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1.
Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of 1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in krad as Fe/(Fe + Mg) increases beyond 0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback. 相似文献
2.
We provide new petrological evidence for the strong influence of water on the formation of the oceanic lithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysis throws new light on the hypothesis that new continental lithosphere was formed by the passage of silicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis, we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcano known as “The Thumb”, located in the center of the Colorado Plateau, western United States. The studied sample collection exhibits multi-stage water enrichment processes along point, line and planar defect structures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorph antigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures act like monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinental mantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plate during the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismological techniques to confirm multi-stage hydration from a depth of ∼150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line and planar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspectroscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at a depth of ∼150 km (T ≈ 1225 °C). These lherzolites originated from the middle part of the Farallon mantle slab; they were released during the break up of the Farallon mantle slab, caused by the instability of the dipping slab. The background hydration levels in homogeneous olivines reached ∼138 ppm wt H2O, and the water concentration at the planar defects could reach up to ∼1000 ppm wt H2O. However, the formation of antigorite in grain boundaries was found to be the primary hydration mechanism for harzburgitic samples originating from the subcontinental mantle (for hydration, T ≈ 600 °C). Additionally, the formation of antigorite in lherzolites could be found in annealed cracks. From these observations, we conclude that hydration induces multi-stage water enrichment of the mantle wedge by a process that is dominated by the growth and movement of ubiquitous cracks, which acts as planar defects. Cracks in the mantle seem to be the an important feature in both the water cycle of the subduction zone and the formation of the continental lithosphere. 相似文献
3.
We use geodynamic models with imposed plate velocities to test the forward-modeled history of subduction based on a particular plate motion model against alternative seismic tomography models. We utilize three alternative published reference frames: a hybrid moving hotspot-palaeomagnetic, a hybrid moving hotspot-true polar wander corrected-palaeomagnetic, and a Subduction Reference Frame, a plate model including longitudinal shifts of subduction zones by matching subduction volumes imaged by P-wave tomography, to assess which model best predicts present day mantle structure compared with seismic tomography and volumetrically derived subduction history. Geodynamic modeling suggests paleo-longitudinal corrections applied to the Subduction Reference Frame result in lower mantle slab material beneath North America and East Asia accumulating up to 10–15° westward of that imaged by tomography, whereas the hybrid models develop material offset by 2–9°. However, the Subduction Reference Frame geodynamic model produces slab material beneath the Tethyan Domain coinciding with slab volumes imaged by tomography, whereas the hybrid reference frame models do not, suggesting regional paleo-longitudinal corrections are required to constrain slab locations. We use our models to test inferred slab sinking rates in the mantle focusing on well-constrained regions. We derive a globally averaged slab-sinking rate of 13 ± 3 mm/yr by combining the ages of onset and cessation of subduction from geological data and kinematic reconstructions with images of subducted slabs in the mantle. Our global average slab-sinking rate overlaps with the 15–20 mm/yr rate implied by mantle convection models using a lower mantle viscosity 100 times higher than the upper mantle. 相似文献
4.
