The Calcium Isotope Composition of Modern Seawater Determined by Thermal Ionisation Mass Spectrometry

The Ca isotopic composition of modern seawater has been determined using a ⁴³Ca-⁴⁸Ca double spike, which was calibrated using a ⁴²Ca/⁴⁴Ca seawater ratio of 0.30587 ± 0.00026. This ratio was determined from a total evaporation experiment in which the ion beam was measured from the beginning to the end of the emission. With integration of the peak intensities, the fractionation effects can be minimised, since total evaporation of the reservoir cancels out the effect of vapour enrichment in the light isotopes. This experiment avoids the gravimetric uncertainty inherent in the double spike calibration. This calibration allows the precise redetermination of the seawater isotopic composition of Ca. A mean ⁴⁰Ca/⁴⁴Ca ratio for two Atlantic water samples of 45.143 (2s_mean= 0.003) was found. The good reproducibility of the Ca isotope ratios in present seawater and the very strong isotopic homogeneity of Ca in the oceans illustrate the advantage of using seawater as the common standard, with the advantage of decreasing interlaboratory bias.     

The lower crust beneath cratonic north-east Europe: isotopic constraints from garnet granulite xenoliths

Lower crustal garnet-bearing mafic granulite xenoliths from beneath the cratonic areas of NE Europe (NW Russia, Belarus, Finland) have unradiogenic ¹⁴³Nd/¹⁴⁴Nd ratios that differ strongly from those of xenoliths from beneath Phanerozoic regions of the European plate and worldwide, but closely resemble xenoliths from other cratonic regions of the world. Phanerozoic lower crustal xenoliths worldwide also show a very limited range of Pb isotope compositions whereas most cratonic lower crustal xenoliths have more varied but usually unradiogenic Pb isotope compositions, plotting to the left of the Geochron. However, many of the xenoliths from beneath NE Europe plot on the right-hand side of the Geochron and also have more radiogenic ²⁰⁸Pb/²⁰⁴Pb ratios. Thus, the lower crust of NE Europe shows characteristics of both cratonic lower crust (unradiogenic Nd isotopes) and Phanerozoic lower crust (radiogenic Pb isotopes). Its present-day low U/Pb and Th/Pb ratios indicate that it has been depleted in heat-producing elements, but the radiogenic Pb isotope ratios show that this depletion occurred relatively recently.     

Application of Laser Ablation ICP-MS to U-Th-Pb Dating of Monazite

Recent advances in laser ablation ICP-MS techniques allow accurate U-Th-Pb age dating of monazites that are as young as several tens of million years to a precision better than 2%. Accuracy of the age determinations has been improved by true real-time mass bias correction via nebulisation of a solution containing enriched ²³³U and natural Tl isotopes. The Tl-U tracer solution eliminates possible effects of variable sample matrices on the precision and accuracy of measured isotopic ratios. Mass bias corrections based on measured ²⁰⁵Tl/²³³U ratios in the tracer solution allow direct measurement of ²³⁵U in monazite. Combined with high-sensitivity laser ablation ICP-MS measurements, direct measurement of ²³⁵U particularly improves the precision of U-Pb dating of young monazites. Correction for laser-induced Pb/U and Pb/Th elemental fractionation is based on a mathematical treatment of time resolved count-rate data that is independent of laser ablation characteristics, does not require external standardisation and allows variable laser pit size or raster patterns for each measurement. The new procedures make the LA ICP-MS technique more flexible for in situ U-Th-Pb analysis.     

The Use of Multiple Reference Samples for the Monitoring of Ion Microprobe Performance During Zircon 207Pb/206Pb Age Determinations

Data from a series of extended analytical sessions using a range of different zircon reference samples have been used to investigate the periodic derivation of abnormal ²⁰⁷Pb/²⁰⁶Pb ages during microbeam analysis%For the Canberra SHRIMP II, this phenomenon probably results from the presence of a signal at mass 204 that is alien to the Pb spectrum; isotopic fractionation is unlikely to be a significant contributor. In contrast, abnormal ²⁰⁷Pb/²⁰⁶Pb ages obtained from SHRIMP I at the same research centre require a different, but as yet unknown explanation. Assessment of the reasons for atypical ^207Pb/²⁰⁶Pb measurements and the means of correcting for them should therefore be independently assessed for individual analytical instruments. The use of reference samples with very different ages is an effective way of doing this, because older reference samples are more useful for documenting isotopic fractionation, whereas younger ones are more sensitive indicators of isobaric interference.     

