Os Isotope Research Articles

The Central Atlantic Magmatic Province (CAMP) in Morocco

AbstractThe Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components.

Development of a multi-segmented proportional gas counter for β-decay spectroscopy at KISS

We developed a multi-segmented proportional gas counter to perform β-decay spectroscopy of neutron-rich nuclei extracted from KEK Isotope Separation System (KISS). The 32 segmented gas counters allow us two-dimensional tracking of the events to identify β-ray events by discriminating background events. The detectors being by low-density material of thin aluminized Mylar foils and P10 gas enabled us to detect low energy electrons down to 100 keV. The absolute detection efficiency was 45% at Qβ=1 MeV, and the measured background event rate was 0.11 cps. The detector was applied to perform β-decay spectroscopy and hyperfine structure measurements of neutron-rich isotopes of Pt, Ir, and Os. We started to develop the gas counter with three-dimensional tracking by replacing the material of anode wire from BeCu to resistive thin carbon to reduce the background event rate below 0.01 cps.

Refinement of the Micro‐Distillation Technique for Isotopic Analysis of Geological Samples with pg‐Level Osmium Contents

In recent years, the 187Re–187Os isotope system has been increasingly used to study samples containing very small quantities of Os. For such samples, optimisation of measurement procedures is essential to minimise the loss of Os before mass spectrometric measurements. Micro‐distillation is a necessary purification step that is applied after the main Os chemical separation procedure, prior to Os isotope ratio measurements by negative‐thermal ionisation mass spectrometry (N‐TIMS). However, unlike the other separation steps, this procedure has not yet been optimised for small samples. In this study, we present a refined micro‐distillation method that achieved higher yields and allowed high‐precision R(187Os/188Os) expressed as 187Os/188Os measurements for small‐sized geological samples that contain only a few pg Os. The Os recovery in the micro‐distillation step was tested by changing the operating conditions including heating time and temperature, and amounts of oxidant and reductant. Recoveries were measured by the isotope dilution ICP‐MS method after the addition of 190Os‐enriched spike solution. We found that the most critical factor controlling the chemical yield of Os during micro‐distillation is the extent of dilution of the reductant (HBr) by H2O evaporated from the oxidant. A refined micro‐distillation method, in which the amount of oxidant solution is reduced from the conventional method, achieved an improved chemical yield of Os (~ 90%). This refined method was applied to the measurement of 187Os/188Os by N‐TIMS of varying test portions of the geological reference material BIR‐1a. The resulting 187Os/188Os ratios of BIR‐1a matched the literature data, with propagated uncertainties of 0.2, 1.1 and 11% digested sample quantities containing 150, 10 and 1 pg of Os, respectively.

Improving confidence in ferromanganese crust age models: A composite geochemical approach

Accurate age models for marine ferromanganese (Fe-Mn) crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity. However, no absolute method exists for dating these deposits beyond the age of 10 Myr, which requires the combination of a number of approaches. Here, we present a composite age model for a 15 cm thick Fe-Mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m, on the summit of Tropic Seamount, in the north-east Atlantic. The age model is based on cross-validation of laser-ablation U-Pb dating, Co-chronometry and Os isotopes. These enable robust calibration of the age-depth model using the Bayesian statistical modelling of Markov Chain Monte Carlo (MCMC) simulations. The results show that this Fe-Mn crust commenced growth in the Late Cretaceous between 73 and 77 Ma, and grew at a rate between 1 and 24 mm/Myr, averaging 4 mm/Myr. The phosphatised carbonate substrate, capping Tropic Seamount and underlying most of the Fe-Mn crusts, yields a U-Pb age of 84 ± 4 Myr, and provides the upper age limit for the model. Less radiogenic excursions of 188Os/187Os in the vertical profile through the crust permit the identification of key inflection points in the Os isotope seawater curve at the Eocene-Oligocene and Cretaceous-Paleogene transitions. Growth rates estimated from the empirical Co-chronometer are combined with the age envelope defined by the Os data and used to validate the MCMC simulations. The model identifies five hiatuses that occurred during the Pliocene (2.5 ± 1.9–5.3 ± 1.7 Ma), Early Miocene (16 ± 1–27 ± 2 Ma), Oligocene (29 ± 2–32 ± 1 Ma), Eocene (41 ± 2–52 ± 0.6 Ma), and the Late Paleocene (55 ± 1–59 ± 1.4 Ma). A major phosphatisation event affecting the Fe-Mn core can be dated to the Late Eocene (38 ± 1.2 Ma), which coincides with a recorded change in the global oceanic system, from warm and sluggish circulation to cold and vigorous thermohaline-driven meridional overturn at the onset of Antarctic glaciation.

