A study on exposure ages of chondrites based on spallogenic 53Mn

Longlived spallogenic ⁵³Mn was determined in 36 individual chondrites. Combining our data with those available so far in literature (in total 91 ⁵³Mn-values of 61 individual stones) a ⁵³Mnproduction rate of 478 ± 84dpm/kg Fe was derived. Based on this figure it was possible to calculate ⁵³Mn-radiation ages for more than 30 stones with T_rad ≤ 10 × 10⁶ yr. A comparison of our results with the respective rare gas- and ²⁶Al-exposure ages (so far available) shows that in general a good agreement exists between ages calculated by these three different methods. Only in the case of ‘finds’, e.g. in Bondoc and some other stones of probably high terrestrial ages. partial losses of ⁵³Mn seem to have occurred. For well preserved objects the ⁵³Mn method is ideal for the estimation of terrestrial residence times.     

The 21Ne production rate in stony meteorites estimated from 10Be and other radionuclides

The ¹⁰Be contents of 28 stony meteorites with known ²¹Ne contents range from 0.97 to 23 dpm/kg and give an average ²¹Ne production rate (P₂₁) of (0.28 ± 0.02) × 10⁸ cm³ STP/g-Myr for shielding conditions corresponding to ${}^{22}\text{Ne}{}^{21}\text{Ne}\text{= 1.114}$ in an H-chondrite. Our $\text{P}{}_{21}\text{(}{}^{10}\text{Be)}$ agrees with others' P₂₁ based on ²²Na, ⁸¹Kr and ⁵³Mn but not on ²⁶A1. Temporal variations in the cosmic ray flux do not explain the disagreement satisfactorily; major errors in the radionuclide half-lives are not indicated. The discrepancy seems rooted in the data selection and the difficulties of making accurate corrections for shielding, chemical composition and other sources of variability.     

Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for ⁵³Mn (t_1.2 = 3.7Myr), ^26Al(t_1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for ²⁶Al and ⁵³Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for ²⁶Al and ⁵³Mn to deduce the respective saturation values ²⁶Al₀ and ⁵³Mn₀ and to obtain ²¹Ne production rates (²¹P).²⁶Al and ⁵³Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of ²¹p based on ⁵³Mn and ²⁶Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity.     

The pore water chemistry of 239,240Pu and 137Cs in sediments of Buzzards Bay,Massachusetts

The collection of large volumes of pore water (1–2 liters per 2 cm horizon of sediment) and low level radiochemical measurements of ^239,240Pu and ¹³⁷Cs have been combined to produce the first study of these fallout artificial radionuclides in marine pore waters. Profiles from box cores taken in June and September 1982 from Buzzards Bay, Mass., are reported along with profiles of many diagenetic constituents (i.e. SO₄^2?, alkalinity, Fe, Mn, DOC, and nutrients).The ^239,240Pu pore water profile is characterized by a subsurface maximum of about 0.28 dpm/100 kg lying between 3–11 cm. Overlying seawater, in contrast, has an activity of $\text{0.01 ± 0.02}$ dpm/100 kg. Below about 11 cm, the pore water ^239,240Pu distribution follows that of the solid phase which decreases rapidly with depth. The pore water profiles of ¹³⁷Cs are characterized by a broad and deeply penetrating maximum where activities of about 35–40 dpm/100 kg extend from 3 to 20 cm. Overlying seawater, in contrast, has an activity of 17–24 dpm/100 kg. The ¹³⁷Cs and ^239,240Pu pore water data show that there is preferential downward transport of ¹³⁷Cs and that ^239,240Pu does not have an active diagenetic chemistry and is not significantly mobile in these coastal sediments.     

