Osmium Isotope Data Research Articles

Enhanced volcanic activity and long-term warmth in the middle Eocene revealed by mercury and osmium isotopes from IODP Expedition 369 Site U1514

Rapid plate reorganization may have influenced global climate during the Eocene; however, its linkage remains poorly constrained, particularly during the middle Eocene. To elucidate this tectonic–climatic relationship, here, we conducted a comprehensive analysis based on high-resolution mercury (Hg) and osmium (Os) abundance and isotope data obtained from the complete Eocene sedimentary sequence of Site U1514, drilled in the Mentelle Basin off southwest Australia. The Hg signals in this sedimentary sequence, which are characterized by significantly high enrichment and insignificant mass-independent fractionation (Δ199Hg) signal, confirm that the middle Eocene (∼45–38 Ma) was a period of persistent, increased volcanism, accompanied by intense tectonic activity. In particular, a remarkable seafloor volcanic eruption persisted for approximately 1.5 million years (∼42.0–40.5 Ma), immediately preceding the Middle Eocene Climate Optimum (MECO). Contemporaneously, the trends toward a slightly more radiogenic seawater 187Os/188Os (Osi) composition denote the prevalence of intensified continental weathering under a warm, humid climate during the middle Eocene, a phenomenon particularly evident during the MECO. Importantly, the Hg and Os records from Site U1514 reveal the occurrence of a multi-million-year warming reversal amid the long-term Eocene cooling trend, which likely contributed to significant CO2 reduction during the late Eocene. These findings significantly enhance our understanding of Eocene climate dynamics, which are fundamentally linked to intensive tectonic-driven volcanic activity and associated continental chemical weathering.

A Permian rhenium–osmium radiometric age for bornite at the Rock Creek deposit, Spar Lake copper–silver district (Montana, USA)—a link to the Sonoma Orogeny and the copper–silver–vanadium Midcontinent Belt?

We report rhenium–osmium (Re–Os) isotope data and age information for bornite, chalcocite, and chalcopyrite from the Spar Lake and Rock Creek copper–silver (Cu–Ag) sediment-hosted deposits in the Spar Lake district, Mesoproterozoic Belt–Purcell Basin, Montana, USA. The Re–Os geochronometers in chalcocite and chalcopyrite do not record plausible mineralization ages at the Spar Lake deposit, which is currently interpreted to have formed at ca. 1410 million years ago (Ma) from prior work. However, a Re–Os isochron date of bornite (253.2 ± 26.9 Ma, n = 6) at Rock Creek with an extremely high initial 187Os/188Os ratio of 19.1 ± 0.2 can be interpreted as resetting of Re–Os in bornite that originally recorded a Mesoproterozoic timing of Cu–Ag mineralization in the Spar Lake district. The new Re–Os age of bornite at Permian–Triassic boundary time is compatible with a resetting at the time of building of the Sonoma Foreland Basin that resulted from the loading of an accretionary prism (i.e., Golconda Allochthon) onto the North American Plate. Our ca. 253 Ma bornite age does not constrain whether Cu was introduced to Spar Lake at this time or simply redistributed, but our findings open the possibility that Cu–Ag mineralization in the Spar Lake district might have taken place not only in the Mesoproterozoic, but also in addition, at Permian–Triassic boundary time. Indeed, numerous Cu–Ag, vanadium–uranium (V–U) sandstone-hosted and black shale-hosted deposits of assumed Permian to Triassic age are documented in the Midcontinent copper belt from western Texas to southern Idaho, including (1) Cu–Ag sediment-hosted deposits in western Colorado, southeastern Utah, and northern Arizona and (2) the Montpelier and Lake Alice sandstone-hosted Cu–Ag deposits in southeastern Idaho and western Wyoming, respectively.