Reconsideration of Neo-Tethys evolution constrained from the nature of the Dazhuqu ophiolitic mantle, southern Tibet

The nature (i.e., sub-oceanic, sub-arc or sub-continental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re–Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr# (~ 0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites; two samples have spinel Cr# higher than 0.6, which is probably ascribed to intense melt–rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnet-facies melting followed by 10–18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites and implies deep initial melting (> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites (commonly < 80 km in origin). The Dazhuqu peridotites have unradiogenic 187Os/188Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle (PUM). All but one samples yield relatively younger Re depletion ages (TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re–Os isotopes and highly siderophile element (HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites and the Oman southern massifs but are distinct from non-cratonic sub-continental lithospheric mantle (SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision.

Black shale xenolith in a Jurassic−Cretaceous kimberlite and organic-rich Upper Ordovician shale on Baffin Island, Canada: A comparison of their organic matter

Abstract Rock-Eval pyrolysis and molecular organic geochemical analyses as well as organic petrographic observations were carried out on a rare black shale xenolith core drilled from a Late Jurassic-Early Cretaceous kimberlite on the Hall Peninsula in southern Baffin Island, Canada. The results are compared with those of outcrop shale samples from the Upper Ordovician Amadjuak Formation, the only organic-rich shale deposit discovered so far in the region. The shale xenolith is found to be organic-rich and thermally mature at the stage of peak oil generation, and contain type II organic matter similar to that of the immature Amadjuak shale. Discounting the differing thermal maturation effect, similar depositional environments and original organic inputs are suggested for both the shale xenolith and the Upper Ordovician shale based on their bulk, molecular and isotopic signatures as well as organic maceral assemblages. The present study cannot exclude the possibility of the shale xenolith being originated from an organic-rich Lower Silurian deposit, as suggested by previous Re Os isotope dating study. It is clear, however, that depending on their areal distribution and thickness, the lithostratigraphic unit from which the organic-rich black shale xenolith is derived would make an excellent petroleum source rock in the study area.

Mercury anomalies across the Palaeocene–Eocene Thermal Maximum

Abstract. Large-scale magmatic events like the emplacement of the North Atlantic Igneous Province (NAIP) are often coincident with periods of extreme climate change such as the Palaeocene–Eocene Thermal Maximum (PETM). One proxy for volcanism in the geological record that is receiving increased attention is the use of mercury (Hg) anomalies. Volcanic eruptions are among the dominant natural sources of Hg to the environment; thus, elevated Hg∕TOC values in the sedimentary rock record may reflect an increase in volcanic activity at the time of deposition. Here we focus on five continental shelf sections located around the NAIP in the Palaeogene. We measured Hg concentrations, total organic carbon (TOC) contents, and δ13C values to assess how Hg deposition fluctuated across the PETM carbon isotope excursion (CIE). We find a huge variation in Hg anomalies between sites. The Grane field in the North Sea, the most proximal locality to the NAIP analysed, shows Hg concentrations up to 90 100 ppb (Hg∕TOC = 95 700 ppb wt %−1) in the early Eocene. Significant Hg∕TOC anomalies are also present in Danish (up to 324 ppb wt %−1) and Svalbard (up to 257 ppb wt %−1) sections prior to the onset of the PETM and during the recovery period, while the Svalbard section also shows a continuous Hg∕TOC anomaly during the body of the CIE. The combination with other tracers of volcanism, such as tephra layers and unradiogenic Os isotopes, at these localities suggests that the Hg∕TOC anomalies reflect pulses of magmatic activity. In contrast, we do not observe clear Hg anomalies on the New Jersey shelf (Bass River) or the Arctic Ocean (Lomonosov Ridge). This large spatial variance could be due to more regional Hg deposition. One possibility is that phreatomagmatic eruptions and hydrothermal vent complexes formed during the emplacement of sills led to submarine Hg release, which is observed to result in limited distribution in the modern era. The Hg∕TOC anomalies in strata deposited prior to the CIE may suggest that magmatism linked to the emplacement of the NAIP contributed to the initiation of the PETM. However, evidence for considerable volcanism in the form of numerous tephra layers and Hg∕TOC anomalies post-PETM indicates a complicated relationship between LIP volcanism and climate. Factors such as climate system feedbacks, changes to the NAIP emplacement style, and/or varying magma production rates may be key to both the onset and cessation of hyperthermal conditions during the PETM. However, processes such as diagenesis and organic matter sourcing can have a marked impact on Hg∕TOC ratios and need to be better constrained before the relationship between Hg anomalies and volcanic activity can be considered irrefutable.

