3He Data Research Articles

Implications of glacial deposit ages for the timing and rate of active crustal faulting in the central Cascade arc, Oregon, USA

Abstract New cosmogenic 3He chronologies and geologic mapping of faulted glacial drift provide new constraints for the slip rates of active faulting in the central Cascade arc, Oregon, USA. The White Branch and Dilman Meadows fault zones cut deposits created by three distinct glacial advances, which provide timing, kinematics, and rate constraints for fault motion. New cosmogenic 3He data from landforms comprising the youngest and most widespread deposits have ages between 19.4 +10.1/–6.2 ka and 21.3 ± 4.9 ka; therefore, they were deposited during the last glacial maximum (LGM). A second, older outwash surface reveals an age of 74.2 ± 3.8 ka, which suggests glaciation possibly associated with marine isotope stage (MIS) 5b. Dip-slip displacement across fault scarps expressed by lidar data reveal similar magnitudes of extensional deformation for LGM and older glacial deposits on the White Branch fault zone, which implies a lack of earthquake ruptures between the oldest and LGM advances. In contrast, scarp profiles along the Dilman Meadows fault zone reveal progressive cumulative slip for surfaces of increasing age. Taken together, our measurements provide the first constraints on the rate of extensional faulting derived from Quaternary geochronology along the White Branch and Dilman Meadows faults, which total 0.1–0.4 mm/yr since ca. 75 ka and 0.6 ± 0.04 mm/yr since the LGM, respectively. The White Branch fault zone accommodates predominately fault-normal extension, whereas right-oblique slip characterizes the Dilman Meadows fault zone. Active deformation across the central Cascade Range thus reflects the combined effects of ongoing crustal block rotation and arc magmatism.

Exposure-age data from across Antarctica reveal mid-Miocene establishment of polar desert climate

Abstract High-elevation rock surfaces in Antarctica have some of the oldest cosmogenic-nuclide exposure ages on Earth, dating back to the Miocene. A compilation of all available 3He, 10Be, and 21Ne exposure-age data from the Antarctic continent shows that exposure histories recorded by these surfaces extend back to, but not before, the mid-Miocene cooling at 14–15 Ma. At high elevation, this cooling entailed a transition between a climate in which liquid water and biota were present and could contribute to surface weathering and erosion, and a polar desert climate in which virtually all weathering and erosion processes had been shut off. This climate appears to have continued uninterrupted between the mid-Miocene and the present.

Complementary use of dating and hydrochemical tools to assess mixing processes involving centenarian groundwater in a geologically complex alpine karst aquifer

AbstractEnvironmental dating tracers (3H, 3He, 4He, CFC‐12, CFC‐11, and SF6) and the natural spring response (hydrochemistry, water temperature, and hydrodynamics) were jointly used to assess mixing processes and to characterize groundwater flow in a relatively small carbonate aquifer with complex geology in southern Spain. Results evidence a marked karst behaviour of some temporary outlets, with sharp and rapid responses to precipitation events, while some perennial springs show buffer and delayed variations with respect to recharge periods. The general geochemical evolution shows a pattern, from higher to lower altitudes, in which mineralization and the Mg/Ca ratio rise, evidencing longer water–rock interaction. The large SF6 concentrations in groundwater suggest terrigenic production, whereas CFC‐11 values are affected by sorption or degradation. The groundwater age in the perennial springs—as deduced from CFC‐12 and 3H/3He—points to mean residence times of several decades, although the large amount of radiogenic 4He in samples indicate a contribution of old groundwater (free of 3H and CFC‐12). Lumped parameter models and shape‐free models were created based on 3H, tritiogenic 3He, CFC‐12, and radiogenic 4He data in order to interpret the age distribution of the samples. Results evidence the existence of two mixing components, with an old fraction ranging between 160 and 220 years in age. The correlation of physicochemical parameters with some dating parameters, derived from the mixing models, serves to explain the hydrogeochemical processes occurring within the system. Altogether, long residence times are shown to be possible in small alpine systems with a clearly karst behaviour if the geological setting features highly tectonized media including units with diverse hydrogeological characteristics. These findings highlight the importance of applying different approaches, including groundwater dating techniques, when studying such groundwater flow regimes.

