Volatile accretion history of the Earth.

It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ∼100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.

Samarium-neodymium direct dating of fluorite mineralization.

The direct dating of many styles of hydrothermal mineralization has proved difficult, limiting understanding of the geological processes that lead to crustal fluid flow and the formation of major ore deposits. The hydrothermal mineral fluorite (CaF(2)) displays large variations in rare earth element (REE) abundance and samarium/neodymium ratios within a single vein. Samarium-neodymium dating of fluorite from the classic granite-hosted tin deposits of southwest England demonstrates its use as a precise chronometer of mineralization. The concentrations of light rare earth elements (LREEs) in the fluorites are highly variable and suggest the coeval precipitation of an LREE-rich phase as the most likely cause of the extreme variation in samarium/neodymium ratios.

Synorogenic collapse: a perspective from the middle crust, the proterozoic grenville orogen.

Structural, petrological, and geochronological studies of the middle to late Proterozoic Grenville orogen in Ontario, Canada, indicate that a major extensional fault developed synchronously with late thrusting. This fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers. The temporal and spatial relations among faulting, metamorphism, and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology. Comparison of Grenville with recent Himalayan orogenic activity suggests that during Proterozoic times physiographic height, crustal thickness, and crustal strength were similar to modern conditions in orogenic belts.

Mechanisms of Argon Retention in Clays Revealed by Laser 40Ar-39Ar Dating.

A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the (40)Ar-(39)Ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because (39)Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic (40)Ar in nature, rather than by direct recoil as is generally assumed.

Orbital forcing of the marine isotope stage 9 interglacial.

Milankovitch orbital forcing theory has been used to assign time scales to many paleoclimate records. However, the validity of this theory remains uncertain, and independent sea-level chronologies used to test its applicability have been restricted largely to the past approximately 135,000 years. Here, we report U-series ages for coral reefs formed on Henderson Island during sea-level high-stands occurring at approximately 630,000 and approximately 330,000 years ago. These data are consistent with the hypothesis that interglacial climates are forced by Northern Hemisphere summer solar insolation centered at 65 degrees N latitude, as predicted by Milankovitch theory.

Oxygen isotopes and the moon-forming giant impact.

We have determined the abundances of 16O, 17O, and 18O in 31 lunar samples from Apollo missions 11, 12, 15, 16, and 17 using a high-precision laser fluorination technique. All oxygen isotope compositions plot within +/-0.016 per mil (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation line within uncertainties. This observation is consistent with the Giant Impact model, provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components. The similarity between the proto-Earth and Theia is consistent with formation at about the same heliocentric distance. The three oxygen isotopes (delta17O) provide no evidence that isotopic heterogeneity on the Moon was created by lunar impacts.

Development and application of new iondashexchange techniques for the separation of the platinum group and other siderophile elements from geological samples.

Two new aniondashexchange techniques have been developed for the separation of the platinum group elements Ru, Pd, Ir, Pt and the siderophile metals Re, Ag, Zn and Cd from geological samples following a NiS fire assay digestion procedure. Both methods are simple and permit the isolation of these elements in sufficient purity for quantitative analysis by isotope dilutiondashinductively coupled plasma mass spectrometry (ID-ICPMS) at yields of 75-95%. The high affinity of the considered elements to anion exchange resins allows the use of small (1.25 ml) columns even for the processing of 5-10 g sized silicate rock samples. Following fire assay digestion and dissolution of the NiS buttons in aqua regia, the samples are loaded onto the resin bed as solutions in 1 M HCl. After elution of the bulk sample matrix with dilute HCl and HNO(3), Zn and Cd are stripped from the column using 0.8 M HNO(3). Small amounts of bromine water are added to the dilute mineral acids for the stabilization of strongly retained Ir(IV). Following this, the iondashexchange techniques permit the sequential elution of Ag, Re and the PGE using 11 M HCl, 8 M HNO(3) and 13.5 M HNO(3). The iondashexchange methods have been applied to separation of Ru, Pd, Re, Ir and Pt from the geological reference material SU-la prior to concentration measurements by ID-ICPMS. Our analytical results are in good agreement with previously published data for this sample and display an external reproducibility (based upon repeat dissolutions) of approximately 2-10% for the elements considered in this study.

Sm-Nd evidence for the age and origin of a Mississippi Valley Type ore deposit.

MISSISSIPPI Valley Type (MVT) ore deposits represent the relatively common product of large-scale fluid transport in the continental lithosphere, yet the models for their genesis have been more controversial and unconstrained than those of any other class of giant ore deposit(1,2). Here we show that Sm-Nd isotope data can be used to determine the age and origin of an MVT deposit. Sm-Nd data for fluorites from the North Pennine orefield are difficult to explain unless some of the mineralization is of Mesozoic rather than the traditionally accepted Palaeozoic age. Furthermore, the Nd and Sr isotopie compositions of the fluorites do not support a variety of recent models that include derivation of the components from the mantle, the Lower Palaeozoic basement or the underlying buried granite which served to focus the flow of hydrothermal fluids.

New half-life measurement of 182Hf: improved chronometer for the early solar system.

The decay of 182Hf, now extinct, into stable 182W has developed into an important chronometer for studying early solar system processes such as the accretion and differentiation of planetesimals and the formation of the Earth and the Moon. The only 182Hf half-life measurements available were performed 40 years ago and resulted in an imprecise half-life of (9+/-2)x10(6) yr. We redetermined the half-life by measuring the specific activity of 182Hf based on two independent methods, resulting in a value of t(1/2)(182Hf)=(8.90+/-0.09)x10(6) yr, in good agreement with the previous value, but with a 20 times smaller uncertainty. The greatly improved precision of this half-life now permits very precise intercalibration of the 182Hf-182W isotopic system with other chronometers.