The silicate weathering feedback in the context of ophiolite emplacement: Insights from an inverse model of global weathering proxies

Silicate weathering provides a stabilizing feedback in the global carbon cycle and has played a key role in keeping surface environments clement for the majority of Earth9s history. It was recently proposed that ophiolite emplacement at low latitude at ∼90 to 70 Ma and ∼50 to 40 Ma enhanced the rate of silicate weathering and led to global cooling that terminated the Early Eocene Climatic Optimum. In this study, we revisit this proposal with a series of models of varying complexities to investigate the likelihood that Earth9s climate was driven largely by ophiolite emplacement in the Paleogene. Our models are constrained by either marine Os isotopic records or the combination of carbon cycle, Sr and Os isotopic records, and solid Earth degassing rates. The proposed changes in ophiolite weathering are inconsistent with our Os isotope mass balance model. With our coupled carbon cycle and weathering proxy model, total global silicate weathering fluxes (including ophiolite weathering) slightly decreased or remained relatively constant from 52 to 42 Ma. This constancy in global silicate weathering fluxes is inconsistent with the idea that enhanced ophiolite weathering directly led to global climate cooling. However, our model results support the idea that the strength of the silicate weathering feedback (weatherability) has played a significant role in regulating the pCO2 levels through the Cenozoic. Further, the modeling toolkit developed here can be used to explore the role of mafic rock weathering in driving climate change in other intervals through Earth9s history.

Effects of multi-stage rifting and metasomatism on HSE-187Os/188Os systematics of the cratonic mantle beneath SW Greenland

We report highly siderophile element (HSE) abundances and Re–Os isotope compositions, obtained by isotope dilution inductively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014–0.62, and have both depleted and enriched 187Os/188Os (0.1139–0.2724) relative to chondrite that are not correlated with 187Re/188Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1–1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<< 1 ppm) during more recent wehrlitisation and phlogopite-introduction, and control of the Os isotopic signature by older Os-rich BMS that precipitated from the basaltic melt. Depletions in compatible HSE (< 0.5 × PUM for Ru, Ir, Os) in all but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic BMS inclusions) and/or loss due to sulphide dissolution into oxidising small-volume melts that invaded the lithosphere during recurrent rifting, the latter supported by similar depletions in published bulk peridotite data. Combined, these multiple metasomatic events destroyed all vestiges of Mesoarchaean or older inheritance in the olivine separates investigated here, and highlight that caution is needed when interpreting Proterozoic Os model ages in terms of Proterozoic lithosphere stabilisation.

An improved in situ technique for the analysis of the Os isotope ratio in sulfides using laser ablation-multiple ion counter inductively coupled plasma mass spectrometry

The 187Os/188Os ratio in low-Os sulfides could be in situ measured precisely by LA-MC-ICP-MS equipping an array of ion counters.