Geochemistry of gas and water discharge from the magmatic-hydrothermal system of Guallatiri volcano, northern Chile

This work presents the first chemical and isotopic (δ13C-CO2, δ13C-CH4, 3He, 4He, 20Ne, 40Ar, 36Ar, δ18O, and δD) data for fluid discharges from Guallatiri volcano, a remote and massive stratovolcano, which is considered as the second most active volcano of the Central Volcanic Zone (CVZ) in northern Chile. Fumarolic gases had outlet temperatures of between 80.2 and 265 °C, and showed a significant magmatic fluid contribution marked by the occurrence of SO2, HCl, and HF that are partially scrubbed by a hydrothermal aquifer. The helium isotope ratios (< 3.2) were relatively low compared to those of other active volcanoes in CVZ, possibly due to contamination of the magmatic source by 4He-rich crust and/or crustal fluid addition to the hydrothermal reservoir. Geothermometry in the H2O-CO2-CO-H2-CH4 system suggests equilibrium temperatures of up to 320 °C attained in a vapor phase at redox conditions intermediate between those typical of hydrothermal and magmatic environments. Thermal springs located 12 km northwest of the volcano’s summit had outlet temperatures of up to 50.1 °C, neutral to slightly basic pH, and a sodium bicarbonate composition, typical of distal fluid discharges in volcanic systems. Cold springs at the base of the volcanic edifice, showing a calcium sulfate composition, were likely produced by interaction of shallow meteoric water with CO2- and H2S-rich gases. A geochemical conceptual model was constructed to graphically represent these results, which can be used as an indication for future geochemical monitoring and volcanic hazard assessment.

Geochemical survey of the Colpitas-Taapaca volcanic-hydrothermal system, northern Chile

This work presents chemical and isotopic (δ13C-CO2, δ13C-CH4, 3He, 4He, 20Ne, 40Ar, 36Ar, δ18O and δD) data of fluid discharges from Colpitas-Taapaca volcanic-hydrothermal system, located close to the Taapaca Volcanic Complex, with the aim to investigate the physical-chemical conditions of the fluid source and to provide a preliminary evaluation of the geothermic potential of the study area. Colpitas thermal springs (to 56 °C) and part of the cold springs (≤18°C) from this area have a Na+-Cl- composition and Total Dissolved Solids (TDS) values (from 6,059 to 19,118 mg/L). Putre springs also show a Na +-Cl -composition, TDS values up to 7,887 mg/L, and outlet temperatures from 21 to 31 °C. Colpitas cold springs, with a Ca2+-SO42- composition and relatively low TDS values (≤1,350 mg/L), are likely produced by interaction of shallow water with uprising H2S- rich hydrothermal gases. This process is likely also controlling the chemistry of Jurase thermal springs, which have the highest outlet temperatures of the study area (up to 68 °C), a Ca2+-SO42- composition and TDS values ≤2,355 mg/L. Eventually, Las Cuevas springs have temperatures up to 36 °C, a Na+-HCO3- composition and low TDS values (≤1,067 mg/L), typical features of springs related to a shallow aquifer. The δ18O-H2O and δD-H2O values indicate that all waters have a dominant meteoric origin. Enrichments in 18O and D shown by Colpitas and Putre thermal waters are likely due to steam loss and water- rock interaction, masking a possible direct steam contribution from magmatic degassing. Gas emissions from Colpitas bubbling pools are dominated by CO2, with significant concentrations of CH4, H2S and H2. The Rc/Ra values (up to 2.04) of Colpitas gases indicate a significant contribution of magmatic to mantle He, whereas the high CO2/ 3He ratios, combined with d 13C-CO2 values ranging from -7.66 to -5.63 ‰ vs. PDB, imply a dominant crustal CO2 source, mostly involving limestone. Estimated temperatures based on the composition of waters and gases from Colpitas are up to 215 °C. Higher temperatures (240 °C) are estimated for Putre thermal waters, although these waters, as well as those from Jurase and Las Cuevas, are too immature for a reliable application of geothermometric techniques. Based on the theoretical reservoir temperature and the measured Cl total output, the thermal energy released from Colpitas thermal area is estimated at up to 13.9 Mw. Such results suggest the occurrence of a promising heat source, possibly related to Taapaca volcanic complex, and encourage the development of future research based on combined geophysical and geochemical approaches, in order to provide a reliable evaluation of the geothermal potential of the whole area.

