Isotopic Zoning Research Articles

Tooth enamel maturation reequilibrates oxygen isotope compositions and supports simple sampling methods

Oxygen isotope and major element zoning patterns of several disparate ungulate teeth were collected to evaluate the timing and geometry of enamel formation, records of isotope zoning, and tooth enamel sampling strategies. Isotopic zoning in mammalian tooth enamel encodes a sub-annual time series of isotopic variation of an animal’s body water composition, with a damping factor that depends on the specifics of how enamel mineralizes. Enamel formation comprises two stages: precipitation of appositional enamel with a high CO3:PO4 ratio, followed by precipitation of maturational enamel with a lower CO3:PO4. If appositional and maturational enamel both contribute to isotope compositions (but with different CO3:PO4), and if isotope compositions vary seasonally, paired δ18O values from CO3 and PO4 profiles should show a spatial separation. CO3 isotope patterns should be shifted earlier seasonally than PO4 isotope patterns. Such paired profiles for new and published data show no resolvable shifts, i.e. CO3 and PO4 δ18O profiles show coincident maxima and minima. This coincidence suggests that enamel maturation reequilibrates appositional isotope compositions. If enamel maturation establishes enamel isotope compositions, the geometry of maturation, not apposition, should be considered when devising sampling protocols. X-ray maps of Ca zoning show that the majority of enamel (inner and middle layers) mineralizes heavily at a high angle to the external tooth surface and the enamel-dentine junction over length scales of 2–4mm, while the outer enamel surface mineralizes more slowly. These data suggest that isotopic sampling strategies should parallel maturational geometry and focus on interior enamel to improve data fidelity. The magnitude of isotopic damping is also smaller than implied in previous studies, so tooth enamel zoning more closely reflects original body water isotopic variations than previously assumed.

Trace element and isotopic geochemistry of Cretaceous magmatism in NE Asia: Spatial zonation, temporal evolution, and tectonic controls

Results of a comprehensive geochemical study (major and trace elements, and isotopes of Sr, Nd, Pb, Hf) of Cretaceous volcanic rocks from the Chukotka area in northeastern Russia are presented. Synthesis of available geological and geochronological data suggests diachronous onset of activity of the Okhotsk–Chukotka volcanic belt (OCVB), the largest magmatic province in the region. The OCVB consists of ca. 106km3 of volcanic rocks. At 106–105Ma, subduction-related magmatism initiated in the southern and central segments of the OCVB. In the Central and Northern Chukotka areas, where the northern OCVB is exposed, onset of arc magmatism occurred ca. 10m.y. after extension-related magmatism of the Chaun igneous province at 109–104Ma. Mafic rocks from the OCVB yield (87Sr/86Sr)80Ma of 0.7033 to 0.7047, εNd80Ma of 0.0 to 7.10, εHf80Ma of 4.12 to 12.88, (206Pb/204Pb)80Ma of 18.11 to 18.42, and (208Pb/204Pb)80Ma of 37.96 to 38.21. Volcanic rocks from the Chaun province, as well as OCVB rocks from Northern Chukotka, originate from a relatively enriched source and have (87Sr/86Sr)80Ma of 0.7088 to 0.7100, εNd80Ma of −5.81 to −3.42, εHf80Ma of −3.40 to −0.25, (206Pb/204Pb)80Ma of 18.69 to 18.90, and (208Pb/204Pb)80Ma of 38.65 to 38.86. No definitive across-arc elemental or isotopic zonation of the OCVB has been revealed, probably because of wide-scale crustal melting and subsequent contamination of mantle-derived melts. However, there is a clear along-arc isotopic zonation. In our interpretation, this results from heterogeneity of the subcontinental lithospheric mantle, which likely was a major contributor to the magma source. The similar isotopic signatures of silicic (dominantly crust-derived) and mafic (mantle-derived) volcanic rocks in each OCVB segment imply that remelting of juvenile mafic underplated material was the main process responsible for the crust-derived magma generation. These data from the major Cretaceous magmatic provinces of northeast Asia are synthesized in context of existing plate tectonic reconstructions.

Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell

The Rb-Sr beta-decay dating system is one of the most attractive tools in geochronology, as Rb is sufficiently abundant in common K-bearing minerals like biotite, muscovite and K-feldspar. This allows dating of a wide variety of rocks (e.g., volcanic, magmatic, metamorphic, sedimentary and hydrothermal environments) without the need of preconcentration, as is often required for U-Pb dating of zircon. However, this advantage was to date negatively counteracted by the lack of a suitable in-situ technique, as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope.A reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer (ICP-MS) allows exactly this, the online chemical separation of two different elements. Coupled to a laser ablation (LA) system, in-situ Rb-Sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates Sr from Rb.We present here a simple procedure in which Rb-Sr ages can be obtained from a suite of individual phases in regular thin sections. Using the most established reaction gas, oxygen (O2), it is possible to react part of the ablated Sr to SrO+ while no RbO+ is formed. Measurements of feldspars and micas with a spot size of 80μm were calibrated against glass standards NIST SRM 610, BCR-2G and biotite from La Posta (California; 91.6Ma). Results are presented for a variety of magmatic rocks with well-established thermal records: a sample each from the Klokken syenodiorite (Greenland; 1161Ma), the Ulvö alkaligabbro (Sweden; 1256Ma) and a pegmatite from the Bohus granite (Sweden; 920Ma). Obtained in situ Rb-Sr isochron ages are accurate <1.5%, while initial 87Sr/86Sr ratios are accurate <0.2% compared to published data.The methodology outlined in this study has significant implications for Rb-Sr studies due to the high spatial resolution: (1) quality of measurements can be better controlled by avoiding alterations and inclusions, (2) large sample numbers can be investigated simply by using regular thin sections, (3) several mineral generations can be targeted that would not be distinguishable in mineral separates, and (4) isotope zonation within single crystals can be revealed. This will open the Rb-Sr dating system to new fields of study. For example, thermochronologic studies can use 87Sr/86Sr and 87Rb/86Sr zonation within crystals to better work out cooling and/or reheating paths. Further applications are in the field of provenance studies and temporal evolution of shear zones.

Microanalysis of carbonate cement δ18O in a CO2-storage system seal: Insights into the diagenetic history of the Eau Claire Formation (Upper Cambrian), Illinois Basin

Oxygen isotope (δ18O) zonation in carbonate mineral cements is often employed as a proxy record (typically with millimeter-scale resolution) of changing temperature regimes during different stages of sediment diagenesis. Recent advances in secondary ion mass spectrometry allow for highly precise and accurate determinations of cement δ18O values to be made in situ on a micrometer scale, thus significantly increasing the spatial resolution available to studies of diagenesis in sandstone–shale and carbonate systems. Chemo-isotopically zoned dolomite–ankerite cements within shaly sandstone beds of the predominantly silty–shaly Eau Claire Formation (Cambrian, Illinois Basin) were investigated, revealing the following: with increasing depth of burial (from <0.5 to ∼2 km [<1500 to 6500 ft]), cement δ18O values decrease from a high of approximately 24‰ down to approximately 14‰ (on the Vienna standard mean ocean water [VSMOW] scale, equivalent to −6.5‰ to −16.5‰ on the Vienna Peedee belemnite [VPDB] scale). The observed cross-basin trend is largely consistent with cements having formed in response to progressive sediment burial and heating. Within the context of independent burial and thermal history models for the Illinois Basin, cementation began soon after deposition and continued intermittently into the mid-Permian. However, temperatures in excess of burial model predictions are inferred at the time of latest ankerite cement precipitation, which we propose overlapped in time with conductive heating of the Eau Claire Formation (a closed system) from under- and overlying sandstone aquifers that channeled the flow of hot, Mississippi Valley–type mineralizing brines during the mid-Permian (ca. 270 Ma).

Oxygen isotopic variations in the outer margins and Wark–Lovering rims of refractory inclusions

Oxygen isotopic variations across the outer margins and Wark–Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16O-rich protosolar gas reservoir and were later exposed to both relatively 17,18O-rich and 16O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time–temperature combinations for their exposure in the nebula. At mean temperatures of 1400K, models that match the isotope gradients in the inclusions yield timescales ranging from 5×103 to 3×105years. Assuming CAIs originated with a relatively 16O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16O-rich but subtly distinct among different CAIs. Melilite is often relatively 16O-poor, but rare relatively 16O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body.

