The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The 187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC)and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the 187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Proterozoic, The two samples with the lowest 187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.     

Ancient Os isotope signatures from the Ontong Java Plateau lithosphere: Tracing lithospheric accretion history

In order to better understand the nature and formation of oceanic lithosphere beneath the Early Cretaceous Ontong Java Plateau, Re–Os isotopes have been analysed in a suite of peridotite xenoliths from Malaita, Solomon Islands. Geological, thermobarometric and petrological evidence from previous studies reveal that the xenoliths represent virtually the entire thickness of the southern part of subplateau lithospheric mantle (< 120 km). This study demonstrates that vertical Os isotopic variations correlate with compositional variations in a stratified lithosphere. The shallowest plateau lithosphere (< 85 km) is dominated by fertile lherzolites showing a restricted range of ¹⁸⁷Os/¹⁸⁸Os (0.1222 to 0.1288), consistent with an origin from ~ 160 Ma Pacific lithosphere. In contrast, the basal section of subplateau lithospheric mantle (~ 95–120 km) is enriched in refractory harzburgites with highly unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios ranging from 0.1152 to 0.1196, which yield Proterozoic model ages of 0.9–1.7 Ga. Although the whole range of Os isotope compositions of Malaita peridotites is within the variations seen in modern abyssal peridotites, the contrasting isotopic compositions of shallow and deep plateau lithosphere suggest their derivation from different mantle reservoirs. We propose that the subplateau lithosphere forms a genetically unrelated two-layered structure, comprising shallower, typical oceanic lithosphere underpinned by deeper impinged material, which included a component of recycled Proterozoic lithosphere. The impingement of residual but chemically heterogeneous mantle, mechanically coupled to the recently formed, thin lithosphere, may have a bearing on the anomalous initial uplift and late subsidence history of the seismically anomalous plateau root.     

Sulphide inclusions in diamonds from the Koffiefontein kimberlite, S Africa: constraints on diamond ages and mantle Re–Os systematics

Re–Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8–8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69±30 Ma with chondritic initial ¹⁸⁷Os/¹⁸⁸Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, ¹⁸⁷Os/¹⁸⁸Os 0.346 to 2.28, and Re–Os model ages from 1.1 to 2.9 Ga. They define an array on a Re–Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05±0.12 Ga, with an elevated initial ¹⁸⁷Os/¹⁸⁸Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re–Os data.     

Os–Hf–Sr–Nd isotope and PGE systematics of spinel peridotite xenoliths from Tok,SE Siberian craton: Effects of pervasive metasomatism in shallow refractory mantle

Os–Hf–Sr–Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70–84%) rocks. Hf–Sr–Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower ¹⁴³Nd / ¹⁴⁴Nd and ¹⁷⁶Hf / ¹⁷⁷Hf and higher ⁸⁷Sr / ⁸⁶Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated ¹⁷⁶Hf / ¹⁷⁷Hf (up to 0.2838) and plot above the Hf–Nd mantle array defined by oceanic basalts.¹⁸⁷Os / ¹⁸⁸Os in the poorly metasomatised, fertile to moderately refractory (Al₂O₃ ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The ¹⁸⁷Os / ¹⁸⁸Os in these rocks show good correlations with partial melting indices (e.g. Al₂O₃, modal cpx); the intercept of the Al–¹⁸⁷Os / ¹⁸⁸Os correlation with lowest Al₂O₃ estimates for melting residues (∼0.3–0.5%) has a ¹⁸⁷Os / ¹⁸⁸Os of ∼0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. ¹⁸⁷Os / ¹⁸⁸Os in the strongly metasomatised, olivine-rich xenoliths (0.6–1.3% Al₂O₃) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt / rock ratios, which may have also variably elevated their ¹⁸⁷Os / ¹⁸⁸Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re–Os system in the shallow lithospheric mantle can be dramatic.     

