Radiogenic Research Articles

Control of climate, sea-level fluctuations and tectonics on the pervasive dolomitization and porosity evolution of the Oligo-Miocene Asmari Formation (Dezful Embayment, SW Iran)

The Asmari reservoir properties in various hydrocarbon fields across the Dezful Embayment (SW Iran) are modified by dolomitization. Petrographic and geochemical analyses of the dolomitized Asmari Formation units highlight their origin in the Bibi Hakimeh and Gachsaran oilfields. The petrographic studies revealed that dolomite in the Asmari Formation can be classified texturally (based on crystal sizes) into three main groups: (i) very fine crystalline (D1, <20 μm), (ii) fine to medium crystalline (D2, 20–150 μm) and (iii) coarse crystalline dolomites (D3, >150 μm). Petrography, fluid inclusions, stable isotopes (δ13C, δ18O), radiogenic isotopes (86Sr/88Sr) and major and trace elemental analyses were used to interpret the origin and paragenetic sequence of each dolomite group. Pervasive dolomitization prevails in the upper part of the Asmari Formation where very fine and medium crystalline dolomites with partially and fully fabric-retentive and fabric destructive textures, replaced micrite and allochems of the platform-top facies, respectively. The parent fluids of the very fine dolomites originated from contemporaneous Oligo-Miocene seawaters produced in a sabkha environment. The fine to medium dolomites (D2) most likely shared the same seawater origin and relate to refluxing, sabkha-derived fluids. The medium to coarse crystalline dolomite has more depleted oxygen isotopes than the former, very fine to fine crystalline, dolomites suggesting a burial dolomitization origin. The estimated high salinities (≈19 to 20 wt% NaCl) together with compatibility of oxygen and strontium isotope data with evaporative seawater values, suggest Oligo-Miocene seawater was the parent fluid for precipitation of coarse and cement phase of the medium crystalline dolomites. This study reveals that reactivated basement faults with associated fractures have probably played an important role in convecting and heating evaporative seawater along their planes. This heated seawater provided the required fluid for recrystallization of coarser matrix and cement phases of medium (D2) and coarse (D3) dolomites at depth. Therefore, the interaction of climate, sea-level changes, and tectonics played a critical role in formation and development of pervasive dolomitization in the Asmari Formation. Additionally, dissolution (partly as a consequence of dolomitization) was one of the key processes during early diagenesis in the formation of different types of porosity such as biomolds, vugs and intraparticles porosity. This study shows that the reservoir quality improves from finely crystalline dolomite towards samples dominated by coarsely crystalline dolomites.

Inter- and intra-crystal quartz δ18O homogeneity at Okataina volcano, Aotearoa New Zealand: Implications for rhyolite genesis

The sources and processes involved in the genesis of the voluminous rhyolitic magmas of cataclysmic caldera-forming eruptions, and the intervening lower-volume intra-caldera extrusions, have been subject to much debate. To better understand generation of high-volume and low-volume silicic eruptions within a single volcanic centre, and how they may differ, we examined ten volumetrically varied high-SiO2 rhyolite eruptions from the Okataina Volcanic Centre (OVC) in Aotearoa New Zealand. The OVC is one of the world's most recurrently active silicic volcanoes. In the last ~600 ky, the OVC was the focus of three known caldera-forming events and numerous intermittent dome-building and fissure eruption episodes, with rhyolitic eruption activity as recent as 1314 CE. To elucidate how mass contributions from the mantle and crust may have fluctuated over the lifespan of the OVC magmatic system, oxygen isotopic ratios (δ18O) of quartz in rhyolites were investigated for the first time at inter-crystal and intra-crystal scales. Quartz crystals from four eruption episodes (two caldera-forming events, Utu, ~557 ka, Rotoiti, ~45 ka, and two intra-caldera dome-building events, Rotoma, ~9.5 ka, and Kaharoa, ~0.7 ka) yielded intra-crystal δ18O isotopic homogeneity (±0.23‰, 2sd) based on secondary ion mass spectrometry (SIMS). These samples also display inter-crystal and inter-unit homogeneity within slightly lower precision (7.6 ± 0.5‰, 2sd). Whole-crystal quartz from the same four units, as well as six other units (two intra-caldera dome-building episodes, Okareka, ~21.8 ka, Whakatane, ~5.5 ka, three pre-Rotoiti extra-caldera domes, Round Hill, Haparangi, Kakapiko, and one immediately post-Rotoiti eruption, Earthquake Flat), were then examined using high-precision laser fluorination. Single crystals also yielded mostly homogenous ratios with average δ18O = 7.6 ± 0.5‰ (2sd), which is consistent with intra-crystal SIMS analyses, albeit for a larger set of samples. Stable and radiogenic isotope mixing models using the newly obtained δ18O ratios demonstrate that OVC rhyolites can be produced by ≥25% assimilation of a regional (Torlesse-like) metasedimentary endmember by a depleted mantle source with slightly variable amounts of subduction flux, and that any incorporation of hydrothermally altered material to the system is limited to <5% in caldera and intra-caldera eruptions. The δ18O records of the OVC are among the most homogenous currently known and indicate stable and consistent mantle and crustal contributions across the lifespan of the magmatic system, with assimilation largely occurring prior to segregation of rhyolitic melts within the silicic reservoir. This isotopic homogeneity may be due to a relatively high-volume and constant magma flux at the OVC, which contrasts to other rhyolitic caldera volcanoes with greater isotopic variability.