A new conceptual model of mantle convection is constructed for consideration of the origin of hotspot plumes, using recent evidence from seismology, high-pressure experiments, geodynamic modeling, geoid inversion studies, and post-glacial rebound analyses. This conceptual model delivers several key points. Firstly, some of the small-scale mantle upwellings observed as hotspots on the Earth's surface originate at the base of the mantle transition zone (MTZ), in which the Archean granitic continental material crust (TTG; tonalite-trondhjemite-granodiorite) with abundant radiogenic elements is accumulated. Secondly, the TTG crust and the subducted oceanic crust that have accumulated at the base of MTZ could act as thermal or mechanical insulators, leading to the formation of a hot and less viscous layer just beneath the MTZ; which may enhance the instability of plume generation at the base of the MTZ. Thirdly, the origin of some hotspot plumes is isolated from the large low shear-wave velocity provinces (LLSVPs) under Africa and the South Pacific. I consider that the conceptual model explains why almost all the hotspots around Africa are located above the margins of the African LLSVP. Because a planetary-scale trench system surrounding a “Pangean cell” has been spatially stable throughout the Phanerozoic, a large amount of the oceanic crustal layer is likely to be trapped in the MTZ under the Pangean cell. Therefore, under Africa, almost all of the hotspot plumes originate from the base of the MTZ, where a large amount of TTG and/or oceanic crusts has accumulated. This conceptual model may explain the fact that almost all the hotspots around Africa are located on margins above the African LLSVP. It is also considered that some of the hotspot plumes under the South Pacific thread through the TTG/oceanic crusts accumulated around the bottom of the MTZ, and some have their roots in the South Pacific LLSVP while others originate from the MTZ. The numerical simulations of mantle convection also speculate that the Earth's mantle convection is not thermally double-layered at the ringwoodite to perovskite + magnesiowüstite (Rw → Pv + Mw) phase boundary, because of its gentle negative Clapeyron slope. This is in contrast with some traditional images of mantle convection that have independent convection cells between the upper and lower mantle. These numerical studies speculate that the generation of stagnant slab at the base of the MTZ (as seismically observed globally) may not be due to the negative Clapeyron slope, and may instead be related to a viscosity increase (i.e., a viscosity jump) at the Rw → Pv + Mw phase boundary, or to a chemically stratified boundary between the upper and the lower mantle, as suggested by a recent high-pressure experiment. 相似文献
5.
《Physics of the Earth and Planetary Interiors》2005,148(2-4):109-130
In an effort to resolve the current conflict between geochemical requirements for an apparently isolated mantle reservoir and recent geophysical evidence for whole-mantle convection, we investigate the possibility that the region above the core-mantle boundary, termed D″, serves as an early-isolated, rare-gas- and incompatible-element-bearing reservoir, and we propose a mechanism for its formation that is a likely outcome of Earth accretion models. In these models, the most cataclysmic event in Earth history, the moon-forming giant impact on the proto-Earth of a Mars-size planet (perhaps as early as 4540 Ma ago) was followed by accretion of smaller bodies long afterwards (until ∼3900 Ma). Some collisions probably triggered melting, metal segregation and degassing. However, the small bodies, fragments, particles, dust, including those of chondrite-like composition, existed on near-earth orbits, were irradiated by intense solar wind, and finally fell on an early-formed, incompatible-element-enriched basaltic crust without causing extensive melting. The respective regions of the crust, loaded with chondrite-like debris, were therefore significantly enriched in iron. When this mixed material was subducted, the bulk density of the subducted lithosphere exceeded that of the bulk silicate mantle, which had already lost its metallic iron to the core. Segregation of this denser material at the base of the mantle was facilitated by the high temperatures at the core-mantle boundary, which greatly reduce the viscosity, as was quantitatively modelled by Christensen and Hofmann (Christensen, U.R., Hofmann, A.W., 1994. Segregation of subducted oceanic-crust in the convecting mantle. J. Geophys. Res.