Sulfur isotope study of vent chimneys from Upper Cretaceous volcanogenic massive sulfide deposits of the eastern Pontide metallogenic belt, NE Turkey

We obtained sulfur isotope analysis results of sulfide samples from hydrothermal vent chimneys of the eastern Pontide volcanogenic massive sulfide (VMS) deposits. The total range of δ34S values for vent chimneys in the eastern Pontide VMS deposits is -2.7 to 6.5 per mil. Sulfide Δ34S values show narrow variation in the Lahanos, Killik, and Kutlular deposits, but wider variation in the Kizilkaya and Cayeli deposits. The δ34S values of sulfides in Cayeli chimney samples gave a slightly higher range than the other Pontide chimney samples. In some samples, a rough isotopic zonation pattern was observed throughout chimney zones. Variations in δ34S values of sulfides within chimney walls were probably caused by chemical reactions of reprecipitation and replacement between vent fluids and earlier sulfide minerals in the chimney. Ranges of Δ34S values of sulfide minerals are similar for different deposits within the same region. Variations in the δ34S values of the Pontide deposits appear to be geographic rather than stratigraphic. The sulfur isotope values of the deposits have a narrow compositional range, indicative of a fairly specific origin. Although deep-seated sulfur may be a potential source in the Pontide district, a significant contribution of seawater sulfate cannot be ruled out. The δ34S values of selected samples from Pontide vent chimneys are within the range of sulfur values obtained from Phanerozoic VMS deposits. The range is similar to, but slightly broader than, the range of values reported for modern vent chimneys and ancient vent chimneys from the Yaman-Kasy deposit.

The effect of long-term regional pumping on hydrochemistry and dissolved gas content in an undeveloped shale-gas-bearing aquifer in southwestern Ontario, Canada

Baseline groundwater geochemical mapping of inorganic and isotopic parameters across 44,000 km2 of southwestern Ontario (Canada) has delineated a discreet zone of natural gas in the bedrock aquifer coincident with an 8,000-km2 exposure of Middle Devonian shale. This study describes the ambient geochemical conditions in these shales in the context of other strata, including Ordovician shales, and discusses shale-related natural and anthropogenic processes contributing to hydrogeochemical conditions in the aquifer. The three Devonian shales—the Kettle Point Formation (Antrim equivalent), Hamilton Group and Marcellus Formation—have higher DOC, DIC, HCO3, CO2(aq), pH and iodide, and much higher CH4(aq). The two Ordovician shales—the Queenston and Georgian-Bay/Blue Mountain Formations—are higher in Ca, Mg, SO4 and H2S. In the Devonian shale region, isotopic zones of Pleistocene-aged groundwater have halved in size since first identified in the 1980s; potentiometric data implicate regional groundwater extraction in the shrinkage. Isotopically younger waters invading the aquifer show rapid increases in CH4(aq), pH and iodide with depth and rapid decrease in oxidized carbon species including CO2, HCO3 and DIC, suggesting contemporary methanogenesis. Pumping in the Devonian shale contact aquifer may stimulate methanogenesis by lowering TDS, removing products and replacing reactants, including bicarbonate, derived from overlying glacial sedimentary aquifers.

Processes and time scales of magmatic evolution as revealed by Fe–Mg chemical and isotopic zoning in natural olivines