Middle Pleistocene formation of the Rio Grande Gorge, San Luis Valley, south-central Colorado and north-central New Mexico, USA: Process, timing, and downstream implications

The Rio Grande is the fourth longest river in North America extending over 3,000 km from the Rocky Mountains to the Gulf of Mexico. The Pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated. Herein, we constrain the middle Pleistocene evolution of the northernmost and largest Rio Grande basin, the San Luis basin, and the timing of incision of the Rio Grande Gorge, based on new geologic mapping, 3He surface exposure dating, and U-series dating on pedogenic carbonates. 3He dating of shoreline deposits of closed basin Lake Alamosa and fluvially scoured bedrock along the Rio Grande gorge rim indicate the San Luis basin was connected with southerly Rio Grande basins and progressively incised since ∼400 ka. Waning tectonic activity, coupled with the hydrologic response to intensified middle Pleistocene glaciation, drove southward spillover of Lake Alamosa ∼400 ka, expanding the river's drainage basin by nearly 24,000 km2, and adding recharge areas in the high-altitude, glaciated San Juan, Tusas, and Sangre de Cristo Mountains. Geologic barriers to streamflow on the Taos Plateau were breached and basin tributary drainages were integrated and inset into their present canyons ∼250-200 ka, with approximately 120 m of incision during ∼400-200 ka. Previous work demonstrates that southerly streamflow through basins south of the SLB terminated into a large, rift-related bolson complex until the middle Pleistocene, and most axial Rio Grande river incision has occurred since 630 ka. We propose that middle Pleistocene integration of the SLB was the major geomorphic event that eroded Plio-Pleistocene tectonic and volcanic barriers and re-established flow of the Rio Grande from the Rocky Mountains to the Gulf of Mexico.

An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body.

The breakup of the L-chondrite parent body in the asteroid belt 466 million years (Ma) ago still delivers almost a third of all meteorites falling on Earth. Our new extraterrestrial chromite and 3He data for Ordovician sediments show that the breakup took place just at the onset of a major, eustatic sea level fall previously attributed to an Ordovician ice age. Shortly after the breakup, the flux to Earth of the most fine-grained, extraterrestrial material increased by three to four orders of magnitude. In the present stratosphere, extraterrestrial dust represents 1% of all the dust and has no climatic significance. Extraordinary amounts of dust in the entire inner solar system during >2 Ma following the L-chondrite breakup cooled Earth and triggered Ordovician icehouse conditions, sea level fall, and major faunal turnovers related to the Great Ordovician Biodiversification Event.

Holocene uplift and rapid fluvial erosion of Iceland: A record of post-glacial landscape evolution

In actively deforming regions fluvial systems are strongly regulated by uplift. River geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief. Iceland's rapidly evolving landscapes provide an opportunity to disentangle histories of uplift generated by postglacial rebound, volcanism, dynamic support, and plate spreading. Broad knickzones observed along Iceland's large rivers, and its powerful waterfalls and deep canyons, hint that regional processes have generated significant relief. We combine high-resolution drone photogrammetry and cosmogenic 3He dating of fluvial terraces to measure the erosional history of one of Iceland's largest knickzones, Jökulsárglúfur, in the northeast part of the island. Progressive younging of terraces indicates knickpoint propagation rates of up to ∼70 cma−1 during the last 8 ka. Knickpoint velocities appear to be controlled partly by toppling of basalt columns. These rates were used to calibrate a model that inverts Iceland's drainage networks for uplift rate histories. Calculated uplift and isostatic calculations indicate that rifting, sub-plate support, and isostatic adjustment resulted in tens to hundreds of meters of regional Holocene uplift. Our results suggest regional uplift and fluvial erosion can rapidly generate hundreds of meters of relief in post-glacial landscapes.

Retention of cosmogenic 3He in calcite

Cosmogenic 3He can be used to date a wide range of mineral phases because it is produced from all target elements and can be readily measured above atmospheric contamination. Calcite is a particularly attractive target mineral due to its natural abundance, large crystal size (>1 mm), and low He closure temperature (<70 °C), which limit non-cosmogenic 3He components (Copeland et al., 2007). However, several recent studies have shown that some calcite may not be retentive to helium, even under surface temperatures (Cros et al., 2014; Copeland et al., 2007). This study thus explores 3He retention and production in natural calcite samples at four different sites. Samples from two high elevation sites appear retentive to 3He over 10 kyr timescales, whereas two additional sites clearly suffer from diffusive loss of 3He. Step-degassing experiments suggest that diffusion in calcite is controlled by multiple diffusion domains, with an apparent activation energy of 25–27 kcal mol−1. Although minor 3He loss is expected from the smallest diffusion domains, the observed kinetics cannot explain the poor retention at all sites. We thus propose that opaque (non-transparent) calcite may be more retentive due to the presence of imperfections in the crystal lattice. We conclude that 3He dating of calcite shows promise in some settings. However, because retention depends on crystallographic variability it must be evaluated on a case-by-case basis until robust criteria for retention can be identified.

