Radiogenic Isotope Data Research Articles

Nucleosynthetic, radiogenic and stable strontium isotopic variations in fine- and coarse-grained refractory inclusions from Allende

We present new nucleosynthetic, radiogenic and stable Sr isotopic data from fifteen previously studied CAIs from the Allende CV3 meteorite, including the highly altered Curious Marie inclusion. We use double-spike TIMS techniques to determine the degrees of isotopic mass fractionation, and also present internally normalised data for the same sample digestions to permit comparisons with previous studies and couple these isotopic data with Rb, Sr, Eu and Th abundance data to consider the origins and relationships of the isotopic variations documented here. Analysed CAIs display elevated μ84Sr anomalies of +58 ppm to +287 ppm, with variability far outside of analytical uncertainties (13 ppm 2 s.d.). We cannot tell at present whether these variations arise from heterogeneities in p-process 84Sr or in the other non-radiogenic isotopes of Sr (86Sr, 88Sr) that are produced by the main s-process, weak s-process, and r-process. All inclusions fall on an offset mass-dependent fractionation line in three-isotope space (δ88/86Sr vs δ84/86Sr) identical within error to that previously defined by bulk undifferentiated meteorites, and have a total range of δ88/86Sr of ∼5.3 ‰ (+1.67 ‰ to −3.67 ‰), reflecting kinetic isotope effects during partial condensation/evaporation and/or low-temperature alteration processes. CI-normalized Sr/Th ratios in our CAIs correlate with normalized Eu/Th ratios with a ∼ 1:1 relationship, regardless of texture or Sr-isotopic values. This indicates that Sr and Eu had similar condensation behaviors with Eu condensing as Eu2+ and having the same chemical behavior in minerals as Sr2+ under conditions relevant to CAI formation in the solar nebula. Rb/Th ratios are highly variable: fine-grained CAIs display elevated Rb/Th ratios, consistent with the introduction of Rb into the CAIs by alkali-rich secondary alteration fluids. The μ84Sr anomalies measured in our CAIs are similar (in magnitude) to those found in carbonaceous chondrites that formed in the outer part of the solar system. A way to reconcile this observation with the formation of CAIs near the Sun would be if the inventories of Sr and other refractory elements in carbonaceous chondrites are dominated by a cryptic refractory dust component (CRD) that was formed early and near the Sun, and was subsequently transported outwards to the carbonaceous chondrite-forming region.

Petrogenesis of end-Cretaceous/Early Eocene lamprophyres from the Deccan Large Igneous Province: Constraints on plume-lithosphere interaction and the post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India

We present petrology, geochemistry and radiogenic isotope (Sr and Nd) data of thirteen post-Deccan lamprophyre dykes in the Narmada rift zone from the Chhotaudepur alkaline province of the Deccan Large Igneous Province (DLIP). Mineralogically, these dykes show affinity towards alkaline (sannaite and camptonite) as well as ultramafic (damtjernite) varieties of lamprophyres. Their major oxides and certain trace element ratios increase with increasing silica content highlighting the strong influence of fractionation processes. Their Nb/U and Ce/Pb ratios are similar to the mantle array defined by MORBs and OIBs and suggests an uncontaminated nature. Major oxide (K2O, Na2O, SiO2 and TiO2) contents show geochemical similarity towards shoshonitic volcanic series, whereas elevated Zr/Hf and Nb/La coupled with suppressed Rb/Nb and Zr/b display their affinity towards HIMU-type intraplate basalts. Their radiogenic initial 87Sr/86Sr (0.706034–0.710582) and sub-chondritic initial ɛNd (−8.6 to 2.1) are akin to those of the (i) ca. 65 Ma Ambadongar carbonatite, NW India, and (ii) ca. 65 Ma orangeites from Bastar Craton, central India, highlighting an enriched lithospheric mantle source. REE inversion modeling suggests ~3% enrichment of an undepleted mantle followed by small degrees of melting of this enriched mantle source are sufficient- as in the case of ocean island basalts (OIB)- to reproduce their observed REE concentrations. Their TDM Nd model ages (564–961 Ma) are consistent with widespread convergent margin-related magmatism during the amalgamation of the Rodinia supercontinent. We propose that enriched lithospheric mantle developed during the Neoproterozoic was metasomatized by small-volume CO2-rich melts imparting a HIMU-type geochemical character during Late Cretaceous, when the mantle plume (viz., Réunion) responsible for the flood basalt eruption, impinged at the base of the NW Indian lithosphere. From the presence of F-rich apatite and high K/Rb in mica, we infer the (i) presence of F-phlogopite in their source regions, and (ii) that the depth of post-Deccan lithosphere-asthenosphere boundary (LAB) beneath NW India was at least ~100 km at ca. 65 Ma.

Radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late Mesoproterozoic Borden Basin, Arctic Canada

Abstract The ca. 1050 Ma Bylot Supergroup in Arctic Canada is one of the best-preserved archives of late Mesoproterozoic geochemistry and biology and offers evidence that this period of Earth history may have been more biogeochemically dynamic than previously appreciated. The Bylot Supergroup was deposited in the Borden Basin and is the most thoroughly studied stratigraphic succession from a series of broadly contemporaneous late Mesoproterozoic intracratonic basins known as the Bylot basins. This ∼6-km-thick mixed carbonate-siliciclastic succession has undergone minimal postdepositional deformation and is now exposed on Baffin and Bylot Islands, Nunavut, Canada. Deep-water and tidally influenced carbonate facies, traditionally interpreted as marine, have yielded important insights into the evolution of Proterozoic seawater chemistry; however, more recent studies indicate that the Borden Basin was restricted marine or lacustrine for portions of its depositional history. Here, we present new multiproxy radiogenic isotope chemostratigraphic data spanning the Bylot Supergroup. A comparison of stratigraphic trends in radiogenic isotope data from hydrogenous (black shale 187Os/188Os and limestone 87Sr/86Sr) and detrital (whole-rock mudstone 143Nd/144Nd) sedimentary phases elucidates the complex hydrologic history of the Borden Basin and reconciles these disparate interpretations. Episodic coupling and decoupling between the composition of basin waters (from Os and Sr isotopes) and the local weathering input to the basin (from Nd isotopes) indicate that depositional environments within the Borden Basin fluctuated between marine and nonmarine (i.e., lacustrine). Variations in basin hydrology controlled secular sedimentation patterns through changes in basin water chemistry. These interpretations help to characterize the environment in which the early red algal fossil Bangiomorpha pubescens evolved. Episodically restricted epeiric seaways, such as within the Borden Basin, were relatively widespread within Rodinia and may have exerted unique selective pressures on eukaryotic evolution in the Mesoproterozoic Era. Hydrogenous and detrital radiogenic isotope chemostratigraphy, as implemented in this study, may provide a useful paleoenvironmental framework for future paleontological studies aimed at testing the role of freshwater environments in eukaryotic evolution. In addition, 87Sr/86Sr compositions from 81 new middle Bylot Supergroup marine limestone samples, calibrated by recent Re-Os geochronology, contribute to the terminal Mesoproterozoic marine 87Sr/86Sr curve. These data display a rise from ∼0.705 to 0.706 that reflects weathering of the active Grenville orogenic belt and demonstrates a global increase in chemical weathering during the amalgamation of Rodinia.

Magnesite formation in playa environments near Atlin, British Columbia, Canada

The hydromagnesite-magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth’s surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg5(CO3)4(OH)2·4H2O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO3·5H2O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10-17 to 10-16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO2 within Mg-carbonate minerals.

Geochemistry, Sr-Nd isotopes and zircon U-Pb geochronology of intrusive rocks: Constraint on the genesis of the Cheshmeh Khuri Cu mineralization and its link with granitoids in the Lut Block, Eastern Iran

The Cheshmeh Khuri prospecting area, part of the Tertiary volcanic-plutonic rocks in the Lut Block (central eastern Iran), comprises Middle Eocene granitoid and volcanic (with basaltic andesite to rhyolite composition) sequences that intruded pyroclastic rocks. Copper mineralization is related to the granitoid rocks. The mafic rocks occur mainly as dykes and are related to a younger magmatic activity (post-mineralization) in the area. The main hydrothermal alteration zones include quartz-sericite-pyrite ± carbonate (QSP ± C), argillic, and propylitic. Mineralization (pyrite, chalcocite, chalcopyrite, sphalerite, galena and magnetite) occurs as disseminated, stockwork and vein-type. The granitoids have features typical of high-K calk-alkaline, metaluminous, and belong to magnetite granitoid series. Primitive mantle normalized trace element spider diagrams display enrichment in LILE, such as Rb, Ba, and Cs, compared to those of HFSE. Chondrite-normalized REE plots show moderately to slightly strong LREE enriched patterns, and a negligible negative Eu anomaly. Rb-Sr whole rock–feldspar–biotite-hornblende and zircon U-Pb ages between ~44.6 and ~43.4 Ma (Middle Eocene) were obtained in granitoid samples. Initial 87Sr/86Sr ratios of granitoids range from 0.7047 to 0.7054, and their εNdi values vary from −1.1 to +0.8. Initial 87Sr/86Sr ratios of mafic rocks range from 0.7043 to 0.7047, and the εNdi values vary from +0.7 to +3.5. The whole set of geochemical data of rocks in the study area indicate an origin of the parental melts in a subduction-modified upper mantle in post-collisional extension-related zone. The primitive magmas of granitoids underwent contamination and assimilation through being exposed to the continental crust, while primitive magmas of the mafic rocks do not have relevant crustal contribution. The geochemical and radiogenic isotope data for the granitoids associated with mineralization in the Cheshmeh Khuri area in comparison with those of other prospects in the Lut Block indicate that mantle has played a crucial role in their petrogeneses and metallogeneses. Due to the crustal contamination and assimilation, the proportion of mantle component that was involved in the granitic magma shows a decrease in the Cheshmeh Khuri area. It seems that the Cheshmeh Khuri area has the lowest potential for a significant amount of Cu-Au agglomeration.