-Solid Earth 99 (B10), 19867–19884). Assuming a basalt/chondrite mass ratio of about 4/1, we obtain a density contrast of ∼7%, which would stabilize the subducted material between the metal core and silicate mantle.Mass balance considerations and preliminary results of geochemical modelling of the above scenario (similar to that performed by Tolstikhin and Marty [Tolstikhin, I.N., Marty, B., 1998. The evolution of terrestrial volatiles, A view from helium, neon, argon and nitrogen isotope modeling. Chem. Geol. 147, 27–52]) show the potential geochemical importance of D″. (1) Modelling of Pu–U–I–Xe isotope systematics predicts formation of this reservoir early in Earth history, ∼100 Ma after formation of the Solar system. (2) The total amount of heat-generating U, Th, K (and other highly incompatible elements) in D″ exceeds 20% of the Earth inventory, and a similar portion of terrestrial heat is being transferred from the core + D″ into the base of the overlying convecting mantle. (3) D″ is enriched in solar implanted rare gases because the small (re)-accreting debris with high surface/mass ratios will have been subjected to intense radiation by the early sun. (4) Rare gases diffuse from D″ into the overlying mantle and are then transferred into upwelling plumes, which originate above D″. In addition, small amounts of D″ material may be entrained by the mantle convective flow as was recently discussed by Schott et al. [Schott, B., Yuen, D.A., Braun, A., 2002. The influences of composition and temperature-dependent rheology in thermal-chemical convection on entrainment of the D″ layer. Physics Earth Planet. Inter. 129, 43–65]. From the rare-gas modelling it follows that initially (∼4500 Ma ago) D″ could have been more massive by a factor of ∼1.2 than at present (about 2 × 1026 g). The present-day mass flux from D″ into the convecting mantle is estimated to be ≤0.05 × 1016 g year−1, a factor of ∼100 less than the rate of ridge magmatism. This small contribution of D″ material makes it difficult to trace fingerprints of D″ even using such sensitive tracers as Pb isotope ratios. (5) The density contrast that stabilizes D″ is maintained by its higher intrinsic density due to the iron-rich chondrite-like component. Subduction of this material, its entrainment by convective mantle flow and mixing could also account for the preservation of the chondritic relative abundances of siderophile elements in the mantle. If D″ is partially molten, the density contrast may be caused by a high-density melt fraction. 相似文献
6.
The Piranshahr metaperidotites in the northwestern end of the Zagros orogen were emplaced following the closure of the Neotethys ocean. The ophiolitic rocks were emplaced onto the passive margin of the northern edge of the Arabian plate as a result of northeastward subduction and subsequent accretion of the continental fragments. The metaperidotites have compositions ranging from low-clinopyroxene lherzolite to harzburgite and dunite. They are mantle residues with distinct geochemical signatures of both mid-ocean ridge and supra subduction zone (SSZ) affinities. The abyssal peridotites are characterized by high Al2O3 and Cr2O3 contents and low Mg-number in pyroxenes. The Cr-number in the coexisting spinel is also low. The SSZ mantle peridotites are characterized by low Al2O3 contents in pyroxenes as well as low Al2O3 and high Cr-number in spinel. Mineral chemical data indicate that the MOR- and SSZ-type peridotites are the residues from ∼15–20% and ∼30–35% of mantle melting, respectively. Considering petrography, mineralogy and textural evidence, the petrological history of the Piranshahr metaperidotites can be interpreted in three stages: mantle stable stage, serpentinization and metamorphism. The temperature conditions in the mantle are estimated using the Ca-in-orthopyroxene thermometer as 1210 ± 26 °C. The rocks have experienced serpentinization. Based on the textural observations, olivine and pyroxene transformed into lizardite and/or chrysotile with pseudomorphic textures at temperatures below 300 °C during the initial stage of serpentinization. Subsequent orogenic metamorphism affected the rocks at temperatures lower than 600 °C under lower-amphibolite facies metamorphism. 相似文献
7.
In this work we have modeled the thermal structure of the lithosphere of the Spanish Central System and the Tajo Basin, and their implications for lithospheric strength. For this, we have used refined heat-producing elements (HPE) values to obtain new estimates of heat production rates in the Spanish Central System and Tajo Basin areas, which have been used joined to the relation between topography and thermal structure of the lithosphere to calculate the best-fit surface heat flows in the study area. Moreover, we have implemented a temperature-dependent thermal conductivity (appropriate for olivine) for the lithospheric mantle to improve the calculations of temperature profiles in the mantle. The geotherms so obtained, together with the implementation of a new rheological law for the upper lithospheric mantle, have been used to calculate refined estimations of the strength and effective elastic thickness of the lithosphere. We have obtained surface heat flow values of 84 mW m−2 and ∼82 mW m−2 for the Spanish Central System and the Tajo Basin, respectively. The thermal state of the lithosphere affects mantle temperatures, and hence may be playing an important role in the uplift and maintenance of the Spanish Central System. 相似文献
8.
