Sr Isotope Systematics Research Articles

Evaluation of Mantle Processes in an Extensional Regime: Insight from In Situ O and Sr Isotope Systematics of Mantle Xenoliths from Ethiopia

AbstractNew O and Sr isotope compositions of minerals from well-characterized and variably metasomatized mantle xenoliths in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia were acquired to further evaluate the nature and effects of metasomatic processes affecting the lithospheric mantle beneath the East African Rift System (EARS), where a transition from plume impingement to rifting is recorded. The studied samples are dominantly fresh spinel lherzolites with subordinate spinel harzburgites and clinopyroxenites. The in situ δ18O composition of olivine is largely homogeneous, with average values of 5.31‰ for Gundeweyn, 5.22‰ for Dillo, and 5.25‰ for Megado xenoliths. The values are within the range of normal mantle olivine (δ18O = 5.1‰–5.4‰), and there is no systematic variation of δ18O values with petrographic characteristics, elemental compositions, Sr isotopic ratios, or tectonic setting (i.e., plateau [Gundeweyn] vs. rift [Dil...

C, O, Sr and Nd isotope systematics of carbonates of Papaghni sub-basin, Andhra Pradesh, India: Implications for genesis of carbonate-hosted stratiform uranium mineralisation and geodynamic evolution of the Cuddapah basin

The Cuddapah basin (CB) is one of a series of Proterozoic basins that overlie the Archaean cratons of India, and contains a unique stratiform carbonate-hosted uranium mineralisation. In the present work, we discuss stable (C, O) and radiogenic (Nd, Sr) isotope systematics of carbonates of the Papaghni sub-basin in order to understand uranium ore forming processes and geodynamic evolution of the CB. Uranium mineralised dolomites (UMDs) of the basal Vempalle Formation show a significantly lighter (~1.5‰) C-isotope signature compared to that of open-marine stromatolitic sub-tidal facies, suggesting input of isotopically lighter carbon through in situ remineralisation of organic matter (OM). This implies deposition in a hydrologically-restricted, redox-stratified lagoonal basin wherein exchange with open oceanic dissolved inorganic carbon (DIC) was limited. Persistent bottom water anoxia was created and maintained through consumption of dissolved oxygen (DO) by decaying OM produced in oxidised surface water zone. Significantly more radiogenic εNd(t) of UMD (−6.31±0.54) compared to that of Dharwar upper crust (−8.64±3.11) indicates that dissolved constituents did not originate from the Dharwar craton, rather were derived from more juvenile exotic sources — possibly from a continental arc. Dissolved uranyl ions (U+6) were introduced to the basin through fluvial run-off and were reduced to immobile uranous ions (U+4) at the redox interface resulting in precipitation of pitchblende and coffinite. Carbonate horizons of upper Vempalle Formation and Tadpatri Formation show progressively more radiogenic Nd isotope compositions signifying increased juvenile arc contribution to the Papaghni sub-basin through time, which is also corroborated by the presence of younger zircons (1923±22Ma) in Pulivendla quartzites. We propose that the Papaghni sub-basin opened as a back-arc extensional basin at ~2Ga as a result of westerly-directed subduction of oceanic crust beneath the eastern Indian continental margin. The ‘Papaghni’ back-arc basin eventually evolved to ‘upper Cuddapah’ foreland basin with collision of the Dharwar craton and Napier block of Antarctica at ~1.6Ga, possibly, during the final stage of amalgamation of the Columbia supercontinent.

Geochemistry, Nd, Pb and Sr isotope systematics, and U–Pb zircon ages of the Neoarchean Bad Vermilion Lake greenstone belt and spatially associated granitic rocks, western Superior Province, Canada