Valanginian climate cooling and environmental change driven by Paraná-Etendeka basalt erosion

Abstract The Valanginian Weissert Event (ca. 134 Ma) has long been linked to emplacement of the Paraná-Etendeka large igneous province (LIP). Although several Mesozoic crises were triggered by volcanic CO2 emissions and global warming, causing oceanic oxygen depletion, the Weissert Event featured climate cooling and limited marine anoxia. Here, the impact of silicate weathering on environmental change during the Weissert Event is investigated by presenting the first osmium isotope (187Os/188Os) data sets for the late Berriasian–Valanginian interval. These records document a possible rise in weathering and nutrient runoff during the latest Berriasian, followed by a switch to Paraná-Etendeka basalt erosion during the Valanginian. The high weatherability of LIP basalts enhanced global silicate weathering despite limited coeval climate warming. This weathering triggered the documented cooling, with consequent micronutrient runoff potentially aiding ocean fertilization, highlighting a mechanism linking LIP formation with environmental change that was uniquely different than that of other Mesozoic crises.

Enhanced magmatism played a dominant role in triggering the Miocene Climatic Optimum

The Miocene Climatic Optimum represents one of the major warming events during the Cenozoic and was accompanied by a positive stable carbon isotope excursion. Here, we report seawater osmium isotope data from Pacific Ocean sediments to determine if carbon dioxide emissions from coeval magmatism could have been the trigger for this event. Our data reveal a negative osmium isotope excursion from 0.80 to 0.72 between about 17 and 15.8 million years ago that can be explained by enhanced magmatism during this period. Simple mass balance calculations suggest an increase of 22‒45% in non-radiogenic osmium input from the mantle into the ocean during this period. Using osmium isotopic composition as a constraint for global carbon cycle modelling, we find that such an increase in magmatism is capable of elevating atmospheric carbon dioxide concentrations by 65‒140 ppmv and of causing a positive excursion in seawater stable carbon isotopes of 0.4‒0.7‰, consistent with geochemical observations. We conclude that it is conceivable that the enhanced magmatism played a dominant role in causing the Miocene Climatic Optimum.

Evidence for a primitive deep mantle component in the source of Marquesas Islands Lavas from Os isotope and highly siderophile element abundance systematics

The Marquesas islands are an age-progressive volcanic chain in French Polynesia formed by partial melting above a mantle plume. Previous studies have demonstrated the presence of an enriched mantle (EM2) endmember mixed with a high-3He/4He (up to 14.4 RA) component in Marquesas post-shield lavas. In contrast, Marquesas shield lavas sample a dilute high-μ (HIMU) type component with lower 3He/4He (∼8 RA). We report new incompatible trace element and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances and osmium isotope data for bulk rocks from the islands of Fatu Hiva, Hiva Oa and Nuku Hiva, for which He, Sr, Nd and Pb isotope compositions have been reported previously (Castillo et al., 2007). As the HSE are chiefly hosted in sulfide phases in the mantle and all but Re are compatible during partial melting, they offer information on both sulfide-rich and melt-depleted mantle lithologies within the Marquesas plume. Primitive mantle-normalized HSE abundances show depletions in Ru and Pd, and fractionated (Ir/Os)N in fourteen of the seventeen analyzed samples. Ruthenium depletions and Ir-Os fractionations are likely caused by parental magma fractional crystallization. Palladium depletions are not easily explained by magmatic or secondary alteration processes and are instead likely to be a feature of the source mantle which was partially depleted in the more incompatible HSE by previous melting events within the Marquesas plume. Osmium isotopic systematics are mildly radiogenic relative to chondrites and are identical within uncertainties for shield-building Fatu Hiva (γOs = +2.9 ± 1.6; 2 SD; n = 7) and post-shield Hiva Oa (+2.7 ± 2.2; n = 9), showing no evidence for significant assimilation of radiogenic oceanic crust. The osmium budget of Marquesas lavas is controlled by a volumetrically dominant peridotite endmember that is incompatible element depleted while the Sr, Nd, and Pb budgets are strongly influenced by a volumetrically minor, incompatible element enriched EM2 component. The Nd-Sr-Os isotope systematics of the lavas can be explained by mixing of 0.3–0.7% of an EM2 type melt into the Marquesas source. Marquesas lavas offer further support that the high-3He/4He “focus zone” (FOZO) mantle has 187Os/188Os mildly more radiogenic than the depleted upper mantle, similar to estimates of the primitive mantle. Osmium isotope systematics offer a useful tool with which to characterize more depleted components within the ocean island basalt-source mantle that may be difficult to detect using lithophile isotope signatures.

