Low Strontium Research Articles

Local structure modulation of the low La-doped layered strontium manganite Sr4Mn3O10 studied by transmission electron microscopy and electron energy-loss spectroscopy

Transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) have been used to characterize the structure and microstructure of the low La-doped layered manganite LaxSr4−xMn3O10 (x ≤ 0.15). The structure of the perfect phase has been identified as an n = 3 layered structure of the hexagonal series by electron diffraction (ED), high-resolution electron microscopy (HRTEM) together with numerical image simulations. The presence of local superstructure modulation associated with La ordering in the compounds was revealed by electron diffraction and high-resolution electron microscopy. Electron diffraction data indicate that this superstructure modulation is two-dimensional, and the modulation plane lies in the ac plane. The two primary modulation vectors are \( q_{1} = (1/4)a^{*} \) and \( q_{2} = (1/2)c^{*} . \) In addition, another type of modulated structure associated with periodic lattice distortion was occasionally visible in the La0.15Sr3.85Mn3O10 phase. Energy-loss spectroscopy data indicate that the periodic lattice distortion is led by ordered oxygen deficiency.

Barium strontium oxide coated carbon nanotubes as field emitters

The authors report a field emitter structure based on oxide coated carbon nanotubes (CNTs). This emitter consists of a thin tungsten ribbon with CNTs on the surface and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT emitter was designed to combine the benefits of the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission properties of the emitters were characterized. A field enhancement factor of 467 and a work function of 1.9eV were obtained for the oxide coated CNTs. Compared to the uncoated CNTs, the field emission from the oxide coated CNTs increased by a factor of 2–3. At 4.4V∕μm, the field emission current of 23.6μA was obtained from an emitting surface of 0.012cm2.

87Sr/ 86Sr record of Permian seawater

A population of 169 Permian articulate brachiopod shells was analysed for their 87Sr/ 86Sr ratios. 51 of these, characterised as well preserved and stratigraphically well defined, are utilized for delineation of the Permian seawater strontium isotope trend. The 87Sr/ 86Sr curve shows values of about 0.7080 in the lowermost Permian (Asselian), followed by a gradual decline to 0.70685 in the Capitanian and a minor rise in the Dzhulfian and lower Dorashamian to values of 0.70715 just below the Permian-Triassic boundary. The Early Permian decrease in the strontium isotope curve that commences in the early Sakmarian is coincident with the advancing deglaciation of the Gondwana and with the increased aridity in large parts of the Pangaea. These factors may have led to a reduced continental weathering of Rb-rich silicate rocks, and thus to the decline in seawater 87Sr/ 86Sr. Starting with the Artinskian, the opening of the Neotethys and the associated widespread basaltic volcanism supplied low radiogenic strontium to seawater from an enhanced hydrothermal flux. In the Capitanian, basaltic volcanism in the entire Palaeotethys ceased and this may have been the reason for a slightly more radiogenic seawater isotopic composition during the Lopingian. Higher input of riverine Sr due to expansion of humid areas may have been a contributory factor.

Thermionic emission from carbon nanotubes with a thin layer of low work function barium strontium oxide surface coating

We have created a thermionic cathode structure that consists of a thin tungsten ribbon, carbon nanotubes (CNTs) on the ribbon surface, and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT cathode was designed to combine the benefits from the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission and thermionic emission properties of the cathode have been characterized. A field enhancement factor of 266 and a work function of 2.1eV were obtained. At 1437K, a thermionic emission current density of 15mA∕cm2 in an electric field of 0.9V∕μm was obtained, which is 50 times greater than the emission current density from the uncoated CNT cathode at the same temperature.

The atomic surface structure of SrTiO 3(0 0 1) in air studied with synchrotron X-rays

The atomic surface structure of single terminated SrTiO 3(0 0 1) (1 × 1) is investigated employing surface X-ray diffraction. In order to obtain these surfaces a special treatment is needed consisting of chemical etching and annealing. Since this is done in an aqueous and subsequently oxygen environment, after which the crystals are kept at ambient conditions, the surface is studied in air. Crystal truncation rods are measured and several models that are proposed in literature in recent years are tested against the experimental data. These models include surface rumpling, low temperature-like distortions, strontium adatom and lateral displacement distortions for both TiO 2 and SrO-terminated surfaces. None of these models represents the data very accurately. A much better fit to the experimental results is obtained by using a model in which a TiO 2-terminated crystal is covered by an oxygen layer.

