Os Isotope Data Research Articles

Re−Os Isotope and PGE Abundance Systematics of Coast Range Ophiolite Peridotites and Chromitite, California: Insights into Fore-Arc Magmatic Processes

Abstract We report platinum-group element (PGE) and Re concentrations, and Re−Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)N ≈ 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)−Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE−Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = −8.3 to 1.4) and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite−magnetite−quartz buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after ~5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has 187Os/188Os value of 0.1250 (γOs = −3.5) and PGE−Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Sources of Ore Material in the Platinum-Group Element Deposits of Polar Siberia and the Middle Urals Based on the Data from Radiogenic (Re–Os, Pt–Os) and Stable (Cu, S) Isotopes

Abstract —Understanding the main events of platinum-group element (PGE) ore formation is impossible without analysis of the sources and behavior of major ore-forming components, namely, platinum, osmium, sulfur, and copper, which are important indicators of magmatic and hydrothermal processes. In contrast to the Re–Os isotope system, the radiogenic Pt–Os isotope system, as well as stable isotopes of Cu and S in PGE deposits, are still relatively understudied. Our comprehensive research is aimed at filling this gap. The paper presents data for the Guli massif of ultramafic and alkaline rocks and carbonatites in Polar Siberia and on the zonal Nizhny Tagil and Svetly Bor clinopyroxenite–dunite massifs in the Middle Urals, which include: (1) the contents of the highly siderophile elements (HSE) in whole rocks and platinum-group minerals (PGM), (2) the Re–Os and Pt–Os isotope systematics of chromitite, Os–Ir alloys, and Ru–Os sulfides, (3) the sulfur isotope composition in Ru–Os and Ir–Rh sulfides in primary and secondary PGM assemblages, and (4) the copper isotope composition in Pt–Fe minerals from chromitites and placers. The research was performed using scanning electron microscopy, electron probe microanalysis, and high-precision isotope-geochemical analysis. The high-precision Re–Os and Pt–Os isotope data show that the HSE contents in chromitites and PGM of the Guli massif were controlled by the composition of the mantle source that evolved with near-chondritic time-integrated Re/Os and Pt/Os ratios, which are also typical of the sources of most komatiites and abyssal peridotites. The δ65Cu values of the studied samples of ferroan platinum and isoferroplatinum are identical within the analytical uncertainty and are close to 0‰, which is typical of high-temperature Cu-containing minerals. The sulfur isotope compositions of the Ir–Rh sulfides of the kashinite–bowieite series and of the Ru–Os sulfides of the laurite–erlichmanite series in the primary PGM assemblages indicate that the source of sulfur has a chondritic isotope composition, which is in agreement with the osmium isotope composition of the Ru–Os sulfides and Os–Ir alloys. The heavy sulfur isotope composition (δ34S = 5.6 ± 1.5‰) of As-containing erlichmanite is consistent with its secondary origin. The new data on the isotope compositions of osmium, copper, and sulfur can be used as new important parameters that characterize the sources of PGE mineralization.

Fine-grained interplanetary dust input during the Turonian (Late Cretaceous): evidence from osmium isotope and platinum group elements

The Turonian age (~ 90–94 Ma) was the hottest geological interval in the Cretaceous and also marked by the K3 event, a pronounced enrichment of 3He in pelagic sediments (i.e., massive input of extraterrestrial materials). Here, we present Os isotopic (187Os/188Os) and platinum group element (PGE) data from Turonian sedimentary records. After a sharp unradiogenic shift during the end-Cenomanian oceanic anoxic event 2, the 187Os/188Os ratios declined continuously throughout the Turonian, which could be ascribed to the formations of several large igneous provinces (LIPs). Because the interval with the most unradiogenic 187Os/188Os ratios (i.e., enhanced LIP volcanism) does not correspond to the warmest interval during the mid-Cretaceous, additional sources of CO2, such as subduction zone volcanism or the kimberlite formation, may explain the Cretaceous Thermal Maximum. As Os isotope ratios do not show any sharp unradiogenic shifts and PGE concentrations do not exhibit a pronounced enrichment, an influx of fine-grained cosmic dust to the Earth’s surface, possibly from the long-period comet showers, can be inferred at the time of the 3He enrichment during the mid-Turonian K3 event. Our findings highlight the different behaviors of 3He and PGE information in the sedimentary rocks during the input of fined-grained extraterrestrial materials.

