Fraction Of Rocks Research Articles

Asphalt Concrete with Fly Ashes of Different Sources as Filler Replacement

Paper presents and discusses data obtained in a testing program on the replacement of limestone powder in asphalt concretes by fly ashes. This would combine economic, environmental and technical benefits. The finest fly ash of Vietnamese origin scored best. This is proposed due to the gap-graded design of the particulate mixture of coarse crushed rock fractions, fine fluvial sand and the mineral admixture. The test program encompassed Marshall tests, creep tests and splitting tensile tests.

Extraction of hazardous elements by water from contaminated rocks: An experimental study

The interaction of contaminated rocks (loam and clay recovered from depths of 1 and 4 m by a borehole drilled in the territory of a metallurgical plant in Ryazan oblast) was examined experimentally, and the concentrations (C) of twenty elements in solutions after the experiments were analyzed. The concentrations lie within ranges typical of the upper aquifer in the contaminated area, with the Al, As, Mo, Na, S, and W concentrations exceeding the maximum permissible concentration values (MPC). The experiments with clay yielded less stable and unambiguous results because a certain portion of clay particles passed through the filter (0.05 μm) when the rock and water were separated after the experiments and thus may contribute to the concentrations of certain elements in the solutions. The experiments led us to derive two types of dependences for each element: (1) C-t (time) dependences within the range of 1–36 days and (2) C-n (number of acts of rock treatment with fresh water portions) dependences. The both types are presented by various mass ratios of rock and water (r/w = 0.5 and 0.1). The character of the dependences led us to classify the elements into two groups. The concentrations of elements of one of the groups (As, Si, and V) shows no systematic dependences of C on t, n, and r/w, and this corresponds to equilibrium reached by fast adsorption-desorption reactions. These dependences in the other group of elements (almost all other elements) testify that the slow dissolution-precipitation reactions proceed away from equilibrium. The C-n dependences were utilized to evaluate the recovery ratios of elements and to identify easily soluble species of Ca, Mg, Mo, Na, S, and Sr. A fraction of ooliths (goethite + SiO2) separated from the clay contains elevated concentrations of certain trace elements. Microprobe analyses of the ooliths show that elements whose concentrations are comparable with the detection limits of microprobe analysis are evenly distributed, with elevated concentrations detected only in two instances: (i) Ce and La (in monazite) and (ii) Co and Ni (in MnO). Having a high isomorphic capacity with respect to several elements, goethite can act as a geochemical barrier and thus constrain the migration of these elements. The correlation dependences of elemental concentrations in rock fractions were utilized to derive further information on the speciation of the elements related to common sources in various rocks and the number of concentrator minerals.

Determination of the composition of ground phosphate rock by chemical and physicochemical methods of analysis

The composition of the phosphate-containing fraction of the ground phosphate rock from Polpino deposit was examined by chemical and physicochemical methods of analysis.

U–Pb chronology of the Solar System's oldest solids with variable 238U/ 235U

Accurate determination of the absolute ages of the oldest Solar System objects — chondrules and Ca–Al-rich inclusions (CAIs), requires knowledge of their 238U/ 235U ratios. This ratio was assumed to be invariant in all U–Pb dating of meteorites so far, but the recent discovery of U isotope variations in CAIs (Brennecka et al., 2010a) shows that this assumption is invalid. We present the first combined high-precision U and Pb isotopic data for a CAI, and U isotopic data for chondrules and whole rock fractions of the Allende meteorite. The Pb–Pb isochron age of the CAI SJ101 is 4567.18 ± 0.50 Ma, calculated using the measured 238U/ 235U = 137.876 ± 0.043 (2σ), reported relative to 238U/ 235U = 137.837 of the CRM 145 standard. Our best current estimate of the average terrestrial value is: 238U/ 235U = 137.821 ± 0.014.The error in the age includes uncertainties in the Pb–Pb isochron intercept and in the 238U/ 235U ratio. Allende bulk rock and chondrules have 238U/ 235U = 137.747 ± 0.017 (2σ), distinctly lower than the CAI. The difference in the 238U/ 235U ratio of 0.129 ± 0.046 (2σ) between the CAI and chondrules and bulk meteorite increases the 207Pb– 206Pb age difference by ~ 1.4 Ma, and eliminates apparent disagreement between the CAI–chondrule formation time interval determinations with the U–Pb and extinct nuclide ( 26Al– 26Mg and 182Hf– 182W) data. We discuss standardisation of 238U/ 235U measurements for U–Pb geochronology and cosmochronology, elemental and isotopic fractionation induced by intensive acid leaching, ages of CAIs in the context of 238U/ 235U variability, and possible causes of U isotopic variations in CAIs and meteorites.

