Os Isotopic Compositions Research Articles

Petrological and Re‐Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north (Dupuis et al., 2005; Yang et al., 2011). In the western part of the YZSZ, the Northern and the Southern sub‐belts form two sub‐parallel zones of mafic – ultramafic rock assemblages with overlapping crystallization ages (Xiong et al., 2011; Hebért et al., 2012; Liu et al., 2015). The upper mantle section of the Cuobuzha ophiolite in the northern sub‐belt of the Yarlung–Zangbo Suture Zone (YZSZ) in SW Tibet comprises mainly clinopyroxene (cpx)–rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). The cpx‐harzburgites plot in a Re vs. Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes (Uysal et al., 2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐ocean ridge type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting (∼5%) of a primitive mantle source, and they were subsequently modified by melt–rock interactions in a mid‐ocean ridge environment. The depleted harzburgites, on the other hand, were produced by re‐melting of the cpx‐harzburgites, which later interacted with MORB‐ or island arc tholeiite (IAT)‐like melts (Fig. 1) possibly in a trench‐distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Timing of mineralization at the Shihu gold deposit in the middle segment of the Taihang Mountain, China

The Shihu gold deposit, located in the middle-south section of the core of the Fuping mantle branch structure, is hosted in the Archean Fuping Group and adjacent to the quartz diorite porphyrite. The gold deposit is the only large gold deposit with reserves of more than 30 tons gold discovered in western Hebei Province so far. In order to constrain the timing of mineralization of this ore deposit, this paper focuses on the isotopic dating of zircon and pyrite. Zircons in gold-bearing quartz veins are magmatic in origin and no hydrothermal zircon has been found in such quartz veins, indicating that zircons were derived from the wall rocks. U–Pb ages of zircons fall mainly in the two domains: 2492 ± 82 and 136 ± 4 Ma, respectively, indicative of the contribution of the Fuping-Group TTG gneiss and Yanshanian igneous rocks, respectively. The Re–Os isotopic compositions of pyrites in the gold-bearing quartz veins yield an isochron age of 127 ± 31 Ma. Combined with other dating results, we suggest that the main metallogenic age of the Shihu gold deposit is 120–127 Ma.

Re–Os isotope evidence from Mesozoic and Cenozoic basalts for secular evolution of the mantle beneath the North China Craton

The mechanism and process of lithospheric thinning beneath the North China Craton (NCC) are still debated. A key criterion in distinguishing among the proposed mechanisms is whether associated continental basalts were derived from the thinning lithospheric mantle or upwelling asthenosphere. Herein, we investigate the possible mechanisms of lithospheric thinning based on a systematic Re–Os isotopic study of Mesozoic to Cenozoic basalts from the NCC. Our whole-rock Re–Os isotopic results indicate that the Mesozoic basalts generally have high Re and Os concentrations that vary widely from 97.2 to 839.4 ppt and 74.4 to 519.6 ppt, respectively. They have high initial 187Os/188Os ratios ranging from 0.1513 to 0.3805, with corresponding variable γOs(t) values (+20 to +202). In contrast, the Re–Os concentrations and radiogenic Os isotope compositions of the Cenozoic basalts are typically lower than those of the Mesozoic basalts. The lowest initial 187Os/188Os ratios of the Cenozoic basalts are 0.1465 and 0.1479, with corresponding γOs(t) values of +15 and +16, which are within the range of ocean island basalts. These new Re–Os isotopic results, combined with the findings of previous studies, indicate that the Mesozoic basalts were a hybrid product of the melting of pyroxenite and peridotite in ancient lithospheric mantle beneath the NCC. The Cenozoic basalts were derived mainly from upwelling asthenosphere mixed with small amounts of lithospheric materials. The marked differences in geochemistry between the Mesozoic and Cenozoic basalts suggest a greatly reduced involvement of lithospheric mantle as the magma source from the Mesozoic to the Cenozoic. The subsequent lithospheric thinning of the NCC and replacement by upwelling asthenospheric mantle resulted in a change to asthenosphere-derived Cenozoic basalts.

Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

Chromitite from the Harold’s Grave locality in the mantle section of the Shetland ophiolite complex is extremely enriched in Ru, Os and Ir, at µg/g concentrations. High-resolution X-ray computed tomography on micro-cores from these chromitites was used to determine the location, size, distribution and morphology of the platinum-group minerals (PGM). There are five generations of PGM in these chromitites. Small (average 5µm in equivalent sphere diameter, ESD) euhedral laurites, often with Os-Ir alloys, are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised. Also within the chromitite there are clusters of larger (50µm ESD) aligned elongate crystals of Pt-, Rh-, Ir-, Os- and Ru-bearing PGM that have different orientations in different chromite crystals. These may have formed either by exsolution, or by preferential nucleation of PGMs in boundary layers around particular growing chromite grains. Thirdly there is a generation of large (100µm ESD) composite Os-Ir-Ru-rich PGM that are all interstitial to the chromite grains and sometimes form in clusters. It is proposed that Os, Ir and Ru in this generation were concentrated in base metal sulfide droplets that were then re-dissolved into a later sulfide-undersaturated magma, leaving PGM interstitial to the chromite grains. Fourthly there is a group of almost spherical large (80µm ESD) laurites, hosting minor Os-Ir-Ru-rich PGM that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer. These may be hosted in an annealed late syn- or post magmatic fracture. Finally a few of the PGM have been deformed in localised shear zones through the chromitites.The vast majority of the PGM – including small PGM enclosed within chromite, larger interstitial PGM and elongate aligned PGM – have Os isotope compositions that give Re-depletion model ages approximately equal to the age of the ophiolite at ∼492Ma. A number of other PGM – not confined to a single textural group – fall to more or less radiogenic values, with four PGM giving anomalously unradiogenic Os corresponding to an older age of ∼1050Ma. The 187Os/188Os isotopic ratios for PGM from Cliff and Quoys, from the same ophiolite section, are somewhat more radiogenic than those at Harold’s Grave. This may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal Os.

Petrogenesis of Cenozoic basalts in central-eastern China: Constraints from Re–Os and PGE geochemistry

We present Re–Os and PGE (platinum group elements), together with major–, trace–element and Sr–Nd isotope data, to further constrain the petrogenesis for the Cenozoic basalts in central-eastern China. The basalts include low-Ca alkali, high-Ca alkali and tholeiitic varieties. Most low-Ca alkali basalts have unradiogenic 187Os/188Os (mostly <0.14), flat chondrite-normalized PGE patterns and depleted Sr–Nd isotopic compositions, indicating that they probably originated from a depleted mantle source with insignificant shallow-level crustal contamination. Minor high-Ca alkali/tholeiitic basalts have low PGE concentrations, particularly Os (down to 0.0056ppb), and highly suprachondritic initial 187Os/188Os ratios (up to >0.4), indicating involvements of sulfide/PGE-alloy segregation–AFC (assimilation–fractionation–contamination) processes. Most tholeiites and high-Ca alkali basalts have relatively high Os concentrations, highly variable and suprachondritic Os isotopic compositions, high Pd/Ir ratios and enriched Sr–Nd isotopic compositions. Modeling results reveal that the Os isotopic variations in these basalts cannot be ascribed to crustal contamination during ascent but mainly reflect source characteristics. Combined with their low CaO, low Mg#, high FC3MS values (FeOT/CaO–3×MgO/SiO2, all in wt.%), and high Fe/Mn signatures, a large portion of pyroxenite is inferred as an enriched component in the mantle sources to account for their highly suprachondritic Os isotopic compositions. The formation of the pyroxenites was probably due to the subducted sediment-bearing Pacific oceanic crust or the recycled lower continental crust resulted from the collision between the Yangtze and North China blocks.