《Earth and Planetary Science Letters》2006,241(3-4):888-900
At longer periods, scattered ScS waves sometimes dominate over coda waves at large lapse times. Examining recordings of seismic envelopes at 9 IRIS seismic stations of regional earthquakes with focal depths deeper than 150 km in periods from 1 to 20 s for a wide lapse time range up to 2000 s, we found significant frequency dependence. The coda decay gradient at short periods is steeper than that at longer periods; however, the change of coda gradient associated with the ScS arrival becomes distinct as the period becomes longer. In particular, a clear offset of coda amplitude appears in central Asia for 10 and 15 s period bands. The multiple isotropic scattering process of S-waves in the heterogeneous mantle can be simply simulated by using the Monte Carlo simulation method based on the radiative transfer theory in scattering media. Assuming a two-plane-layer attenuation structure and smoothed velocity model of the PREM, we estimated the average total scattering coefficients of S-waves such as 7.52 × 10− 4∼1.32 × 10− 3 km− 1 and 2.08 × 10− 4∼6.23 × 10− 4 km− 1 at 4 s, and 4.51 × 10− 4∼7.37 × 10− 4 km− 1 and 2.80 × 10− 5∼2.71 × 10− 4 km− 1 at 10 s, for the lithosphere and the upper mantle and for the lower mantle, respectively. Our results indicate that scattering occurs mostly in the lithosphere and the upper mantle and support that medium heterogeneity spreads over the whole mantle though its scattering power is small. Strong scattering occurs beneath central Asia and Papua New Guinea, whereas the scattering beneath Italy and regions of east Russia is much weaker. The numerical calculation enables us to confirm that much stronger scattering than intrinsic attenuation causes the offset behavior with coda decay gradient change after the ScS arrival for 4 and 10 s period bands in some regions. 相似文献
9.
Yongjun Gao Jonathan E. Snow John F. Casey Junbao Yu 《Earth and Planetary Science Letters》2011,301(1-2):231-240
Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7 ppm and δ7Li values of 3 to 5‰, which fall within the range of reported normal pristine “MORB mantle” values. Lithium concentrations vary in the order cpx (2.1–4.7 ppm) > opx (0.9–1.7 ppm) ≥ olivine (0.4–0.9 ppm), the opposite found in “equilibrated” mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with δ7Liolivine (7.14‰–15.09‰) > δ7Liopx (1.81‰–3.66‰) > δ7Licpx (?2.43‰ ? ?0.39‰). The δ7Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δolivine–cpx (δ7Liolivine ? δ7Licpx) and ol/cpxD (Liolivine/Licpx).Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks. 相似文献
10.
《Earth and Planetary Science Letters》2006,241(1-2):65-79
The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable 4He / 3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / Ra ∼8 to 18) and they show primitive neon isotopic ratios (average 21Ne / 21Neext ∼0.044). The neon isotopic systematics and the 4He / 3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar 3He / 22Ne and with a higher (U, Th) / 3He ratio compared to typically high R / Ra hotspot basalts such as those from Iceland and Hawaii. 相似文献
11.