The role of mantle and crust in the generation of calc-alkaline Variscan magmatism and its tectonic setting in the Eastern Pyrenees

The lower structural levels of the Variscan orogen exposed in the Eastern Pyrenees reveal three genetically associated magmatic suites: (i) a batholitic sized calc-alkaline granitoid (Sant Llorenç – La Jonquera, SL–LJ); (ii) minor mafic intrusions with local ultramafic cumulates (Ceret and Mas Claret mafic complexes); and (iii) peraluminous leucogranite bodies. These suites were emplaced in a syn- to post-collisional setting during the Late Carboniferous – Early Permian (ca. 315–290 Ma) in an Upper Proterozoic-Upper Ordovician metasedimentary sequence. We carried out field and petrographic work and whole-rock geochemistry (including SrNd radiogenic isotopes) in order to constrain the petrogenetic relationships between them and to determine the interplay of the igneous suites with the country rock. We compare geochemical data to those from the neighbouring massifs of the Pyrenees and Catalan Coastal Ranges. The granitoids and the mafic complexes underwent variable degrees of lower crustal assimilation as demonstrated by the Sr and Nd isotopic ratios of SL–LJ granitoids and mafic rocks. Contaminated gabbro-diorites are high in Fe and Zr and contain magmatic garnet in equilibrium with an FeMg amphibole. A supra-subduction metasomatized mantle source for the mafic complexes is inferred. The magma that formed the SL–LJ granitoids was of intermediate composition and may have formed by differentiation of of magmas derived from partial melting of a subduction-metasomatized mantle caused by active subduction or mantle delamination or by partial melting of the lower crust triggered by underplating of mantle-derived mafic magmas. Leucogranite magmas formed later by partial melting of crustal rocks with compositions similar to the outcropping metapelites and orthogneisses. We deduce a retrograde cooling decompression path from 0.75 GPa down to 0.55 GPa from phase equilibria for a garnet-bearing diorite. This path is compatible with a local extensional context linked to a regional dextral strike-slip regime that provided space for the ascent and emplacement of the pluton. This strike-slip system is consistent with late-Variscan shear zones displacing Gondwana to the west with respect to Laurasia during the orogenic collapse.

Mantle sources of ocean islands basalts revealed from noble gas isotope systematics

