Vesicle-Specific Noble Gas Analyses of "Popping Rock": Implications for Primordial Noble Gases in Earth

Gases trapped in individual vesicles in the volatile-rich basaltic glass "popping rock" were found to have the same carbon dioxide, helium-4, and argon-40 composition, but a variable 40Ar/36Ar ratio ( approximately 4000 to >/=40,000). The argon-36 is probably surface-adsorbed atmospheric argon; any mantle argon-36 trapped in the vesicles cannot be distinguished from an atmospheric contaminant. Consequently the 40Ar/36Ar ratios and 3He/36Ar ratios (1.45) determined are minimum estimates of the upper mantle composition. Heavy noble gas relative abundances in the mantle resemble solar noble gas abundance patterns, and a solar origin may be common to all primordial mantle noble gases.     

Primordial noble gases in chondrites: the abundance pattern was established in the solar nebula

Ordinary chondrites, like carbonaceous chondrites, contain primordial noble gases mainly in a minor phase comprising 相似文献   

Retention of Solar Helium and Neon in IDPs in Deep Sea Sediment

It was recently proposed that subduction of interplanetary dust particles (IDPs) contained in deep sea sediments could have introduced substantial solar helium and neon to the Earth's mantle. However, it is not certain if IDPs would retain solar noble gases during subduction. A diffusion experiment that examined He and Ne in IDPs in a magnetic separate from Pacific Ocean sediments showed that He and Ne would be lost from IDPs within 3 years at 500 degrees C, and possibly within 10(5) years at 200 degrees C, which suggests that they would be lost from subducting slabs at shallow depths.     

Trapped solar wind noble gases, kr81/kr exposure ages and k/ar ages in apollo 11 lunar material

Grain size and etching experiments show that the fine lunar material contains large amounts of trapped solar wind particles. Elemental and isotopic compositions of the noble gases in solar material and in the terrestrial atmosphere are significantly different, except for the Ar(36)/ Ar(38) and the Kr isotope ratios. Exposure ages of two rocks and of the fine material are between 380 and 510 x 10(6) years. Feldspar concentrates give K/Ar ages of 3220 and 3300 x 10(6) years, significantly higher than the unseparated rock.     

Cooling of Tropical Brazil (5{degrees}C) During the Last Glacial Maximum

A 30,000-year paleotemperature record derived from noble gases dissolved in carbon-14-dated ground water indicates that the climate in lowland Brazil (Piaui Province, 7 degrees S, 41.5 degrees W; altitude, 400 meters) was 5.4 degrees +/- 0.6 degrees C cooler during the last glacial maximum than today. This result suggests a rather uniform cooling of the Americas between 40 degrees S and 40 degrees N. A 5.4 degrees C cooling of tropical South America is consistent with pollen records, snow line reconstructions, and strontium/calcium ratios and delta(18)O coral records but is inconsistent with the sea-surface temperature reconstruction of CLIMAP (Climate: Long-Range Investigation, Mapping and Prediction). On the basis of these results, it appears that the tropical Americas are characterized by a temperature sensitivity comparable to that found in higher latitudes.     

Venus upper atmosphere neutral composition: preliminary results from the pioneer venus orbiter

Measurements in situ of the neutral composition and temperature of the thermosphere of Venus are being made with a quadrupole mass spectrometer on the Pioneer Venus orbiter. The presence of many gases, incluiding the major constituents CO(2), CO, N(2), O, and He has been confirmed. Carbon dioxide is the most abundant constituent at altitudes below about 155 kilometers in the terminator region. Above this altitude atomic oxygen is the major constituent, with O/CO(2) ratios in the upper atmosphere being greater than was commonly expected. Isotope ratios of O and C are close to terrestrial values. The temperature inferred from scale heights above 180 kilometers is about 400 K on the dayside near the evening terminator at a solar zenith angle of about 69 degrees . It decreases to about 230 K when the solar zenith angle is about 90 degrees .     

Composition and structure of the venus atmosphere: results from pioneer venus

Results from the Pioneer Venus sounder probe neutral mass spectrometer indicate that there is no difference in the isotopic ratios of carbon and oxygen between Venus and Earth to within +/- 5 percent. The mixing ratio of nitrogen is 3.5(+3)(-2) percent with an isotopic ratio within 20 percent of that of Earth. The ratio of argon-36 to argon-40 is 85 percent, and the ratio of argon-38 to argon-36 is 20 percent. The mixing ratios of argon-36 and argon-40 are approximately 40 and 50 parts per million, respectively, with an error of about a factor of 2 (mainly toward a lesser amount) resulting from uncertainty in the response of the ion pump to rare gases. Hydrogen chloride cannot account for more than a few percent of the 36 mass peak, and therefore the large excess of primordial argon is a reasonable conclusion. The ratio of neon-20 to argon-36 of 0.5 +/- 0.3 is definitely terrestrial in character rather than solar. These results indicate that there is a large excess of all primordial noble gases on Venus relative to Earth. There appears to be a considerably higher abundance of sulfur compounds below 20 kilometers than in or above the main cloud layer. The 32 and 60 mass peaks show a sharp increase below 22 kilometers, indicating the possible production of sulfur and carbon oxysulfide (COS) at the expense of sulfur dioxide.     