The ca. 2720Ma Neoarchean Bad Vermilion Lake (BVL) greenstone belt, in the western Superior Province, Canada, is composed of a suite of tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gabbros, and Timiskaming-type siliciclastic sedimentary rocks. The greenstone belt was intruded by Neoarchean granitic rocks, and underwent greenschist facies metamorphism and intense deformation, resulting in mobilization of many elements (e.g., Rb, Ba, Sr, K, U, Pb).The high-field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks, and gabbros are consistent with subduction zone geochemical signatures, suggesting that the BVL greenstone belt formed in a magmatic arc setting. On the basis of lithological associations and trace element systematics, the BVL greenstone belt is defined as a fragment of a Neoarchean subduction-related ophiolite. Three rhyolite samples from the belt have yielded 2722±18Ma, 2706±13Ma and 2710±28Ma U–Pb zircon ages, representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western Wabigoon terrane to the north and the Wawa-Abitibi terrane to the south. The intrusion of the ca. 2664±15Ma late- to post-tectonic, potassic Ottertail Lake granite marks the end of tectonic accretion in the study area.Both the volcanic rocks and gabbros display large ranges of Nd (143Nd/144Nd=0.511600–0.512849; εNd (2720Ma)=+0.8 to+4.0), Pb (206Pb/204Pb=13.80–60.67) and Sr (87Sr/86Sr=0.701481–1.01154) isotopic compositions, suggesting that these isotope systems were variably affected by post-magmatic element mobility. Neither the Sm–Nd (2921±200Ma) nor Rb–Sr (2130±610Ma) system has yielded reliable regression (isochron) ages, reflecting the open-system behavior of these systems during metamorphism. Despite large uncertainties, Pb–Pb regression ages yielded by all rock types (2661±60Ma), and basalts and gabbros (2725±83Ma) agree with the zircon U–Pb ages of the rhyolites, suggesting that the U–Pb system was the most robust among all three systems.

Fluid sources in the Twangiza–Namoya Gold Belt (Democratic Republic of Congo): Evidence from tourmaline and fluid compositions, and from boron and Rb–Sr isotope systematics

The Twangiza–Namoya Gold Belt (TNGB) developed in a Proterozoic, variably composed sedimentary sequence that is associated with felsic and mafic igneous rocks. During polyphase tectono-metamorphic episodes the basement was exposed to repeated periods of heat influx that mobilised hydrothermal fluids and, at the Meso- to Neoproterozoic boundary, led to the formation of “G4” granites and pegmatites. The evolution of the TNGB hydrothermal system and its associated gold mineralisation are poorly understood. Here we investigate the tourmaline major elemental and the boron isotopic composition from one pegmatite and seven tourmaline-bearing hydrothermal veins sampled from the Kamituga and the Lugushwa gold deposits.Tourmalines from the hydrothermal veins are essentially dravitic with variable Ca contents. In contrast, the pegmatitic tourmaline is schorl–elbaite dominated and largely Ca-free. The boron isotopic signature of hydrothermal tourmaline from the Kamituga deposit (δ11B: −18.4‰ to −15.3‰) is distinctly lighter than that from Lugushwa (δ11B: −13.0‰ to −8.9‰). Pegmatitic tourmaline from Kamituga shows intermediate δ11B values between −14.2‰ and −13.1‰ with an average of −13.9‰. The pegmatitic fluid most likely had a δ11B composition in the range of −12.8‰. The isotopically significantly lighter boron in tourmaline from hydrothermal veins from Kamituga cannot have formed by fractionation from this pegmatitic source.Using the Rb–Sr mineral isochron method we have dated the Kamituga pegmatite as 981±16Ma, which correlates in time with the regional late-Kibaran “G4” granites. Tourmaline-bearing veins from Lugushwa are significantly younger and formed during the early stages of the Pan-African orogenic period (677±17Ma; Rb–Sr mineral data).Fluid inclusions in quartz that are texturally associated with hydrothermal tourmaline contain abundant saline brines and CO2-rich fluids that are commonly associated with CH4 and in places with amorphous carbon and N2. We interpret these fluids as having been derived from the hosting sedimentary sequence. This model is in agreement with the observed tourmaline composition, which are most readily affiliated with a variably Ca- and Al-rich pelitic source. Our model based on the isotopic, age and mineral compositional evidence indicates a sedimentary source of fluids forming the tourmaline-bearing hydrothermal veins in Kamituga and Lugushwa.

In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult.In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~1060°C and 43kbar (~140–145km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca–Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na–K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts.The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.