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.

Pelagic responses to oceanic anoxia during the Carnian Pluvial Episode (Late Triassic) in Panthalassa Ocean

The Carnian Pluvial Episode (CPE) was a short interval of extreme rainfall in the Late Triassic that caused significant changes in marine ecosystems. Global warming induced by Wrangellia volcanism is thought to have resulted in oceanic anoxia during the CPE, but the global extent, duration, and severity of anoxia, and its effects on major marine taxa, remain unclear. To address this, we examined an equatorial record of conditions in the Panthalassa Ocean during the CPE, focusing on marine Os isotope data, redox conditions, and conodont and radiolarian biostratigraphy. The results show that Wrangellia volcanism peaked in the latest Julian (early Carnian), coinciding with development of reducing conditions in the deep-sea Panthalassa. A strong conodont turnover occurred during the period of oceanic anoxia, whereas radiolarians were less affected and their diversity increased after the recovery from anoxia. The increased radiolarian diversity during the early Tuvalian (late Carnian) can be attributed to chemical weathering and enhanced nutrient fluxes associated with global warming and the more humid climate of Pangea.

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Organic matter accumulation and hydrothermal effect of the Barney Creek formation in the Glyde Sub-basin, northern Australia

The Glyde Sub-basin is a large petroliferous unit in the McArthur Basin, northern Australia, and it is situated ∼100 km southeast to the world-class McArthur River Zn–Pb hydrothermal deposit. The Paleoproterozoic Barney Creek Formation (BCF) is the hydrocarbon source rock in the sub-basin, and also the host rock for the McArthur River deposit. The controlling factors of organic matter accumulation and the hydrothermal effects on organic matter thermal maturity are not well-constrained for the BCF in the Glyde Sub-basin, hindering petroleum and mineral explorations. To address these issues, rock eval pyrolysis, reflectance measurement, trace element analysis, Nd–Hf and Re–Os isotopic analysis were conducted for the BCF samples taken from the drill-hole GRNT-79-5 in the Glyde Sub-basin. Analysis of trace elements and Nd–Hf isotopic ratios shows that cuttings of the BCF were derived from a mafic-dominant source (La/Ni = 0.89–2.27, La/Cr = 0.63–1.04, La/Sc = 3.16–4.95, La/V = 0.23–0.81, ϵNd = −5.01 to −2.98, ϵHf = −2.27 – 1.8), which may have contributed to a pronounced phosphorous delivery to the basin, resulting in enhanced primary productivity of seawater. Therefore, the high organic matter abundance in the BCF (TOC = 0.86%–5.04%) is most likely controlled by the primary productivity of seawater. The 187Re/188Os ratios (34.954–432.95) of a suite of samples negatively correlate with proportions of kerogen in organic matter (0%–94%), suggesting that burial temperature has reset the Re–Os isotope system. Hydrocarbon generation may have been promoted by hydrothermal fluid flows which relate to the McArthur River mineralization because the Re–Os isotope data yield an isochron age similar to the age of mineralization. The Re–Os isotope data provide direct evidence that the BCF in the Glyde Sub-basin was affected by hydrothermal fluids. However, the co-variation between thermal maturity indicators (Tmax = 436–444 °C; reflectance of solid bitumen = 0.34%–0.66%) and burial depth (97–512 m) suggests that organic matter thermal maturation primarily resulted from burial diagenesis rather than hydrothermal fluid flows. This indicates that temperatures of hydrothermal fluids was not high enough to significantly alter the thermal regime of the studied region.