The Aksug Porphyry Cu–Mo Deposit (Northeastern Tuva): Chronology of Magmatism and Ore Formation Processes (U–Pb and Re–Os Isotope Data) and Metallogenic Implications

Abstract —The Aksug porphyry Cu–Mo deposit is located in a region of long-lasting magmatic activity. Gabbroids of the Khoito-Oka complex are the earliest intrusive rocks, in which the Aksug granitoid pluton hosting ore-bearing small porphyry intrusions is localized. The intrusive activity was terminated with emplacement of late leucogranite dikes. There are different viewpoints on the age of magmatism and mineralization of the Aksug deposit, with the concept of their Devonian age prevailing. To solve the debatable issue, we performed isotope geochronological studies and analyzed new results of U–Pb (SHRIMP-II) zircon dating and previously published Re–Os molybdenite dates (518 ± 2, 516 ± 2, and 511 ± 2 Ma). The concordant U–Pb zircon ages for igneous rocks are younger than the Re–Os age for mineralization. New U–Pb dating of Khoito-Oka gabbro-diorites has yielded an age of 503 ± 2 Ma. The U–Pb SHRIMP zircon age of tonalites from the Aksug pluton has been estimated at 504 ± 5 Ma. The U–Pb zircon ages for ore-related tonalite porphyry I and tonalite porphyry II are 500 ± 6 and 499 ± 6 Ma, respectively. The obtained SHRIMP age for leucogranite dike is 509 ± 4 Ma. Two groups of U–Pb dates have been obtained for each of the analyzed zircon samples: close to the Re–Os dates (518–511 ± 2 Ma) and younger (507–486 Ma). The weighted average zircon ages calculated for early and late populations from post-ore leucogranites are 515 ± 4 and 500 ± 4 Ma, respectively. We suggest that zircons with an age close to the Re–Os dates found in post-ore leucogranites were assimilated from the underlying substrate and zircons with an age of 500 ± 4 Ma crystallized from melt. The oldest U–Pb dates (509–515 Ma) of individual zircon grains from ore-bearing tonalite porphyry are consistent with the Re–Os molybdenite ages. Zircons from tonalite, tonalite porphyry, and Khoito-Oka gabbroids sometimes show internal textures indicating secondary alteration. The younger U–Pb concordia zircon ages relative to the Re–Os dates might be due to the influence of late thermal processes on the U–Pb isotopic system. The younger dates (486–507 Ma) of individual zircon grains probably reflect the time of the impact of a thermal fluid process. The weighted average of these younger dates (502 ± 2 Ma) falls within the weighted average age of post-ore leucogranites (500 ± 4 Ma). According to the Re–Os dates, the Aksug deposit formed at the end of the early Cambrian. Ore occurrences similar in magmatism and mineralization to the Aksug ore deposit are widespread in Tuva and in the Lake Zone in Mongolia. Therefore, it is necessary to reassess the role of the Cambrian and Devonian magmatism in the development of porphyry Cu–Mo mineralization both in Tuva and in the Altai–Sayan orogenic area.

Triassic marine Os isotope record from a pelagic chert succession, Sakahogi section, Mino Belt, southwest Japan

Pelagic cherts preserved in accretionary complexes represent former seafloor sediment that can retain geochemical evidence of paleoceanographic conditions that predate the oldest extant oceanic crust. The ratio of Os isotopes in seawater, in particular, is of wide interest as a source of insight into notable geologic events such as oceanic anoxic events, rapid global warming episodes, and eruption of large igneous provinces, but marine Os isotope records from before 80 Ma are scarce. We present a record of secular variations of marine Os isotope ratios from a thick, continuous succession of Middle and Upper Triassic pelagic chert in the Sakahogi section of the Mino Belt accretionary complex. Initial Os isotope ratios (187Os/188Os) have a wide range in this succession, from 0.197 to 0.813. The variations in this Os record are quite similar to those in the marine Sr isotope record for the same time period, although their amplitudes differ owing to the differing residence times of Os and Sr in the ocean. Together, our results and previous data trace a continuous Os isotope record for ca. 50 Myr starting in Anisian time. We document the following six notable features in the Os record, which also appear in the Sr record, and their interpretation: (1) a negative (unradiogenic) shift associated with euxinia in the latest Anisian reflecting late-stage Siberian Traps eruptions, (2) wide fluctuations from the latest Anisian to Ladinian induced by volcanism and by the late Ladinian humid pulse and volcanism of the Italian Volcanic Center, (3) a continuous decrease in the early Carnian during eruptions of the Wrangellia large igneous province, (4) stable unradiogenic values from the early to early middle Norian associated with a prolonged arid period, (5) a negative spike in the middle Norian associated with the Manicouagan impact event, and (6) a broad rise and fall from the earliest late Norian to the Rhaetian ages, attributed to the Cimmerian-Indosinian orogeny and subsequent volcanism of the Central Atlantic Magmatic Province.