Direct Determination of Lead Isotope Ratios by Laser Ablation-Inductively Coupled Plasma-Quadrupole Mass Spectrometry in Lake Sediment Samples

A direct method for the determination of lead isotopic ratios by laser ablation-inductively coupled plasma-quadrupole mass spectrometry (LA-ICP-QMS) is presented. Samples of lake sediments were ground and pressed as pellets before being analysed. Mass bias was corrected by analysing an external calibration sample manufactured with pure silica doped with NIST SRM 981 solution. The efficiency of the mass bias correction was checked by comparing the ICP-MS data with lead isotopic ratios determined by thermal ionisation mass spectrometry (TIMS). The average long term reproducibilities were 0.40%, 0.40%, 0.20% and 0.30% (RSD) respectively for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb ratios. The method was applied to the study of lake sediment samples, in order to determine the amount and origin of historical contamination by lead.     

大别山北缘中生代火山—侵入岩铅同位素组成特征及其地质意义

大别山北缘中生代火山－侵入岩的１４个夺铅同位素组成表现为高的非放射性成因＾２０４Ｐｂ,较低的＾２０６Ｐｂ／＾２０４Ｐｂ比值和低且相对一致的＾２０７Ｐｂ／＾２０４Ｐｂ比值,属于明显的低Ｕ／Ｐｂ体系,并暗示它们由具低μ值的源岩衍生的。     

Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA

Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in southern Washington represent fragments of mantle lithosphere from the back-arc side of the Cascade arc front. Previous studies have shown that metasomatism by either silica-rich fluids or hydrous melts crystallized phlogopite, imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and more radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re–Os, Sm–Nd, Sr, and U–Th–Pb isotopic data shed further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb=18.63–19.55, ²⁰⁷Pb/²⁰⁴Pb=15.56–15.63, ²⁰⁸Pb/²⁰⁴Pb=38.22–38.87). The most radiogenic Pb isotopic compositions extend beyond the most radiogenic Pb isotopic compositions for the Cascade arc lavas and display a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at the high ²⁰⁶Pb/²⁰⁴Pb end of this array. Therefore, models for rapid transfer of components from the slab to the mantle lithosphere are not viable in this case. Instead, a multi-stage model is preferred. In the first stage, the slab component is transferred via fluid or melt into, and reacts with the hanging wall mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting of the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures. In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compositions of the clinopyroxenes, and the presence of high alkali glasses in the xenoliths, are consistent with the metasomatic agent derived from the hybridized hanging wall being alkali-rich, and possibly similar to potassic-rich lavas found in arc and back-arc settings. These data therefore demonstrate the importance of the hybridized hanging wall mantle above slabs as a source for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at least tens of million years.     

A lead isotopic study of the Stillwater Complex,Montana: constraints on crustal contamination and source regions

Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater Complex indicate a narrow range of apparent initial isotopic compositions (²⁰⁶Pb/ ²⁰⁴Pb=13.95; ²⁰⁷Pb/²⁰⁴Pb=14.95–15.01; ²⁰⁸Pb/²⁰⁴Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher ²⁰⁷Pb/²⁰⁴Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas. Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values, but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73–2.79 Ga), Late Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about 0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2–26) but often high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were generated at different levels within this crust/mantle system, before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs. This situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the Stillwater magmas. The late Archean crustal complex and the Stillwater Complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the Middle to Early Archean crust present in the region.     

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.     

