Radiogenic Sr Isotopic Compositions Research Articles

Rare earth element and isotope (C, O, Sr) characteristics of hydrothermal carbonates: genetic implications for dolomite-hosted talc mineralization at Göpfersgrün (Fichtelgebirge, Germany)

The talc deposit at Göpfersgrün (Fichtelgebirge, Germany) was formed by hydrothermal alteration of the Wunsiedel marble probably during Permian times. The Wunsiedel marble consists of calcite (cc1) and dolomite (do1) layers. Low REE contents, flat shale-normalized REE n patterns with a negative Ce anomaly, δ 13 C PDB values of −1.5 to 2‰ and a low 87 Sr/ 86 Sr ratio (0.7086) of the hydrothermally unaltered marble are typical for a sedimentary protolith formed within a marine environment. Hydrothermal alteration and talc mineralization of the marble occurs along a major fault zone and is associated with formation of massive saddle dolomite 2 (do2) replacing cc1 and do1. Massive dolomitization is followed by vug-filling carbonates, dolomite 3, calcite 2 and calcite 3. Each carbonate generation has a distinct chemical composition. Dolomite 2 is enriched in REE, depleted in 18 O ( δ 18 O SMOW =10.8 to 11.9‰) and shows a higher 87 Sr/ 86 Sr value (0.7103) compared to the Wunsiedel marble ( δ 18 O SMOW =14.3 to 18.1‰). Late stage vug-filling carbonates differ from the preceding (replacement) dolomite 2 by higher REE contents, a clear roof-shaped REE n pattern, enhanced radiogenic Sr isotope composition ( 87 Sr/ 86 Sr =0.7115 and 0.7117) and lower δ 13 C PDB values (−2.6 to −11.5‰). Main talc mineralization result from decarbonation reactions at low X CO 2 and temperatures between 250 and 400°C. It is proposed that main stage hydrothermal dolomitization, talc mineralization and late stage carbonate vug-fillings are related to formation brines or crustal fluids that interact with graphite-bearing metapelites under acid conditions. Changes in chemical composition between (early) replacement dolomite and (late) vug-filling carbonates are mainly ascribed to increasing fluid/rock ratios and decreasing temperatures.

Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the87Sr/86Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between87Sr/86Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low87Sr/86Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and87Sr/86Sr in the headwaters on a basin-wide scale, however, the high87Sr/86Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of87Sr/86Sr and Ca/Sr data of the source waters show that they diverge from a low87Sr/86Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO2 consumption rates and their global significance.

Petrogenesis of 1.9 Ga limestones and dolostones and their record of Paleoproterozoic environments

Fibrous calcite cements from limestones of the Paleoproterozoic Pethei Group in northwest Canada have relatively constant δ 13C values, near 1‰, that indicate the C exogenic cycle of ca 1.9 Ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the Phanerozoic. 87Sr/ 86Sr ratios of these cements are near 0.7051, and support a progressively increasing influx of radiogenic Sr from continental weathering into the Paleoproterozoic oceans. δ 18O values of the cements vary from −14 to −8.5‰, although the original values of the cements were probably higher because the cements have been subseqently altered, as evidenced by their variable trace element contents. Proposed higher temperatures or substantially lower δ 18O values of ancient seawater based on such values may be in error. Fabric retentive dolomites, generally regarded as syndepositional, also have variable δ 18O values, the highest of which are ca 30 greater than the highest values of the fibrous calcite cements. The dolomites have more radiogenic Sr isotopic compositions indicating that they formed from fluids that interacted with rocks such as those of the clastic sediments which underlie the limestones. These fluids may have partially altered the δ 18O values and chemical compositions of the fibrous calcite cements. Late-stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar δ 18O values and radiogenic Sr isotopic ratios as the fabric retentive dolomites. If these potentially syndepositional dolomites have had their primary isotopic compositions reset through interaction with diagenetic fluids, this would have significant implications to other studies using early dolomite to constrain ancient environmental conditions.

Rare earth element behavior and Pb, Sr, Nd isotope systematics in a heavy metal contaminated soil

The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure. The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO 3. The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The 87Sr/ 86Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO 3 extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr. The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and Fe Mn oxides for the REE in the HCl and HNO 3 extracts. A part of the HNO 3 extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by 143Nd/ 144Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.