Rhenium‐Osmium Isotope Constraints on the Origin of the Tianyu Cu‐Ni Deposit in the East Tianshan Orogenic Belt, Xinjiang, NW China

AbstractThe Tianyu Cu‐Ni sulfide deposit occurs in the north margin of the Central Tianshan Arc in East Tianshan orogenic belt, Xinjiang, NW China. The intrusions consist of gabbro, peridotite, and olivine pyroxenite. The peridotite and pyroxenite are the main host rock for the Cu‐Ni ores. Rhenium and osmium isotopic analyses of Ni‐ and Cu‐bearing sulfide minerals from the deposit have been used to determine the source of osmium, and by inference, the sources of ore metals. Sulfide ore samples have Os and Re concentrations varying in the ranges 1.85 to 4.58 ppb and 93.56 to 146.00 ppb, respectively. An initial 187Os/188Os ratio ranges from 0.86 to 1.23 for the ores and the γOs values from 592 to 2227. Osmium isotopic data suggest that the Tianyu intrusion and associated Cu‐Ni mineralization has derived from crustal‐contaminated mantle melts. The intrusions early show island‐arc geochemical signatures, which indicate that the Hulu mafic–ultramafic intrusions, along with the Cu‐Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian.

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

Marine osmium isotope record during the Carnian “pluvial episode” (Late Triassic) in the pelagic Panthalassa Ocean

The Carnian Pluvial Episode (CPE) was a global environmental change and biotic crisis that occurred during the Carnian (Late Triassic). The climate during the CPE was characterized by a short-lived period of extreme rainfall, and an extinction of marine taxa is known to have occurred during the latest Julian (i.e. early Carnian). Although these events are considered to have been caused by the Wrangellia Flood Basalt (FB) volcanism, existing studies have found little direct evidence to support this. We investigated the temporal relationship between the eruption of Wrangellia FB and CPE using high-resolution microfossil biostratigraphy and paleo-seawater Os isotope data of an Upper Triassic bedded chert succession from an accretionary complex in Japan, which accumulated in a pelagic deep-sea environment in an equatorial region of the Panthalassa Ocean. Our biostratigraphic analysis, based on conodonts and radiolarians, and osmium isotope data show: (i) a continuous decline of initial Os isotope ratios (187Os/188Osi) in the early Julian; (ii) low 187Os/188Osi ratios during the late Julian; and (iii) an abrupt increase in 187Os/188Osi ratios at the end of the Julian. The decrease in 187Os/188Osi ratios throughout the Julian suggests an increased input of unradiogenic Os from the eruption of the Wrangellia FB into the ocean. Moreover, redox-sensitive elements, such as V and U, increased abruptly at the end of the Julian, which is the first evidence of reducing conditions during the CPE within the pelagic deep-sea Panthalassa Ocean. Marine anoxic event in the late Julian has been recognized from widespread deposition of black shales and organic-rich marls in intermediate to shallow water Tethyan sections. Thus, oxygen-depleted conditions occurred at the Tethyan shallow continental margin, as well as in the pelagic deep-sea Panthalassa Ocean, at the end of Wrangellia FB volcanism.