Formation and Evolution of Chromitites in the Hongshishan Complex, Beishan Orogenic Collage, NW China: Constraints from Mineralogical Compositions, Re‐Os Isotopes and Platinum‐Group Element Geochemistry

AbstractThe Hongshishan chromitite deposits are situated to the north of the Beishan orogenic collage, in the southern part of the Central Asian Orogenic Belt. This study describes the mineral chemistry, Re‐Os isotopes and platinum‐group elements geochemistry of the Hongshishan chromitites for the purpose of constraining the origin, evolution and composition of their parental melts. The restricted ranges of Al2O3, Cr2O3 and Cr#‐Mg# variation of chromite‐cores and chromites fall within the field of the mid‐ocean ridge and ophiolitic podiform chromite settings. The (187Os/188Os)i ratios of the chromitites are in the range of 0.12449–0.12745 (average 0.12637) and the γOs are from –1.92 to –0.06 (average –0.83). In the Re‐Os isotope diagrams, all the samples fall in the field of chromitites and show a residual peridotitic trend. The range of Os isotopic compositions and γOs values indicate that they overlap the depleted MORB mantle (DMM) as well as being close to global Os isotopic data and γOs of ophiolite chromitites. The characteristics of the PGE contents can be roughly subdivided into two groups: podiform chromitites and Ural‐Alaskan type complexes. For the ferritchromite cores, the calculated Al2O3 concentrations of the parental melt are higher (average 16.65 wt%) in high‐Cr than high‐Al chromitite (average 16.17 wt%) and for the chromite, the calculated Al2O3 concentrations are even higher (average 16.48 wt%) in the high‐Cr than the high‐Al examples (average 15.38 wt%). In the (TiO2)melt vs. TiO2 diagrams, most high‐Al melts fall in the MORB, while the high‐Cr melts fall in the ARC field. The calculated FeO/MgO ratios for the parental melt show the closest resemblance to a MORB magma composition. The inferred parental melt composition for studied chromitites falls in the field of mid‐ocean ridge basalt (MORB) magmas and far away from boninite. The calculated degrees of partial melting producing the chromitites are 16%–22% (average 19%), which is around the range of those of the MORB magmas. The chromitites are suggested to have been formed in a MORB setting. The chromites and ferritchromite cores are mostly scattered along the MORB and SSZ harzburgite–dunite fields. Ferritchromite rims and ferritchromites with high YFes formed as a result of alteration during serpentinization. .

Subduction-related mantle accretion and makeover revealed by mantle xenoliths at the Pacific margin of NE Eurasia

Subduction is thought to be a major factor in the origin and makeover of continental lithospheric mantle, but essential aspects of these processes continue to be debated. Here, we report textural, chemical and Re Os isotope data on mantle xenoliths from far eastern Russia located at different distances from the Asian Pacific coast to examine how the proximity to subduction zones affects lithospheric mantle compositions. The xenoliths come from the Khanka block close to the border with China and several sites in the Sikhote-Alin orogenic belt (SAOB) near the Japan Sea coast. The inboard Khanka xenoliths are spinel lherzolites with chemical compositions, suggesting that they are residues of low- to moderate-degrees of melt extraction from fertile mantle and essentially unaffected by later events for 1.9 Ga. Such rocks are also common among peridotites in the SAOB, located closer to the subduction margins, but the SAOB suite also includes: a) heterogeneous rocks with veins and patches of pyroxenites or hydrous minerals, and b) rocks enriched in iron (8.8–13.5 wt%), Al 2 O 3 and incompatible minor elements. The positive correlation between elevated 187 Os/ 188 Os ratios (>0.135) and FeO contents in some Fe-rich SAOB xenoliths is interpreted to indicate that the metasomatic melts were derived from the asthenosphere hybridized with recycled materials from the Pacific slab. The lack of correlation between Re -depletion Os model ages (T RD ) and depletion indices, as well as the similarity of Os isotopic compositions between the SAOB peridotites and abyssal peridotites are interpreted to show that the SAOB mantle possibly formed and accreted to Eurasia from the asthenosphere during eastward migration of the Paleo-Pacific subduction trench. Based these results, we infer that the SAOB lithospheric mantle experienced multi-stage metasomatism after its formation, the most significant of which is localized infiltration of subduction-related basaltic melts with recycled characteristics induced by the extension of Japan Sea and release of volatiles from the slab. By contrast, the inboard Khanka lithospheric mantle shows little, if any, evidence for metasomatism. Such contrasting metasomatic modification implies different degrees and styles of makeover processes at different distances relative to the subduction margins. • The Khanka mantle was little affected by metasomatism since it was formed at ~1.9 Ga. • The Sikhote-Alin lithospheric mantle was formed by subduction and accretion. • Mantle metasomatism beneath Sikhote-Alin was caused by melts with recycled signature. • The nature and degrees of metasomatism are related to the distance from subduction front.

