Cosmic ray exposure ages of chondrites,pre-irradiation and constancy of cosmic ray flux in the past

A systematic calibration of the production rate of one specific cosmic-ray-produced nuclide in chondrites, that of²¹Ne, was achieved by using four independent methods:P₂₁(1.11) = 0.507 ± 0.039, 0.302 ± 0.013, 0.312 ± 0.017and0.292 ± 0.019 (in units of 10^?8 cm³ STP/g My) based on²⁶Al-age,⁵³Mn-age,⁸¹Kr-⁸³Kr and²²Na-²²Ne methods, respectively. These production rates are all normalized to a shielding parameter ratio²²Ne/²¹Ne= 1.11 and to the chemical composition of L chondrites. The results obtained by the latter three methods are in good agreement, but they disagree in a systematic way with the²⁶Al-age calibration. Based on these results, we recommend a valueP₂₁(1.11) = 0.31 and a production rate equation:P₂₁ = 4.845 P₂₁ (1.11) F[21.77(²²Ne/²¹Ne) ? 19.32]^?, whereF = 1.00 for L and LL, andF = 0.93 for H chondrites, for the calculation of cosmic ray exposure ages on the basis of Ne concentrations. In an attempt to assess possible causes for this discrepancy, we discuss the²⁶Al half-life measurements, we evaluate effects resulting from pre-irradiation of meteorites, and we discuss the evidence regarding the constancy of the cosmic ray flux in the past, in the light of some recent astronomical observations.     

Cosmic-ray exposure age of Martian meteorite GRV 99027

We have determined the concentrations of 10Be and 26Al in GRV 99027 recovered by the 16th Chinese Antarctic expedition team, which are 14.1 ± 0.6 dpm/kg and 67.5 ± 3.4 dpm/kg, respectively. From the concentration of 10Be, we calculate a cosmic-ray exposure age of 4.4 ± 0.6 Ma for GRV 99027. The concentration of 26Al is too high compared to the 10Be exposure age, indicating extra production from solar ray. The exposure ages, petrologic and geochemical characteristics of mantle-derived Martian meteorites GRV 99027, LEW 88516, Y-793605, NWA 1950 and ALHA77005 are very similar, suggesting that these meteorites most probably were ejected from Mars in the same impact event.     

26Al in iron meteorites and the constancy of cosmic ray intensity in the past

Cosmic-ray-produced²⁶Al in iron meteorites has been measured by low-level γγ-coincidence counting. The²⁶Al activities, in dpm/kg, are: Aroos3.0 ± 1.0, Braunau2.6 ± 0.5. Kayakent4.6 ± 1.5, N'Goureyma4.4 ± 1.1, Okahandja3.6 ± 0.9, Treysa4.0 ± 0.5. Exposure ages based on²⁶Al/²¹Ne are in agreement, within experimental error(±20%), with those based on³⁶Cl/³⁶Ar and³⁹Ar/³⁸Ar but the ages based on⁴⁰K/⁴¹K are higher by about 50%. The difference in exposure ages is probably caused by a real change of the cosmic ray intensity in the inner solar system.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

The reaction Mg(n,α)Ne at 14.1 and 14.7 MeV: cross sections and implications for meteorites

Mass spectrometric analyses of neutron-irradiated targets of natural magnesium yield cross sections of59 ± 14,160 ± 8, and11.0 ± 3.3mb for²⁰Ne,²¹Ne, and²²Ne, respectively, at 14.1 MeV and of94 ± 8,152 ± 12, and13.0 ± 2.0mb at 14.7 MeV. With the incorporation of these cross sections, calculations modeling cosmic-ray interactions in stony meteoroids of radii 20 and 26 cm predict that between the surface and center the²²Ne/²¹Ne ratio falls more than 10% while the²¹Ne production rate rises about 30%. The reaction²⁴Mg(n,α)²¹Ne predominantly controls these trends: the²²Ne/²¹Ne ratio due to magnesium decreases over 15% while that due to silicon remains constant with increasing depth. The calculations are compared with published neon measurements for the Keyes and St. Séverin meteorites.     

