Evaluation of cosmogenic 3He and 21Ne production rates in olivine and pyroxene from two Pleistocene basalt flows,western Grand Canyon,AZ, USA

Combining cosmogenic ³He and ²¹Ne (³He_c and ²¹Ne_c) measurements on both pyroxene and olivine from the Pleistocene Bar Ten flows (85–107 ka) greatly increases our ability to evaluate the accuracy of ³He_c and ²¹Ne_c production rates and, therefore, ³He_c and ²¹Ne_c surface exposure ages. Comparison of ³He_c and ²¹Ne_c age-pairs yielded by experimentally determined production rates and composition-based model calculations indicates that the former give more accurate surface exposure ages than the latter in this study. However, experimental production rates should be adjusted to the composition of the minerals being analyzed to obtain the best agreement between ³He_c and ²¹Ne_c ages for any given sample. ²¹Ne_c/³He_c values are 0.400 ± 0.029 and 0.204 ± 0.014 for olivine and pyroxene, respectively, in Bar Ten lava flows, in agreement with previously published values, and indicate that ²¹Ne_c/³He_c in olivine and pyroxene is not affected by erosion and remains constant with latitude, elevation, and time (up to 10 Myr). Samples with ²¹Ne_c/³He_c that do not agree with these values may indicate the presence of non-cosmogenic helium and/or neon. The neon three-isotope diagram can also indicate whether or not all excess neon in mineral separates comes from cosmogenic sources. An error-weighted regression for olivine defines a spallation line [y = (1.033 ± 0.031)x + (0.09876 ± 0.00033)], which is indistinguishable from that for pyroxene (Schäfer et al., 1999). We have derived a production rate of 25 ± 8 at/g/yr for ²¹Ne_c in clinopyroxene (En_43–44) based on the ⁴⁰Ar/³⁹Ar age of the upper Bar Ten flow. Our study indicates that the production rate of ²¹Ne_c in olivine may be slightly higher than previously determined. Cosmogenic ³He and ²¹Ne remain extremely useful, particularly when paired, in determining accurate eruption ages of young olivine- and pyroxene-rich basaltic lava flows.     

Cosmogenic 21Ne exposure dating of young basaltic lava flows from the Newer Volcanic Province,western Victoria,Australia

Cosmogenic ²¹Ne was utilised to determine exposure ages of young subaerial basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia. The ages (36–53 ka) determined from co-existing cosmogenic ²¹Ne and ³He in olivines separated from basalts are consistent within analytical uncertainties with ages previously determined by cosmogenic ³⁶Cl exposure dating. This paper illustrates the potential of cosmogenic neon exposure ages in studying the eruption, surface morphology, and erosion history of young volcanic rocks, which are difficult to date using other conventional methods, such as K-Ar or ⁴⁰Ar/³⁹Ar dating. The present study demonstrates that combined cosmogenic ³He and ²¹Ne dating, specifically measured cosmogenic ³He/²¹Ne ratios, on the same samples, is powerful for evaluating the validity of calculated cosmogenic ³He and ²¹Ne surface exposure ages.     

Cosmogenic 3He and 21Ne dating of biotite and hornblende

Stable cosmogenic isotopes such as ³He and ²¹Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for ³He and ²¹Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic ³He and ²¹Ne can be readily measured in biotite and hornblende. ²¹Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure ²¹Ne_hbl/²¹Ne_qtz = 1.35 ± 0.03 and ²¹Ne_bio/²¹Ne_qtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g^? 1 yr^? 1 relative to a ²¹Ne_qtz production rate of 19.0 ± 1.8 at g^? 1 yr^? 1. We show that nucleogenic ²¹Ne concentrations produced via the reaction ¹⁸O(α,n)²¹Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic ²¹Ne by comparison to nucleogenic ²²Ne produced from the reaction ¹⁹F(α,n)²²Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic ³He dating, but that ³He is lost from biotite at Earth surface temperatures. Comparison of ³He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for ³He production via the reaction ⁶Li(n,α)³H → ³He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced ³He represents ~ 40% of total ³He production in our samples, and must be considered on a sample-specific basis if ³He dating in hornblende is to be widely implemented.     