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.

An Isotope Study of the Dzhida Mo–W Ore Field (Western Transbaikalia, Russia)

The Dzhida ore field includes Pervomaika (Mo), Inkur (W) and Kholtoson (W) deposits. This article presents stable and radiogenic isotopic data (O, C, D, S, Sr and Nd) in an attempt to better understand the petrogenetic processes and the problem concerning the sources of ore-forming fluids. Granites from the Pervomaika deposit, which includes Mo-ores, as well as the syenite dikes that precede W-mineralization, have low δ18O values (about 5‰ and 4‰ respectively), and low initial ratios 87Sr/86Sr (0.704–0.705). The εNd (T) values (+0.9–−1.1) in granites and syenites are close to the evolution trend of the mantle-derived source. It was determined that a mantle-derived source was involved in ore-forming processes. It was also confirmed that δ34S values in sulfide minerals (molybdenite, pyrite, sphalerite, galena, and chalcopyrite) were close to the meteoric standard (from −2‰ to +2‰). The δ13C and δ18O values in carbonate minerals (rhodochrosite and ankerite) of the Kholtoson deposit are located within the primary igneous carbonatite (PIC)-square, as a possible juvenile source of CO2. This was also confirmed by the δ18O and δD values in muscovite from greisens (4.2‰–6.5‰ δ18O, –78.8‰ … –84.0‰ δD). The δ18O values calculated in a fluid equilibrated with hydrothermal minerals indicated a meteoric origin.

Age and genesis of polymetallic veins in the Freiberg district, Erzgebirge, Germany: constraints from radiogenic isotopes

The Freiberg mining district in the Erzgebirge hosts three principal types of polymetallic veins. These are (1) the quartz-bearing polymetallic sulfide type, (2) the carbonate-bearing polymetallic sulfide type, and (3) the barite-fluorite-sulfide type. We investigated the genesis of each vein-type using Rb-Sr sphalerite geochronology, Sm-Nd fluorite geochronology, and Pb, Sr, and Nd isotope systematics of ore and gangue minerals. Field relationships and the Rb-Sr and Pb isotope systematics of sulfides from quartz-bearing polymetallic sulfide veins and carbonate-bearing polymetallic sulfide veins confirm their close genetic affiliation and yield a combined Rb-Sr errorchron age of 276 ± 16 Ma. The high mean squared weighted deviation (MSWD) value of 42 on the regression is considered to reflect initial isotopic heterogeneity, which is probably related to fluid-rock interaction during the hydrothermal mineralization process. Although some sphalerites from barite-fluorite-sulfide veins have strongly disturbed Rb-Sr isotope systematics, six sphalerites and one co-genetic fahlore yield a robust isochron age of 121.3 ± 4.2 Ma with an MSWD of 2.9. This age is supported by a fluorite Sm-Nd isochron age of 101 ± 18 Ma (MSWD = 1.3). The new ages and radiogenic isotope data place robust constraints on the long-held hypothesis that veins in the Freiberg district formed during two hydrothermal events. The Lower Permian age of first stage quartz-bearing polymetallic sulfide veins and carbonate-bearing polymetallic sulfide veins coincides with post-Variscan crustal reorganization and Rotliegend volcanism. The Mid-Cretaceous age of second stage barite-fluorite-sulfide veins coincides with opening of the North Atlantic Ocean during the break-up of Pangea.

