Abstract: Rhenium (Re) is one of the least abundant elements in Earth, averaging 0.28 ppb in the primitive mantle. The unique occurrence of rheniite ReS2 (74.5 wt% of Re) in Kudryavy volcano precipitates raises questions about recycling of Re-rich reservoirs within the KurileâKamchatka volcanic Island arc setting. The sources of this unique Re enrichment have been inferred from studies of ReâOs isotope systematic and trace elements in volcanic gases, sulphide precipitates and host volcanic rocks. The fumarolic gas condensates are enriched in hydrophile trace elements relative to fluid-immobile elements and exhibit high Ba/Nb (133â204), Rb/Y (16â406) and Th/Zr (0.01â0.25) ratios. They are characterised by high Re (7â210 ppb) and Os abundances (0.4â0.9 ppb), with 187Os/188Os ratios in a range 0.122â0.152. This Os isotopic compositional range is similar to that of the peridotite xenoliths from the metasomatised mantle wedge above the subducted Pacific plate, the radiogenic isotopic signature of which is probably due to radiogenic addition from a slab-derived fluid.
Re- and Os-rich sulphide and oxide minerals precipitate from volcanic gases within fumarolic fields. Molybdenite (MoS2), powellite (CaMoO4) and cannizzarite (Pb4Bi6S13) contain 1.5â1.7 wt%, 10 ppm, and 65â252 ppb of Re, respectively. Both molybdenite and rheniite contain normal Os concentrations, with total Os abundances in a range from 0.6 to 3.1 ppm for molybdenite, and 2.3â24.3 ppb for the rheniite samples. Repeated analyses of osmium isotope ratios for two rheniite samples form a best-fit line with an initial 187Os/188Os ratio of 0.32 ± 0.15 and an age of 79 ± 11 yr, which is the youngest age ever measured in natural samples. The high Re contents in molybdenite and rheniite led to high radiogenic 187Os values, even in the limited period of time, with 187Os/188Os ratios up to 3.3 for molybdenite and up to 4.4 for rheniite.
The Os isotopic compositions of andesiteâbasaltic rocks from the Kudryavy volcano (187Os/188Os up to 0.326) are more radiogenic than those of residual peridotites and fumarolic gas condensates that are mainly constituted from magmatic vapor. Such radiogenic values can be attributed either to the addition of a radiogenic Os-rich subduction component to the depleted mantle, or to the assimilation of older dacitic caldera walls (187Os/188Os = 0.6) during arc magma ascent and emplacement. The latter hypothesis is supported by the correlation between 187Os/188Os ratio and indicators of fractionation such as MgO or Ni, and by low contents of potentially hydrophile trace elements such as Ba, Rb and Th relative to fluid-immobile elements such as Nb, Zr and Y. The high Re flux in the Kudryavy volcano (estimated at 46 kg/yr) can be explained by remobilisation of Re by Cl-rich water from an underplated mantle wedge and subducted organic-rich sediments of the Pacific plate.
Notes: Related paper: Yudovskaya M.A., Tessalina S., Distler V.V., Chaplygin I.V., Chugaev A.V. and Dikov Y.P. (2008) Behavior of highly-siderophile elements during magma degassing: A case study at the Kudryavy volcano. Chemical Geology 248: 318-341.
Abstract: The capacity of natural vapor phase to transport metallic elements is not unambiguously established relative to that of a liquid hydrothermal phase. We measured highly-siderophile element (HSE) and Au abundances in gas condensates and mineralized rocks in order to examine the geochemical behavior of these elements during magma degassing at the Kudryavy volcano, Kurile Arc. Gas condensates of the Kudryavy volcano are enriched with Re, Os and Au (to 210 ppb Re, 0.907 ppb Os, 2.4 ppb Au, 0.49 ppb Pt, 0.4 ppb Pd, 0.04 ppb Ir, 0.07 ppb Rh, 0.009 ppb Ru). The measured enrichment factors demonstrate that Os is the element that is most strongly compatible with fluid. Fluid compatibility decreases in the sequence: OsNReNAuNPtNPd over the temperature
range from 480 to 850 °C. The mobility of HSE and Au in fluid is confirmed by the sublimation of their compounds, amongst which rheniite ReS2 and K perrhenate KReO4, native Pt, PtâPd selenide and various Au alloys have been identified with a scanning electron microscope [Nature 369 (1994) 51; Miner. Deposita 40 (2006) 828]. In addition, new HSE compounds, including ReO2, ReO3, Pt(OH)2 and metal-chloro-organic complexes, were detected in the sublimates using X-ray photoelectron spectroscopy. In contrast to the chalcophile behavior of Pb, Re and Os exhibit a dual behavior in the gaseous phase, since both sulfide
and oxide phases containing these metals precipitate throughout the entire temperature range. However, available mineralogical, experimental and thermodynamic modeling data indicate that Re and Os are preferentially transported as oxygen-bearing species. Data on metal contents in fumarolic crusts of the volcano confirm that a high-temperature low-density fluid can concentrate these metals to economic grade.