Neon measurements of individual Greenland sediment particles: proof of an extraterrestrial origin and comparison with EDX and morphological analyses

The work reported here confirms the existence of (largely) unmelted micrometeorites in sediments from seasonal lakes on the Greenland ice sheet. Approximately1/3 of the dark and irregularly shaped particles selected from disaggregated sediments had sufficient contents of Ne to permit measurement of isotopic compositions. The sediments consisted of a mixture of biologically derived contaminants and minor amounts of terrestrial minerals and extraterrestrial fragments. Although terrestrial mineral contamination varied by a factor of 20 for the two different sediments, a yield of 1000 cosmic particles/kg of wet cryoconite was found for both samples. The isotopic compositions of the Ne in both rounded and irregular (25% of total) particles that contained enough gas to measure were non-terrestrial, plotting in a field defined by solar wind (SW), solar energetic particle (SEP) and spallation. Exposure ages ranged from < 0.5 Ma to 20 Ma, consistent with previous estimates from¹⁰Be and²⁶Al measurements of larger spherules removed from similar sediments. Comparison of the Ne data with compositional-morphological data demonstrates the general validity of criteria previously used to identify extraterrestrial particles; however, two particles identified as of probable terrestrial origin by three independent, experienced meteorite petrologists were shown by the Ne data to be extraterrestrial and probably extraterrestrial, respectively. Future studies of Greenland micrometeorites are outlined and improved collection techniques are discussed.     

Noble gases,KrKr ages, andBe of chrondrites from China

A comprehensive study of the cosmic-ray exposure history of five ordinary chondrites from China was carried out using measurements of the noble gas isotopic abundances and¹⁰Be concentrations. The following average cosmic-ray exposure ages, based on cosmogenic²¹Ne and on⁸¹KrKr dating were obtained: Zhaodong (L4) — 15.7 ± 3.0 m.y., Nan Yang Pao (L6) — 48 ± 10.0 m.y., Guangrao (L6) — 16.8 ± 3.5 m.y., and Lunan (H6) — 26.7 ± 5.0 m.y. The H5 chondrite Zaoyang was exposed for only 0.90 ± 0.12 m.y. to galactic cosmic rays as calculated from the¹⁰Be activity and from the low amounts of cosmic-ray-produced noble gases. The Zhaodong chondrite contains large amounts of⁸⁰Kr and⁸²Kr produced by neutron capture of bromine. From the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite.     

Effects of long soil surface residence times on apparent cosmogenic nuclide denudation rates and burial ages in the Cradle of Humankind,South Africa

In situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil-bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10-Beryllium (¹⁰Be) in river sediments to estimate denudation rates and the ratio of 26-Aluminium (²⁶Al) to ¹⁰Be (²⁶Al/¹⁰Be), to constrain ages of sediment burial. Here, we use ¹⁰Be and ²⁶Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent ¹⁰Be-derived denudation rates obtained from pebble- to cobble-sized clasts and coarse-sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2-3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average ²⁶Al/¹⁰Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the ¹⁰Be concentrations accumulated in soils during the long near-surface residence times can potentially cause underestimation of single-nuclide (¹⁰Be) catchment-wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand-size quartz grains could be overestimated if a pre-burial ²⁶Al/¹⁰Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd.     

The unique cosmic-ray history of the Malakal chondrite

Cosmogenic radionuclides, including²²Na,²⁶Al and⁵⁴Mn, were measured in a sample of the recently-fallen Malakal hypersthene chondrite. The high²⁶Al activity, 79 ± 2 dpm/kg, greatly exceeds the levels expected from elemental production rates, shielding considerations, or comparisons with other ordinary chondrites, and can only be explained by exposure to a uniquely high cosmic-ray flux. Calculations including noble gas,³H, and⁵³Mn data from other laboratories require a two-stage irradiation. Malakal's most probable history is: exposure in excess of 4 m.y. to an effective cosmic-ray flux approximately three times that experienced by other chondrites, an orbit change (very possibly caused by a collision), and a final period of about 2 m.y. during which it was exposed to a “normal” cosmic-ray flux.     

The origin ofBe in island-arc volcanic rocks

The presence of cosmogenic¹⁰Be (t_1/2 = 1.5 × 10⁶ y) in island-arc volcanic rocks has been interpreted as indicating that sediments are subducted to the depths of island-arc magma genesis [1,2]. We have measured¹⁰Be and⁹Be (the stable common isotope) in phenocryst and groundmass fractions separated from four Aleutian volcanic rocks.¹⁰Be and⁹Be covary linearly in three of the rocks proving that the¹⁰Be was incorporated prior to the formation of phenocrysts. In the other rock¹⁰Be and⁹Be do not covary; the lack of covariation indicates either that the rock incorporated¹⁰Be during weathering or that⁹Be-rich xenocrysts were incorporated by mechanical mixing. Our results strongly support the hypothesis of Brown et al. [1] and Tera et al. [2] that sediments containing¹⁰Be are an integral part of island-arc magmatic systems. As we cannot identify the depth at which the¹⁰Be was incorporated we still cannot completely rule out the possibility of assimilation of sediments or altered crust near the Earth's surface as a source of¹⁰Be.     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.     