Growth of a Pleistocene giant carbonate vein and nearby thermogene travertine deposits at Semproniano, southern Tuscany, Italy: Estimate of CO2 leakage

A giant carbonate vein (≥50m thick; fissure ridge travertines) and nearby travertine plateaus in the Semproniano area (Mt. Amiata geothermal field, southern Tuscany, Italy) are investigated through a multidisciplinary approach, including field and laboratory geochemical analyses (U/Th geochronology, C, Nd, O and Sr isotope systematics, REE abundances, and fluid inclusion microthermometry). The main aim of this work is to understand: (1) modes and rates for the growth of the giant vein and nearby travertine deposits within a Quaternary volcano-tectonic domain; (2) implications in terms of the CO2 leakage; and (3) possible relationships with Quaternary paleoclimate and hydrological oscillations. Results show that the giant vein was the inner portion of a large fissure ridge travertine and grew asymmetrically and ataxially through repeated shallow fluid injections between >650 and 85ka, with growth rates in the 10−2–10−3mm/a order. The giant vein developed mainly during warm humid (interglacial) periods, partially overlapping with the growth of nearby travertine plateaus. Estimated values of CO2 leakage connected with the vein precipitation are between about 5×106 and 3×107mola−1km−2, approximately representing one millionth of the present global CO2 leakage from volcanic areas. Temperature estimates obtained from O-isotopes and fluid inclusion microthermometry indicate epithermal conditions (90–50°C) for the circulating fluid during the giant vein growth, with only slight evidence of cooling with time. Geochemical and isotope data document that the travertine deposits formed mainly during Pleistocene warm humid periods, within a tectonically-controlled convective fluid circuit fed by meteoric infiltration and maintained by the regional geothermal anomaly hosted by the carbonate reservoir of the Mt. Amiata field.

He, Ne and Ar isotope signatures of mid-ocean ridge basalts and their implications for upper mantle structure: A case study from the Mid-Atlantic Ridge at 4–12°S

The geochemical structure of the upper mantle in general and its noble gas isotopic structure in particular have been the subject of countless studies, as both provide important insights into mantle dynamic processes and are essential for the formulation of mantle geodynamic models. This contribution presents a noble gas study of basaltic glasses derived from the Mid-Atlantic Ridge (MAR) between 4° and 12°S, an area well known for its high degree of lithophile isotope heterogeneity and its anomalous crustal thickness. The Sr, Nd, Pb and Hf isotopic systematics along this segment of the MAR range from strongly depleted (i.e. more depleted than N-MORB) to highly enriched. Different concepts have been proposed to explain the observed isotopic signatures. Here we show that the high degree of heterogeneity is not confined to the isotopes of the lithophile elements, but is also shown by the noble gas isotopes and noble gas interelement ratios, such as 3He/22NeM or 4He/40Ar∗. Helium isotopic ratios (3He/4He), 21Ne/22Neextra and 40Ar/36Ar range from 7.34±0.06 to 9.38±0.08RA, from 0.039±0.020 to 0.075±0.007, and from 346±5 to 37,400±1300, respectively. However, the majority of the Ne isotope data are clearly aligned along a single mixing line in the Ne three-isotope diagram, represented by the equation 20Ne/22Ne=70.5×21Ne/22Ne+7.782, with a slope distinctly less than that of the MORB line. This indicates that the strongly depleted material is characterised by a significantly more nucleogenic Ne isotopic composition than the normal depleted mantle. We show, based on covariations between 3He/4He and 21Ne/22Neextra with 206Pb/204Pb and 178Hf/177Hf that the strongly depleted material erupted at this MAR segment was most likely produced by an ancient melting event. This implies that isotopic heterogeneities in the upper mantle are not solely caused by the injection of enriched materials from deep-seated mantle plumes or by crustal recycling but may also be due to differences in the depth and degree of melting of upper mantle material over the lifetime of our planet. In addition, the observed along-ridge and off-axis isotope patterns in conjunction with spatial distribution patterns of off-axis seamounts in the area show that the upper mantle can be extremely heterogeneous in space and time even at small scales. This implies that the mixing efficiency of either large- or small-scale convection processes in the upper mantle might be lower than assumed.