Recipes for Spatial Statistics with Global Datasets: A Martian Case Study

The Mars Odyssey Gamma Ray Spectrometer has yielded planetary data of global extent. Such remote-sensing missions usually assign the value of a continuous-valued geospatial attribute to a uniform latitude-longitude grid of bins. Typical attributes include elemental-mass fraction, areal fraction of a mineral type, areal fraction of rocks, thermal inertia, etc. The fineness of the grid is chosen according to the spatial resolution of the orbiter and concomitant data processing. We describe methods to maximize the information extracted from both bin and regional data. Rigorous use of statistical parameters and related methods for inter- and intra- regional comparisons are also discussed. While we discuss results from the Mars Odyssey mission, the techniques we describe are applicable whenever continuous-valued attributes of a planet's surface are characterized with bins and regions. Our goal is to distill the simplest statistical methods for regional comparisons that would be intuitively accessible to planetary scientists.

Comparative analysis and geological significance of kerogen isolated using open-system (palynological) versus chemically and volumetrically conservative closed-system methods

Characterization of kerogen, the insoluble organic fraction of sedimentary rocks, is of key importance in investigations of the geochemistry of ancient depositional systems, paleo climates, basin thermal history, and petroleum generation systematics. Furthermore, exploitation of organic-rich oil shales as unconventional petroleum resources requires understanding of the physical chemistry of the kerogen conversion reactions and the coking behavior. These issues can be effectively addressed only if pure, representative kerogen is obtained from the rock matrix using chemically and volumetrically conservative methods that preclude losses, alteration, or fractionation of the organic matter. Isolation of representative and pure kerogen concentrates from the rock matrix is difficult. Many methods have been used over the years with various degrees of success. Most are based on traditional palynological processing approaches in open systems that commonly yield fractionated products, generally contaminated with pyrite and insoluble neo-fluorides. A few are based on complex sealed-system procedures that purport to conserve the entire kerogen with no fractionation, no alteration and no contamination. This study focuses on a comparative analysis of kerogens isolated with the conventional open-system approach (HCl/HF/heavy liquid separation) and kerogens concentrated with a fully automated, conservative, closed-system method (HCl/HF/CrCl 2) under an inert N 2 atmosphere. Source-rock samples of various richness, kerogen types, ages, depositional environments, lithologies and thermal histories were selected for kerogen isolation using the two approaches. Kerogen recovery efficiency, progress of the processes and quality of the isolated kerogens were monitored via mass balance calculations, organic elemental analysis and X-ray diffraction analysis. The inherent inadequacy in the open-system approach for geochemical research and the improved features of the chemically and volumetrically conservative method are documented.

A three-stage experimental constructed wetland for treatment of domestic sewage: First 2 years of operation