This paper reports ten new surface heat-flow density (qs) values for central and southern Israel (central Sinai Microplate), whose crystalline crust and lithosphere formed as part of the Neoproterozoic Arabian-Nubian Shield. Heat flow was calculated in Mesozoic sediments using the classical approach of heat-flow determination by implementing in the analysis high-precision continuous temperature logs obtained in air- and/or water-filled boreholes. Thermal conductivity (TC) measured for a large suite of rock samples of lithotypes making up the sequence was assigned to temperature gradients in intervals for which the lithology was known. The heat-flow values obtained for different depth intervals in a borehole as well as the average values for the individual borehole locations cover a narrow range, attesting heat-conduction conditions. A steady-state thermal model along an E–W crustal cross section through the area shows that the observed systematic spatial distribution of the qs values, which range between 50 and 62 mW m−2, can primarily be explained by variations in the thickness of the upper crust and in the ratio between sedimentary and crystalline rocks therein. Given the time lapse of thermal heat transfer through the lithosphere, the qs data monitor the crustal thermal conditions prior to rift- and plume-related lithospheric thermal perturbations that have started in the larger area ca. 30 Ma ago. Observed and modeled qs display the best fit for a pre-Oligocene lithosphere–asthenosphere boundary (LAB) at ∼150 km, which would be at the upper end of LAB depths determined from stable areas of the Arabian Shield (150–120 km) not affected by the young, deep-seated thermal processes that have caused a further uprise of the LAB. Our data imply or predict that the surface heat flow of the Sinai Microplate generally tends to increase along N–S and W–E traverses, from ∼45–50 mW m−2 to ∼55–60 mW m−2. Surface heat flows on the order of 55–60 mW m−2 may be common in the northern Arabian Shield, where it exhibits typical lithosphere structure and composition and is unaffected by young heating processes, compared to values of ≤45 mW m−2 recently determined in the southern Arabian Plate for the Arabian Platform. 相似文献
12.
Kay L. Achenbach Michael J. Cheadle Ulrich Faul Peter Kelemen Susan Swapp 《Earth and Planetary Science Letters》2011,301(1-2):199-212
Eleven harzburgites and one dunite from Ocean Drilling Program Leg 209 Hole 1274A preserve high-temperature mantle textures. Electron backscatter diffraction (EBSD) analysis shows moderately developed crystal lattice preferred orientations (LPOs) in olivine and orthopyroxene (M-indices ≈ 0.1) indicative of crystal-plastic deformation at ~ 1250 °C. These rocks preserve a protogranular texture with a weak olivine foliation, a very weak or absent orthopyroxene foliation that may be decoupled from the orthopyroxene LPO, and minor interstitial clinopyroxene and spinel. Olivine grain size distributions, along with melt-related microstructures in orthopyroxene, clinopyroxene and spinel suggest that high-temperature deformation textures have been overprinted by pervasive post-deformation melt-rock interaction. Paleomagnetic data constrain the olivine [100] axes to be subhorizontal and oriented at low angle (≤ 28.6° ± 10.6°) to the ridge axis at the onset of serpentinization. This orientation is consistent with either complex 3-D mantle upwelling or 2-D mantle upwelling coupled with complex 3-D tectonic emplacement to the seafloor. 相似文献
13.
《Earth and Planetary Science Letters》2006,241(1-2):11-20