Three terrane-defining thrusts of the Kumaun Himalaya: controversies on position and nomenclature

Three of the five thrust fault systems defining the boundaries of four lithotectonic terranes of the Himalaya and involved in controversies related to their positions and nomenclature, are the objects of discussion in this paper. Youngest of the five terrane-defining faults, the Himalayan Frontal Fault (HFF) is a series of reverse faults that demarcates the boundary of the Siwalik front of the Himalayan province with the alluvial expanse of the Indo-Gangetic Plains. Over large tracts, it is either concealed under younger sediments or has as yet not reached the ground surface and is therefore a blind fault. The nature of this frontal fault varies along its length. Where the hidden ridges of the Indo-Gangetic basement impinge the Himalaya, the mountain front is ruptured and the HFF is repeatedly reactivated. In the sectors intervening these ridges, it is not expressed on the surface, but the ground of the adjoining Indo-Gangetic Plain is sinking, the rivers are shifting their courses and large tracts of land are waterlogged and characterized by The Vaikrita Thrust is the plane that marks pronounced metamorphic break and abrupt change in style and orientation of structures within the succession of crystalline rocks that build the bulk of the snowy ranges in the Kumaun Himalaya. Not only is there a jump of pressure of the order of 4 kb and a temperature rise of >200 oC, but also is there a conspicuous change of neodymium isotope value across the tectonic plane that separates the low-grade metarnorphics in the lower part from the high-grade metamorphic rocks of the upper part of the Great Himalayan succession. The Vaikrita Thrust is therefore recognised as the Main Central Thrust (MCT). While the basal low-grade metamorphic assemblage comprises 1900±100 Ma old highly tectonised porphyritic granite characterised by low initial strontium isotope ratio, the upper high -grade metamorphic group is intruded by 20±1 Ma old anatectic granites characterised by garnet, kyanite, sillimanite and cordierite, and a high but variable value of strontium isotope ratio. Moreover, the anatectic Lower Miocene granites are singularly absent in the succession of the Lesser Himalayan nappes. Probably it is this thrust that has flexed downwards the plane of decoupling and displacement between the under thrusting Indian plate and the overlying Himalayan mass.
 The terrane-defining fault between the high-grade metamorphics with granitic rocks of the Himadri (i.e., Great Himalaya) and the Tethyan sedimentary pile was recognised as the Malari Thrust Fault in the northern Kumaun Himalaya in the early seventies, as the South Tibetan Detachment System in southern Tibet adjoining north-eastern Nepal in the early eighties and as the Zanskar Shear Zone in north-western Himachal Pradesh in the late eighties. Not only is there an abrupt change in the metamorphic grade across the tectonic plane, but also an attenuation and wholesale elimination of some lithostratigraphic formations of the hanging wall besides the difference in the style of deformation. Exhibiting predominant dip-slip movement in the central sector of the Himalayan arc, the Trans-Himadri Fault (T-HF) was formed as a consequence of the Tethyan sedimentary cover detaching from its rigid foundation of the basement complex that was squeezed up following blocking or slowing down of tectonic movements related to India-Asia convergence. The sedimentary cover lagging behind the thrust­ up basement complex slid down and toppled over northward and gave rise to back-folds and back-thrusts. In Kashmir, western Himachal Pradesh, central Nepal and north-western Bhutan, in sharp contrast, there was very strong compression, so that the Tethyan sedimentary rocks-along with a slice of the low-grade metamorphic basement in the hanging wall advanced southwards across the Himadri and were emplaced as nappes and klippen south of the Main Central Thrust. The T-HF movement occurred approximately around 20.9 Ma, although the movement had started quite earlier in some places. Quaternary reactivation resulted in river ponding and development of huge lakes such as the Garbyang palaeolake in the Kali valley and the ~ 40,000 year-old Goting palaeolake in the Western Dhauli Valley.