Hybrid constructed wetland systems have recently been used to treat wastewaters where high demand for removal of ammonia is required. However, these systems have not been used too often for small on-site treatment systems. This is because in many countries ammonia is not limited in the discharge from small systems. Hybrid systems have a great potential to reduce both ammonia and nitrate concentrations at the same time. In our study we employed a three-stage constructed wetland system consisting of saturated vertical-flow (VF) bed (2.5 m 2, planted with Phragmites australis), free-drained VF bed (1.5 m 2, planted with P. australis) and horizontal-flow (HF) bed (6 m 2, planted with Phalaris arundinacea) in series. All wetlands were originally filled with crushed rock (4–8 mm). However, nitrification was achieved only after the crushed rock was replaced with sand (0–4 mm) in the free-drain wetland. Also, original size of crushed rock proved to be too vulnerable to clogging and therefore, in the first wetlands the upper 40 cm was replaced by coarser fraction of crushed rock (16–32 mm) before the second year of operation started. The system was fed with mechanically pretreated municipal wastewater and the total daily flow was divided into two batches 12 h apart. The evaluation of the results from the period 2007 to 2008 indicated that such a system has a great potential for oxidation of ammonia and reduction of nitrate. The ammonia was substantially reduced in the free-drained VF bed and nitrate was effectively reduced in the final HF bed. The inflow mean NH 4-N concentration of 29.9 mg/l was reduced to 6.5 mg/l with the average removal efficiency of 78.3%. At the same time the average nitrate-N concentration rose from 0.5 to only 2.7 mg/l at the outflow. Removal of BOD 5 and COD amounted to 94.5% and 84.4%, respectively, with respective average outflow concentrations of 10 and 50 mg/l. Phosphorus was removed efficiently despite the fact that the system was not aimed at P removal and therefore no special media were used. Phosphorus removal amounted in 2008 to 65.4%, but the average outflow concentration of 1.8 mg/l is still high. The results of the present study indicate very efficient performance of the hybrid constructed wetlands, but optimal loading parameters still need to be adjusted. The capital cost of the experimental system is comparable to the conventional on-site treatment plant but the operations and maintenance costs are about one third of the conventional plant.

Enhancement of the bentonite sorption properties

The almost monomineral fraction of bentonite rock–montmorillonite was modified by magnetic particles to enhance its sorption properties. The method of clay modification consists in the precipitation of magnetic nanoparticles, often used in preparing of ferrofluids, on the surface of clay. The influence of the synthesis temperature (20 and 85 °C) and the weight ratio of bentonite/iron oxides (1:1 and 5:1) on the composite materials properties were investigated. The obtained materials were characterized by the X-ray diffraction method and Mössbauer spectroscopy. Changes in the surface and pore properties of the magnetic composites were studied by the low nitrogen adsorption method and the electrokinetic measurements. The natural bentonite and magnetic composites were used in sorption experiments. The sorption of toxic metals (zinc, cadmium and nickel) from the model solutions was well described by the linearized Langmuir and Freundlich sorption model. The results show that the magnetic bentonite is better sorbent than the unmodified bentonite if the initial concentration of studied metals is very low.

Rare earth elements in the clay fraction of coaliferous sediments of the Arkagalinskoe (Magadan district) and Dolinskoe (Sakhalin Island) coalfields

The study of REE distribution in the clay fraction of sedimentary rocks from two coalfields made it possible to distinguish several types of REE distribution, which correlate with their mineral composition. It is shown that the REE fractionation was related to the mineral reconstruction of the primary clay fraction and some detrital minerals in the course of postsedimentary transformations of rocks during diagenesis, early catagenesis, and beginning of late catagenesis. These transformations were governed by several factors, such as the composition of sediments; hydrochemical features of accumulation environment; the chemical composition, dynamics, and feeding sources of pore solutions; the porosity and permeability of sediments and rocks; and the content of organic matter and its reaction ability.

Separation performance of double deck banana screens – Part 1: Flow and separation for different accelerations

Banana screens are often used for high capacity separation of iron ore, coal and aggregates into different size fractions. They consist of one or more curved decks that are fitted with screen panels with arrays of square or rectangular holes. The screen structure is vibrated at high frequency to generate peak accelerations of around 4–6 g which separates particles flowing over each screen according to their size. Screens are often used to close comminution circuits and return specific size fractions of rock to different destinations such as pebble mills, crushers and back into the mills. All multi-deck screens are difficult to sample for intermediate products which makes measurement and optimization very difficult. Banana screens are even more difficult because the screen cut size varies with the varying slope of the decks. In this paper, the discrete element method (DEM) is used to simulate a full industrial scale double deck banana screen for a range of accelerations. The nature of the particle flow through this complex machine is explored for a range of peak accelerations. Critical aspects of the flow are linked to the separation performance.

Mechanisms for incompatible-element enrichment on the Moon deduced from the lunar basaltic meteorite Northwest Africa 032

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb–Sr and Sm–Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb–Sr and Sm–Nd ages. Rubidium–Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/ 86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/ 86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium–neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial ε Nd of +9.71 ± 0.74 corresponding to a source region with 147Sm/ 144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts. Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.