Direct coupling between carbon release and weathering during the Toarcian oceanic anoxic event

AbstractSilicate weathering represents a major feedback mechanism in the Earth’s climate system, helping to stabilize atmospheric CO2 levels and temperature on million-year time scales. On shorter time scales of greater relevance to understanding the fate of anthropogenic CO2, the efficacy and responsiveness of weathering is less clear. Here, we present high-resolution osmium-isotope data that reflect global chemical weathering from a stratigraphically thick record of the early Toarcian oceanic anoxic event (T-OAE; ca. 182 Ma). A pronounced decrease in the carbon-isotope composition of exogenic carbon reservoirs during this event has been linked to the large-scale release of 12C-enriched carbon. Our data indicate that the flux of radiogenic osmium to the oceans increased in lockstep with the decrease in carbon-isotope values, demonstrating a geologically synchronous coupling between massive carbon release and enhanced global continental crust weathering. We show that abrupt shifts in carbon isotopes, previously interpreted as millennial-scale methane hydrate melting or terrestrial carbon-release events, are coeval with rapid increases in weathering. Global weathering may have increased by >40% across each of these intervals of rapid carbon injection. Our results help to reconcile previous estimates of weathering change during the T-OAE, and support the view that, overall, global weathering rates may have increased six-fold through the entire event.

A Palaeoproterozoic diamond-bearing lithospheric mantle root beneath the Archean Sask Craton, Canada

The recently recognised Sask Craton, a small terrane with Archean (3.3–2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9–1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites.The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re–Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9–1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.

Melt-modified lithosphere beneath Ross Island and its role in the tectono-magmatic evolution of the West Antarctic Rift System

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1–88.2), lherzolite (Fo90.6–87.4), and pyroxenite xenoliths (Fo89.3–87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo89.7–88.0), with more ferroan matrix olivine grains (Fo83.7–81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ± 0.3RA; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with an initial 187Os/188Os ratio of 0.1286 ± 0.0001. The initial 187Os/188Os ratio is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279–0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land.

Tracing the Impact of Coastal Water Geochemistry on the Re‐Os Systematics of Macroalgae: Insights From the Basaltic Terrain of Iceland

AbstractThis study presents rhenium (Re) and osmium (Os) elemental and isotope data for macroalgae, dissolved load, and bed load from Icelandic coastal and/or river waters, an environment adjacent to predominantly basaltic terrains, ranging in age from historic to ~12 Ma. Both the Re (0.1 to 88.4 ppb) and Os (3.3 to 254.5 ppt) abundance in macroalgae are shown to be primarily controlled by uptake from the dissolved load of local seawater and are largely dependent on the relative influence of local freshwater inputs. Incorporation of Re and Os into macroalgae appears to be complicated by additional Re and Os uptake from the bed load. The 187Os/188Os (0.16 to 0.99) composition of macroalgae is highly variable and is explained in terms of an unradiogenic 187Os/188Os contribution from rivers draining younger basaltic catchments that have undergone congruent weathering (and/or hydrothermal input) and a radiogenic 187Os/188Os contribution from two distinct sources: rivers draining older catchments that have undergone incongruent weathering of radiogenic primary basaltic minerals and North Atlantic seawater. The 187Re/188Os composition (~65 to 40,320) of macroalgae traces that of water, with higher values associated with higher salinity waters, but far exceeds the 187Re/188Os of water due to the preferential uptake of Re over Os by macroalgae in areas of high dissolved and/or bed load Re abundances. This study substantiates the utility of macroalgae as a proxy for the long‐term (months to years) average 187Os/188Os composition of seawater, which holds the potential to elucidate a range of Earth system and anthropogenic processes.

Evidence for rapid weathering response to climatic warming during the Toarcian Oceanic Anoxic Event

Chemical weathering consumes atmospheric carbon dioxide through the breakdown of silicate minerals and is thought to stabilize Earth’s long-term climate. However, the potential influence of silicate weathering on atmospheric pCO2 levels on geologically short timescales (103–105 years) remains poorly constrained. Here we focus on the record of a transient interval of severe climatic warming across the Toarcian Oceanic Anoxic Event or T-OAE from an open ocean sedimentary succession from western North America. Paired osmium isotope data and numerical modelling results suggest that weathering rates may have increased by 215% and potentially up to 530% compared to the pre-event baseline, which would have resulted in the sequestration of significant amounts of atmospheric CO2. This process would have also led to increased delivery of nutrients to the oceans and lakes stimulating bioproductivity and leading to the subsequent development of shallow-water anoxia, the hallmark of the T-OAE. This enhanced bioproductivity and anoxia would have resulted in elevated rates of organic matter burial that would have acted as an additional negative feedback on atmospheric pCO2 levels. Therefore, the enhanced weathering modulated by initially increased pCO2 levels would have operated as both a direct and indirect negative feedback to end the T-OAE.