Provenance ages and alteration histories of shales from the Middle Archean Buhwa greenstone belt, Zimbabwe: Nd and Pb isotopic evidence

Shales of the ca. 3.0 Ga Buhwa Greenstone Belt, Zimbabwe, were derived from a compositionally diverse provenance whose ages, determined by ion probe analyses of detrital zircons in interbedded sandstones, range from 3.8 to 3.1 Ga. Geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source. REE concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed, with components of felsic (tonalite and alkalic granitoid) and mafic rocks. Sm/Nd and Nd isotopic compositions of these rocks can be used to model initial Nd isotopic ratios at the time of sedimentation (ε_Ndsed), as well as model crustal formation ages (T_DM). The former, at the age of sedimentation, range from +0.6 to −10.8, consistent with a range of provenance ages. The latter range from 4.46 Ga to 2.99 Ga. The oldest crustal formation ages, up to 0.7 Ga older than known detrital components, are interpreted here to indicate that the Sm-Nd system of the sediments experienced open system behavior. The implied alteration would have included an increase in Sm/Nd by about 20-25 percent, probably in the form of preferential loss of Nd with respect to Sm. The Pb isotopic compositions of whole rock samples are quite radiogenic, with a range of ²⁰⁶Pb/²⁰⁴Pb from 25.5 to 154. An array of ten samples lies scattered about a line with a ²⁰⁷Pb/²⁰⁴Pb -²⁰⁶Pb/²⁰⁴Pb slope age of about 2.73 Ga. Five individual samples were sequentially leached to further test the timing and characteristics of this U-Th-Pb alteration event. These arrays of a whole rock, three leach steps, and a residue also form linear Pb-Pb arrays (one is more scattered) with ages ranging from 2260 ± 360 Ma to 2824 ± 170 Ma, suggesting that all samples experienced a latest Archean to earliest Proterozoic enrichment in U/Pb. This age range also may be the approximate age of Sm/Nd enrichment for the shales. All samples, both whole rocks and leached samples, lie grouped on a ²⁰⁸Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagram around a line with ²³²Th/²³⁸U = 3.5 (2.9 to 3.9). Because of the lack of large differences in the Th/U of the samples through large ranges of U/Pb, we interpret this consistency in Th/U to mean that the shales of the Buhwa belt experienced Pb loss, rather than U and Th gain. Circumstances that may be responsible for Pb loss in a sedimentary basin include loss of saline fluids during basin dewatering. Such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent Limpopo Belt, suggesting that uplift, dewatering, and geochemical and isotopic alteration can be genetically related.     

Natural Cadmium Isotopic Variations in Eight Geological Reference Materials (NIST SRM 2711, BCR 176, GSS-1, GXR-1, GXR-2, GSD-12, Nod-P-1, Nod-A-1) and Anthropogenic Samples, Measured by MC-ICP-MS

In this study, the Cd isotopic composition of various geological reference materials and anthropogenic samples was investigated. The measurements were made by multicollector ICP-MS and instrumental mass fractionation was controlled using a "sample-standard bracketing" technique. Cadmium isotopic data are reported relative to an internal Cd solution (Cd Spex) and expressed as the ¹¹⁴ Cd/¹¹⁰Cd delta value. Two other Cd solutions (Prolabo and JMC) were analysed and yielded the same 0% delta value. A fractionated Cd metal sample (Münster Cd) was used as a secondary reference material for Cd isotopic measurements and we obtained a ¹¹⁴ Cd/¹¹⁰ Cd delta value of 4.48% relative to Cd Spex solution. As opposed to multi-stage Cd purification previously published in the literature, a new one step anionic exchange purification using dilute HCl for the analysis of Cd isotopes in geological samples was developed. This method enabled a high recovery (> 95%) and effective separation of the sample matrix to be achieved. The long-term external reproducibility was evaluated at 0.12% (2 standard deviations) for the ¹¹⁴ Cd/¹¹⁰Cd ratio, based on reference solutions and replicated measurements of samples over one year. The variation of Cd isotopic composition of natural terrestrial samples is restricted to a small range of 0.4%, which is similar to previously reported results. In contrast, large variations of Cd isotopic composition were found for anthropogenic samples with values as low as −0.64% for a dust sample issued from a lead smelter and values as high as +0.50% for NIST SRM 2711 (metal-rich soil). These variations are 10 times larger than the reproducibility and suggest that Cd isotopes can be useful as tracers of anthropogenic sources of Cd in the environment.     