Cosmic-ray-produced53Mn in thirty-one meteorites

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.     

Al–Mg and Pb–Pb systematics of Allende CAIs: Canonical solar initial Al/Al ratio reinstated

The precise knowledge of the initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of ²⁷Al/²⁴Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined ²⁶Al–²⁶Mg fossil isochron with an (²⁶Al/²⁷Al)₀ of (5.23 ± 0.13) × 10^− 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (²⁶Al/²⁷Al)₀ = (4.96 ± 0.25) × 10^− 5, anchored to our precisely determined absolute ²⁰⁷Pb–²⁰⁶Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” (²⁶Al/²⁷Al)₀ of 5 × 10^− 5 for the early Solar System. The uncertainty in (²⁶Al/²⁷Al)₀ corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the ²⁶Al–²⁶Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” ²⁶Al/²⁷Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical ²⁶Al/²⁷Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS.The radiogenic ²⁰⁸Pb/²⁰⁶Pb ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated ²³²Th/²³⁸U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.     

Cosmic-ray-produced36Cl and53Mn in Allan Hills-77 meteorites

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been determined by neutron activation in nine Allan Hills-77 meteorites. Additionally,³⁶Cl (_1/2 = 3.0 × 10⁵years) has been measured in seven of these objects using tandem accelerator mass spectrometry. These results, along with¹⁴C (t_1/2 = 5740years) and²⁶Al (7.2 × 10⁵ years) concentrations determined elsewhere, yield terrestrial ages ranging from 0.1 × 10⁵ to 7 × 10⁵ years. Weathering was not found to result in⁵³Mn loss.     

Cosmogenic21Ne and22Ne depth profiles in chondrites

The production rate profiles of²¹Ne and²²Ne as a function of depth in meteoroids due to spallation by solar flare cosmic rays (SCR) and galactic cosmic rays (GCR) are calculated and their dependence on size and composition of meteoroids has been evaluated. The GCR production rate at a given depth increases with size for radii<25cm and then decreases whereas the²²Ne²¹Ne ratio (NeR) generally decreases with size and depth. The calculated GCR production rates and NeR are consistent with the measurements in several Chondrites. A plot of track production rate vs. NeR shows that some chondrites have NeR values smaller than those expected for their sizes. Thes obeervation suggestsat least a two-stage irradiation for such meteorites; the meteoroid exposure as a small body in the interplanetary space must have been preceded by exposure under deep shielding, possibly in its parent body.     

Meteoritic and bedrock constraints on the glacial history of Frontier Mountain in northern Victoria Land, Antarctica

In 2001, a small H4 chondrite, Frontier Mountain (FRO) 01149, was found on a glacially eroded surface near the top of Frontier Mountain, Antarctica, about 600 m above the present ice level. The metal and sulphides are almost completely oxidized due to terrestrial weathering. We used a chemical leaching procedure to remove weathering products, which contained atmospheric ¹⁰Be and ³⁶Cl in a ratio similar to that found in Antarctic ice. The FRO 01149 meteorite has a terrestrial age of 3.0 ± 0.3 Myr based on the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al and ³⁶Cl. This age implies that FRO 01149 is the oldest stony meteorite (fossil meteorites excluded) discovered on Earth. The noble gas cosmic ray exposure age of FRO 01149 is ~ 30 Myr. The meteorite thus belongs to the 33 Myr exposure age peak of H-chondrites.The bedrock surface on which FRO 01149 was found has wet-based glacial erosional features recording a former high-stand of the East Antarctic ice sheet. This ice sheet evidently overrode the highest peaks (> 2800 m a.s.l.) of the inland sector of the Transantarctic Mountains in northern Victoria Land. We argue that FRO 01149 was a local fall and that its survival on a glacially eroded bedrock surface constrains the age of the last overriding event to be older than ~ 3 Myr. The concentrations of in-situ produced cosmogenic ¹⁰Be, ²⁶Al and ²¹Ne in a glacially eroded bedrock sample taken from near the summit of Frontier Mountain yield a surface exposure age of 4.4 Myr and indicate that the bedrock was covered by several meters of snow. The exposure age is also consistent with bedrock exposure ages of other summit plateaus in northern Victoria Land.     

14C-39ArMe correlations in chondrites and their pre-atmospheric size

Cosmogenic¹⁴C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of¹⁴C.In eight cases³⁹Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR₀ of the meteoroids and depth of burial of the samples investigated. Values ofR₀ between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published¹⁴C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.     

