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1.
Gannoun A Burton KW Thomas LE Parkinson IJ Van Calsteren P Schiano P 《Science (New York, N.Y.)》2004,303(5654):70-72
Radiogenic isotope variations in mid-ocean ridge basalts (MORB) are commonly attributed to compositional variations in Earth's upper mantle. For the rheniumosmium isotope system, constituent MORB phases are shown to possess exceptionally high Re/Os (parent/daughter) ratios, consequently radiogenic 187Os is produced from the decay of 187Re over short periods of time. Thus, in the absence of precise age constraints, Os isotope variations cannot be unambiguously attributed to their source, although Re-Os isotope data for constituent minerals can yield crystallization ages, details of equilibration, and initial Os isotope values that relate directly to the mantle source. 相似文献
2.
Seismic tomography reveals garnet-rich crust and mantle lithosphere descending into the upper mantle beneath the southeastern Sierra Nevada. The descending lithosphere consists of two layers: an iron-rich eclogite above a magnesium-rich garnet peridotite. These results place descending eclogite above and east of high P wave speed material previously imaged beneath the southern Great Valley, suggesting a previously unsuspected coherence in the lithospheric removal process. 相似文献
3.
The seismologically delineated transition zone, at depths between 400 and 670 kilometers, is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle. Xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite. These xenoliths show evidence for the high-pressure (90 to 120 kilobars) transformation of pyroxene to a solid solution of pyroxene in garnet (majorite) and silicon in octahedral coordination; low-pressure (less than 80 kilobars) exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved. Although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths, it is likely that both majorite and beta-spinel help produce the observed seismic gradient of the transition zone. 相似文献
4.
Hotspots, basalts, and the evolution of the mantle 总被引：2，自引：0，他引：2
Anderson DL 《Science (New York, N.Y.)》1981,213(4503):82-89
The trace element concentration patterns of continental and ocean island basalts and of mid-ocean ridge basalts are complementary. The relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment-depletion patterns. Their combined volume occupies most of the mantle above the 670 kilometer discontinuity. The source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt. The mid-ocean ridge basalts source evolved from an eclogite cumulate that lost its late-stage enriched fluids at various times to the shallower mantle and continental crust. The mid-ocean ridge basalts source is rich in garnet and clinopyroxene, whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment. These relationships are consistent with the evolution of a terrestrial magma ocean. 相似文献
5.
Geophysical discontinuities in Earth's upper mantle and experimental data predict the structural transformation of pyroxene to garnet and the solid-state dissolution of pyroxene into garnet with increasing depth. These predictions are indirectly verified by omphacitic pyroxene exsolution in pyropic garnet-bearing xenoliths from a diamondiferous kimberlite. Conditions for silicon in octahedral sites in the original garnets are met at pressures greater than 130 kilobars, placing the origin of these xenoliths at depths of 300 to 400 kilometers. These ultradeep xenoliths support the theory that the 400-km seismic discontinuity is marked by a transition from peridotite to eclogite. 相似文献
6.
Luth RW 《Science (New York, N.Y.)》1993,261(5117):66-68
The reaction dolomite + 2 coesite --><-- diopside + 2 diamond + 2O(2) defines the coexistence of diamond and carbonate in mantle eclogites. The oxygen fugacity of this reaction is approximately 1 log unit higher at a given temperature and pressure than the oxygen fugacities of the analogous reactions that govern the stability of diamond in peridotite. This difference allows diamond-bearing eclogite to coexist with peridotite containing carbonate or carbonate + diamond. This potential coexistence of diamond-bearing eclogite and carbonate-bearing peridotite can explain the presence of carbon-free peridotite interlayered with garnet pyroxenites that contain graphitized diamond in the Moroccan Beni Bousera massif at the Earth's surface and the preferential preservation of diamond-bearing eclogitic relative to peridotitic xenoliths in the Roberts Victor kimberlite. 相似文献
7.
Mid-ocean ridge basalts (MORBs) and ocean island basalts (QIBs) are derived by partial melting of the upper mantle and are marked by systematic excesses of thorium-230 activity relative to the activity of its parent, uranium-238. Experimental measurements of the distribution of thorium and uranium between the melt and solid residue show that, of the major phases in the upper mantle, only garnet will retain uranium over thorium. This sense of fractionation, which is opposite to that caused by clinopyroxene-melt partitioning, is consistent with the thorium-230 excesses observed in young oceanic basalts. Thus, both MORBs and QIBs must begin partial melting in the garnet stability field or below about 70 kilometers. A calculation shows that the thorium-230-uranium-238 disequilibrium in MORBs can be attributed to dynamic partial melting beginning at 80 kilometers with a melt porosity of 0.2 percent or more. This result requires that melting beneath ridges occurs in a wide region and that the magma rises to the surface at a velocity of at least 0.9 meter per year. 相似文献
8.
