Os Isotope Study Research Articles

Origin and sources of recent continental alkaline volcanism in the Southwest Ankaratra volcanic field, Central Madagascar: Insights from major and trace element and Sr, Nd, Pb, Hf, Os isotopes

We conducted 40Ar/39Ar, major element, trace element and Sr, Nd, Pb, Hf, Os isotopic studies of basaltic lavas from the Southwest Ankaratra volcanic field, in order to assess the origin and sources of recent monogenetic volcanism in the central highlands of Madagascar. All samples in this study yielded ages less than 110 ka, providing further evidence for Pleistocene volcanism in the central highlands of Madagascar including the Southwest Ankaratra volcanic field. The Southwest Ankaratra magmas are characterized by significant chemical and isotopic heterogeneity across the volcanic field and within an individual eruptive center (Iavoko cinder cone), similar to that observed in other monogenetic volcanic fields. The data suggest two distinct geochemical groups within the Quaternary Ankaratra magmas: trachybasalts and basanites. All of the samples have trace element abundances and patterns similar to those observed in ocean island basalts (OIB). The trachybasalts are characterized by more evolved compositions, yet display lower trace element abundances, than the basanites. Furthermore, the trachybasalts are characterized by more radiogenic 87Sr/86Sr and lower 143Nd/144Nd than the basanites, and are displaced to slightly lower 206Pb/204Pb, but similar 207Pb/204Pb and 208Pb/204Pb, compared to the basanites.Samples from the Southwest Ankaratra volcanic field display suprachondritic 187Os/188Os values that can be explained by minor crustal assimilation, but their trace element and Sr-Nd-Pb-Hf isotopic characteristics are incompatible with crustal assimilation, and instead represent features of their mantle sources. Our data show that the magmas are compositionally distinct from those expected for subcontinental lithospheric mantle (SCLM)-derived basalts, and likely derive from an asthenospheric mantle source polluted by plume materials and delaminated SCLM. Chemical and isotopic studies of Pleistocene alkaline basalts from SW Ankaratra suggest the involvement of the African superplume C in their origins, and further suggest that superplume materials must be widely distributed in the upper mantle beneath the entire EARS from the Red Sea to Madagascar, both near to and distal from the low seismic velocity mantle regions.

Mineralogical and Re-Os isotope constraints on fluid– and melt–rock interactions and the origin of mantle peridotites from the Amdo ophiolite, northern Tibet

Ophiolites from the Bangong–Nujiang Suture Zone in the central part of the Tibetan Plateau represent relics of Neo-Tethyan oceanic lithosphere. Here we present the results of a detailed mineralogical and Os isotopic study of mantle peridotites of the Amdo ophiolite, which provide new insights into the origin of the peridotites and their diverse melt/fluid–peridotite reactions. The Amdo ophiolite comprises mantle peridotites, mafic–ultramafic cumulates, massive gabbros, diabases, and pillow basalts. The mantle peridotites are mainly refractory harzburgites, composed of olivine (Ol, 65%–85%), orthopyroxene (Opx, 12%–35%), clinopyroxene (Cpx, <4%), and Cr-spinel (Cr-Spl, ~2%). Most olivine grains (Ol-I) are protogranular, locally enclosed in orthopyroxene and Cr-spinel, and have high Fo values (91.1–92.9) and NiO contents (0.29–0.44 wt%), typical of residual origin. In contrast, interstitial olivines (Ol-II) have low Fo values (67.2–84.7) and NiO contents (<0.05 wt%), but extremely high MnO contents (0.28–1.65 wt%), suggesting metasomatism by carbonate-bearing fluids. Hypidiomorphic-granular clinopyroxene grains (Cpx-I) have high Mg# values (~94.5) and low Al2O3 (average 1.26 wt%) and Na2O (<0.19 wt%) contents, typical of residual phases after melting. However, clinopyroxene coronas (Cpx-II) around orthopyroxene have higher Na2O contents (0.24–1.06 wt%), indicative of mantle metasomatism by melts. Osmium isotopic compositions in the harzburgites vary from subchondritic to suprachondritic (187Os/188Os ratios: 0.11716–0.13284). The subchondritic Os isotopic compositions (0.11716–0.12367) correspond to TRD model ages of 0.76–1.56 Ga, suggesting Mesoproterozoic mantle depletion events. On the other hand, several harzburgites with low Os (0.29–0.86 ppb) and suprachondritic 187Os/188Os ratios (0.12823–0.13284) reflect the overprinting of original Os budgets and Os isotopic compositions by percolating melts or fluids. The combined mineralogical and Os isotopic compositions suggest that the Amdo peridotites represent a mixture of ancient subcontinental lithospheric mantle (SCLM) and Mesozoic oceanic lithosphere, which were subsequently infiltrated by carbonate-bearing fluids and silicate melts in a supra-subduction zone.

