ca. 300 °C. This indicates a considerable pre-Permian uplift of at least 7 km for parts of the Saxothuringian basement and supports depth estimates from gravity data. The heat supply of granitic intrusions by radiogenic decay is modeled considering several scenarios for the geological conditions recovered by the Obernsees borehole (model for conductive heat transfer). The 1390 m-deep drillhole is in a marginal position to the most-pronounced negative Bouguer anomaly. It could be shown that the Saxothuringian basement, including heat-producing granites (heat production rates: 4–6 μW/m3), covered by the insulating sedimentary rocks (1.35 km of Permian to Lower Jurassic units), can account for the enhanced geothermal gradient (38 °C/km) that were measured in the borehole.

Dynamical investigation of a thickening ice-shell: Implications for the icy moon Europa

Current astrobiological exploration of the outer solar system is largely driven by understanding planetary ice-ocean systems such as those of Europa and Enceladus. The ice-shell thickness that affects the dynamics of the ice-ocean system is determined by the balance between heat loss via convection and conduction, and the heat production via tidal dissipation and radiogenic decay. In this study, we investigate how ice-shells of such ocean moons grow, and how convection inside them evolves as the system cools. We perform geodynamical calculations to investigate the ice-shell thickness and growth rate as the system cools from a warm initial condition. We study both diffuse and localized end member modes of tidal dissipation within the ice-shell. We find that as the ice-shell cools and thickens, there is a reconfiguration of the convection patterns, increasing the size of convection cells that leads to a relatively significant increase in the ice-shell growth rate.

Evolutionary models of cold and low-mass planets: cooling curves, magnitudes, and detectability

Context.Future instruments like the Near Infrared Camera (NIRCam) and the Mid Infrared Instrument (MIRI) on theJames WebbSpace Telescope (JWST) or the Mid-Infrared E-ELT Imager and Spectrograph (METIS) at the European Extremely Large Telescope (E-ELT) will be able to image exoplanets that are too faint (because they have a low mass, and hence a small size or low effective temperature) for current direct imaging instruments. On the theoretical side, core accretion formation models predict a significant population of low-mass and/or cool planets at orbital distances of ~10–100 au.Aims.Evolutionary models predicting the planetary intrinsic luminosity as a function of time have traditionally concentrated on gas-dominated giant planets. We extend these cooling curves to Saturnian and Neptunian planets.Methods.We simulated the cooling of isolated core-dominated and gas giant planets with masses of 5M⊕–2M♃. The planets consist of a core made of iron, silicates, and ices surrounded by a H/He envelope, similar to the ice giants in the solar system. The luminosity includes the contribution from the cooling and contraction of the core and of the H/He envelope, as well as radiogenic decay. For the atmosphere we used grey,AMES-Cond,petitCODE, andHELIOSmodels. We considered solar and non-solar metallicities as well as cloud-free and cloudy atmospheres. The most important initial conditions, namely the core-to-envelope-mass ratio and the initial (i.e. post formation) luminosity are taken from planet formation simulations based on the core accretion paradigm.Results.We first compare our cooling curves for Uranus, Neptune, Jupiter, Saturn, GJ 436b, and a 5M⊕planet with a 1% H/He envelope with other evolutionary models. We then present the temporal evolution of planets with masses between 5M⊕and 2M♃in terms of their luminosity, effective temperature, radius, and entropy. We discuss the impact of different post formation entropies. For the different atmosphere types and initial conditions, magnitudes in various filter bands between 0.9 and 30 micrometer wavelength are provided.Conclusions.Using blackbody fluxes and non-grey spectra, we estimate the detectability of such planets with JWST. We found that a 20 (100)M⊕planet can be detected with JWST in the background limit up to an age of about 10 (100) Myr with NIRCam and MIRI, respectively.

