Primordial noble gases in separated meteoritic minerals,II

Eight silicate samples from the Orgueil carbonaceous chrondrite were analyzed for He, Ne, Ar, and Xe by a stepwise heating technique. Six of the samples, including two etched with NaOH, were density fractions covering the following ranges: < 2.35, 2.35–2.45, 2.45–2.48, and > 2.48 g/cm³. Two others were grain-size fractions, separated according to their ability to form a colloid at pH 11.5.All fractions are grossly deficient in cosmogenic neon, having retained only 8–33% of their normal complement. Retentivity increases with density.All fractions give low²⁰Ne/²²Ne ratios above 950°C, suggestive of D.C. Black's exotic “Neon-E” component of²⁰Ne/²²Ne ≤ 3.4. The lowest ratios were found in the low-density and especially the non-colloidal fractions. This suggests that the host phase of Ne-E is a clay mineral of lower iron content and coarser grain size than the main silicates of Orgueil.The main fraction,ρ = 2.35–2.45g/cm³, is inhospitable to Xe; it contains less Xe and releases it more readily at low temperatures (30–35% in 1 hour at 550°C) than do any of the other fractions.     

Dissolved noble gases in marine waters

The supersaturation of neon in southeastern Pacific deep sea water is unexpectedly high, 6.6% on the average, while surface waters have values about a factor of two lower. A comparison of neon and argon concentrations indicates the presence of injected air in the deep water. The results from surface samples cannot be reconciled with air injection.     

Anomalous noble gases in josephinite and associated rocks?

In this paper we report Ne, Ar, Kr and Xe analyses of josephinite, Josephine Peridotite, and serpentinized Josephine Peridotite. In all three samples the elemental abundance patterns resemble patterns associated with surface waters, the Ne data do not exhibit the large²¹Ne enrichments observed earlier, and the Kr and Xe compositions are indistinguishable from atmospheric composition at all isotopes, including¹²⁹Xe. Our data thus offer no significant evidence for isotopic anomalies in the noble gases. We also argue that the previous claims for primordial atmospheric-like Ar, anomalous Kr and Xe, excess¹²⁹Xe, and 4.6 × 10⁹-year age are all questionable interpretations which cannot be defended against more prosaic alternatives. This leaves excess²¹Ne as the only noble gas argument for exotic origin; we suggest that this might be an experimental artifact. Until the²¹Ne question can be settled by more definitive experimentation, we feel that noble gas data cannot be used to support arguments that the origin of josephinite is more exotic than crustal serpentinization.     

Cosmogenic and radiogenic noble gases in the Dhajala chondrite

Whole rock and chondrules of the Dhajala chondrite were analyzed for Ne, Ar, Kr and Xe by total melting as well as by stepwise heating techniques. The cosmic ray exposure ages for the whole rock and the chondrules are6.2 ± 0.8 and6.3 ± 1.0m.y. as determined by the²¹Ne method and4.8 ± 1.5 and4.2 ± 2.0m.y. by the³⁸Ar method, respectively. The K-Ar age of the whole rock is4.2 ± 0.4b.y. The elemental composition of the trapped gas in this chondrite is of “planetary” type. The radiogenic¹²⁹Xe contents in the whole rock and chondrules are similar and this component is very retentively sited in the chondrules.     

The origins and concentrations of water,carbon, nitrogen and noble gases on Earth