88Sr/86Sr fractionation in inorganic aragonite and in corals

Conflicting results have been reported for the stable Sr isotope fractionation, specifically with respect to the influence of temperature. In an experimental study we have investigated the stable Sr isotope systematics for inorganically precipitated and biogenic (coral) aragonite (natural and laboratory-cultured). Inorganic aragonite precipitation experiments were performed from natural seawater using the CO2 diffusion technique. The experiments were performed at different temperatures and different carbonate ion concentrations. 88Sr/86Sr of the inorganic aragonite precipitated in the experiments are 0.2‰ lighter than seawater, but showed no correlation to the water temperature or to CO32− concentration. Similar observations are made in different coral species (Cladocora caespitosa, Porites sp. and Acropora sp.), with identical fractionation from the bulk solution and no correlation to temperature or CO32− concentration. The lack of 88Sr/86Sr variability in corals at different environmental parameters and the similarity to the 88Sr/86Sr fractionation in inorganic aragonite may indicate a similar Sr incorporation mechanism in corals skeleton and inorganic aragonite, and therefore the previously proposed Rayleigh-based multi element model (Gaetani et al., 2011) cannot explain the process of Sr incorporation in the coral skeletal material. It is proposed that the relatively constant 88Sr/86Sr fractionation in aragonite can be used for paleo reconstruction of seawater 88Sr/86Sr composition. The seawater 88Sr/86Sr ratio reconstruction can be further used in calcite samples to reconstruct paleo precipitation rates.

Revealing magma degassing below closed-conduit active volcanoes: Geochemical features of volcanic rocks versus fumarolic fluids at Vulcano (Aeolian Islands, Italy)

The elemental and isotopic compositions of noble gases (He, Ne, and Ar) in olivine- and clinopyroxene-hosted fluid inclusions have been measured for rocks at various degrees of evolution and belonging to high-K calcalkaline–shoshonitic and shoshonitic–potassic series in order to cover the entire volcanological history of Vulcano Island (Italy). The major- and trace-element concentrations and the Sr- and Pb-isotope compositions for whole rocks were integrated with data obtained from the fluid inclusions. 3He/4He in fluid inclusions is within the range of 3.30 and 5.94R/Ra, being lower than the theoretical value for the deep magmatic source expected for Vulcano Island (6.0–6.2R/Ra). 3He/4He of the magmatic source is almost constant throughout the volcanic history of Vulcano. Integration of the He- and Sr-isotope systematics leads to the conclusion that a decrease in the He-isotope ratio of the rocks is mainly due to the assimilation of 10–25% of a crustal component similar to the Calabrian basement. 3He/4He shows a negative correlation with Sr isotopes except for the last-erupted Vulcanello latites (Punta del Roveto), which have anomalously high He isotope ratios. This anomaly has been attributed to a flushing process by fluids coming from the deepest reservoirs, since an input of deep magmatic volatiles with high 3He/4He values increases the He-isotope ratio without changing 87Sr/86Sr. A comparison of the He-isotope ratios between fluid inclusions and fumarolic gases shows that only the basalts of La Sommata and the latites of Vulcanello have comparable values. Taking into account that the latites of Vulcanello relate to one of the most-recent eruptions at Vulcano (in the 17th century), we infer that the most probable magma which actually feeds the fumarolic emissions is a latitic body that ponded at about 3–3.5km of depth and is flushed by fluids coming from a deeper and basic magma.

Strontium isotope systematics of scheelite and apatite from the Felbertal tungsten deposit, Austria – results of in-situ LA-MC-ICP-MS analysis

The in-situ Sr isotopic systematics of scheelite and apatite from the Felbertal W deposit and a few regional Variscan orthogneisses (“Zentralgneise”) have been determined by LA-MC-ICP-MS. The 87Sr/86Sr ratios of scheelite and apatite from the deposit are highly radiogenic and remarkably scattering. In the early magmatic-hydrothermal scheelite generations (Scheelite 1 and 2) the 87Sr/86Sr ratios range from 0.72078 to 0.76417 and from 0.70724 to 0.76832, respectively. Metamorphic Scheelite 3, formed by recrystallisation and local mobilisation of older scheelite, is characterised by even higher 87Sr/86Sr values between 0.74331 and 0.80689. Statistics allows discriminating the three scheelite generations although there is considerable overlap between Scheelite 1 and 2; they could be mixtures of the same isotopic reservoirs. The heterogeneous and scattering 87Sr/86Sr ratios of the two primary scheelite generations suggest modification of the Sr isotope system due to fluid-rock interaction and isotopic disequilibrium. Incongruent release of 87Sr from micas in the Early Palaeozoic host rocks of the Habach Complex contributed to the solute budget of the hydrothermal fluids and may explain the radiogenic Sr isotope signature of scheelite. Spatially resolved analyses revealed isotopic disequilibrium even on a sub-mm scale within zoned Scheelite 2 crystals indicating scheelite growth in an isotopic dynamical hydrothermal system. Zoned apatite from the W mineralised Early Carboniferous K1-K3 orthogneiss in the western ore field yielded 87Sr/86Sr of 0.72044–0.74514 for the cores and 0.74535–0.77937 for the rims. Values of magmatic apatite cores from the K1-K3 orthogneiss are comparable to those of primary Scheelite 1; they are too radiogenic to be magmatic. The Sr isotopic composition of apatite cores was therefore equally modified during the hydrothermal mineralisation processes, therefore supporting the single-stage genetic model in which W mineralisation is associated with the intrusion of the K1-K3 metagranitoid at Felbertal. The subsequent regional metamorphic overprint of the deposit caused redistribution of 87Sr as a consequence of metamorphic reactions involving Rb and Sr-bearing minerals. Metamorphic Scheelite 3 and apatite rims (e.g., in the K1-K3 orthogneiss) generally became more radiogenic during this process. However, local recrystallisation of primary scheelite under closed conditions (without addition of 87Sr by the metamorphic fluid) is also documented. The latter process resulted in a homogenisation of the isotope composition of Scheelite 3. Further increase in 87Sr/86Sr ratios in Scheelite 3 and apatite rims is attributed to Late Alpine (?) metamorphic recrystallisation and redistribution of 87Sr by metamorphic fluids.