The location in the Barberton Greenstone Belt (Kaapvaal Craton) of ∼3.26–3.24 Ga asteroid impact ejecta units at, and immediately above, a sharp break between a > 12 km-thick mafic–ultramafic volcanic crust (Onverwacht Group ∼3.55–3.26 Ga, including the ∼3.298 > 3.258 Ga Mendon Formation) and a turbidite–felsic volcanic rift-facies association (Fig Tree Group ∼3.258–3.225 Ga), potentially represents the first documented example of cause–effect relations between extraterrestrial bombardment and major tectonic and igneous events [D.R. Lowe, G.R. Byerly, F. Asaro, F.T. Kyte, Geological and geochemical record of 3400 Ma old terrestrial meteorite impacts, Science 245 (1989) 959–962; D.R. Lowe, G.R. Byerly, F.T. Kyte, A. Shukolyukov, F. Asaro, A. Krull, Spherule beds 3.47–3.34 Ga-old in the Barberton greenstone belt, South Africa: a record of large meteorite impacts and their influence on early crustal and biological evolution, Astrobiology 3 (2003) 7–48; A.Y. Glikson, The astronomical connection of terrestrial evolution: crustal effects of post-3.8 Ga mega-impact clusters and evidence for major 3.2 ± 0.1 Ga bombardment of the Earth–Moon system, J. Geodyn. 32 (2001) 205–229]. Here we correlate this boundary with a contemporaneous break and peak magmatic and faulting events in the Pilbara Craton, represented by the truncation of a 3.255–3.235 Ga-old volcanic sequence (Sulphur Springs Group—SSG) by a turbidite-banded iron formation–felsic volcanic association (Pincunah Hill Formation, basal Gorge Creek Group). These events are accompanied by ∼3.252–3.235 Ga granitoids (Cleland plutonic suite). The top of the komatiite–tholeiite–rhyolite sequence of the SSG is associated with a marker chert defined at 3.238 ± 3–3.235 ± 3 Ga, abruptly overlain by an olistostrome consisting of mega-clasts of felsic volcanics, chert and siltstone up to 250 × 150 m-large, intercalated with siliciclastic sedimentary rocks and felsic volcanics (Pincunah Hill Formation-basal Gorge Creek Group-GCG [R. M. Hill, Stratigraphy, structure and alteration of hanging wall sedimentary rocks at the Sulphur Springs volcanogenic massive sulphide (VMS) prospect, east Pilbara Craton, Western Australia. B.Sc Hon. Thesis, University of Western Australia (1997) 67 pp.; M.J. Van Kranendonk, A.H. Hickman, R.H. Smithies, D.R. Nelson, Geology and tectonic evolution of the Archaean north Pilbara terrain, Pilbara Craton, Western Australia, Econ. Geol. 97 (2002) 695–732; M.J. Van Kranendonk, Geology of the North Shaw 1 : 100 000 Sheet. Geological Survey Western Australia 1 : 100 000 Geological Series (2000) 86 pp., R. Buick, C.A.W. Brauhart, P. Morant, J.R. Thornett, J.G. Maniew, J.G. Archibald, M.G. Doepel, I.R. Fletcher, A.L. Pickard, J.B. Smith, M.B. Barley, N.J. McNaughton, D.I. Groves, Geochronology and stratigraphic relations of the Sulphur Springs Group and Strelley Granite: a temporally distinct igneous province in the Archaean Pilbara Craton, Australia, Precambrian Res. 114 (2002) 87–120]). The structure and scale of the olistostrome, not seen elsewhere in the Pilbara Craton, is interpreted in terms of intense faulting and rifting, supported by topographic relief represented by deep incision of overlying arenites (Corboy Formation) into underlying units [M.J. Van Kranendonk, Geology of the North Shaw 1 : 100 000 Sheet. Geological Survey Western Australia 1 : 100 000 Geological Series (2000) 86 pp.]. The age overlaps between (1) 3.255 ± 4–3.235 ± 3 Ga peak igneous activity represented by the SSG and the Cleland plutonic suite (Pilbara Craton) and the 3.258 ± 3 Ga S2 Barberton impact unit, and (2) 3.235 ± 3 Ga top SSG break and associated faulting and the 3.243 ± 4 S3–S4 Barberton impact units may not be accidental. Should correlations between the Barberton S2–S4 impact units and magmatic and tectonic events in the Pilbara Craton be confirmed, they would imply impact-triggered reactivation of mantle convection, crustal anatexis, faulting and strong vertical movements in Archaean granite–greenstone terrains associated with large asteroid impacts, culminating in transformation from sima-dominated crust to continental rift environments. 相似文献
14.
In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed. 相似文献
15.