Evaluated gas production cross-section data for natural titanium irradiated with protons at energies up to 3 GeV

Abstract Evaluated proton, deuteron, triton, 3He and 4He production cross-section data were prepared for natural titanium irradiated with protons at the energy up to 3 GeV. The evaluated data were obtained from the analysis of results of nuclear model calculations, available measured, and systematic data. The calculations of gas production cross-sections were carried out using the intranuclear cascade model (INC), the pre-equilibrium exciton model, geometry dependence hybrid (GDH) model, Weisskopf-Ewing (WE) model, and the Hauser-Feshbach model. CASCADE, TALYS, ALICE/ASH, and ALICE/HMS codes were used for calculations. Available experimental and systematic data, model calculations were used to evaluate gas production cross-section. The evaluation was carried out using statistical methods implemented in the BEKED code package developed at KIT/INR. The evaluated cross-sections were compared with the data from evaluated data libraries.

Strong proximal earthquakes revealed by cosmogenic 3He dating of prehistoric rockfalls, Christchurch, New Zealand

The 2011 rupture of previously undetected blind faults beneath Christchurch, New Zealand, in moment magnitude (M w ) 6.2 and 6.0 earthquakes triggered major rockfalls that caused fatalities and infrastructure damage. Here we use field, geospatial, seismologic, numerical modeling, and cosmogenic 3 He data to provide first evidence for prehistoric rockfall ca. 8– 6 ka, and a possible preceding event ca. 14– 13 ka, at a site where extensive rockfall occurred in the Christchurch earthquakes. The long (∼7 ± 1 k.y.) time intervals between successive rockfall events and the high peak ground velocity thresholds required for rockfall initiation at this site (∼20–30 cm/s) preclude earthquakes from major identified seismic sources, including the plate boundary Alpine fault, as likely rockfall triggering sources. Rockfalls were probably triggered by strong paleoearthquakes sourced from active faults proximal (i.e., <10–20 km) to Christchurch, including the sources of the 2011 Christchurch earthquakes and/or other currently unidentified faults. Given the inherent incompleteness of seismic source catalogues and challenges in obtaining earthquake chronologies for blind faults, high scientific priority should be given to the search for, and analysis of, geologic records of strong earthquake shaking near populated areas.

An inter-laboratory comparison of cosmogenic 3He and radiogenic 4He in the CRONUS-P pyroxene standard

This study reports an inter-laboratory comparison of the 3He and 4He concentrations measured in the pyroxene material CRONUS-P. This forms part of the CRONUS-Earth and CRONUS-EU programs, which also produced a series of natural reference materials for in situ produced 26Al, 10Be, 14C, 21Ne and 36Cl.Six laboratories (GFZ Potsdam, Caltech Pasadena, CRPG Nancy, SUERC Glasgow, BGC Berkeley, Lamont New York) participated in this intercomparison experiment, analyzing between 5 and 22 aliquots each. Intra-laboratory results yield 3He concentrations that are consistent with the reported analytical uncertainties, which suggests that 3He is homogeneous within CRONUS-P. The inter-laboratory dataset (66 determinations from the 6 different labs) is characterized by a global weighted mean of (5.02 ± 0.12) × 109 at g−1 with an overdispersion of 5.6% (2σ). 4He is characterized by a larger variability than 3He, and by an inter-lab global weighted mean of (3.60 ± 0.18) × 1013 at g−1 (2σ) with an overdispersion of 10.4% (2σ).There are, however, some systematic differences between the six laboratories. More precisely, 2 laboratories obtained mean 3He concentrations that are about 6% higher than the clustered other 4 laboratories. This systematic bias is larger than the analytical uncertainty and not related to the CRONUS-P material (see Schaefer et al., 2015). Reasons for these inter-laboratory offsets are difficult to identify but are discussed below. To improve the precision of cosmogenic 3He dating, we suggest that future studies presenting cosmogenic 3He results also report the 3He concentration measured in the CRONUS-P material in the lab(s) used in a given study.