26Al and rare gases in Allende,Bereba and Juvinas: implications for production rates

The ²⁶Al, light rare gas and major and minor element contents of Al-rich and poor samples separated from Allende. Bereba and Junivas have been measured. The production rate of ²¹Ne from Al (²¹P_Al) is $\text{(1.9 ± 0.6) ×}{}^{21}\text{P}{}_{\mathrm{Si}}$ and ${}^{\mathrm{<mtext>22</mtext><mtext>21</mtext>}}\text{P}{}_{\mathrm{Al}}\text{= 1.4 ± 0.4.}$ The ³He, ²¹Ne and ³⁸Ar exposure ages of the eucritic pyroxenes agree suggesting complete cosmogenic gas retention. The eucritic feldspars have lost virtually all ³He and most radiogenic ⁴He. The equation ²⁶Al = 0.42 ± 0.41 Mg + 2.74 ± 0.21 Si + 4.92 ± 0.51 Al + 1.33 S + 0.24 Ca + 0.03 Fe reproduces within 15% our ²⁶Al measurements and the average values measured in ordinary chondrites without recourse to unusual cosmic-ray effects.     

210Pb activities in and fluxes to sediments of the Washington continental slope and shelf

Surface sediments of the Washington coast have ²¹⁰Pb activites which average 104 ± 48dpm/g for submarine canyon and slope regimes and 18 ± 12dpm/g for the continental shelf regime. ²¹⁰Pb sedimentary fluxes are also higher in canyons, averaging 18 ± 13dpm/cm² per yr, compared to 5.2 ± 3.1 dpm/cm² per yr for slope and 4.8 ± 1.8dpm/cm² per year for shelf regions. These ²¹⁰Pb activities and fluxes are 2–7 times greater than those reported for other coastal regions. Inputs from the atmosphere and the Columbia River are not sufficient to supply the ²¹⁰Pb, but advection of seawater containing dissolved ²¹⁰Pb produced in situ from ²²⁶Ra provides an input several times larger than the sedimentary fluxes. The sedimentary ²¹⁰Pb flux is limited by scavenging reactions rather than by supply of dissolved ²¹⁰Pb.Calculations of maximum biological uptake and fluxes of ²¹⁰Pb and ‘selective’ chemical leaching experiments all show that the primary scavenging processes are due to hydrous Mn and Fe oxides rather than biological phases. The pattern of higher ²¹⁰Pb depositional fluxes in canyons than in nearby open slope areas of comparable water depth is most reasonably explained by enhanced scavenging of dissolved ²¹⁰Pb near the sea floor, rather than by processes operating throughout the water column. Relatively rapid removal of dissolved ²¹⁰Pb from the near bottom nepheloid layer to slope and canyon sediments is shown by its mean residence time of less than two years in this layer.     

Effects of shock pressure on 40Ar -39Ar radiometric age determinations

The relation of shock to the drop in the ⁴⁰¹Ar/³⁹¹Ar ratio seen at high release temperatures in some neutron-irradiated lunar samples is investigated through measurements of the ⁴⁰¹Ar/³⁹¹Ar ratio in gas samples released by stepwise heating of rock samples previously subjected to shock, either in the laboratory or in nature.Explosives were used to shock solid pieces and powder of a basalt from a diabase dike in Liberia to calculated pressures of 65, 150 and 270 kbar. These, an unshocked sample of the powder, two naturally shocked samples from the Brent impact crater in Canada, one unshocked sample from near the crater, and appropriate monitors were irradiated. Ar from stepwise heating was analyzed.The unshocked basalt shows a good ⁴⁰¹Ar/³⁹¹Ar plateau at age 198 ± 9 m.y. in agreement with a previous result of 186 ± 2 m.y. The shocked samples contain varying amounts of implanted atmospheric Ar, the isotopes of which have experienced mass fractionation. This effect is small enough in four samples so that the linearity of their graphs of ³⁹¹Ar/⁴⁰Ar vs ³⁶Ar/⁴⁰Ar is evidence of a plateau. The ages of these samples are then 201 ± 10, 205 ± 11, 205 ± 12 and 201 ± 9 m.y. It appears that the shock has had little effect on the ⁴⁰Ar-³⁹Ar age spectrum, although the release patterns of the ³⁹¹Ar are shifted downward by the order of 200°C. Shock implantation of Ar was at lower shock pressure, in the presence of less Ar, and into a less porous material than previously demonstrated.The Brent Crater samples do not all show good plateaus, but do indicate an age of ~420 m.y. for the crater event and 795 ± 24 m.y. for the rock formation, in agreement with previous results.None of the ⁴⁰¹Ar/³⁹¹Ar profiles shows a drop at high temperature, but a possible role of shock implantation of Ar is indicated in the production of this effect. Further experiments are suggested.     