The isotopic compositions of terrestrial hydrogen and nitrogen are clearly different from those of the nebular gas from which the solar system formed, and also differ from most of cometary values. Terrestrial N and H isotopic compositions are in the range of values characterizing primitive meteorites, which suggests that water, nitrogen, and other volatile elements on Earth originated from a cosmochemical reservoir that also sourced the parent bodies of primitive meteorites. Remnants of the proto-solar nebula (PSN) are still present in the mantle, presumably signing the sequestration of PSN gas at an early stage of planetary growth. The contribution of cometary volatiles appears limited to a few percents at most of the total volatile inventory of the Earth. The isotope signatures of H, N, Ne and Ar can be explained by mixing between two end-members of solar and chondritic compositions, respectively, and do not require isotopic fractionation during hydrodynamic escape of an early atmosphere.The terrestrial inventory of ⁴⁰Ar (produced by the decay of ⁴⁰K throughout the Earth's history) suggests that a significant fraction of radiogenic argon may be still trapped in the silicate Earth. By normalizing other volatile element abundances to this isotope, it is proposed that the Earth is not as volatile-poor as previously thought. Our planet may indeed contain up to ~ 3000 ppm water (preferred range: 1000–3000 ppm), and up to ~ 500 ppm C, both largely sequestrated in the solid Earth. This volatile content is equivalent to an ~ 2 (± 1) % contribution of carbonaceous chondrite (CI-CM) material to a dry proto-Earth, which is higher than the contribution of chondritic material advocated to account for the platinum group element budget of the mantle. Such a (relatively) high contribution of volatile-rich matter is consistent with the accretion of a few wet planetesimals during Earth accretion, as proposed by recent dynamical models.The abundance pattern of major volatile elements and of noble gases is also chondritic, with two notable exceptions. Nitrogen is depleted by one order of magnitude relative to water, carbon and most noble gases, which is consistent with either N retention in a mantle phase during magma generation, or trapping of N in the core. Xenon is also depleted by one order of magnitude, and enriched in heavy isotopes relative to chondritic or solar Xe (the so-called “xenon paradox”). This depletion and isotope fractionation might have taken place due to preferential ionization of xenon by UV light from the early Sun, either before Earth's formation on parent material, or during irradiation of the ancient atmosphere. The second possibility is consistent with a recent report of chondritic-like Xe in Archean sedimentary rocks that suggests that this process was still ongoing during the Archean eon (Pujol et al., 2011). If the depletion of Xe in the atmosphere was a long-term process that took place after the Earth-building events, then the amounts of atmospheric ¹²⁹Xe and ^131–136Xe, produced by the short-lived radioactivities of ¹²⁹I (T_1/2 = 16 Ma) and ²⁴⁴Pu (T_1/2 = 82 Ma), respectively, need to be corrected for subsequent loss. Doing so, the I–Pu–Xe age of the Earth becomes ≤ 50 Ma after start of solar system formation, instead of ~ 120 Ma as computed with the present-day atmospheric Xe inventory.     

Hot springs of Rhodesia: Their noble gases,isotopic and chemical composition

Four of Rhodesia's hottest spring complexes (54–100°C) were studied. Tritium contents were very low, compared to adjacent rivers, indicating that the samples studied were indigenous, deep-seated water which had undergone negligible intermixing with surface water.The noble-gas measurements revealed: (1) the waters are meteoric; (2) the noble gases were kept in closed-system conditions in the ground; (3) paleotemperatures are 26–31°C; (4) the boiling Binga springs lost part of their noble gases.Independently, measurements of stable isotopes indicate the meteoric origin of the springs. The chemical composition of the waters clearly reflects their origin from two groups of rocks — one from Karroo sediments and one from crystalline rocks.     

Implanted radiogenic and other noble gases in crustal diamonds from Northern Kazakhstan

Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of ⁴He in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that ⁴He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of ⁴He(5.6 × 10⁻⁴ cm³ STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of ²³⁸U accompanies the radiogenic ⁴He; the ¹³⁶Xe_f/⁴He ratio of (2.5 ± 0.3) × 10⁻⁹ agrees well with the production ratio of 2.3 × 10⁻⁹ expected in a reservoir where Th/U 3.3. Radiogenic ⁴⁰Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation.

³He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10⁻¹² cm³ STP/g, with a ³He/⁴He ratio of 1 × 10⁻⁸. Excess ²¹Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.     

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in ³He and ⁴He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in ³He/⁴He should be correlated with larger variations (factor of 5) in ⁴He/⁴⁰Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s ⁴He/⁴⁰Ar and ³He/⁴He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of ³He relative to ⁴He and of ⁴He relative to ⁴⁰Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low ⁴He/⁴⁰Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low ⁴He/⁴⁰Ar, the complement of the high ⁴He/⁴⁰Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high ³He/⁴He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.     

Characteristics and origins of primary fluids and noble gases in mantle-derived minerals from the Yishu area, Shandong Province, China

springerlink.com Studies of mantle fluids are currently one of the hot topics in the earth science, greatly contributing to re-vealing origins and evolutions of fluids. In general, the concept of mantle fluids refers to their active compo-nents, such as CO2, H2O, N2, etc., while the noble gases inert in chemical properties belong to another research system. Due to their marked differences in various fluid sources of the Earth[1], the isotopic sig-natures of He and Ar have been widely used a…     