Fossil subduction zone origin for magmas in the Ferrar Large Igneous Province, Antarctica: Evidence from PGE and Os isotope systematics in the Basement Sill of the McMurdo Dry Valleys

Mantle plumes provide an attractive mechanism for generating short-duration, voluminous magmas in large igneous provinces (LIPs) while at the same time providing an explanation for the frequently associated break-up of supercontinents. This model has also been invoked for the Ferrar large igneous province (FLIP) in Antarctica, which zircon and baddeleyite U–Pb dating shows was emplaced over a short duration at 182.7 ± 0.5 Ma, contemporaneously with fragmentation of the supercontinent Gondwanaland. Here, we present platinum-group-element (PGE) and Os-isotopic data for the Basement Sill in the McMurdo Dry Valleys – a part of the FLIP – that challenge the plume interpretation. The Basement Sill samples studied are cumulate-textured gabbro to norite, and pyroxenite with minor ferro- or leuco-lithofacies with MgO ranging from 2 to 19 wt%. The 187Os/188Os values range from 0.1609 ± 0.003 (2σ) to 8.100 ± 1.600 (2σ); the minimum value overlaps with a previously published estimated initial 187Os/188Os ratio for Ferrar magmas of 0.145 ± 0.049 (2σ). The PGE abundance patterns for the Basement Sill define positive, convex-shaped slopes between the IPGE (Os, Ir and Ru) and PPGE (Pt, Pd and Rh). The most significant feature of the entire data set is the extreme sub-chondritic Os/Ir ratios (<0.33), values which are atypical of plume-derived magmas. These low Os/Ir ratios are more consistent with the alternative view that FLIP resulted from the decompression melting of mantle with a fossil subduction zone signature along the proto-Pacific margin of Gondwanaland, disaggregated by rifting related to plate rearrangements during supercontinent break-up. We propose that hydrated fossil subduction zones elsewhere on Earth might account for other short-lived voluminous magmatic events that form LIPs. The remarkably short duration of these events may be due to rapid decompression of hydrated mantle allowing instantaneous large-volume melting which then peters out quickly (<1 Myr) as H2O is expelled from the source rocks and into the melt.

The 825 Ma Yiyang high–MgO basalts of central South China: Insights from Os–Hf–Nd data

High–MgO basaltic lavas, including komatiites and picrites, have long been used as probes of both the chemical and thermal evolution of the mantle through time. The ca. 825 Ma Yiyang high–MgO basalts in the central South China Block (SCB) were considered to be the first evidence for >1500 °C mantle melts by a Rodinian mantle plume. However, later studies proposed that they may have been generated within an arc setting and can be classified as boninites. Here we present a comprehensive Os–Hf–Nd isotopic study of the Yiyang basalts. The Yiyang basalts have high MgO (>10%), Ni (>182 ppm), Cr (>667 ppm) and Os contents (mostly >0.3 ppb) with radiogenic Os isotopes ((187Os/188Os)i = 0.134 to 0.282). They have relatively uniform whole–rock Nd and Hf isotopes with εNd(t) and εHf(t) values ranging from −3.7 to −1.3 and from +2.9 to +4.3, respectively. Our new data indicate that although crustal assimilation and fractional crystallization (AFC) may have played a role in the geochemical diversity of the Yiyang basalts, the enrichment in light rare earth elements (LREE) and large–ion lithophile elements (LILE) relative to high field strength elements (HSFE) most likely are features of the primary magma. The Yiyang high–MgO basalts, along with contemporary high–MgO basalts in the central SCB, have lower SiO2 contents, and Al2O3/TiO2 ratios, but higher TiO2, Zr, Nb, and Nd contents and Th/U ratios than typical boninites. The presence of negative Ba anomalies and depletion in middle REE (MREE) to heavy REE (HREE) in these high-MgO basaltic rocks are also distinct from what is observed in typical boninites. Therefore, the primary magma of the Yiyang basalts had similar whole–rock geochemical compositions to that of komatiites and plume–derived basalts. Their decoupled HfNd isotopes indicate that recycled oceanic sediments may have been incorporated into their source. They have primitive mantle–like Al2O3/TiO2 ratios (21−23) and flat HREE patterns. However, unlike other Al–undepleted or Munro–type komatiites, their mantle source did not undergo earlier melting events. Our new results support the view that the Yiyang basalts are komatiitic and not boninitic, and that they were likely derived from a Rodinian mantle plume.

