Tibor J. Dunai

Scaling factors for production rates of in situ produced cosmogenic nuclides: a critical reevaluation

Abstract New scaling factors are presented describing the altitude and latitudinal dependence of production rates for in situ produced cosmogenic nuclides. The new factors incorporate the influence of the non-dipole contributions to the geomagnetic field on the cosmic ray flux. The currently used scaling factors of Lal [Earth Planet. Sci. Lett. 104 (1991) 424–439] are based on the assumption of a dipole field. The overall strategy used in deriving the new scaling factors is, however, very similar to that of Lal and relies on the same and/or similar nuclear data. In this reevaluation, data are only used if the effective geomagnetic parameters (inclination, horizontal field strength) can be reconstructed for the time of measurement. The absorption free pathlengths Λ for cosmic rays selected for this study are based on observational data at altitudes relevant for exposure age dating (sea level to 7000 m). At sea level and latitudes between 20° and 40°, the new factors are up to 18% lower and at high altitudes more than 30% higher than those of Lal. In addition to accounting for the influence of the effective geomagnetic field and providing more applicable estimates of Λ, the new factors also allow for correction for the significant deviations from the standard pressure–altitude relationship that exist in the atmosphere (≤10%).

Oligocene-Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms

The age of onset of hyperaridity in the Atacama Desert, Chile, which is needed to validate geological and climatological concepts, has been heretofore uncertain. Measurement of cosmogenic 21 Ne in clasts from erosion-sensitive sediment surfaces in northern Chile shows that these surfaces have been barely affected by erosion since 25 Ma. Surface exposure ages of sediment clasts give replicate values at 25, 20, and 14 Ma and individual values at 37 and 9 Ma. Predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on Earth. Our findings are compatible with the hypothesis that the onset of aridity in the Atacama Desert could be the reason for, rather than the consequence of, uplift of the high Andes.

Solving the production-diffusion equation for finite diffusion domains of various shapes Part I. Implications for low-temperature (U-Th)/He thermochronology

Abstract We propose an accurate, fast and easy-to-use method to derive numerical solutions for production–diffusion equations for finite diffusion domains of various shapes and arbitrary cooling histories. Previous studies provide solutions for spheres, infinite cylinders and infinite sheets. We extend this range and provide solutions for finite bodies, i.e. finite cylinders and rectangular blocks of any aspect ratio. This approach is important as recently, it has become clear that, for example, the physical grain is the diffusion domain for He diffusion in apatite and titanite [J. Geophys. Res. 105 (2000) 2903; Geochim. Cosmochim. Acta 63 (1999) 3845]. We discuss the use of the new approach for forward modelling (U–Th)/He production–diffusion in apatite. Taking results with finite cylinders as a good approximation for apatite crystals, it is found that approximating instead with spheres or infinite cylinders having the same radius yields differences in calculated ages that can easily be as large as 20–35%. The relative differences are most pronounced in thermal histories that spend significant time at or near the closure temperature. On the other hand, reasonable agreement is found with spheres having the same surface to volume ratio.

Solving the production -diffusion equation for finite diffusion domains of various shapes part 2, Application to cases with Alpha ejection and non-homogeneous distribution of the source

Abstract In this paper, numerical methods derived in Part I (Chem. Geol. (2002)) are expanded to accommodate the effects of α-emission and zonation of U and Th as relevant for (U–Th)/He thermochronology. Expressions for the required coefficients are derived for spheres, finite and infinite cylinders and rectangular blocks, for parent nuclide distributions, which are either uniform or have stepwise zoning. We show that the effects of α-emission and zonation on the He retention in a nonspherical crystal of modest aspect ratio can be approximated by a sphere of identical surface-to-volume ratio ( S / V ) if the zonation is transposed on to the sphere. The currently usual correction method is to perform first a diffusive calculation without considering α-emission and zonation and to multiply afterward with a correction factor that has been derived originally for nondiffusive calculations. Our results demonstrate that more accurate calculations yield different results and that the differences are often too large to be ignored in practice. Especially for thermal histories that include a protracted residence in the partial retention zone, the difference between the usual correction and our correction can be as high as ∼20% for homogeneous sources. For zoned crystals, the difference can be even larger.