Decoupling of the Lu–Hf, Sm–Nd, and Rb–Sr isotope systems in eclogites and a garnetite from the Sulu ultra-high pressure metamorphic terrane: Causes and implications

The whole-rock Hf, Sr and Nd isotope data of five high-Fe–Ti eclogites, nine high-Al eclogites, and a garnetite from the Sulu ultrahigh pressure (UHP) metamorphic terrane at eastern China were analyzed to resolve the causes for the decoupling of the Lu–Hf, Sm–Nd, and Rb–Sr isotope systems in these UHP rocks and to infer their protolith characteristics. Seven of the nine high-Al eclogites define an 87Rb/86Sr–87Sr/86Sr errorchron age of 192±43Ma (MSWD=2.8), which is within the time span of retrograde metamorphism despite the large uncertainty. The high-Fe–Ti eclogites and garnetite, however, have low 87Rb/86Sr ratios of <0.031 with scattered 87Sr/86Sr ratios of 0.7042–0.7058. Accordingly, it is inferred that the Rb–Sr isotope system in the samples reflects the effects of processes postdating the UHP metamorphism. Despite having different constituent mineral assemblages and whole rock geochemistry, the samples, however, define a 147Sm/144Nd–143Nd/144Nd errorchron age of 232±36Ma (MSWD=4.6). Although the uncertainty of ±36Ma implies incomplete Nd isotope equilibrium among the samples, the large overlap between this errorchron age span and the mineral isochron ages of 245–210Ma for the UHP metamorphism indicates the control of peak metamorphism on the Sm–Nd isotope system. The incomplete Nd isotope re-equilibration was accompanied by metamorphic modification on the Sm/Nd ratios as indicated by the U-shaped LREE patterns. The initial εNd(780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial εNd(t)–εHf(t) trend to significantly lower values, consistent with the metamorphic increase in the Sm/Nd ratios. In contrast, the Lu/Hf ratios are generally within the range for basalts and do not vary systematically with the 176Hf/177Hf ratios. The protolith εHf(780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial εHf(t) values of the ~780Ma magmatic zircon cores from the Yangtze craton, suggesting the dominance of protolith characteristics on the Lu–Hf isotope system. Being controlled by different processes, the Rb–Sr, Sm–Nd, and Lu–Hf isotope systems of the investigated samples are therefore decoupled. The εNd(220)–εHf(220) compositions of the samples deviate from the εNd(t)–εHf(t) trend of intraplate lavas to high εHf(220) at a given εNd(220) and are within the arc lava field. The protolith εNd(780)–εHf(780) compositions calculated from metamorphic initials using igneous Sm/Nd and Lu/Hf ratios show a similar distribution pattern, strengthening the arc affinity of the eclogites. These arc signatures together with the felsic–mafic bimodal geochemical features of the UHP rocks from the Sulu terrane are explained as the characteristics of protoliths generated by backarc rifting.