Laboratory studies of the diagenesis and mobility of 239,240pu and 137Cs in nearshore sediments

Controlled laboratory experiments have been used to study the diagenetic chemistry of ^239,240Pu ¹³⁷Cs, and ⁵⁵Fe. Experiments using Buzzards Bay sediments in small tanks show that sulfate reduction is accompanied by the production of large pore water concentration gradients of alkalinity, phosphate, ammonia and dissolved organic carbon and the formation of subsurface maxima in Fe and Mn. These pore water profiles demonstrate that bacterially-mediated processes of organic matter degradation and redox reactions can be simulated in the laboratory.A vertical profile of ⁵⁵Fe in pore waters is reported for the first time: it follows the profile of stable Fe and as such has a large (200 dpm/100 kg) subsurface maximum between 2–4 cm depth. Comparison of ⁵⁵Fe/Fe ratios in sediments and pore waters shows that there is preferential solubilization of ⁵⁵Fe over stable Fe.The pore water activities of ^239,240Pu show no gradients within the large uncertainties of the counting statistics, but are two to four times higher than Buzzards Bay seawater (0.05 dpm/100 kg).The activity of ¹³⁷Cs in the pore water profile is constant (40 dpm/100 kg) within the large counting uncertainties and is twice that of Buzzards Bay seawater. Cs-137 does not appear to be involved in diagenetic chemistry but may increase in pore waters as a result of ion exchange reactions.Flux estimates based on the pore water data show that remobilization and transport of ^239,240 Pu in coastal sediments are not significant processes while the transport of ^l37Cs may be.     

Implications of the carbonaceous chondrite Mn-Cr isochron for the formation of early refractory planetesimals and chondrules

An excellent ⁵³Mn-⁵³Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca-Al-rich inclusions (CAIs), which is considered to be the start of the solar system (t₀). However, carbonaceous chondrites were not assembled until at least 1.5-5 Myr after t₀, to judge by the ²⁰⁷Pb-²⁰⁶Pb and ²⁶Al-²⁶Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of ²⁶Al. Presumably, therefore, these meteorites inherited their bulk Mn-Cr isochron from precursor materials which experienced Mn-Cr fractionation at t₀. As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t₀ of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial ε⁵³Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debris-rich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes.     

暴露测年样品中26Al和10Be分离及其加速器质谱测定

在已有实验流程基础上,建立并优化了石英样品中Be和Al提取、纯化等实验流程,设计的流程条件实验包括实验试剂、器皿和离子交换柱选择、离子交换树脂分离Be和Al时酸浓度选择等。结果表明,选择钢铁研究总院研制的⁹Be标准溶液作为¹⁰Be样品制备的载体;使用一次性实验器皿;选用4 cm规格的离子交换柱;用0.05 mol/L草酸和0.75 mol/L盐酸混合溶液洗脱吸附于阴离子树脂上的Al,可有效提取、纯化样品中的Be和Al。加速器质谱(AMS)测量结果显示,13组化学空白的¹⁰Be/⁹Be和²⁶Al/²⁷Al比值平均值分别为7.48×10^-15和1.96×10^-15,与国内已有宇宙成因核素实验室的结果(5×10^-15~8×10^-15)具有可比性。电感耦合等离子体发射光谱(ICP-AES)的测量结果表明,Be、Al回收率分别达90%和60%。基于新建立的实验流程分析了祁连山北侧金佛寺的一个岩石样品,获得了¹⁰Be和²⁶Al的暴露年代分别为(10.7±1.0) ka和(10.0±1.2) ka,与前人研究结果一致。     