In this study, we applied high-precision in situ Fe and Mg isotope analyses by femtosecond laser ablation (fs-LA) MC-ICP-MS on chemically zoned olivine xeno- and phenocrysts from intra-plate volcanic regions in order to investigate the magnitude of Fe and Mg isotope fractionation and its suitability to gain information on magma evolution. Our results show that chemical zoning (i.e., Mg#) in magmatic olivines is commonly associated with significant zoning in δ56Fe and δ26Mg (up to 1.7‰ and 0.7‰, respectively). We explored different cases of kinetic fractionation of Fe and Mg isotopes by modeling diffusion in the melt or olivine and simultaneous growth or dissolution. Combining the information of chemical and isotopic zoning in olivine allows to distinguish between various processes that may occur during magma evolution, namely diffusive Fe–Mg exchange between olivine and melt, rapid crystal growth, and Fe–Mg inter-diffusion simultaneous to crystal dissolution or growth. Chemical diffusion in olivine appears to be the dominant process that drives isotope fractionation in magmatic olivine. Simplified modeling of Fe and Mg diffusion is suitable to reproduce both the chemical and the isotopic zoning in most of the investigated olivines and, additionally, provides time information about magmatic processes. For the Massif Central (France), modeling of diffusive re-equilibration of mantle olivines in basanites revealed a short time span (<2years) between the entrainment of a mantle xenolith in an intra-plate basaltic magma and the eruption of the magma. Furthermore, we determined high cooling rates (on the order of a few tens to hundreds of °C per year) for basanite samples from a single large outcrop in the Massif Central, which probably reflects the cooling of a massive lava flow after eruption. Results from the modeling of Fe and Mg isotope fractionation in olivine point to a systematic difference between βFe and βMg (i.e., βFe/βMg≈2), implying that the diffusivity ratio of 54Fe and 56Fe (i.e., D54Fe/D56Fe) is very similar to that of 24Mg and 26Mg, despite the smaller relative mass difference for the 54Fe–56Fe pair. This study demonstrates that a combined investigation of Fe–Mg chemical and isotopic zoning in olivine provides additional and more reliable information on magma evolution than chemical zoning alone.

North American precipitation isotope (δ18O) zones revealed in time series modeling across Canada and northern United States

AbstractDelineating spatial patterns of precipitation isotopes (“isoscapes”) is becoming increasingly important to understand the processes governing the modern water isotope cycle and their application to migration forensics, climate proxy interpretation, and ecohydrology of terrestrial systems. However, the extent to which these patterns can be empirically predicted across Canada and the northern United States has not been fully articulated, in part due to a lack of time series precipitation isotope data for major regions of North America. In this study, we use multiple linear regressions of CNIP, GNIP, and USNIP observations alongside climatological variables, teleconnection indices, and geographic indicators to create empirical models that predict the δ18O of monthly precipitation (δ18Oppt) across Canada and the northern United States. Five regionalization approaches are used to separate the study domain into isotope zones to explore the effect of spatial grouping on model performance. Stepwise regression‐derived parameterizations quantified by permutation testing indicate the significance of precipitable water content and latitude as predictor variables. Within the Canadian Arctic and eastern portion of the study domain, models from all regionalizations capture the interannual and intraannual variability of δ18Oppt. The Pacific coast and northwestern portions of the study domain show less agreement between models and poorer model performance, resulting in higher uncertainty in simulations throughout these regions. Long‐term annual average δ18Oppt isoscapes are generated, highlighting the uncertainty in the regionalization approach as it compounds over time. Additionally, monthly time series simulations are presented at various locations, and model structure uncertainty and 90% bootstrapped prediction bounds are detailed for these predictions.

Trans-Atlantic application of the Baltic Middle and Upper Ordovician carbon isotope zonation

Application of the recently introduced Baltic δ 13 C isotope zonation to a composite North American Darriwilian through Hirnantian succession shows that in most intervals there is good trans-Atlantic agreement not only between the isotope zones but also with the available biostratigraphic data. This indicates that this isotope zonation is a useful tool for improving previously uncertain long-distance correlations.