Temperatures of equilibration for the majority (81 percent) of the eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 1000 degrees and 1250 degrees C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 10(9) years) of these eclogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle convection for the earth. 相似文献
9.
Measurement of rhenium (Re) and osmium (Os) concentrations and Os isotopic compositions in Ivory Coast tektites (natural glasses with upper crustal compositions that are ejected great distances during meteorite impact) and rocks from the inferred source crater, Lake Bosumtwi, Ghana, show that these tektites incorporate about 0.6 percent of a meteoritic component. Analysis of elemental abundances of noble metals alone gives equivocal results in the detection of meteoritic components because the target rocks already have relatively large amounts of noble metals. The Re-Os system is ideally suited for the study of meteorite impacts on old continental crust for three reasons. (i) The isotopic compositions of the target rocks and the meteoritic impactor are significantly different. (ii) Closed-system mixing of target rocks and meteorites is linear on Re-Os isochron diagrams, which thus permits identification of the loss of Re or Os. (iii) Osmium isotopic compositions are not likely to be altered during meteorite impact even if Re and Os are lost. 相似文献
10.
Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite (187)Re/(186)Os and (l87)Os/(l86)Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula. 相似文献
11.
AD Brandon RJ Walker JW Morgan MD Norman HM Prichard 《Science (New York, N.Y.)》1998,280(5369):1570-1573
Osmium isotopic analyses of picritic lavas from Hawaii show enrichments in the osmium-186/osmium-188 ratio (186Os/188Os) of 0. 008 to 0.018%, relative to a chondritic upper mantle, that are positively correlated with enrichments in 187Os/188Os of 5.4 to 9.0%. The most viable mechanism to produce these coupled 186Os and 187Os enrichments is by addition of 0.5 to 1 weight percent of outer core metal to a portion of the D" layer and subsequent upwelling of the mixture. These data suggest that some plumes originate at the core-mantle boundary and that Os isotopes may be used to distinguish plumes derived from shallow versus deep mantle sources. 相似文献
12.
The amount of recycled crust in sources of mantle-derived melts 总被引：5，自引：0，他引：5
Sobolev AV Hofmann AW Kuzmin DV Yaxley GM Arndt NT Chung SL Danyushevsky LV Elliott T Frey FA Garcia MO Gurenko AA Kamenetsky VS Kerr AC Krivolutskaya NA Matvienkov VV Nikogosian IK Rocholl A Sigurdsson IA Sushchevskaya NM Teklay M 《Science (New York, N.Y.)》2007,316(5823):412-417
Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting. 相似文献
13.
Thermobarometric and Os isotopic data for peridotite xenoliths from late Miocene and younger lavas in the Sierra Nevada reveal that the lithospheric mantle is vertically stratified: the shallowest portions (<45 to 60 kilometers) are cold (670 degrees to 740 degrees C) and show evidence for heating and yield Proterozoic Os model ages, whereas the deeper portions (45 to 100 kilometers) yield Phanerozoic Os model ages and show evidence for extensive cooling from temperatures >1100 degrees C to 750 degrees C. Because a variety of isotopic evidence suggests that the Sierran batholith formed on preexisting Proterozoic lithosphere, most of the original lithospheric mantle appears to have been removed before the late Miocene, leaving only a sliver of ancient mantle beneath the crust. 相似文献
14.
Ancient mantle in a modern arc: osmium isotopes in izu-bonin-mariana forearc peridotites 总被引：2，自引：0，他引：2
Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized. 相似文献
15.
