Isotopic Homogenization Research Articles

New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions

We report oxygen, calcium, titanium and 26Al–26Mg isotope systematics for spinel-hibonite inclusions (SHIBs), a class of calcium–aluminum-rich inclusions (CAI) common in CM chondrites. In contrast to previous studies, our analyses of 33 SHIBs and four SHIB-related objects obtained with high spatial resolution demonstrate that these CAIs have a uniform Δ17O value of approximately −23‰, similar to many other mineralogically pristine CAIs from unmetamorphosed chondrites (e.g., CR, CV, and Acfer 094). Five SHIBs studied for calcium and titanium isotopes have no resolvable anomalies beyond 3σ uncertainties. This suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where SHIBs with uniform Δ17O formed. We established internal 26Al–26Mg isochrons for eight SHIBs and found that seven of these formed with uniformly high levels of 26Al (a multi-CAI mineral isochron yields an initial 26Al/27Al ratio of ∼4.8×10−5), but one SHIB has a smaller initial 26Al/27Al of∼2.5×10−5, indicating variation in 26Al/27Al ratios when SHIBs formed. The uniform calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low initial 26Al/27Al ratios allow for two interpretations. (1) If subcanonical initial 26Al/27Al ratios in SHIBs are due to early formation, as suggested by Liu et al. (2012), our data would indicate that the CAI formation region had achieved a high degree of isotopic homogeneity in oxygen and refractory elements before a homogeneous distribution of 26Al was achieved. (2) Alternatively, if subcanonical ratios were the result of 26Al–26Mg system resetting, the clustering of SHIBs at a Δ17O value of ∼−23‰ would imply that a 16O-rich gaseous reservoir existed in the nebula until at least ∼0.7Ma after the formation of the majority of CAIs.

New triple oxygen isotope data of bulk and separated fractions from SNC meteorites: Evidence for mantle homogeneity of Mars

AbstractWe report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty–Nakhla–Chassigny (SNC) meteorites using enhanced laser‐assisted fluorination technique. This study shows that SNCs have remarkably identical Δ17O and a narrow range in δ18O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px‐ol and mask‐ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass‐dependent fractionation line for bulk materials with a slope of 0.526 ± 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 ± 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 ± 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with Δ17O = 0.318 ± 0.016‰ (SD) for bulk materials and 0.316 ± 0.009‰ (SD) for separated fractions. The conformity in slopes and Δ17O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.

Uranium and Sm isotope studies of the supergiant Olympic Dam Cu–Au–U–Ag deposit, South Australia

The Olympic Dam Cu–U–Au–Ag deposit in the Archean–Proterozoic Gawler Craton (South Australia) is a type example of the iron oxide–copper–gold (IOCG) spectrum of deposits and one of the largest Cu–U–Au resources known. Mineralization is hosted in a lithologically and texturally diverse, hematite-rich breccia complex developed within a granite of the 1.59Ga Gawler Silicic Province. Emerging evidence indicates that both the breccia complex and its metal content developed over ∼1000Ma, responding to major tectonic events, e.g., at 1300–1100, 825 and 500Ma. However, metal sources and exact mechanism/s of ore formation remain poorly known.New high-precision 238U/235U data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range (δ238UCRM112a=−0.56‰ to +0.04‰). At the scale of sampling, there is no correlation of δ238U with lithology, degree of alteration or U mineralogy, although ores with U>5wt.% have subtly higher δ238U values (−0.20‰ to 0.00) than the majority of samples (<0.7wt.% U, −0.56‰ to −0.23‰). The new U isotope data are consistent with published data for uraninites from Olympic Dam, and with published results from high-temperature U deposits. They overlap completely with the range of δ238U values in granitoids (including the host granite, −0.18‰ to −0.32‰) and with estimates of the upper continental crust in general. This similarity suggests that Olympic Dam δ238U values reflects the crustal sources of U, which probably include felsic volcanic rocks and granitoids. The isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of U; alternatively, primary fractionation signatures may have been erased during the long history of the U mineralization.High-grade U ores may record isotopic neutron-capture effects related to fissionogenic neutrons. High-precision Sm isotope data for five high-U (>5wt.%U, U/Sm≫500) Olympic Dam ores define a neutron capture line, with correlated depletions in 149Sm (up to ∼2εunits) and excesses in 150Sm (up to ∼ 4εunits), but fission fragment contributions to Sm are below detection. These observations provide evidence for small-scale neutron-capture effects, with calculated neutron fluences of 1015 to 1016ncm−2, similar to those observed in several Proterozoic and Phanerozoic U deposits. The apparent lack of fission fragment contributions in Olympic Dam high-grade ores can be explained with an age of U deposition, or re-deposition that is substantially younger than the initial 1.59Ga age of the oldest IOCG-style mineralization.The results presented here thus (i) suggest uranium sources in common (likely igneous) upper crustal lithologies, (ii) support geochronological evidence for gradual addition of U in several stages over 1000Ma at elevated temperatures of mineralization, and (iii) do not show the high δ238U signatures expected from low-temperature reworking of older low- δ238U ores.