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif, Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

O, H and Sr Isotopes Tracing the Migration Distance of Mining, Ore Processing and Metallurgical Activities End-products in a River Basin (Subarnarekha River, India)

This study reports on the geochemistry of surface- and groundwaters along the Subarnarekha River system (north-eastern India). Multi-isotopic approaches (Sr, O, H) have been applied for tracing the water origin and pathways and investigate how far pollutants released by mining, ore processing and metallurgical activities can migrate at the catchment scale. Results of the Sr isotope systematics did not provide any evidence of mining processes-derived minerals impacts on the water. Pollution from mining activities entering the river are transported to the estuary as non-impacting fine fractions.

Fluid inclusions and isotopic characteristics of the Jiawula Pb–Zn–Ag deposit, Inner Mongolia, China

The large Jiawula Pb–Zn–Ag deposit is located in the Derbugan metallogenic belt of the northern Great Xing’an Range. The vein style orebodies of the deposit occur along NWW- to NNW-trending fault zones. The ore-forming process at the deposit can be divided into three stages: an early quartz–pyrite–pyrrhotite–chalcopyrite stage, a middle quartz–carbonate–pyrite–galena–sphalerite stage, and a late quartz–carbonate–pyrite stage. Sulfide Rb–Sr dating indicates that the Jiawula deposit formed at ca. 143–142Ma. Four types of fluid inclusions have been distinguished in quartz veins including liquid-rich, gas-rich, H2O–CO2, and daughter mineral-bearing inclusions. The fluid inclusions of the early stage are mainly liquid-rich, gas-rich, and H2O–CO2 types, with a small number containing daughter minerals. Cumulatively, the types have homogenization temperatures, densities, and salinities of 304–438°C, 0.35–1.37g/cm3, and 0.8–44.6wt.% NaCl eqv., respectively. Inclusions of the middle stage are mainly liquid-rich and gas-rich types, with a small amount of H2O–CO2 and daughter mineral-bearing types; their homogenization temperatures, densities, and salinities vary from 242°C to 297°C, 0.71 to 1.44g/cm3, and 0.4wt.% to 36.8wt.% NaCl eqv., respectively. The late stage only comprises liquid-rich inclusions with homogenization temperatures, densities, and salinities of 181–238°C, 0.81–0.90g/cm3, and 0.2–1.9wt.% NaCl eqv., respectively. The ore-forming fluids of the Jiawula deposit are generally characterized by moderate temperature and low salinity and density, and belong to an H2O–NaCl–CO2±CH4 system. The δ18Owater values calculated for ore-bearing quartz vary from −13.4‰ to −9.1‰, and the δDV-SMOW values from bulk extraction of fluid inclusion waters vary from −166‰ to −133‰, suggesting that the ore-forming fluids mainly consist of meteoric water with a small amount of magmatic water. The δ34SV-CDT values range from 1.2‰ to 8.4‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ore minerals are in the ranges of 18.319–18.377, 15.499–15.596, and 38.094–38.314, respectively. Initial 87Sr/86Sr ratios of sulfides range from 0.712381 to 0.712770. The data for the S, Pb, and Sr isotopic systems indicate that the ore-forming metals and sulfur came from Mesozoic magma. The Jiawula deposit is a low-sulfidation epithermal Pb–Zn–Ag deposit, and fluid boiling is the dominant mechanism for the deposition of ore-forming materials.

Petrology and geochemistry of the Cretaceous granitoid magmatism of Central Kamchatka, exemplified by the Krutogorova and Kol’ intrusive complexes

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ∼80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.

Tracking mobility using human hair: What can we learn from lead and strontium isotopes?

The isotope ratios of strontium (Sr) and lead (Pb) in water derive from local geology and their isotopic signature can be linked to the age and type of underlying bedrocks and soils (if applicable), and are transferred up the food-chain. Both elements are transferred to human blood through diet and water, and some of it will ultimately be incorporated into the hair structure, making Sr and Pb isotopes interesting tools for tracing human mobility. In this study, we analyzed both the elemental concentration and isotope ratios of Sr and Pb from four different women of different ages to monitor their permanent relocation from central France to Eastern Canada during the summer 2012. For comparison, we also characterized bulk hair samples of sedentary individuals and local tap waters from their regions of origin and of settlement. Our results indicate that the 87Sr/86Sr and 206Pb/204Pb ratios are significantly modified by the change of environment, but also confirm that human hair compositions are impacted by other external factors (such as dust). Sr and Pb isotope systematics demonstrate their added value for detecting human mobility, but require further studies to better constrain the main sources and processes controlling their respective budgets in human hair for provenancing purposes.