Newly obtained data on the Pb and SmâNd isotopic composition of volcanic gas condensates and host rocks were correlated with available data on Re and Os abundances and with the ReâOs isotopic composition of the same sample set in order to identify the possible sources of the magmatic melts. The homogeneity of the Pb and Nd isotopic composition of volcanic rocks (206Pb/204Pb: 18.33â18.41, 207Pb/204Pb: 15.52â15.54, 206Pb/204Pb: 38.19â38.24; n=6; 143Nd/144Nd: 0.513067â0.513118; n=5) indicates that the main source of the melts was metasomatised depleted MORB mantle. This is consistent with the relatively low radiogenic 187Os/188Os isotope ratios of younger basaltic andesites and fumarolic gas condensates, but is inconsistent with the radiogenic Os isotope characteristics of the acid volcanic rocks and the high Re abundance in rocks and fluids [Geochem. Cosmochem. Acta 72 (2008) 889]. The results of this study suggest that similar elemental and isotope HSE signature can be characteristic of HSE fractionation in other environments of low-density oxidizing fluid stability.
Notes: Available online at www.sciencedirect.com
www.elsevier.com/locate/chemgeo
Related paper: Tessalina S., Yudovskaya M.A., Chaplygin I.V., Birck J.-L. and Capmas F. (2008) Sources of unique rhenium enrichment in fumaroles and sulphides at Kudryavy volcano. Geochimica et Cosmochimica Acta 72: 889â909.
Abstract: The ReâOs distribution and isotopic composition have been studied within different ore facies, host-rocks and sediments from the Alexandrinka volcanogenic hydrothermal massive sulphide deposit, Southern Urals, Russia. The osmium contents increase and the initial 187Os/188Os isotopic compositions decrease in the ore facial range: sulphide chimneysâstockwork zoneâseafloor massive and clastic sulphidesâmetalliferous sediments. This range reflects variable degrees of reduced hydrothermal fluidâoxidized seawater mixing during the hydrothermal ore-forming process. The Os isotopic composition of the hydrothermal sulphide chimney (187Os/188Os = 1.3) is estimated to be a minimum value of the Devonian hydrothermal fluid, which is an intermediate between initial Os isotopic compositions of island-arc volcanics and interlayered sediments. The initial Os isotopic composition of metalliferous sediments (187Os/188Os 0.17â0.2) possibly reflects that of the Devonian seawater. The low rhenium concentrations in metalliferous sediments (about 1â2 ppb) could indicate oxic formation conditions.
The ReâOs isotope data define a best-fit line corresponding to a Late Devonian age of 355 ± 15 Ma (2Ï) with initial 187Os/188Os of 0.12 ± 0.19. This age could indicate a late Os isotope reequilibration due to ongoing hydrothermal fluid flow from the Givetian (stratigraphic age 375 Ma) until the closure of the Ural paleoocean in the Late Devonian. The Os contents are higher and Re/Os ratio is lower within Palaeozoic island-arc hosted Urals VHMS deposits compared with TAG deposit in MOR setting.
