Uranium-series disequilibrium studies in phosphorite nodules from the west coast of South America

Sedimentary phosphorites occurring on the sea floor off Peru and Chile have been analyzed for U and Th isotopes, to establish their ages and hence obtain clues for their mode of formation. Fission-track distribution studies indicate that the U is primarily associated with the apatite fraction. Uranium-series disequilibrium methods, therefore, should be applicable, if the U incorporation is syngenetic with the apatite.The fractionation of U isotopes between oxidation states in the relatively young phosphorites from South America is low compared to that in older deposits. This supports the contention of Kolodny and Kaplan (1970) that the major mechanism of ${}^{234}\text{U}{}^{238}\text{U}$ fractionation is displacement of ²³⁴U atoms into sites where they are more ‘oxidizable’ than the ²³⁸U parent. Age estimates based on ²³⁴U(IV) and ²³⁰Th contents are internally consistent and range from late Pleistocene to Recent.The results indicate that marine phosphorites are currently forming in this area of intense oceanic upwelling. The age pattern during the last 150,000 yr suggests a correlation with eustatic high sea level stands and implies that conditions were more favorable for apatite genesis in this area during interglacials rather than during glacial times.     

The phosphate supply system in the Pacific region

Phosphate trade in the Pacific Basin is dominated by imports from outside the region. The principal suppliers within the area have been Nauru and Christmas Islands and the now depleted supplies of Makatea and Ocean Islands. Rapidly dwindling reserves on these islands coupled with expectations of a four to five fold increase in demand of phosphate fertilizer by the end of this century place considerable stress on the phosphate supply system in the Pacific. Anticipated developments in production from inactive deposits in Australia, Peru, and possibly Baja California, Mexico may eventually hold a major share in the Pacific markets. Other possibilities in the region include contributions from new discoveries and utilization of submarine deposits, especially those of the Chatham Rise and Baja California areas. Increased interest in small-scale mining for use in local agricultural operations may result in increased utilization of smaller deposits found throughout the Pacific.     

Actinide microdistributions in the enstatite meteorites

Oldhamite is a major Th and U bearing phase in the enstatite meteorites. Oldhamite from E-6 chondrites has mean Th and U abundances of 1550 ± 80 ppb Th and 410 ± 20 ppb U, with $\text{Th}\text{U}\text{= 3.8 ± .2}$. With the exception of ferroan alabandite which contains 25 ± 1 ppb U, no other Th or U enriched phases were located in the E-6 chondrites, and nearly all of the total rock Th and U can be accounted for by oldhamite. In Khairpur (E6), excess fossil fission tracks were observed in enstatite grains in contact with oldhamite which indicates the presence of ²⁴⁴Pu in oldhamite. Oldhamite from St. Mark's (E5) and Abee (E4) also shows actinide enrichments but at levels about half the E-6 results. Niningerite in Abee contains 45 ± 5 ppb U and due to its high reported modal abundance is an important U reservoir in Abee. The U content of oldhamite from the aubrite Peña Blanca Spring is 1920 ± 100 ppb. All $\text{Th}\text{U}$ values measured in this study cluster tightly around a value of 4 which indicates a lack of ThU fractionation in both oldhamite and in the enstatite meteorites, themselves. This lack of fractionation, along with the presence of ²⁴⁴Pu in oldhamite and reported rare earth enrichments also in oldhamite, suggests that the enstatite chondrites may be well-suited for PuU chronology and for providing the initial PuU value in the early solar system.     

Actinide crystal-liquid partitioning for clinopyroxene and Ca3(PO4)2

Using fission and alpha track radiography techniques, we have measured partition coefficients (D) for the actinide elements Th, U and Pu between diopsidic clinopyroxene, whitlockite [β-Ca₃ (PO₄)₂] and silicate liquid at 20kbar. Equilibrium partitioning at the crystal-liquid interface is assumed, and corrections for actinide zoning have been applied to the measured D values. Reproducibility for both actinide and minor element D values is carefully examined as a criterion for crystal-liquid interface equilibrium. The data are mostly compatible with interface equilibrium except for experiments at high cooling rates ( ? 30 deg/hr). Partition coefficients for Th/U/Pu of about 0.002/0.002/0.06 are measured for clinopyroxene and 1.2/0.5/3.4 for whitlockite. At an oxygen fugacity of 10^?8.5, Pu is much more readily incorporated into the crystalline phases than is U or Th because of the importance of trivalent Pu. The D_Pu(cpx) is similar to D(cpx) of the light rare earths supporting the concept of Pu/(rare earth) dating.     