Hydrothermal copper mineralization in the Mesoproterozoic Huashugou banded iron formation, Northwest China: Characteristics, timing of formation and genesis

Precambrian banded iron formation (BIF)-related copper deposits constitute a rare category in the world with their origins remaining equivocal. Here we investigate the Huashugou Fe-Cu deposit in the North Qilian Orogenic Belt (NQOB), Northwest China, containing approximately 428 Mt iron ore @ 40 wt% Fe and 0.2 Mt Cu (metal) @ 1.82 wt% Cu. The deposit consists of the upper BIF and the lower copper orebodies, which are hosted in the dominantly clastic-carbonate sedimentary Mesoproterozoic Jingtieshan Group. The distribution of the copper orebodies is controlled by NNW-SSE-trending faults. The BIF and phyllite hosted mineralization includes abundant Cu-sulfide minerals distributed along foliation planes, as transgressive veinlets, fissures, and irregular lenses and aggregates, with or without quartz and siderite. Sulfides (chalcopyrite and pyrite) from the copper ores show large variation in δ34S values (−2.7‰ to 32.2‰), which partly overlap with the δ34S values of pyrite and slightly lower than δ34S of barite from the Jingtieshan Group rocks. Most of the sulfides from copper ores have high 207Pb/204Pb and 208Pb/204Pb ratios relative to the country rocks although there are minor overlaps. The lead isotope data show linear correlation between 207Pb/204Pb and 206Pb/204Pb. The sulfur and lead isotope compositions suggest that the ore-forming materials were derived from the host strata. Fluid inclusion studies constrain the trapping temperature of copper mineralization as 180–200 °C, and are related to low salinity (<10.2 wt% NaCl equiv) fluids. The δ18O and δD values of quartz (13.3‰–17.3‰ and −82‰ to −63‰, respectively) and δ13C and δ18O of siderite (−6.7‰ to −4.3‰ and 15.4‰–18.7‰, respectively) intergrown with sulfides, along with the characteristics of the wall rock alteration in the Huashugou area, suggest that strata-sourced ore-forming fluids with a contribution of meteoric water remobilized sulfur and copper from the country rocks. Chalcopyrite from the ores yield Re–Os isotope age of 583.6 ± 7.9 Ma (2σ, MSWD = 1.6), providing tight constraints on the timing of the copper mineralization. We classify the late Neoproterozoic Cu deposit associated with Huashugou BIF as an epigenetic hydrothermal-type deposit.

Sulphide Re-Os geochronology links orogenesis, salt and Cu-Co ores in the Central African Copperbelt

The origin of giant, sedimentary rock-hosted copper-cobalt (Cu-Co) provinces remains contentious, in part due to the lack of precise and reliable ages for mineralisation. As such, no consensus has been reached on the genetic model for ore formation, and the relationships between tectonism, palaeo-fluid circulation and mineralisation. Here, we link the timing of Cu-Co mineralisation in the Central African Copperbelt to compressional tectonics during the Lufilian Orogeny by using new ca. 609–473 Ma ages given by rhenium-osmium (Re-Os) isotope data for individual Cu-Co sulphides (carrolite and bornite) from the Cu-Co Kamoto deposit. The initial Os isotope composition of carrolite is compatible with the leaching of Os and Cu(-Co) from Mesoproterozoic Cu sulphide deposits hosted in fertile basement. In contrast, the ca. 473 Ma Cu-Au mineralisation stage, which is coeval with late- to post-compressional deformation, may be a distal expression of fluid flow and heat transfer caused by magmatic intrusions in the core of the collisional orogen. The Re-Os ages support a model for mineralisation driven by evaporite dissolution and percolation of large volumes of dense brines in the Katangan Basin during the Lufilian Orogeny.