Modification of an ancient subcontinental lithospheric mantle by continental subduction: Insight from the Maowu garnet peridotites in the Dabie UHP belt, eastern China

Orogenic mantle-derived peridotites commonly originate from the subcontinental lithospheric mantle (SCLM) and thus provide a key target to investigate the modification of the SCLM by a subducting slab. The Maowu ultramafic rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt have formerly been debated as representing cumulates or mantle-derived peridotites. Detailed petrological and geochemical data presented in this study provide new constraints on the origin and formation of the peridotites involving melt depletion in the ancient SCLM and deep crustal metasomatism. The Maowu garnet dunites have refractory bulk compositions characterized by high Mg# (91.9–92.0) and Ni (2537–2892ppm) values and low Al2O3 (0.26–0.76wt.%), CaO (0.05–0.32wt.%), TiO2 (<0.03wt.%), Pd/Ir (0.40–0.46) and 187Os/188Os (minimum 0.11461) values. The Paleoproterozoic model ages (TRD=2.1Ga, TMA=2.3Ga) of the most refractory dunites represent minimum estimates for the age of the initial melt extraction. The extremely depleted nature, high olivine Fo (92.7–93.9), high Cr# (82–87) of spinel, and Re–Os isotopic data suggest that the Maowu garnet dunites are the residues of ~40% partial melting and represent a Paleoproterozoic fragment of the SCLM beneath the southeastern margin of the North China craton. Many garnet orthopyroxenite veins crosscutting the Maowu dunites preserve abundant metasomatic textures and show variable enrichment in incompatible elements. Mineral and whole-rock chemistry indicate that these veins represent metasomatic products between the wall dunites and silica-rich hydrous melts under UHP conditions. The veins show large variations in platinum-group element (PGE) signatures and Re–Os isotopes. The garnet-poor orthopyroxenite veins are characterized by low Al2O3 (<2wt.%) and S (<31ppm) contents and have PGE patterns and 187Os/188Os ratios similar to the wall dunites, whereas the garnet-rich orthopyroxenite veins have high Al2O3 (>6wt.%) and S (99–306ppm) contents and show melt-like PGE patterns and high 187Os/188Os ratios (up to 0.36910). These features, combined with the occurrence of interstitial sulfides in the garnet-rich orthopyroxenite veins, suggest that crust-derived sulfur-saturated silicate melts may have significantly modified the PGE signature and destroyed the Re–Os systematics of the SCLM. However, when the crust-derived silicate melts became sulfur-depleted, such melts would not significantly modify the PGE patterns, radiogenic Os-isotope compositions or the Re-depletion model ages of the SCLM. Consequently, deep crust-mantle interactions in continental subduction zones could induce high degrees of Os isotopic heterogeneity in the SCLM wedge.

Osmium uptake, distribution, and 187Os/188Os and 187Re/188Os compositions in Phaeophyceae macroalgae, Fucus vesiculosus: Implications for determining the 187Os/188Os composition of seawater

The osmium isotopic composition (187Os/188Os) of seawater reflects the balance of input from mantle-, continental- and anthropogenic-derived sources. This study utilizes the Phaeophyceae, Fucus vesiculosus, to analyse its Os abundance and uptake, as well as to assess if macroalgae records the Os isotope composition of the seawater in which it lives. The data demonstrates that Os is not located in one specific biological structure within macroalgae, but is found throughout the organism. Osmium uptake was measured by culturing F. vesiculosus non-fertile tips with different concentrations of Os with a known 187Os/188Os composition (∼0.16), which is significantly different from the background isotopic composition of local seawater (∼0.94). The Os abundance of cultured non-fertile tips show a positive correlation to the concentration of the Os doped seawater. Moreover, the 187Os/188Os composition of the seaweed equalled that of the culture medium, strongly confirming the possible use of macroalgae as a biological proxy for the Os isotopic composition of the seawater.