Noble gas isotope systematics, particular those of He, have been fundamental in showing that some ocean island basalts (OIB) were sourced from deep mantle plumes. Relationships between He, W, Os, Sr, Nd and Pb isotopes in Hawaiian, Samoan, Galapagos, and Icelandic lavas have been suggested to reflect contributions from less degassed lower mantle sources, and perhaps even materials advected from the core-mantle boundary. This study reviews the noble gas (He-Ne-Ar-Xe) isotope systematics of major OIB suites. Important in this evaluation are considerations of spatial and temporal variations, including the sample media (glass, minerals, hydrothermal gases and fluids) used for analyzing noble gases, as well as the degree of partial melting experienced to produce OIB. Limited availability of gas-rich samples means patchy coverage in definition of OIB Ne, Ar, Xe isotope compositions. Additionally, low-degree partial melting will lead to preferential sampling of more fusible, generally more enriched components which could affect noble gas isotope systematics. OIB with low- (<8RA) to MORB-like 3He/4He (8 ± 2RA) dominantly sample convecting mantle domains and can also contain some relatively undegassed (solar) components. Their range in Sr-Nd-Os-Pb isotope compositions reflect a strongly recycled crustal and/or lithospheric heritage. Intermediate (>10RA) to high-3He/4He (>25RA) OIB (Loihi, Hawaii; Iceland; Fernandina, Galapagos; Ofu, Samoa) sample a reservoir that has been relatively isolated since ~4.5 billion years as shown by noble gases (He, Ne, Ar, Xe), and by W isotopes. However, this reservoir is not pristine and these OIB show evidence for containing depleted and enriched recycled components from Xe isotopes as well as lithophile-siderophile radiogenic isotope systematics. Linking the highest-3He/4He OIB to a putative Focus Zone (FOZO) reservoir is also problematic; FOZO is not a reservoir least affected by recycling of crust, rather a mantle reservoir that contains recycled components, including depleted lithosphere, and that has been oxidized. This distinction in definition means that, while the FOZO reservoir is almost certainly sampled by the deepest, hottest mantle plumes, its status as a primitive reservoir is unsubstantiated. Models of mantle convection can satisfy seismic constraints on Earth's deep mantle and reveal extensive mixing throughout with enriched and depleted lithologies formed by partial melting processes at Earth's surface. Ancient deep isolated reservoirs are likely to be relatively minor present-day mantle features. More remarkable is the predicted abundance of strongly refractory mantle material. Focus on the noble gas, radiogenic and stable isotope attributes of such refractory reservoirs are required for a fuller understanding of mantle processes.

Evolution of mafic lavas in Central Anatolia: Mantle source domains

Abstract We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas, Develidağ, Erciyes, and Erkilet volcanic complexes in central Turkey and Tendürek in eastern Turkey to evaluate the mantle sources for volcanism in the context of the geodynamic evolution of the Anatolian microplate. Early Miocene through Quaternary volcanism in Western Anatolia and latest Miocene through Quaternary activity in Central Anatolia were dominated by contributions from two distinct source regions: heterogeneous metasomatized or subduction-modified lithosphere, and roughly homogeneous sublithospheric ambient upper mantle; we model the source contributions through mixing between three end members. The sublithospheric mantle source plots close to the Northern Hemisphere reference line (NHRL) with radiogenic 206Pb/204Pb of ∼19.15, while the other contributions plot substantially above the NHRL in Pb isotope space. The lithospheric source is heterogeneous, resulting from variable pollution by subduction-related processes likely including direct incorporation of sediment and/or mélange; its range in radiogenic isotopes is defined by regional oceanic sediment and ultrapotassic melts of the subcontinental lithospheric mantle. The geochemical impact of this contribution is disproportionately large, given that subduction-modified lithosphere and/or ocean sediment dominates the Pb isotope signatures of mafic Anatolian lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked crustal shortening, took place throughout the region until ca. 16–17 Ma, after which time broad delamination of the thickened lower crust and/or the Tethyan slab beneath Central Anatolia allowed for sediment and/or mélange and slab-derived fluids to be released into the overlying evolving modified mantle. Aggregation of melts derived from both mantle and lithospheric domains was made possible by upwelling of warm asthenospheric material moving around and through the complexly torn younger Aegean-Cyprean slab that dips steeply to the north beneath southern Anatolia.

Disentangling diagenetic and biogenic trace elements and Sr radiogenic isotopes in fossil dental enamel using laser ablation analysis

The reconstruction of life histories traits such as the ontogenic evolution of diet or sequences of mobility can be achieved for fossil mammals thanks to laser ablation analysis of trace elements concentration and radiogenic Sr isotope composition in dental enamel. However, a major limitation for the use of laser ablation to study fossil tooth enamel is that the analysis must be carried out on bulk sample hence prohibiting any ad hoc leaching step to remove diagenetic compounds. Biogenic and diagenetic trace elements along with the 87Sr/86Sr ratio must be monitored and post hoc correlation tests processed to isolate areas free of diagenesis. The present study combines a unique combination of biogenic (Sr and Ba), diagenetic (Mn and U) trace elements, major elements (Mg and Ca) and 87Sr/86Sr ratio time series obtained by means of laser ablation ICPMS and MC-ICPMS on a large number (n = 94) of fossil tooth enamel samples of humans and animals from a set of neighboring Neolithic sites in France. Bone fragments sampled from the same specimens are also analyzed for comparison. Trends between diagenetic and biogenic trace and major elements are first identified using inclusive data analysis. The trends are then validated at the scale of the laser ablation raster, and a protocol of data cleaning is developed to identify areas free of diagenesis. Non-significantly altered Sr/Ca, Ba/Ca and 87Sr/86Sr average values are thus gathered for each cleaned tooth enamel samples and are discussed, along with previously measured bone collagen δ13C and δ15N values, in terms of diet and mobility in the context of the Neolithic revolution.