Nuclear effects and neutron structure in deeply virtual Compton scattering off 3He

The study of nuclear generalized parton distributions (GPDs) could be a crucial achievement of hadronic physics since they open new ways to obtain new information on the structure of bound nucleons, in particular, to access the neutron GPDs. Here, the results of calculations of 3He GPDs in Impulse Approximation (IA) are presented. The calculation of the sum of GPDs H + E, and , with the correct limits, will be shown. These quantities, at low momentum transfer, are largely dominated by the neutron contribution so that 3He is an ideal target for these kind of studies. Nevertheless the extraction of neutron information from future 3He data could be non trivial. A procedure, which takes into account nuclear effects encoded in IA, is presented. The calculation of H, E and allows also to evaluate the cross section asymmetries for deeply virtual compton scattering (DVCS) at Jefferson Lab kinematics. Thanks to these observations, DVCS off 3He could be an ideal process to access the neutron information in the next future.

Pyroxene separation by HF leaching and its impact on helium surface-exposure dating

Cosmogenic 3He surface-exposure dating has become an important tool in earth surface sciences. Pyroxene, together with olivine, is the principal mineral phase for this technique, making 3He particularly appropriate for volcanic lithologies. However, two important factors that affect the viability of the method are the extensive, thus expensive, preparation procedure and the often under-constrained purity of pyroxene separates. Here, we present an approach to preparing pyroxenes for 3He dating, adapted from the quartz-separation method for 10Be and 26Al analyses, which utilises hydrofluoric-acid leaching to improve pyroxene purity and streamline the pyroxene separation procedure. In addition to producing abundant sample in relatively little time, the results of two experiments demonstrate that acid leaching (i) yields samples of higher purity than conventional methods, reflected in slightly elevated 3He concentrations, and (ii) reduces 4He concentrations, in turn elevating the 3He/4He ratio. This new protocol for preparing pyroxenes thus has the potential to increase the precision and accuracy of cosmogenic 3He surface-exposure dating.

Progressive glacial retreat in the Southern Altiplano (Uturuncu volcano, 22°S) between 65 and 14 ka constrained by cosmogenic 3He dating

This work presents the first reconstruction of late Pleistocene glacier fluctuations on Uturuncu volcano, in the Southern Tropical Andes. Cosmogenic 3He dating of glacial landforms provides constraints on ancient glacier position between 65 and 14ka. Despite important scatter in the exposure ages on the oldest moraines, probably resulting from pre-exposure, these 3He data constrain the timing of the moraine deposits and subsequent glacier recessions: the Uturuncu glacier may have reached its maximum extent much before the global LGM, maybe as early as 65ka, with an equilibrium line altitude (ELA) at 5280m. Then, the glacier remained close to its maximum position, with a main stillstand identified around 40ka, and another one between 35 and 17ka, followed by a limited recession at 17ka. Then, another glacial stillstand is identified upstream during the late glacial period, probably between 16 and 14ka, with an ELA standing at 5350m. This stillstand is synchronous with the paleolake Tauca highstand. This result indicates that this regionally wet and cold episode, during the Heinrich 1 event, also impacted the Southern Altiplano. The ELA rose above 5450m after 14ka, synchronously with the Bolling–Allerod.

Mapping modern CO2 fluxes and mantle carbon content all along the mid-ocean ridge system

Quality criteria have been used to select ∼400 vesicularity measurements on zero-age mid-ocean ridge glasses from ∼600 data available in the literature published over the past ∼30 years. At face value, observations show that for a given depth of sampling, enriched basalts from slow spreading ridge segments are more vesicular than those from depleted and intermediate or fast spreading ridges. A shallower depth of eruption enhances these effects because lower hydrostatic pressure favours bubble expansion. In order to get an insight into these complex and intermingled processes, we used empirical and semi-quantitative approaches based on a limited number of inputs (segment depth D, spreading rate τ and K2O/TiO2 ratios). Both models give equivalent results and predict vesicularities within ±50%. From these calculations, we compute the equivalent CO2 concentration at the depth of eruption all along the oceanic ridge system. The total calculated CO2 fluxes are low ranging from 6.5±1.8 to 8.7±2.8×1011 mol/yr and the CO2 mantle content displays large variabilities from 66−19+27 to 78−40+82 ppm. In order to test these results, the mantle 3He fluxes have been evaluated using the calculated CO2 fluxes and a CO2/3He ratio of 2.2×109. These fluxes range from 295±82 to 395±127 mol/yr and are close to the values reported by Jean-Baptiste (1992) (267–534 mol/yr) and the most recent estimate (Bianchi et al., 2010, ∼527±102 mol/yr) using box-model of the three main ocean basins constrained by measurements of 3He and radiocarbon data. As these independent methods give similar helium fluxes at regional and global scales, it provides strong support to a low and heterogeneous mantle carbon concentration and distribution. Finally, the calculated volcanic CO2 emissions at oceanic ridges correspond to ∼30 seconds of anthropogenic emissions, at current rates.