Origin of temporal compositional trends in monogenetic vent eruptions: Insights from the crystal cargo in the Papoose Canyon sequence, Big Pine Volcanic Field, CA

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.

Paleoceanographic conditions on the São Paulo Ridge, SW Atlantic Ocean, for the past 30 million years inferred from Os and Pb isotopes of a hydrogenous ferromanganese crust

Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn–Fe–10×(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained 187Os/188Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over ~30Myr with growth rates of 0.5–3mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past ~30Myr. The Pb isotopic compositions show very limited ranges (e.g., 206Pb/204Pb=18.80–18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since ~30Ma.

Petrogenesis of Late Jurassic granodiorites from Gutian, Fujian Province, South China: Implications for multiple magma sources and origin of porphyry Cu–Mo mineralization

The Gutian porphyry Cu–Mo deposit is a newly proved porphyry copper deposit in the coastal South China associated with granodioritic porphyries. In this study, zircon U–Pb ages and Hf isotope data, as well as geochemical and Sr–Nd–Pb–Re–Os isotopic compositions, are reported for these intrusions and minerals. Both zircon U–Pb and molybdenite Re–Os dating suggest that the Gutian granodiorite porphyries and related mineralization formed at ~160Ma. The Gutian granodiorites show a low-Mg adakitic geochemical affinity, with relatively high K2O but low Cr and Ni contents. These rocks have initial (87Sr/86Sr)i ratios of 0.7085 to 0.7097, negative εNd(t) values (−12.5 to −7.8), (206Pb/204Pb)t ratios of 18.048 to 18.241, (207Pb/204Pb)t ratios of 15.609 to 15.628, and (208Pb/204Pb)t ratios of 38.494 to 38.667. Zircons from the granodiorites have negative εHf(t) values of −15.7 to −8.5, which are close to those of Cathaysia crust-derived melts. Geochemical and Sr–Nd–Pb–Hf isotopic compositions suggest that they may be derived from Late Jurassic thickened juvenile lower crust. These lower crustal magma sources may not only contain pre-Proterozoic basement rocks, but also involve Triassic and Middle–Late Jurassic arc magmas within the lower crust, which were likely derived from an enriched mantle source associated with paleo-Pacific Plate subduction from the Middle to Late Jurassic. The Gutian ore-related granodiorites represent a new example for significant contributions of ancient subduction melts and enriched mantle-derived sources for porphyry-type magmatism and Cu-Mo mineralization, which occurred in response to an arc regime during the Middle to Late Jurassic in South China.

Type Ib diamond formation and preservation in the West African lithospheric mantle: Re–Os age constraints from sulphide inclusions in Zimmi diamonds

Ten sulphide inclusions in three diamonds from the Zimmi (West Africa) alluvial diamond locality were analysed for their bulk Fe–Cu–Ni–Co contents and Re–Os isotopic compositions. The host diamonds are exceptionally rare, Ib types that still preserve isolated nitrogen (C centres), rather than more common nitrogen pairs (A centres) and nitrogen aggregates (B centres). C centres in Zimmi diamonds require that they did not experience temperatures above 850°C for any extended period. Such diamonds make up less than 0.1% of natural gem diamonds and have never before been dated. The sulphides are pyrrhotite-rich, have low Ni and Os contents, and radiogenic 187Os/188Os, all features characteristic of eclogitic sulphides. Each diamond has 3–4 individual inclusions. 187Re/188Os and 187Os/188Os data fall along three individual ∼650Ma age arrays that represent essentially 3-point or 4-point mineral isochrons for each diamond – unambiguously dating the time of diamond formation. The ∼650Ma age correlates with the timing of Neoproterozoic assembly of Gondwana, recorded in the Rokelide orogen along the SW margin of the West African craton. The initial 187Os/188Os of the three age arrays fall between 1.6 and 2.2 and are highly radiogenic compared to chondritic mantle at 650Ma. Along with low Re/Os ratios, this data suggests that sulphides were not derived from Neoproterozoic subducting slabs, but rather from older eclogitic material already present in the West African lithospheric mantle. The age of the diamonds and their nitrogen substitutional characteristics, along with their residence in a lithospheric mantle with a normal cratonic geotherm (determined here from Koidu clinopyroxene xenocrysts) suggests that after diamond formation they were rapidly exhumed to shallower depths in the lithosphere. This likely occurred through tectonic uplift following Neoproterozoic continental collision.