Large Lithium Isotopic Variations in Minerals from Peridotite Xenoliths from the Eastern North China Craton

To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and δ7Li of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) for six peridotite xenoliths from the eastern North China Craton. These xenoliths contain two lherzolites, two Cpx-rich lherzolites, and two wehrlites and are variably metasomatized. Lithium isotope zonation is observed in most peridotite minerals. The majority of Cpx grains display isotopically light cores with lower Li concentrations than the heavy rims. However, the Opx grains show a different style of zonation from Cpx, where higher Li concentrations in the cores are associated with much lighter δ7Li. Olivines in most peridotites have a restricted range of Li concentration and δ7Li within individual grains, whereas the olivines in a lherzolite show isotopically light cores (−10.3) with high Li concentrations (2.3 ppm) and heavy rims (5.5) with low Li concentrations (1.7 ppm). These Li isotopic variations in mineral phases may reflect the combined effects of diffusion-driven kinetic fractionation of Li isotopes during melt/fluid-peridotite interactions and slow cooling. Intersample heterogeneity of Li isotopes is also apparent. Olivine with forsterite (Fo) content of 91.3 in one lherzolite sample has “normal” mantle-like Li concentrations (1.1∼2.4 ppm) and light δ7Li (−10.3∼5.5), while Ol with Fo content of 89.7 in another lherzolite has slightly high Li concentrations (2.0∼3.0 ppm) but similar δ7Li (1.6∼6.4) relative to normal mantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier δ7Li (5.0∼22.0) than those in lherzolites and normal mantle. The δ7LiOl value correlates positively with Li concentration and negatively with Fo from lherzolites to Cpx-rich lherzolites, indicating a reaction between lherzolites and melts with isotopically heavy Li- and Fe-rich signatures. By contrast, olivines in wehrlites have extremely lower Fo contents (82.2∼83.2) and higher Li concentrations (2.4∼4.2 ppm) than those in normal mantle, while their δ7Li values are within the range of normal mantle, reflecting metasomatism of the peridotites by asthenospheric melt. Overall, the large intragrain and intersample variations in Li concentrations and isotopic compositions reflect kinetic isotope fractionation during multiple metasomatisms.

High‐Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA‐MC‐ICP‐MS and by Solution Nebulisation MC‐ICP‐MS

In this study, a technique for high precision in situ Fe and Mg isotope determinations by femtosecond‐laser ablation‐multi collector‐ICP‐MS (fs‐LA‐MC‐ICP‐MS) was developed. This technique was employed to determine reference values for a series of common reference glasses that may be used for external standardisation of in situ Fe and Mg isotope determinations in silicates. The analysed glasses are part of the MPI‐DING and United States Geological Survey (USGS) reference glass series, consisting of basaltic (BIR‐1G, BCR‐2G, BHVO‐2G, KL2‐G, ML3B‐G) and komatiitic (GOR128‐G and GOR132‐G) compositions. Their Fe and Mg isotope compositions were determined by in situ fs‐LA‐MC‐ICP‐MS and by conventional solution nebulisation multi‐collector ICP‐MS. We determined δ56Fe values for these glasses ranging between ‐0.04‰ and 0.10‰ (relative to IRMM‐014) and δ26Mg values ranging between ‐0.40‰ and ‐0.15‰ (relative to DSM‐3). Our fs‐LA‐MC‐ICP‐MS results for both Fe and Mg isotope compositions agreed with solution nebulisation analyses within analytical uncertainties. Furthermore, the results of three USGS reference glasses (BIR‐1G, BHVO‐2G and BCR‐2G) agreed with previous results for powdered and dissolved aliquots of the same reference materials. Measurement reproducibilities of the in situ determinations of δ56Fe and δ26Mg values were usually better than 0.12‰ and 0.13‰ (2s), respectively. We further demonstrate that our technique is a suitable tool to resolve isotopic zoning in chemically‐zoned olivine crystals. It may be used for a variety of different applications on isotopically‐zoned minerals, e.g., in magmatic or metamorphic rocks or meteorites, to unravel their formation or cooling rates.

Sr Isotope Zoning in Plagioclase from Parinacota Volcano (Northern Chile): Quantifying Magma Mixing and Crustal Contamination