Pb Isotopic Composition and Metal Sources of Au and Ag Deposits of the South Verkhoyansk Region (Yakutia,Russia) According to High-Precision MC-ICP-MS Data

The paper considers the results of high-precision Pb–Pb isotopic analysis of 120 galena samples from 27 Au and Ag deposits of the South Verkhoyansk Synclinorium (SVS) including large Nezhdaninsky deposit (628.8 t Au). The Pb isotopic composition is analyzed on a MC-ICP-MS NEPTUNE mass-spectrometer from solutions with an error of no more than ±0.02% (2σ). Four types of deposits are studied: (i) stratified vein gold–quartz deposits (type 1) hosted in metamorphosed Upper Carboniferous–Lower Permian terrigenous rocks and formed during accretion of the Okhotsk Block to the North Asian Craton synchronously with dislocation metamorphism and related granitic magmatism; (ii) vein gold–quartz (Nezhdaninsky type) deposits also hosted in Lower Permian metasedimentary rocks; (iii) Au–Bi deposits localized at the contact zones of the Late Cretaceous granitic plutons; and (iv) Sn–Ag polymetallic deposits related to granitic and subvolcanic rocks of the Okhotsk Zone of the SVS. The deposits of types 2, 3, and 4 are postaccretionary. The general range of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios is 18.1516–18.5903 (2.4%), 15.5175–15.6155 (0.63%), and 38.3010–39.0481 (2.0%), respectively. In ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb diagrams, the data points of Pb isotopic compositions of all deposits occupy restricted, partly overlapping areas along a general elongated trend. The various SVS Au–Ag deposits can be classified according to the Pb isotopic composition in accordance with all three Pb ratios. Deposits of the same type show distinct Pb isotopic compositions that strongly exceed the scale of analytical error (±0.02%). The differences in Pb isotopic composition within specific deposits are low and subordinate and have little effect on variations in the Pb isotopic composition of the SVS deposits. The μ₂ values (Stacey–Kramers model), which characterize the ²³⁸U/²⁰⁴Pb ratios of ore lead sources of the SVS deposits, widely vary from 9.7 to 9.38. The ω₂ values (²³²Th/²⁰⁴Pb) are 39.82–36.61, whereas the Th/U ratios are 4.04–3.86. The content of all three radiogenic Pb isotopes and μ₂ values of feldspars from SVS intrusive rocks are strongly distinct from those of galena of stratified gold–quartz and vein gold–quartz deposits and are identical to Pb of galena from Au–Bi and Sn–Ag polymetallic deposits, indicating a mostly magmatic origin for the Pb of these deposits. Detailed isotopic study of the Nezhdaninsky deposit shows different Pb isotopic composition of two consecutive mineral assemblages (gold–sulfide and Ag polymetallic): ~0.30, ~0.07, and ~0.22% for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios, respectively. These differences are interpreted as a result of involvement of at least two metal sources during the evolution of an ore-forming system: (i) host Lower Permian terrigenous rocks and (ii) a magmatic source similar in Pb isotopic composition to that of Sn–Ag polymetallic deposits. The Pb isotopic composition and μ₂ and Th/U values show that lead of stratified gold–quartz deposits combines isotopic tracers of lower and upper crustal sources (Upper Carboniferous–Lower Permian terrigenous rocks), lead of which was mobilized by ore-bearing fluids. The high ²⁰⁸Pb/²⁰⁶Pb ratios and Th/U evolutionary parameter are common to all Pb isotopic composition of all studied Au–Ag deposits and SVS Cretaceous intrusive rocks and indicate that Pb sources were depleted in U relative to Th. Taking into account the structure of the region and conceptions on its evolution, we can suggest that the magma source was related to lower crustal subducted rocks of the Archean (~2.6 Ga) North Asian Craton and the Okhotsk terrane.     