Boron, carbon, oxygen and radiogenic isotope investigation of carbonatite from the Miaoya complex, central China: Evidences for late-stage REE hydrothermal event and mantle source heterogeneity

The Miaoya carbonatite complex (MCC) is located within the southern edge of the Qinling orogenic belt in central China, and is associated with significant rare earth element (REE) and Nb mineralization. The MCC consists of syenite and carbonatite that were emplaced within Neo- to Mesoproterozoic-aged supracrustal units. The carbonatite intruded the associated syenite as stocks and dikes, and is mainly composed of medium- to fine-grained calcite and abundant REE-bearing minerals. Carbonatite melt generation and emplacement within the MCC occurred during the Silurian (at ~440 Ma), and was subsequently impacted by a late-stage hydrothermal event (~232 Ma) involving REE-rich fluids/melt.This study reports trace element and stable (B, C, and O) and radiogenic (Nd, Pb, and Sr) isotope data for the MCC carbonatite, and these have been subdivided into three groups that represent different REE contents, interpreted as varying degrees of hydrothermal interaction. Overall, the group of carbonatites with the lowest enrichment in LREEs (i.e., least affected by hydrothermal event) is characterized by δ11B values that vary between −7 (typical asthenospheric mantle) and + 4‰; δ11B values and B abundances (~0.2 to ~1 ppm) do not correlate with LREE contents. The Sm-Nd and Pb-Pb isotope systems have both been perturbed by the late-stage, REE-rich hydrothermal activity and corroborate open-system behavior. Contrarily, initial 87Sr/86Sr ratios (vary between ~0.70355 and 0.70385) do not correlate significantly with both LREEs and Sr abundances, nor with initial 143Nd/144Nd ratios. The late-stage hydrothermal event overprinted the Nd and Pb isotope compositions for most of the carbonatite samples examined here, whereas a majority of the samples preserve their variable B and Sr isotope values inherited from their mantle source. The B and Sr isotope data for carbonatites exhibiting the least LREE enrichment correlate positively and suggest carbonatite melt generation from a heterogenous upper mantle source that records the input of recycled crustal material. This finding is consistent with those previously reported for young (<300 Ma old) carbonatites worldwide.

Early earth geodynamics: cross examining the geological testimony.

Many studies link the presence of continents on Earth to the operation of plate tectonics. Radiogenic isotope data have, however, long consigned the bulk of crust generation and preservation to the murky realm of the Precambrian Earth, where the prevailing geodynamic systems are highly uncertain due to the sparse and complex nature of the geological record of these early eons. The purpose of this paper is to examine the nature of this geological record, considering the biases and artefacts that may undermine its fidelity, and to assess what are the most robust lines of evidence from which meaningful geodynamic inferences can be drawn. This is pursued with reference to Hadean detrital zircons, Archean gneiss complexes and Archean granite-greenstone terranes, and by considering isotopic proxies of crust-mantle interaction. The evidence reinforces long held views that the formation of some of the oldest continental nuclei involved a distinctive mode of planetary geodynamics that rests uneasily within definitions of modern style plate tectonics. A detailed interrogation of the oldest rocks, integrating multi-scale information from the best preserved whole-rock and mineral archives, and emphasizing careful selection at the sampling and analytical stages, will lead to the most robust input data for petrological and thermodynamic models of early Earth processes.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Radiogenic isotopes record a ‘drop in a bucket’ – A fingerprint of multi-kilometer-scale fluid pathways inferred to drive fault-valve behavior

Minerals precipitated from fluids carry a chemical signature of fluid origin and transport history. We use this concept to test the hypothesis that a subset of veins precipitated in the Loma Blanca fault zone, Rio Grande rift, NM, USA, record fault-valve behavior. Previous studies of this site show syntectonic calcite veins formed in response to post-seismic fluid discharge following repeated ground-rupturing earthquakes. U-Th ages on veins constrain the timing of fault slip over a period of >400,000 years. The radiogenic isotope (Sr, Pb, and U) data we report here demonstrate that a cluster of earthquakes that occurred at ∼430 ka, when fault-slip frequency increased by nearly one order of magnitude, corresponds with a brief period of particularly large and rapid variations in post-seismic fluid chemistry. Isotopic variations are consistent with fluid input from the Socorro magma body, a large, active, mid-crustal basalt intrusion 19 km below the surface. The short duration of the earthquake cluster (<50,000 years) precludes a role of thermal weakening of the crust in increased extension. We suggest that volatile exsolution and heating of ambient fluids during magma crystallization increased fluid volume and thereby pore-fluid pressure at depth, weakening the fault and driving more frequent failure through fault-valve action. This conclusion could not have been reached using traditional approaches in structural geology, stable isotope geochemistry, or geochronology alone, illustrating the utility of integrated data sets including radiogenic isotope analyses in studies of fluid-fault interactions.