The transient distributions of nuclear weapon-generated tritium and its decay product &amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt; He in the Mediterranean Sea, 1952–2011, and their oceanographic potential

Abstract. We present a comprehensive account of tritium and 3He in the Mediterranean Sea since the appearance of the tritium generated by the atmospheric nuclear-weapon testing in the 1950s and early 1960s, based on essentially all available observations. Tritium in surface waters rose to 20–30 TU in 1964 (TU = 1018 × [3H]/H]), a factor of about 100 above the natural level, and thereafter declined 30-fold up to 2011. The decline was largely due to radioactive tritium decay, which produced significant amounts of its stable daughter 3He. We present the scheme by which we separate the tritiugenic part of 3He and the part due to release from the sea floor (terrigenic part). We show that the tritiugenic component can be quantified throughout the Mediterranean waters, typically to a ± 0.15 TU equivalent, mostly because the terrigenic part is low in 3He. This fact makes the Mediterranean unique in offering a potential for the use of tritiugenic 3He as a tracer. The transient distributions of the two tracers are illustrated by a number of sections spanning the entire sea and relevant features of their distributions are noted. By 2011, the 3He concentrations in the top few hundred metres had become low, in response to the decreasing tritium concentrations combined with a flushing out by the general westward drift of these waters. Tritium-3He ages in Levantine Intermediate Water (LIW) were obtained repeated in time at different locations, defining transit times from the LIW source region east of Rhodes. The ages show an upward trend with the time elapsed since the surface-water tritium maximum, which arises because the repeated observations represent increasingly slower moving parts of the full transit time spectrum of LIW. The transit time dispersion revealed by this new application of tritium-3He dating is considerable. We find mean transit times of 12 ± 2 yr up to the Strait of Sicily, 18 ± 3 yr up to the Tyrrhenian Sea, and 22 ± 4 yr up into the Western Mediterranean. Furthermore, we present full Eastern Mediterranean sections of terrigenic 3He and tritium-3He age in 1987, the latter one similarly showing an effect of the transit time dispersion. We conclude that the available tritium and 3He data, particularly if combined with other tracer data, are useful for constraining the subsurface circulation and mixing of the Mediterranean Sea.

Risk assesment of an urban aquifer based on environmental tracers

Groundwater from a Pleistocene sandy-gravel aquifer is a drinking water resource for the Union brewery, located near the centre of Ljubljana (Slovenia). A large part of the aquifer recharge area is highly urbanized, which represents a great risk for the groundwater quality assurance. The groundwater dating techniques were used to study the contamination risk of this drinking-water resource. The application of chlorofluorocarbons (CFCs), sulphur hexafiuoride (SF6) and tritium-helium-3 (3H-3He) age indicator was tested, as they haven't been used in Slovene urban areas so far. The results reflect that the 3H-3He dating technique is the most suitable for a groundwater age determination in the study urban aquifer, since SF6 and particularly CFCs concentrations could be affected by the local contaminations. They indicated that average groundwater residence times range from 10 to 30 years. Boreholes that are more distant from the Šišenski hrib hill are more vulnerable to contamination due to flow of young groundwater from a direction of the main aquifer, which is exposed to the urban pollution. The presented results were supplemented with chemical investigations of groundwater organic pollutants. An unknown trace organic pollutant with a base ion mass-to-charge ratio 147 was put into focus. Its identification based on chromatographic separation and a mass spectrometric detection with GC-MS, LC-MS and LC-TOF MS techniques. The newly detected trace organic pollutant in sampled groundwater represents together with the 3H and 3He data a new technique to study the flow paths and contaminant transport in the urban aquifer in both, the lateral and vertical directions.