Petrogenesis of peridotites in the Dargai Complex ophiolite, Indus Suture Zone, Northern Pakistan: Implications for two stages of melting, depletion, and enrichment of the Neo-Tethyan mantle

The upper mantle section of the Dargai ophiolite along the Indus Suture Zone, also known as the Main Mantle Thrust of northern Pakistan, consists mainly of clinopyroxene-bearing (Cpx)-harzburgites, depleted harzburgites, and dunites. Here we document a two-stage origin for these ultramafic rocks using new mineral and whole-rock geochemistry, integrated with Re -Os isotopic data of these upper mantle peridotites. Chromian spinel in the Cpx-harzburgites have significantly lower Cr# values (15.2–20.7) than the depleted harzburgites (38.4–75.2) and dunites (75.0–81.9). The Cpx-harzburgites are characterized by high average Al 2 O 3 (2.04 wt%), CaO (2.26 wt%), ∑REE (24.3 ppb), and 187 Os/ 188 Os values ranging between 0.12495 and 0.12997. In contrast, the depleted harzburgites and dunites are characterized by lower average Al 2 O 3 (0.51 wt% and 0.14 wt%, respectively), CaO (0.68 wt% and 0.19 wt%, respectively), and ∑REE (3.41 ppb and 0.92 ppb, respectively). The 187 Os/ 188 Os of the depleted harzburgites range between 0.11699 and 0.12612, values that are less radiogenic than those of the Cpx-harzburgites. The mantle-normalized trace element patterns of the Cpx-harzburgites reflect low degrees of partial melting (~10–15%), whereas those of the depleted harzburgites and dunites record somewhat higher degrees of partial melting (~15–23% additional melting of the Cpx-harzburgites). The low degree of melting, coupled with low degrees of melt extraction, therefore point to a spreading center at amid-ocean ridge (MOR) or in a distal forearc basin. In contrast, the depleted harzburgites and dunites formed during the second stage of melting and related refertilization, and were clearly linked to a supra-subduction zone (SSZ) setting. The Re -Os isotopic compositions of the Dargai Complex peridotites provide model ages of ca. 250 and ca. 450 Ma recording protracted tectonic events in close association with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans. The extremely heterogeneous Os isotopic compositions of this part of the Neo-Tethyan upper mantle reflect multiple melt production and melt extraction processes throughout its tectonic evolution. • Peridotites in the Dargai Complex are formed by episodic melt-rock reaction. • The Dargai Complex peridotites experienced two stage of depletion and enrichment. • Peridotites were formed in the Indus Suture Zone after low-high degrees partial melting. • Re-Os isotopic composition infer derivation from heterogeneously depleted mantle.

Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history.

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.

Evolution of a deeply-buried oil reservoir in the north Shuntuoguole Low Uplift, Tarim Basin, western China: Insights from molecular geochemistry and Re–Os geochronology

The recently discovered Shunbei Oilfield is perceived as a remarkable breakthrough in deep marine carbonate petroleum exploration in the Tarim Basin, western China. Constraining the absolute timing of petroleum system evolution is crucial for further hydrocarbon exploration and development in the area. Here, we integrated molecular geochemical correlation, stable carbon isotopes of bulk oil (bitumen) and Re–Os geochronology to trace the petroleum sources, to constrain the absolute timing of hydrocarbon generation, and to reconstruct the petroleum system evolution of the deeply-buried oil accumulations in carbonate reservoirs. Biomarker parameters derived from isoprenoids, terpanes, steranes and aromatic compounds, as well as stable carbon isotopic compositions, revealed that the Ordovician reservoir oils and bitumens were all originated from the same source rock, deposited under a highly-reduced environment with organic matters being predominantly contributed by algae and bacteria. The calculated equivalent vitrinite reflectance (Rc) values of reservoir bitumen and oil samples indicate that the Shunbei Oilfield has experienced at least two hydrocarbon charges. Geochemical characteristics and thermal maturity suggest that the bitumen within the Ordovician Yijianfang Formation reservoirs in the north Shuntuoguole Low Uplift is not originated from thermal cracking, thermochemical sulfate reduction, biodegradation, nor gas washing. Re–Os isotopic data from six reservoir bitumen samples yield an isochron age of 399 ± 12 Ma with an initial 187Os/188Os ratio (Osi) of 1.26 ± 0.11. This age is interpreted to represent the timing of oil generation when compared with the thermal evolution of possible source rocks and other radiometric dating results in the region. A comparison of 187Os/188Os of the 399 Ma-old bitumen and that of possible source rocks at the time of oil generation suggests that the reservoir oil was possibly originated from the lower Cambrian Yuertusi source rock. This work highlights that Re–Os geochronology combining with the initial 187Os/188Os ratio can be used to effectively constrain the timing of hydrocarbon generation in petroleum systems with complex tectonic history, multiple oil charges and alterations.