Surface exposure dating of young basalts (1–200 ka) in the San Francisco volcanic field (Arizona,USA) using cosmogenic 3He and 21Ne

K–Ar ages of young basalts (<500 ka) are often higher than the actual eruption age, due to low potassium contents and the frequent presence of excess Ar in olivine and pyroxene phenocrysts. Geological studies in the San Francisco and Uinkaret volcanic fields in Arizona have documented the presence of excess ⁴⁰Ar and have concluded that K–Ar ages of young basalts in these fields tend to be inaccurate. This new study in the San Francisco volcanic field presents ³He_c and ²¹Ne_c ages yielded by olivine and pyroxene collected from three Pleistocene basalt flows – the South Sheba (∼190 ka), SP (∼70 ka), and Doney Mountain (∼67 ka) lava flows, – and from one Holocene basalt, the Bonito Lava Flow (∼1.4 ka) at Sunset Crater. These data indicate that, in two of three cases, ⁴⁰Ar/³⁹Ar and K–Ar ages of the young basalts agree well with cosmic-ray surface exposure ages of the same lava flow, thus suggesting that excess ⁴⁰Ar is not always a problem in young basalt flows in the San Francisco volcanic field. The exposure age of the Bonito lava flow agrees within uncertainty with dendrochronological and archeological age determinations. K–Ar and cosmogenic ³He and ²¹Ne ages from the SP flow are in agreement and much older than the OSL age (5.5–6 ka) reported for this lava flow. Furthermore, if the non-cosmogenic ages are assumed to be accurate, the subsequent calculated production rates at South Sheba and SP flow sample sites agree well with values in the literature.     

Cosmogenic 21Ne analysis of individual detrital grains: Opportunities and limitations

We use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper Gaub River catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic ²¹Ne (²¹Ne_c) concentrations in individual detrital grains. The numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total ²¹Ne_c concentration that each grain acquires along its pathway. The model fully accounts for variations in nuclide production due to changes in latitude, altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates. Model results show that the form of the frequency distribution of ²¹Ne_c concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment's source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment. The results also show that lithology can affect the form of the ²¹Ne_c concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment. Model results further indicate that the form of the distribution of ²¹Ne_c concentrations in the exported sediment can also be affected by the acquisition of ²¹Ne_c after detachment from bedrock, in the diffusive (hillslope) and/or advective (fluvial) domains. However, for such post‐detachment nuclide acquisition to be important, this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock. Copyright © 2009 John Wiley and Sons, Ltd.     

Interlaboratory comparison of cosmogenic 21Ne in quartz

We performed an interlaboratory comparison study with the aim to determine the accuracy of cosmogenic ²¹Ne measurements in quartz. CREU-1 is a natural quartz standard prepared from amalgamated vein clasts which were crushed, thoroughly mixed, and sieved into 125–250 μm and 250–500 μm size fractions. 50 aliquots of CREU-1 were analyzed by five laboratories employing six different noble gas mass spectrometers. The released gas contained a mixture of 16–30% atmospheric and 70–84% non-atmospheric (predominantly cosmogenic) ²¹Ne, defining a linear array on the ²²Ne/²⁰Ne-²¹Ne/²⁰Ne three isotope diagram with a slope of 1.108 ± 0.014. The internal reproducibility of the measurements is in good agreement with the formal analytical precision for all participating labs. The external reproducibility of the ²¹Ne concentrations between labs, however, is significantly overdispersed with respect to the reported analytical precision. We report an average reference concentration for CREU-1 of 348 ± 10 × 10⁶ at [²¹Ne]/g[SiO₂], and suggest that the 7.1% (2σ) overdispersion of our measurements may be representative of the current accuracy of cosmogenic ²¹Ne in quartz. CREU-1 was tied to CRONUS-A, which is a second reference material prepared from a sample of Antarctic sandstone. We propose a reference value of 320 ± 11 × 10⁶ at/g for CRONUS-A. The CREU-1 and CRONUS-A intercalibration materials may be used to improve the consistency of cosmogenic ²¹Ne to the level of the analytical precision.     

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).     

Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago,Indian Ocean

The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable ⁴He / ³He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / R_a ∼8 to 18) and they show primitive neon isotopic ratios (average ²¹Ne / ²¹Ne_ext ∼0.044). The neon isotopic systematics and the ⁴He / ³He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar ³He / ²²Ne and with a higher (U, Th) / ³He ratio compared to typically high R / R_a hotspot basalts such as those from Iceland and Hawaii.     