The distribution of U and Pu in the St. Severin chondrite

A detailed study of the U distribution of the St. Severin chondrite has been made by fission track radiography in order to clarify the interpretation of fission Xe thermal release data in terms of the mineralogical location of the fission Xe within the meteorite. This is of importance because the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin has been widely adopted as the average solar system value. The U contents of the constituent minerals cannot account for the total rock U which, instead, appears to be primarily localized on grain boundaries. The greatest localizations of U are in olivine-poor, orthopyroxene-rich ‘clasts’. Our data coupled with those of Podosek (1970a) show that ²⁴⁴Pu in St. Severin was also located on grain boundaries and that the bulk of Pu and U are unfractionated within this meteorite. Due to recoil, the ²⁴⁴Pu fission Xe is found in 10 micron surface layers on major phases. Assuming that the grain boundaries (on which the Pu was located) was formed during metamorphism, the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin applies to a time subsequent to the textural recrystallization of the meteorite. Our data support the interpretation of Podosek and our best estimate of the solar system ${}^{244}\text{Pu}{}^{238}\text{U}$ is 0.015.     

Radiographic study of the distribution of boron in recent mollusc shells

Studies have been made of the boron content and distribution in recent Mytilus edulis shells by mapping the alpha particle distribution produced by the ¹⁰B (n,α)⁷Li reaction using cellulose nitrate track detectors. Boron is concentrated in the inorganic parts of the shell matrix and is enriched in aragonite relative to calcite by a factor of 1.5–2. The B contents in different parts of individual shells and in different specimens from a given locality are well defined, particularly for the aragonite layer. A correlation was found between the B in the aragonite layer and the salinity of the water in which the shells grew. The method shows promise as a means of using the boron content of fossil shells to determine paleosalinity.     

Growth rates of Pacific manganese nodules as deduced by uranium-series and hydration-rind dating techniques

Growth rates of four Pacific ferromanganese deposits have been determined by uranium-series and hydration-rind dating techniques. This represents the first time these two methods have been applied to the same samples. Of the four samples analyzed, two agree in both techniques and two show a wide disparity. The samples that do not agree have uranium-series rates which are low compared to those deduced by the hydration-rind techniques. The same samples also display steep²³²Th activity gradients in the outer layers of the ferromanganese material. Since the half-life of²³²Th is extremely long (10¹⁰ yr), these gradients could not be due to radioactive decay. This suggests that the distribution of²³⁰Th, at least in these cases, may also be a result of some process other than radioactive decay.One of our samples which has a steep²³²Th gradient produces a²³⁰Th “exposure age” which is indistinguishable to the calculated hydration age within the analytical uncertainty. The rate of growth calculated on the basis of the slope of the decline of²³⁰Th within the deposit, however, is low by over one order of magnitude when compared to the rate determined by the hydration-rind method. Although previous results indicate that the distribution of²³²Th is usually quite random within ferromanganese deposits, our results indicate that caution should be exercised when assessing their growth rates on the basis of uranium-series results.     

The lateral and vertical growth of laterite weathering profiles,Hawaiian Islands,USA

The Hawaiian Islands permit investigation of tropical chemical weathering rates and processes on a single rock type, basalt. Chronosequences are investigated as a function of rainfall due to the varying age of each island, including Kauai (~4 Ma), Oahu (~2 Ma), and Hawaii's Kohala Peninsula (~0.3 to 0.17 Ma). Understanding tropical critical zone (CZ) development is vital given the large populations in developing countries that rely on it. HVSR (horizontal-to-vertical spectral ratio) seismic soundings on Kauai indicate that ~60% of the variability in laterite thickness is due to gradients in precipitation, with errors in erosion corrections and variability in the original permeability structure of the volcanic sequence playing important roles. Basalts have higher horizontal than vertical hydraulic conductivity (K_h > K_v) , and local variability in likely drives much of the remaining differences in laterite thickness. HVSR is well suited for estimating laterite thickness as it has been shown to reliably detect the base of the weathering profile, is rapid (20 min/sounding), highly portable, and occupies a very small footprint. Comparison of Kauai and Oahu weathering profiles suggests that the Oahu laterites are fully or nearly fully formed, despite being half the age of Kauai. By contrast, the young laterites on Kohala (~170 to ~300 ka) exhibit greatly contrasting thicknesses, where coastal laterites are thick and interior laterites are thin, suggesting that early weathering on shield volcanoes produces wedge-shaped laterites near the coast. With time, the thick (coastal) end of the wedge propagates upslope such that a fully developed, constant-thickness laterite carapace can form in ~2 Ma or less. The development of thickened coastal laterites on young substrates depends on greater water–rock ratios as vertically infiltrating water upslope is diverted laterally. This view of laterite development is very different compared to endmember models of continental weathering and CZ development. © 2020 John Wiley & Sons, Ltd.     

Enhanced production of stratospheric OH from methane oxidation at elevated reactive chlorine levels in northern midlatitudes

Examination of odd-hydrogen production and loss processes in the lower stratosphere reveals that the large abundances of midlatitude ClO recently measured requires a reassessment of the OH yield from methane oxidation due to a more rapid initiation of the methane oxidation sequence by atomic chlorine. Using a steady state calculation for OH and an iterative procedure for evaluating odd-hydrogen production from the methane oxidation sequence, we demonstrate that both OH and reactive chlorine are amplified below 25 km. The amplified values of OH and ClO are found to be consistent with recent measurements of ClO concentrations and of OH column abundances.