The U–Pb systematics of angrite Sahara 99555

Angrite Sahara 99555 (hereafter SAH), precisely dated by Baker et al. (Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127–1131), has been proposed as a new reference point for the early Solar System timescale and for calculation of the revised minimum age of our Solar System. The Pb–Pb age of SAH of 4566.18 ± 0.14 Ma, reported by Baker et al., differs from the Pb–Pb age of D’Orbigny, another basaltic angrite, of 4564.42 ± 0.12 Ma (Amelin Y. (2008) U–Pb ages of angrites. Geochim. Cosmochim. Acta 72, 221–232), despite the fact that the relative 53Mn– 53Cr and 182Hf- 182W ages of these meteorites are identical. Here I report U–Pb data for 21 whole rock and pyroxene fractions from SAH, analyzed using the same approach as D’Orbigny (Amelin, 2008). These fractions contain between 1.3 and 8.9 pg of total common Pb, slightly more than analytical blank. Measured 206Pb/ 204Pb ratios are between 625 and 2817 for D’Orbigny, blank-corrected 206Pb/ 204Pb ratios are between 1173 and 6675. Eight acid-washed whole rock fractions yielded an isochron age of 4564.86 ± 0.38 Ma, MSWD = 1.5. Data for pyroxene fractions plot mostly above the whole rock isochron, and do not form a linear array in 207Pb/ 206Pb vs. 204Pb/ 206Pb isochron coordinates. The 207Pb ∗/ 206Pb ∗ model dates of the pyroxene fractions vary from 4563.8 ± 0.3 to 4567.1 ± 0.5 Ma. The difference between whole rock and pyroxene U–Pb systematics may be a result of re-distribution of radiogenic Pb at a mineral grain scale several million years after crystallization. Complexities of Sm–Nd, Lu–Hf, and possibly 26Al– 26Mg mineral systematics of SAH, described previously, may be related to the same process that caused the re-distribution of radiogenic Pb. Disturbance of isotopic chronometers renders SAH an imperfect anchor for the early Solar System timescale. The problems with age determination revealed by the studies of SAH call for greater attention in Pb-isotopic dating of angrites and other achondrites.

Áreas Fontes dos Minerais Pesados e sua Distribuição sobre a Plataforma Continental Sul-brasileira, Uruguaia e Norte-argentina

The heavy mineral fraction of sedimentary rocks and sediments can provide important information about provenance in sedimentary basins studies because some minerals or group of minerals can be traced to a source rock or terrain of specific composition. This work is a provenance study based on heavy mineral analyses of samples collected on the Rio Grande do Sul (southern Brazil), Uruguay and north of Argentine continental shelf (between 29º30´ and 37º 30´ south latitude and 48º 30´ and 56º 30´ west longitude). The fine and very fine sand fractions (2-4 Φ) were selected for heavy mineral analysis employing bromoform (S.G.= 2.65). The quantitative analysis was made using a microscope for the mineral identification and the counting of 300 grains from each sample, in a total of 106 samples. Multivariate data analysis was applied for mineralogical analysis, principally the RQmode vector. Four principal assemblages were defined applying this technique revealing the main source as well as the dispersal patterns for the sediments. The first component was dominated by augite indicating a panpean-patagonic source. The second component was dominated by hornblende and hypersthene minerals, indicating a provenance from Precambrian terrains with basaltic influence. The third component was marked by the dominance of tourmaline, staurolite, epidote and kyanite reflecting a source from the Precambrian metamorphic terrains. Finally, the four components with apatite, zircon and sillimanite minerals were dispersed by the Rio de La Plata system.