On the changing petroleum generation properties of Alum Shale over geological time caused by uranium irradiation

An interdisciplinary study was carried out to unravel organic–inorganic interactions caused by the radiogenic decay of uranium in the immature organic-rich Alum Shale (Middle Cambrian-Lower Ordovician).Based on pyrolysis experiments, uranium content is positively correlated with the gas-oil ratios and the aromaticities of both the free hydrocarbons residing in the rock and the pyrolysis products from its kerogen, indicating that irradiation has had a strong influence on organic matter composition overall and hence on petroleum potential. The Fourier Transform Ion Cyclotron Resonance mass spectrometry data reveal that macro-molecules in the uranium-rich Alum Shale samples are less alkylated than less irradiated counterparts, providing further evidence for structural alteration by α-particle bombardment. In addition, oxygen containing-compounds are enriched in the uranium-rich samples but are not easily degradable into low-molecular-weight products due to irradiation-induced crosslinking.Irradiation has induced changes in organic matter composition throughout the shale’s entire ca. 500 Ma history, irrespective of thermal history. This factor has to be taken into account when reconstructing petroleum generation history. The Alum Shale’s kerogen underwent catagenesis in the main petroleum kitchen area 420–340 Ma bp. Our calculations suggest the kerogen was much more aliphatic and oil-prone after deposition than that after extensive exposure to radiation. In addition, the gas sorption capacity of the organic matter in the Alum Shale can be assumed to have been less developed during Palaeozoic times, in contrast to results gained by sorption experiments performed at the present day, for the same reason. The kerogen reconstruction method developed here precludes overestimations of gas generation and gas retention in the Alum Shale by taking irradiation exposure into account and can thus significantly mitigate charge risk when applied in the explorations for both conventional and unconventional hydrocarbons.

Synthesis and characterisation of the hollandite solid solution Ba1.2-xCsxFe2.4-xTi5.6+xO16 for partitioning and conditioning of radiocaesium

The geological disposal of high level radioactive waste requires careful budgeting of the heat load produced by radiogenic decay. Removal of high-heat generating radionuclides, such as 137Cs, reduces the heat load in the repository allowing the remaining high level waste to be packed closer together therefore reducing demand for repository space and the cost of the disposal of the remaining wastes. Hollandites have been proposed as a possible host matrix for the long-term disposal of Cs separated from HLW raffinate. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the hollandite structure, including iron. A range of Cs containing iron hollandites were synthesised via an alkoxide-nitrate route and the structural environment of Fe in the resultant material characterised by Mössbauer and X-ray Absorption Near Edge Spectroscopy. The results of spectroscopic analysis found that Fe was present as octahedrally co-ordinated Fe (III) in all cases and acts as an effective charge compensator over a wide solid solution range.

Measurement of Pb208(n,γ)Pb209 Maxwellian averaged neutron capture cross section

The doubly magic $^{208}\mathrm{Pb}$ nucleus is a bottleneck at the termination of the $s$-process path due to its very low neutron capture cross section. This cross section is also important for the decomposition of $s$, $r$ processes and U/Th radiogenic decay contributions to the Pb-Bi solar abundances. The $^{208}\mathrm{Pb}(n,\ensuremath{\gamma})^{209}\mathrm{Pb}$ cross section was measured at the Soreq Applied Research Accelerator Facility Phase I using an intense quasi-Maxwellian neutron source produced by irradiation of the liquid-lithium target with a 1.5-mA continuous-wave proton beam at 1.94 MeV. The cross section was measured by counting the $\ensuremath{\beta}$ activity from the irradiated lead target. The measurement allowed us to evaluate the Maxwellian averaged cross section (MACS) at 30 keV obtaining a value of 0.33(2) mb. This has been compared with the earlier activation and time-of-flight measurements found in the literature. The MACS cross-sectional value of the $^{63}\mathrm{Cu}(n,\ensuremath{\gamma})^{64}\mathrm{Cu}$ reaction was determined in the same experiment and is compared to a recent published value.