Inert gases in twelve particles and one “dust” sample from Luna 16

The inert gases were measured mass-spectrometrically in 12 fragments and 1 “dust” sample from Luna 16. The fragments were classified petrologically by microscopic inspection. The major petrologic types were breccias and basalts. The former were much richer in trapped gases than the latter, and were apparently formed by the welding of local fines. However, there was no clear-cut difference in gas content of either breccias or basalts between zone A (top) and zone G (bottom). The⁴He/²⁰Ne ratio of the breccias (average 49) was systematically smaller than that of the basalts (average 78), probably because of He-Ne fractionation during or after the formation of the breccias. We suggest that the⁴He/²⁰Ne ratios of bulk fines in general may reflect the proportions of basaltic and breccia (plus cindery glasses) fragments in the fines. Substantial variations of⁴He/³He were found, which could not be explained with the presence of variable proportions of cosmogenic³He_c. Either the solar-wind value has changed in time, or the fragments with the small ratios were exposed to solar flares rich in³He and/or⁴He. Exposure ages of four fragments are several hundred million years. The⁴⁰Ar/³⁶Ar slopes of breccias and basalts are identical: 0.65.     

Inter-comparison of cosmogenic in-situ He, Ne and Cl at low latitude along an altitude transect on the SE slope of Kilimanjaro volcano (3°S, Tanzania)

Because the intensity and energy spectrum of the cosmic ray flux are affected by atmospheric depth and geomagnetic-field strength, cosmogenic nuclide production rates increase considerably with altitude and to a lesser degree with latitude. The scaling methods used to account for spatial variability in production rates assume that all cosmogenic nuclides have the same altitude dependence. In this study we evaluate whether the production rates of cosmogenic ³⁶Cl, ³He and ²¹Ne change differently with altitude, which is plausible due to the different threshold energies of their production reactions. If so, nuclide-specific scaling factors would be required.Concentrations of the three cosmogenic nuclides were determined in mafic phenocrysts over an altitude transect between 1000 and 4300 m at Kilimanjaro volcano (3°S). Altitude dependence of relative production rates was assessed in two ways: by determination of concentration ratios and by calculation of apparent exposure age ratios for all nuclide pairs. The latter accounts for characteristics of ³⁶Cl that the stable nuclides ³He and ²¹Ne do not possess (radioactive decay, high sensitivity to mineral composition and significant contributions from production reactions other than spallation). All ratios overlap within error over the entire transect, and altitudinal variation in relative production rates is not therefore evident. This suggests that nuclide-specific scaling factors are not required for the studied nuclides at this low-latitude location. However, because previous studies have documented anomalous altitude-dependent variations in ³He production at mid-latitude sites, the effect of latitude on cross-calibrations should be further evaluated.We determined cosmogenic ²¹Ne/³He concentration ratios of 0.1864 ± 0.0085 in pyroxenes and 0.377 ± 0.018 in olivines, agreeing with those reported in previous studies.Despite the absence of independently determined ages for the studied lava surfaces, the consistency in the dataset should enable progress to be made in the determination of the production rates of all three nuclides as soon as the production rate of one of the nuclides has been accurately defined.To our knowledge this is the first time that ³⁶Cl has been measured in pyroxene. The Cl extraction method was validated by measuring ³⁶Cl in co-existing plagioclase phenocrysts in one of the samples.     

Cosmogenic neon produced from sodium in meteoritic minerals

Cosmogenic neon in sodium-rich oligoclase feldspar from the ordinary chondrites St. Severin and Guaren?a is characterized by an unusually high²²Ne/²¹Ne = 1.50 ± 0.02. This high ratio is due to the cosmogenic²²Ne/²¹Ne production ratio in sodium which is 2.9 ± 0.3, two to three times the production ratio in any other target element. The relative production rate of²¹Ne per gram sodium is one quarter the production rate per gram magnesium. The striking enrichment of²²Ne relative to²¹Ne in sodium arises from enhanced indirect production from²³Na via²²Na.The unusual composition of cosmogenic neon in sodium and sodium-rich minerals explains the high²²Ne/²¹Ne ratios observed in inclusions of the Allende carbonaceous chondrite, and observed during low-temperature extraction of neon from ordinary chondrites. The isotopic composition of cosmogenic neon released during the stepwise heating of a trapped gas-rich meteorite containing sodium-rich phases can be expected to vary, and use of a constant cosmogenic neon composition to derive the composition of the trapped gas may not be justified. Preferential loss of this²²Ne-enriched cosmogenic neon from meteoritic feldspar can result in a 2–3% drop in the measured cosmogenic²²Ne/²¹Ne ratio in a bulk meteorite sample. This apparent change in composition can lead to overestimation of the minimum pre-atmospheric mass of the meteorite by a factor of two.     

Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds

Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as⁴He, ⁴⁰Ar, ²¹Ne_excess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. ²⁰Ne/²²Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. ²¹Ne/²²Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic ²¹Ne from ¹⁸O(α, n)²¹Ne and ²⁴Mg(n, α)²¹Ne reactions. The difference in ²⁰Ne/²²Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earth's history. ³⁸Ar/³⁶Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for ²³⁸U- or ²⁴⁴Pu-fission Xe, the ^131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (^124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained.In a ¹³⁶Xe/¹³⁰Xe−¹²⁹Xe/¹³⁰Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORB's appears to suggest that the progenitor of the excess^131–136Xe is ²⁴⁴Pu, but not²³⁸U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.     

An important source ofHe (andHe) in diamonds

A large data base has recently accumulated on the concentrations of helium isotopes in diamonds mined from various regions. It was noted earlier (Ozima et al. (1985) [1]; Lal et al. (1989) [2]) that the frequency distribution of the⁴He concentrations is a fairly narrow one, whereas that of³He concentrations is a broad one with no pronounced peaks. The ratios ³He/⁴He, on the other hand show a broad maximum around 2 R_a (R_a equals atmospheric ³He/⁴He ratio, = 1.40 × 10⁻⁶) with a slow decrease over two orders of magnitude on either side. Does this imply that the diamonds sample a wide variety of helium reservoirs having a range of ³He/⁴He ratios but somehow attain similar⁴He concentrations? We propose that in a majority of the diamonds studied,⁴He is primarily due to implantation of radiogenic alpha particles from the host material after emplacement in the crust, usually kimberlite, and that the concentrations of⁴He in diamonds often get appreciably altered by this process. Thus the⁴He trapped in the diamond at the time of its crystallization is usually overwhelmed by the implanted helium and the measured ³He/⁴He ratios do not generally correspond to any “sources” in the mantle. However, the implanted⁴He resides in the outer 16 μm of the diamond, and the intrinsic⁴He and ³He/⁴He ratios in the diamond can be studied if its outer layers are removed.The wider implications of diamond being the “target” material for nuclear reaction products from the host material are discussed. Radiogenic³He produced in the host material is also implanted in the diamond, but this contribution is small on a gross basis. However, since the depth of implantation of³He is greater than that of⁴He, some of the very high ³He/⁴He ratios observed in diamonds could be due to the “implantation” of radiogenic³He. The radiogenic reactions in the host material can also contribute to appreciable²¹Ne in diamonds.     

The Antarctic meteorite Yamato 74123 — a new ureilite

The rare gases He, Ne, Ar, Kr and Xe were measured in bulk samples of Yamato 74123. The ³He and ²¹Ne exposure ages are found to be 5.50 Ma and 2.83 Ma, respectively. In addition to the cosmogenic component the samples contain primordial rare gases of the fractionated type in amounts typical of ureilites. In a three-isotope plot neon turns out to be a mixture of planetary neon and cosmogenic neon.The elements Na, Mg, Al, Si, P, S, K, Ca, Cr, Mn, Fe, Co, and Ni have been determined by spark source mass spectrometry in Yamato 74123 and for comparison in the ureilites Haveröand Kenna. The chemical composition as well as the noble gas abundance pattern identify Yamato 74123 as an ureilite.     

Crustal neon: a striking uniformity

By combining data from a diverse suite of crustal fluid samples representing a broad geographical distribution, we have identified a well-defined nucleogenic (crustal) neon component. The neon is produced from (α, n) and (n, α) nuclear interactions involving nuclei of O, Mg, and F [1]. In the limiting case of ²⁰Ne/²²Ne = 0, the composition is: ²¹Ne/²²Ne = 0.47 ± 0.01 and ²¹Ne/⁴He = (0.46 ± 0.08) × 10⁻⁷. A crustal O/F ratio of 110 (atomic) calculated from the ²¹Ne/²²Ne ratio is 4–10 times less than the average crustal O/F ratio. The discrepancy can be accounted for by an enhanced O/F ratio within the 10–40 μm range of the U-Th-generated α-particles.     