Triggers on sulfide saturation in Fe–Ti oxide-bearing, mafic-ultramafic layered intrusions in the Tarim large igneous province, NW China

Three Fe–Ti oxide-bearing layered intrusions (Mazaertag, Wajilitag, and Piqiang) in the Tarim large igneous province (NW China) have been investigated for understanding the relationship of sulfide saturation, Platinum-group element (PGE) enrichment, and Fe–Ti oxide accumulation in layered intrusions. These mafic-ultramafic layered intrusions have low PGE concentrations (<0.4 ppb Os, <0.7 ppb Ir, <1 ppb Ru, <0.2 ppb Rh, <5 ppb Pt, and <8 ppb Pd) and elevated Cu/Pd (2.2 × 104 to 3.3 × 106). The low PGE concentrations of the rocks are mainly attributed to PGE-depleted, parental magma that was produced by low degrees of partial melting of the mantle. The least contaminated rocks of the Mazaertag and Wajilitag intrusions have slightly enriched Os isotopic compositions with γOs(t = 280 Ma) values ranging from +13 to +23, indicating that the primitive magma may have been generated from a convecting mantle, without appreciable input of lithospheric mantle. The Mazaertag and Wajilitag intrusions have near-chondritic γOs(t) values (+13 to +60) against restricted e Nd(t) values (−0.4 to +2.8), indicating insignificant crustal contamination. Rocks of the Piqiang intrusion have relatively low e Nd(t) values of −3.1 to +1.0, consistent with ∼15 to 25 % assimilation of the upper crust. The rocks of the Mazaertag and Wajilitag intrusions have positive correlation of PGE and S, pointing to the control of PGE by sulfide. Poor correlation of PGE and S for the Piqiang intrusion is attributed to the involvement of multiple sulfide-stage liquids with different PGE compositions or sulfide-oxide reequilibration on cooling. These three layered intrusions have little potential of reef-type PGE mineralization. Four criteria are summarized in this study to help discriminate between PGE-mineralized and PGE-unmineralized mafic-ultramafic intrusions.

Mantle source of the 2.44–2.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions

Aims. To identify the range of emotional labour employed by healthcare professionals in a healthcare setting and implications of this for staff and organisations. Background. In a healthcare setting, emotional labour is the act or skill involved in the caring role, in recognizing the emotions of others and in managing our own. Design. A thematic synthesis of qualitative studies which included emotion work theory in their design, employed qualitative methods and were situated in a healthcare setting. The reporting of the review was informed by the ENTREQ framework. Data sources. 6 databases were searched between 1979–2014. Review methods. Studies were included if they were qualitative, employed emotion work theory and were written in English. Papers were appraised and themes identified. Thirteen papers were included. Results. The reviewed studies identified four key themes: (1) The professionalization of emotion and gendered aspects of emotional labour; (2) Intrapersonal aspects of emotional labour – how healthcare workers manage their own emotions in the workplace; (3) Collegial and organisational sources of emotional labour; (4) Support and training needs of professionals Conclusion. This review identified gendered, personal, organisational, collegial and socio-cultural sources of and barriers to emotional labour in healthcare settings. The review highlights the importance of ensuring emotional labour is recognized and valued, ensuring support and supervision is in place to enable staff to cope with the varied emotional demands of their work.