Development of a complex arc–back-arc basin system within the South Tianshan Ocean: Insights from the Wuwamen ophiolitic peridotites

Ophiolites within the South Tianshan orogenic belt directly record the nature and tectonic evolution of the South Tianshan Ocean (STO), which is key for understanding the accretionary history of the Central Asian Orogenic Belt. Here we present petrological and Os isotopic data for the Wuwamen mantle peridotites from the South Tianshan orogenic belt, which provide new insights into the tectonic origin of this ophiolite and evolution of the STO. The lherzolites have low spinel Cr# (0.1–0.2), similar to those of fertile oceanic or continental peridotites that have undergone low degrees of melting. However, the extremely high Na2O (1.5–1.8 wt%) and Al2O3 (5.5–6.2 wt%) contents of the clinopyroxenes are atypical of oceanic peridotites, and resemble those of subcontinental peridotites. The harzburgites have more refractory compositions, suggesting higher degrees of melt extraction. Both the lherzolites and harzburgites have low initial 187Os/188Os ratios (< 0.121), which reflect Proterozoic melt depletion and a long-term lithospheric evolution. These data confirm an ancient subcontinental lithospheric mantle origin for the Wuwamen peridotites. The replacive dunites have moderate spinel Cr# (0.3–0.4), reduced Os and enriched 187Os/188Os (0.142–0.160), indicative of reaction between host lherzolites and percolating mid-ocean ridge basalt (MORB)-type melts with radiogenic Os isotopic signatures, presumably in a subduction-related setting. This and the associated back-arc basin basalt (BABB)–type mafic crust suggest the Wuwamen ophiolite formed in a continental margin back-arc setting. A comparison with other ophiolites in the South Tianshan orogenic belt further indicates that a complex arc–back-arc system existed along the southern margin of the Central Tianshan, which was associated with northward subduction within the STO.

The Tethyan Himalaya Igneous Province: Early Melting Products of the Kerguelen Mantle Plume

AbstractThe Kerguelen large igneous province (LIP) has been related to mantle plume activity since at least 120 Ma. There are some older (147–130 Ma) magmatic provinces on circum-eastern Gondwana, but the relationship between these provinces and the Kerguelen mantle plume remains controversial. Here we present petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb–Os isotopic data for high-Ti mafic rocks from two localities (Cuona and Jiangzi) in the eastern Tethyan Himalaya igneous province (147–130 Ma). Zircon grains from these two localities yielded concordant weighted mean 206Pb/238U ages of 137.25 ± 0.98 Ma and 131.28 ± 0.78 Ma (2σ), respectively. The analyzed mafic rocks are enriched in high field strength elements and have positive Nb–Ta anomalies relative to Th and La, which have ocean island basalt-like characteristics. The Cuona basalts were generated by low degrees of melting (3–5 %) of garnet lherzolites (3–5 vol% garnet), and elsewhere the Jiangzi diabases were formed by relatively lower degrees of melting (1–3%) of garnet lherzolite (1–5 vol% garnet). The highly radiogenic Os and Pb isotopic compositions of the Jiangzi diabases were produced by crustal contamination, but the Cuona basalts experienced the least crustal contamination given their relatively low γOs(t), 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi values. Major and trace element geochemical and Sr–Nd–Hf–Pb–Os isotope data for the Cuona basalts are similar to those for products of the Kerguelen mantle plume head. Together with high mantle potential temperatures (&amp;gt;1500 °C), this suggests that the eastern Tethyan Himalaya igneous province (147–130 Ma) was an early magmatic product of the Kerguelen plume. A mantle plume initiation model can explain the temporal and spatial evolution of the Kerguelen LIP, and pre-continental breakup played a role in the breakup of eastern Gondwana, given the &amp;gt;10 Myr between initial mantle plume activity (147–130 Ma) and continental breakup (132–130 Ma). Like studies of Re–Os isotopes in other LIPs, the increasing amount of crustal assimilation with distance from the plume stem can explain the variations in radiogenic Os.