Gravitational separation of gases and isotopes in polar ice caps

Atmospheric gases trapped in polar ice at the firn to ice transition layer are enriched in heavy isotopes (nitrogen-15 and oxygen-18) and in heavy gases (O(2)/N(2) and Ar/N(2) ratios) relative to the free atmosphere. The maximum enrichments observed follow patterns predicted for gravitational equilibrium at the base of the firn layer, as calculated from the depth to the transition layer and the temperature in the firn. Gas ratios exhibit both positive and negative enrichments relative to air: the negative enrichments of heavy gases are consistent with observed artifacts of vacuum stripping of gases from fractured ice and with the relative values of molecular diameters that govern capillary transport. These two models for isotopic and elemental fractionation provide a basis for understanding the initial enrichments of carbon-13 and oxygen-18 in trapped CO(2), CH(4), and O(2) in ice cores, which must be known in order to decipher ancient atmospheric isotopic ratios.     

Infrared spectroscopy of comet 81P/Wild 2 samples returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.     

Rogue mantle helium and neon

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.     

Flow of mantle fluids through the ductile lower crust: helium isotope trends

Heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust. Minimum 3He/4He ratios in surface fluids from the northern Basin and Range Province, western North America, increase systematically from low crustal values in the east to high mantle values in the west, a regional trend that correlates with the rates of active crustal deformation. The highest ratios occur where the extension and shear strain rates are greatest. The correspondence of helium isotope ratios and active transtensional deformation indicates a deformation-enhanced permeability and that mantle fluids can penetrate the ductile lithosphere, even in regions where there is no substantial magmatism. Superimposed on the regional trend are local, high 3He/4He anomalies indicating hidden magmatic activity and/or deep fluid production with locally enhanced permeability, identifying zones with high resource potential, particularly for geothermal energy development.     

Noble gas partitioning between metal and silicate under high pressures

Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earth's core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.     

Subduction and recycling of nitrogen along the Central American margin

We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta15N values are positive (i.e., greater than air) and N2/He ratios are high (up to 25,000). In contrast, Costa Rican N2/He ratios are low (maximum 1483) and delta15N values are negative (minimum -3.0 per mil). The results identify shallow hemipelagic sediments, subducted into the Guatemalan mantle, as the transport medium for the heavy N. Mass balance arguments indicate that the subducted N is efficiently cycled to the atmosphere by arc volcanism. Therefore, the subduction zone acts as a "barrier" to input of sedimentary N to the deeper mantle.     

Isotopic composition of rare gases in lunar samples

Data from total melt and step-by-step heating experiments on the Apollo 11 lunar samples suggest a close affinity between lunar and meteoritic rare gases. Trapped neon-20/neon-22 ratios range from 11.5 to approximately 15, resembling those for the gas-rich meteorites. Trapped krypton and xenon in the lunar fines and in the carbonaceous chondrites are similar except for an interesting underabundance of the heavy isotopes in both lunar gases which suggests that the fission component found in carbonaceous chondrites is depleted in lunar material. Spallation gases are in most cases quite close to meteoritic spallation gases in isotopic composition.     

Lead and Helium Isotope Evidence from Oceanic Basalts for a Common Deep Source of Mantle Plumes

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.     

Carbyne forms of carbon: do they exist?

Almost 15 years have passed since carbynes entered the literature as new forms of elemental carbon. They recently attracted attention as possible interstellar dust constituents and as carriers of presolar noble gases in meteorites. Their existence and that of the related mineral chaoite are questioned, and a reevaluation of previous data is suggested.     

Surface of young Jupiter family comet 81P/Wild 2: view from the Stardust Spacecraft

Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.     

Noble gases in stratospheric dust particles: confirmation of extraterrestrial origin

Noble gas elemental and isotopic ratios were measured in a group of 13 "chondritic" stratospheric dust particles. Neon and argon are present in "solar" proportions; xenon appears to be dominated by contributions from "planetary" sources. The apparent xenon concentration is higher than that measured in any bulk meteorite, approaching the concentration found in the noble gas-rich, acid-insoluble residues from carbonaceous chondrites.     

High-3He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.     

Tunable quasi-two-dimensional electron gases in oxide heterostructures

We report on a large electric-field response of quasi-two-dimensional electron gases generated at interfaces in epitaxial heterostructures grown from insulating oxides. These device structures are characterized by doping layers that are spatially separated from high-mobility quasi-two-dimensional electron gases and therefore present an oxide analog to semiconducting high-electron mobility transistors. By applying a gate voltage, the conductivity of the electron gases can be modulated through a quantum phase transition from an insulating to a metallic state.