Recycling of ancient subduction-modified mantle domains in the Purang ophiolite (southwestern Tibet)

Ophiolites in the Indus–Yarlung Zangbo (IYZ) suture (southern Tibet) have been interpreted as remnants of the Neo-Tethyan lithosphere. However, the discovery of diamonds and super-reducing, ultra-high pressure (SuR-UHP) mineral assemblages (e.g., coesite after stishovite, olivine after wadsleyite, native metals, alloys, and moissanite) in some of these massifs and associated chromitites requires a re-evaluation of their origin and evolution. A new petrological and geochemical study of the Purang ophiolite in the western IYZ suture sheds new lights on these issues.The depleted harzburgites of the Purang massif have low modal contents of clinopyroxene (<2%), and high Cr# [100*Cr3+/(Cr3++Al3+)] in spinel (>40~70) and pyroxenes (>16 in orthopyroxene, and >20 in clinopyroxene), suggesting high degrees of melt extraction (>20%). These features are not consistent with formation in a (ultra-) slow-spreading mid-ocean ridge. These peridotites have high modal contents of orthopyroxene; this, and the extremely high Cr# of spinels in these peridotites, suggests modification in a subduction zone. The clinopyroxene-rich harzburgites and lherzolites contain rare spinel–pyroxene symplectites after garnet. Their clinopyroxenes have low MREE-to-HREE ratios ((Sm/Yb)N<0.1) at relatively high HREE concentrations, and are Na-rich but Nd-poor. The relatively enrichment of Na but depletion of Nd in clinopyroxene cannot be explained by refertilization with MORB melts but are consistent with an origin from Na-rich subcontinental lithospheric mantle (SCLM). All lines of evidence suggest that these peridotites underwent initial melting in the stability field of garnet–facies peridotites, followed by additional melting in the spinel–facies mantle.Whole-rock Os isotopic compositions of the Purang peridotites give ancient TRD model ages (up to 1.3Ga), indicating that the formation of these ancient depletion residues predated the opening of Neo-Tethyan Ocean. These observations, together with recent studies on other IYZ peridotites, suggest that the Purang peridotites are genetically unrelated to the associated mafic crust. Instead, they represent ancient SCLM domains, initially formed beneath a continental margin, and then modified by subduction, before they were incorporated into the Neo-Tethyan ocean basin. This model is consistent with the deep-mantle-recycling model for the presence of SuR-UHP phases in the IYZ ophiolites. The infiltration of MORB melts through these ancient depleted peridotites during their final exhumation in a (ultra-) slow-spreading center may have refertilized them to produce the clinopyroxene-rich peridotites.

Os isotopic constraints on crustal contamination in Auckland Volcanic Field basalts, New Zealand

The Auckland Volcanic Field (AVF) represents the youngest and northernmost of three subjacent Quaternary intraplate basaltic volcanic fields in the North Island, New Zealand. Previous studies on AVF eruptive products suggested that their major- and trace-element, and Sr-, Nd- and Pb-isotopic signatures primarily reflect their derivation from the underlying asthenospheric and lithospheric mantle. All AVF lavas however ascend through a ca. 20–30km thick continental crust, and some do carry crustal xenoliths, posing the question whether or not crustal contamination plays a role in their formation. Here we present new Os and Pb isotopic data, and Os and Re concentrations for 15 rock samples from 7 AVF volcanic centres to investigate mantle and crustal petrogenetic processes. The samples include the most primitive lavas from the field (Mg# 59–69) and span a range of eruption sizes, ages, locations, and geochemical signatures. The data show a large range in Os concentrations (6–579ppt) and 187Os/188Os isotope ratios from mantle-like (0.123) to highly radiogenic (0.547). Highly radiogenic Os signatures together with relatively low Os contents in most samples suggest that ascending melts experienced contamination primarily from metasedimentary crustal rocks with high 187Os/188Os ratios (e.g., greywacke). We further demonstrate that <1% metasedimentary crustal input into the ascending melt can produce the radiogenic Os isotope signatures observed in the AVF data. This low level of crustal contamination has no measurable effect on the corresponding trace element ratios and Sr-Nd-Pb isotopic compositions. In addition, high Os contents (195–578ppt) at slightly elevated but mantle-like Os isotopic compositions (187Os/188Os=0.1374–0.1377) in some samples suggest accumulation of xenocrystic olivine-hosted mantle sulphides from the Permian-Triassic ultramafic Dun Mountain Ophiolite Belt, which traverses the crust beneath the Auckland Volcanic Field. We therefore infer that the AVF Os isotopic compositions and Os contents reflect contamination from varying proportions of heterogeneous crustal components, composed of Waipapa and Murihiku terrane metasediments, and ultramafic rocks of the Dun Mountain Ophiolite Belt. This demonstrates, contrary to previous models that primitive lavas from the Auckland Volcanic Field do show evidence for variable interaction with the crust.