Fe/Sr ratio effect on magnetic properties of strontium ferrite powders synthesized by microwave-induced combustion process

Strontium hexaferrite nano-particles were successfully synthesized by microwave-induced combustion process. Magnetic properties of strontium ferrite powders with Fe/Sr ratios varying from 11 to 12 and different annealing temperatures range of 850 to 1050 °C were studied. The resultant powders were investigated by XRD, TEM, SEM, VSM, TG/DTA, and surface area measurement. The optimum magnetic properties of strontium ferrite powders were as follows: the composition with an Fe/Sr ratio of 11.6 annealed at 1000 °C for 2 h possessed a saturation magnetization of 62 emu/g and an intrinsic coercive force of 1950 Oe. Moreover, the microwave-heated strontium ferrite powders with a Fe/Sr ratio of 11.6 annealed at various temperatures had particle size ranging from 52 to 78 nm. Low-coercivity strontium ferrite powders can be obtained by microwave-induced combustion process without doping with metal elements such as Co–Ti, Co–Sn, Zn–Sn, etc. These low coercive strontium ferrite powders are suitable for magnetic recording applications in hard disks, floppy disks, video tapes, etc.

Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO 23Al 2O 31.5P 2O 53CaO2CaF 2, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca 5(PO 4) 3F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The 29Si, 27Al and 31P NMR spectra for the glasses with different strontium contents were identical. The 19F spectra indicated the presence of F–Ca(n) and Al–F–Ca(n) species in the calcium glasses and in the strontium glasses F–Sr(n) and Al–F–Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium–strontium apatite and the fully strontium substituted glass to strontium fluorapatite.

The effect of strontium and chromate ions on the inhibition of zinc

The corrosion and inhibition of zinc in artificial acid rain solution (pH 4.5) at a low strontium chromate concentration has been evaluated using electrochemistry, solution analysis and XPS. These techniques have been used to estimate inhibitor efficiency and to determine a possible inhibition mechanism. The results show that strontium chromate at low concentration inhibited significantly more of the cathodic corrosion process, by reduction of Cr 6+ to a solid species containing Cr 3+. The surface film contained a mixture of oxidised zinc and chromium, probably in the form of basic zinc carbonate (hydrozincite) and hydrated chromia. Importantly strontium ions were found to play no significant role in the inhibition process.

A low temperature nano-structured SrTiO 3 thick film oxygen gas sensor

A low temperature nano-structured Strontium Titanate (SrTiO 3) thick film oxygen gas sensors were obtained by high-energy ball milling technique and thick-film screen-printing technique. SrTiO 3 nano-sized powders of around 20 nm were produced from commercial SrTiO 3 and synthesized SrTiO 3. The material micro-structural properties and sensor oxygen sensing properties were characterized using X-ray diffraction (XRD) and gas sensing measurements respectively. Experimental results show that the sensing property of the synthesized SrTiO 3 sensors with an annealing temperature of 400 °C is much better than the commercial SrTiO 3 sensors (both milled and not milled materials). Especially, their optimal operating temperature of around 40 °C is the lowest reported value for such metal oxide semiconductor gas sensors ever achieved.

Strontium Isotopes in the Investigation of Early Glass Production: Byzantine and Early Islamic Glass from the Near East*

87Sr/86Sr ratios have been determined for glasses from four production sites, dated to between the sixth and the 11th centuries, in the Eastern Mediterranean region. On the basis of elemental analyses, the glasses at each location are believed to have been melted from different raw materials. Two glass groups, from Bet Eli‘ezer and Bet She‘an, in Israel, are believed to have been based upon mixtures of Levantine coastal sands and natron, and have 87Sr/86Sr ratios close to 0.7090, plus high elemental strontium, confirming a high concentration of modern marine shell (87Sr/86Sr ~ 0.7092) in the raw materials. The isotopic compositions of these two groups of glasses differ slightly, however, probably reflecting a varying ratio of limestone to shell because the sands that were utilized were from different coastal locations. Natron‐based glasses from a workshop at Tel el Ashmunein, Middle Egypt, have 87Sr/86Sr values of 0.70794–0.70798, and low elemental strontium, consistent with the use of limestone or limestone‐rich sand in the batch. High‐magnesia glasses based on plant ash, from Banias, Israel, have 87Sr/86Sr values of 0.70772–0.70780, probably reflecting the isotopic composition of the soils that were parental to the plants that were ashed to make the glass. Strontium and its isotopes offer an approach to identifying both the raw materials and the origins of ancient glasses, and are a potentially powerful tool in their interpretation.

