Antonio Simonetti

U–Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array

This study reports U–Pb geochronological data for zircon obtained by laser ablation-multi-collector-ICP-MS using a new collector block design that includes three ion counters and twelve Faraday buckets. The collector configuration allows for simultaneous detection of ion signals from mass 238U to 203Tl, an important factor for the achievement of highly precise and reproducible Pb–Pb and Pb–U ratios. The main advantage of the multiple ion counting system is the capability to readily measure low Pb ion signals ( 50) of zircon grains.

A Hf‐Nd isotopic correlation in ferromanganese nodules

The 176Hf/177Hf ratio was measured on 34 ferromanganese nodules, mostly from the Atlantic ocean. The different ocean basins are isotopically distinct with the extreme compositions being less radiogenic in the Atlantic (єHf ∼ +1) than in the Pacific (єHf ∼ +9). A good correlation of єHf and єNd is observed amongst most samples which supports that Hf isotopic compositions in nodules reflect those of ambient seawater. For a given єNd, єHf is more radiogenic in ferromanganese nodules than in rocks from either the mantle or the crust. This correlation makes the coupled Hf-Nd systems a potential paleoceanographic tool. It is argued that a zircon-free clayish component of probable eolian origin may account for the radiogenic Hf in nodules.

Symmetry versus Minimal Pentagonal Adjacencies in Uranium‐Based Polyoxometalate Fullerene Topologies

C U soon: Clusters containing 60, 44, and 36 uranyl peroxide hydroxide polyhedra (see picture) adopt fullerene topologies of maximum symmetry. The largest of these, denoted U60, is topologically identical to C(60) with no pentagonal adjacencies and the highest possible symmetry. U44 adopts the topology with maximum symmetry rather than that with the lowest number of pentagonal adjacencies.

Hf isotope evidence for a hidden mantle reservoir

High-precision Hf isotopic analyses and U-Pb ages of carbonatites and kimberlites from Greenland and eastern North America, including Earth9s oldest known carbonatite (3 Ga), indicate derivation from an enriched mantle source. This previously unidentified mantle reservoir—marked by an unradiogenic Hf isotopic composition and preserved in the deep mantle for at least 3 b.y.—may account for the mass imbalance in Earth9s Hf-Nd budget. The Hf isotopic data presented here support a common mantle source region and genetic link between carbonatite and some oceanic- island basalt volcanoes.

Probing Archean lithosphere using the Lu^Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

Abstract A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth’s mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites ( 1100°C; 0.004–0.03, 0.28265–0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu–Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ∼150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the Somerset low-temperature peridotites indicate that part of the mantle root beneath the North American craton is characterized by a more radiogenic Hf isotope signature than that estimated for a typical ‘depleted’ mantle.

Direct dating of Archean microbial ichnofossils

Well-preserved Archean pillow lavas from the ca. 3.35 Ga Euro Basalt of the Pilbara Craton, Western Australia, contain micron-sized tubular structures mineralized by titanite (CaTiSiO 4 ) with residual organic carbon preserved along their margins. Direct U-Pb dating of titanite in the tubular structures demonstrates an Archean age. These tubular microstruc- tures are identical to microbial ichnofossils in modern basalts, ophiolites, and greenstone belts, and are interpreted as a biogenic signature in these ancient rocks. Microbial colonization of basaltic glass thus appears to have been part of a deep subsurface biosphere established early in Earths history.

In situ 87Sr/86Sr investigation of igneous apatites and carbonates using laser-ablation MC-ICP-MS

Abstract In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (≪1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured 87 Sr/ 86 Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses ( n = 107) of apatite and carbonate yield 87 Sr/ 86 Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in 87 Sr/ 86 Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle.

Pb and Sr isotopic compositions of snowpack from québec, canada: inferences on the sources and deposition budgets of atmospheric heavy metals