Geological and isotopic constraints on ore genesis, Huangjindong gold deposit, Jiangnan Orogen, southern China

The Huangjindong gold deposit, with gold resources of ∼80 t, is hosted in Neoproterozoic slates along the Chang-Ping Fault in the Jiangnan Orogen, southern China. The orebodies are dominated by auriferous arsenopyrite-pyrite-quartz-(scheelite) veins; quartz-rich breccias that are cemented by late arsenopyrite, pyrite, muscovite, and quartz; auriferous stibnite-quartz veins; and disseminated gold in arsenopyrite-pyrite-sericite-quartz altered rocks. Neither significant alteration zoning nor ore-element zoning are recognized within the orebodies, although the orebodies at shallower crustal levels are slightly more stibnite-rich. At the regional scale, the first-order control on the distribution of gold deposits is the NNE- to NE-trending crustal-scale, sinistral strike-slip Chang-Ping Fault. As in other gold provinces globally, this controlling fault is not gold mineralized. The gold occurrs in WNW to E-W-trending zones of thrusts and fracture arrays associated with tight ‘locked-up’ asymmetrical folds. The broadly E-W-trending orebodies formed independently within approximately NE-trending fault-bounded corridors defined by the first-order Chang-Ping, and second-order Ni-Wan and Keng-Shang/Chang-Long Faults. Hydrothermal activity was initiated at the ductile-brittle transition, but evolved to brittle-dominated deformation, all within a transpressional environment.Oxygen and hydrogen isotope compositions of fluids (δD = −44.4‰ to −34.2‰; δ18O = +8.6‰ to +9.5‰) calculated from gold-related hydrothermal quartz and hydrothermal sericite, respectively, are consistent with a metamorphic source. Sulfur isotope compositions of the gold-bearing arsenopyrite and pyrite (median values between −8.0‰ and −6.0‰) are generally distinct from δ34S of the regional Neoproterozoic slates in the Jiangnan Orogen, but overlap the compositions of pyrites in global sediment-hosted orogenic gold deposits. Lead isotope compositions of the gold-bearing arsenopyrite and pyrite (206Pb/204Pb: 17.049 to 18.417; 207Pb/204Pb: 15.263 to 15.712; 208Pb/204Pb: 37.597 to 39.220) suggest that the lead was sourced from metamorphic rocks involved in the orogenic events. Therefore, both the stable and radiogenic isotopic data indicate that ore-forming fluids and metals were derived from a metamorphic source, although the precise nature of that source is unclear.On the basis of these geological and isotopic data, Huangjindong is interpreted as an orogenic gold deposit. The gold ore bodies show classic structural controls, being adjacent at the province scale to the crustal-scale Chang-Ping Fault but located in thrusts and associated tight anticlinal fold hinges that lie in corridors between markedly oblique, relatively brittle fault arrays that parallel the Chang-Ping Fault.

Zircon petrochronology reveals the timescale and mechanism of anatectic magma formation

Igneous rocks of intermediate to acidic composition commonly exhibit considerable degrees of isotope variability preserved at the crystal and sub-crystal scale, as well as a significant U–Pb age range, reflecting protracted timescales of zircon crystallization and long magma residence times. The association of high-precision U–Pb zircon dates with stable and radiogenic isotope data represents a powerful tool to unravel the petrological evolution of granitic rocks, hence allowing a better understanding of the processes that led to the formation and reworking of the continental crust.In this case study, we combine high-precision U–Pb dates with stable and radiogenic isotope data from zircon crystals in the Larderello–Travale (Italy) shallow-level granites. These rocks are peraluminous two-mica, cordierite-bearing granites and represent pure crustal anatectic magmas, generated in a post-collisional extensional setting. As such, they are ideal candidates to investigate the timing, rates and mechanisms of melt production during anatectic magma formation, giving insights into the process of intracrustal differentiation. Magmatic zircon crystals from the Larderello–Travale granites contain δ18O values ranging from 8.6 to 13.5‰ and crystals from individual samples exhibit inter- and intra-grain oxygen isotope variability exceeding 3‰. The analysed crystals have εHf values ranging between −7.4 and −12.4, with moderate, intra-sample εHf isotope variability. All CA-ID-TIMS (chemical abration isotope-dilution thermal ionization mass spectrometry) 206Pb/238U zircon ages range from 4.5 to 1.6 Ma and suggest four pulses of magmatic activity at ∼3.6, 3.2, 2.7 and 1.6 Ma. More importantly, zircon crystals from individual samples typically exhibit an age spread as large as 300–500 ka. This age dispersion suggests that most of the zircon did not crystallize at the emplacement level but in the middle crust and were subsequently recycled and juxtaposed during ascent and emplaced at shallow level. When plotted against age, stable and radiogenic isotope data suggest the co-existence of multiple and isotopically distinct magma batches produced by partial melting of different crustal domains. This requires coeval magma batches that are physically separated and evolve independently for hundreds of thousands of years before coalescing during ascent and emplacement. The involvement of multiple sources in the production of crustal anatectic magmas reflects the inherent heterogeneous nature of the continental crust and result from the interplay between the rise and evolution of the geotherms through the crust and the composition of the fertile source rocks. Finally, the isotopically diverse zircon-bearing magma batches mixed and assembled into shallow-level intrusions generated during the four major magma pulses.