Sulfur Isotope Constraints on the Petrogenesis of the Kimberley Kimberlites

AbstractCretaceous kimberlites in southern Africa have been suggested to host deeply subducted material in their mantle sources based on radiogenic isotope systematics. However, potential subducted material contributions to the volatile budget, including sulfur, of these kimberlites is unclear. Here we report new petrographic, geochemical, and isotopic data on sulfides and sulfates in sub‐volcanic kimberlites from Kimberley, South Africa. The examined kimberlites were divided into four groups based on their sulfide mineralogy, sulfur contents, and isotopic compositions. None of these groups exhibit clear signs of mass‐independent fractionation. Three samples contain sphalerite, have moderate bulk‐sulfide S concentrations (203–329 μg/g) and highly negative bulk‐sulfide δ34S values (−10 to −13‰). Four samples have moderate‐to‐high bulk‐sulfide S contents (220–745 μg/g), positive δ34Ssulfide values (+0.2 to +14‰), and contain galena, pyrite or secondary Cu‐sulfides as the dominant sulfides. These groups of S‐rich kimberlites were probably contaminated by fluids sourced from local country rocks. The remaining eight samples contain negligible amounts of crustal sulfides (e.g., sphalerite, galena), have lower bulk‐sulfide S concentrations (≤111 μg/g), and display a different δ34Ssulfide range (−5.7 to +1.1‰) compared to the S‐rich groups. By considering only the five samples with fresh primary Cu‐Fe‐Ni sulfides, the δ34S range contracts to between −5.7 and −3.4‰, which is considered representative of the mantle source composition. This range indicates the presence of a deeply recycled sedimentary component in the melt source. The combination of detailed sulfide petrography and S isotope geochemistry in fresh kimberlite rocks provides a further tool to investigate mantle chemical geodynamics through time.

Stable and radiogenic strontium isotope cycling in a representative karst forest ecosystem, Southwest China

Stable and radiogenic strontium isotope cycling in a representative karst forest ecosystem, Southwest China

Geochemistry and Sr–Nd isotopic studies of Paleoproterozoic ( c. 2.3 Ga) meta-lamprophyre from the Rapuru area, Nellore Schist Belt, southern India: implications for back-arc basin magmatism and its relevance to the Columbia supercontinent assembly

Abstract This study reports on two lamprophyre dykes from the Rapuru area along the margin of the Eastern Dharwar Craton (EDC) and the Nellore Schist Belt (NSB). The Rapuru lamprophyre (RL) dykes are situated along the southern extension of the Prakassam Alkaline Province (PAP). The RL dykes are deformed, yet still preserve a porphyritic–panidiomorphic texture, with mica phenocrysts, and amphibole and feldspars in the groundmass. Geochemically, the RL dykes have a low Mg# (0.28–0.37), and Ni (30–60 ppm) and Cr (119–228 ppm) concentrations that indicate their evolved nature, such as for other reported lamprophyres from the PAP and EDC. This is further supported by Sr–Nd isotopic ratios that show an affinity towards a mid-ocean ridge basalt (MORB)–ocean island basalt (OIB)-like signature and a juvenile magmatic nature. The RL seems to have been affected by two major influences, namely, the primary source region, which is geochemically juvenile similar to the compositional field of enriched-MORB, and the continental lithosphere. Such magmas are known to have formed in a back-arc-basin environment. The initial 87 Sr/ 86 Sr ratio ( c. 0.7012–0.7045) and initial ε Nd ratios (3.13–7.93) are in line with back-arc basin basalts recorded in other parts of the world. The field observations and bulk-rock Sr–Nd radiogenic isotope values in the present study support the Paleoproterozoic nature of the RL. This concurrence of juvenile radiogenic isotopes and fluid-related trace element compositions apparently suggest dehydration of a subducted-slab-triggered metasomatism of the overlying mantle wedge in a subduction-related geodynamic setting. Such intrusive lamprophyre rocks of older ages are limited in India as well as other parts of the world. The 2.1 and 1.8 Ga rocks are widely considered to represent the initial accretion and final break-up of an erstwhile Columbia supercontinent assembly. We argue that the RL were formed in the Paleoproterozoic during the waxing stages of the Columbia supercontinent assembly in a back-arc basin environment, most probably due to the low degree of partial melting of the asthenosphere–lithospheric interaction caused by the introduction of an influx of subduction components into the arc–back-arc basin system.