Abstract: Some Cu-rich, maficâultramafic- and ultramafic-hosted massive sulfide deposits from the southern segment of the Main Uralian Fault Zone (Ivanovka and Ishkinino deposits, southern Urals) show unusual characteristics. Their major features include: (i) relatively high Co (Ni, Au), very low Zn and negligible Pb grades; (ii) a pyrrhotite-dominated mineralization, locally characterized by the presence of open-latticework aggregates of lamellar pyrrhotite with Mg-saponite ± Mg-chlorite and carbonate matrix; (iii) hydrothermal alteration of ultramafic host rocks into talc ± carbonate ± quartz ± chlorite and of mafic host rocks into chloritites; (iv) the presence of clastic facies with reworked sulfide and ultramafic or mafic components; (v) the widespread occurrence of sulfide-associated chromite; (vi) the specific mineralogy of Co, Ni, Fe and As, including sulfoarsenides, mono- and diarsenides, and Co-rich pentlandite and pyrite; (vii) the supra-subduction-zone geochemical signature of the host serpentinites and volcanic rocks. Although some of these features have been separately reported in certain modern ocean-seafloor and ophiolite-hosted fossil deposits, a true equivalent has yet to be found. Based on recognized partial analogies with a few modern seafloor examples, the arc tholeiiticâboninitic geochemical signature of sulfide-associated volcanic rocks and the highly refractory compositions of sulfide-hosted chromite relicts, the studied deposits are believed to have formed by seafloorâsubseafloor hydrothermal processes in an oceanic island arc setting. Possible tectonostratigraphic correlation of sulfide-associated units with infant, non-accretionary arc volcanic units of the adjacent Magnitogorsk oceanic island-arc system suggests formation of the studied deposits during the earliest stages of Devonian subduction-related volcanism.
Abstract: ReâOs and SmâNd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions.
The ReâOs and SmâNd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals.
The oldest ReâOs age of 1250 ± 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent.
The younger SmâNd age of Mindyak peridotites (882 ± 83 Ma) and ReâOs age of associated gabbros (804 ± 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian arc in the Southern Urals.
A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation.
The combination of 187Reâ187Os and 147Smâ143Nd systematics for peridotites, maficâultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation.
Abstract: VMS deposits of the South Urals developed within the evolving Urals palaeo-ocean between Silurian and Late Devonian times. Arc-continent collision between Baltica and the Magnitogorsk Zone (arc) in the south-western Urals effectively terminated submarine volcanism in the Magnitogorsk Zone with which the bulk of the VMS deposits are associated. The majority of the Urals VMS deposits formed within volcanic-dominated sequences in deep seawater settings. Preservation of macro and micro vent fauna in the sulphide bodies is both testament to the seafloor setting for much of the sulphides but also the exceptional degree of preservation and lack of metamorphic overprint of the deposits and host rocks. The deposits in the Urals have previously been classified in terms of tectonic setting, host rock associations and metal ratios in line with recent tectono-stratigraphic classifications. In addition to these broad classes, it is clear that in a number of the Urals settings, an evolution of the host volcanic stratigraphy is accompanied by an associated change in the metal ratios of the VMS deposits, a situation previously discussed, for example, in the Noranda district of Canada.
Two key structural settings are implicated in the South Urals. The first is seen in a preserved marginal allochthon west of the Main Urals Fault where early arc tholeiites host CuâZn mineralization in deposits including Yaman Kasy, which is host to the oldest macro vent fauna assembly known to science. The second tectonic setting for the South Urals VMS is the Magnitogorsk arc where study has highlighted the presence of a preserved early forearc assemblage, arc tholeiite to calc-alkaline sequences and rifted arc bimodal tholeiite sequences. The boninitc rocks of the forearc host Cuâ(Zn) and CuâCo VMS deposits, the latter hosted in fragments within the Main Urals Fault Zone (MUFZ) which marks the line of arc-continent collision in Late Devonian times. The arc tholeiites host CuâZn deposits with an evolution to more calc-alkaline felsic volcanic sequences matched with a change to ZnâPbâCu polymetallic deposits, often gold-rich. Large rifts in the arc sequence are filled by thick bimodal tholeiite sequences, themselves often showing an evolution to a more calc-alkaline nature. These thick bimodal sequences are host to the largest of the CuâZn VMS deposits.
The exceptional degree of preservation in the Urals has permitted the identification of early seafloor clastic and hydrolytic modification (here termed halmyrolysis sensu lato) to the sulphide assemblages prior to diagenesis and this results in large-scale modification to the primary VMS body, resulting in distinctive morphological and mineralogical sub-types of sulphide body superimposed upon the tectonic association classification.
It is proposed that a better classification of seafloor VMS systems is thus achievable using a three stage classification based on (a) tectonic (hence bulk volcanic chemistry) association, (b) local volcanic chemical evolution within a single edifice and (c) seafloor reworking and halmyrolysis.