Re–Os and Sr Isotopic Study of Permian–Triassic Sedimentary Rocks from the Himalaya: Shale Chronology and Carbonate Diagenesis

Sedimentary rocks from the Himalayas are well-preserved archives of the Neo-Tethys oceanic conditions. In this contribution, Re–Os isotopic systematics of black shales from the Gungri Formation, Spiti valley and siltstones from the Khunamuh Formation, Guryul Ravine have been investigated to constrain their depositional ages. The Re–Os isochron for the Gungri shales yields a depositional age of 255 ± 22 Ma (2σ; n = 8; MSWD (Mean Square Weighted Deviation) = 5.7), consistent with available biostratigraphic information. The initial 187Os/188Os ratio (0.60 ± 0.13) is similar to that reported for the Late Permian shales, indicating the connection of the Neo-Tethys with the global ocean. In contrast, the Re–Os systematic is found to be non-isochronous for the Guryul Ravine section, a proximal site with a strong influence of seismic/Tsunami events. Global compilation of 187Re/188Os ratios in Late Permian shales and bathymetric distribution of the Re/Os ratios point to strong role of Re/Os uptake by macroalgae, in addition to oceanic pH and redox state, in regulating the Re–Os systematic in shales. The 87Sr/86Sr ratios for the Induan carbonates from the Spiti (0.71551–0.71837) are higher than to that expected for the Lower Triassic ocean (~0.707). Co-variations of Sr and 87Sr/86Sr with Mn concentrations establish the diagenetic alteration of these carbonates.

The Metallogenic Setting of the Jiangjiatun Mo Deposit, North China: Constraints from a Combined Zircon U–Pb and Molybdenite Re–Os Isotopic Study

The Jiangjiatun Mo deposit is a recently discovered molybdenum deposit in the easternmost area of the Yan-Liao metallogenic belt, North China Craton. Quartz vein-type Mo mineralization at Jiangjiatun is associated with the granitic porphyry stock. In this study, we performed a combined zircon U–Pb and molybdenite Re-Os dating study on the Jiangjiatun Mo deposit to constrain its mineralization age and metallogenic setting. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) zircon U–Pb analyses suggest that the granitic porphyry was formed during the Late Jurassic, with a weighted mean 206Pb/238U age of 154 ± 1 Ma (2σ). Seven molybdenite samples from the Jiangjiatun deposit yield a 187Re–187Os isochron age of 157.5 ± 0.5 Ma (2σ). The discrepancy between the U–Pb and Re–Os ages may be explained (1) by the “2 sigma” measurement uncertainty, or (2) by the different closure temperature of the Re–Os isotopic system of molybdenite and the U–Pb isotopic system of zircon. Even though there is a small difference between the zircon U–Pb and molybdenite Re–Os ages, we can clearly identify a Late Jurassic Mo mineralization event at Jiangjiatun in the easternmost area of the Yan-Liao metallogenic belt. The moderate Re concentrations (13 to 73 ppm) in molybdenite from the Jiangjiatun Mo deposit are indicative of the involvement of the mantle materials into the Mo mineralization. The Jiangjiatun Mo deposit is likely the result of the subduction of the paleo-Pacific plate beneath the North China Craton during the Late Jurassic. Combined with the available published regional robust geochronological data, we proposed that the Mo mineralization in the Yan-Liao belt is in good agreement with the tectonic transition from Late Triassic post-collision extensional setting due to the closure of the paleo-Asian ocean to the Yanshanian (J–K1) continental arc setting in response to the subduction of the paleo-Pacific Plate. The study highlights that regional mineralization may provide an excellent constraint on tectonic change.

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

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.