Compositional and Sr–Nd–Hf isotopic variations of Baijingsi eclogites from the North Qilian orogen, China: Causes, protolith origins, and tectonic implications

Eclogites from the Baijingsi area in the North Qilian orogen at NW China were analyzed for major and trace element abundances as well as Sr, Nd, and Hf isotope ratios to evaluate the impacts from subduction processes on these compositional parameters and to reveal the complexity in protolith characteristics. The major element compositions of the Baijingsi eclogites are within the ranges for basalts. However, the absence of systematic variations among major oxides indicates varying extents of metamorphic modifications. Seven samples are characterized by profound Nb and Ta depletions. Six of them form coherent trends in the La versus Th, HFSE, and LREE plots. They are classified as the Group 1 samples. Other eight samples, referred to as the Group 2 samples, define distinct La–Zr and La–Sm trends and have flat variation patterns with slight Th–Nb–Ta–LREE depletions in the primitive mantle-normalized multiple-element diagram. Group 1 dominates the 87Sr/86Sr–87Rb/86Sr, 143Nd/144Nd–147Sm/144Nd and 176Hf/177Hf–176Lu/177Hf trends for relatively larger variations in the isotope and abundance ratios. Based on the errorchron ages from the samples, closure temperatures of the isotope systems, and isotope ratio versus 1/X plots (e.g., 87Sr/86Sr–1/Sr), it is inferred that the Rb–Sr isotope system was subjected to metamorphic modification, whereas the Sm–Nd and Lu–Hf isotope systems mainly reflect protolith characteristics. In addition to the Nb and Ta depletions, the Group 1 samples generally have εHf(510) values (protolith initial at 510Ma) higher than those of MORB at a given εNd(510), reflecting derivation from relatively high Lu/Hf ratio sources. They also plot within the fields for arc lavas in the Ta/Yb–Th/Yb and Th–Ta–Hf/3 diagrams. All these features are consistent with derivation from protoliths of an arc origin. In contrast, the Group 2 samples have εNd(510) values slightly higher than those of the Group 1 samples at a given εHf(510), plotting on the low εNd(510) margin of the MORB field. They also deviate from the Ta/Yb–Th/Yb trend defined by MORB and OIB toward higher Th/Yb values within the fields of back-arc basin basalts. Therefore, it is inferred that the protoliths of the Group 2 samples were generated as back-arc basin basalts possibly associated with the protoliths of the Group 1 samples in a common supra-subduction zone. The εNd(510)–εHf(510) relationship of the Baijingsi eclogites shows an affinity to the Indian ocean type mantle, implying protolith generation at the southern hemisphere on the margin or a fragment of the Gondwana continent, most likely in the Qilian Block. Based on the current tectonic configuration, a southward subduction is required for protolith generation and might eclogitize the arc protoliths by subduction erosion. Alternatively, the northward subduction in a bidirectional subduction system might become dominant during the amalgamation of the Qilian Block and the North China Craton to eclogitize the arc protoliths in the Qilian Block. Resolving these two possibilities requires more petrological, geochemical, and structural evidence from the North Qilian orogen.

Mineralogy of high-field-strength elements (Y, Nb, REE) in the world-class Vergenoeg fluorite deposit, South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040±46Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE+Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE+Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE+Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE+Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).

Sr, Nd, Pb and Os Isotope Systematics of CAMP Tholeiites from Eastern North America (ENA): Evidence of a Subduction-enriched Mantle Source