Predictability of the dispersion of Fukushima-derived radionuclides and their homogenization in the atmosphere

Long-range simulation of the dispersion of air pollutants in the atmosphere is one of the most challenging tasks in geosciences. Application of precise and fast numerical models in risk management and decision support can save human lives and can diminish consequences of an accidental release. Disaster at Fukushima Daiichi nuclear power plant has been the most serious event in the nuclear technology and industry in the recent years. We present and discuss the results of the numerical simulations on dispersion of Fukushima-derived particulate 131I and 137Cs using a global scale Lagrangian particle model. We compare concentrations and arrival times, using two emission scenarios, with the measured data obtained from 182 monitoring stations located all over the Northern Hemisphere. We also investigate the homogenization of isotopes in the atmosphere. Peak concentrations were predicted with typical accuracy of one order of magnitude showing a general underestimation in the case of 131I but not for 137Cs. Tropical and Arctic plumes, as well as the early detections in American and European midlatitudes were generally well predicted, however, the later regional-scale mixing could not be captured by the model. Our investigation highlights the importance of the parameterization of free atmospheric turbulence.

MKED1: A new titanite standard for in situ analysis of Sm–Nd isotopes and U–Pb geochronology

Titanite (sphene) has great potential as a petrogenetic indicator and mineral geochronometer, as it can host high concentrations of a range of trace elements and it occurs in diverse rock types. In this paper we describe a natural titanite standard material that may be used to calibrate chemical and isotopic analysis of titanite of varying age and origin. Through comprehensive bulk analysis of mm-size crystal fragments and in situ microanalysis, we show that the titanite, named MKED1, is largely free of inclusions and is homogenous at the level of analytical precision for major element, U–Pb isotope and Sm–Nd isotope composition. There is some minor zoning in trace element composition, but these zones are easily recognised using backscattered electron imaging and the trace element concentrations of each of these zones are also very homogenous. MKED1 has relatively high levels of high field strength elements (Nb, Ta, Zr, Hf, Sn), rare earth elements (REE), Th, U, and radiogenic Pb, but very low levels of common Pb. Uranium–Pb isotope data show MKED1 to be concordant with 206Pb/207Pb, 207Pb/235U, and 206Pb/238U ages (ID-TIMS) of 1521.02±0.55Ma, 1518.87±0.31Ma, and 1517.32±0.32Ma, respectively. Cross calibration with other titanite standards demonstrates that MKED1 can be used as a primary standard for determining U–Pb ages of titanite ranging in age from Precambrian to Neogene.We show that MKED1 is also suitable as a Sm–Nd isotope reference material due to its relatively high REE concentrations and homogenous 147Sm/144Nd (0.127) and 143Nd/144Nd (0.5116303±0.0000025) content. Isotopic homogeneity is demonstrated through extensive in situ microanalysis, which returned an average initial εNd value of −6.30±0.65 that is comparable to the value obtained by TIMS of −6.13±0.05.We suggest that MKED1 can be employed as a U–Pb isotope and Sm–Nd isotope reference material for in situ or bulk analytical methods, including techniques that allow simultaneous collection of multiple elemental and/or isotopic data sets in situ. MKED1 may also be used as a standard for in situ trace element microanalysis on the provision that locations for analytical sampling are selected with consideration to grain-scale elemental zoning. To demonstrate the potential value of titanite analysis for resolving geological problems we present case studies from two very different geological settings; the first examines the timing and origin of Cu–Au-REE skarn mineralisation from the Mount Isa Inlier, Australia, and the second study investigates the timing and origin of rift related volcanism and sedimentation in the western branch of the East African Rift System.

Isotopic homogenization and scrambling associated with oxygen isotopic exchange on hot platinum: studies on gas pairs (O2, CO2) and (CO, CO2)

The catalytic exchange between O2 and CO2 on hot platinum leads to isotope scrambling in CO2 and homogenization of the oxygen isotopes in the two phases even though they could be different in isotope ratios from each other before the exchange.

Sulfur isotope homogeneity of lunar mare basalts

We present a new set of high precision measurements of relative 33S/32S, 34S/32S, and 36S/32S values in lunar mare basalts. The measurements are referenced to the Vienna-Canyon Diablo Troilite (V-CDT) scale, on which the international reference material, IAEA-S-1, is characterized by δ33S=−0.061‰, δ34S≡−0.3‰ and δ36S=−1.27‰. The present dataset confirms that lunar mare basalts are characterized by a remarkable degree of sulfur isotopic homogeneity, with most new and published SF6-based sulfur isotope measurements consistent with a single mass-dependent mean isotopic composition of δ34S=0.58±0.05‰, Δ33S=0.008±0.006‰, and Δ36S=0.2±0.2‰, relative to V-CDT, where the uncertainties are quoted as 99% confidence intervals on the mean. This homogeneity allows identification of a single sample (12022, 281) with an apparent 33S enrichment, possibly reflecting cosmic-ray-induced spallation reactions. It also reveals that some mare basalts have slightly lower δ34S values than the population mean, which is consistent with sulfur loss from a reduced basaltic melt prior to eruption at the lunar surface. Both the sulfur isotope homogeneity of the lunar mare basalts and the predicted sensitivity of sulfur isotopes to vaporization-driven fractionation suggest that less than ≈1–10% of lunar sulfur was lost after a potential moon-forming impact event.