The ReOs systematics of the Ronda Ultramafic Complex of southern Spain

The Os isotopic compositions of twelve ultramafic and six mafic layer samples from the Ronda Ultramafic Complex of southern Spain have been determined. Among the ultramafic rocks, ¹⁸⁷Os/¹⁸⁶Os varies from 0.98 to 1.12. A weak correlation is observed between ¹⁸⁷Os/¹⁸⁶Os and Re/Os. A much stronger correlation exists between Os isotopic ratio and Mg#, suggesting that the Re/Os ratios have been perturbed to some extent. Two alternatives are proposed to explain the relationship between Os composition and Mg#: (1) Continuous processes in the convecting mantle; (2) Radiogenic ingrowth since an ancient melt depletion event. No relationship is observed between ¹⁸⁷Os/¹⁸⁶Os and ¹⁴³Nd/¹⁴⁴Nd. This is probably because the Nd systematics were strongly affected by a recent metasomatic event, which apparently had little effect on the Os isotopic compositions.The Os isotopic ratios of the mafic layers range from 1.7 to 47.9. Within a single thick layer, the ratios vary from 16.5 to 47.9. These high ratios demonstrate that the layers are ancient features. Among the mafic samples, Os isotopic ratio is found to decrease strongly with increasing Os concentration, which ranges from 0.009 ppb to 1.16 ppb. One layer, which had a SmNd model age of less than 200 Ma, yielded a ReOs model age of about 2 Ga. This implies that neither system can be trusted to give accurate information about the time of mafic layer formation.     

Rhenium–osmium isotope and elemental behaviour during subduction of oceanic crust and the implications for mantle recycling

This study presents major-, trace-element, and rhenium–osmium (Re–Os) isotope and elemental data for basalts and gabbros from the Zermatt-Saas ophiolite, metamorphosed to eclogite-facies conditions during the Alpine orogeny. Igneous crystallisation of the gabbros occurred at 163.5 ± 1.8 Ma and both gabbro and basalt were subject to ‘peak’ pressure–temperature (P–T) conditions of > 2.0 GPa and ~ 600 °C at about 40.6 ± 2.6 Ma.Despite such extreme P–T conditions, Re–Os isotope and abundance data for gabbroic rocks suggest that there has been no significant loss of either of these elements during eclogite-facies metamorphism. Indeed, ¹⁸⁷Re–¹⁸⁷Os isotope data for both unaltered gabbros and gabbroic eclogites lie on the same best-fit line corresponding to an errorchron age of 160 ± 6 Ma, indistinguishable from the age of igneous crystallisation. In contrast, metamorphosed basalts do not yield age information; rather most possess ¹⁸⁷Re/¹⁸⁸Os ratios that cannot account for the measured ¹⁸⁷Os/¹⁸⁸Os ratios, given the time since igneous crystallisation. Taken with their low Re contents these data indicate that the basalts have experienced significant Re loss (∼ 50–60%), probably during high-pressure metamorphism. Barium, Rb and K are depleted in both gabbroic and basaltic eclogites. In contrast, there is no evident depletion of U in either lithology.Many ocean-island basalts (OIB) possess radiogenic Os and Pb isotope compositions that have been attributed to the presence of recycled oceanic crust in the mantle source. Published Re–Os data for high-P metabasaltic rocks alone (consistent with this study) have been taken to suggest that excessive amounts of oceanic crust are required to generate such signatures. However, this study shows that gabbro may exert a strong influence on the composition of recycled oceanic crust. Using both gabbro and basalt (i.e. a complete section of oceanic crust) calculations suggest that the presence of ≥ 40% of 2 Ga oceanic crust can generate the radiogenic Os compositions seen in some OIB. Furthermore, lower U/Pb ratios in gabbro (compared to basalt) serve to limit the ²⁰⁶Pb/²⁰⁴Pb ratios generated, while having a minimal effect on Os ratios. These results suggest that the incorporation of gabbro into recycling models provides a means of producing a range of OIB compositions having lower (and variable) ²⁰⁶Pb/²⁰⁴Pb ratios, but still preserving ¹⁸⁷Os/¹⁸⁸Os compositions comparable to HIMU-type OIB.     