Re–Os depositional age for black shales from the Kaimur Group, Upper Vindhyan, India

Proterozoic sedimentary deposits from the Vindhyan Supergroup, central India are excellent archives of paleo-environmental conditions. Meaningful interpretation of these geological signatures and their global correlation requires depositional age information for the sediments, data on which are limited for the Upper Vindhyan sediments. In this work, Re–Os isotopic study of the Bijaigarh shale, Kaimur Group from the Upper Vindhyan has been carried out to infer their depositional age and paleo-hydrological conditions. The Re–Os isochron for the Bijaigarh shale, a lagoonal deposit with an open ocean connection, provides a depositional age of 1210±52Ma (Model 3; MSWD=69; n=15; 2σ). This age for the Kaimur Group is younger by ~400Ma than the underlying Semri Group (age ~1600Ma), confirming existence of a long sedimentary hiatus between the two groups. The isochron-derived initial 187Os/188Os (Osi) ratio for these shales (~0.6) is higher than those reported for Mesoproterozoic marine shales (~0.3) from other global margins. Osi ratios of shales could be regulated by regional paleo-hydrological conditions; osmium mass balance calculations for the restricted basin of the Bijaigarh shale show that a minor increase in freshwater influx (as low as ~2.4% i.e. salinity change by ~0.8psu) to the euxinic deep water of the basin is sufficient to elevate Osi ratio from 0.3 (marine value) to ~0.6 (Osi ratio of the lagoonal shale). In contrast to lagoonal shales, the Osi ratios of marine shales seem to mimic the seawater 187Os/188Os during their deposition period. Compilation of marine Osi ratios shows coarse synchrony between temporal changes in Osi during the Proterozoic eon and the atmospheric oxygen level, pointing to the dominant role of oxidative continental weathering on paleo seawater 187Os/188Os ratio.

Understanding Re–Os systematics and model ages in metamorphosed Archean ultramafic rocks: A single mineral to whole-rock investigation

187Rhenium–187Os model ages are used to infer the timing of crust-mantle differentiation events and evolution of lithospheric mantle. However, ancient samples often have long and complex histories of metamorphism, metasomatism and deformation that may be problematic if these processes caused disturbance the Re–Os system. Such processes have been shown to disturb the Re–Os systematics of ophiolitic chromitites, but the effect on stratiform chromitites has not previously been investigated. Here we present a detailed petrographic and Re–Os isotopic study of chromitites, chromite-bearing meta-peridotites and single chromite grains from the Ujaragssuit nunât layered ultramafic intrusion, southwest Greenland, which has experienced intense deformation and at least two episodes of metamorphism up to amphibolite facies. We report the first ever Re–Os isotope and platinum-group element concentration data for single chromite grains achieved by single grain dissolution and isotope dilution. Micrometric Os-, Ir-, Ru – sulphide and sulpharsenide inclusions in chromite occur in the massive chromitites; these accessory phases host a significant portion of the Os, Ir and Ru in these samples. The remaining Os-Ir-Ru budget, along with Pt and Pd, appears to be homogeneously distributed within the chromite, occurring either in the lattice itself, as evenly distributed sub-micrometric alloy inclusions in chromite (unresolvable using the techniques applied in this study) or a combination of both. Rhenium is hosted in silicates, as predicted by previous studies. Both single-grain and whole-rock Re–Os isotope systematics yield unfeasibly young Re–Os model ages due to their radiogenic 187Os/188Os compositions. This could result from country rock contamination of the original melt from which the Ujaragssuit nunât intrusion crystallised, and/or from disturbance of the Re–Os isotope systematics of these rocks during regional metamorphic events at ∼2.8 and/or ∼3.75Ga. We conclude that it is vital to establish a complete sample history before interpreting Re–Os isotope data and the validity of model ages, as the Re–Os system is sensitive to disturbance. It is feasible that in some cases single chromite grains or chromite mineral separates may offer a more robust and meaningful isotopic record than silicates, because Re is preferentially hosted in silicates relative to chromite. Alternatively, the 190Pt–186Os decay system, applied to whole-rock samples, may provide a more reliable chronometer than 187Re–187Os model ages in samples that have experienced crustal contamination and/or metamorphic disturbance.

Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re‐Pt‐Os Isotope Systematics in Mafic‐Ultramafic Rocks