HETEROGENEITY OF S ISOTOPE COMPOSITIONS RECORDED IN THE SUDBURY IGNEOUS COMPLEX, CANADA: SIGNIFICANCE TO FORMATION OF Ni-Cu SULFIDE ORES AND THE HOST ROCKS

Sulfur isotope ratios of sulfide ores and their host rocks, and the overlying main mass sequence of norites, quartz gabbros, and granophyres from the Sudbury Igneous Complex have been analyzed to establish the extent of isotopic heterogeneity of the Complex and the impact melt sheet from which it was produced. Noritic rocks located above the sublayer in the lower portion of the main mass of the Sudbury Igneous Complex are characterized by uniform sulfur isotope ratios with δ 34S values of 2.59 ± 0.48‰. Rocks from the granophyric portion of the main mass are characterized by elevated and variable δ 34S values from 4.4 to 14.1‰. The more 34S-enriched S in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled. Footwall and contact-style mineralization from the sublayer show variable δ 34S values from 0.2 to 4.3‰. South Range mineralization is characterized by low ratios from 0.2 to 2.5‰. North and Northeast Range mineralization exhibits locally elevated but variable δ 34S values; for example mineralization from Whistle mine varies from 3.1 to 4.3‰, whereas mineralization from Victor varies between 2.4 and 2.9‰. There is typically only a small difference in S isotope ratio of contact and proximal footwall mineralization. Country rocks in the area of the Sudbury Igneous Complex are also characterized by a wide range in δ 34S values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources. However, the variability present between deposits at the lower contact of the Complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet. There is no evidence that deposit size correlates with δ 34S value; variations between deposits may involve local S isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall. This study illustrates the application of S isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet.

Behavior of Re and Os during contact between an aqueous solution and oil: Consequences for the application of the Re–Os geochronometer to petroleum

Several recent studies have raised the exciting possibility that oils can be dated using the Re–Os radioisotope system. However the exact nature of the events dated by this technique often remains unclear. Geochronologic interpretation of Re–Os data for oils is hampered by our limited knowledge of how these metals behave in petroleum systems. In particular, it is difficult to understand how isotopic homogenization, an essential prerequisite for the development of an isochronal relationship, can be achieved at the scale of a petroleum basin. The mechanisms capable of fractionating the Re/Os ratio in a suite of oils are also poorly understood. For this reason, we have performed an experimental study aimed at investigating the behavior of Re and Os during a particularly widespread phenomenon in petroleum systems, the interaction of formation waters with oils during migration. Contact experiments between natural oils and aqueous solutions enriched in Re and/or Os were carried out for varying lengths of time (6h to 5months), at different temperatures (25–150°C), over a wide range of metal concentrations in the enriched solution (0.001–100μg/g for Re; 1 and 10ng/g for Os). In addition, the effect of oil composition on Re–Os exchange at the water–oil interface was examined by testing two oils with very different properties.All of our results demonstrate that Re and Os are transferred massively and very rapidly from the aqueous solution to the organic phase. This is true regardless of temperature or oil composition. It is also true for a very wide range of metal concentrations in the aqueous solution, up to an apparent saturation level that exceeds natural concentrations in oils by several orders of magnitude. Given the efficiency of Re and Os transfer from water to oil demonstrated here, and assuming that our findings are applicable to natural conditions, water/oil ratios of only about 250 would be needed to explain the Re and Os contents of most oils, based on the measured concentrations of Re and Os in groundwaters. Thus transfer of Re and Os from formation waters could plausibly provide a source for these metals in oils. Waters from a given basin may have roughly homogeneous Os isotopic compositions, approximating the average value of the source rocks. On the other hand, variable Re/Os ratios might be expected to develop, as a result of interaction with various phases (e.g. Fe–Mn oxides or authigenic pyrite) encountered by the water as it flows through the reservoir. If homogeneous 187Os/188Os ratios coupled with variable Re/Os ratios are transferred to the oils with which the waters come in contact, the isotopic geochronometer could be reset.

Mantle sources of quaternary volcanism on Zhokhov Island (De Long Islands, East Arctic): Isotope-geochemical features of the basalts and spinel lherzolite xenoliths

This paper reports the results of isotope-geochemical studies and distribution patterns for trace elements and rare earth elements in a collection of specimens of Cenozoic alkaline olivine basalts and spinel lherzolite xenoliths sampled at Zhokhov Island, De Long Archipelago (New Siberian Islands), East Arctic. In spite of various contributions of xenogenic minerals of mantle lherzolite xenoliths (olivine, pyroxene and spinel), the bulk-rock isotope composition of the studied specimens deviates within insignificant limits. This evidences the isotopic and geochemical homogeneity of the mantle source and the absence of a significant effect of processes of contamination by crustal host rocks, which frequently occur within the basalts as xenoliths. On the other hand this reflects the short lifetime of existence and evolution of the mantle melt source. The studied basalts by their isotope and geochemical characteristics resemble volcanics of oceanic islands, intraplate continental volcanics, and those of ocean rises, relate to the activity of mantle plumes. The lack of elevated U/Pb in the studied specimens permits us to correlate the occurrence of the Cenozoic volcanism at De Long Islands of East Arctic with intraplate continental plume volcanism of the rifting stage (analogous to plume magmatism of the East African Rift, Mesozoic alkaline ultramafic volcanism of Eastern India, and others).