Sources and genesis of the Chinkuashih Au-Cu deposits in northern Taiwan: constraints from Os and Sr isotopic compositions of sulfides

The Chinkuashih district at northern Taiwan hosts one of the largest Au deposits in the western Pacific gold province. Gold were precipitated from hydrothermal solutions as native gold or incorporated into sulfides at a temperature range of 200-350 °C. The sulfides in ore mines have ¹⁸⁷Os/¹⁸⁸Os ratios varying from 0.139 to 0.249. The positive ¹⁸⁷Os/¹⁸⁸Os-1/Os correlation is consistent with derivation from the hybrid fluids containing various proportions of mantle and crustal components. The crustal component was the meteoric water that had acquired its Sr and Os isotopic signatures from the local sedimentary formations and dacitic intrusions. The mantle component was the magmatic fluid segregated from the dacitic magma by fractional crystallization. Based on the ¹⁸⁷Os/¹⁸⁸Os-1/Os correlation, the hybrid fluids forming the Chinkuashih sulfides contained less than 30% magmatic fluid, except for one sulfide sample from Hsumei, which required >40% magmatic fluid. Compared to meteoric water, the magmatic fluid contained a higher Os content (130 times higher) and was enriched in Os relative to Sr with an Os/Sr ratio two orders higher than that of the crustal fluid. Consequently, the Os budget in the hybrid fluids was controlled by the magmatic fluid, although the meteoric water was volumetrically dominated. If gold and Os behave similarly in chemistry, the Chinkuashih gold deposits are of mantle origin and the area where sulfides with the greatest mantle Os signature may host undiscovered gold deposits. Finally, the ¹⁸⁷Os/¹⁸⁸Os ratios of sulfides show no relationship with the mineral assemblages of sulfides, implying that the sulfide mineral assemblages reflect local surfacial redox conditions rather than the chemical characteristics of parental fluids.     

The southeast Australian lithospheric mantle: isotopic and geochemical constraints on its growth and evolution

Trace elements and isotopic compositions of whole rocks and mineral separates are reported for 15 spinel-bearing harzburgite and lherzolite xenoliths from southeastern Australia. These samples have an exceedingly large range in isotopic compositions, with⁸⁷Sr/⁸⁶Sr ranging from 0.70248 to 0.70834 and ε_Nd values ranging from +12.7 to −6.3. This range in isotopic compositions can be found in xenoliths from a single locality. The isotopic compositions of clinopyroxene separates and their whole rocks were found to be different in some xenoliths. Samples containing small glass pockets, which replace pre-existing hydrous minerals, generally show only small differences in isotopic composition between clinopyroxene and whole rock. In a modally metasomatized peridotite, significant differences in the Sr and Nd isotopic compositions of a coexisting phlogopite-clinopyroxene pair are present. Coexisting clinopyroxenes and orthopyroxenes from an anhydrous lherzolite have Sr isotopic compositions that are significantly different (0.70248 versus 0.70314), and yield an apparent age of 625 Ma, similar to that found previously by Dasch and Green [1]. However, the Nd isotopic compositions of the clinopyroxene and orthopyroxene are identical indicating recent (within 40 Ma) re-equilibration of Nd.Sr and Nd concentrations in the whole rocks and clinopyroxenes show an excellent positive correlation, and have an average Sr/Nd ratio of 15. This ratio is similar to the primitive mantle value, as well as that found in primitive MORBs and OIBs, but is much lower than that measured in island arc basalts and what might be predicted for a subduction zone-derived fluid. This indicates that a significant proportion of the Sr and Nd in these peridotites is introduced as a basaltic melt with intraplate chemical characteristics.The isotopic compositions of the peridotites reflect long-term, small-scale heterogeneities in the continental lithospheric mantle, and are in marked contrast to the near uniform isotopic compositions of the host alkali basalts (⁸⁷Sr/⁸⁶Sr= 0.7038–0.7041andε_Nd = +3.6 to +2.9). A minimum of three evolutionary stages are identified in the growth of the continental lithospheric mantle: an early basalt depletion event, recording the initial development and stabilization of the lithospheric mantle, followed by at least two enrichment episodes. These observations are consistent with continental lithospheric mantle growth involving the underplating of refractory peridotite diapirs.     