Abstract: Copper and Zn isotope ratios of well-characterized samples from three ore facies in the Devonian Alexandrinka volcanic-hosted massive sulphide (VHMS) deposit, southern Urals, were measured using multi collector ICP-MS (MC-ICP-MS) and show variations linked to depositional environment and mineralogy. The samples analysed derived from: a) hydrothermalâmetasomatic vein stockwork, b) a hydrothermal vent chimney, and c) reworked clastic sulphides. As the deposit has not been significantly deformed or metamorphosed after its formation, it represents a pristine example of ancient seafloor mineralization. Variations in δ65Cu (where δ65Cu = [(65Cu / 63Cu)sample / (65Cu / 63Cu)standard â 1] * 1000) and δ66Zn (where δ66Zn = [(66Zn / 64Zn)sample / (66Zn / 64Zn)standard â 1] * 1000) of 0.63 and 0.66â°, respectively, are significantly greater than analytical uncertainty for both isotope ratios (± 0.07â°, 2Ï). Very limited isotopic fractionation is observed in primary Cu minerals from the stockwork and chimney, whereas the Zn isotopic composition of the stockwork varies significantly with the mineralogy. Chalcopyrite-bearing samples from the stockwork have lighter δ66Zn by 0.4â° relative to sphalerite dominated samples, which may be due to equilibrium partitioning of isotopically light Zn into chalcopyrite during its precipitation. δ66Zn also showed significant variation in the chimney, with an enrichment in heavy isotopes toward the chimney rim of 0.26â°, which may be caused by changing temperature (hence fractionation factor), or Raleigh distillation. Post-depositional seafloor oxidative dissolution and re-precipitation in the clastic sediments, possibly coupled with leaching, led to systematic negative shifts in Cu and Zn isotope compositions relative to the primary sulphides. Copper shows the most pronounced fractionation, consistent with the reduction of Cu(II) to Cu(I) during supergene mineralization. However, the restricted range in δ65Cu is unlike modern sulphides at mid oceanic ridges where a large range of Cu isotope, of up to 3â° has been observed [Rouxel et al., 2004 and Zhu et al., 2000].
Abstract: We have studied textural relationships and compositions of phyllosilicate minerals in the maficâultramafic-hosted massive-sulfide deposit of Ivanovka (Main Uralian Fault Zone, southern Urals). The main hydrothermal phyllosilicate minerals are Mg-rich chlorite, variably ferroan talc, (Mg, Si)-rich and (Ca, Na, K)-poor saponite (stevensite), and serpentine. These minerals occur both as alteration products after mafic volcanics and ultramafic protoliths and, except serpentine, as hydrothermal vein and seafloor mound-like precipitates associated with variable amounts of (Ca, Mg, Fe)-carbonates, quartz and Fe and Cu (Co, Ni) sulfides. Brecciated mafic lithologies underwent pervasive chloritization, while interlayered gabbro sills underwent partial alteration to chlorite + illite ± actinolite ± saponite ± talc-bearing assemblages and later localized deeper alteration to chlorite ± saponite. Ultramafic and mixed ultramaficâmafic breccias were altered to talc-rich rocks with variable amounts of chlorite, carbonate and quartz. Chloritization, locally accompanied by formation of disseminated sulfides, required a high contribution of Mg-rich seawater to the hydrothermal fluid, which could be achieved in a highly permeable, breccia-dominated seafloor. More evolved hydrothermal fluids produced addition of silica, carbonates and further sulfides, and led to local development of saponite after chlorite and widespread replacement of serpentine by talc. The Ivanovka deposit shows many similarities with active and fossil hydrothermal sites on some modern oceanic spreading centers characterized by highly permeable upflow zones. However, given the arc signature of the ore host rocks, the most probable setting for the observed alterationâmineralization patterns is in an early-arc or forearc seafloorâsubseafloor environment, characterized by the presence of abundant maficâultramafic breccias of tectonic and/or sedimentary origin.