We present analyses of Sr isotope zoning by microdrilling and thermal ionization mass spectrometry in plagioclase crystals from Parinacota volcano (Central Volcanic Zone, northern Chile), which were analysed for major and minor element zoning in a previous study. Although the isotopic range of the bulk-rock samples is small at this volcano (0·7067–0·7070, except for one flow of mafic andesite at 0·7061), significant variations are seen (0·70649–0·70700) within and between plagioclase crystals. A general negative correlation is observed between Sr isotope composition and Sr concentration in the liquid in equilibrium with each plagioclase zone, as calculated from chemical zoning data and partition coefficients. Additional scatter is superimposed on this general trend, indicating a decoupling between isotopic and chemical variations for Sr. In one dacite sample a detailed isotopic profile shows increasing contamination during crystal growth, except for an abrupt decrease correlated with a dissolution surface and interpreted as a recharge event. We apply energy-constrained recharge, assimilation and fractional crystallization modelling to the melt evolution recorded in the chemical and isotopic zoning in this crystal. Results suggest 20% assimilation of the local wall-rock gneiss, at high initial temperatures. The isotopic data confirm the involvement of two contrasting mafic magmas, which are sampled at flank cinder cone vents. One (Lower Ajata) has a low Sr content with high 87Sr/86Sr, the other (Upper Ajata) has a high Sr content with lower 87Sr/86Sr. In some samples from Parinacota, the isotopic composition of plagioclase crystal rims or groundmass crystals is significantly higher than that of the high 87Sr/86Sr mafic magma. In others, where chemical zoning profiles suggest that recharge was from the low 87Sr/86Sr magma, the 87Sr/86Sr of the groundmass and crystal rims is higher than expected. This indicates either additional parent magmas to the two previously identified, or further crustal assimilation, either at lower crustal depths, before crystallization of plagioclase, or just after the last recharge. Our results illustrate the complexity of magma–crust interaction beneath Parinacota, which is likely to be representative of many other Central Andean volcanoes formed on thick crust. Such complex interactions can be revealed by combined study of chemical and isotopic zoning in plagioclase (in a textural petrographic context), despite a small whole-rock isotopic range. The distinct contamination patterns of various samples suggest an important role for the geometry, location and evolution of the magma plumbing system and, in general, variations of the thermal and compositional structure of the crust underneath the volcano.

Field and microanalytical isotopic investigation of ultradepleted in 18O Paleoproterozoic “Slushball Earth” rocks from Karelia, Russia

The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow δ^(18)O and δD (as low as −27.3‰ and −235‰ standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic “Slushball Earth” glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of ^(18)O-depleted rocks, we performed detailed field mapping in Khitostrov, where δ^(18)O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of δ^(18)O depletion is greater than 6 × 2 km in areal extent, ∼10 times larger than previously thought. Relationships between δ^(17)O versus δ^(18)O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in δ^(18)O (5‰–7‰). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal δ^(18)O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in δ^(18)O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U <0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The δ^(18)O-age relationship for metamorphic rims in zircon and corundum grains suggests that δ^(18)O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ∼3‰ intracrystalline δ^(18)O isotope zonation from –24 to –21‰, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high-δ^(18)O values, suggesting that the low δ^(18)O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme δ^(18)O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low-δ^(18)O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic “Slushball Earth” glaciations between ca. 2.4 and ca. 2.23 Ga.

Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals

Radiogenic isotopes are widely used to investigate crustal evolutionary processes, however recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability of such information. We have conducted an in situ Sm–Nd isotope study of apatite, allanite, titanite, xenotime and monazite in metasedimentary rocks of different metamorphic grade to test Nd isotope equilibrium during metamorphism. Our results show that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500 °C before being isotopically homogenised, irrespective of textural context within the rock. Once equilibrated, apatite retains its Nd isotope signature throughout anatexis. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400 °C. REE-rich accessory minerals in high-grade rocks (∼600°C) show very similar initial εNd values at the time of metamorphism. We conclude that under these metamorphic conditions Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Intra-grain Nd isotope zoning of monazite indicates that partial melting was open system, involving the injection of externally-derived melt into migmatites. This process, likely to be common in anatectic terranes but not always obvious at hand-specimen scale or from bulk rock geochemical data, can produce isotope variation that could potentially be misinterpreted as disequilibrium between the melt and its protolith.