Rutile-bearing refractory eclogites: missing link between continents and depleted mantle 总被引：9，自引：0，他引：9
A mass imbalance exists in Earth for Nb, Ta, and possibly Ti: continental crust and depleted mantle both have subchondritic Nb/Ta, Nb/La, and Ti/Zr, which requires the existence of an additional reservoir with superchondritic ratios, such as refractory eclogite produced by slab melting. Trace element compositions of minerals in xenolithic eclogites derived from cratonic lithospheric mantle show that rutile dominates the budget of Nb and Ta in the eclogites and imparts a superchondritic Nb/Ta, Nb/La, and Ti/Zr to the whole rocks. About 1 to 6 percent by weight of eclogite is required to solve the mass imbalance in the silicate Earth, and this reservoir must have an Nb concentration >/= 2 parts per million, Nb/La >/= 1.2, and Nb/Ta between 19 and 37-values that overlap those of the xenolithic eclogites. As the mass of eclogite in the continental lithosphere is significantly lower than this, much of this material may reside in the lower mantle, perhaps as deep as the core-mantle boundary. 相似文献
16.
Subduction recycles aqueous fluids from slab and sediment to the mantle. Subduction zones are long-lived, but time scales for fluid-rock interaction within subduction complexes are uncertain. Large-ion lithophile elements (potassium and barium) were added to eclogite (subducted basalt) during high pressure/temperature metamorphism via phengite crystallization from subduction zone fluids. Phengite grains from eclogite blocks and their metasomatic selvages yielded 40Ar/39Ar ages across grains and between samples that indicate 25 and 60 million years of fluid-rock interaction in the Samana Complex, Dominican Republic, and the Franciscan Complex, California, respectively. 相似文献
17.
Luguet A Graham Pearson D Nowell GM Dreher ST Coggon JA Spetsius ZV Parman SW 《Science (New York, N.Y.)》2008,319(5862):453-456
To explain the elevated osmium isotope (186Os-187Os) signatures in oceanic basalts, the possibility of material flux from the metallic core into the crust has been invoked. This hypothesis conflicts with theoretical constraints on Earth's thermal and dynamic history. To test the veracity and uniqueness of elevated 186Os-187Os in tracing core-mantle exchange, we present highly siderophile element analyses of pyroxenites, eclogites plus their sulfides, and new 186Os/188Os measurements on pyroxenites and platinum-rich alloys. Modeling shows that involvement in the mantle source of either bulk pyroxenite or, more likely, metasomatic sulfides derived from either pyroxenite or peridotite melts can explain the 186Os-187Os signatures of oceanic basalts. This removes the requirement for core-mantle exchange and provides an effective mechanism for generating Os isotope diversity in basalt source regions. 相似文献
18.
Lambert DD Morgan JW Walker RJ Shirey SB Carlson RW Zientek ML Koski MS 《Science (New York, N.Y.)》1989,244(4909):1169-1174
Isotopic data for the Stillwater Complex, Montana, which formed about 2700 Ma (million years ago), were obtained to evaluate the role of magma mixing in the formation of strategic platinum-group element (PGE) ore deposits. Neodymium and osmium isotopic data indicate that the intrusion formed from at least two geochemically distinct magmas. Ultramafic affinity (U-type) magmas had initial epsilon(Nd) of -0.8 to -3.2 and a chondritic initial (187)Os/(186)Os ratio of approximately 0.88, whereas anorthositic affinity (A-type) magmas had epsilon(Nd) of -0.7 to +1.7 and an initial (187)Os/(186)Os ratio of approximately -1.13. These data suggest that U-type magmas were derived from a lithospheric mantle source containing recycled crustal materials whereas A-type magmas originated either by crustal contamination of basaltic magmas or by partial melting of basalt in the lower crust. The Nd and Os isotopic data also suggest that Os, and probably the other PGEs in ore horizons such as the J-M Reef, was derived from A-type magmas. The Nd and Os isotopic heterogeneity observed in rocks below the J-M Reef also suggests that A-type magmas were injected into the Stillwater U-type magma chamber at several stages during the development of the Ultramafic series. 相似文献
19.
Recycled subducted ocean crust has been traced by elevated 187Os/188Os in some studies and by high nickel and low manganese contents in others. Here, we show that these tracers are linked for Quaternary lavas of Iceland, strengthening the recycling model. An estimate of the osmium isotopic composition of both the recycled crust and the mantle peridotite implies that Icelandic Quaternary lavas are derived in part from an ancient crustal component with model ages between 1.1 _ 109 and 1.8 _ 109 years and from a peridotitic end-member close to present-day oceanic mantle. 相似文献
20.
Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle (182)W isotopic heterogeneities within Earth's mantle. Tungsten isotopic data for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved (182)W excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined (182)W, (186,187)Os, and (142,143)Nd isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir-which likely formed within the first 30 million years of solar system history-indicates that the mantle may have never been well mixed. 相似文献