The mafic alkaline volcanism of SW Madagascar (Ankililoaka, Tulear region): 40 Ar/ 39 Ar ages, geochemistry and tectonic setting

High-precision 40 Ar/ 39 Ar ages, major and trace element, and radiogenic isotope data are presented for the basanites and alkali basalts forming the southwesternmost monogenetic volcanic field in Madagascar. The volcanic rocks were erupted along fissure zones and aligned cones in a nearly flat area covered by the Cenozoic sediments of the Morondava basin. The high-precision 40 Ar/ 39 Ar ages constrain the beginning of the magmatism in the Ankililoaka area to about 12 Ma, significantly earlier than suggested by previously published K/Ar ages. The Ankililoaka basanites include primitive compositions (MgO &gt;10 wt%, Ni &gt;200 ppm and Cr &gt;400 ppm), whereas other basanites and alkali basalts experienced limited removal of olivine, chromiferous spinel and clinopyroxene. Initial Sr and Nd isotope ratios of the basanites are 0.70343–0.70445 and 0.51279–0.51282, respectively. The Pb isotope compositions are in the ranges 206 Pb/ 204 Pb = 19.08–19.38, 207 Pb/ 204 Pb = 15.61–15.64 and 208 Pb/ 204 Pb = 39.1–39.4. The alkali basalts have similar 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and 207 Pb/ 204 Pb, but slightly lower 206 Pb/ 204 Pb and 208 Pb/ 204 Pb than the basanites. The isotopic composition of the Ankililoaka rocks partially overlaps with that of the Cenozoic volcanic mafic rocks of northern Madagascar, and differs significantly from that of the mafic volcanic rocks of central Madagascar, which have lower 206 Pb/ 204 Pb and 207 Pb/ 204 Pb, and higher 87 Sr/ 86 Sr. Major and trace element systematics and geochemical modelling suggest that the Ankililoaka mafic alkaline rocks are low-degree melts of an incompatible element enriched peridotite source starting from depths where garnet is stable. Crustal contamination during ascent was insignificant. We argue that the genesis of the Ankililoaka alkaline magmas was triggered by melting an enriched, volatile-rich lithospheric mantle uplifted in the Cenozoic. Supplementary material: Analytical techniques, X-ray fluorescence whole-rock data, mineral compositions, 40 Ar/ 39 Ar dataset, mantle source modelling and figures showing volcanological features and thin sections are available at https://doi.org/10.6084/m9.figshare.c.4065743

Variable sulfur isotope composition of sulfides provide evidence for multiple sources of contamination in the Rustenburg Layered Suite, Bushveld Complex

The Rustenburg Layered Suite (RLS) of the Bushveld Complex (BC) is famous for its platinum group element (PGE) ore, which is hosted in sulfides. The source of sulfur necessary to generate this type of mineralization is inferred to be the host rock of the intrusion. The RLS has a sulfur isotopic signature that indicates the presence of Archean surface-derived material (Δ33S≠0) in the magma. This signature, with an average value of Δ33S=0.112±0.024‰, deviates from the expected Δ33S value of the mantle of 0±0.008‰. Previous work suggested that this signature is uniform throughout the RLS, which contrasts with radiogenic isotopes which vary throughout the igneous stratigraphy of the RLS.In this study, samples from key intervals within the igneous stratigraphy were analyzed, showing that Δ33S values vary in the same stratigraphic levels as Sr and Nd isotopes. However, the variation is not consistent; in some levels there is a positive correlation and in others a negative correlation. This observation suggests that in some cases distinct magma pulses contained assimilated sulfur from different sources. Textural analysis shows no evidence for late addition of sulfur. These results also suggest that it is unlikely that large-scale assimilation and/or efficient mixing of host rock material in a single magma chamber occurred during emplacement.The data do not uniquely identify the source of sulfur in the different layers of the RLS, but the variation in sulfur isotope composition and its relationship to radiogenic isotope data calls for a reevaluation of the models for the formation and evolution of the RLS, which has the potential to impact the knowledge of how PGE deposits form.