Quantitative Separation of Molybdenum and Rhenium from Geological Materials for Isotopic Determination by MC-ICP-MS

We have developed a new chemical procedure for the quantitative separation of molybdenum (Mo) and rhenium (Re) from a wide variety of geological samples. A single pass anion exchange separation provided complete recovery of pure Mo and Re in a form that was ideal for subsequent isotope and abundance determination by multi-collector inductively coupled plasma-mass spectroscopy (MC-ICP-MS). An enriched ¹⁰⁰Mo-⁹⁷Mo solution, mixed with the sample before digestion, enabled natural mass-dependant isotopic fractionation of Mo to be determined with an external reproducibility of < 0.12‰ (δ⁹⁸Mo/⁹⁵Mo, 2 s ). Determination of the concentration of Mo and Re in the same sample was achieved by isotope dilution, with instrumental mass-fractionation of Re being corrected by the simultaneous measurement of the ¹⁹¹Ir/¹⁹³Ir ratio. We have applied the new procedure to a variety of samples, including seawater, basalt and organic-rich mudrock. The procedure is ideally suited to palaeoredox studies requiring the precise determination of the Mo isotope composition and the Re/Mo ratio from the same sample.     

Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits,East Fennoscandia,Kola Peninsula,Russia

The Lovozero alkaline massif—an agpaitic nepheline syenite layered intrusion—is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376?±?28 Ma, based on a ²⁰⁶Pb/²⁰⁴Pb versus ²³⁸U/²⁰⁴Pb isochron and 373?±?9 Ma, from a ²⁰⁸Pb/²⁰⁴Pb versus ²³²Th/²⁰⁴Pb isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The ²⁰⁶Pb/²⁰⁴Pb values fall within the depleted MORB field, with some rocks having lower ²⁰⁷Pb/²⁰⁴Pb but higher ²⁰⁸Pb/²⁰⁴Pb values. Together with other related carbonatites having both lower and higher ²⁰⁶Pb/²⁰⁴Pb values, the combined KACP rocks form an extended linear array defining either a?~2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4–2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5–10% contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.     

Sr-Nd-Pb isotope ratios, geochemical compositions, and 40Ar/39Ar data of lavas from San Felix Island (Southeast Pacific): Implications for magma genesis and sources

We present the first trace element and age data combined with new Sr, Nd, and Pb isotope ratios on lavas from San Felix Island in the Southeast Pacific. A ⁴⁰Ar/³⁹Ar plateau age of 421 ± 18 ka implies young intraplate volcanic activity in this region relative to the ∼22 Ma old volcanism on the neighbouring Easter seamount chain (ESC). The incompatible element compositions of the San Felix magmas are similar to those of EM1-type basalts from Gough, although the isotopic compositions differ. San Felix formed some 20 Ma after the ESC plume affected the plate in this region but no chemical signature of the ESC material is observed in the young volcanic rocks. The composition of the San Felix basalts indicates a mantle source containing old continental lithospheric material from either metasomatized mantle or recycled sediments, which ascends in a weak mantle plume.     

A double spike for osmium analysis of highly radiogenic samples

Geologic samples containing highly radiogenic Os (molybdenites and low-level, highly radiogenic (LLHR) samples) have no internal means by which to correct for mass fractionation during isotopic measurement by mass spectrometry. We describe a double spike for use with highly radiogenic samples, created by combining isotopically enriched ¹⁸⁸Os and ¹⁹⁰Os. Spiking molybdenite and other highly radiogenic minerals with this tracer allows for a fractionation correction, as well as a more reliable determination of common Os relative to analysis using single spikes.

The precise isotopic composition of the double spike is determined by a calibration against natural Os, in which two separate measurements are necessary: one each for the pure double spike and the spike–standard mixture. An estimate of the true composition of the spike is obtained by least squares approximation, and the errors are obtained by Monte Carlo methods. Sample analyses are then much more straightforward than the calibration because isotopic compositions of all components are known a priori.