Abstract: Maficâultramafic-hosted hydrothermal FeâCuâ(NiâCo) sulfide ores from the Main Uralian Fault Zone (MUFZ), South Urals (Ivanovka and Ishkinino ore fields), contain a relatively large (up to 3%) proportion of chromite. This association is common for magmatic FeâNiâCu sulfides, but definitely unusual for hydrothermal sulfides. Textural, morphological and compositional data are used here to gain an insight into the origin and significance of this unusual chromiteâsulfide association. The studied chromites occur both as broken fragments and as euhedral or subhedral crystals, which are included in the sulfides or scattered in their talc±chlorite±saponite±quartz±carbonate matrix. They are characterized by high Cr/(Cr+Al) ratios (0.58â0.85) and range in composition from magnesiochromite to chromite sensu stricto. Textural, morphological and compositional features, as well as the occurrence of relatively high-silica, low-Ti, low-K melt inclusions in some of the crystals, indicate that the ore-associated chromites (i) are a mixed population of grains derived from maficâultramafic mantle and crustal magmatic rocks and mantle peridotite melting residua, (ii) have no genetic relation with the host sulfides and (iii) represent relicts derived from the hydrothermally altered country rocks. The compositions of the chromites and of the melt inclusions denote a clear supra-subduction zone signature. The melts parent to the cumulitic chromites had an arc tholeiitic to, possibly, boninitic affinity. These data suggest that the host maficâultramafic complexes formed in an early arc or forearc setting and do not represent obducted portions of MORB oceanic lithosphere. Hence, contrary to previous interpretations, the associated massive sulfides could not originate on a mid-ocean ridge, but rather in an early arc or forearc environment. Given the relatively short life of the western Uralian arc system, the most probable time window for sulfide ore deposition is confined to Early to Middle Devonian time.
Abstract: Rhenium and osmium elemental and isotopic data have been obtained for the two maficâultramafic hosted volcanogenic massive sulphide (VMS) deposits of Dergamish and Ivanovka from the south Urals. The associated ophiolitic blocks belong to the Main Uralian Fault (MUF) melange zone considered to represent obducted early Palaeozoic oceanic crust. Despite their close geographical proximity, the two ore bodies are morphologically, mineralogically and isotopically quite different. Sulphides from Ivanovka possess higher Ni and Os and lower Re and Cu relative to those from Dergamish.
The Re and Os isotope data for Dergamish define a best-fit line corresponding to a Late Devonian age of 366±2 Ma (2Ï) with an MSWD of 4.6. This age is some 40 My younger than the inferred Silurian crystallisation age of the associated maficâultramafic rocks, but in good agreement with the previously published RbâSr and ArâAr ages of 360â380 Ma corresponding to the high-pressure metamorphic age of the adjacent Maksyutov metamorphic complex. These data suggest that ReâOs systematics of the Dergamish sulphide deposit were reset, either by diffusion or recrystallisation, during high-pressure metamorphism or subsequent cooling.
The preservation of unradiogenic Os isotopic ratios in some of the Ivanovka samples and the near chondritic initial Os isotopic composition obtained for the Dergamish samples indicates that most of the Os in the massive sulphides was ultimately derived from the mantle. The corresponding tectonic setting equates to an area with submarine high-level mantle rocks. In contrast, sulphides from Ivanovka have experienced continued re-equilibration and have been modified by post-depositional processes at least some of which occurred relatively recently.
Abstract: Rhenium loss through magma degassing could be partly balanced by rhenium enrichment in fumarolic magmatic gases and Re-bearing precipitates, as may be the case for the Kudriavy volcano associated with an active subduction zone. The relatively unradiogenic 187Os/188Os isotope ratios (0.122 up to 0.152) and high Os contents (averaging 0.6 ppb) of fumarolic gas condensates imply that significant Re and Os are remobilised from depleted MORB mantle. Involvement of a Re-rich component is evident from high Re concentrations in high-temperature gas condensates, ranging from 7 to 200 ppb. Indeed, Re-rich Os-poor components such as organic-rich subducted sediments and volcanic rocks do not significantly shift the isotopic composition of fumarolic products. The relatively radiogenic composition of the dacite-andesite-basaltic arc volcanics (187Os/188Os ratio up to 0.58), however, could result from significant Os (and Re) input from subducted sediments.
Abstract: The isotopic composition of lead from a total of 53 samples of galena from 18 VHMS deposits shows a range between 17.437 and 18.111 for 206Pb/204Pb; 15.484 and 15.630 for 207Pb/204Pb and 37.201 â 38.027 for 208/204Pb. The results show a systematic trend with the leads of the Sibay, Barsuchiy Log and Djusa deposits being most radiogenic by comparison with those of Bakr-Tau and Oktiabrskoe which are the least radiogenic deposits. The Bakr-Tau and Oktiabrskoe deposits occur within most primitive fore-arc rocks at the lower part of the Baymak-Buribay formation, which contain lavas of boninitic affinity. The Sibay, Barsuchiy Log and Djusa deposits are found in intra- and back-arc setting and are hosted by a sequence of bimodal tholeiites. The deposits in âarcâ setting such as the Balta- Tau, Gai and Alexandrinka deposits occupy an intermediate position. This trend is explained in term of mixing between mantle wedge and continental blocks.
