Zaicong Wang

Ratios of S, Se and Te in the silicate Earth require a volatile-rich late veneer

The excess of highly siderophile (iron-loving) elements (HSEs) and the chondritic ratios of most HSEs in the bulk silicate Earth (BSE) may reflect the accretion of a chondritic ‘late veneer’ of about 0.5 per cent of Earth’s mass after core formation. The amount of volatiles contained in the late veneer is a key constraint on the budget and the origin of the volatiles in Earth. At high pressures and temperatures, the moderately volatile chalcogen elements sulphur (S), selenium (Se) and tellurium (Te) are moderately to highly siderophile; thus, if depleted by core formation their mantle abundances should reflect the volatile composition of the late veneer. Here we report ratios and abundances of S, Se and Te in the mantle determined from new isotope dilution data for post-Archaean mantle peridotites. The mean S/Se and Se/Te ratios of mantle lherzolites overlap with CI (Ivuna-type) carbonaceous chondrite values. The Se/Te ratios of ordinary and enstatite chondrites are significantly different. The chalcogen/HSE ratio of the BSE is similar to that of CM (Mighei-type) carbonaceous chondrites, consistent with the view that the HSE signature of the BSE reflects a predominance of slightly volatile-depleted, carbonaceous-chondrite-like material, possibly with a minor proportion of non-chondritic material. Depending on the estimates for the abundances of water and carbon in the BSE, the late veneer may have supplied 20 to 100 per cent of the budget of hydrogen and carbon in the BSE.

Multiple isotope composition (S, Pb, H, O, He, and Ar) and genetic implications for gold deposits in the Jiapigou gold belt, Northeast China

The Jiapigou gold belt (>150xa0t Au), one of the most important gold-producing districts in China, is located at the northeastern margin of the North China Craton. It is composed of 17 gold deposits with an average grade around 10xa0g/t Au. The deposits are hosted in Archean gneiss and TTG rocks, and are all in shear zones or fractures of varying orientations and magnitudes. The δ34S values of sulfide from ores are mainly between 2.7u2009‰ and 10u2009‰. The Pb isotope characteristics of ore sulfides are different from those of the Archean metamorphic rocks and Mesozoic granites and dikes, and indicate that they have different lead sources. The sulfur and lead isotope compositions imply that the ore-forming materials might originate from multiple, mainly deep sources. Fluid inclusions in pyrite have 3He/4He ratios of 0.6 to 2.5xa0Ra, whereas their 40Ar/36Ar ratios range from 1,444 to 9,805, indicating a dominantly mantle fluid with a negligible crustal component. δ18O values calculated from hydrothermal quartz are between −0.2u2009‰ and +5.9u2009‰, and δD values of the fluids in the fluid inclusions in quartz are from −70u2009‰ to −96u2009‰. These ranges suggest dominantly magmatic water with a minor meteoric component. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a dominantly mantle source with minor crustal addition.

The distribution of lead and thallium in mantle rocks: Insights from the Balmuccia peridotite massif (Italian Alps)

Abstract Sulfides in mantle rocks sometimes have unradiogenic Pb isotopic compositions and, assuming specific conditions, may represent a low U/Pb reservoir that might balance the radiogenic Pb isotope reservoirs of the silicate Earth. A critical requirement to test this hypothesis is knowledge of typical Pb contents in sulfides from different types of mantle rocks and estimates of their contribution to the Pb budget of the mantle rocks. However, data on the distribution of Pb between mantle minerals in mantle rocks from different geologic settings are scarce. In this study, new Pb and Tl concentration data from well-characterized unserpentinized spinel-facies peridotites and pyroxenites from the Balmuccia mantle tectonite (Ivrea-Verbano Zone, Italian Alps) are presented as an example to better understand the Pb distribution in minerals and rocks of the upper mantle. Most peridotites display variable bulk-rock Pb contents (13–97 ng/g), which tend to be lower than Pb contents in the websterites (60–254 ng/g) and clinopyroxenites (174–657 ng/g). The pyroxenites show broadly positive correlations of Pb with Al2O3, Ce, and also S contents. In situ laser ablation-ICP-MS data indicate low Pb contents in olivine, orthopyroxene, and spinel (mostly below the detection limits of 50 ng/g); whereas Pb contents are higher in clinopyroxene (from <50 to 920 ng/g) and in sulfides (typically a few micrograms per gram and sometimes higher in chalcopyrites). Mass-balance calculations indicate that silicates predominantly control Pb contents in bulk rocks (>70–80% of the Pb), with a minor role for sulfides (mostly <20%). This result from Phanerozoic subcontinental lithosphere mantle rocks is consistent with data on abyssal peridotites. As in some previous studies, bulk-rock Pb contents calculated from constituent phases of peridotites are often lower than the measured values. This imbalance mainly reflects trapped fluid inclusions in silicate minerals and, perhaps also fine-grained crystallization products of trapped melt along grain boundaries. Bulk-rock Tl contents in peridotites (0.05–3.5 ng/g) are lower than in pyroxenites (0.66–7.9 ng/g) and display no correlations with Al2O3 and S contents. The bulk-rock data probably reflect the highly heterogeneous distribution of Tl in sulfides (<0.01–110 μg/g), and, as for Pb, the effect of trapped fluid inclusion. Because the Pb budget in mantle rocks is mainly controlled by silicates, mantle sulfides with unradiogenic Pb isotopic compositions likely cannot balance radiogenic Pb isotopic compositions of oceanic basalts.