Lithium isotopes in hydrothermally altered basalts from Hengill (SW Iceland)

The Li isotope signatures of hydrothermal fluids are remarkably constant (δLi7=8.0±1.9‰) irrespective of the water/rock ratio (W/R), permeability, temperature or fluid involved (seawater or meteoric). High temperature hydrothermal fluids represent the second most significant source of Li to the ocean, yet the homogeneity of the Li isotopic signatures of this source remains to be explained and in this context, the lack of data for the corresponding altered phases is problematic. We measured Li contents and Li isotope signatures (as well as mineralogy, composition and local fluid temperature) in hyaloclastites collected from a borehole in the Hellisheidi geothermal system (Iceland) which have been altered by high temperature aqueous fluids (from 170 to 300 °C). Li is more enriched in the solid phases than the other alkali metals, highlighting its greater ability to be incorporated into secondary phases, especially at high temperatures (>250 °C). Mass balance calculations show that the low Li concentrations in hydrothermal fluids are best explained by a high water/rock ratio and a high permeability of this system. The Li isotopic signature of the altered hyaloclastites (δLi7 between +1.9 and +4.0‰) remains close to the fresh basalt at deep levels and high temperatures (290–300 °C) (as measured δLi7 range between +3.7 and +4.0‰), and decreases at shallower depths and lower temperatures (150–270 °C) (δLi7 between +1.9 and +3.1‰). A mass balance model involving basalt dissolution, secondary phase formation, and successive isotope equilibrium during the migration and the cooling of the percolating fluid was developed. The corresponding apparent mineral-fluid Li isotope fractionation factors resulting from precipitation of secondary phases (ΔLiminerals-fluid7) range between 0‰ at 300 °C and −8.5‰ at 170 °C and highlight a key role of chlorite. Applying the same approach to mid-ridge oceanic hydrothermal systems allows the relatively homogeneous isotope signatures of high temperature fluids of various locations to be explained.

Texture-specific Si isotope variations in Barberton Greenstone Belt cherts record low temperature fractionations in early Archean seawater

Sedimentary cherts are unusually abundant in early Archean (pre-3.0Ga) sequences, suggesting a silica cycle that was profoundly different than the modern system. Previously applied for the purpose of paleothermometry, Si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration, precipitation, diagenesis, and metamorphism. Early Archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition, cementation, silicification, and recrystallization. To add a new layer of insight into the chemistry of early cherts, we have used wavelength-dispersive spectroscopy and then secondary ion mass spectrometry (SIMS) to produce elemental and Si and O isotope ratio data from banded black-and-white cherts from the Onverwacht Group of the Barberton Greenstone Belt, South Africa. This geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations. SIMS allows the comparison of Si and O isotope ratios of distinct silica phases, including black carbonaceous chert beds and bands (many including well-defined sedimentary grains), white relatively pure chert bands including primary silica granules, early cavity-filling cements, and later quartz-filled veins. Including all chert types and textures analyzed, the δ30Si dataset spans a range from −4.78‰ to +3.74‰, with overall mean 0.20‰, median 0.51‰, and standard deviation 1.30‰ (n=1087). Most samples have broadly similar δ30Si distributions, but systematic texture-specific δ30Si differences are observed between white chert bands (mean +0.60‰, n=750), which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements (mean −1.41‰, n=198). We observed variations at a ∼100μm scale indicating a lack of Si isotope homogenization at this scale during diagenesis and metamorphism, although fractionations during diagenetic phase transformations may have affected certain textures. We interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated. SIMS δ18O values fall in a range from 16.39‰ to 23.39‰ (n=381), similar to previously published data from bulk gas source mass spectrometry of Onverwacht cherts. We observed only limited examples of texture-related variation in δ18O and did not observe correlation of δ18O with δ30Si trends. This is consistent with hypotheses that Si isotope ratios are more resistant to alteration under conditions of rock-buffered diagenesis (Marin-Carbonne et al., 2011). Our results indicate that low temperature processes fractionated silicon isotopes in early Archean marine basins, a behavior that probably precludes the application of chert δ30Si as a robust paleothermometer. The values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30Si-rich than that expected for bulk silicate Earth. This is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30Si-enriched cherts and 30Si-depleted iron formation lithologies. Precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30Si-depleted sink, with the corollary of an important non-carbonate alkalinity sink consuming cations released by silicate weathering.