CRONUS-Earth calibration samples from the Huancané II moraines,Quelccaya Ice Cap,Peru

The Huancané II moraines deposited by the Quelccaya Ice Cap in southern Peru were selected by the CRONUS-Earth Project as a primary site for evaluating cosmogenic-nuclide scaling methods and for calibrating production rates. The CRONUS-Earth Project is an effort to improve the state of the art for applications of cosmogenic nuclides to earth-surface chronology and processes. The Huancané II moraines are situated in the southern Peruvian Andes at about 4850 m and ∼13.9°S, 70.9°W. They are favorable for cosmogenic-nuclide calibration because of their low-latitude and high-elevation setting, because their age is very well constrained to 12.3 ± 0.1 ka by 34 radiocarbon ages on peat bracketing the moraines, and because boulder coverage by snow or soil is thought to be very unlikely. However, boulder-surface erosion by granular disintegration is observed and a ∼4% correction was applied to measured concentrations to compensate. Samples from 10 boulders were analyzed for ¹⁰Be, ²⁶Al, and ³⁶Cl. Interlaboratory bias at the ∼5% level was the largest contributor to variability of the ¹⁰Be samples, which were prepared by three laboratories (the other two nuclides were only prepared by one laboratory). Other than this issue, variability for all three nuclides was very low, with standard deviations of the analyses only slightly larger than the analytical uncertainties. The site production rates (corrected for topographic shielding, erosion, and radionuclide decay) at the mean site elevation of 4857 m were 45.5 ± 1.6 atoms ¹⁰Be (g quartz)⁻¹ yr⁻¹, 303 ± 15 atoms ²⁶Al (g quartz)⁻¹ yr⁻¹, and 1690 ± 100 atoms ³⁶Cl (g K)⁻¹ yr⁻¹. The nuclide data from this site, along with data from other primary sites, were used to calibrate the production rates of these three nuclides using seven global scaling methods. The traditional Lal formulation and the new Lifton-Sato-Dunai calibrations yield average ages for the Huancané samples that are in excellent-to-good agreement with the radiocarbon age control (within 0.7% for ¹⁰Be and ³⁶Cl and 6% for ²⁶Al). However, all of the neutron-monitor-based methods yielded ages that were too young by about 20%. The nuclide production ratios at this site are 6.74 ± 0.34 for ²⁶Al/¹⁰Be in quartz and 37.8 ± 2.3 (atoms ³⁶Cl (g K)⁻¹) (atom ¹⁰Be (g SiO₂)⁻¹)⁻¹ for ³⁶Cl/¹⁰Be, in sanidine and quartz, respectively.     

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO₂-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides¹³⁷Cs,^239,240Pu and²⁴¹Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and²⁴¹Am are supplied to the deep water by advection.The depth distributions of the naturally occurring radionuclides²³²Th,²²⁸Th and²³⁰Th reflect their sources to the oceans.²³²Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of²³²Th decrease with depth to values 0.01 dpm/1000 l.²²⁸Th shows high activities in near surface and near bottom water, due to the distribution of its parent,²²⁸Ra. Dissolved²³⁰Th, produced throughout the water column from²³⁴U decay, increases with depth to 3000 m. Values in the deep water (> 3000 m) are nearly constant ( 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and²⁴¹Am data.The ratio of particulate to dissolved activity for both²³⁰Th and²²⁸Th is 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths²²⁸Th tends to have a higher particulate/dissolved ratio than²³⁰Th, suggesting uptake and release of²³⁰Th and²²⁸Th by different processes. This could occur if²²⁸Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking.²³⁰Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths.²³⁰Th,²⁴¹Am and^239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with 15% of the²³⁰Th on particles compared with 7% for Am and 1% for Pu. Distribution coefficients (K_d) are 1.3–1.6 × 10⁷ for Th, 5–6 × 10⁶ for Am and 7–10 × 10⁵ for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show 85% oxidized (V + VI) Pu. However, theK_d value for Pu may be an upper limit because Pu, like²²⁸Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.     

Cosmogenic and radiogenic noble gases in the Dhajala chondrite

Whole rock and chondrules of the Dhajala chondrite were analyzed for Ne, Ar, Kr and Xe by total melting as well as by stepwise heating techniques. The cosmic ray exposure ages for the whole rock and the chondrules are6.2 ± 0.8 and6.3 ± 1.0m.y. as determined by the²¹Ne method and4.8 ± 1.5 and4.2 ± 2.0m.y. by the³⁸Ar method, respectively. The K-Ar age of the whole rock is4.2 ± 0.4b.y. The elemental composition of the trapped gas in this chondrite is of “planetary” type. The radiogenic¹²⁹Xe contents in the whole rock and chondrules are similar and this component is very retentively sited in the chondrules.