Svecofennian Orogen of the Fennoscandian Shield: Compositional and isotopic zoning and its tectonic interpretation

Endogenic events in the form of intrusive activity and regional metamorphism developed asynchronously in various parts in the Svecofennian Orogen of Fennoscandia. The Early and Late Svecofennian stages of regional high-temperature metamorphism and related plutonism are distinguished from isotopic evidence. The composition, structural features, and asynchronous peaks of endogenic activity within the orogen indicate that at least two zones (inner and outer) should be distinguished in the Svecofennides. The lateral heterogeneity of the orogen in present-day coordinates is traced southward from the margin of the Archean craton. The conjugation zone of the Svecofennian Orogen and the Archean Karelian Craton is characterized by transition from negative to positive eNd (1.9 Ga) values as evidence for a decreased contribution of Archean crustal material to the source of Proterozoic granitoids from the north toward the Proterozoic domain in the south. With allowance for lateral compositional and isotopic heterogeneity of the Svecofennian Orogen and asynchronous culmination of endogenic events in different parts of this orogen, a new scheme of tectonic regionalization has been proposed.

Contrasting behavior of oxygen and iron isotopes in banded iron formations revealed by in situ isotopic analysis

In situ O and Fe isotope measurements of magnetite and hematite in banded iron formations (BIFs) from the 2.5 Ga Dales Gorge member of the Brockman Iron Formation, Hamersley Group, Western Australia, document distinct fine-scale isotopic zonation. For hematite, δO18 values (VSMOW) range from −7.1 to −0.6‰, and δFe56 values range from −0.50 to +1.53‰. Magnetite has a δO18 range of −5.6 to +7.0‰ and a δFe56 range of −0.76 to +1.33‰. Notably, magnetite shows significant O isotope variability at a <100 μm scale in individual magnetite grains or layers, where up to 6‰ difference in δO18 values exists between low-Si core and Si-rich overgrowth. Iron isotope compositions are homogeneous to ±0.2‰ in δFe56 values within individual grains or layers. Hematite is always isotopically heavier than co-existing magnetite by 0.1–0.4‰ in δFe56 within individual samples, and there is a large variation (>2‰) in δFe56 values between BIF samples at the > m-scale (e.g., between meso- or macro-bands of the Dales Gorge Member). In situ isotopic results highlight the distinct behavior of O and Fe isotopes during interaction with post-deposition diagenetic or metamorphic fluids. The large variations in δO18 values of iron oxides likely reflect exchange with fluids during post-depositional events at temperatures up to ca. 280 °C, and the lower δO18 values of Fe oxides are interpreted to reflect less isotopic modification and lower temperatures. δO18 variations support a model where initial Fe(OH)3 precipitates that formed in the photic zone were converted to hematite during very early diagenesis, followed by production of magnetite over a range of temperatures during later diagenesis/metamorphism. In contrast, magnetite and hematite seem to have preserved the initial δFe56 values of early Fe(OH)3 precipitates, despite significant changes in O isotope compositions in magnetite. The consistency in apparent Fe isotope fractionation between co-existing hematite and magnetite suggests that they share a common precursor, where magnetite was formed in soft sediment through in situ reduction by microbial processes, and possibly including reaction with aqueous Fe(II). Combined in situ Fe–O isotope analyses allow distinctions to be made between near-primary and post-depositional signatures in BIFs, which bears on the use of BIFs as paleo-environmental proxies and recorders of microbial processes.

Genetic implications of new Sr and Nd isotopic data of the intrusive rocks from the Laramide Arc in Northern Sonora, Mexico.

The Laramide Intrusive Arc constitutes a wide intrusive belt broadly parallel to the actual Sonora coastline. It was formed by the subduction of the Farallon Plate beneath the North-American Plate during the Late Cretaceous-Early Tertiary period. New isotopic data on rocks of this arc show initial 87Sr/86Sr and eNd isotopic values of 0.7066 to 0.7070 and -5 to -6, respectively, for two samples from Bacanora area; as well as 0.7074 to 0.7081 and -3 to -5.5 respectively, for five samples from Cananea, Mariquita and La Caridad areas. Isotopic ages were determined by U/Pb in zircons (95 Ma) and Ar/Ar in potassic feldspar (56 to 71 Ma) from a quartz monzonite porphyry, and by Ar/Ar in potassic feldspar (56 Ma) from another plutonic granodiorite, both from Bacanora. Initial 87Sr/86Sr and eNd values for samples reported in this study suggest that the Laramidic magmas had a large influence from the Proterozoic basement in northeastern Sonora. Four different isotopic zones are proposed for Sonora, according with Sr-Nd data of laramidic rocks and the substratum intruded.

