The Fernando de Noronha archipelago (southwest Atlantic, 345 km from the coast of Brazil) is considered as the result of mantle plume activity. However, data on the isotopic composition of helium and ...neon, which are, perhaps, the only unambiguous geochemical criterion for deep mantle plumes have not been published yet for the region. In this paper, we present the first data on the isotopic composition of helium, neon, argon, and nitrogen, obtained by stepwise crushing of mantle xenoliths from the basanites of the San José Formation. The results obtained may indicate that fluid inclusions contain the very first portions of the exsolved gases—they are ultra-depleted in helium in relation to neon and especially argon. This conclusion is also supported by He–Ar–CO
2
systematics. The isotopic composition of helium (
4
He/
3
He = 31 879 ± 6796) and neon (
21
Ne/
22
Ne
(mantle)
= 0.0453 ± 0.0012) indicates that it was indeed a mantle plume, identical in noble gas composition to the Kerguelen plume. According to the Ar–Ne isotope systematics
40
Ar/
36
Ar
(mantle)
= 7455 ± 2290. Nitrogen is characterized by a heavy isotopic composition (δ
15
N = +5.4 ± 0.2‰), which corresponds to the hypothesis of the subduction nature of nitrogen in deep mantle plumes.
We report the results of petrological, geochemical, and geochronological investigations of the unusual K‐rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly ...fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K‐rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K‐rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago.
Here we present summarizing of isotopic compositions and element ratios of noble gases, nitrogen, carbon and hydrogen in carbonatites of different generations of the Guli massif (West Siberia, ...Russia) obtained by stepwise crushing. The data point to the subcontinental lithospheric mantle (SCLM) as a primary source of the fluid phase in Guli carbonatites. However, the estimated
40
Ar/
36
Ar ratio in the Guli mantle source of about 5400 is similar to the Kola plume value of 5000 ± 1000 (Marty et al., 1998). One explanation of such a low estimated
40
Ar/
36
Ar ratio in the mantle end-member with SCLM type helium (
4
Не/
3
Не ~ 120000) and neon (
21
Nе/
22
Nе
mantle
~ 0.7) is an admixture of atmospheric argon to the local mantle source. This assumption is supported by the Ar-Ne systematics as well as by the data for hydrogen isotopic composition. Early carbonatite differs significantly from the later ones by the concentration of highly volatile components, as well as by the isotopic compositions of carbon (CO
2
), argon, and hydrogen (H
2
O). The mantle component dominated in fluids at the early formation stages of the Guli massif rocks, whereas the late stages of carbonatite formation were characterized by an additional fluid source, which introduced atmospheric argon and neon, and most likely a high portion of CO
2
with isotopically heavy carbon. The argon-neon-hydrogen isotope systematics suggest that the most plausible source of these late stage fluids are high temperature paleometeoric waters. The absence of a plume signature could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.
We analyze the dynamics and composition of Saturnian stream particles measured by the Cosmic Dust Analyser (CDA) onboard the Cassini spacecraft. To reconstruct the dynamical properties of Saturnian ...stream particles, we adopt a backward tracing method with in situ solar wind measurements to filter out the influence of the interplanetary magnetic field. Our results show that stream particles from Saturn have sizes ranging from 2 to 8 nm (radius) with ejection velocities between 50 and 200 kms−1. Moreover, the derived “ejection region” of stream particles in the outer part of Saturn's E ring is indicative of the dust charging condition profile in the planet's magnetosphere. By using the Cassini magnetospheric plasma measurements as input, our ejection model considers stochastic charging and well reproduces the dynamical properties of stream particles derived from backward simulations. An updated analysis of CDA stream‐particle mass spectra confirms that the silicateous material is the most probable composition of Saturnian stream particles, in contrast to E ring particles whose composition is dominated by water ice. This compositional discrepancy can be reproduced by our model if the different sputter efficiencies of silicateous material and water ice are considered. We suggest that silicateous impurities released from icy grains at the outer E ring are the most probable source of Saturnian stream particles. Finally, we discuss the role of dust particles as a mobile neutral reservoir in Saturn's magnetosphere which may be responsible for certain features in the Cassini O and O2+ measurements.
Key Points
Electrodynamics of nanodust particles in Saturn's magnetosphere
Composition discrepancy between stream particles and the rings of Saturn
Interactions between dust and the magnetosphere
The article considers different points of view on the genesis of extraterrestrial trapped argon with a composition different from primordial/solar in meteorites and lunar rocks. An alternative ...hypothesis of the origin of this component is discussed.
The role of micrometeorite bombardment in space weathering on asteroid surfaces was studied using a 2 MV Van de Graaff accelerator. About 90,000‐ to 100,000‐micron‐ to submicron‐sized copper ...particles with a mass and velocity distribution similar to the interplanetary dust population were fired onto the surfaces of polished Allende CV3 chondrite and eucrite NWA 6966 samples at speeds between 1 to 70 kilometers per second. We find a clear relationship between microparticle bombardment, infrared reflectance decrease, and overall spectral reddening. Differences in impact effects due to variable particle speed, size, and structure are observed. Some Cu particles form large clusters that break up upon impact and disperse. Other impactors leave imprints on the surface, and implant or generate typical craters with rims and spallation features. Very small, fast particles generate small craters without spallation or significant crater rim. Mid‐infrared (IR) spectra (bulk and microscopic measurements of individual components), 3‐D laser microscopic images and XRD spectra from the processed and unprocessed samples were collected. Mid‐IR spectra (700–6,000 cm−1) over the entire sample surface show overall darkening of features. Microscopic IR spectra show the damage seen as reflectance decrease and spectral reddening, which is variable in the micrometer range, depending on impact density and target properties (mineralogic composition). The fine‐grained Allende matrix with predominantly Fe‐rich olivine seems less affected than coarse‐grained chondrules with Mg‐rich silicates, where darkening can reach 60%. X‐ray diffraction analysis also suggests chemical and crystallographical differences in the bombarded sample, due to impact shock.