Properly combining highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data, obtained by isotope dilution, with corresponding 187Os/188Os and 186Os/188Os measurements of rocks requires efficient digestion of finely‐ground powders and complete spike‐sample equilibration. Yet, because of the nature of commonly used methods for separating Os from a rock matrix, hydrofluoric acid (HF) is typically not used in such digestions. Consequently, some silicates are not completely dissolved, and HSE residing within these silicates may not be fully accessed. Consistent with this, some recent studies of basaltic reference materials (RMs) have concluded that an HF‐desilicification procedure is required to fully access the HSE (Ishikawa et al. (2014) Chemical Geology, 384, 27–46; Li et al. (2015) Geostandards and Geoanalytical Research, 39, 17–30). Highly siderophile element abundance and Os isotope studies of intraplate basalts typically target samples with a range of MgO contents (&lt; 8 to &gt; 18% m/m, or as mass fractions, &lt; 8 to &gt; 18 g per 100 g), in contrast to the lower MgO mass fractions (&lt; 10 g per 100 g) of basalt and diabase RMs (i.e., BIR‐1, BHVO‐2, TDB‐1). To investigate the effect of HF‐desilicification on intraplate basalts, experiments were performed on finely ground Azores basalts (8.1–17 g per 100 g MgO) using a ‘standard acid digestion’ (2:1 mixture of concentrated HNO3 and HCl), and a standard acid digestion, followed by HF‐desilicification. No systematic trends in HSE abundances were observed between data obtained by standard acid digestion and HF‐desilicification. Desilicification procedures using HF do not improve liberation of the HSE from Azores basalts, or some RMs (e.g., WPR‐1). We conclude that HF‐desilicification procedures are useful for obtaining total HSE contents of some young lavas, but this type of procedure is not recommended for studies where Re‐Pt‐Os chronological information is desired. The collateral effect of a standard acid digestion to liberate Os, followed by HF‐desilicification to obtain Re and Pt abundances in samples, is that the measured Re/Os and Pt/Os may not correspond with measured 187Os/188Os or 186Os/188Os.

Re–Os pseudo-isochron of disseminated ore from the Kalatongke Cu–Ni sulfide deposit, Xinjiang, Northwest China: Implications for Re–Os dating of magmatic Cu–Ni sulfide deposits

Re–Os dating of disseminated ore from the Kalatongke Cu–Ni sulfide mineral deposit, Xinjiang, Northwest (NW) China, yields an apparent isochron age of 433±31Ma with an apparent initial 187Os/188Os (433Ma) ratio of 0.197±0.027. This apparent age is older than not only the zircon U–Pb age of the host intrusion (287±5Ma, Han et al., 2004) but also the stratigraphic age of the intruded country rock. Thus, the regression line is a pseudo-isochron. However, previous Re–Os dating of massive ores of the same deposit yielded an age that is consistent, within analytical uncertainty, with the zircon U–Pb age (Zhang et al., 2008). This relationship is similar to that observed in the Jinchuan deposit, NW China. Therefore, we suggested that the same mechanism, post-segregation diffusion of Os (Yang et al., 2008), is applicable to the Kalatongke deposit.Re–Os isotopic studies of Kalatongke, Jinchuan and representative magmatic Cu–Ni sulfide deposits suggest that the massive ores of mafic–ultramafic-rock-associated Cu–Ni sulfide deposits would yield geologically meaningful Re–Os age, whereas a pseudo-isochron would be obtained for the disseminated ores. Therefore, to obtain a geologically meaningful Re–Os age, the type of the deposit, the type of the ore and the ore-forming process should be taken into account.

Re–Os isotope study of Indian kimberlites and lamproites: Implications for mantle source regions and cratonic evolution

We report the first Re–Os isotope data for samples from Mesoproterozoic kimberlite and lamproite occurrences from the Eastern Dharwar craton and end-Cretaceous Kodomali orangeite from the Bastar craton, India. The Re concentrations of the kimberlites (n=10) range from 0.077 to 0.290 parts per billion (ppb) whereas the Os contents vary from 0.312 to 1.98ppb. The measured 187Os/188Os ratios range from 0.1143 to 0.1655. The calculated initial 187Os/188Os ratios range from 0.0967 to 0.1260, with low values indicating Re mobility on the whole-rock scale since crystallisation. The orangeite has a measured 187Os/188Os ratio of 0.1342 and a slightly higher initial 187Os/188Os value of 0.1330. On the other hand, the lamproites (n=3) display lower Re (0.031 to 0.279ppb) and Os (0.060 to 0.129ppb) contents. Their measured 187Os/188Os ratios (0.2398 to 0.7521) and their initial 187Os/188Os ratios (0.1891 to 0.5499) are significantly higher. A majority of the kimberlites display γOsi of −2.7 to −22.7 and imply their interaction with a depleted or unradiogenic source such as cratonic lithosphere. A regression age (involving 187Re/188Os vs 187Os/188Os) of 1008±410Ma for eight least altered kimberlites corresponds, within the error limits, to their emplacement age (1.1Ga) and negates mixing of a highly radiogenic low Os melt, similar to the lamproites, and lithospheric mantle in their genesis. The most Re-depleted kimberlite sample with the lowest present-day 187Os/188Os (0.1143) yields a model age (TMA) of 3.2Ga and a Re-depletion model age (TRD) of 2Ga. The TRD age reveals: (i) the involvement of Proterozoic lithosphere in the genesis of the kimberlites, (ii) coupling of the continental crust of the Eastern Dharwar craton and the underlying sub-continental lithospheric mantle from 2.5Ga to at least 1.1Ga, and (iii) its similarity with the emplacement age of large igneous provinces of similar age and magmatism in the Eastern Dharwar and Bastar cratons (India), the Superior Craton (Canada) and the Kaapvaal craton (southern Africa). However, two of the kimberlite samples (from Raichur and Narayanpet kimberlite fields) show positive γOs values ranging from 2.9 to 3.6 suggesting involvement of multiple sources (e.g., subduction, plume or metasomatised lithosphere) in their genesis. An enriched mantle source for the Kodomali orangeite (γOs=3) is also supported by its lower εNdi values. The lamproites have very radiogenic γOsi values of 56 to 355, similar to those displayed by the lamproites of the Italian peninsula, and indicate a source with elevated Re/Os, possibly a subducted component. Our study provides strong evidence for contrasting mantle sources for kimberlites and lamproites in the Eastern Dharwar craton, southern India.