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 10e7km2. Here we document the geochemical characteristics of CAMP basalts from Triassic-Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr-Nd-Pb initial (t=201Ma) isotopic compositions (206Pb/204Pbini. = 18·155-18·691, 207Pb/204Pbini. = 15·616-15·668, 208Pb/204Pbini. = 38·160-38·616, 143Nd/144Ndini. = 0·512169-0·512499). On the basis of stratigraphy, rare earth element (REE) chemistry and Sr-Nd-Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206Pb/204Pbini. >18·5 and 143Nd/144Ndini.>0·51238, comprises all the latest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206Pb/204Pbini.>18·5 and 0·51233>143Nd/144Ndini.>0·51225, comprises the intermediate units. The Orange Mountain group has the highest La/Yb ratios and 143Nd/144Ndini.<0·51235 and involves all the earliest and stratigraphically lowest units, including the entire North Mountain basalts from Nova Scotia. In this last group, three sub-groups may be distinguished: the Rapidan sill, which has 206Pb/204Pbini. higher than 18·5, the Shelburne sub-group, which has 143Nd/144Ndini.<0·51225, and the remaining Orange Mt samples. With the exception of one sample, the Eastern North America (ENA) CAMP basalts display initial 187Os/188Os ratios in the range of mantle-derived magmas (<0·15). Simple modelling shows that the composition of the ENA CAMP basalts cannot plausibly be explained solely by crustal contamination of oceanic island basalt (OIB), mid-ocean ridge basalt (MORB) or oceanic plateau basalt (OPB) magmas. Mixing of such magma compositions with sub- continental lithospheric mantle (SCLM)-derived melts followed by crustal contamination, by either assimilation-fractional crystallization (AFC) or assimilation through turbulent ascent (ATA) pro- cesses is somewhat more successful. However, this latter scenario does not reproduce the REE and isotopic composition of the ENA CAMP in a fully satisfactory manner. Alternatively, we propose a model in which asthenospheric mantle overlying a subducted slab (i.e. mantle wedge) was enriched during Cambrian to Devonian subduction by sedimentary material, isotopically equivalent to Proterozoic-Lower Paleozoic crustal rocks. Subsequently, after subduction ceased, the isotopic composition of this mantle evolved by radioactive decay for another 170 Myr until the CAMP magmatic event. Varying amounts and compositions of the incorporated sedimentary component coupled with radiogenic ingrowth over time can account for the main geochemical characteristics of the ENA CAMP (enriched incompatible element patterns, negative Nb anomalies, enriched Sr-Nd-Pb isotopic composition) and the differences between the three chemical groups.

Sr, Nd, Hf and Pb isotope systematics of postshield-stage lavas at Kahoolawe, Hawaii

We report high-precision Sr, Nd, Hf and Pb isotope compositions for twenty-one postshield-stage and two post-caldera lavas from Kahoolawe, a Loa-trend Hawaiian volcano. Kahoolawe postshield- and shield-stage lavas have overlapping though highly heterogeneous Sr, Nd, Hf and Pb isotope compositions, implying that the shield- and postshield-stage volcanism at Kahoolawe sampled the same isotopically heterogeneous mantle source. This differs from that of most other Hawaiian volcanoes, such as Haleakala, Mauna Kea, and Hualalai, whose shield-to-postshield transitions are characterized by shifts to lower 87Sr/86Sr and higher 143Nd/144Nd. There are correlations between CaO, Sc and V contents and radiogenic isotope compositions within Kahoolawe postshield-stage lavas. For example, Sc abundance is negatively correlated with 87Sr/86Sr, and positively correlated with εNd and εHf; V abundance is positively correlated with εNd, εHf, and 206Pb/204Pb. Element-isotope correlations are also observed in Mauna Kea postshield-stage lavas: Sc and V abundances are negatively correlated with εHf and 206Pb/204Pb, and positively correlated with εNd. These trends may be due to magma–magma mixing. That is, in addition to clinopyroxene fractionation to account for the low CaO, Sc and V contents in some postshield-stage lavas, partial melts of eclogite/garnet pyroxenite, characterized by low CaO, Sc and V contents, may also be part of the petrogenesis of Kahoolawe postshield-stage lavas.It is well established that lavas erupted at the geographically defined Loa- and Kea-trend volcanoes have different isotopic and geochemical compositions. Specifically, compared to the Kea-trend lavas, Loa-trend lavas have higher 208Pb/204Pb at a given 206Pb/204Pb. However, cases exist of both shield- and postshield-stage volcanism where Kea-type isotopic signatures are present in Loa-trend volcanoes and the reverse. We propose that Loa- and Kea-type source components are present beneath both Loa- and Kea-trend volcanoes in such a way that the average source compositions of Loa-trend volcanoes have a Loa-type isotopic signature, and that of the Kea-trend volcanoes have a Kea-type isotopic signature. When the size of the magma capture zone is much larger than that of the source components, the erupted lavas have the average compositions of the source. If the size of the magma capture zone is comparable to that of the source components, the erupted lavas could have either Loa- or Kea-type isotopic signatures.