Geochemical exploration for buried sandstone-hosted uranium mineralization using mobile U and Pb isotopes: case study of the REB deposit, Great Divide Basin, Wyoming

Mobile radiogenic lead isotopes ( 206 Pb, 207 Pb, 208 Pb and 210 Pb) represent products of radioactive decay of their parental uranium and thorium isotopes ( 238 U, 235 U, 232 Th), and are considered potential geochemical pathfinders of buried sandstone-type uranium deposits. Soil samples collected along a geochemical traverse intersecting buried uranium roll front mineralization at the REB deposit in the Great Divide Basin, Wyoming, USA were studied. Mineralization of this deposit is hosted in weakly lithified arkosic sands, at a depth of 120–200 m, without a strong surficial expression of its presence at depth, which makes discovery of this deposit type difficult, slow and expensive. All soil samples have been analysed for ratios of the mobile long-lived Pb isotopes and their parental U and Th isotopes, determined from partial leach products obtained using a weak acid leaching technique. The samples were also analysed for trace elements, assayed both in the partial leach products and using conventional whole-soil sample assays. Ratios of the mobile radiogenic Pb isotopes to their parental U and Th isotopes ( 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th) determined in the partial leach products exhibit anomalous contents in the soil samples collected above the uranium rolls. The anomalous values are several times greater than background values, to lateral distances of 350–400 m outside of the roll fronts. Notably, conventional whole-soil assays have failed to detect the anomalies that were detected using mobile Pb isotopes. Supplementary material : Minor and trace elements assays of the geochemical samples are available at https://doi.org/10.6084/m9.figshare.c.5610980

Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria

AbstractGenerally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i= 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i= 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.

Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.

Radiogenic and stable Sr isotopes constrain weathering processes in rapidly eroding Taiwan catchments

Small mountainous rivers (SMRs) in subtropical Taiwan supply a disproportionately large quantities of suspended and dissolved flux to the ocean due to extraordinarily high denudation rates in the world. We present major elements, 87Sr/86Sr, and δ88/86Sr data of river water and sediment from two typical Taiwan SMRs, Zhuoshui (Choshui) and Liwu Rivers. High contents of dissolved Ca and Mg are generally accompanied by high 87Sr/86Sr ratios in waters of the Zhuoshui and Liwu Rivers (0.710721–0.715551 and 0.710461–0.711618). Sequential extraction experiments of river sediments reveal that Sr mainly resides in the residual silicate and acid-soluble fractions. High 87Sr/86Sr ratios of 0.709908–0.714168 in the acid-soluble fraction of the Zhuoshui River sediments indicate that disseminated calcite is present in silicate rocks. Rapid weathering of disseminated calcite in response to high denudation rate determines the high 87Sr/86Sr ratios in the upper Zhuoshui River water. Its downstream decreasing of 87Sr/86Sr ratios across major lithology could be explained by isotopic exchange between river water and sediment leaching fractions. However, all the 87Sr/86Sr ratios of the sequential leaching sediments from the Liwu River largely overlap along its course. The relatively low 87Sr/86Sr ratios of 0.708747–0.711070 in the acid-soluble fraction of the Liwu River sediments could be sourced from disseminated calcite and/or Tailuko marble, thus supplying low 87Sr/86Sr ratios to river waters.Spatial variations in river water δ88/86Sr values mainly occur between the mainstram and the tributary. The relation between δ88/86Sr and 87Sr/86Sr in both rivers conservatively traces water-mass mixing, while the stable Sr isotope fractionation processes (e.g., biological uptake, clay reactions, and calcite precipitation) during transport in streams is negligible. Therefore, it is suggested that combining radiogenic and stable Sr isotopes in river water and/or sequential extracted sediment is an effective means to constrain chemical weathering as well as fractionation processes in a multi-lithological mountainous catchment with high denudation rate and wet climate.