High-precision osmium isotopes in enstatite and Rumuruti chondrites

Isotopic heterogeneity within the solar nebula has been a long-standing issue. Studies on primitive chondrites and chondrite components for Ba, Sm, Nd, Mo, Ru, Hf, Ti, and Os yielded conflicting results, with some studies suggesting large-scale heterogeneity. Low-grade enstatite and Rumuruti chondrites represent the most extreme ends of the chondrite meteorites in terms of oxidation state, and might thus also present extremes if there is significant isotopic heterogeneity across the region of chondrite formation. Osmium is an ideal tracer because of its multiple isotopes generated by a combination of p-, r-, and s-process and, as a refractory element; it records the earliest stages of condensation. Some grade 3–4 enstatite and Rumuruti chondrites show similar deficits of s-process components as revealed by high-precision Os isotope studies in some low-grade carbonaceous and ordinary chondrites. Enstatite chondrites of grades 5–6 have Os isotopic composition identical within error to terrestrial and solar composition. This supports the view of digestion-resistant presolar grains, most likely SiC, as the major carrier of these anomalies. Destruction of presolar grains during parent body processing, which all high-grade enstatite chondrites, but also some low-grade chondrites seemingly underwent, makes the isotopically anomalous Os accessible for analysis. The magnitude of the anomalies is consistent with the presence of a few ppm of presolar SiC with a highly unusual isotopic composition, produced in a different stellar environment like asymptotic giant branch stars (AGB) and injected into the solar nebula. The presence of similar Os isotopic anomalies throughout all major chondrite groups implies that carriers of Os isotopic anomalies were homogeneously distributed in the solar nebula, at least across the formation region of chondrites.

Geophysical and geological tests of tectonic models of the North China Craton

The geometry and timing of amalgamation of the North China Craton have been controversial, with three main models offering significantly different interpretations of regional structure, geochronology, and geological relationships. One model suggests that the Eastern and Western Blocks of the NCC formed separately in the Archean, and an active margin was developed on the Eastern Block between 2.5 and 1.85 Ga, when the two blocks collided above an east-dipping subduction zone. A second presumes the Eastern Block rifted from an unknown larger continent at circa 2.7 Ga, and experienced a collision with an arc (perhaps attached to the western block) above a west-dipping subduction zone at 2.5 Ga, and the 1.85 Ga metamorphism is related to a collision along the northern margin of the craton when the NCC joined the Columbia supercontinent. A third model suggests two collisions in the Central Orogenic Belt, at 2.1 and 1.88 Ga, but recognizes an early undated deformation event. Recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models, showing that subduction beneath the Central Orogenic Belt was west-directed, and that there is a second, west-dipping paleosubduction zone located to the east of the COB dipping beneath the Western Block (Ordos Craton). The boundaries identified through geophysics do not correlate with the boundaries of the Trans-North China Orogen suggested in the first model, and the subduction polarity is opposite that predicted by that model. High-pressure granulite facies metamorphism at 1.85 Ga is not restricted to the “TNCO” as suggested by the first model, but is documented across the NCC, as predicted by the second model, suggesting a major continent–continent collision along the north margin of the craton at 1.85 Ga. Further, it has recently been shown that in the southern “TNCO”, there is no record of metamorphism at circa 1.85 Ga, but only at 2.7–2.5 Ga, showing that the “TNCO”, as defined as a circa 1.85 Ga orogen, does not exist. This is further confirmed by recent Re–Os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern COB is late Archean in age, and that a province in the northern NCC is circa 1.8 Ga, correlating with the proposed collision belt of the NCC with the Columbia supercontinent across the entire NCC. The COB is an Archean convergent belt, re-worked in the Paleoproterozoic, and the Paleoproterozoic tectonism is widespread across the NCC, as predicted by the model whereby the previously amalgamated Eastern and Western Blocks experienced a continental collision with Columbia at circa 1.85 Ga, but uplift/exhumation rates are slow, necessitating a re-evaluation of the tectonic models of the NCC.

