Helium-3 from the mantle: primordial signal or cosmic dust?

Helium-3 in hotspot magmas has been used as unambiguous evidence for the existence of a primordial, undegassed reservoir deep in the Earth's mantle. However, a large amount of helium-3 is delivered to the Earth's surface by interplanetary dust particles (IDPs). Recycling of deep-sea sediments containing these particles to the mantle, and eventual incorporation in magma, can explain the high helium-3/helium-4 ratios of hotspot magmas. Basafts with high helium-3/helium-4 ratios may represent degassing of helium introduced by ancient (probably 1.5 to 2.0 billion years old) pelagic sediments rather than degassing of primordial lower mantle material brought to the surface in plumes. Influx of IDPs can also explain the neon and siderophile compositions of mantle samples.     

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.     

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.     

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.     

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.     

Stable compounds of helium and neon: he@c60 and ne@c60

It is demonstrated that fullerenes, prepared via the standard method (an arc between graphite electrodes in a partial pressure of helium), on heating to high temperatures release (4)He and (3)He. The amount corresponds to one (4)He for every 880,000 fullerene molecules. The (3)He/(4)He isotopic ratio is that of tank helium rather than that of atmospheric helium. These results convincingly show that the helium is inside and that there is no exchange with the atmosphere. The amount found corresponds with a prediction from a simple model based on the expected volume of the cavity. In addition, the temperature dependence for the release of helium implies a barrier about 80 kilocalories per mole. This is much lower than the barrier expected from theory for helium passing through one of the rings in the intact structure. Amechanism involving reversibly breaking one or more bonds to temporarily open a "window" in the cage is proposed. A predicted consequence of this mechanism is the incorporation of other gases while the "window" is open. This was demonstrated through the incorporation of (3)He and neon by heating fullerene in their presence.     

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.     

Mantle plumes and entrainment: isotopic evidence

Many oceanic island basalts show sublinear subparallel arrays in Sr-Nd-Pb isotopic space. The depleted upper mantle is rarely a mixing end-member of these arrays, as would be expected if mantle plumes originated at a 670-kilometer boundary layer and entrained upper mantle during ascent. Instead, the arrays are fan-shaped and appear to converge on a volume in isotopic space characterized by low (87)Sr/(86)Sr and high (143)Nd/(144)Nd, (206)Pb/(204)Pb, and (3)He/(4)He ratios. This new isotopic component may be the lower mantle, entrained into plumes originating from the core-mantle boundary layer.     

Helium Isotopic Evidence for a Lower Mantle Component in Depleted Archean Komatiite

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.     

Rare gases in lunar samples: study of distribution and variafions by a microprobe technique

The rare gas distribution in lunar soil, breccias, and rocks was studied with a micro-helium-probe. Gases are concentrated in grain surfaces and originate from solar wind. Helium-4 concentrations of different mineral components vary by more than a factor of 10 apart from individual fluctuations for each type. Also grains with no detectable helium-4 exist. Titanium-rich components have the highest, calcium-rich minerals the lowest concentrations. The solar wind was redistributed by diffusion. Mean gas layer thicknesses are 10, 6, and 5 microm for helium, neon, and argon respectively. Lithic fragments in breccias contain no solar gases. Glass pitted surfaces of crystalline rocks contain about 10(-2) cubic centimeter of helium-4 per square centimeter. Etched dust grains clearly show spallogenic and radiogenic components. The apparent mean exposure age of dust is approximately 500 x 10(6) years, its potassium-argon age is approximately 3.5 x 10(9) yerars. Cavities of crystalline rocks contain helium-4, radiogenic argon, H(2), and N(2).     

Neodymium isotope evidence for a chondritic composition of the Moon

Samarium-neodymium isotope data for six lunar basalts show that the bulk Moon has a 142Nd/144Nd ratio that is indistinguishable from that of chondritic meteorites but is 20 parts per million less than most samples from Earth. The Sm/Nd formation interval of the lunar mantle from these data is 215(-21)(+23) million years after the onset of solar system condensation. Because both Earth and the Moon likely formed in the same region of the solar nebula, Earth should also have a chondritic bulk composition. In order to mass balance the Nd budget, these constraints require that a complementary reservoir with a lower 142Nd/144Nd value resides in Earth's mantle.     

Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples

Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray-produced (21)Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.     

Genetic relations of oceanic basalts as indicated by lead isotopes

The isotopic compositions of lead and the concentrations lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206: lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206: lead-204 ratios between 19.3 and 20.4 It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from and original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.     

Rutile-bearing refractory eclogites: missing link between continents and depleted mantle

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.     

Lunar rock compositions and some interpretations

Samples of igneous "gabbro," "basalt," and lunar regolith have compositions fundamentally different from all meteorites and terrestrial basalts. The lunar rocks are anhydrous and without ferric iron. Amounts of titanium as high as 7 weight percent suggest either extreme fractionation of lunar rocks or an unexpected solar abundance of titanium. The differences in compositions of the known, more "primitive" rocks in the planetary system indicate the complexities inherent in defining the solar abundances of elemizents and the initial compositions of the earth and moon.     

142Nd evidence for early (>4.53 Ga) global differentiation of the silicate Earth

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.     