40Ar/39Ar dating of the SP and Bar Ten lava flows AZ, USA: Laying the foundation for the SPICE cosmogenic nuclide production-rate calibration project

Precise 40Ar/39Ar age determinations made on basalt groundmass collected from the SP and upper and lower Bar Ten lava flows in the San Francisco and Uinkaret volcanic fields of Arizona, USA, yield ages of 72 ± 4, 97 ± 10, and 123 ± 12 ka (2σ; relative to Renne et al., 2010, 2011, full external precision), respectively. Previous ages of the SP lava flow include a K–Ar age of 70 ± 8 ka and OSL ages of 5.5–6 ka. 40Ar/39Ar age constraints, relative to the optimization model of Renne et al. (2010, 2011), of 81 ± 50 and 118 ± 64 ka (2σ; full external precision) were previously reported for the upper and lower Bar Ten lava flows, respectively. The new 40Ar/39Ar ages are within uncertainty of previous age constraints, and are more robust, accurate, and precise. Preliminary cosmogenic 3He and 21Ne production rates from the Bar Ten flows reported by Fenton et al. (2009) are updated here, to account for the improved quality of the 40Ar/39Ar data. The new 40Ar/39Ar age for the SP flow yields cosmogenic 3He and 21Ne production rates for pyroxene (119 ± 8 and 26.8 ± 1.9 at/g/yr; error-weighted mean, 2σ uncertainty; Dunai (2000) scaling method) that are consistent with production rate values reported throughout the literature. The 40Ar/39Ar and cosmogenic 3He and 21Ne data support field observations indicating the SP flow has undergone negligible erosion. The SP flow contains co-existing phenocrysts of olivine and pyroxene, as well as xenocrysts of quartz in a fine-grained groundmass facilitating cross-calibration of cosmogenic production rates and production-rate (3He, 10Be, 14C, 21Ne, 26Al, and 36Cl). Thus, we propose the SP flow is an excellent location for a cosmogenic nuclide production-rate calibration site (SPICE: the SP Flow Production-Rate Inter-Calibration Site for Cosmogenic-Nuclide Evaluations).

(U–Th)/He ages of phosphates from St. Séverin LL6 chondrite

We obtained single-grain (U–Th)/He ages from 14 merrillite and five chlorapatite aggregates from St. Séverin to constrain its low-T thermal history. In addition, we performed 3He/4He stepped heating diffusion experiments for merrillite and chlorapatite crystals from Guareña chondrite to better constrain He diffusion properties in chondritic phosphates in general. Among the 19 phosphate grains from St. Séverin, the five oldest merrillites and four oldest chlorapatites yielded weighted mean ages of 4412±75Ma (1σ: MSWD=0.34) and 4152±70Ma (MSWD=0.48), respectively. These weighted mean ages overlap with the peaks of the corresponding probability density plots, thus likely represent the most pristine (U–Th)/He ages of St. Séverin. The radiogenic 4He and proton-induced 3He diffusion experiments on Guareña chondrite resulted in two well-defined linear trends in Arrhenius plot for merrillite (radius=∼59μm) and chlorapatite (∼43μm) grains. Error-weighted linear regressions of the 3He data yielded following diffusion parameters: Ea=135.8±4.8kJ/mol, and ln(Do/a2)=5.83±0.66ln(s−1) for merrillite; and Ea=109.3±9.7kJ/mol, and ln(Do/a2)=8.15±1.93 ln(s−1) for chlorapatite. Assuming the analyzed fragments approximate the diffusion domain, logDo was calculated as (−1.93±0.29) log(cm2/s) and (−1.19±0.84) log(cm2/s) for merrillite and chlorapatite, respectively. These results indicate higher closure T for merrillite than chlorapatite. Assuming these results also apply to St. Séverin and that crystal dimensions define the limiting diffusive length-scale, the closure temperatures of merrillite and chlorapatite in St. Séverin are estimated to be 94–112°C (for cooling rates of 0.3–2.6°C/Ma) and 18°C (for 0.3°C/Ma), respectively. The new single-grain (U–Th)/He ages and 3He/4He stepped heating diffusion results, combined with previously reported Pb/Pb and 40Ar/39Ar (with updated λ–tFCs pairs) data indicate that St. Séverin experienced relatively rapid cooling from 477°C down to ∼100°C at a cooling rate of 2.6°C/Ma. The identified thermal history is generally consistent with the reported cooling path from Pu fission track data. These results suggest that St. Séverin experienced relatively slow cooling compared to H chondrites, which may have resulted from its parent body size being larger than that of the H chondrites.