The influence of small-scale mantle heterogeneities on Mid-Ocean Ridge volcanism: Evidence from the southern Mid-Atlantic Ridge (7°30′S to 11°30′S) and Ascension Island

Volcanism along Mid-Ocean Ridges is known to exhibit significant isotopic and elemental variations, traditionally regarded as reflecting global scale variations in mantle depletion history and refertilization processes. The ∼ 400 km long Mid-Atlantic Ridge (MAR) between the Ascension and Bode Verde Fracture Zones (7°30′S to 11°30′S) has been sampled at high spatial resolution (c. 10 km along-axis scale) and large elemental and isotopic variations have been identified covering almost the entire compositional spectrum of MAR basalts. In this paper we employ a multi-isotope (Sr–Nd–Hf–Pb) and trace element approach in order to explore the geodynamic implications of along-ridge compositional variations and their relationship to off-axis volcanism on nearby Ascension Island, commonly regarded as plume-related. The studied portion of the MAR consists of four major segments. Isotopic data from the deeply incised northern and southern segments A1 and A4 define a trend involving a high-εHf depleted mantle endmember. This isotopic signature is inferred to result from an ancient melting event in the garnet stability field, causing high time-integrated Lu/Hf and elevated 176Hf/ 177Hf. In contrast, a low-εHf depleted mantle endmember is indicated by the compositional trend for samples from the topographically elevated central portion (segments A2 and A3). Both trends converge at a common enriched endmember with least diluted compositions represented by samples from the subaerial eruptive stage of nearby Ascension Island. Because linear co-variations between isotope compositions and the respective parent–daughter ratios cannot be explained as representing “mantle isochrons” we infer that our data reflect arrays related to physical mixing of depleted and enriched mantle domains. This implies that distinctive mantle domains at kilometer scale may survive convection processes over time spans in the order of 1–2 Ga. This finding corroborates the results of recent Os-isotope studies of abyssal peridotite which returned similar conclusions. The systematics of along-axis isotopic variations show that mantle upwelling is highly variable. A systematic decrease of the enriched isotopic signature from the central segment A3 toward the northern termination of the segment A2 suggests a northward flow of enriched mantle material. In marked contrast, the compositional variations along the marginal segments A1 and A4 are random indicating heterogeneous mixing of mantle domains at a spatial resolution of < 50 km to 10 km (along-axis scale). With regard to Ascension Island we show that the submarine volcanic stage, sampled by a 3216 m long drill hole, was fed by a distinctive enriched mantle source currently inaccessible to partial melting. We interpret this observation within the framework of recent local-scale geophysical investigations and infer an on-axis origin of the bulk of the Ascension volcanic edifice at around 5 to 6 Ma, synchronous with the surrounding oceanic crust. Off-axis partial melting of the current common enriched mantle endmember accounts for rejuvenation of volcanism on Ascension generating the volumetrically subordinate subaerial portion of the island.

The continental lithosphere–asthenosphere boundary: Can we sample it?