Early metal-silicate differentiation during planetesimal formation revealed by acapulcoite and lodranite meteorites

In order to establish the role and expression of silicate-metal fractionation in early planetesimal bodies, we have conducted a highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and 187Re-187Os study of acapulcoite-lodranite meteorites. These data are reported with new petrography, mineral chemistry, bulk-rock major and trace element geochemistry, and oxygen isotopes for Acapulco, Allan Hills (ALHA) 81187, Meteorite Hills (MET) 01195, Northwest Africa (NWA) 2871, NWA 4833, NWA 4875, NWA 7474 and two examples of transitional acapulcoite-lodranites, Elephant Moraine (EET) 84302 and Graves Nunataks (GRA) 95209. These data support previous studies that indicate that these meteorites are linked to the same parent body and exhibit limited degrees (<2–7%) of silicate melt removal. New HSE and osmium isotope data demonstrate broadly chondritic relative and absolute abundances of these elements in acapulcoites, lower absolute abundances in lodranites and elevated (>2×CI chondrite) HSE abundances in transitional acapulcoite-lodranite meteorites (EET 84302, GRA 95209). All of the meteorites have chondritic Re/Os with measured 187Os/188Os ratios of 0.1271±0.0040 (2 St. Dev.). These geochemical characteristics imply that the precursor material of the acapulcoites and lodranites was broadly chondritic in composition, and were then heated and subject to melting of metal and sulfide in the Fe-Ni-S system. This resulted in metallic melt removal and accumulation to form lodranites and transitional acapulcoite-lodranites. There is considerable variation in the absolute abundances of the HSE, both among samples and between aliquots of the same sample, consistent with both inhomogeneous distribution of HSE-rich metal, and of heterogeneous melting and incomplete mixing of silicate material within the acapulcoite-lodranite parent body. Oxygen isotope data for acapulcoite-lodranites are also consistent with inhomogeneous melting and mixing of accreted components from different nebular sources, and do not form a well-defined mass-dependent fractionation line. Modeling of HSE inter-element fractionation suggests a continuum of melting in the Fe-Ni-S system and partitioning between solid metal and sulfur-bearing mineral melt, where lower S contents in the melt resulted in lower Pt/Os and Pd/Os ratios, as observed in lodranites. The transitional meteorites, EET 84302 and GRA 95209, exhibit the most elevated HSE abundances and do not follow modelled Pt/Os and Pd/Os solid metal-liquid metal partitioning trends. We interpret this to reflect metal melt pooling into domains that were sampled by these meteorites, suggesting that they may originate from deeper within the acapulcoite-lodranite parent body, perhaps close to a pooled metallic ‘core’ region. Petrographic examination of transitional samples reveals the most extensive melting, pooling and networking of metal among the acapulcoite-lodranite meteorites. Overall, our results show that solid metal-liquid metal partitioning in the Fe-Ni-S system in primitive achondrites follows a predictable sequence of limited partial melting and metal melt pooling that can lead to significant HSE inter-element fractionation effects in proto-planetary materials.

Re–Os isotope geochronology of the Shangbao pyrite–flourite deposit in southeastern Hunan, South China: Evidence for multiple mineralization events and the role of crust–mantle interaction in polymetallic deposits