Geochemical and petrological characteristics of Eppawala phosphate deposits, Sri Lanka

The apatite-bearing carbonate rocks at Eppawala, Sri Lanka occur as massive, discontinuous bodies in a Precambrian, high-grade metamorphic terrain, which weather to form economically important phosphate deposits. The ore bodies at Eppawala contain ≤42% P2O5, and citric acid solubility of different components varies from 4 to 6%. The parent rocks are mainly made up of calcite, dolomite and apatite, with lesser amounts of ilmenite, magnetite, pyrite, forsterite, phlogopite, enstatite, magnesite, diopside, tremolite and spinel. Most of minerals show an euhedral habit, with a wide range of crystal sizes (from a few millimetres to several decimetres). The Eppawala rocks are characterised by low silica (≤0.41%), high phosphorous (≤10.58%) and high strontium content (2,960–6,819 ppm). Concentrations of light rare-earth elements in these rocks are comparably higher than those of marbles. The REE fractionation of these rocks is pronounced, and La/Yb ratios vary between 14 and 43. Both apatite and calcite show markedly elevated strontium levels (≤0.6%). The δ 13CPDB and δ 18OSMOW values of the carbonates are in the range of –3.4 to –2.2 and 7.7 to 16.4‰ respectively. The euhedral habit, as well as the presence of major quantities of apatite and considerable amounts of iron-bearing minerals suggest that the ore host rock has genetic links to an igneous source rather than to an intensely metamorphosed limestone. The higher light REE contents of the rocks, compared to marbles, also argue against a metamorphic or sedimentary origin. The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively, suggesting that they were formed in a carbonatite magma. The markedly increased REE concentrations in the bulk chemistry of the rocks have been shown to be mainly controlled by the content of phosphate minerals. Compared to most carbonatites, the Eppawala rocks are generally depleted in selected trace elements, particularly Ba, Nb, Th, V, U and Zr. This depletion may be due to either a primary infertility of the parent magma with regard to such trace elements, or it is a result of fractional crystallisation during the rock formation. The stable isotope ratios do not plot within the defined "mantle carbonatite box", but still lie within the broader range of carbonatitic rocks. With these data at hand, it can be readily argued that the mode of occurrence, petrography and geochemistry of the Eppawala apatite-bearing carbonates provide conclusive evidence of their carbonatitic origin.

Effects of Octahedral Tilting on the Piezoelectric Properties of Strontium/Barium/Niobium‐Doped Soft Lead Zirconate Titanate Ceramics

(Pb1−x−ySrxBay)(Zr0.976−zTizNb0.024)O3 solid solutions have been investigated to understand the relationship between structural changes caused by isovalent strontium and barium substitution on the A‐site and dielectric and piezoelectric properties. As strontium and barium were substituted for lead, the zirconium:titanium (Zr:Ti) ratio was modified so that all compositions had an optimized piezoelectric coefficient (d33). The value of d33 was at a maximum in the tetragonal phase near, but not at, the morphotropic‐phase boundary (MPB). The real MPB was taken as the Zr:Ti ratio at which X‐ray diffraction patterns appeared either pseudocubic or a mixture of rhombohedral and tetragonal. As strontium content increased, optimized d33 also increased from 410 pC/N (x= 0) to 640 pC/N (x= 0.12), commensurate with a decrease in the paraelectric‐to‐ferroelectric phase transition temperature (TC) from 350°C (x= 0) to 175°C (x= 0.12). However, for ceramics where x > 0.12, optimized d33 decreased even though the phase‐transition temperature was ∼150°C. Low strontium concentration ceramics (x= 0–0.08) contained 80 nm ferrroelectric domains typical of PZT, but high strontium concentration ceramics (x= 0.12–0.16) contained fine‐scale domains (20 nm) in some regions of the microstructure. In addition, [110] pseudocubic electron diffraction patterns revealed superlattice reflections at 1/2{hkl} positions associated with rotations of the octahedra in antiphase. Co‐doping ceramics with strontium (x= 0.06) and barium (y= 0.06) resulted in the disappearance of the 1/2{hkl} reflections. Optimized d33 (∼520 pC/N, TC∼ 205°C) for this composition was similar to that of ceramics where x= 0.08, y= 0, which had a TC of ∼250°C.