Petroleum source rock identification of United Kingdom Atlantic Margin oil fields and the Western Canadian Oil Sands using Platinum,Palladium, Osmium and Rhenium: Implications for global petroleum systems

This study demonstrates that petroleum and source rocks are enriched in Pt and Pd to the ppb level, and that the ¹⁸⁷Os/¹⁸⁸Os composition coupled with the Pt/Pd value permits the fingerprinting of petroleum to its source. Oils from the United Kingdom Atlantic Margin (sourced from the Upper Jurassic Kimmeridge Clay Fm.) as well as source rock samples have been analysed for Pt and Pd. When the Pt/Pd value is compared with ¹⁸⁷Os/¹⁸⁸Os (calculated at the time of oil generation; Os_g) the values from both the known source and the oils are similar, demonstrating that they can be used as an oil to source fingerprinting tool. This inorganic petroleum fingerprinting tool is particularly important in heavily biodegraded petroleum systems where traditional fingerprinting techniques (e.g. biomarkers) are severely hampered, e.g. the world's largest oil sand deposit, the West Canadian Oil Sands (WCOS). This has caused the source of the WCOS to be hotly debated, with no present day consensus between inputs from potential source units e.g. Exshaw and Gordondale Fms. ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd fingerprinting of the oil sands shows that the majority of the petroleum have similar ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd values, supporting the hypothesis of one principal source. Analysis of the potential source rocks establishes that the principal source of the oil sands to be from the Jurassic Gordondale Fm., with a minor Exshaw Fm. input. Thus, the combination of previously pioneered Re–Os petroleum geochronology with ¹⁸⁷Os/¹⁸⁸Os and Pt/Pd values of petroleum permits both a temporal and spatial understanding of petroleum systems.     

Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using 238U–230Th disequilibria and Os isotopes

A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (²³⁰Th/²³⁸U) > 1 and span a large range in (²³⁰Th/²³²Th) ratios, and most basalts have higher (²³⁰Th/²³²Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate ²³⁰Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by ¹⁸⁷Os/¹⁸⁸Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large ²³⁰Th excesses (25–48%) and high ¹⁸⁷Os/¹⁸⁸Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization.     

The Late Eocene 187Os / 188Os excursion: Chemostratigraphy,cosmic dust flux and the Early Oligocene glaciation

High resolution records (ca. 100 kyr) of Os isotope composition (¹⁸⁷Os / ¹⁸⁸Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene ¹⁸⁷Os / ¹⁸⁸Os excursion and confirm that the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink [Earth Planet. Sci. Lett. 210 (2003) 151–165], is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os minimum can be placed at 34.5 ± 0.1 Ma in the marine records. In addition, two other distinct features of the ¹⁸⁷Os / ¹⁸⁸Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. ± 0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO₂. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the ¹⁸⁷Os / ¹⁸⁸Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; ¹⁸⁷Os / ¹⁸⁸Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of ¹⁸⁷Os / ¹⁸⁸Os records with high resolution benthic foraminiferal δ¹⁸O records across the Eocene–Oligocene transition suggests that ¹⁸⁷Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater ¹⁸⁷Os / ¹⁸⁸Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine ¹⁸⁷Os / ¹⁸⁸Os records are obscured by recovery from the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.     