Establishing radiogenic strontium isotope signatures for Chavín de Huántar, Peru

The current manuscript reports the first bioavailable radiogenic strontium isotope (87Sr/86Sr) data for Chavín de Huántar, a U.N.E.S.C.O. World Heritage site long recognized as one of the most important ceremonial centers of the Andean Formative Period. 87Sr/86Sr ratios were measured in local soil, vegetation, and archaeological and modern fauna from in and around the archaeological site. 87Sr/86Sr signatures from human tooth enamel from five Mariash-Recuay era (1–700 CE) individuals also were generated. Results indicate that 87Sr/86Sr values among soil, plants, and animals are relatively uniform, although the range of 87Sr/86Sr ratios within each category is broad. While in the present study, non-migrants and migrants were distinguishable based on their 87Sr/86Sr values, the wide range of Chavín's bioavailable 87Sr/86Sr signature might hinder the differentiation of local and non-local individuals in subsequent analyses. We caution that future 87Sr/86Sr applications at Chavín should consider both bioavailable 87Sr/86Sr signatures calculated from fauna, plants, and soil along with statistically analyzed human data when investigating evidence for residential mobility in the archaeological record.

Refractory organic matter in coastal salt marshes-effect on C sequestration calculations

The age and ability of salt marshes to accumulate and sequester carbon is often assessed using the carbon isotopic signatures (Δ14C and δ13C) of sedimentary organic matter. However, transfers of allochthonous refractory carbon (CRF) from the watershed to marshes would not represent new C sequestration. To better understand how refractory carbon (CRF) inputs affect assessments of marsh age and C sequestration, Δ14C and δ13C of both total organic carbon (TOC), CRF, and non-CRF organic matter fractions were measured in salt marshes from four contrasting systems on the North Atlantic coast. To our knowledge, no salt marsh sediment study has considered refractory or allochthonous carbon in carbon budget calculations or the impact on chronologies. Stable and radiogenic isotope data suggest that while TOC was dominated by autochthonous plant inputs, CRF was dominated by locally recycled or allochthonous C, the delivery of which was controlled by the size and slope of each watershed. Steep-gradient rivers analyzed delivered Δ14C-depleted CRF to their estuarine marshes, while the site located in the low-gradient river was associated with larger CRF content. Finally, the marsh isolated from riverine input contained the least fraction of TOC as CRF. Laterally transported CRF caused only a small offset in Δ14C in relation to TOC in low-gradient systems (average Δ14C offset was −44.4 and −24.2‰ at each location). However, the presence of allochthonous Δ14C-depleted CRF in sediments of steep-gradient rivers led to large overestimates of the time of organic matter deposition (i.e. apparent age was older than the ‘true’ time of deposition) (Δ14C offset ranged from −170.6 to −528.9‰). Further, reliance on TOC or loss on ignition analyses to calculate C sequestration by marshes might produce overestimates of at least as much as 10 to 20% since neither account for the lateral transport of allochthonous carbon.

Magnesium Isotopes as a Tracer of Crustal Materials in Volcanic Arc Magmas in the Northern Cascade Arc

Fifteen North Cascade Arc basalts and andesites were analyzed for Mg isotopes to investigate the extent and manner of crustal contributions to this magmatic system. The δ26Mg of these samples vary from within the range of ocean island basalts (the lightest being -0.33 ± 0.07‰) to heavier compositions (as heavy as -0.15 ± 0.06‰). The observed range in chemical and isotopic composition is similar to that of other volcanic arcs that have been assessed to date in the circum-pacific subduction zones and in the Caribbean. The heavy Mg isotope compositions are best explained by assimilation and fractional crystallization within the deep continental crust with a possible minor contribution from the addition of subducting slab-derived fluids to the primitive magma. The bulk mixing of sediment into the primitive magma or mantle source and the partial melting of garnet-rich peridotite are unlikely to have produced the observed range of Mg isotope compositions. The results show that Mg isotopes may be a useful tracer of crustal input into a magma, supplementing traditional methods such as radiogenic isotopic and trace element data, particularly in cases in which a high fraction of crustal material has been added.