Ion microprobe survey of the grain-scale oxygen isotope geochemistry of minerals in metamorphic rocks

The oxygen isotope compositions of calcite, diopside, dolomite, forsterite, garnet, K-feldspar, kyanite, plagioclase, quartz, and wollastonite were analyzed in suites of contact and regional metamorphic rocks using an ion microprobe. Spatial resolution was ∼10μm. Precision, measured as the standard deviation of working standards averaged over the entire project, was 0.13–0.18‰ for three carbonate standards and 0.11–0.12‰ for two silicate standards. A total of 1176 analyses (excluding standards) were made of 73 minerals in 23 samples. Both intercrystalline and intracrystalline variability in δ18O is greater in contact than in regional metamorphic rocks. Of 27 minerals analyzed in contact metamorphosed rocks, 70% exhibit statistically significant grain-to-grain variability in δ18O over areas ⩽1.41cm2 with the largest range in silicates and carbonates in a single sample of 7.4‰ (forsterite) and 10.6‰ (dolomite). Of 88 grains analyzed in two or more places in contact metamorphosed rocks, 32% exhibit statistically significant intracrystalline variability in δ18O with the largest range in a single silicate and carbonate grain of 3.1‰ (forsterite) and 10.1‰ (dolomite). In contrast, 44% of 45 minerals in regional metamorphic rocks exhibit significant grain-to-grain variability in δ18O over areas ⩽1.17cm2 with the largest range in silicates and carbonates in a single sample of only 1.1‰ (plagioclase) and 0.9‰ (calcite). Only 6% of 144 grains analyzed in two or more places in regional metamorphic rocks exhibit significant intracrystalline variability in δ18O with the largest range in a single silicate and carbonate grain of only 1.5‰ (diopside) and 0.7‰ (calcite). The difference in intercrystalline and intracrystalline variability in δ18O between contact and region metamorphic rocks is explained by the longer duration and slower reaction rates of regional metamorphism rather than to differences in temperature. There is no significant difference in intercrystalline and intracrystalline variability in δ18O in regional metamorphic rocks among samples from the biotite, garnet, and kyanite zones. Calcite inclusions in forsterite, and calcite and quartz inclusions in garnet either have δ18O that is statistically indistinguishable from δ18O of the same mineral occurring as nearby matrix grains or have statistically significant lower δ18O. No reversed isotope fractionations were measured between coexisting mineral pairs. Minerals in individual samples, however, exhibit a wide range in the degree to which they attained and preserve oxygen isotope fractionations consistent with metamorphic temperatures recorded by mineral equilibria. Processes that account for grain-scale departures from isotope exchange equilibrium include: (a) overstepping of prograde mineral reactions, (b) growth zoning in low-diffusivity minerals, (c) interaction of rocks with fluids at the peak of metamorphism and/or during cooling, (d) retrograde mineral reactions, and (e) closed-system isotope exchange between coexisting minerals during cooling. This study provides new information about (1) the degree to which a variety of textural changes experienced by rocks during metamorphism are associated with changes in δ18O, (2) oxygen isotope homogenization at the outcrop scale among contrasting lithologies, (3) changes in δ18O with increasing grade of regional metamorphism, and (4) time scales of metamorphic process.

EOCENE ZIRCON REFERENCE MATERIAL FOR MICROANALYSIS OF U-Th-Pb ISOTOPES AND TRACE ELEMENTS

Eocene gem zircons from Reddistone Creek, Glen Innes, Australia, have been assessed for isotopic and chemical homogeneity by Secondary Ion Mass Spectrometry (SIMS), Isotope Dilution Thermal Ionization Mass Spectrometry (ID TIMS), Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), and Wavelength Dispersive X-ray analysis (WDS). Sensitive High-Resolution Ion Microprobe (SHRIMP) SIMS data show that the U-Pb and Th-Pb isotope systems are homogeneous at the ion microprobe spot mass of a few ng for two zircons, AusZ2 and AusZ5, and that AusZ2 has good homogeneity in U, Th, and Th/U ratio. Twelve ID U-Pb TIMS analyses of separate 200–400 μm chips, measured in two different laboratories, show that AusZ2 has remarkable isotopic homogeneity at the sub-permil level, is concordant, and has a 206 Pb- 238 U age of 38.8963 ± 0.0044 Ma (2σ; n = 12, MSWD = 1.01). A similar data set for AusZ5 shows that this gem has a similarly precise and reproducible 206 Pb- 238 U age of 38.9022 ± 0.0035 Ma (2σ; n = 12, MSWD = 2.4), but exhibits scatter in 207 Pb- 235 U that is in excess of that expected based on the uncertainty of individual analyses (weighted mean 207 Pb- 235 U date of 39.162 ± 0.054 Ma, 2σ; n = 12, MSWD=10.0), with two concordant analyses and 10 slightly discordant analyses (0.7%). The slight discordance of AusZ5 may reflect an initial excess of 231 Pa. The AusZ2 and AusZ5 ID TIMS data sets demonstrate the excellent reproducibility of measurements between labs using the EARTHTIME calibration spikes and solutions. LA-ICP-MS trace element data indicate that AusZ2 is also very homogeneous for the REE and other trace elements, whereas AusZ5 shows trace element heterogeneity. WDS analysis confirms the major element homogeneity of these zircons. Our data show that the AusZ2 zircon will be an excellent Reference Material (RM) for ID TIMS, SIMS, and LA-ICP-MS microanalysis of U-Pb isotopes and trace elements in Cainozoic zircons.