Does large igneous province volcanism always perturb the mercury cycle? Comparing the records of Oceanic Anoxic Event 2 and the end-Cretaceous to other Mesozoic events

Mercury (Hg) is increasingly being used as a sedimentary tracer of Large Igneous Province (LIP) volcanism, and supports hypotheses of a coincidence between the formation of several LIPs and episodes of mass extinction and major environmental perturbation. However, numerous important questions remain to be answered before Hg can be claimed as an unequivocal fingerprint of LIP volcanism, as well as an understanding of why some sedimentary records document clear Hg enrichment signals whilst others do not. Of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle, as well as the role played by depositional processes in recording global Hg-cycle perturbations. Here, new mercury records of Cretaceous Oceanic Anoxic Event 2 (OAE 2: ∼94 Ma) and the latest Cretaceous (∼67–66.0 Ma) are presented. OAE 2 is associated with the emplacement of multiple, predominantly submarine, LIPs; the latest Cretaceous with subaerial volcanism of the Deccan Traps. Both of these connections are strongly supported by previously published trends towards unradiogenic osmium- (Os) isotope values in globally distributed sedimentary records. Hg data from both events show considerable variation between different locations, attributed to the effectiveness of different sediment types in registering the Hg signal, with lithologically homogeneous records documenting more clear Hg enrichments than sections with major changes in lithology such as limestones to claystones or organic-rich shales. Crucially, there is no geographically consistent signal of sedimentary Hg enrichment in stratigraphic records of either OAE 2 or the latest Cretaceous that matches Os-isotope evidence for LIP emplacement, indicating that volcanism did not cause a global Hg perturbation throughout the entire eruptive history of the LIPs formed at those times. It is suggested that the discrepancy between Os-isotope and Hg trends in records of OAE 2 is caused by the limited dispersal range of Hg emitted from submarine volcanoes compared to the global-scale distribution of Os. A similar lack of correlation between these two proxies in uppermost Cretaceous strata indicates that, although subaerial volcanism can perturb the global Hg cycle, not all subaerial eruptions will do so. These results highlight the variable impact of different volcanogenic processes on the efficiency of Hg dispersal across the globe. Factors that could influence the impact of LIP eruptions on the global mercury cycle include submarine versus subaerial volcanism, volcanic intensity or explosivity, and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic-rich sediments.

The relationship between stratabound Pb–Zn–Ag and porphyry–skarn Mo mineralization in the Laochang deposit, southwestern China: Constraints from pyrite Re-Os isotope, sulfur isotope, and trace element data

The Laochang polymetallic deposit is located in the Sanjiang Tethyan metallogenic province of southwestern China. This deposit contains stratabound (and locally vein-type) Pb–Zn–Ag mineralization and underlying porphyry–skarn Mo mineralization. The stratabound Pb–Zn–Ag mineralization is characterized by massive sulfide ores composed mainly of pyrite, sphalerite, galena, and minor chalcopyrite, whereas the underlying porphyry–skarn Mo mineralization is characterized by quartz–sulfide veins comprising mainly pyrite, chalcopyrite, molybdenite, and minor sphalerite and galena. The age and origin of the stratabound Pb–Zn–Ag mineralization has long been debated, and its relationship to the porphyry–skarn Mo mineralization remains unknown. Integrated pyrite Re-Os isotope, sulfur isotope, and in situ trace element data are used here to constrain the age and origin of both types of mineralization and their genetic relationships.Pyrite related to porphyry–skarn Mo mineralization yields a Re-Os isochron age of 47.3 ± 4.8 Ma, which is consistent with a previous molybdenite Re-Os age of the quartz–sulfide ores and zircon U-Pb ages of the granite porphyry (~44–50 Ma). Pyrite from the stratabound Pb–Zn–Ag mineralization failed to yield a geologically meaningful age, but associated sphalerite and galena have a Re-Os isochron age of 308 ± 25 Ma (Liu et al., 2015), which is consistent with the Carboniferous age of the host basalts. Pyrite, sphalerite, galena, and chalcopyrite from the stratabound Pb–Zn–Ag mineralization have δ34S values (−2.1 to 0.5‰; average 0.48‰) indistinguishable from those of sulfides from the porphyry–skarn Mo mineralization (−4 to 1.9‰; average 0.52‰). Initial Os isotope ratios indicate a mainly crustal origin for both types of mineralization. The calculated (Se/S)fluid and Co/Ni values of pyrite from the stratabound Pb–Zn–Ag mineralization indicate a mixed sedimentary and hydrothermal origin, with a significant magmatic contribution. The identification of a magmatic component, Eocene sulfide mineralization, and phyllic and propylitization alteration in stratabound Pb–Zn–Ag ores indicates that the magmatic–hydrothermal component reflects the overprinting of Carboniferous volcanogenic massive sulfide mineralization by magmatic–hydrothermal fluids derived from an Eocene granite porphyry.