As anomalias geológicas e geofísicas localizadas ao norte de Itapororoca (PB), folha Guarabira

Anomalies concerning the litho-stratigraphic, geomorphic and hydrogeologic contexts were detected on the region north of Itapororoca-PB. These anomalies are concentrated in the area coined as "Cretacic volcanics" by the system DNPM-CPRM (1974), in the basement of the Barreiras Group. In this region, two classic terranes of the Borborema Province were recognized: S. José do Campestre (SJC, Paleoproterozoic) and Alto Pajeú (TAP, Eoneoproterozoic, respectively to the north and to the south of the Patos Lineamment (LP). Along LP there is preserved part of the cover rocks of the SJC, phyllonitized and drastically folded, of the Seridó Group. These terranes and cover rocks are pierced by Brasiliano plutons that display very distinct petrogenetic and isotopic characteristics. The fist case of of juvenile tonalitic plutonism of the Province is there recorded. A silicious duricrust, with many textural facies is present in the area, as relicts of previous plateau, between LP and the Mamanguape River (aligned by an E-NE normal fault). Anomalies of F, Cr and Ni were detected in this duricrust, in which some sparce fractions of ultramafic and alkaline rocks could be identified. Besides the geological mapping, a gravity survey of 548 new gravity stations was performed for the purpose in addition to 452 prexisting stations of the Observatório Nacional and of the UFRN, so covering the area of the duricrust as well as the area of some circular structures, in the surroundings.

As anomalias geológicas e geofísicas localizadas ao norte de Itapororoca (PB), folha Guarabira

Anomalies concerning the litho-stratigraphic, geomorphic and hydrogeologic contexts were detected on the region north of Itapororoca-PB. These anomalies are concentrated in the area coined as “Cretacic volcanics” by the system DNPM-CPRM (1974), in the basement of the Barreiras Group. In this region, two classic terranes of the Borborema Province were recognized: S. Jose do Campestre (SJC, Paleoproterozoic) and Alto Pajeu (TAP, Eoneoproterozoic, respectively to the north and to the south of the Patos Lineamment (LP). Along LP there is preserved part of the cover rocks of the SJC, phyllonitized and drastically folded, of the Serido Group. These terranes and cover rocks are pierced by Brasiliano plutons that display very distinct petrogenetic and isotopic characteristics. The fist case of of juvenile tonalitic plutonism of the Province is there recorded. A silicious duricrust, with many textural facies is present in the area, as relicts of previous plateau, between LP and the Mamanguape River (aligned by an E-NE normal fault). Anomalies of F, Cr and Ni were detected in this duricrust, in which some sparce fractions of ultramafic and alkaline rocks could be identified. Besides the geological mapping, a gravity survey of 548 new gravity stations was performed for the purpose in addition to 452 prexisting stations of the Observatorio Nacional and of the UFRN, so covering the area of the duricrust as well as the area of some circular structures, in the surroundings.

Sorption of 237Np(V), 238U(VI), and 137Cs on clays: Role of surface films of Fe(III) compounds

Mineralogical and sorption properties of sandy-argillaceous rocks proposed as a material of protecting barriers in near-surface repositories of nuclear wastes were studied. Different sorption behavior with respect to Np(V), U(VI), and 137Cs is caused by formation of iron-containing films on the particle surface of coarse (>0.25 mm) and fine (<0.01 mm) rock fractions. The presence of Fe in the compositions of different minerals contained in the rock (chlorite, illite, montmorillonite, and hematite) was determined by scanning electron microscopy with X-ray probe microanalysis (SEM-XPMA), transmitting electron microscopy with electron energy loss spectroscopy (TEM-EELS), and Mossbauer spectroscopy. Hematite is present in the form of both separate particles and films on quartz grains. The Fe fraction in these formations was estimated.

On a Hamiltonian PDE arising in magma dynamics

In this article we discuss a new Hamiltonian PDE arising from a class of equations appearing in the study of magma, partially molten rock in the Earth's interior. Under physically justifiable simplifications, a scalar, nonlinear, degenerate, dispersive wave equation may be derived to describe the evolution of $\phi$, the fraction of molten rock by volume, in the Earth. These equations have two power nonlinearities which specify the constitutive realitions for bulk viscosity and permeability in terms of $\phi$. Previously, they have been shown to admit solitary wave solutions. For a particular relation between exponents, we observe the equation to be Hamiltonian; it can be viewed as a generalization of the Benjamin-Bona-Mahoney equation. We prove that the solitary waves are nonlinearly stable, by showing that they are constrained local minimizers of an appropriate time-invariant Lyapunov functional. A consequence is an extension of the regime of global in time well-posedness for this class of equations to (large) data which includes a neighborhood of a solitary wave. Finally, we observe that these equations have <em>compactons</em>, solitary traveling waves with compact spatial support.