Helium and neon abundances and compositions in cometary matter

Materials trapped and preserved in comets date from the earliest history of the solar system. Particles captured by the Stardust spacecraft from comet 81P/Wild 2 are indisputable cometary matter available for laboratory study. Here we report measurements of noble gases in Stardust material. Neon isotope ratios are within the range observed in "phase Q," a ubiquitous, primitive organic carrier of noble gases in meteorites. Helium displays 3He/4He ratios twice those in phase Q and in Jupiter's atmosphere. Abundances per gram are surprisingly large, suggesting implantation by ion irradiation. The gases are probably carried in high-temperature igneous grains similar to particles found in other Stardust studies. Collectively, the evidence points to gas acquisition in a hot, high ion-flux nebular environment close to the young Sun.     

Preliminary examination of lunar samples from apollo 14

The major findings of the preliminary examination of the lunar samples are as follows: 1) The samples from Fra Mauro base may be contrasted with those from Tranquillity base and the Ocean of Storms in that about half the Apollo 11 samples consist of basaltic rocks, and all but three Apollo 12 rocks are basaltic, whereas in the Apollo 14 samples only two rocks of the 33 rocks over 50 grams have basaltic textures. The samples from Fra Mauro base consist largely of fragmental rocks containing clasts of diverse lithologies and histories. Generally the rocks differ modally from earlier lunar samples in that they contain more plagioclase and contain orthopyroxene. 2) The Apollo 14 samples differ chemically from earlier lunar rocks and from their closest meteorite and terrestrial analogs. The lunar material closest in composition is the KREEP component (potassium, rare earth elements, phosphorus), "norite," "mottled gray fragments" (9) from the soil samples (in particular, sample 12033) from the Apollo 12 site, and the dark portion of rock 12013 (10). The Apollo 14 material is richer in titanium, iron, magnesium, and silicon than the Surveyor 7 material, the only lunar highlands material directly analyzed (11). The rocks also differ from the mare basalts, having much lower contents of iron, titanium, manganese, chromium, and scandium and higher contents of silicon, aluminum, zirconium, potassium, uranium, thorium, barium, rubidium, sodium, niobium, lithium, and lanthanum. The ratios of potassium to uranium are lower than those of terrestrial rocks and similar to those of earlier lunar samples. 3) The chemical composition of the soil closely resembles that of the fragmental rocks and the large basaltic rock (sample 14310) except that some elements (potassium, lanthanum, ytterbium, and barium) may be somewhat depleted in the soil with respect to the average rock composition. 4) Rocks display characteristic surface features of lunar material (impact microcraters, rounding) and shock effects similar to those observed in rocks and soil from the Apollo 11 and Apollo 12 missions. The rocks show no evidence of exposure to water, and their content of metallic iron suggests that they, like the Apollo 11 and Apollo 12 material, were formed and have remained in an environment with low oxygen activity. 5) The concentration of solar windimplanted material in the soil is large, as was the case for Apollo 11 and Apollo 12 soil. However, unlike previous fragmental rocks, Apollo 14 fragmental rocks possess solar wind contents ranging from approximately that of the soil to essentially zero, with most rocks investigated falling toward one extreme of this range. A positive correlation appears to exist between the solar wind components, carbon, and (20)Ne, of fragmental rocks and their friability (Fig. 12). 6) Carbon contents lie within the range of carbon contents for Apollo 11 and Apollo 12 samples. 7) Four fragmental rocks show surface exposure times (10 x 10(6) to 20 x 10(6) years) about an order of magnitude less than typical exposure times of Apollo 11 and Apollo 12 rocks. 8) A much broader range of soil mechanics properties was encountered at the Apollo 14 site than has been observed at the Apollo 11, Apollo 12, and Surveyor landing sites. At different points along the traverses of the Apollo 14 mission, lesser cohesion, coarser grain size, and greater resistance to penetration was found than at the Apollo 11 and Apollo 12 sites. These variations are indicative of a very complex, heterogeneous deposit. The soils are more poorly sorted, but the range of grain size is similar to those of the Apollo 11 and Apollo 12 soils. 9) No evidence of biological material has been found in the samples to date.     

18O/16O, 30Si/28Si, D/H, and 13C/12C Studies of Lunar Rocks and Minerals

The delta (18)O of minerals from lunar gabbros and basalts are: plgioclases +6.06 to +6.33), pyroxenes (+5.70 to +5.95), and ilmenites (+3.85 to +4.12). The uniformity of these results indicates isotopic equilibrium in the mineral assemblages. Estimated plagioclase-ilmenite temperautres range from 1150 degrees C to 1340 degrees C. The bulk (18)/ (16)O and (30)Si/ (28)Si ratios of these lunar rocks are identical with ratios of terrestrial basalts, but the lunar glass, breccia, and dust are slightly enriched in the heavier isotopes. The lunar hydrogen (formed from solar wind) has a delta D/H of less than-873 per mil and the value may be even lower, as it is probably contaminated with terrestrial hydrogen. The delta (13)C of lunar dust and breccia is unusually high relative to reduced carbon in meteorites or on earth.     

The nature of pristine noble gases in mantle plumes

High-precision noble gas data show that the Hawaiian and Icelandic mantle plume sources contain uniquely primitive neon that is composed of moderately nucleogenic neon-21 and a primordial component indistinguishable from the meteoritic occurrence of solar neon. This suggests that Earth's solar-type rare gas inventory was acquired during accretion from small planetesimals previously irradiated by solar wind from the early sun. However, nonradiogenic argon, krypton, and xenon isotopes derived from the mantle display nonsolar compositions and indicate an atmosphere-like fingerprint that is not due to recent subduction.