Life-time of a stratified magma chamber recorded in ultramafic xenoliths from Ichinomegata volcano,northeastern Japan

Variations in major-element chemistry and modal compositions of the mantle xenolith-bearing calc-alkalic ejecta from Ichinomegata volcano are inferred to be due to mixing of three magmatic end members: Basalt I (SiO₂ 51 wt% , MgO 8.5 wt%), Basalt II (SiO₂ 54 wt%, MgO 5 wt%), and Dacite (SiO₂ 65 wt%, MgO₂ wt%). Ultramafic xenoliths are found in mafic mixtures of Dacite-Basalt I and Dacite-Basalt II. The thermal histories of the xenoliths in both mixtures are compared with each other. Chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in Basalt I were equilibrated at about 800 °C, while those in Basalt II were also equilibrated originally at about 800 °C but were subsequently annealed at about 1000 °C for more than 10^2–3 years prior to the eruption.The chemical composition of Basalt I indicates that it can coexist with upper mantle peridotite and it is an appropriate candidate for a carrier of ultramafic xenoliths from the upper mantle. On the other hand, Basalt II is fractionated and it cannot be directly derived from the upper mantle. Two pulses of xenolith-bearing basalt injection into a dacite magma chamber are inferred to have occurred. The first injection did not lead to eruption and subsequently formed a dacite/basalt stratified magma chamber. In the lower layer, the basalt was slightly differentiated to become Basalt II and ultramafic xenoliths carried by the first pulse were annealed at the bottom of the layer. The duration of the annealing of the xenoliths implies a minimum life-time of the Dacite-Basalt II stratification in the magma chamber beneath Ichinomegata of 10^2–3 years. The second injection of the xenolith-bearing basalt (Basalt I) was immediately followed by eruption, and all the magmas were effused with mixing in a conduit. Consequently, the ultramafic xenoliths carried by the second pulse are not annealed.     

New insights into the Re-Os systematics of sub-continental lithospheric mantle from in situ analysis of sulphides

The Os isotopic compositions of mantle rocks generally are considered to be established during melt-depletion events and to be robust to subsequent disturbances (e.g. metasomatism). Consequently, Os isotopes are used to date the main melting event that a mantle section has undergone, i.e. transformation of fertile asthenospheric material into a depleted, buoyant lithosphere. However, Os resides almost entirely in Fe-Ni-Cu sulphides, which can be very mobile under mantle conditions. In situ laser ablation multi-collector ICP-MS measurement of Re/Os isotopic ratios in sulphides from spinel peridotite xenoliths demonstrates that whole-rock Os-isotope signatures record the mixing of multiple sulphide populations. Sulphides residual after melting events have unradiogenic Os isotopic compositions reflecting ancient melt depletion; those introduced by later metasomatism events contain more radiogenic Os. Therefore, the whole-rock Os isotopic signature can be strongly altered by metasomatic processes, and studies of mantle-derived xenoliths show that such disturbance is quite common in the lithospheric mantle. Because melt-depletion ages estimated from individual sulphide inclusions are systematically older than those obtained from whole-rock analysis, caution is essential in the interpretation of the Os model ages derived from whole-rock analysis, and their use and abuse in geodynamic models. This work suggests that sulphide could become a key phase in unravelling the formation and evolution of the lithosphere.     

Noble gases in the earth's mantle

The abundances and isotopic compositions of noble gases in two samples from ultramafic xenoliths in alkali basalt, a young kaersutitic amphibole separated from a peridotite xenolith from Dish Hill, California and an ancient whole-rock lherzolite xenolith from Baja California, are reported and compared with the results of analyses on other mantle samples. In addition to previously recognized excesses of ³He and ¹²⁹Xe, our results indicate that ambient gases in the mantle show a general enrichment of the lighter-mass nonradiogenic isotopes of Ar, Kr and Xe, and Ar with ⁴⁰Ar/³⁶Ar = 3 · 10².     