Effective global mixing of the highly siderophile elements into Earth’s mantle inferred from oceanic abyssal peridotites

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al2O3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth’s mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues.

Decoupling between Oxygen and Radiogenic Isotopes: Evidence for Generation of Juvenile Continental Crust by Partial Melting of Subducted Oceanic Crust

Decoupling between Oxygen and Radiogenic Isotopes: Evidence for Generation of Juvenile Continental Crust by Partial Melting of Subducted Oceanic Crust

Radiogenic (Sr-Nd-Hf-Os) isotope data for neogene volcanic rocks from the rif–betic orogen and lanzarote (Canary Islands) in the western Mediterranean

In this communication we present new radiogenic Sr-Nd-Hf-Os isotope data for Neogene volcanic rocks from the Rif-Betic Orogen and Lanzarote Island in the western Mediterranean along with some previously published data. Major and trace element data for the Rif samples are provided as a data supplement. Geological context and petrographic details for equivalent rocks have been published previously but the new data, especially the Hf and Os isotope data should be of use for future appraisals of the distribution of sub-crustal mantle domains in the region. Upon inspection, it would appear that alkali basalts, shoshonites and lamproites sample very different sub-continental lithospheric mantle sources in Spain and Morocco whereas there are close similarities between Lanzarote basalts and mantle xenoliths from the Atlas Mountains in Morocco.

Corrigendum to “Stromatolites as geochemical archives to reconstruct microbial habitats through deep time: Potential and pitfalls of novel radiogenic and stable isotope systems” [Earth Science Reviews 218 (2021) 103683

Corrigendum to “Stromatolites as geochemical archives to reconstruct microbial habitats through deep time: Potential and pitfalls of novel radiogenic and stable isotope systems” [Earth Science Reviews 218 (2021) 103683

Stable and Radiogenic Isotope Analysis of Faunal Remains from a Western Iowa Oneota Complex Site: An Investigation of Diet and Mobility Variation in the Late Prehistoric Period of the Upper Midwest

AbstractThis study provides evidence of the value of using isotopic data from faunal remains to understand human diet and mobility patterns when human remains are not available for examination. In this research, bone apatite, bone collagen, and enamel apatite from fauna recovered from recent excavations of the Dixon site (13WD8), an Oneota complex site (AD 1300–1400) in western Iowa, were analyzed for δ13C, δ15N, δ18O, and 87Sr/86Sr values. The goals of this study were to gather information about human and animal diet and mobility and faunal procurement strategies of humans in the late prehistoric period of upper midwestern North America and to contribute to the growing literature using domesticated dogs as surrogates for humans in isotopic studies of dietary patterns. The results of this study find that the people occupying the Dixon site were subsisting on agricultural products, including maize, in conjunction with the gathered wild resources and hunted fauna, which included both large and small local game. While the Oneota complex is thought to be associated with some amount of seasonal migration, there is no evidence of these movements offered via this study's data nor is there strong evidence of long-distance hunting. Domesticated canids were an important part of the Dixon settlement and were fed human foodstuffs and scraps, including maize. At times, these canids were also a source of food. As a substitute for analyses of human remains, this study uses the canine surrogacy approach (CSA) and argues that the canid data would be similar to the human data from the Dixon settlement. A Bayesian stable-isotope mixing model (MixSiar) was used to quantitatively interpret the stable-isotope values of the Dixon canids, and it suggests that bison hunting was a specialization of the human population occupying the Dixon site.