Noble gases in six ordinary chondrites: comparison of exposure ages from noble gases with 26Al ages

Noble gases were redetermined in six ordinary chondrites in order to clarify some discrepancies between cosmic-ray exposure ages from noble gas isotopes and ²⁶Al activity. Generally, samples used for noble-gas analyses were from the same specimen which earlier had been used for ²⁶Al determinations in this laboratory. The high ²¹Ne ages (> 3 × 10⁶ y) for Dimmitt, Menow and Pierceville are consistent with ²⁶A1 activity at saturation level. Variations in both ²¹Ne_c age and ²⁶Al activity, in earlier measurements on Dimmitt, were due to “shielding”; very low noble-gas ages in earlier analyses on Menow and Pierceville were due to mis-labelling of samples. Earlier observation of very low noble-gas ages for Seres, which is inconsistent with its saturation activity of ²⁶Al, is confirmed. Ladder Creek yields consistent ages both from noble gases and ²⁶Al. Malotas contains a substantial excess of ²¹Ne, as revealed by a higher ²¹Ne age, compared to its ²⁶Al age; the excess ²¹Ne is probably due to “pre-irradiation”. Isotopic compositions for trapped Kr and Xe in these chondrites, excepting Seres, are very similar to compositions determined earlier for ordinary chondrites. The Xe in Seres is enriched in light isotopes and is similar to solar Xe.     

The 57Fe Mössbauer parameters of pyrite and marcasite with different provenances

Eighteen pyrite and twelve marcasite samples which have different provenances have been investigated to determine the systematics of the influence of mineralogical and geological factors on the ⁵⁷Fe Mössbauer spectra at 298 K. The following results have been obtained: there is no ambiguity in distinguishing single phase pyrite from single phase marcasite by means of ⁵⁷Fe Mössbauer spectroscopy at 298 K. At 298 K the average electric quadrupole splitting, 〈ΔE_Q〉, and average isomer shift, 〈δ〉, with respect to Fe metal, are 0.6110 ± 0.0030 mm s^?1 and 0.313 ± 0.008 mm s^?1, respectively, for the 18 pyrites; 〈ΔE_Q〉 = 0.5030 ± 0.0070 mm s^?1 and 〈δ〉 = 0.2770 ± 0.0020 mm s^?1 for the 12 marcasites. At 77 K, ΔE_Q is 0.624 mm s^?1 for pyrite and 0.508 mm s^?1 for marcasite. In distinguishing pyrites from marcasites, spectra obtained at 77 K are not warranted.The Mössbauer parameters of pyrite and marcasite exhibit appreciable variations, which bear no simple relationship to the geological environment in which they occur but appear to be selectively influenced by impurities, especially arsenic, in the pyrite lattice. Quantitative and qualitative determinations of pyrite/marcasite mechanical mixtures are straightforward at 298 K and 77 K but do require least-squares computer fittings and are limited to accuracies ranging from ±5 to ±15 per cent by uncertainties in the parameter values of the pure phases. The methodology and results of this investigation are directly applicable to coals for which the presence and relative amounts of pyrite and marcasite could be of considerable genetic significance.     

Variability of the He3 and Ne21 production rates in ordinary chondrites

Al²⁶ and noble gas contents of 6 ordinary chondrites with $\text{He}{}^3\text{Ne}{}^{21}$ ratios above 6.0 or below 4.0 are used to infer the variability of the production rates of He³ and Ne²¹ (P_He³ and P_Ne²¹). The ratio of the observed Al²⁶ content to a calculated, normal value is taken as a measure of the change of P_Ne²¹ from its normal value. The corresponding change in P_He³ is then computed from the observed $\text{He}{}^3\text{Ne}{}^{21}$ ratio and an average value of P_He³.According to these calculations which exclude orbital effects, P_He³ will be near the average value in meteorites with high $\text{He}{}^3\text{Ne}{}^{21}$ ratios, while P_Ne²¹ will be about 30 per cent below normal. In meteorites with low $\text{He}{}^3\text{Ne}{}^{21}$ ratios, P_He³ may be depressed by as much as 25 per cent from normal while P_Ne²¹ may be 15–20 per cent above the average.     