Detailed mapping of geomorphological and biological sea-level markers around the Capo Vaticano promontory (western Calabria, Italy), has documented the occurrence of four Holocene paleo-shorelines raised at different altitudes. The uppermost shoreline (PS1) is represented by a deeply eroded fossiliferous beach deposit, reaching an elevation of ∼2.2 m above the present sea-level, and by a notch whose roof is at ∼2.3 m. The subjacent shoreline PS2 is found at an elevation of ∼1.8 m and is represented by a Dendropoma rim, a barnacle band and by a wave-cut platform. Shoreline PS3 includes remnants of vermetid concretions, a barnacle band, a notch and a marine deposit, and reaches an elevation of ∼1.4 m. The lowermost paleo-shoreline (PS4) includes a wave-cut platform and a notch and reaches an elevation of ∼0.8 m. Radiocarbon dating of material from individual paleo-shorelines points to an average uplift rate of 1.2–1.4 mm/yr in the last ∼6 ka at Capo Vaticano. Our data suggest that Holocene uplift was asymmetric, with a greater magnitude in the south-west sector of the promontory, in a manner similar to the long-term deformation attested by Pleistocene terraces. The larger uplift in the south-western sector is possibly related to the additional contribution, onto a large-wavelength regional signal, of co-seismic deformation events, which are not registered to the north-east. We have recognized four co-seismic uplift events at 5.7–5.4 ka, 3.9–3.5 ka, ∼1.9 ka and <1.8 ka ago, superposed on a regional uplift that in the area, is occurring at a rate of ∼1 mm/yr. Our findings places new constrains on the recent activity of border faults south of the peninsula and on the location of the seismogenic source the 1905 destructive earthquake. 相似文献
16.
Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of hazardous and radioactive wastes. Cement is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill and liner materials) of repositories for radioactive waste. In this study, bulk-X-ray absorption spectroscopy (XAS) was used to investigate the uptake mechanism of Nd on the crystalline C–S–H phase 11 Å tobermorite in the presence of Zn (co-absorbing metal), and vice versa, as potential competitor under strongly alkaline conditions (pH = 12.5–13.3). The Zn and Nd concentration in all samples was 50,000 ppm, whereas the reaction times varied from 1 to 6 months.Extended X-ray absorption fine structure (EXAFS) data of the Nd LIII-edge indicate that the local structural environment of Nd consists of ∼7–8 O atoms at 2.42 Å, ∼7–8 Si at ∼3.67 Å and ∼5–6 Ca at ∼3.8 Å, and that this environment remains unchanged in the presence and absence of Zn. In contrary, Zn K-edge EXAFS data exhibit distinct differences in the presence and absence of Nd as co-absorbing element. Data analysis indicates that Zn is tetrahedrally coordinated (∼4 O at ∼1.96 Å) and the obtained structural data in the simultaneous presence of Nd and Zn are consistent with the formation of mixed Zn surface complexes and Zn bound in the interlayer remaining in these positions also with prolonged reaction times (up to 6 months). However, without the co-absorbing element Nd, strong structural changes in the uptake mechanisms of Zn are observable, e.g., after 3 month reaction time Zn–Zn backscattering pairs can be observed. These findings suggest that Nd has an influence on the incorporation of Zn in the tobermorite structure. In addition, the results of this study indicate that competitive uptake of metal cations with similar sorption behaviour by C–S–H phases can take place, deserving further attention in future assessments of the safe disposal of radioactive wastes in cement-based repositories. 相似文献
17.
Motohiko Murakami Yasuo Ohishi Naohisa Hirao Kei Hirose 《Earth and Planetary Science Letters》2009,277(1-2):123-129
The aggregate shear wave velocities of MgO (periclase) have been determined throughout Earth's lower mantle pressure regime approaching 130 GPa using Brillouin spectroscopy in conjunction with synchrotron X-ray diffraction technique in a diamond anvil cell apparatus. We found that the extrapolations of the high-pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies. However, the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus (dG/dP = G0′) of MgO is 1.92(2), which is distinctly lower than that of previous lower-pressure experiments. Compared with the previous results on (Mg,Fe)O ferropericlase, there is no clear correlation between iron content and G0′. We calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower G0′ value of periclase can remarkably well reproduce the global seismological 1-D velocity profile model with uniform composition model. The best-fitting result indicates the possibility of a lower mantle mineralogy with ~ 92 vol.% silicate perovskite phase, implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic. The present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the Earth's lower mantle. 相似文献
18.