CHeAr variations within a dunite nodule as a function of fluid inclusion morphology

Combined laser extraction and manometric-quadrupole-mass spectroscopic measurements permit the simultaneous analysis of CO 2, He and Ar in selected fluid inclusion generations trapped in a dunite nodule from Réunion Island. Variations in 40 Ar 36 Ar in primary fluid inclusions (1000–8500) implies that atmospheric Ar is present in all the trapped fluids, possibly introduced by percolation of seawater into the magma from which the olivine crystallised. Later inclusion generations have lower 40 Ar 36 Ar (500–3500). CO 2 3 He ratios ((1–9.6) × 10 9) are at the lower range of reported values from high 3 He 4 He xenoliths ((1–20) × 10 9) but show no systematic variation with inclusion generation. 4 He 40 Ar∗ values are constant (2.4 ± 0.1) for all inclusion generations and over wide ranges in CO 2 He ((0.2–8.2) × 10 5). This is not consistent with the relative solubilities of CO 2, He and Ar in basaltic melts but implies that ƒ CO 2 is determined by some other process, possibly the partitioning of CO 2 into an immiscible phase. Mixing between high 3 He 4 He , high He abundance mantle (primitive He mantle) with a degassed and incompatible element enriched component can generate the coupled high 3 He 4 He -radiogenic Sr and Pb isotopic compositions of Réunion lavas. The range in C 3 He likely for the primitive He mantle ((1–20) × 10 9) spans the estimated C 3 He of chondrites (≈ 2 × 10 9). The primitive He component cannot represent undegassed primitive mantle as the C 36 Ar values of Réunion (> 1 × 10 9) are higher than those of chondrites (8 × 10 7).

The role of the crust in the magmatic evolution of the island of Lipari (Aeolian Islands, Italy)

Volcanic rocks on the island of Lipari show the entire range of Sr, Nd, Pb isotopic compositions displayed by other islands in the Aeolian archipelago. The rapid isotopic evolution of subaerial volcanic rocks on Lipari towards crustal values together with the appropriate isotopic composition of the neighbouring Calabrian crust (Serre) indicate that many geochemical characteristics observed in the lavas can be attributed to contamination and mixing with crustal materials and melts. Interpretation of the data is complicated by the fact that underplating onto the crust-mantle boundary and the specific lithologies present in the crustal section differ underneath each individual sector of the island. In the central and northern parts of the island, metapelitic rocks were incorporated to provide the more radiogenic Sr isotopic compositions of some lavas. The products from M. Guardia in the southern part of Lipari, where activity is restricted to the last 30–40 ka, bear geochemical similarities to the island of Vulcano, where it is proposed that considerable remobilization of the crust took place in the presence of mafic mantle-derived melts. On Lipari the petrogenetic processes of magma mixing and assimilation dominate over fractional crystallization, and the observed increase of K2O over Na2O can be correlated with contributions from metapelitic crustal lithologies. It is suggested that the variability in isotopic composition and the budget of alkalis (Na2O versus K2O) in the lavas can be explained by invoking a heat source from an intruding asthenospheric MORB-type mantle into a cooler lithospheric crust/mantle during the opening of the Tyrrhenian basin.

Isotopic geochemistry of Fernando de Noronha

Volcanic and hypabyssal rocks ranging in age from 12 to 3 Ma from the Fernando de Noronha archipelago in the western equatorial Atlantic Ocean can generally be divided into two age-compositional groups that have variable and distinct isotopic compositions. Predominantly older alkali basalts and trachytes are generally characterized by more radiogenic Sr-isotopic ( 87Sr/ 86Sr= 0.70457–0.70485 ) compositions and less radiogenic Nd-isotopic ( 143/Nd 144Nd= 0.51271–0.51281 ) and Pb-isotopic ( 206Pb/ 204Pb= 19.132–19.282 ) compositions relative to the generally younger, more alkaline Si-undersaturated rocks which include nephelinites, ankaratrites, and melilitites ( 87Sr/ 86Sr= 0.70365–0.70418, 143Nd/ 144Nd= 0.51277–0.51290, 206Pb/ 204Pb= 19.317–19.565 ). These variations suggest the influence of at least two separate components in the source(s) of both series. One component is characterized by high Rb/Sr and low μ, possibly derived from delaminated subcontinental lithosphere, whereas the other has high μ and low Rb/Sr similar to the source of St. Helena lavas. A third component is suggested by correlated compositions in the latest alkaline, Si-undersaturated lavas, and this component may be derived from depleted mantle. These isotopic variations in conjunction with the generally increasing degree of alkalinity with time are consistent with the temporal depletion of a low-μ, high Rb/Sr component and increasing contributions from a high-μ component in the sources of the volanic rocks of Fernando de Noronha.