In South China, both crustal reworking and crust–mantle interaction were important geological processes during the Paleozoic and Mesozoic eras. However, the relationships between these two processes and metal mineralization are still unknown. Here we report rhenium and osmium isotopic data for pyrite grains from a pyrite deposit associated with a granite intrusion in the Shangbao area, southeastern Hunan Province (South China). Two pyrite samples, both containing many euhedral pyrite grains, were collected from the same locality, but the samples yield distinct ages. Six euhedral pyrite grains from one sample yield an isochronal age of 279 ± 12 Ma, with an initial 187Os/188Os ratio of 0.39 ± 0.71, and Re and Os concentrations of 0.12–63.5 ppb and 2.14–185 ppt, respectively. This Early Permian age is in good agreement with the age of the strata that host the pyrite deposit. Five euhedral pyrite grains from the other sample yield an isochronal age of 75.2 ± 4.3 Ma, with an initial 187Os/188Os ratio of 0.141 ± 0.030 and Re and Os concentrations of 0.15–0.43 ppb and 1.0–39.9 ppt, respectively. If one pyrite grain with the highest 187Re/188Os and 187Os/188Os ratios is excluded, other four pyrite grains give an isochronal age of 85 ± 13 Ma. The Late Cretaceous age (75–85 Ma) is consistent with the zircon U–Pb age of the Shangbao granites (80.1 ± 0.3 Ma) to within uncertainties. Considering also the relatively lower radiogenic initial 187Os/188Os ratio of this sample, we suggest that the later stage pyrite ore was probably formed through crystallization from the magmatic hydrothermal fluids. Combined with other geological and associated magmatic data, we propose a skarn-related fluid–ore interaction process to explain the second stage of metallogenesis in the Shangbao pyrite deposit. The Early Permian pyrite ore was deposited in a brine basin with evaporites during the Early Permian. Later magmatic hydrothermal fluids originating from the Shangbao granites, which included mantle components, interacted with the strata and the Early Permian pyrite ore during the Late Cretaceous and precipitated a later stage pyrite ore. During the Late Mesozoic, the roll-back of subducted Paleo-Pacific plate caused lithospheric extension in South China, triggering the upwelling and partial melting of the asthenosphere. The resulting underplating of mantle-derived magmas provided a vast amount of heat and materials for the formation of the granites and polymetallic deposits in South China. Given that the multiple mineralization events were spatially and temporally associated with the Paleozoic–Mesozoic magmatism, the Re–Os isotopic dating of euhedral pyrite grains has been shown to be a viable method for unveiling the evolutionary history of ore-deposits. Skarn development caused by granite and mafic dike emplacement resulting from crust–mantle interaction explains the occurrence of two mineralization episodes at the same locality.

Closed-system behaviour of the Re–Os isotope system recorded in primary and secondary platinum-group mineral assemblages: Evidence from a mantle chromitite at Harold's Grave (Shetland Ophiolite Сomplex, Scotland)

This study evaluates in detail the mineral chemistry, whole-rock and mineral separate Os-isotope compositions of distinct platinum-group mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Os-rich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of ‘unradiogenic’ 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from ‘primary’ PGM assemblage (0.12473–0.12488) and similar ‘unradiogenic’ 187Os/188Os values for both ‘primary’ and ‘secondary’ PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re–Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a supra-subduction zone setting shortly before obduction.

Big insights from tiny peridotites: Evidence for persistence of Precambrian lithosphere beneath the eastern North China Craton

Previous studies have shown that the eastern North China Craton (NCC) lost its ancient lithospheric mantle root during the Phanerozoic. The temporal sequence, spatial extent, and cause of the lithospheric thinning, however, continue to be debated. Here we report olivine compositions, whole-rock Re–Os isotopic systematics, and platinum-group element abundances of small (<2cm in maximum dimension) mantle peridotite xenoliths from two basalt localities from the eastern NCC, Wudi (Cenozoic) and Fuxin (Cretaceous). These locations lie far (~150–200km) from the Tan–Lu fault, which has been linked to lithospheric replacement in the eastern NCC. Peridotites at both locations have fertile to moderately refractory compositions (Fo<91.5), while highly refractory (Fo>92) lithospheric mantle is largely absent. Osmium isotopic data suggest the Wudi peridotites experienced melt depletion primarily during the Paleoproterozoic (~1.8Ga), although an Archean Os model age for one xenolith indicates incorporation of a minor component of Archean lithospheric mantle. These data suggest that a previously unrecognized Paleoproterozoic orogenic event removed and replaced the original Archean lithospheric mantle beneath the sedimentary basin at the southern edge of the Bohai Sea. By contrast, the Fuxin peridotites, entrained in Cretaceous basalts that crop out along the northern edge of the eastern NCC, document the coexistence of both ancient (≥2.3Ga) and modern lithospheric mantle components. Here, the original Late Archean–Early Paleoproterozoic lithospheric mantle was, at least partially, removed and replaced prior to 100Ma. Combined with literature data, our results show that removal of the original Archean lithosphere occurred within Proterozoic collisional orogens, and that replacement of Precambrian lithosphere during the Mesozoic may have been spatially associated with the collisional boundaries and the strike–slip Tan–Lu fault, as well as the onset of Paleo-Pacific plate subduction.