Abstract: The Re-Os distribution and isotope composition have been studied within different ore facies of
the Alexandrinka and Dergamish VHMS deposits, Southern Urals. The osmium contents increase and the
187Os/188Os isotope composition decrease in the ore facial range: stockwork zone â sulphide chimneys â
coarse-clastic ore â fine-clastic ore â submarine alteration zone on the hanging wall. This range reflects the
degrees of hydrothermal fluid â seawater mixing during the hydrothermal ore-forming process. The Re-Os
isotope signature has been perturbed in the Alexandrinka hydrothermal system, which could result from a late
addition of rhenium.
Abstract: The European MinUrals project is focusing on the South Urals mining sector, in order to improve
local socio-economic conditions, through:
1) The reinterpretation of the geodynamics of South Urals and of the different types of ore deposits and the
development of tools for mineral exploration (new geophysical and geochemical technology). The convergence
setting and the formation of arc, fore-arc and back-arc systems explain the volcano-sedimentary and
structural features. This geodynamic setting largely controls the distribution and characteristics of the different
types of mineralisation;
2) The evaluation of local mining-related risks to the environment, with a development of methodologies for
assessing and reducing the environmental impact and localizing areas of high metal potential/low environmental
constraints. Three pilote sites were investigated: Sibay and Uchaly (with mining installations), and
Karabash (with mining installations and smelter);
3) The implementation of a Geographical Information System taking into account the mineral potential and
the environmental constraints that, through data ranking and combining the key parameters of the areas with
high metal potential and environmental constraints, will enable the production of a Mineral Potential and Environmental
Constraints Map of the South Urals;
4) The elaboration of recommendations for a suitable environmentally aware mining-industry legislation,
based on a comparison with the European legislation, to be adressed to the Commission on the demarcation of
powers and subjects between the federal government, governments of the subjects of the Russian Federation
and local authorities.
Abstract: Application of the 147Sm-143Nd and 146Sm-142Nd chronometers suggests that mantle depletion and related crust extraction had started during the first hundred million years of Earth history (Caro et al., 2006), but there is limited evidence for the presence of an enriched silicate reservoir before 4.0 Ga (Harrison et al., 2006). Here, we report new 147Sm-143Nd data from low-grade metamorphosed Paleoarchean volcanic and sedimentary rocks of the Warrawoona Group, Pilbara Craton, Western Australia. A Sm-Nd isochron age of 3.48 ± 0.09 Ga is in an agreement with the U-Pb zircon age from intercalated volcanoclastic rocks and indicates the preservation of their
Sm-Nd isotope signature since the Archean. The initial εNd value of -3.3 ± 0.5 is out of the range (+1< εNd < +3)
established for the Archean cratons based on whole-rock Sm-Nd isochrons with ages agreeing with independent chronological constraints. This data is best explained by the assimilation of older continental crust, as shown by significant Ta-Nb depletion associated with LILE, Th, U and LREE enrichment (Green et al., 2000). Geochemical modeling suggests the assimilation of 10% to 20% of crustal material that may be either Archean granites, shales, or average upper continental crust (Condie, 1993). Assimilation of this crustal material with εNd values of â6 (granite), â8 (shales) and/or â10 (upper CC) by a mantle-derived melt accounts for the
observed Nd isotope composition of basalts. This crustal component, with 147Sm/144Nd ratios of 0.09â0.12, would have been isolated from the depleted mantle at 4.0â4.4 Ga, which is consistent with the Nd model ages derived from the studied rocks. Our observations, albeit indirect, clearly show the existence of a an Hadean crustal reservoir. Its presence is confirmed by negative εNd of Archean seawater, inferred from
carbonate rocks with seawater-like REE patterns. The presence of a crustal basement involved in plume magmatism indicates eruption of flood basalts onto a submerged continental platform, analogous with the Phanerozoic Kerguelen plateau (Van Kranendonk and Pirajno, 2004). This could have been an important mode of crustal production in the Archean.