Rhenium–osmium geochronology and oil–source correlation of the Duvernay petroleum system, Western Canada sedimentary basin: Implications for the application of the rhenium–osmium geochronometer to petroleum systems

The Re–Os geochronometer has been applied to many petroleum systems worldwide. However, it is debated if the Re–Os systematics in petroleum actually record the timing of oil generation. Here, we investigate the Re–Os isotope systematics of the Duvernay petroleum system in the Western Canada sedimentary basin, which has been shown to be a relatively simple petroleum system that is associated with oil generated during the Late Cretaceous–early Eocene Laramide orogeny from a single source. The organic geochemistry of the Duvernay oils (pristane/phytane ratios of ∼1.5, smooth homohopane profile, C29 > C27 > C28 regular sterane distribution, and predominance of diasteranes over regular steranes) strongly suggests the oil source to be that of type I–II marine organic matter of the Upper Devonian Duvernay Formation. The asphaltene fraction Re–Os isotope data of the Duvernay oil yield an age of 66 ± 31 Ma, which is in excellent agreement with the timing of the main-stage hydrocarbon generation of the Duvernay Formation based on basin models. Further, the similarity between the 187Os/188Os compositions of the Duvernay Formation source rock (0.46–1.48) and the oil (0.55–1.06) at the time of oil generation supports the hypothesis that the 187Os/188Os composition of an oil is inherited from the source unit at the time of oil generation and therefore shows no, or limited, influence from interaction with basin fluids. This study supports that the Re–Os isotope systematics of an oil can yield the timing of oil generation and be used to trace its source.

Re-Os and HSE in individual base metal sulfide grains: Evaluating micro-analytical procedures using a sulfide reference material

Two Fe-Ni sulfides, resembling the chemical composition of a typical mantle base metal sulfide (BMS), were synthesized and used to test micro-analytical procedures (sample digestion and chemical separation) to determine 187Os/188Os and highly siderophile element (HSE) mass fractions in single BMS grains. The bulk 187Os/188Os and HSE mass fractions of the synthetic sulfides were independently determined after high pressure asher (HPA) digestion and conventional HSE separation (Os solvent extraction and Ir, Ru, Pt, Pd, Re separation via anion exchange chromatography), while the homogenous distribution of HSE was assessed via LA-ICP-MS.Tests were performed following the protocol of Pearson et al. (1998), where sulfides are digested in H2SO4/CrO3 and Os is simultaneously extracted as OsO4. Additional tests were performed adding a pre-digestion step in HBr and/or HCl. The duration and the temperature of the pre-digestion and micro-distillation steps, as well as different chromatographic separations of the HSE were also evaluated. While the majority of the tests yielded 187Os/188Os in agreement with that obtained after HPA digestion, HSE mass fractions show large deviations from the reference content, depending on the used procedure. Such variations are interpreted as the result of incomplete sulfide digestion, Os spike loss, and the possible presence of undigested sub-micrometric platinum group minerals (PGM). Overall, the bulk sulfide HSE mass fractions obtained after HPA digestion are best reproduced (mean deviations ≤10%) using a pre-digestion step in HBr + HCl at 120 °C.This study highlights the need for matrix-matched sulfide reference materials for routine use in laboratories determining HSE mass fractions and Os isotope ratios on single BMS grains. Such an approach is fundamental for comparing and compiling Re-Os ages collected on BMS in many different laboratories. This study also demonstrates that there is need for further testing and for methodological developments/improvement of the analytical procedure(s) used for Re-Os dating and HSE mass fraction determinations in single BMS grains.