Elemental concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained along two main transects from the province of Quebec (Canada); one north-south (between 47°N and 55°N; 1994) and the other within the St. Lawrence Valley (1997). Median enrichment factors (relative to upper crustal abundances) for Cd, Cu, Mn, Pb and Zn for all samples range from ≈300 to ≈42,000 and are indicative of an anthropogenic origin. Pb isotope ratios for snow samples retrieved in 1994 are highly variable (206Pb/207Pb = 1.148 to 1.193) and are characterized by the most radiogenic Sr isotope values (87Sr/86Sr ≥ 0.710). In contrast, the Pb and Sr isotope results for 1997 snow samples collected along the St. Lawrence Valley (below latitude 47°N), yield the most radiogenic Pb isotope ratios (206Pb/207Pb = 1.180 to 1.190) and 87Sr/86Sr ratios between 0.708 and 0.710. The former indicate that the atmospheric pollution in this region of Quebec is dominated by a mixture of anthropogenic emissions from U.S. (206Pb/207Pb ≈ 1.20) and Canadian (206Pb/207Pb ≈ 1.15) sources. Pb isotope ratios (206Pb/207Pb = 1.160 to 1.180) for 1997 samples collected north of latitude 47°N indicate input of an additional anthropogenic component, possibly that of Eurasian pollution being transported over the high Arctic during the winter season. A comparison of the Pb isotope results between lichens and snow samples from identical sample locations indicate that these either overlap (along St. Lawrence Valley), or are significantly different (north–south transect). The latter discrepancy may be attributed to either: (1) different time scales for the integration of the atmospheric signal (months for snow vs. years for lichens); (2) recording of the atmospheric signal at substantially different altitudes; or (3) the presence of an important, local point source of atmospheric pollution. Annual depositional budgets have been estimated for Pb, Cd, Zn, Cu, and Mn, and average values (g km−2 yr−1) are 1500, 130, 196,000, 1900, and 6400, respectively. Compared to previous depositional fluxes (1993–1994) estimated from adjacent regions in North America, those reported here are slightly lower with the exception of Mn and Zn. The nondecrease in depositional fluxes of Mn may be attributed to combustion of Mn-bearing fossil fuels by automotive vehicles. The exact cause for the elevated annual depositional values for Zn, however, remains enigmatic.

Small-scale Sr isotope investigation of clinopyroxenes from peridotite xenoliths by laser ablation MC-ICP-MS-implications for mantle metasomatism

Although numerous studies have shown that mantle xenoliths are isotopically heterogeneous on a hand specimen scale, experimental data imply that isotopic equilibrium should be attained within single mineral grains in the upper mantle. Until recently, this prediction has not been verified because of the difficulty of analyzing individual grains. We report in situ 87 Sr/ 86 Sr compositions of clinopyroxenes in garnet peridotite xenoliths from the Nikos kimberlite, Somerset Island (Arctic Canada) obtained by laser ablation MC-ICP-MS. Results for five different peridotites indicate the existence of large Sr isotope variations within individual xenolith samples, varying from 0.5xto as much as 1.1x(3 to 8 parts in 7000). This study is the first to document isotopic heterogeneity in peridotite xenoliths at the scale of individual grains. Multiple analyses of the same grain, however, indicate intra-grain Sr isotopic equilibrium. The Sr isotopic ratios for individual clinopyroxene grains correlate with major element abundances such as SiO2 and TiO2, and trend towards the composition of the host Nikos kimberlite. It thus appears likely that the Sr isotope heterogeneities recorded by the clinopyroxenes are the result of metasomatic interaction with the host kimberlite magma. The preservation of Sr isotopic variability in clinopyroxene separates from different peridotite samples and between clinopyroxene grains of individual xenoliths suggests that metasomatism occurred just prior to or during kimberlite transport. The Sr isotope heterogeneities documented here for a single peridotite phase suggest that isotopic compositions obtained on mineral separates from peridotite xenoliths likely represent weighted averages and could be less homogenous than previously assumed. D 2003 Elsevier Science B.V. All rights reserved.

Tracing sources of atmospheric pollution in Western Canada using the Pb isotopic composition and heavy metal abundances of epiphytic lichens

The Pb isotopic composition and trace metal concentrations of epiphytic lichens collected from tree branches within northwestern North America are reported here, with a latitudinal coverage extending from the Beaufort Sea (Arctic circle) to the Canada–USA border. Overall, the trace metal concentrations and Pb isotope compositions correlate with latitudinal position, since lichens retrieved north of latitude 60°N are characterized by low enrichment factors (EF) (mainly between 10 and 30) for heavy metals (i.e. Pb, Zn) and radiogenic 206Pb/207Pb isotope values (∼1.170–1.180). Samples collected further south are characterized by higher EF for heavy metals and much lower Pb isotopic compositions (i.e. 206Pb/207Pb⩽1.150). Lichens retrieved in the immediate vicinity of major urban centers (i.e. Calgary, Alberta and Victoria, British Columbia) record distinct Pb isotopic values compared to the regional signal measured in adjacent (remote) samples. The total variation defined by the Pb isotopic compositions of the lichens may be attributed to the mixing of atmospheric particulates and aerosols derived from at least four end-member components, three anthropogenic and one natural. The latter is the predominant signal recorded in lichens retrieved north of 60°N, and is similar in Pb isotopic composition to a natural component identified in aerosols collected during the autumn season of 1994 at Alert (Canadian High Arctic). In contrast, samples collected further south reflect in-part mixing between Canadian and USA anthropogenic sources of atmospheric Pb. The third (unradiogenic) anthropogenic end-member most probably represents atmospheric emissions originating from one of the worlds major Zn/Pb smelters located at Trail, British Columbia.