Lead isotope constraints on the mantle sources involved in the genesis of Mesozoic high-Ti tholeiite dykes (Urubici type) from the São Francisco Craton (Southern Espinhaço, Brazil)

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb] &lt; 7 µg/g.

Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

Ultramafic cumulate rocks form during intrusive crystallization of high-MgO magmas, incorporating relatively high abundances of compatible elements, including Cr and Ni, and high abundances of the highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re). Here, we utilize a suite of cumulate xenoliths from Piton de la Fournaise, La Réunion (Indian Ocean), to examine the mantle source composition of the Réunion hotspot using HSE abundances and Os isotopes. Dunite and wherlite xenoliths and associated lavas from the Piton de la Fournaise volcanic complex span a range of MgO contents (46 to 7 wt.%), yet exhibit remarkably homogeneous 187Os/188Os (0.1324±0.0014, 2σ), representing the Os-isotopic composition of Réunion hotspot primary melts. A significant fraction of the xenoliths also have primitive upper-mantle (PUM) normalized HSE patterns with elevated Ru and Pd (PUM-normalized Ru/Ir and Pd/Ir of 0.8–6.3 and 0.2–7.2, respectively). These patterns are not artifacts of alteration, fractional crystallization, or partial melting processes, but rather require a primary magma with similar relative enrichments. Some highly olivine-phyric (>40 modal percent olivine) Piton de la Fournaise lavas also preserve these relative Ru and Pd enrichments, while others preserve a pattern that is likely related to sulfur saturation in evolved melts.The estimate of HSE abundances in PUM indicates high Ru/Ir and Pd/Pt values relative to carbonaceous, ordinary and enstatite chondrite meteorite groups. Thus, the existence of cumulate rocks with even more fractionated HSE patterns relative to PUM suggests that the Réunion hotspot samples a yet unrecognized mantle source. The origin of fractionated HSE patterns in Réunion melts may arise from sampling of a mantle source that experienced limited late accretion (<0.2% by mass) compared with PUM (0.5–0.8%), possibly involving impactors that were distinct from present-day chondrites, or limited core–mantle interactions. Given the remarkably homogeneous Os, Pb, and noble-gas isotopic signatures of Réunion, which plot near the convergence point of isotopic data for many hotspots, such a conclusion provides evidence for an early differentiated and subsequently isolated mantle domain that may be partially sampled by some ocean island basalts.

Re–Os and Lu–Hf isotopic constraints on the formation and age of mantle pyroxenites from the Bohemian Massif