HFSE (High Field Strength Elements)-transport and U–Pb–Hf isotope homogenization mediated by Ca-bearing aqueous fluids at 2.04 Ga: Constraints from zircon, monazite, and garnet of the Venetia Klippe, Limpopo Belt, South Africa

Results from laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and isotope dilution (ID) analyses of minerals and rocks from a single outcrop of the Venetia Klippe of the Limpopo Belt indicate that the U–Pb and Hf isotope system homogenized on the decimetre scale under amphibolite-facies conditions of ⩽645±25°C and ⩽7.0±1.1kbar, i.e. in the presence of an aqueous fluid phase. For a metabasite sample, homogenization is supported by isotope analyses of metamorphic zircon, garnet, and whole rock, which yield a six-point Lu–Hf isochron age of 2039.7±3.4Ma, with initial 176Hf/177Hf of 0.28126±0.00001, and a U–Pb zircon age of 2042±10Ma. The occurrence of a few inherited magmatic zircon cores with ages up to 2705Ma, and with significantly lower initial 176Hf/177Hf of 0.28112, however, indicate that homogenization was incomplete. For a chlorite–biotite–garnet schist isotope homogenization is reflected by within error identical zircon and monazite U–Pb ages of 2045±10Ma and 2041±8Ma, respectively, and by a zircon-garnet-whole rock Lu–Hf isochron age of 2083±63Ma, with an initial 176Hf/177Hf of 0.28140±0.00003. Contemporaneous formation of metamorphic zircon, monazite and garnet in the chlorite schist is not only supported by the isotope data, but also by chlorite inclusions in all three minerals, and by inclusions of metamorphic zircon in garnet. The inclusion textures and the identical initial 176Hf/177Hf support the conclusion that metamorphic zircon grains precipitated from an aqueous fluid phase, after dissolution of zirconium-bearing phases elsewhere, followed by a major HFSE transport, and Hf isotope homogenization. This fluid perhaps was Ca-bearing, as is suggested by the fact that garnet in the schist sample is the only Ca-bearing phase, and that metamorphic monazite, dating the metamorphic peak, is partially replacement by apatite. The fact that the metamorphic zircon rims in the metabasite sample have significantly lower initial 176Hf/177Hf than expected from the Lu–Hf isotope analyses of relic zircon cores and whole rock additionally hint that Ca-HFSE-bearing fluids infiltrated from surrounding quartzo-feldspathic gneisses, which had much lower 176Hf/177Hf at the time of metamorphism.

Origin of gem corundum in calcite marble: The Revelstoke occurrence in the Canadian Cordillera of British Columbia

The calcite marble-hosted gem corundum (ruby, sapphire) occurrence near Revelstoke, British Columbia, Canada, occurs in the Monashee Complex of the Omineca Belt of the Canadian Cordillera . Corundum occurs in thin, folded and stretched layers with green muscovite + Ba-bearing K-feldspar + anorthite (An 0.85–1 ) ± phlogopite ± Na-poor scapolite . Other silicate layers within the marble are composed of: (1) diopside + tremolite ± quartz and (2) garnet (Alm 0.7–0.5 Grs 0.2–0.4 ) + Na-rich scapolite + diopside + tremolite + Na,K-amphiboles. Non-silicate layers in the marble are either magnetite- or graphite-bearing. Predominantly pink (locally red or purple) opaque to transparent corundum crystals have elevated Cr 2 O 3 (≤ 0.21 wt.%) and variable amounts of TiO 2 ; rare blue rims on the corundum crystals contain higher amounts of TiO 2 (≤ 0.53 wt.%) and Fe 2 O 3 (≤ 0.07 wt.%). The associated micas have elevated Cr, V, Ti, and Ba contents. Petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism (~ 650–700 °C at 8.5–9 kbar). Because the marble is almost pure calcite (dolomite is very rare), the corundum was preserved because it did not react with dolomite to spinel + calcite during decompression. The scapolite-bearing assemblages formed during or after decompression of the rock at ~ 650 °C and 4–6 kbar. Gem-quality corundum crystals formed especially on borders of the mica-feldspar layers in an assemblage with calcite. Whole rock geochemistry data show that the corundum-bearing silicate (mica-feldspar) layers formed by mechanical mixing of carbonate with the host gneiss protolith ; the bulk composition of the silicate layers was modified by Si and Fe depletion during prograde metamorphism . High element mobility is supported by the homogenization of δ 18 O and δ 13 C values in carbonates and silicates for the marble and silicate layers. The silicate layers and the gneiss contain elevated contents of Cr and V due to the volcanoclastic component of their protolith. • First detailed characterization of prograde gem corundum formation in calcite marble • Corundum present in mica-feldspar layers in calcite marble • Common protolith of Cr,V-bearing marble assemblages and host rock • Isotope homogenization between the host rock and the marble • Corundum formed by prograde muscovite breakdown reactions