Deglacial history of the West Antarctic Ice Sheet in the Weddell Sea embayment: Constraints on past ice volume change

The retreat history of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum is important for understanding the process of rapid deglaciation, constraining models that seek to predict the future trajectory of the ice sheet, and for estimating rates of sea-level change. Here we report new glacial geologic data from the southwestern Weddell Sea embayment that demonstrate that this part of the WAIS was thinner than previously suggested, and that there was progressive thinning of the ice sheet by 230–480 m since ca. 15 ka. We use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the Weddell Sea at the Last Glacial Maximum. The volume of this ice would have added between 1.4 and 2.0 m to postglacial sea-level rise and would not have been sufficient to contribute significantly to meltwater pulse 1A, a rapid rise in sea level ∼14,200 yr ago.

Long-term rates of denudation in t he Dry Valleys, Transantarctic Mountains, southern Victoria Land, Antarctica bases on in-situ-produced cosmogenic 21Ne.

Abstract Concentrations of cosmogenic 21 Ne ( 21 Ne c ) measured in quartz have been used to estimate long-term rates of denudation for contrasting landscape components in the Dry Valleys area of the Transantarctic Mountains, southern Victoria Land, Antarctica. Samples of Beacon Supergroup sandstones and granitic basement were collected from two contrasting landscape elements—low-relief, high-elevation surfaces and rectilinear slopes—to assess variations in rates of denudation with topographic position. The sample sites for rectilinear slopes were selected because of proximity to 40 Ar / 39 Ar-dated lavas and ash-avalanche deposits. All 21 Ne / 20 Ne ratios are significantly greater than the atmospheric value, and concentrations of 21 Ne c were calculated from the measured 21 Ne values assuming an atmospheric composition for the trapped component. Apparent exposure ages calculated from the concentrations of 21 Ne c , assuming no denudation since exposure, range from 3.78–4.66 Ma for samples from the high-elevation plateau surface, and 0.61–2.48 Ma for samples from the rectilinear slopes. Exposure ages for 3 He c were 2 to 42 times lower than those derived from the abundances of 21 Ne c because of preferential diffusive loss of 3 He c from the quartz lattice; concentrations of 3 He c were, therefore, not used in the calculation of rates of denudation. We interpret the abundances of 21 Ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure (>15 Ma) of bedrock surfaces at the sample sites. Calculated maximum rates of denudation range from 0.26–1.02 m Ma−1 for the rectilinear slopes, down to only 0.133–0.164 m Ma−1 for the high-elevation surface sites. These rates are comparable to other estimates of denudation rates for the Dry Valleys derived from analyses of cosmogenic isotopes, but are around two orders of magnitude lower than the long-term mean rate over the past ∼50 Ma estimated from fission-track thermochronology. Combined with the preservation of volcanic deposits dating back to the mid-Miocene, these 21 Ne c data demonstrate that only minimal modification of the landscape has occurred in the Dry Valleys over at least the past ∼15 Ma. This significant conclusion supports the view that the East Antarctic Ice Sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the Pliocene, as has previously been suggested.

Long-term cosmogenic 3He production rates (152 ka - 1.35 Ma) from 40Ar/39Ar dated basalt flows at 29ºN latitude

A set of time integrated cosmogenic 3He production rates in olivines for the last 1.35 Ma are presented. We investigated three basaltic lava flow tops from Lanzarote, Canary Islands, Spain. The 40Ar/39Ar ages determined for those basalt flows by incremental laser heating of leached groundmass samples are 152±26 ka, 281±19 ka and 1.35±0.01 Ma (±2σ). Three or four different olivine phenocryst samples have been analyzed from each flow for their cosmogenic 3He abundances. The resulting 3He production rates in olivine at sea level at 29° latitude are 82±14 and 82±8 atoms g−1 a−1, as obtained from the 152 ka and 281 ka old flows, respectively. Considering effects of erosion on the 1.35 Ma old flow we find that the production rate of 82±8 atoms g−1 a−1 is consistent with the cosmogenic 3He production during the last 1.35 Ma. There appears to be a 14% discrepancy between previously published production rates derived at higher latitudes and altitudes if the scaling factors of Lal are used to compare results. This discrepancy is greatly reduced, however, if the revised scaling factors of Dunai (this issue) are applied. Using the new scaling factors we derive a production rate for cosmogenic 3He in olivine at sea level and high latitudes (>60°) of 118±11 atoms g−1 a−1 (±2σ). The correspondingly revised value of Cerling and Craig, and Ackert et al.) is 123±6 atoms g−1 a−1 (±2σ). The mean value of these two calibrations is 121±6 atoms g−1 a−1 (±2σ). We suggest that the production rate of 121±6 atoms g−1 a−1 at sea level and high latitudes may be applied to the complete time range where paleomagnetic data indicate that there was no long-term averaged intensity variation in the Earth’s magnetic field, i.e. over the last 10 Ma.

