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1.
Compositional stratification in the deep mantle 总被引：2，自引：0，他引：2
A boundary between compositionally distinct regions at a depth of about 1600 kilometers may explain the seismological observations pertaining to Earth's lower mantle, produce the isotopic signatures of mid-ocean ridge basalts and oceanic island basalts, and reconcile the discrepancy between the observed heat flux and the heat production of the mid-ocean ridge basalt source region. Numerical models of thermochemical convection imply that a layer of material that is intrinsically about 4 percent more dense than the overlying mantle is dynamically stable. Because the deep layer is hot, its net density is only slightly greater than adiabatic and its surface develops substantial topography. 相似文献
2.
Measurements of the partitioning of rhenium (Re) between garnet and silicate liquid from 1.5 to 2.0 gigapascals and 1250 degrees to 1350 degreesC show that Re is compatible in garnet. Oceanic island basalts (OIBs) have lower Re contents than mid-ocean ridge basalt, because garnet-bearing residues of deeper OIB melting will retain Re. Deep-mantle garnetite or eclogite may harbor the missing Re identified in crust-mantle mass balance calculations. Oceanic crust recycled into the upper mantle at subduction zones will retain high Re/Os (osmium) ratios and become enriched in radiogenic 187Os. Recycled eclogite in a mantle source should be easily traced using Re abundances and Os isotopes. 相似文献
3.
Mantle oxidation state and its relationship to tectonic environment and fluid speciation 总被引：5，自引：0，他引：5
The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth. 相似文献
4.
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
5.
Linear arrays in lead isotope space for mid-ocean ridge basalts (MORBs) converge on a single end-member component that has intermediate lead, strontium, and neodymium isotope ratios compared with the total database for oceanic island basalts (OIBs) and MORBs. The MORB data are consistent with the presence of a common mantle source region for OIBs that is sampled by mantle plumes. 3He/4He ratios for MORBs show both positive and negative correlation with the 206Pb/204Pb ratios, depending on the MORB suite. These data suggest that the common mantle source is located in the transition zone region. This region contains recycled, oceanic crustal protoliths that incorporated some continental lead before their subduction during the past 300 to 2000 million years. 相似文献
6.
Albarède F 《Science (New York, N.Y.)》2008,319(5865):943-945
The canonical model of helium isotope geochemistry describes the lower mantle as undegassed, but this view conflicts with evidence of recycled material in the source of ocean island basalts. Because mantle helium is efficiently extracted by magmatic activity, it cannot remain in fertile mantle rocks for long periods of time. Here, I suggest that helium with high 3He/4He ratios, as well as neon rich in the solar component, diffused early in Earth's history from low-melting-point primordial material into residual refractory "reservoir" rocks, such as dunites. The difference in 3He/4He ratios of ocean-island and mid-ocean ridge basalts and the preservation of solar neon are ascribed to the reservoir rocks being stretched and tapped to different extents during melting. 相似文献
7.
Lee CT Luffi P Chin EJ Bouchet R Dasgupta R Morton DM Le Roux V Yin QZ Jin D 《Science (New York, N.Y.)》2012,336(6077):64-68
Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation. 相似文献
8.
Island arc lavas have radium-226 excesses that extend to higher values than those observed in mid-ocean ridge or ocean island basalts. The initial ratio of radium-226 to thorium-230 is largest in the most primitive lavas, which also have the highest barium/thorium ratios, and decreases with increasing magmatic differentiation. Therefore, the radium-226 excesses appear to have been introduced into the base of the mantle melting column by fluids released from the subducting plate. Preservation of this signal requires transport to the surface arguably in only a few hundreds of years and directly constrains the average melt velocity to the order of 1000 meters per year. Thus, melt segregation and channel formation can occur rapidly in the mantle. 相似文献
9.
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
10.
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
11.
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
12.
Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts 总被引：1，自引：0，他引：1
Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield (40)Ar/(39)Ar dates of 68.53 +/- 0.16 and 68.57 +/- 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an (40)Ar/(39)Ar date of 64.96 +/- 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a (3)He/(4)He ratio of 14.0 times the air ratio, an initial (87)Sr/(86)Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-(3)He mantle plume before the rapid eruption of the Deccan CFB. 相似文献
13.
New high-precision samarium-neodymium isotopic data for chondritic meteorites show that their 142Nd/144Nd ratio is 20 parts per million lower than that of most terrestrial rocks. This difference indicates that most (70 to 95%) of Earth's mantle is compositionally similar to the incompatible element-depleted source of mid-ocean ridge basalts, possibly as a result of a global differentiation 4.53 billion years ago (Ga), within 30 million years of Earth's formation. The complementary enriched reservoir has never been sampled and is probably located at the base of the mantle. These data influence models of Earth's compositional structure and require revision of the timing of global differentiation on Earth's Moon and Mars. 相似文献
14.