Lewisian Complex of Strath Dionard–Rhiconich and its significance in the early history of the NW Highlands of Scotland

Synopsis Remapping of a quartzofeldspathic gneiss-dominated part of the Lewisian Complex in the NW Highlands of Scotland, originally mapped by the Geological Survey in the latter part of the nineteenth century, has added new information about the lithologies and their distribution, and established a tectonothermal–igneous history. Linking new data for the gneisses of Strath Dionard–Rhiconich to the results of isotopic studies points to the development of their protolith in late Archaean times as the result of arc magmatism similar to that at a modern ocean–continent margin. Compositionally, most of the gneisses, and those southwards towards Laxford, correspond to granodiorite with adakitic affinities. Using a comparison with a well-substantiated model for coeval late Archaean crust formation of part of the Wyoming Craton, they are interpreted as representing a stage in a tectonomagmatic cycle when the products of metasomatism and melting in the mantle were emplaced as sheet-like banded plutons at middle crustal levels. Less common granitic gneisses represent products of further melting in the thickened middle crust. Xenolithic blocks of subducted older sedimentary rocks, carried upwards by the magmas, are now seen as elongate lensoid masses of calc-silicate rock and quartzite that are grossly concordant with the banding in the gneisses. Also generally concordant with this banding are elongate lensoid masses of amphibolite, the protolith of which is considered to have been the product of mafic magmatism during the convergent margin arc activity. In early Proterozoic times the late Archaean assemblage was extensively reworked. The development of penetrative foliation in both gneisses and amphibolites was associated with isotopic homogenization that, in the southerly continuation of the domain towards the type locality of the Laxfordian orogenic belt at Laxford, has been dated at c . 1860 Ma. This preceded the imposition of the current major structural pattern that included the formation of the Strath Dionard antiform and the parasitic Gharbh Mhòr folds. Abundant granites and pegmatites, subsequently emplaced in an injection complex at Strath Dionard, are linked with the c . 1855 Ma suite of acidic intrusions that are prominent at Laxford. The rock assemblage and sequence of development are like those of other early Proterozoic mobile belts formed as the result of extensive reworking of late Archaean cratons. Limited mineralogical and structural modification of the ≥1850 Ma gneiss–amphibolite–granite assemblage, and pegmatite emplacement associated with (micro)-plate collision, took place during the development of the c . 1750 Ma Calvian orogenic belt. Further limited modification and pegmatite emplacement was at c. 1670 Ma. The structural–metamorphic–igneous histories described, integrated with isotopic data, do not support interpretations proposing a transition between the Strath Dionard–Rhiconich (–Laxford) gneiss–amphibolite domain and the adjacent granulite–dolerite dyke domain; or that the quartzofeldspathic gneiss–amphibolite domains in some other parts of the NW Highlands can be correlated with the type area of the Laxfordian orogenic belt.

Deciphering the geochronology of a large granitoid pluton (Karkonosze Granite, SW Poland): an assessment of U–Pb zircon SIMS and Rb–Sr whole-rock dates relative to U–Pb zircon CA-ID-TIMS

Granitoid plutons are often difficult to radiometrically date precisely due to the possible effects of protracted and complex magmatic evolution, crustal inheritance, and/or partial re-setting of radiogenic clocks. However, apart from natural/geological issues, methodological and analytical problems may also contribute to blurring geochronological data. This may be exemplified by the Variscan Karkonosze Pluton (SW Poland). High-precision chemical abrasion (CA) ID-TIMS zircon data indicate that the two main rock types, porphyritic and equigranular, of this igneous body were both emplaced at ca. 312 Ma, while field evidence points to a younger age for the latter. This is in contrast to the earlier reported SIMS (SHRIMP) zircon dates that scattered mainly between ca. 322 and 302 Ma. In an attempt to overcome this dispersion, at least in part caused by radiogenic lead loss, the CA technique was used before SHRIMP analysis. The 206Pb/238U age obtained in this way from a sample of porphyritic granite is 322 ± 3 Ma, ~16 Ma older than the untreated zircons; another porphyritic sample yielded a mean age of 319 ± 3 Ma, and the mean age was 318 ± 4 Ma for an equigranular granite sample – all three somewhat older than the age obtained by ID-TIMS. Older SIMS dates of ca. 318–322 Ma might indicate either faint inheritance or that zircon domains crystallized during earlier stages of Karkonosze igneous evolution. The ID-TIMS results have been used to re-assess the whole-rock Rb–Sr data. Excluding a porphyritic granite with excess radiogenic 87Sr, it appears that isotopic homogeneity was achieved for most samples during the 312 Ma event, as shown by a pooled 21-point isochron with an age of 311 ± 3 Ma and an initial 86Sr/86Sr of 0.7067 ± 4. Local crustal contamination by stopping of metapelitic material might account for the more radiogenic Sr isotope signature observed in biotite-rich schlieren. A critical re-evaluation of all available SHRIMP data using the ID-TIMS age of 312 Ma as a benchmark suggests that the observed scatter may be partly attributed to analytical and methodological problems, in particular failing to distinguish subtly discordant spots from truly concordant ones, which is a serious limitation of the microbeam analytical approach. Other likely pitfalls contributing to geochronological scatter are identified in the published Re–Os ages on molybdenite and the 40Ar/39Ar data on micas. A scenario postulating a 15–20 milliion year evolution of the Karkonosze Pluton cannot be established on the basis of available geochronological data, which rather supports a brief igneous event, although a more protracted pre-emplacement evolution is possible. A short timescale for crystallization of large igneous bodies, as suggested by the ID-TIMS data from the Karkonosze Granite, is in line with models of transport of granitic magmas through dikes to form large plutons.

