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1.
This study evaluates in detail the mineral chemistry, whole-rock and mineral separate Os-isotope compositions of distinct platinum-group mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Os-rich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of ‘unradiogenic’ 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from ‘primary’ PGM assemblage (0.12473–0.12488) and similar ‘unradiogenic’ 187Os/188Os values for both ‘primary’ and ‘secondary’ PGM assemblages (0.1242 ± 0.0008 and 0.1245 ± 0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240 ± 0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re–Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a supra-subduction zone setting shortly before obduction. 相似文献
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Origin and recharge estimates of groundwater in the ordos plateau,People’s Republic of China 总被引:2,自引:3,他引:2
Groundwater is a valuable resource in the semiarid Ordos Plateau region where abundant mineral resources, such as coal, natural
gas, and halite, are present. With resources development, groundwater demand will increase dramatically. The origin identification
and recharge estimates of groundwater are significant components of sustainable groundwater development in the Ordos Plateau.
Groundwater and precipitation samples were taken and the isotopic compositions δ2H, δ18O, and chloride were analyzed to identify groundwater origins and to estimate recharge rates. The δ2H and δ18O of the groundwater show that the groundwater recharge is of meteoric origin. The chloride mass balance (CMB) method was
used to quantify recharge rates of groundwater in the Ordos Plateau, which varies from 2.93 to 22.11% of the effective annual
rainfall. Recharge rates estimated by CMB were compared with values obtained from other methods and were found to be in good
agreement. This study can be used to develop effective programs for groundwater management and development. 相似文献
5.
Sylvie Demouchy Stephen J. Mackwell David L. Kohlstedt 《Contributions to Mineralogy and Petrology》2007,154(3):279-289
Interdiffusion of Fe and Mg in (Mg,Fe)O has been investigated experimentally under hydrous conditions. Single crystals of
MgO in contact with (Mg0.73Fe0.27)O were annealed hydrothermally at 300 MPa between 1,000 and 1,250°C and using a Ni–NiO buffer. After electron microprobe
analyses, the dependence of the interdiffusivity on Fe concentration was determined using a Boltzmann–Matano analysis. For
a water fugacity of ∼300 MPa, the Fe–Mg interdiffusion coefficient in Fe
x
Mg1−x
O with 0.01 ≤ x ≤ 0.25 can be described by with and C = −80 ± 10 kJ mol−1. For x = 0.1 and at 1,000°C, Fe–Mg interdiffusion is a factor of ∼4 faster under hydrous than under anhydrous conditions. This enhanced
rate of interdiffusion is attributed to an increased concentration of metal vacancies resulting from the incorporation of
hydrogen. Such water-induced enhancement of kinetics may have important implications for the rheological properties of the
lower mantle.
相似文献
Sylvie DemouchyEmail: |
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Maria Dolores Ruiz Cruz 《Mineralogy and Petrology》2011,101(3-4):245-261
Atoll-shaped and normal garnets from schists of the Betic Cordillera (Spain) were studied by electron microprobe, scanning electron microscopy and microstructural electron backscattered scanning diffraction (EBSD). Medium-grade schists contain a textural variety of atoll garnets, characterized by the presence of muscovite, annite, and quartz as main ??core?? phases. Zoning patterns, EBSD analyses and themobarometric data indicate that the micaceous intergrowths formed in most cases through breakdown of a first garnet generation, with orientation of micas being controlled by garnet (c*mica//[111]*Grt and c*mica//[110]*Grt as main crystallographic relationships). Rings formed from multiple nucleation and coalescence, with orientation being controlled, in some cases, by the relics of the initial garnet and more generally by mica orientation. P-T estimates indicate that the first stage of garnet growth occurred at relatively high P (9?C12?kbar/500?C550°C) whereas the second metamorphic stage occurred at lower P and slightly higher T conditions (5?C7?kbar/500?C600°C). 相似文献
8.