Osmium isotopes as petrogenetic and geological tracers

Using terrestrial osmium-enriched samples of known ages, we have shown that¹⁸⁷Os/¹⁸⁶Os varies with time in result of the¹⁸⁷Re β^? radioactivity. Such a variation in the earth's mantle can be fitted by a straight line corresponding to¹⁸⁷Re/¹⁸⁶Os = 3.15 for the mantle, comparable to C1 carbonaceous chondrites. Using this result and the Re and Os contents of various rocks, several theoretical considerations and predictions can be deduced for the chemical evolution of the earth, such as a method for distinguishing between different processes of development of the continental crust. The special result of¹⁸⁷Os/¹⁸⁶Os from Bushveld is discussed with respect to the possible existence of an “enriched” subcontinental mantle.     

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.     

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly <5 vol.% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al₂O₃ contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO₂-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.     

Effects of melt percolation on the Re-Os systematics of continental mantle lithosphere: A case study of spinel peridotite xenoliths from Heilongjiang,NE China

Os isotope ratios of mantle peridotites have been considered to be largely immune to recent melt-rock interaction. However, Os isotope ratios and PGE (Platinum group elements) concentrations of the Yong’an xenoliths have been significantly modified by melt percolation, and are not suitable for determining the formation age of lithosphere mantle in Yong’an. In this study, the Yong’an spinel peridotite xenoliths are divided into two groups: N-Type and E-Type. The N-Type group including cpx (clinopyroxene)-poor lherzolite and harzburgite, shows a large variation of Cr#(sp) (13.2-48) and sulfur contents (from 171 ppm to below detection limit), whereas the E-Type peridotites are mainly refractory harzburgites and are characterized by high Cr#(sp) (35.3-42.2) and overall low sulfur contents (below 51 ppm). Both types show similar major and REE (rare earth element) patterns. Furthermore, the N-Type peridotites display a restricted range of iridium-group PGE (IPGE), Os/Ir and Ru/Ir ratios (Os/Ir = 0.64-1.12, Ru/Ir = 1.52-1.79) and variable palladium-group PGE (PPGE) contents (3.4-14.9 ppb), whereas the E-Type peridotites show a large variation of Os/Ir and Ru/Ir ratios (Os/Ir = 0.33-0.84, Ru/Ir = 0.94-1.6), and a restricted range of PPGE (4.3-6.9 ppb). 187Os/188Os ratios of E-Type peridotites are higher than those of N-Type peridotites at comparable fertility levels. These results suggest that N-Type peridotites may have been overprinted by metasomatism via small melt fractions, in which the percolation of the volatile-rich, small melt fractions only resulted in LILE (large ion lithophile element) enrichment of clinopyroxene, and their whole rock PGE contents and Re-Os isotope values were little changed. Moreover, E-Type peridotites may have been modified by melt-rock reaction involving relatively large melt fractions, which may result in the formation of secondary cpx and olivine and the removal of IPGE-bearing minerals such as Ru-Os-(Ir) alloys or laurite, followed by precipitation of secondary sulfides from melt with radiogenic isotopic signature.     

Precise Re–Os mineral isochron and Pb–Nd–Os isotope systematics of a mafic–ultramafic sill in the 2.0 Ga Onega plateau (Baltic Shield)