New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

Recognising variations in the carbon isotope compositions of marine organic-rich sedimentary rocks can provide insight into changes in ocean chemistry throughout geological time. Further, identification of global excursions in the carbon isotope record has proved to be valuable as a chronostratigraphic correlation tool.This investigation presents new high-resolution organic carbon isotope data (δCorg13) for marine sediments from 2 regions in North America (Last Creek, British Columbia, Canada and Five Card Draw, Nevada, USA). The carbon isotope profiles demonstrate that there were significant differences between the carbon reservoirs at Five Card Draw and Last Creek, notably in the upper part of the Leslei Zone. The δCorg13 values show a gradual positive CIE (∼2‰) at Last Creek in the upper part of the Leslei Zone. This corresponds to a coeval positive CIE of similar duration in Dorset, UK (upper Turneri Zone; Jenkyns and Weedon, 2013), suggesting that this may be a global marine carbon isotope signature, and likely reflects a widespread increase in primary productivity during the Early Sinemurian. In addition, a brief negative CIE is observed in the uppermost Lower Sinemurian at Last Creek. This negative excursion is not recorded in the Dorset section, suggesting localised upwelling of 12C-rich bottom-waters at Last Creek. Further, the signals identified at Last Creek are not present in coeval sections at Five Card Draw, thus highlighting a significant difference between these localities. Osmium (Os) isotope data (initial 187Os/188Os values) provide a quantitative determination of the contrasting depositional environments of Five Card Draw and Last Creek (at least partially restricted with high levels of continental inundation and open-ocean, respectively). This demonstrates that basinal restriction may act as a major factor that controls isotopic stratigraphic signatures, thus preventing the identification of global or widespread regional excursions.

SIMS U-Pb zircon dating and Re-Os isotopic analysis of the Hulu Cu-Ni deposit, eastern Tianshan, Central Asian Orogenic Belt, and its geological significance

*Corr esponding author Exploration of the Hulu Cu–Ni–Co sulfide deposit in Eastern Tianshan, Central Asian Orogenic Belt, has detected horizons of sulfide mineralization in pyroxenite and peridotite units. SIMS U–Pb zircon ages of the gabbro-hosting Cu–Ni–Co sulfide deposit indicate that the Hulu intrusion was emplaced at 282.3 ± 1.2 Ma (95% confidence level, MSWD = 3.30, n = 13). Osmium isotopic data suggest that the Hulu intrusion and associated Cu–Ni mineralization were derived from crustally-contaminated mantle melts. The intrusions clearly show island-arc geochemical signatures, such as negative Nb, Ta, Zr and Ti anomalies and enrichment in LILE. These geochemical tracers indicate that the Hulu mafic–ultramafic intrusions, along with the Cu–Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian. The arc-related geodynamic setting of the Hulu Cu–Ni deposit is unusual for magmatic sulfide deposits. This new type of ore deposit offers an alternative perspective for Cu and Ni exploration.

Sr–Nd–Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) 87Sr/86Sr ratios of 0.70538–0.70642, 143Nd/144Nd of 0.51233–0.51218, 206Pb/204Pb of 17.74–18.25, 207Pb/204Pb of 15.51–15.57, and 208Pb/204Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γOs values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.