Magmatic oxide mineralization widely developed in syenite–gabbro intrusive complexes is an important Fe and Ti resource. However, its origin is hotly debatable. Some researchers believe that the oxide ores were formed through precipitation of dense Ti-magnetite in an initial ferrogabbroic magma (Bai et al., 2012), whereas others consider them as a product of immiscible splitting of Fe-rich liquid during crystallization of Fe–Ti basaltic magma (Zhou et al., 2013). We consider this problem with a study of the Middle Paleoproterozoic (2086 ± 30 Ma) Elet’ozero Ti-bearing layered intrusive complex in northern Karelia (Baltic Shield). The first ore-bearing phase of the complex is mainly made up of diverse ferrogabbros, with subordinate clinopyroxenites and peridotites. Fe–Ti oxides (magnetite, Ti-magnetite, and ilmenite) usually account for 10–15 vol %, reaching 30–70% in ore varieties. The second intrusive phase is formed by alkaline and nepheline syenites. Petrographical, mineralogical, and geochemical data indicate that the first phase of the intrusion was derived from a moderately alkaline Fe–Ti basaltic melt, while the parental melt of the second phase was close in composition to alkaline trachyte. The orebodies comprise disseminated and massive ores. The disseminated Fe–Ti oxide ores make up lenses and layers conformable to general layering. Massive ores occur in subordinate amounts as layers and lenses, as well as cross-cutting veins. Elevated Nb and Ta contents in Fe–Ti oxides makes it possible to consider them complex ores. It is shown that the Fe–Ti oxide mineralization is related to the formation of a residual (Fe,Ti)-rich liquid, which lasted for the entire solidification history of the first intrusive phase. The liquid originated through multiple enrichment of Fe and Ti in the crystallization zone of the intrusion owing to the following processes: (1) precipitation of silicate minerals in the crystallization zone with a corresponding increase in the Fe and Ti contents in an interstitial melt; and (2) periodic accumulation of the residual melt in front of this zone. Unlike liquid immiscibility leading to melt splitting into two phases, this liquid dissolved the residual components of the melt. Correspondingly, such an Fe-rich liquid has unusual properties and requires further study. 相似文献
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E. I. Marchenko N. N. Eremin A. Yu. Bychkov A. E. Grechanovskii 《Moscow University Geology Bulletin》2017,72(5):299-304
Semi-empirical and quantum chemical studies of Al atom energy in CaSiO3 and MgSiO3 with the perovskite-type structure at pressures and temperatures of the Earth’s mantle are reported. The phase diagram for CaSiO3 is reproduced and refined. Probable mechanisms of Al incorporation in the structures studied are considered. According to the results of the calculations, Al is preferably incorporated into MgSiO3, rather than into CaSiO3. Evaluation of the isomorphic capacity of perovskite phases in relation to Al shows that the Al content in MgSiO3 may reach 2.4 mol % at 120 GPa and 2400 K. CaSiO3 cannot be a source of Al atoms in the Earth’s mantle. 相似文献
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The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO2 and periclase–wüstite solid solutions, (MgO · FeO)ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO2 react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO2–CaSiO3 system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO3 (bridgmanite) + L = SiO2 + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO3 (bridgmanite) + (MgO · FeO)ss (ferropericlase), and basic minerals, (FeO · MgO)ss (magnesiowüstite) + SiO2 (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions. 相似文献
13.
Platinum group element (PGE) geochemistry to understand the chemical evolution of the Earth’s mantle
Sisir K. Mondal 《Journal of the Geological Society of India》2011,77(4):295-302
Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical
and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements
show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle
xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate
minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle
harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the
basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process
of the mantle that helped to shape the chemical evolution of the Earth. 相似文献
14.
S. Siegesmund C.-H. Friedel J. Vogel S. Mosch D. Naumann A. Peter H. Giesen 《Environmental Earth Sciences》2011,63(3):641-659
The St. Servatius Church in Quedlinburg (UNESCO’s World Heritage Site, Germany) is characterised by long-standing stability
problems and structural damages, which have been known over the last several centuries. The monotonous Cretaceous sandstone
with its poor lithification is considered to be the main factor. The sandstone is characterised by a high porosity of around
30 Vol.% (max. ca. 35 Vol.%) and a corresponding high w-value. The porosity and the degree of cementation are responsible for the very low compressive strength of around 8 MPa at
maximum, whereas under moisture these values are significantly reduced up to 40%. The freeze–thaw tests indicate a very poor
resistance to frost weathering, which may explain the near-surface softening of the sandstone. Direct shear experiments with
an approximate 60° angle of friction on the sandstone clearly demonstrate the safety margins. Initial geotechnical modelling
does not favour the hypothesis that a landslide of the hill parallel to the southward dipping bedding planes (i.e. shear failure
along weak zones) can occur. A prominent clay layer (also with a southward dip) below the entire castle hill is monitored
to determine the possible amount of movement with respect to the geological discontinuities (e.g. joints, fractures). At present,
a combination of foundation problems are being considered for the stability situation at the church. These include construction
deficiencies due to deformation or softening of the foundation of the forerunner churches and missing or faulty connections
from building additions. Geological factors responsible for the stability problems include the softening of the sandstone
by the influence of weathering and penetrating water as well as the presence of possible shear planes and joints. 相似文献
15.
Kamil Yılmaz 《Mineralogy and Petrology》2010,99(3-4):219-239
The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling. 相似文献
16.
Claudio Marchesi William L. Griffin Carlos J. Garrido Jean-Louis Bodinier Suzanne Y. O’Reilly Norman J. Pearson 《Contributions to Mineralogy and Petrology》2010,159(3):315-330
The western part of the Ronda peridotite massif (Southern Spain) consists mainly of highly foliated spinel-peridotite tectonites
and undeformed granular peridotites that are separated by a recrystallization front. The spinel tectonites are interpreted
as volumes of ancient subcontinental lithospheric mantle and the granular peridotites as a portion of subcontinental lithospheric
mantle that underwent partial melting and pervasive percolation of basaltic melts induced by Cenozoic asthenospheric upwelling.