Noble gas study of the Reunion hotspot: Evidence for distinct

We present an extensive He, Ne and Ar isotope data set from the Reunion hotspot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. 3He/4He ratios of the two volcanoes on Reunion (Piton de la Fournaise, <0.53 Ma; Piton des Neiges, 2–0.44 Ma) are uniform 12–13.5 Ra regardless of 4He concentration and sample age. The shield-building Older Series of Mauritius (5–8 Ma) has a constant 3He/4He ratio around 11.5 Ra. The similarity of 3He/4He and Sr–Nd isotope ratios among them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. 20Ne/22Ne and 21Ne/22Ne of these volcanoes define a linear trend on a Ne three-isotope diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between 20Ne/22Ne and 40Ar/36Ar. In contrast, Intermediate (2–3 Ma) and Younger Series (<1 Ma) of Mauritius and Rodrigues (1.5 Ma) have 3He/4He ratios similar to MORB and Sr and Nd isotope ratios closer to MORB than lavas from Reunion and Older Series of Mauritius. These Intermediate and Younger Series lavas therefore record a late stage thermal rejuvenation beneath Mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reunion magmatism are relatively low 3He/4He (13 Ra), an intermediate slope in a Ne three-isotope diagram and relatively radiogenic Sr isotope ratios. These source characteristics cannot be explained by either crustal contamination or MORB source mixing with Loihi-type primitive mantle. Thus He–Ne–Ar–Sr–Nd isotopes demonstrate that this plume source is distinct from the source of other large plumes (Loihi and Iceland), clearly showing that the mantle contains several relatively less-degassed reservoirs and not a single primitive source. Two possible models can account for the different isotopic signature of Reunion and Hawaii hotspots; (1) the Reunion source contains more recycled material than Loihi source and (2) the Reunion source experienced stronger degassing/differentiation than the Loihi source in the early stage of mantle evolution. In both cases a convection mode in the mantle is required that isolates and preserves several less-degassed reservoirs in the convectively stirred lower mantle.

Late Miocene to present exhumation in the Ligurian Alps (southwest Alps) with evidence for accelerated denudation during the Messinian salinity crisis

Detrital apatite (U-Th)/He and fission-track thermochronometry are applied to constrain the exhumation history of the Ligurian Alps (southwest Alps). Exhumation of the Ligurian Alps is recorded from ca. 11 Ma and continues until present. More than 4 km of exhumation is recorded at an average rate of 0.4 ± 0.2 mm/yr. Exhumation is accommodated along a north-dipping thrust at the foot of the Ligurian margin. Combined (U-Th)/He and fission-track thermal modeling show a phase of accelerated denudation between 7 and 5 Ma, associated with the base-level drop during the Messinian salinity crisis.

Alzamiento litoral Pleistoceno del norte de Chile: edades 21Ne de la terraza costera más alta del área deCaldera-Bahía Inglesa

En este trabajo se presenta la edad de la terraza costera emergida mas alta del area de Caldera-Bahia Inglesa, localizada a una altura de 224±6 m s.n.m. La metodologia empleada consiste en el uso de edades de exposicion de clastos de cuarzo mediante 21Ne de origen cosmogenico. La edad obtenida fue de 0,86 ± 0,11 Ma. Dentro de este rango de edad, se desarrollan tres interestadiales fuertes correspondientes a los estadios isotopicos MIS 19 (780 ka), MIS 21 (860 ka) y MIS 25 (950 ka) en los cuales se pudo formar esta terraza costera. La tasa de alzamiento promedio considerando estos tres casos es de 0,28±0,02 mm/a, cuyo valor es menor que el determinado por autores previos para los ultimos 400 ka. Entre 750 y 400 ka no se desarrollaron terrazas costeras en el area de estudio. Esto seria un indicador de tasas de alzamiento bajas e interestadiales debiles durante ese periodo, y la fortaleza del interestadial desarrollado durante el estadio MIS 11 (400 ka) que provoco una importante erosion de la topografia litoral incluyendo las terrazas costeras que pudieron haberse formado entre 750 y 400 ka. El alzamiento litoral en el area de estudio es de caracter regional y estaria relacionado con procesos generados como consecuencia de la convergencia de las placas de Nazca y Sudamericana; e.g., los terremotos de subduccion