We report on the Lu–Hf and Re–Os isotope systematics of a well-characterized suite of spinel and garnet pyroxenites from the Gföhl Unit of the Bohemian Massif (Czech Republic, Austria). Lu–Hf mineral isochrons of three pyroxenites yield undistinguishable values in the range of 336–338Ma. Similarly, the slope of Re–Os regression for most samples yields an age of 327±31Ma. These values overlap previously reported Sm–Nd ages on pyroxenites, eclogites and associated peridotites from the Gföhl Unit, suggesting contemporaneous evolution of all these HT–HP rocks. The whole-rock Hf isotopic compositions are highly variable with initial εHf values ranging from −6.4 to +66. Most samples show a negative correlation between bulk rock Sm/Hf and εHf and, when taking into account other characteristics (e.g., high 87Sr/86Sr), this may be explained by the presence of recycled oceanic sediments in the source of the pyroxenite parental melts. A pyroxenite from Horní Kounice has decoupled Hf–Nd systematics with highly radiogenic initial εHf of +66 for a given εNd of +7.8. This decoupling is consistent with the presence of a melt derived from a depleted mantle component with high Lu/Hf. Finally, one sample from Bečváry plots close to the MORB field in Hf–Nd isotope space consistent with its previously proposed origin as metamorphosed oceanic gabbro. Some of the websterites and thin-layered pyroxenites have variable, but high Os concentrations paralleled by low initial γOs. This reflects the interaction of the parental pyroxenitic melts with a depleted peridotite wall rock. In turn, the radiogenic Os isotope compositions observed in most pyroxenite samples is best explained by mixing between unradiogenic Os derived from peridotites and a low-Os sedimentary precursor with highly radiogenic 187Os/188Os. Steep increase of 187Os/188Os at nearly uniform 187Re/188Os found in a few pyroxenites may be connected with the absence of primary sulfides, but the presence of minor late stage sulfide-bearing veinlets likely associated with HT–HP metamorphism at crustal conditions.

Platinum group placer minerals in ancient gold artifacts – Geochemistry and osmium isotopes of inclusions in Early Bronze Age gold from Ur/Mesopotamia

One of the most significant characteristics of the gold artifacts from the Early Dynastic Royal Tombs of Ur, Mesopotamia are numerous inclusions consisting of the platinum group elements (PGE) osmium–iridium–ruthenium. In nature, minerals of PGE (PGM) are enriched along with gold and other heavy minerals in placer deposits. During metallurgical gold extraction from placer material and subsequent production of artifacts, PGMs were incorporated in the gold artifacts due to their refractoriness almost unmodified. In order to evaluate their potential for provenance studies of gold, the PGE inclusions were analyzed for their chemical and Os-isotope compositions. They contain highly variable concentrations of Os (26–70 wt.%), Ir (14–62 wt.%) and Ru (0.4–45 wt.%). 187Os/188Os isotope ratios vary between 0.118 und 0.178. Due to the high Ru content of the alloys, the chemical composition point to a geological context of ophiolite complexes. Os isotope ratios are a powerful tool to narrow down the potential ore sources for the gold. However, the interpretation of calculated model ages is difficult due to the unknown genesis of the parental magma. Calculated ages (290–610 Ma) for measured 187Os/188Os of 0.125 using different reference values could indicate placers close to Paleozoic ophiolites like Samti (Takhar) in Northern Afghanistan and Zarshouran (Western Azerbaijan) in Iran, but need to be confirmed by additional measurements of their Os isotope signature in the future. Other archaeological relevant sources of PGM and gold could be excluded by direct comparison of their Os isotope data: 1.) old Neoproterozoic ophiolites from the Eastern Desert type (750–800 Ma), Egypt, 2.) young Mesozoic ophiolites from the Samail complex (96 Ma) in Oman. Thus, in combination with other tracers the Os isotope ratio is a valuable source for provenance studies.

Rhenium–osmium isotopes in pervasively metasomatized mantle xenoliths from the Bohemian Massif and implications for the reliability of Os model ages

Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re–Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history. Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (<1ppb). Rhenium concentrations in the whole suite are below the primitive mantle value. A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187Os/188Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tectonic events to reset the original Os isotopic systematics. The calculated mantle extraction ages (TMA) range from <0.1 to 2.1Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion ages (TRD) vary between <0.1 and ~1.6Ga. However, the TRD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (TRDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite. A prominent peak in the calculated TRDII ages ranges between 0.5 and 0.6Ga which corresponds to the Cadomian orogenic cycle.