Results obtained with a mixed Re-double Os spike demonstrate an improved reproducibility over individual ¹⁸⁵Re and ¹⁹⁰Os spikes. For an Archean in-house molybdenite standard we now observe a reproducibility of 0.08%. The ability to make a fractionation correction is essential for Os measurements made by ion counting. With the double Os spike, young samples and those with low Re contents (i.e., LLHR) can now be accurately analyzed. The ¹⁸⁸Os–¹⁹⁰Os double spike also allows a determination of the common Os contents of highly radiogenic samples. Common Os is poorly determined for ancient samples with high concentrations of ¹⁸⁷Os, which fortunately are not sensitive to estimates of common Os. Common Os can be reasonably well determined for younger samples and those with low Re contents. We report a common Os concentration of 0.4±0.1 ppb for an 11 Ma molybdenite. Consideration of common Os content is important for age determination of young samples and LLHR samples, and is not possible by other published means of Os analysis.     

The isotopic composition of oxygen and carbon in the subfossil mollusc shells of the Baltic Sea as an indicator of palaeosalinity

The carbon (δ¹³ C) and oxygen (δ¹⁸O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ¹⁸O values of water show a good correlation with the salinity of the Baltic. This correlation served as a basis for reconstructing palaeosalinity and for stratifying the marine sediments according to the δ¹⁸O values of the carbonate skeletons of subfossil shells. The δ¹³C values in shells are mainly determined by the isotopic composition of land-originating bicarbonate, especially in the carbonate skeleton of Lymnaea balthica , which inhabits the immediate coastal zone. According to the δ¹⁸O data, salinity in the investigated area (the coastal area of W and NW Estonia) was highest (about 9–11%) during the Littorina stage. The Limnae a stage had, in general, a salinity similar to the contemporary one, but during some phases possibly exceeding it by 2–3%.     

A 3.1 billion year old marble and the 87Sr/86Sr of late-Archean seawater

A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ₁ indicate an older continental crustal source for the Nd and Pb. The initial ⁸⁷Sr/⁸⁶Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre-3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured ⁸⁷Sr/⁸⁶Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.     

Nd,Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan,Egypt): high-μ signatures from the mantle lithosphere

The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = ²³⁸U/²⁰⁴Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, ²⁰⁶Pb/²⁰⁴Pb ca. 19.5–20.5, ²⁰⁷Pb/²⁰⁴Pb ca. 15.63–15.73, ²⁰⁸Pb/²⁰⁴Pb ca. 39–40 and ⁸⁷Sr/⁸⁶Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

K-Ar ages and Pb,Sr isotopic characteristics of Cenozoic volcanic rocks in Shandong Province,China

28 samples of Cenozoic volcanic rocks collected from Shandong Province have been dated by K-Ar method. They are mainly Neogene with an age range of 4–19 m. y. The basalts from Linqu and Yishui in west Shandong Province are Miocene and those from Penglai and Qixia in east Shandong Province are Miocene and Pliocene in age. The basalts from Wudi in north Shandong Province are Middle-Early Pleistocene in age. In each area the duration of volcanic eruption was estimated at about 2–3 m. y. Pb and Sr isotopic compositions and U, Th, Pb, Rb, Sr, and major elements in most of the samples were determined. The isotopic compositions are:²⁰⁶Pb/²⁰⁴Pb—16.92-18.48,²⁰⁷Pb/²⁰⁴Pb—15.30-15.59,²⁰⁸Pb/²⁰⁴Pb—37.83-38.54, and (⁸⁷Sr/⁸⁶Sr)_i—0.70327-0.70632. There are some positive or negative linear correlations between²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb, Pb isotopes and Pb content, Pb isotopes and Sr isotopes, and Sr isotopes and other elements. The basaltic rocks from east and west Shandong Province have somewhat differences in isotopic composition and element content. The basalts probably are products of multi-stage evolution of the mantle. They have preserved the primary features of the source, although they were influenced, to some extent, by the contamination of crustal materials.