Zircon U–Pb Geochronology and Hf–O Isotope Characteristics of Granitoids from the Capricorn Orogen, Western Australia

AbstractThe Capricorn Orogen, Western Australia, is a complex orogenic zone that records the convergence and collision of the Archaean Yilgarn and Pilbara cratons in forming the West Australian Craton (WAC), then over one billion years of subsequent intracontinental reworking. Granites associated with these tectonothermal events (the Dalgaringa, Bertibubba, Moorarie, Durlacher and Thirty Three supersuites) are exposed in the western part of the Capricorn Orogen. This study integrates radiogenic (U–Pb and Hf) and stable isotope (O) analysis of zircon grains from granitic rocks in the Capricorn Orogen to determine their ages and magmatic sources, including the relative contributions of mantle versus crustal material. Granites from the margin of the Yilgarn Craton record periods of crustal growth and reworking during the Archaean that influenced later Proterozoic magmatic events. Components of the Capricorn Orogen, collectively termed the Glenburgh Terrane, have previously been considered to be exotic to the adjacent Pilbara and Yilgarn cratons. However, new U–Pb zircon geochronology and Lu–Hf isotope compositions of basement rocks in the Glenburgh Terrane (the Halfway Gneiss) have similarities to some terranes of the Yilgarn Craton, and are interpreted to represent a reworked portion of the craton that was re-accreted during the Glenburgh Orogeny. Arc magmatism during the Ma Glenburgh Orogeny resulted in a period of crustal growth, with magmas representing a mixture of 50–90 % mantle-derived magmas and 50–10 % magmas derived from an evolved crustal component with an isotopic composition equivalent to that of the Halfway Gneiss. Following assembly of the WAC, granite magmatism in the Capricorn Orogen records a significant change from one dominated by mantle-derived magmatism to one dominated by crustal melting and an increased contribution from metasedimentary material. This transition reflects a geodynamic evolution from subduction–accretion to collision and intracratonic reworking. The isotopic characteristics of granites from the Moorarie Supersuite indicate three distinct sources: (1) a metasedimentary component; (2) an evolved crustal component, comparable with the Glenburgh Terrane; (3) a mafic juvenile component. Following this, the Hf–O compositions of the Durlacher Supersuite indicate that they were derived from reworking of the Moorarie Supersuite granites, and require no juvenile contribution or any additional sedimentary source. The isotopic compositions of the Thirty Three Supersuite pegmatites indicate that they were largely derived from reworking of the Moorarie and Durlacher supersuites.

Hf-Nd isotopes from ultramafic and mafic rocks in the western Dharwar Craton, India, record early Archean mantle heterogeneity

Both primary mantle differentiation and early extraction of oceanic and continental crust can have resulted in a chemically heterogeneous Archean mantle. Evidence for heterogeneity of the Archean mantle is preserved in the correlated Hf and Nd isotopes in mafic (basaltic) magmatic rocks and their ‘decoupling’ in ultramafic rocks, especially in komatiites, from the Archean Dharwar Craton, India. The komatiites and komatiitic basalts from the western Dharwar Craton were emplaced at ~3.25 Ga and have large spread in initial radiogenic isotope compositions with initial ɛHf (ranging from +0.9 to +21.3) and initial ɛNd from +0.2 to +7.4 that do not correlate. In contrast, the ɛHfi and ɛNdi in contemporaneous mafic (ɛHfi = +5.9 ± 11.2; ɛNdi = +3.2 ± 7.6) and felsic (ɛHfi = +4.3 ± 4.6; ɛNdi = +2.7 ± 4.2) rocks in the same area correlate, as is typical of rocks that are the product of magmatic processes involving melting and magmatic differentiation. The high initial ɛHf and the lack of correlation (‘decoupling’) of ɛHf and ɛNd in the komatiites and komatiitic basalts indicate that the source had superchondritic Lu/Hf which is attributed to early mantle differentiation at great depth, possibly in the perovskite stability field. This finding indicates that the primordial mantle was mineralogically and chemically layered and this layering was the result of fractionation processes very early in Earth's history, probably during magma ocean solidification. During crystallization of a primordial magma ocean, fractionation of Mg-perovskite with minor Ca-perovskite component could have resulted in a melt with high Lu/Hf and nearly unfractionated Sm/Nd. The correlation of the initial Hf isotope composition of the komatiites and komatiitic basalts with their MgO content, and thus melting temperatures, indicates the presence of a highly fractionated component derived from the deep mantle. The coupling of ɛHf-ɛNd in mafic to felsic igneous rocks and the decoupling in contemporaneous ultramafic rocks is thus the result of mantle material originating from different depths involving different ancient fractionated reservoirs. The existence of a chemical and isotopically heterogeneous Archean mantle that supplied material to form the continental crust has to be accommodated in crustal growth and preservation models that commonly assume a nearly homogeneous and gradually depleting mantle reservoir as the source of the continental crust.