Kinetics of isotopic exchange between strontium polymolybdate and strontium ions in aqueous solution

A heterogeneous isotopic exchange reaction of strontium polymolybdate in strontium chloride solution was studied using 90Sr as a tracer. The effects of low and high strontium chloride concentration on the rate and mechanism of the isotopic exchange reaction were investigated. It was found that, at high concentrations, the rate is independent of strontium concentration, but, at low concentrations, the rate is proportional to the strontium concentration. These results support a hypothesis that, at low concentrations, the rate is controlled by film diffusion, whereas at high concentrations it is controlled by particle diffusion. Experiments were performed at 293, 303 and 313 K. Activation energy of isotopic exchange reaction and thermodynamic parameters Δ H *, Δ S *, and Δ G * were calculated using the Arrhenius and Eyring equations. The results also indicated that recrystallization is a predominant factor in the present exchange reaction.

Idade pb-pb e assinatura isotópica rb-sr e sm-nd do magmatismo sienítico paleoproterozóico no Sul do cinturão móvel Salvador-Curaçá: maciço sienítico de São Félix, Bahia

The Sao Felix Syenitic Massif (MSSF) has a tabular shape with about 32 km 2 that represents the south expression of the aligned syenitic plutonism, which occur in the middle part of Salvador-Curaca mobile belt (CMSC). Single zircon dating by stepwise Pb evaporation methodology yields an age of 2098 ± 1 Ma to SFSM. This data correlate the emplacement of the SFSM with the late stages of SCMB stabilization. This massif is isotopically characterized by negative epsilon neodymium values (-1.45 to -2.89) and low initial strontium ratio (0.701 to 0.704). SFSM isotopic signature is similar to the ones displayed by the others syenites from the belt and reflects an enriched source which should be related to a metasomatic enriched mantle.

Mechanical Properties of La1‐xSrxCo0.2Fe0.8O3 Mixed‐Conducting Perovskites Made by the Combustion Synthesis Technique

This paper examined the room‐temperature mechanical properties of a mixed‐conducting perovskite La1–xSrxCo0.2Fe0.8O3 (x= 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations (x= 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m1/2) for low strontium (x= 0.2) content and a lower value (∼1.1 MPa·m1/2) for the other batches (x= 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.

Rb-Sr study of Au-Ag Shkol'noe deposit (Kurama Mountains, north Tadjikistan): age of mineralization and time scale of hydrothermal processes

The epithermal Au-Ag Shkol'noe deposit is located in the Kandjol ore field, Kurama Mountains. This region is a part of the east-west trending Late Hercynian Bel'tau-Kurama volcanic belt, an Andean-style collisional margin. The deposit comprises a number of quartz-carbonate veins hosted by the syn-subductional Middle Carboniferous Karamazar granodiorites. The Au-Ag mineralization is considered to be the result of the earliest hydrothermal event in the region. The Rb-Sr isochron age 296.3 ± 1.3 Ma and an initial 87Sr/86Sr0=0.7071 ± 2 ratio were obtained for an adularia-sericite-quartz-calcite sample from Au-Ag mineralization. The 87Sr/86Sr ratio range from 0.70645 ± 10 to 0.70741 ± 10 was obtained for the calcites from the earlier and later mineral assemblages. The Rb-Sr age is interpreted as a real geological age of the Au-Ag mineralization. It corresponds to the initial stage of the Late Carboniferous – Early Permian collision following the main syn-subduction stage of Bel'tau-Kurama volcanic belt evolution. The comparison of the Rb-Sr age with previously obtained 40Ar-39Ar and K-Ar data for adularia from the Au-Ag mineralization implies that gangue minerals of the Shkol'noe deposit bears the fingerprint of at least three events in its history. They are (1) Au-Ag mineralization at 296.3 ± 1.3 Ma; and (2) two subsequent thermal pulses at 277 ± 4 and 263–267 ± 8 Ma. The minimum time scale for the hydrothermal activity within the Shkol'noe deposit is thus approximately 30 million years. A general uniformity of the strontium source during the hydrothermal processes within the Au-Ag Shkol'noe deposit (87Sr/86Sr0=0.70645 ± 10 to 0.70741 ± 10) is suggested as well as within the Bel'tau-Kurama belt (87Sr/86Sr0=0.7051–0.707). The slight shift into a higher strontium isotope composition of the hydrothermal minerals of the Shkol'noe deposit in comparison with other deposits and rocks of the Bel'tau-Kurama belt may be ascribed to the contribution of relatively radiogenic strontium from the Karamazar-type granitoids. The mobilization of low radiogenic strontium during propylitic alteration of diabase dikes emplaced after the Au-Ag mineralization could be responsible for comparatively low 87Sr/86Sr ratios in some of the latest post-dike carbonates.