The lithosphere–asthenosphere boundary (LAB) represents the base of the Earth's lithosphere, the rigid and relatively cool outer shell characterised by a conductive thermal regime, isolated from the convecting asthenosphere. Chemically, the LAB should divide a lithospheric mantle that is variably depleted in basaltic components from a more fertile asthenosphere. In xenolith suites from cratonic areas, the bottom of the depleted lithosphere is marked by a rapid downward increase in elements such as Fe, Ca, Al, Ti, Zr and Y, and a rapid decrease in the median Mg# of olivine, reflecting the infiltration of mafic melts and related fluids. Eclogites and related mafic and carbonatitic crystallisation products are concentrated at the same depths as the maximum degrees of metasomatism, and may represent the melts responsible for this refertilisation of the lithosphere. This refertilised zone, at depths of ca 170–220 km beneath Archean and Proterozoic cratons, is unlikely to represent a true LAB. Re–Os isotopic studies of the deepest fertile peridotite xenoliths show that they retain evidence of ancient depletion events; seismic tomography data show high-velocity material extending to much greater depths beneath cratons. The cratonic “LAB” probably represents a level where asthenospheric melts have ponded and refertilised the lithosphere, rather than marking a transition to the convecting asthenosphere. Our only deeper samples are rare diamond inclusions and some xenoliths inverted from majoritic garnet, which are unlikely to represent the bulk composition of the asthenosphere. In younger continental regions the lithosphere–asthenosphere boundary is shallower (commonly at about 80–100 km). In regions of extension and lithosphere thinning (e.g. eastern China, eastern Australia, Mongolia), upwelling asthenosphere may cool to form the lowermost lithosphere, and may be represented by xenoliths of fertile garnet peridotites in alkali basalts. The LAB is a movable boundary. It may become shallower due to thermal and chemical erosion of the lithosphere, assisted by extension. Refertilised lithospheric sections, especially where peridotites are intermixed with eclogite, may be capable of gravitational delamination. The lithosphere–asthenosphere boundary may also be deepened by subcretion of upwelling hot mantle (e.g. plumes). This process may be recorded in the strongly layered lithospheric mantle sections seen in the Slave Craton (Canada), northern Michigan (USA) and the Gawler Craton (Australia).

A major element, PGE and Re–Os isotope study of Middle Atlas (Morocco) peridotite xenoliths: Evidence for coupled introduction of metasomatic sulphides and clinopyroxene

We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco. Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al 2O 3 abundances in these xenoliths appear elevated relative to primitive mantle (P rima). The Al 2O 3 abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO 2 in these peridotites is distinctly different from fO 2 conditions observed in subduction zones. The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to P rima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO 2, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al 2O 3, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from 187Os/ 188Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from 187Re/ 188Os but, together with Re abundances, also exhibit a good correlation with Al 2O 3, MgO and Cu. The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.

Re–Os systematics of the Raobazhai peridotite massifs from the Dabie orogenic zone, eastern China

The Raobazhai ultramafic massif of the ultrahigh pressure Sulu–Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re–Os isotopic study of peridotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and 187Os/ 188Os ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. 187Os/ 188Os is roughly correlated with 187Re/ 188Os, and strongly correlated with Al 2O 3. These correlations can be explained by radiogenic ingrowth of 187Os since an ancient partial melting event. T MA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the 187Os/ 188Os vs. Al 2O 3 correlation at ~1% Al 2O 3. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re–Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the 187Os/ 188Os vs. 187Re/ 188Os correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids. Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between 187Os/ 188Os and Al 2O 3, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones.

An integrated petrological, geochemical and Re–Os isotope study of peridotite xenoliths from the Argyle lamproite, Western Australia and implications for cratonic diamond occurrences

An integrated study of the petrology and Re–Os geochemistry of a suite of peridotite xenoliths, some carrying abundant diamonds, from the richly diamondiferous Argyle AK1 lamproite pipe provides definitive evidence for a depleted lithospheric root of Neoarchean age (T RD eruption ∼ 2.2–3.1 Ga) beneath the Proterozoic Halls Creek Orogen at the margin of the Kimberley Craton, Western Australia. The microdiamonds from the peridotitic xenoliths are similar in their properties to the minor population of small, commercial sized, peridotitic diamonds from Argyle, both formed in the Archean from isotopically mantle-like carbon. The major element bulk chemistry and mineral chemistry of the Argyle peridotites are slightly less depleted than Archean cratonic peridotites as a whole but similar to those reported from Neoarchean–Paleoproterozoic cratonic provinces. The Argyle peridotite xenoliths were derived from within the diamond stability field (1050–1300 °C and 4.9–5.9 GPa) near the base of the lithosphere (typically 160–200 km depth) with a geothermal gradient of 41.5 mW/m 2. This thick diamondiferous lithosphere, estimated at up to 225 km thick from present day seismic S-wave tomography, appears to have persisted since the time of eruption of the Argyle lamproite (∼ 1180 Ma). The existence of late Archean age lithosphere beneath the Argyle diamond pipe, in a region where no crustal rocks of Archean age are known, suggests a decoupling of the crust and mantle in the region of the Halls Creek Orogen, perhaps as a consequence of Paleoproterozoic (∼ 1.85 Ga) reworking and/or subduction at the margin of the Kimberley Craton. The confirmation of an Archean lithospheric root beneath the Argyle pipe at the margin of the Kimberley Craton seemingly conforms with “Clifford's Rule”, regarding the restriction of economic diamond deposits to those underlain by Archean cratons. However, Argyle owes its rich diamond grades not to its Neoarchean mantle roots but to the presence of richly diamondiferous eclogitic material accreted to the craton root during the Proterozoic.