Rare earth element sorption by basaltic rock: Experimental data and modeling results using the “Generalised Composite approach”

Sorption of the 14 rare earth elements (REE) by basaltic rock is investigated as a function of pH, ionic strength and aqueous REE concentrations. The rock sample, originating from a terrestrial basalt flow (Rio Grande do Sul State, Brazil), is composed of plagioclase, pyroxene and cryptocrystalline phases. Small amounts of clay minerals are present, due to rock weathering. Batch sorption experiments are carried out under controlled temperature conditions of 20 °C with the <125 μm fraction of the ground rock in solutions of 0.025 M and 0.5 M NaCl and at pH ranging from 2.7 to 8. All 14 REEs are investigated simultaneously with initial concentrations varying from 10 −7 to 10 −4 mol/L. Some experiments are repeated with only europium present to evaluate possible competitive effects between REE. Experimental results show the preferential retention of the heavy REEs at high ionic strength and circumneutral pH conditions. Moreover, results show that REE sorption increases strongly with decreasing ionic strength, indicating two types of sorption sites: exchange and specific sites. Sorption data are described by a Generalised Composite (GC) non-electrostatic model: two kinds of surface reactions are treated, i.e. cation exchange at >XNa sites, and surface complexation at >SOH sites. Total site density (>XNa + >SOH) is determined by measuring the cation exchange capacity (CEC = 52 μmol/m 2). Specific concentrations of exchange sites and complexation sites are determined by fitting the Langmuir equation to sorption isotherms of REE and phosphate ions. Site densities of 22 ± 5 and 30 ± 5 μmol/m 2 are obtained for [>XNa] and [>SOH], respectively. The entire set of REE experimental data is modeled using a single exchange constant (log K ex = 9.7) and a surface complexation constant that progressively increases from log K = −1.15 for La(III) to −0.4 for Lu(III). The model proves to be fairly robust in describing other aluminosilicate systems. Maintaining the same set of sorption constants and only adjusting the site densities, we obtain good agreement with the literature data on REE/kaolinite and REE/smectite sorption. The Generalised Composite non-electrostatic model appears as an easy and efficient tool for describing sorption by complex aluminosilicate mineral assemblages.

U–Pb ages of angrites

Precise U–Pb ages, determined with double spike ( 202Pb– 205Pb) thermal ionization m1ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D’Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D’Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured 206Pb/ 204Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160–4500 for LEW, and 608–8500 for D’Orbigny. Blank-corrected 206Pb/ 204Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible 207Pb ∗/ 206Pb ∗ dates with the average values of 4557.65 ± 0.13 Ma for AdoR, 4558.55 ± 0.15 Ma for LEW, and 4564.42 ± 0.12 Ma for D’Orbigny. Pb–Pb isochrons including data with slightly elevated common Pb, and U–Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopic ages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of “slowly cooled” angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of 53Mn– 53Cr “absolute” ages by 0.7 Ma. Third, a precise age of D’Orbigny allows consistent linking of the 53Mn– 53Cr and 26Al– 26Mg extinct nuclide chronometers to the absolute lime scale.

Uranium–lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

Uranium–lead ratios (commonly represented as 238U/ 204Pb = μ) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of μ values represented by the source regions of martian basalts, we completed U–Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range ( 206Pb/ 204Pb = 11.16–11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in 206Pb/ 204Pb– 207Pb/ 204Pb– 208Pb/ 204Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1–7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a 206Pb– 207Pb array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 ± 10 Ma derived from Rb–Sr and Sm–Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb–Sr and Sm–Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta 61, 4915–4931). Despite the contamination, and accepting 327 ± 10 Ma as the crystallization age, we use the U–Pb data to determine the initial 206Pb/ 204Pb of QUE 94201 to be 11.086 ± 0.008 and to calculate the μ value of its mantle source to be 1.82 ± 0.01. The μ value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that μ values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source μ values indicates that the μ value of bulk silicate Mars is approximately three. The amount of variation in the μ values of the mantle sources (μ ∼ 2–4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U–Pb fractionation during formation of the mantle sources of martian basalts.