Dating Holocene lavas on Stromboli,Italy using cosmogenic He

In-situ cosmogenic ³He exposure ages of pyroxene phenocrysts from basalts from the Upper Neostromboli formation in southwest Stromboli date its eruption at 7.0 ± 0.3 ka (1σ, n = 3, Ginostra site) and 6.8 ± 0.2 ka (1σ, n = 10, Timpone del Fuoco site) respectively. Correlation of our new data to previous K/Ar and palaeomagnetic ages from the northwestern Neostromboli phase suggests that it erupted within a confined period between roughly 6 and 14 ka. The low uncertainty on the ³He_cos ages as well as on individual exposure ages (4.4–8.7%) demonstrates that ³He_cos exposure dating is a viable tool for dating Holocene basalt lavas. The ages compare favourably to uncertainties obtained for radiocarbon dating of similar rocks.     

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including ³He/⁴He, ⁴⁰Ar/³⁶Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount.Glass and coexisting olivine crystals have similar ³He/⁴He ratios (2.8–3.4) × 10^?5, 20 to 24 times the atmospheric ratio (R_A), but different ⁴⁰Ar/³⁶Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not.The apparent high ³He/⁴He ratio (3 × 10^?5; = 21 R_A) coupled with a relatively high ⁴⁰Ar/³⁶Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma.Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component.Excess ¹²⁹Xe has not been observed due to apparent large mass fractionation among Xe isotopes.     

Al andBe production in iron meteorites

We have measured by accelerator mass spectrometry the²⁶Al contents of 20 and the¹⁰Be contents of 14 iron meteorites. The²⁶Al contents are typically 30% or more lower than values obtained by counting techniques; the¹⁰Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the⁴He/²¹Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP₂₆(Fe) as 3–4 dpm/kg andP₁₀(Fe) as 4–5 dpm/kg. The average P/₁₀P₂₆ activity ratio is close to 1.5. Exposure ages calculated from²¹Ne/²⁶Al ratios cannot be calibrated so as to agree with both⁴⁰KK/ ages and ages based on the shorter-lived nuclides³⁹Ar and³⁶Cl. If agreement with the latter is forced, then the disagreement with⁴⁰KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10⁷ a.     

Toward the feldspar alternative for cosmogenic 10Be applications

The possibility of quantifying surface processes in mafic or volcanic environment using the potentialities offered by the in situ-produced cosmogenic nuclides, and more specifically by the in situ-produced ¹⁰Be, is often hampered by the rarity of quartz minerals in the available lithologies. As an alternative to overcome this difficulty, we explore in this work the possibility of relying on feldspar minerals rather that on quartz to perform in situ-produced ¹⁰Be measurements in such environments. Our strategy was to cross-calibrate the total production rate of ¹⁰Be in feldspar (P_10fsp) against the total production rate of ³He in pyroxene (P_3px) by measuring ³He and ¹⁰Be in cogenetic pyroxene (³He_px) and feldspar (¹⁰Be_fsp). The samples were collected from eight ignimbritic boulders, exposed from ca 120 to 600 ka at elevations ranging from 800 to 2500 m, along the preserved rock-avalanche deposits of the giant Caquilluco landslide (18°S, 70°W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22°S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of ³He_px - ¹⁰Be_fsp total production ratios whose weighted-mean is 35.6 ± 0.5 (1σ). The obtained weighted-mean ³He_px - ¹⁰Be_fsp total production ratio combined with the local ³He_py total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL ¹⁰Be in situ-production rate in feldspar mineral (P_10fsp) of 3.57 ± 0.21 at.g⁻¹.yr⁻¹ (scaled for the LSD scaling scheme, the ERA40 atm model and the VDM of Lifton, 2016).Despite the large elevation range covered by the whole dataset (800–4300 m), no significant variation of the ³He_px - ¹⁰Be_fsp total production ratios in pyroxene and feldspar was evidenced. As an attempt to investigate the effect of the chemical composition of feldspar on the total ¹⁰Be production rate, major and trace element concentrations of the studied feldspar samples were analyzed. Unfortunately, giving the low compositional variability of our dataset, this issue is still pending.