Plain Language Summary
Weathering processes in space affect the surfaces of, for example, asteroids, due to the constant bombardment with electrons, ions, and photons emanating from the sun, and the ubiquitously present cosmic dust. This bombardment leads to changes in the physical and chemical properties of affected surface materials. The research presented in this paper focuses on the alteration of meteorite samples via dust bombardment. We shot nanometer‐ to micrometer‐sized copper particles with velocities of up to 70 km/s onto meteorite samples that resemble asteroid surfaces, using a dust accelerator. The samples were analyzed with mid‐infrared reflectance spectroscopy and spectra of processed and unprocessed samples were compared. Hypervelocity impacts result in a variety of impact shock phenomena, depending on the mass and velocity of an impactor. Dents, implanted particles, craters, cracks, and ejected surface material can be observed. Impact shock effects are also observed to spread over a wide area surrounding the impact site, indicating structural and chemical changes in the material, for example, the formation of glasses. The damage and accompanying optical changes due to impact are visible in mid‐infrared reflectance spectra, mainly as a reduction of reflectance, the flattening of band peaks, and the shifting of bands toward longer wavelengths.
Key Points
We simulated particle impacts onto meteorite targets with a dust accelerator
Mid‐infrared spectroscopy was performed on impacted Allende meteorite and NWA 6966 eucrite
This study aimed at investigating the role of micrometeoroid impacts in space weathering
The Earth's mantle contains a mixture of primordial noble gases, in particular solar-type helium and neon, and radiogenic rare gases from long-lived U,
232Th,
40K and short-lived
129I,
244Pu. Rocks ...derived from deep mantle plume magmatism like on Hawaii or Iceland contain a higher proportion of primordial nuclides than rocks from the shallow upper mantle, e.g. mid ocean ridge basalts (MORBs). This is widely regarded as the key evidence for survival of a less degassed and more “primitive” reservoir within the lower mantle. We present an evaluation of noble gas composition showing the shallow mantle to have about five times more radiogenic (relative to primordial) isotopes than Hawaii/Iceland-type plume reservoirs, no matter if short- or long-lived decay systems are considered. This fundamental property suggests that both MORB and plume-type noble gases are mixtures of: (1) a homogeneous radiogenic component present throughout most of the mantle and (2) a uniform primordial noble gas component with very minor radiogenic ingrowth. This conclusion depends crucially on the observed excess of radiogenic Xe in plume-derived rocks, and is only valid if this Xe excess is inherent to the plume sources.
Possible sources of the primordial component of mantle plume reservoirs—and possibly also the MORB mantle—could be mantle reservoirs that remained relatively isolated over most of Earth's history (“blobs”, a deep abyssal layer, or the D” layer), but these need a considerable concentration of primordial gases to compensate U, Th, K decay over 4.5
Ga. Earth's core is evaluated as an alternative viable source feeding primordial nuclides into mantle reservoirs: even low metal–silicate partitioning coefficients allow sufficient primordial noble gases to be incorporated into the early forming core, as the undifferentiated proto-Earth was initially gas-rich. Massive mantle degassing soon after core formation then provides the opposite concentration gradient that allows primordial noble gases reentering the mantle at the core-mantle boundary, probably via partial mantle melts. Another possible source of primordial noble gases in Earth's mantle are subducted sediments containing extraterrestrial dust with solar He and Ne, but this supply mechanism crucially depends on largely unconstrained parameters. The latter two scenarios do not require the preservation of a “primitive” mantle reservoir over 4.5
Ga, and can potentially better reconcile increasing geochemical evidence of recycled lithospheric components in mantle plumes and seismic evidence for whole mantle convection.
For better understanding of the fluid phase sources of carbonatites of Guli alkaline-ultrabasic intrusion (Maymecha-Kotuy complex) we have studied isotope composition of He and Ne in the carbonatites ...of different formation stages. The data definitely point to the subcontinental lithospheric mantle (SCLM) as a primary source of fluid phase of Guli carbonatites. The absence of plume signature in such a plume-like object (from petrological point of view) could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.
We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle ...xenoliths from Réunion Island. He and Ne isotopic compositions were similar to previously reported Réunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated
129Xe/
130Xe and
136Xe/
130Xe ratios following the mantle trend with maximum values of 6.93
±
0.14 and 2.36
±
0.06, respectively. High-resolution argon analyses resulted in maximum
40Ar/
36Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of
40Ar/
36Ar vs.
20Ne/
22Ne. Using a mantle
20Ne/
22Ne of 12.5 (Ne–B) a consistent
40Ar/
36Ar value of 11,053
±
220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle
20Ne/
22Ne ratio of 13.8 (solar wind) would lead to a much higher
40Ar/
36Ar ratio of 75,000, far above observed maximum values. This favours a mantle
20Ne/
22Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated
40Ar/
36Ar ratios of the Réunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected
21Ne/
22Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Réunion–MORB) as observed for
4He/
3He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.
Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several ...dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.