Timing and extent of Mg and Al isotopic homogenization in the early inner Solar System

The first million years of the Solar System is a key period when the first solids were formed from the nebula gas. The chronology of the different processes at the origin of these solids is still largely unknown and relies strongly on the assumption made of homogeneous distribution for short-lived radioactive nuclides such as 26Al. This assumption is questioned. In this study, in situ 26Al–26Mg isotope systematics was studied with high precision in 12 calcium, aluminum-rich inclusions (CAIs) (1 type A, 2 type B, 5 type C, and 4 fine grained spinel-rich), 2 amoeboid olivine aggregates (AOAs), and 2 Al-rich chondrules from Efremovka and Vigarano. The (26Al/27Al)i in these early Solar System solids (the subscript ‘i’ stands for the initial isotope ratio obtained from the mineral 26Al isochron) range from ∼6.5×10−5 to 0.2×10−5 with δMgi⁎26 from −0.08 to +0.37‰. The (26Al/27Al)i and δMgi⁎26 of CAIs and chondrules can be explained by formation of their precursors from a homogeneous reservoir (Solar System Initial, noted hereafter SSI) with initial magnesium isotopic composition of δMgSSI⁎26=−0.052±0.013‰ and initial (26Al/27Al)SSI abundance of (5.62±0.42)×10−5. The high precision magnesium isotope data obtained in the present study and literature data allows identifying a few epochs of formation/reprocessing of CAIs. The time periods of these epochs correspond well with the median life times of the pre-main sequence evolution of stars of Solar mass if we anchor the (Al26/Al27)SSI=(5.62±0.42)×10−5 to the beginning of class I phase. This provides a natural explanation to the range of (26Al/27Al)i – (∼6 to 0.02) × 10−5 seen in corundum grains, FUN (Fractionation and Unidentified Nuclear Effects) CAIs, ultrarefractory CAIs, normal CAIs, and chondrules, and suggests a possible relationship between the astrophysical conditions and the formation of these early solids. Corundum grains, FUN CAIs, ultrarefractory CAIs would have formed during the class 0 of the pre main-sequence, during the collapse of the parent molecular cloud, before homogenization was complete while normal CAIs and chondrules formed later during class I to the beginning of class III. Alternatively, if one wants to explain the present data without calling for an early homogenization of Al and Mg isotopes in the forming Solar System, at least four distinct isotopic reservoirs are required to produce by mixing the range observed in δMgi⁎26 and (26Al/27Al)i. There is no obvious astrophysical explanation of the existence of such distinct reservoirs, which makes this hypothesis quite ad hoc and thus very unlikely.

Systematic variations in the composition of volcanic rocks in tectono-magmatic seamount chaines in the Brazil Basin

Data on the composition of rocks in linear tectono-magmatic rises in the Brazil Basin indicate that volcanic rocks in the Vitoria—Trindade seamount chain were derived from a mantle reservoir unevenly enriched in phosphorus under the effect of melts close to subalkaline picrobasalt. These melts contained much of the EM I mantle component because the plume material was contaminated with continental lithospheric component. A long-lived isotopic homogeneity of the source is typical of the whole structure, including the Trindade and Martin Vaz Islands and the Abrolhos Plateau. The analogous isotopic ratios of rocks at the Fernando de Noronha Islands are reportedly explained by a similar mechanism of melt derivation and the similar evolution of the mantle plume material, which was originally situated beneath the South American continent. Compared to the melts of volcanic rocks of all other seamounts discussed herein, the parental melts of volcanics at the Victoria—Trindade Seamounts were derived at lower degrees of melting of enriched source material at a greater depth. The overwhelming majority of volcanic rocks at the northern chain of the Bahia Seamounts were produced by melts generated with the involvement of material of the HIMU type. At the same time, one of our rock samples was derived from a source of composition close to DM with a certain admixture of enriched material like EM I. The mantle source of rocks of the Pernambuco Seamounts consisted of a mixture of DM and HIMU material with a certain admixture of EM I (or, perhaps, EM II). The 10°–11° S Seamounts were formed near the MAR axial zone at the decompressional melting of chemically homogeneous mantle source that consisted of DM material with an admixture of EM I (or, perhaps, EM II) component.