Metasomatized mantle as the source of Mid-Miocene-Quaternary volcanism in NW-Iranian Azerbaijan: Geochronological and geochemical evidence

Middle Miocene to Quaternary volcanic rocks cover large areas of the Azerbaijan Province in NW Iran. This study reports two separate age clusters out of 23 new LA-ICP-MS U-Pb zircon ages: (1) Middle Miocene (16.2–10.6Ma) and (2) Latest Miocene-Late Pleistocene (5.5–0.4Ma). Major and trace element bulk rock geochemistry and initial Sr, Nd, Pb radiogenic isotope data on the dated rocks provide new constraints on the Mid-Miocene to Quaternary volcanism in this region. The analyses are distributed over a large compositional range from low-K to high-K calc-alkaline andesites and dacites/rhyolites to more alkaline trachybasalts and dacites with shoshonitic affinities. Chondrite-normalized REE patterns are steep with significant enrichment in LREE and low abundances of HREE indicating a garnet control. Plots of primitive mantle-normalized trace elements show negative Ti and Nb-Ta anomalies indicative of an arc signature. The wide compositional range and the ubiquitous presence of an arc signature reveal that the source mantle is heterogeneous and metasomatically altered. Sr, Nd and Pb radiogenic isotope data further point towards an enriched mantle source and/or crustal contamination. Crustal contamination is best recognized by inherited zircon cores, which yield Late Neoproterozoic to Early Cambrian ages typical for the Iranian basement. The occurrence of adakite-like compositions with elevated magnesium numbers, Cr and Ni concentrations argue against a fractionation-driven process but point to a subcrustal origin. Overall, the analyzed lavas show no spatial and temporal relation to a potential subduction zone, confirming the dated volcanics to be post-collisional and not related to singular processes such as slab retreat or delamination of a continuous lower crustal sliver. We propose three hypotheses to explain the reported disparity in distribution, age and composition and favour small-scale sublithospheric convection or incorporation of crustal material into the metasomatized mantle. The discovery of the late Miocene time gap is in line with previously advocated exhumation pulses and coincides with a major tectonic reorganization in the Arabian-Eurasian realm at this time.

Geology and Isotope Geochemistry of the Wainaulo Cu-Au Porphyry Deposit, Namosi District, Fiji

The late Miocene, calc-alkalic, Wainaulo Cu-Au porphyry deposit of the Namosi district, Fiji, hosts distinct styles of alteration and mineralization that overlapped to produce a substantial porphyry Cu-Au resource. The early stages produced medium-grade Cu, low-grade Au and concentric calc-potassic to propylitic alteration that is zoned around the early-stage diorite intrusions. Discrete zones of high-grade Cu and Au and calc-sodic alteration were then superimposed during the intrusion of the main-stage quartz diorites and the formation of quartz-sulfide and epidote-sulfide veins. As the magmatic-hydrothermal system waned, lower Cu and Au grades, with a weaker intensity of calc-sodic alteration and lower density of veins, were produced coincident with emplacement of subsequent quartz diorite intrusions. Late-stage anhydrite-pyrite veins and chlorite-illite alteration overprinted the quartz diorite intrusive complex. The final hydrothermal event consisted of argillic alteration that was concentrated in and around steeply dipping, ENE-trending shears. These structures appear to have controlled the emplacement of the quartz diorite complex and distribution of high-grade Cu-Au mineralization, suggesting they were active during the pre- and synmineralization stages. Stable and radiogenic isotopic data provide evidence for direct seawater contributions to the magmatichydrothermal system. Measured δ34Ssulfide (–5.0 to 3.8‰) and δ34Ssulfate (9.0–16.8‰) values are consistent with a predominantly magmatic source, whereas an elevated bulk sulfur composition (6.7‰) suggests mixing with an isotopically heavy fluid (e.g., seawater). Estimates of δDfluid derived from epidote (–9.1 to 11.3‰) and δ18Ofluid from epidote and anhydrite (–0.2 to 4.7‰) approach that of Vienna standard mean ocean water, and the initial Sr isotope ratios of epidote (0.70364–0.70378) suggest a component of seawater Sr ranging from 3.2 to 5.8%. These results are consistent with the inferred submarine paleogeographic setting and may explain the abundance of albite- and epidote-rich alteration assemblages at Wainaulo.