Osmium isotopes in peridotite xenoliths reveal major mid-Proterozoic lithosphere formation under the Transantarctic Mountains

Osmium isotopes, whole rock and mineral geochemical data from peridotite xenoliths from two Miocene McMurdo Volcanic Group cinder cones in the Transantarctic Mountains (TAM) in Southern Victoria Land, Antarctica, reveal that the underlying mantle preserves evidence for major mid-Proterozoic lithosphere formation despite the crust being dominated by late Neoproterozoic-Ordovician (~0.65–0.47 Ga) rocks. The Hooper Crags xenolith suite is dominated by harzburgites with highly refractory olivine Mg# (up to 92.3) and depleted bulk rock major and platinum group element + Re abundances, with 187Os/188Os ratios indicating depletion in the mid-Proterozoic. Pipecleaner Glacier xenoliths, 18 km distant, are lherzolites with olivine Mg# (<91) and fertile major and platinum group element abundances, with Os isotope abundances defining an aluminochron that also indicates mid-Proterozoic depletion. Although exposed crust along this portion of Antarctica reveals only minor evidence for Proterozoic magmatism, the major episode of lithosphere formation indicated by the Os isotope data is supported by published bulk rock Sm-Nd isotope and zircon εHf mantle model ages of Neoproterozoic to Ordovician plutonic rocks. The heterogeneous circum-cratonic mid-Proterozoic mantle under Southern Victoria Land has therefore persisted on a Ga timescale, including through the formation and destruction of Rodinia and Gondwana supercontinents as well as extensive crustal melting and emplacement of the Ferrar large igneous province. This longevity may be due to the thick (>250 km) East Antarctic Craton lithosphere shielding the immediately adjacent circum-cratonic mantle from being affected by convective asthenosphere-driven erosion. This contrasts with mantle lithosphere accreted distally to the East Antarctic Craton (represented by the now-detached Zealandia continent), which did not attain extreme thickness and has therefore been more susceptible to tectonic reworking and lateral translation.

Long-term Aptian marine osmium isotopic record of Ontong Java Nui activity

AbstractThe early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ∼5 m.y. during the early to mid-Aptian.

Highly siderophile element and osmium isotope systematics of basaltic volcanics: A different approach to petrological processes

The highly siderophile element (HSE) or platinum group element (PGE) and Os isotope systematics of basaltic volcanics have recently received a significant attention because of their potential to constrain the petrological processes on magma generation and evolution. The HSE and Os isotope data, which are generally observed at very low concentrations in basalts and obtained by modern enrichment and analytical techniques, are frequently used in petrological studies. The HSE contents and ratios from whole-rock analysis of basalts, and combined evaluation with the theoretical knowledge and modelling of HSE behaviour during the partial melting of mantle and the differentiation of basaltic magma would provide opportunity for geochemical modelling on mantle melting. Besides, HSE contents and Pd-PGE/Ir-PGE ratios are important indicators for the nature of mantle sulfides, the sulfur saturation conditions of the mantle source, sulfide segregation, fractional crystallization, crustal assimilation and partial melting degrees in the origin and evolution of mantle-derived magmas. Therefore, in addition to the traditional whole-rock geochemical data obtained from Cenozoic aged basalts observed widely in Turkey, HSE and Os isotope systematics of these basalts can contribute to define the geochemical features of the mantle source, and to model petrological processes which are effective in the magma evolution.

Origin of Ru-Os Sulfides from the Verkh-Neivinsk Ophiolite Massif (Middle Urals, Russia): Compositional and S-Os Isotope Evidence

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.