The Re–Os isotopic signature of sulfides from the granular domain and the recrystallization front mostly coincides with that
of grains in the spinel tectonites. This indicates that the Re–Os radiometric system in sulfides was highly resistant to partial
melting and percolation of melts induced by Cenozoic lithospheric thermal erosion. The Re–Os isotopic systematics of sulfides
in the Ronda peridotites thus mostly conserve the geochemical memory of ancient magmatic events in the subcontinental lithospheric
mantle. Os model ages record two Proterozoic melting episodes at ~1.6 to 1.8 and 1.2–1.4 Ga, respectively. The emplacement
of the massif into the subcontinental lithospheric mantle probably coincided with one of these depletion events. A later metasomatic
episode caused the precipitation of a new generation of sulfides at ~0.7 to 0.9 Ga. These Proterozoic Os model ages are consistent
with results obtained for several mantle suites in Central/Western Europe and Northern Africa as well as with the Nd model
ages of the continental crust of these regions. This suggests that the events recorded in mantle sulfides of the Ronda peridotites
reflect different stages of generation of the continental crust in the ancient Gondwana supercontinent. 相似文献
17.
L. E. Sobisevich K. Kh. Kanonidi A. L. Sobisevich O. I. Miseyuk 《Doklady Earth Sciences》2013,449(1):324-327
18.
The assemblage NiO+Ni-Pd alloy has been calibrated as a precise oxygen fugacity sensor in the temperature range 850–1250 K at 1 bar, using an electrochemical technique with oxygen-specific CSZ electrolytes, and Ni+NiO and Cu+Cu2O as the reference electrodes. Nine compositions were studied, ranging from 0.12 to 0.83 X
Ni
alloy
. Steady EMFs, implying equilibrium, were rapidly achieved in all cells, and were found to be reversible on increasing and decreasing temperature with a precision approaching 0.1 mV. The estimated accuracy of the measurements on each cell is ±0.2 mV (1, corresponding to ±0.003 log-bar units in fo2 at 1273 K). Compositions of the Ni-Pd alloys were measured after each run by electron microprobe, and these compositions were then checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Nickel-rich alloys show positive deviations from ideality and endothermic enthalpies of mixing, but palladium-rich compositions have exothermic enthalpies of mixing and strong negative deviations from ideality. The excess entropies of mixing are positive for all compositions, and correlate approximately with the excess volumes of mixing. The highly asymmetrical deviations from ideality are well described by a polynomial expression of the Redlich-Kister form, with three terms for the enthalpies, and two for the excess entropies and volumes of mixing. The experimental data from this study have been used to re-formulate the Ni-Pd oxygen fugacity sensor to give an expression;
O2
ss
=
O2
NNO
– 2RT ln X
Ni
alloy
– [2 · (1 – X
Ni
alloy
)2 · [(–2165–7.958 · T) + (9409 – 0.888 · T) · (4 X
Ni
alloy
– 1) + 2089 · (6 X
Ni
alloy
– 1) · (2 X
Ni
alloy
– 1)]](850<T<1300) where
O2
ss
is in J mol-1, T is in kelvins, and the expression for
O2
NNO
is that given by O'Neill and Pownceby (1993). Values in terms of log fo2 may be obtained from the above by dividing by RT ln 10. The estimated standard error in
O2
ss
is on the order of ±200 J mol-1, which is approximately ±0.01 log-bar units in fo2 at 1273 K. 相似文献
19.
E. G. Reinhardt M. Little S. Donato D. Findlay A. Krueger C. Clark J. Boyce 《Environmental Geology》2005,47(5):729-739
Frenchmans Bay, on the northern shore of Lake Ontario, has been negatively impacted by eutrophication in the last 50 years through urbanization and the use of chemical fertilizers. Eutrophication began with wholesale land clearance and agricultural practises beginning in the mid-nineteenth century and reached a peak with urbanization after World War II. Eutrophication and the effects of land-use changes on the watershed were investigated by a combined analysis of the sediment magnetic properties and arcellacean (thecamoebian) microfauna. Micropaleontological analyses were conducted on two 2-m-long cores (FMB1 and FMB2) every 10 cm (42 samples) and magnetic susceptibility was measured at 2-cm intervals. Both cores showed a distinct correlative transition at 60 cm and 110 cm that was marked by a rapid increase in thecamoebian concentrations (from approx. 10×103 to 30×103 specimens per cc) and a large increase in Cucurbitella tricuspis (from approx. 10–20% to 40–70%). This transition correlated with a marked increase in magnetic susceptibility (150–200×10–8 m3 Kg–1) at the same depth, which was attributed to elevated levels of detrital magnetic minerals derived from land clearance and soil erosion in the watershed. It was indicated by 210 Pb dates indicate that there was a gradual onset of eutrophication in the mid-nineteenth century (AD 1850±56) and a more rapid rise in the mid-1940s to late 1950s. The initial increase in eutrophication was due to land clearing, agricultural development and increased nutrient loadings. The major eutrophication increase in the 1950s was from urbanization and storm sewer discharge loaded with high yield chemical fertilizers from lawns and gardens. This high concentration of nutrients has led to an unprecedented level of eutrophication within the wetland. 相似文献