We present Re–Os, Sm–Nd and Pb–Pb isotope and trace element data for the Konchozero sill, a layered mafic–ultramafic intrusion in the Early Proterozoic Onega plateau, one of the oldest continental flood basalt provinces on Earth. The Sm–Nd and Pb–Pb combined mineral and whole-rock isochron ages of 1988±34 and 1985±57 Ma for the sill coincide with the age of ferropicrites from Pechenga (the Kola Peninsula). The lithostratigraphic, chemical and isotope evidence suggest the derivation of Pechenga lavas and the Onega plateau volcanics from a single mantle plume. Peridotite and gabbro whole-rock samples, and primary ulvospinel and ilmenite mineral separates from the sill yield a Re–Os isochron with a slope corresponding to an age of 1969±18 Ma, γ_Os(T) =−0.61±5.9. This age is consistent with the other isotope data, and indicates the closed-system behavior of Re and Os in the rocks. The peridotites and ulvospinel have high Os concentrations (2.5–14 ppb) and low ¹⁸⁷Re/¹⁸⁸Os ratios (0.35–1.1), thus allowing a more accurate determination of the weighted average initial ¹⁸⁷Os/¹⁸⁸Os of 0.1144±0.0019 (2σ_pop), γ_Os(T) =+0.77±1.7. This value is lower than that determined by Walker et al. (Geochim. Cosmochim. Acta 61 (1997) 3145–3160) for the Pechenga lavas (γ_Os(T) =+6.1±0.7), and implies a substantial Os-isotope heterogeneity in this ancient plume. Compared to the Onega plateau primary basalt magma, Pechenga ferropicrites are relatively enriched in iron and Ni, have lower (Nb/Th)_N ratios (2.1 vs 1.1) and less radiogenic Nd-isotope compositions (_Nd(T) = +3.1 and +1.4, respectively), but share similar low-radiogenic Pb-isotope characteristics (μ₁=8.57 and 8.60). Incorporation of small amounts (1.5%) of outer core material into the hotter central part of the plume and subsequent contamination of the Pechenga ferropicritic magmas with the 2.9 Ga Belomorian gneisses can explain the observed chemical and isotope variations in the two provinces provided that the core had <0.25 ppm of Pb.     

Os isotope heterogeneity of the upper mantle: Evidence from the Mayarí–Baracoa ophiolite belt in eastern Cuba

In an attempt to quantify the extent of geochemical heterogeneity within a restricted and well dated portion of the upper mantle, 27 chromite separates from the 90 My old chromite deposits in the Mayarí–Baracoa ophiolite belt in eastern Cuba have been investigated for platinum group element (PGE) concentrations and Re–Os isotopic systematics. The samples are characterized by systematically subchondritic initial ¹⁸⁷Os/¹⁸⁸Os ratios and substantial heterogeneity. The initial ¹⁸⁷Os/¹⁸⁸Os ratios vary with chromite chemistry and with geographical distribution, reflecting differences in the Os isotopic evolution for the different upper mantle sections represented by the ophiolite. Accordingly, the Os isotope data might be divided into three groups. In the Moa–Baracoa district, where the chromite bodies are located in the mantle–crust transition zone, the calculated initial γOs values average − 0.97 ± 0.69 (n = 13). In the Sagua de Tanamo district, where chromite chemistry is highly variable and their location in relation the mantle sequence is less clear, the initial γOs values are intermediate, with an average of − 1.77 ± 0.80 (n = 7). In the Mayarí district, where the chromite bodies are located in the lower part of the mantle sequence, initial γOs values average − 2.66 ± 0.29 (n = 7). These subchondritic (i.e. negative) initial γOs values are most simply explained by Re depletion during ancient partial melting and/or melt percolation events.The Os isotope heterogeneity documented here indicates a high degree of geochemical complexity on small to intermediate length scales in the upper mantle. Our results, in combination with data on chromites from the literature, show that an “average present-day Os isotopic composition” for the hypothetical depleted MORB mantle (DMM) reservoir cannot be precisely established beyond the statement that it is “broadly chondritic”. Indeed, the upper mantle cannot be considered a sufficiently homogeneous geochemical “reservoir” to serve meaningfully as a baseline against which geochemical “anomalies” are evaluated. On the other hand, our findings are consistent with the “Statistical Upper Mantle Assemblage” or “SUMA”-concept, according to which a high level of geochemical heterogeneity is maintained in the upper mantle at all relevant length scales, as a result of the plate-tectonic cycle and intra-mantle processes such as melt-migration and metasomatism.