Chromite and PGE deposits of mesoarchaean ultramafic-mafic suites within the greenstone belts of the Singhbhum craton, India: Implications for mantle heterogeneity and tectonic setting

The Archaean cratonic nuclei of the continents are important as they contain the most significant evidences for the evolution of Earth e.g. the greenstone sequences. In the Indian Shield, one of the important cratons is the Singhbhum craton, where nearly 95% of the Indian chromite deposits and only PGE deposits are located which are hosted within Mesoarchaean ultramafic-mafic rock sequences. The ultramafic units occur as sill like intrusions within the Iron Ore Group (IOG) greenstone belts and often associated with gabbroic intrusions. In the Nuasahi and Sukinda mining districts of these occurrences, detailed petrological, geochemical and isotopic studies have been carried out in the last decades. Petrological and geochemical studies indicate a supra-subduction zone (SSZ) tectonic settings in Archaean for the origin of these ultramafic-mafic sequences. The Os isotopic and platinum group element (PGE) geochemical studies of chromites from the two mining districts indicate presence of a subchondritic source mantle domain beneath and within the Singhbhum craton similar to the Zimbabwean craton of southern African continent. The Os model age calculation indicates melt extraction from a subcontinental lithospheric mantle (SCLM) before 3.7 Ga which is similar to the other ancient cratons. As a whole the study supports the premise that India was part of the African continent in pre-Gondwana times and even in early Archaean and suggest possible amalgamation and building up of a supercontinent during late Archaean. However, in comparison with other occurrences, the Singhbhum craton of the Indian Shield and the Zimbabwean craton in southern Africa are characterized by the presence of subchondritic lithospheric mantle domains within the SCLM, which were developed prior to 3.7 Ga.

Os isotope systematics of mesoarchean chromitite-PGE deposits in the Singhbhum Craton (India): Implications for the evolution of lithospheric mantle

Os isotope studies of Mesoarchean (3.2 Ga) chromitite and associated gabbro–breccia hosted platinum-group element (PGE) deposits of the Singhbhum Craton in eastern India provide evidence for a complex history involving source mantle heterogeneity and hydrothermal processes. The average initial 187Os/ 188Os compositions of unaltered chromites of massive chromitite layers from two mining districts are 0.1031 +/− 0.0007 (Nuasahi) and 0.1029 +/− 0.0005 (Sukinda), with negative γOs values of − 1.78 +/− 0.65 and − 2.04 +/− 0.43, respectively. Mantle extraction ages (T MA) for these chromites indicate depletion from primitive mantle beginning as early as ∼ 3.7 Ga, some 400 Ma before the crystallization of the respective chromites. The subchondritic signatures of Os in the studied chromites are comparable with results of age-constrained chromites from ultramafic intrusions in the Zimbabwe Craton [Nägler, T.F., Kramers, J.D., Kamber, B.S., Frei, R., Prendergast, M.D.A., 1997. Growth of subcontinental lithospheric mantle beneath Zimbabwe started at or before 3.8 Ga: Re–Os study on chromites. Geology 25, 983–986.], interpreted to reflect sequential extraction from evolving Archean subcontinental lithospheric mantle (SCLM). The similarity of depleted Os isotopic signatures of chromites from the Singhbhum Craton with those from the Zimbabwean occurrences, and their compatibility with the proposed Kaapvaal Craton SCLM model, leads us to propose that SCLM beneath the Singhbhum Craton also started to form in the early Archean. Up to now, early Archean continental crustal fragments, expected to have formed with the SCLM in the Singhbhum Craton have not been found, and early Archean detrital zircons [Mishra, S., Deomurari, M.P., Wiedenbeck, M., Goswami, J.N., Ray, S., Saha, A.K., 1999. 207Pb/ 206Pb zircon ages and the evolution of the Singhbhum Craton, eastern India: an ion microprobe study. Precambrian Res. 93, 139–151.] remain the only evidence for their existence. An alternative possibility is that the early Archean depleted mantle region could have remained within the convecting mantle for a few hundred million years before being added to the SCLM beneath the Singhbhum Craton. Re–Os isotopic data of chromites of massive chromitites from PGE and base metal sulfide (BMS) mineralized breccia zones are extremely heterogeneous. Perturbations of the Re–Os isotope systematics in these chromites are best explained as having resulted from interaction with hydrothermal fluids percolating in the shear zones, possibly genetically related to the emplacement of 3.1 Ga gabbroic suites. These fluids played a major role for the genesis and modification of the PGE mineralization.