The neutron capture process in the He shell in core-collapse supernovae: Presolar silicon carbide grains as a diagnostic tool for nuclear astrophysics

Carbon-rich presolar grains are found in primitive meteorites, with isotopic measurements to date suggesting a core-collapse supernovae origin site for some of them. This holds for about 1–2% of presolar silicon carbide (SiC) grains, so-called Type X and C grains, and about 30% of presolar graphite grains. Presolar SiC grains of Type X show anomalous isotopic signatures for several elements heavier than iron compared to the solar abundances: most notably for strontium, zirconium, molybdenum, ruthenium and barium. We study the nucleosynthesis of zirconium and molybdenum isotopes in the He-shell of three core-collapse supernovae models of 15, 20 and 25M☉ with solar metallicity, and compare the results to measurements of presolar grains. We find the stellar models show a large scatter of isotopic abundances for zirconium and molybdenum, but the mass averaged abundances are qualitatively similar to the measurements. We find all models show an excess of 96Zr relative to the measurements, but the model abundances are affected by the fractionation between Sr and Zr since a large contribution to 90Zr is due to the radiogenic decay of 90Sr. Some supernova models show excesses of 95,97Mo and depletion of 96Mo relative to solar. The mass averaged distribution from these models shows an excess of 100Mo, but this may be alleviated by very recent neutron-capture cross section measurements. We encourage future explorations to assess the impact of the uncertainties in key neutron-capture reaction rates that lie along the n-process path.

Impact of Radiogenic Heating on the Formation Conditions of Comet 67P/Churyumov–Gerasimenko

Abstract Because of the high fraction of refractory material present in comets, the heat produced by the radiogenic decay of elements such as aluminum and iron can be high enough to induce the loss of ultravolatile species such as nitrogen, argon, or carbon monoxide during their accretion phase in the protosolar nebula (PSN). Here, we investigate how heat generated by the radioactive decay of 26Al and 60Fe influences the formation of comet 67P/Churyumov–Gerasimenko, as a function of its accretion time and the size of its parent body. We use an existing thermal evolution model that includes various phase transitions, heat transfer in the ice-dust matrix, and gas diffusion throughout the porous material, based on thermodynamic parameters derived from Rosetta observations. Two possibilities are considered: either, to account for its bilobate shape, 67P/Churyumov–Gerasimenko was assembled from two primordial ∼2 km sized planetesimals, or it resulted from the disruption of a larger parent body with a size corresponding to that of comet Hale–Bopp (∼70 km). To fully preserve its volatile content, we find that either 67P/Churyumov–Gerasimenko’s formation was delayed between ∼2.2 and 7.7 Myr after that of Ca–Al-rich Inclusions in the PSN or the comet’s accretion phase took place over the entire time interval, depending on the primordial size of its parent body and the composition of the icy material considered. Our calculations suggest that the formation of 67P/Churyumov–Gerasimenko is consistent with both its accretion from primordial building blocks formed in the nebula or from debris issued from the disruption of a Hale–Bopp-like body.

Geochemistry of two contrasting deep fluids in the Sardinia microplate (western Mediterranean): Relationships with tectonics and heat sources

The Sardinia microplate in the western Mediterranean represents an ideal example for examining the relationship between fluid geochemistry, tectonic and heat sources in hydrothermal circuits. It consists of a portion of Variscan basement partly covered by sedimentary (mainly carbonate) and volcanic successions that record significant Permian to Pliocene geodynamic events within the southern European margin. The regional structure of the northern Sardinia is dominated by Tertiary ENE–WSW trending strike-slip and NNW–SSE trending normal faults, both capable of controlling deep and shallow fluid circulation. In this paper, results of a detailed geochemical investigation of waters and gases coming from a W-E trending section of central-north Sardinia are presented in order to explain the contrasting thermal and geochemical features of two - already known groups - of fluids. The Volcanic Logudoro Waters (VLW) is a group of cold to hypothermal Na–HCO3 waters characterised by high CO2 contents and mantle-derived He, that are localized in the volcanic-dominated Tertiary grabens. The He mantle signature within the VLW waters is associated with Plio-Pleistocene Quaternary volcanism where the outgassing of mantle-derived fluids is reasonably due recently active magma sources at depth. The currently active emission of mantle-derived gas linked to cold and hypothermal waters, provides evidence that the heat diffusion associated with the Plio-Pleistocene volcanism has already ended. In contrast, the Granite Variscan Basement Waters (GBW) group is characterised by hot-NaCl-rich waters, containing high concentrations of both dissolved N2 and radiogenic 4He. The high contribution of 4He produced by radiogenic decay of U and Th in the crust indicates a supply of radiogenic heat to the hydrothermal system localized within the granitic basement or in the tectonic contact between granite and Tertiary covers.