40Ar-39Ar ages and trace element contents of Apollo 14 breccias; an interlaboratory cross-calibration of 40Ar-39Ar standards

Rare gas data are presented from step-wise heatings of lunar breccias 14066 and 14318 and from an interlaboratory cross-calibration of five standards used in ⁴⁰Ar-³⁹Ar dating. Four samples of 14066 all show depressed ⁴⁰¹Ar/³⁹¹Ar ratios at high temperatures, thus making age interpretation uncertain. While different in detail, the Ar release patterns in the four samples yield indistinguishable plateau ages of $\text{3.93 ± 0.03}$ b.y. and > 400°C total ages of $\text{3.87 ± 0.06}$ b.y. Concentrations of K, Ca, Ba, Br, U and I are given for 14318 and 14066. We also present an updating of all of the ⁴⁰Ar-³⁹Ar ages and trace element concentrations previously published by this laboratory.⁴⁰Ar-³⁹Ar dating standards from Menlo Park, Pasadena, Stony Brook, Toronto and Berkeley are calibrated against each other and the internal homogeneity of their ⁴⁰¹Ar/K ratios is tested. The Berkeley standard (from the St. Severin meteorite) has an age of $\text{4.504 ± 0.020}$ b.y. from this intercalibration.⁸⁰Kr from capture of lunar neutrons is detected in 14318. A comparison of the release pattern of the ⁸⁰Kr produced by lunar neutrons with the ^80,82Kr produced by pile neutrons in 14318, indicates that 14318 has lost approximately 35 per cent of the ⁸⁰Kr produced by lunar neutrons.     

Aluminum-26 in meteorites—VII. Ureilites,their unique radiation history

Cosmogenic ²⁶A1 activities have been measured by γ-γ coincidence counting in the three ureilites which had not previously been studied. The values in dpm/kg are: Dingo Pup Donga, 38.4 ± 2.4; North Haig, 39.3 ± 4.8; Dyalpur, 55.8 ± 4.8. Five of the six known ureilites thus have lower ²⁶A1 contents, 63 per cent to 77 per cent, than the calculated saturation values, in marked contrast to most other stony meteorites. This cannot be attributed to short cosmic ray exposure ages. Nor do size and depth effects account for the narrow range of ²⁶A1 activities, because a nuclear particle track study indicates that preatmospheric radii were highly variable, from ≥ 40 cm for Goalpara to only a few cm for Dingo Pup Donga. By default, the most likely explanation is that the ureilites had much smaller or much larger orbits than all other stony meteorites.     

Spallation production of 3He, 21Ne,and 38Ar from target elements in the Bruderheim chondrite

The concentrations of noble gas isotopes of He, Ne and Ar have been measured in eight mineral separates of the Bruderheim chondrite. The cosmic-ray-produced nuclides ²¹Ne and ³⁸Ar were correlated by a computer least-squares fitting program with the elemental composition in each separate of potential targets for nuclear production yielding the following production equations: $\text{[}{}^{21}\text{Ne, 10}{}^{\mathrm{?8}}\text{cm}{}^3\text{/g] = k(0.45[Mg] + 0.085[Si] + 0.060[S] + 0.017[Ca] + 0.0044[Fe + Ni])}$; $\text{[}{}^{38}\text{Ar, 10}{}^{\mathrm{?8}}\text{cm}{}^3\text{/g] = k(2.6[K] + 0.37[Ca] + 0.08[Ti + Cr + Mn] + 0.021[Fe + Ni])}$ with elemental concentrations in weight per cent and k equal to the reciprocal of the cosmic-ray exposure age of Bruderheim. The P(S)/P(Cr + Mn + Fe + Ni) weight production ratio for ³He was determined to be 1.53; relative productions of ³He from O, Mg and Si and ²¹Ne from Al proved to be incalculable.     