Isotope record of environmental changes at the Skaliska Basin during the Late Glacial and Holocene

ABSTRACT The results of isotopic investigations of the Skaliska Basin sediments are presented. Stable isotope analyses were done for authigenic carbonates from three profiles: W1 - Piotrowo-Ławniki, W2 - Sąkieły Małe, and W4 - Budzewo. The profiles contain carbonate silts at the bottom, then calcareous detritus gyttja and organic silts and peat at the top. Palynological data indicate that sediment were accumulated in the Skaliska Basin from the final phase of the Younger Dryas to the Subboreal period. The values δ18O change from ca −9.4 to −4.5‰, and δ13C values varies from −5.1 to +0.1‰. Such a large range of isotopic data reflects changing conditions in the basin during accumulation of deposits (different water levels, water temperatures, and bioproduction). Based on the results of stable isotope analyses of the carbonates, isotopic zones (Is) were defined and characterized for each profile. The results of isotopic analysis enabled reconstruction of varying environmental conditions connected with lake deepening and/or influx of water enriched in light isotopes, climatic warming, and increase of biological activity. The Late Glacial deposits are characterized by δ18O values of ca −6.5-6‰ and δ13C of ca −1‰. In the transitional time between the Late Glacial and the Holocene, δ18O systematically falls below −8‰, reflecting a substantial change in the isotopic composition of the lake water, likely caused by to inflow of melt water. During the Preboreal period the δ18O as well as the δ13C values systematically decrease and reach minima (−8.4-9.4‰ for δ18O and ca −3.5-4.7‰ for δ13C). At the beginning of the Boreal period, an increase of ca 2‰ in δ18O is noted and is associated with climatic warming. During the Atlantic period the varying trends in the δ18O record are likely connected with changing precipitation/evaporation ratios, causing changes in the isotope composition of the water. The fluctuations of the isotopic values in the upper parts of the successions probably point to shallowing of the lake due to sedimentary infill.

Strontium isotope zoning in garnet: implications for metamorphic matrix equilibration, geochronology and phase equilibrium modelling

AbstractIn principle, garnet growth rates may be calculated from 87Rb/86Sr and 87Sr/86Sr measurements in garnet subsamples and the surrounding rock matrix. Because of low Rb/Sr, garnet should passively record the matrix decay of 87Rb to 87Sr as a progressive increase in 87Sr/86Sr from core to rim. This concept was tested by collecting Rb‐Sr data for five garnet grains from four major orogenic belts: eastern Vermont (c. 380 Ma), western New Hampshire (c. 320 Ma), southern Chile (c. 75 Ma) and northwestern Italy (c. 35 Ma). Both normal Sr isotope zoning (increasing 87Sr/86Sr from core to rim) and inverse Sr zoning (decreasing 87Sr/86Sr from core to rim) were observed. Garnet and matrix isotope data commonly yielded grossly inaccurate model ages. Incomplete Rb and Sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v. measured zoning patterns and the age disparities. Initially, the reactive matrix is dominated by rapidly equilibrating Rb‐rich mica, which imparts high 87Sr/86Sr values in garnet cores. Progressive participation of slower equilibrating Sr‐rich plagioclase buffers or even reduces 87Sr/86Sr, possibly leading to flat or decreasing 87Sr/86Sr from garnet cores to rims. Unusually high 87Sr/86Sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages, so metamorphic ages, growth rates, and associated heating and loading rates are likely suspect. Although Rb‐Sr may be the most susceptible because of the profound disparities between mica and feldspar, zircon reactivity might influence the Lu‐Hf system by up to a few per cent. The Sm‐Nd system seems generally immune to these effects. Pseudosection analysis and conventional garnet geochronology, which presume complete matrix equilibration during metamorphism, may require modification to account for differences between whole‐rock v. reactive matrix compositions.