Bimodal volcanism in the Santa Maria Province of west-central California occurred when segments of the East Pacific Rise interacted with a subduction zone along the California margin during the Early Miocene (about 17 million years ago). Isotopic compositions of neodymium and strontium as well as trace-element data indicate that these volcanic rocks were derived from a depleted-mantle (mid-ocean ridge basalt) source. After ridge-trench interactions, the depleted-mantle reservoir was juxtaposed beneath the continental margin and was erupted to form basalts. It also assimilated and partially melted local Jurassic-Cretaceous sedimentary and metasedimentary basement rocks to form rhyolites and dacites. 相似文献
15.
Flood basalts and hot-spot tracks: plume heads and tails 总被引：4，自引：0，他引：4
Continental flood basalt eruptions have resulted in sudden and massive accumulations of basaltic lavas in excess of any contemporary volcanic processes. The largest flood basalt events mark the earliest volcanic activity of many major hot spots, which are thought to result from deep mantle plumes. The relative volumes of melt and eruption rates of flood basalts and hot spots as well as their temporal and spatial relations can be explained by a model of mantle plume initiation: Flood basalts represent plume "heads" and hot spots represent continuing magmatism associated with the remaining plume conduit or "tail." Continental rifting is not required, although it commonly follows flood basalt volcanism, and flood basalt provinces may occur as a natural consequence of the initiation of hot-spot activity in ocean basins as well as on continents. 相似文献
16.
Bonifacie M Jendrzejewski N Agrinier P Humler E Coleman M Javoy M 《Science (New York, N.Y.)》2008,319(5869):1518-1520
Chlorine stable isotope compositions (delta37Cl) of 22 mid-ocean ridge basalts (MORBs) correlate with Cl content. The high-delta37Cl, Cl-rich basalts are highly contaminated by Cl-rich materials (seawater, brines, or altered rocks). The low-delta37Cl, Cl-poor basalts approach the composition of uncontaminated, mantle-derived magmas. Thus, most or all oceanic lavas are contaminated to some extent during their emplacement. MORB-source mantle has delta37Cl 相似文献
17.
Archean magnesium-rich komatiites require hot and presumably deep mantle sources, but their trace-element composition and radiogenic isotope composition are similar to those of modern mid-ocean ridge basalts, which originate in the upper mantle. The isotopic composition of helium extracted by sequential crushing of fresh olivines separated from two Archean and one mid-Proterozoic komatiites varies over three orders of magnitude, between a radiogenic end-member rich in helium-4 and a component rich in helium-3. Such helium-3 enrichment suggests the presence of a lower mantle component in Archean komatiites. 相似文献
18.
Murthy VR Evensen NM Jahn BM Coscio MR Dragon JC Pepin RO 《Science (New York, N.Y.)》1972,175(4020):419-421
The lunar mare basalt 15555 from the edge of Hadley Rille has been dated at 3.3x10(9) years by both rubidium-strontium and potassium-argon techniques. Age and trace element abundances closely resemble those of the Apollo 12 mare basalts. Data from lunar basalts obtained thus far indicate that they cannot be derived by simple fractionation from a homogeneous source. 相似文献
19.
Previous studies have suggested that melting processes are responsible for the trace element variability observed in olivine-hosted basaltic melt inclusions. Melt inclusions from three individual lava samples (two from Mangaia, Cook Islands, and one from Tahaa, Society Islands) have heterogeneous Pb isotopic compositions, even though the erupted lavas are isotopically homogeneous. The range of Pb isotopic compositions from individual melt inclusions spans 50 percent of the worldwide range observed for ocean island basalts. The melt inclusion data can be explained by two-component mixing for each island. Our data imply that magmas with different isotopic compositions existed in the volcanic plumbing system before or during melt aggregation. 相似文献
20.
Six garnet pyroxenites from Beni Bousera, Morocco, yield a mean lutetium-hafnium age of 25 +/- 1 million years ago and show a wide range in hafnium isotope compositions (varepsilonHf = -9 to +42 25 million years ago), which exceeds that of known basalts (0 to +25). Therefore, primary melts of garnet pyroxenites cannot be the source of basalts. The upper mantle may be an aggregate of pyroxenites that were left by the melting of oceanic crust at subduction zones and peridotites that were contaminated by the percolation of melts from these pyroxenites. As a consequence, the concept of geochemical heterogeneities as passive tracers is inadequate. Measured lutetium-hafnium partitioning of natural minerals requires a reassessment of some experimental work relevant to mantle melting in the presence of garnet. 相似文献