Mn-Cr systematics of carbonates in CM chondrites: Implications for the timing and duration of aqueous alteration

CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The ⁵³Cr excesses are correlated with the ⁵³Mn/⁵⁵Mn ratio and result from the in situ decay of ⁵³Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial ⁵³Mn/⁵⁵Mn ratios ((⁵³Mn/⁵⁵Mn)₀) of (4.1 ± 1.2) × 10⁻⁶ and (5.1 ± 1.7) × 10⁻⁶, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial ⁵³Mn/⁵⁵Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10⁻⁶ to (4.8 ± 2.1) × 10⁻⁶ for QUE 93005 and from (3.1 ± 1.6) × 10⁻⁶ to (1.3 ± 0.5) × 10⁻⁵ for ALH 83100. A possible interpretation for the ranges in (⁵³Mn/⁵⁵Mn)₀ could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.     

Manganese-chromium isotope systematics of enstatite meteorites

We present results of a study of the ⁵³Mn-⁵³Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide ⁵³Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial ⁵³Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic ⁵³Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of ⁵³Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial ⁵³Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic ⁵³Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of ⁵³Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic ⁵³Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ ⁵³Mn-⁵³Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the ¹²⁹I-¹²⁹Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the ¹²⁹I-¹²⁹Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years.     

The stability of UO2OH+ and UO2[HPO4]2−2 complexes at 25°C

The cumulative association constant (β₂) for the geochemically important aqueous complex UO₂[HPO₄]^2?₂ has been determined by potentiometric titration in Na₂HPO₄-UO₂(NO₃)₂ solutions in the pH range 3.9–4.7, at ionic strengths below 0.024 molal with the Newton-Raphson method used to compute β₂ from the chemical analytical data. Based on 25 measurements we obtain logβ₂ = 18.3 ± 0.2 at 25°C. From the same experiments we compute that the association constant of UO₂OH⁺ is 8.9 ± 0.1, in disagreement with the value of 8.3 ± 0.3 for this constant given by Baes and Mesmer (1976).     

The use of 210Pb as a heavy metal tracer in the Susquehanna River system

Analysis of soil profiles and shallow ground water in the Susquehanna River basin, northeastern U.S.A., indicates that the atmospheric flux of ²¹⁰Pb is efficiently scavenged by the organic-rich horizons of the soils. This atmospherically supplied ²¹⁰Pb in soil profiles can only be lost from the system by soil erosion. Based on the annual sediment yield of the Susquehanna River and the excess ²¹⁰Pb concentration in particulate matter, a mean residence time of 2000 yr is calculated for metals similar to Pb in soil profiles.The West Branch of the Susquehanna River (WBSR) is strongly affected by acid mine drainage and is low in pH and high in dissolved ( <0.4 μm) ²¹⁰Pb, Fe and Mn. Along its course iron hydroxide is precipitating at a pH of between 4 and 4.5 and the ²¹⁰Pb supplied by the acid mine water is diminished by about 25% as a result of dilution. As the WBSR enters the Valley and Ridge Province of the Appalachians it has a ²¹⁰Pb concentration of ~ 0.2 dpm/l. At this juncture it receives a considerable influx of alkalinity from tributaries draining carbonate terranes, resulting in neutralization of the sulfuric acid and increase of the river pH to around 6.5–7. This pH adjustment is accompanied by the precipitation of Fe and Mn. Due to the slow rate of Mn removal from solution, the Mn precipitation extends a considerable distance down river from the point of acid neutralization. Analyses for ²¹⁰Pb in the river at points in or below the region of Mn precipitation show that ²¹⁰Pb is rapidly scavenged from solution onto suspended particles. From the data it is possible to calculate the removal rate of Pb from water in the presence of Fe and Mn hydroxides and other particles. At a pH of 4–4.5 Pb removal is nonexistent relative to the river flow rate, but at a pH of 6.5–7 the ²¹⁰Pb data indicate a residence time of <0.7 day for dissolved Pb.