Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with delta(18)O(VSMOW) values as low as + 9 parts per thousand, delta(13)C(VPDB) = -9 parts per thousand, and Sr-87/Sr-86 as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of C-13/C-12 fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals. (C) 2003 Published by Elsevier B.V.

U-Pb zircon ages from the exposed Sask craton are 2450-3100 Ma, from the Peter Lake Domain 2575-2640 Ma, and from rocks of the Trans-Hudson orogen 1840-1880 Ma. U-Pb monazite and zircon ages of post-orogenic pegmatites and aplites are 1770-1800 Ma. Common Pb and Sm-Nd isotopic compositions of post-orogenic intrusions, as probes of crust beneath the orogen, were compared to Sask craton rocks and ca. 1850 Ma orogenic rocks to infer the origin and subsurface distribution of the Sask craton within the internides of the Trans-Hudson orogen. Results show that post-orogenic intrusions within most of the Glennie Domain and Hanson Lake block were derived, at least in part, from Archean source materials, demonstrating that the Sask craton lies beneath Paleoproterozoic orogenic rocks present at the surface. In contrast, common Pb and Sm-Nd isotopic compositions from pegmatites and aplites of the La Ronge Domain are essentially identical with those of the Paleoproterozoic orogenic rocks into which they are intruded, indicating derivation by partial melting of similar rocks. Thus, if the Sask craton extended to the west beneath the La Ronge Domain, it was beneath the zone of melting that produced the post-orogenic intrusions, making it unlikely that the Sask craton is a detached part of the Hearne craton. Many samples from the Sask craton have elevated Pb-208/Pb-204 ratios, unlike Superior craton or Hearne craton rocks, suggesting that the Sask craton was derived from an exotic source, such as the Wyoming craton, which shares similar elevated Pb-208/Pb-204 ratios.

This paper presents an improved technique for the precise and accurate determination of Ag isotopic composition in geologic samples. Following the separation of Ag from the rock matrix with a three-stage ion exchange procedure, the isotopic composition is measured by MC-ICPMS. Instrumental mass bias is corrected externally using a Pd standard, which was added to each sample, and sample-standard bracketing. The use of wet plasma significantly improves the analytical precision by eliminating variations in fractionation between Pd and Ag. A virtually identical behavior of the mass bias for Pd and Ag during the analyses is a prerequisite for the external correction method. Replicate dissolutions of the carbonaceous chondrite Allende with <100 ng Ag yield an external reproducibility (2 S.D.) of 53 ppm for Ag-107/Ag-109. Since the ion exchange procedure provides a good separation from matrix elements such as Ti and Fe, the method is suitable for the analysis of stony and iron meteorites as well as sulfide minerals and terrestrial basalts. The technique thus can be applied to investigate Ag isotopic variations in a large variety of solar system materials. (C) 2006 Elsevier B.V. All rights reserved.

The extinct radionuclide Pd-107 decays to Ag-107 (half-life of 6.5 Ma) and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system. Here, a comprehensive Ag isotope study of carbonaceous and ordinary chondrites is presented. Carbonaceous chondrites show limited variations (epsilon(107) Ag = -2.1 to +0.8) in Ag isotopic composition that correlate with the Pd/Ag ratios. Assuming a strictly radiogenic origin of these variations, a new initial Pd-107/Pd-108 of 5.9 (+/- 2.2) x 10(-5) for the solar system can be deduced. Comparing the Pd-Ag and Mn-Cr data for carbonaceous chondrites suggests that Mn-Cr and Pd-Ag fractionation took place close to the time of calcium-aluminium-rich inclusion (CAI) and chondrule formation similar to 4568 Ma ago. Using the new value for the initial Pd-107 abundance, the revised ages for the iron-rich meteorites Gibeon (IVA, 8.5 +3.2/-4.6 Mal, Grant (IIIAB, 13.0 +3.5/-4.9 Ma) and Canyon Diablo (IA, 19.5 +24.1/-10.4 Mal are consistent with cooling rates and the closure temperature of the Pd-Ag system. In contrast to carbonaceous chondrites, ordinary chondrites show large stable isotope fractionation of order of 1 permil for Ag-107/Ag-109. This indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites. Nebular processes and accretion, as experienced by carbonaceous chondrites, did not led to significant Ag isotope fractionation, while the significant Ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism. (C) 2008 Elsevier Ltd. All rights reserved.

Several models exist to describe the growth and evolution of Earth; however, variables such as the type of precursor materials, extent of mixing, and material loss during accretion are poorly constrained. High-precision palladium-silver isotope data show that Earth's mantle is similar in (107)Ag/(109)Ag to primitive, volatile-rich chondrites, suggesting that Earth accreted a considerable amount of material with high contents of moderately volatile elements. Contradictory evidence from terrestrial chromium and strontium isotope data are reconciled by heterogeneous accretion, which includes a transition from dominantly volatile-depleted to volatile-rich materials with possibly high water contents. The Moon-forming giant impact probably involved the collision with a Mars-like protoplanet that had an oxidized mantle, enriched in moderately volatile elements.

Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for Pt-190-Os-186 and Re-187-Os-187 systematics. They have uniform Os-186/Os-188 ratios with a mean of 0.1198353 +/- 7, identical to the mean of 0.1198340 +/- 12 for Os-Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about +/- 30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The Os-187/Os-188 of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing D-Rc, reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and Os-187/Os-188 results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent meIting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and Os-187/Os-188. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to > 1 Ga, Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic Os-187/Os-188 are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and Os-187/Os-188 of about 0.128-0.129, consistent with estimates for the primitive upper mantle. (C) 2000 Published by Elsevier Science B.V. All rights reserved.