Geological and Geochemical Characteristics and Genesis of the Shaxi Porphyry Copper (Gold) Deposits, Anhui Province

Abstract The Shaxi porphyry copper (gold) deposits are a typical example of porphyry copper deposits associated with diorite in eastern China. Quartz diorite, which hosts the deposits, has a Rb‐Sr isochron age of 127.9 ± 1.6 Ma. Geochemically, the rock is rich in alkalis (especially sodium), light rare earth elements (LREE) and large‐ion lithophile elements (LILE), and has a relatively low initial strontium isotopic ratio (Isr=0.7058); thus it is the product of differentiation of crust‐mantle mixing source magma. The model of alteration and mineralization zoning is similar to the Hollister (1974) diorite model. The ore fluids have a relatively high salinity and contain significant amounts of CO2, Ca2+, Na+ and ***CI−. The homogenization temperatures of fluid inclusions for the main mineralization stage range from 280 to 420°C, the δ18O values of the ore fluids vary from 3.51 to 5.52 %, the δD values are in the range between −82.4 and −59.8 %, the δ34S values of sulphides vary from −0.3 to 2.49 %, and the δ13C values of CO2 in inclusions range between −2.66 and −6.53 %. Isotope data indicate that the hydrothermal ore fluids and ore substances of the Shaxi porphyry copper (gold) deposits were mainly derived from magmatic systems.

The origin of U-shaped rare earth patterns in ophiolite peridotites: assessing the role of secondary alteration and melt/rock reaction

Ten samples of serpentinized harzburgite and lherzolite from the Trinity ophiolite complex (California) can be classified into two groups using their REE patterns and initial neodymium and strontium isotopic ratios. Samples with the most refractory major element compositions (Al 2O 3 = 0.6–1.8%; CaO < 1.2%) have U-shaped REE patterns and show low to very low ε Nd(T) values (+2 to −7), but high to very high 87Sr/ 86Sr(T) ratios (up to 0.715), whereas rocks with more fertile major element compositions (Al 2O 3 = 2–3.2%; CaO = 2–2.6%) give REE patterns showing increasing depletion from Lu to La and have high ε Nd(T) values (+6 to +8), but low 87Sr/ 86Sr(T) ratios (0.703–0.704). The low neodymium and high strontium isotopic ratios of those peridotites having U-shaped REE patterns demonstrate that the LREE enrichments observed in these rocks are the result of contamination by continental crust and not the consequence of peculiar fractionation during melting or chromatographic effects during mantle ascent. A clinopyroxene concentrate prepared from a bulk sample of lherzolite with ε Nd(T) = −7 and 87Sr/ 86Sr(T) = 0.708 gives MORB-like isotopic signatures: ε Nd(T) = +14 and 87Sr/ 86Sr(T) = 0.7035. Furthermore, two highly serpentinized samples with U-shaped REE patterns show strongly negative δD values (−95 and −110‰) much typical of peridotites altered by continental metamorphic fluids or/and meteoritic waters. From these results, we conclude that the crustal contamination corresponds to a low-temperature alteration event which took place during or after obduction of the Trinity ophiolite complex onto the continent and is not the consequence of any high-temperature invasion of the ultramafic rocks by melts/fluids derived from subducted continental materials.