《Earth and Planetary Science Letters》2006,241(3-4):932-951
We report volatile (H2O, CO2, F, S, Cl) and trace element data for submarine alkalic basalt glasses from the three youngest Samoan volcanoes, Ta'u, Malumalu and Vailulu'u. Most samples are visibly sulfide saturated, so have likely lost some S during fractionation. Cl / K ratios (0.04–0.15) extend to higher values than pristine MORBs, but are suspected to be partly due to source differences since Cl / K roughly varies as a function of 87Sr / 86Sr. There are no resolvable differences in the relative enrichment of F among sources, and compatibility of F during mantle melting is established to be nearly identical to Nd. Shallow degassing has affected CO2 in all samples, and H2O only in the most shallowly erupted samples from Vailulu'u. Absolute water contents are high for Samoa (0.63–1.50 wt.%), but relative enrichment of water compared to equally incompatible trace elements (Ce, La) is low and falls entirely below normal MORB values. H2O / Ce (58–157) and H2O / La (120–350) correlate inversely with 87Sr / 86Sr compositions (0.7045–0.7089). This leads us to believe that, because of very fast diffusion of hydrogen in mantle minerals, recycled lithospheric material with high initial water and trace element content will lose water to the drier ambient mantle during storage within the inner Earth. The net result is the counter-intuitive appearance of greater dehydration with greater mantle enrichment. We expect that subducted slabs will experience a two-stage dehydration history, first within subduction zones and then in the ambient mantle during long-term convective mixing. 相似文献
19.
In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5 cm/yr in upper reaches and ∼13.5 cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1 m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1 ka) and differing incision rates of the surface of same terrace (T2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1 ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12 mm/yr and 7 mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27 mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan. 相似文献
20.
《Earth and Planetary Science Letters》2007,253(1-2):1-16
Energy dispersive spectrometry (EDS), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) track analyses of chlorite-dominated quench-textured microkrystite spherules and LA-ICPMS spot analyses of intra-spherule Ni-rich skeletal quench chromites from the 3243 ± 4 Ma Barberton S3 impact fallout unit (lower part of the Mapepe Formation, Fig Tree Group, Barberton greenstone belt, Kaapvaal Craton, South Africa) reveal fractionated siderophile and PGE trace element patterns corresponding to chondrite-contaminated komatiite/basalt compositions. The chlorites, interpreted as altered glass, contain sharp siderophile elements and PGE spikes inherited from decomposed metal and Ni-rich chromite particles. LA-ICPMS spot analysis identifies PGE-rich micronuggets in Ni–chromites (Ir ∼ 12–100 ppm, Os ∼ 9–86 ppm, Ru ∼ 5–43 ppm) and lower levels of the volatile PGEs (Rh ∼ 1–11 ppm, Pd ∼ 0.68–0.96 ppm). Previously reported PGE anomalies in the order of hundreds of ppb in some Barberton microkrystite spherules are accounted for in terms of disintegration of PGE-rich micronuggets. Replacement of the Ni-chromites by sulphide masks primary chondritic patterns and condensation element distribution effects. High refractory/volatile PGE ratios pertain to both the chlorites and the Ni-rich chromites, consistent with similar compositional relations in microkrystite spherules from other impact fallout units in the Barberton greenstone belt and the Pilbara Craton, Western Australia. The near-consistent low Pt/Re and high V/Cr and V/Sc ratios in chlorite of the spherules, relative to komatiites, are suggestive of selective atmospheric condensation of the spherules which favored the relatively more refractory Re and V. Selective condensation may also be supported by depletion in the volatile Yb relative to Sm. Ni–Cr relationships allow estimates of the proportion of precursor crustal and meteoritic components of the spherules. Mass balance calculations based on the iridium flux allow estimates of the order of magnitude of the diameter of the chondritic projectile. 相似文献