On the distribution of noble gases in Allende: a differential oxidation study

We have developed a closed-system combustion technique and utilized it to progressively oxidize a gas-rich, highly carbonaceous acid residue and a fine-grained (<4 μm) matrix sample from the Allende C3V meteorite and analyze the released gases mass spectrometrically. For the residue complete gas mobilization occurs at temperatures below 600°C. The temperature interval over which most of the gases are released coincides with that for combustion of most of the carbon. Release is primarily due to chemical attack rather than thermal activation of the gases. There are somewhat different oxidation thresholds for the heavy gases (Ar, Kr, and Xe) and the light gases (He, Ne), indicating chemically different sites for the two groups. Relative enhancement of isotopically anomalous components near 600°C is as large as in any open-system oxidation method. Differential combustion of the matrix sample reveals three distinct outgassing peaks, the first matching the release from the carbonaceous residue (“combustibles”), the second attributed to sulfides, and the third tentatively assigned to silicates. They comprise about 53%, 7% and 40% of the total heavy gases respectively. While the “sulfides” exhibit a small fission-like component, the Xe in the “silicates” appears isotopically uniform with roughly AVCC composition. The “combustibles” of the matrix contain relatively less excess¹²⁹Xe than the residue, perhaps indicating that ～10% of the total¹²⁹Xe in the residue was acquired from “silicates” by redistribution during acid treatment. We cannot rule out the possibility that planetary gases assigned to “sulfides” or “silicates” actually reside in carbonaceous phases somehow sheltered within soluble mineral assemblages, or in acid-soluble carbonaceous phases resistant to oxidation. Integrated releases below and above 600°C during the matrix combustion exhibit virtually identical heavy gas elemental composition, implying similar fractionation during planetary gas entrapment in various materials or in the same material in various environments.     

Infiltration of river water to a shallow aquifer investigated with H/He, noble gases and CFCs

Noble gas isotopes (³He, ⁴He, Ne, Ar, Kr, Xe), tritium (³H), chlorofluorocarbons (CFCs) and dissolved oxygen (O₂) were seasonally measured in a small groundwater system recharged by infiltration of river water at Linsental, northeastern Switzerland. All Groundwater samples contained an excess of atmospheric noble gases (‘excess air’) usually with an elemental composition equal to air. The concentrations of atmospheric noble gases in the groundwater were used to calculate the excess air component and the water temperature at recharge. The noble gas temperatures (NGTs) in the boreholes close to the river vary seasonally, however, the average NGT of all samples lies close to the mean annual temperature of the river water. Groundwater ages were calculated using the tritium/helium-3 (³H/³He) dating method. The water ages of the samples obtained near the river depend on the amount of recently infiltrated river water and are young during times of active river discharge. In contrast, the mean water age of about 3 years of the deep aquifer remained nearly constant over the sampling period. The observed CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂) concentrations are significantly higher than the atmospheric equilibrium concentrations and therefore CFCs do not provide any direct information on the residence time of the groundwater. Nevertheless, the CFC excess in the groundwater shows a linear increase with the ³H/³He age. Additionally, both accumulation of radiogenic He (⁴He_rad) and O₂ consumption are strongly correlated with residence time. All these correlations can be interpreted either in terms of mixing of recently infiltrated river water with older groundwater or in terms of accumulation/consumption rates.     

Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds

Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as⁴He, ⁴⁰Ar, ²¹Ne_excess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. ²⁰Ne/²²Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. ²¹Ne/²²Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic ²¹Ne from ¹⁸O(α, n)²¹Ne and ²⁴Mg(n, α)²¹Ne reactions. The difference in ²⁰Ne/²²Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earth's history. ³⁸Ar/³⁶Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for ²³⁸U- or ²⁴⁴Pu-fission Xe, the ^131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (^124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained.In a ¹³⁶Xe/¹³⁰Xe−¹²⁹Xe/¹³⁰Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORB's appears to suggest that the progenitor of the excess^131–136Xe is ²⁴⁴Pu, but not²³⁸U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.     

The case for a martian origin of the shergottites: nitrogen and noble gases in EETA 79001

Nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite EETA 79001. A nitrogen component trapped in the glass, but not present in the matrix, has a δ¹⁵N value at least as high as +190‰. Ratios of⁴⁰Ar/¹⁴N and¹⁵N/¹⁴N in the glass are consistent with dilution of a martian atmospheric component (δ¹⁵N = 620 ± 160‰,⁴⁰Ar/¹⁴N= 0.33 ± 0.03) by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock. Trapped noble gases in the glass reproduce, within error, the elemental and isotopic compositions measured in Mars' atmosphere by Viking, and are in general agreement with previous measurements except for much lower abundances of neutron-generated krypton and xenon isotopes. The most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the EETA 79001 glass, and that this meteorite, and thus the shergottites and probably the nakhlites and chassignites as well, originated on Mars.Nitrogen in the non-glassy matrix of EETA 79001 amounts to less than 0.5 ppm and has a spallation-corrected δ¹⁵N value in the range 0 to ?20‰; it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen. Spallogenic noble gases yield single-stage exposure ages between 400,000 and 900,000 years, depending on irradiation geometry. Trapped argon may have an unusually low³⁶Ar/³⁸Ar ratio. Trapped krypton, except for a small excess at⁸⁰Kr, is smoothly mass-fractionated with respect to either terrestrial or chondritic Kr. The trapped xenon composition is consistent with addition of neutron-capture, radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric Xe. The elemental⁸⁴Kr/¹³²Xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio.     

Isotope systematics of noble gases in the Earth's mantle: possible sources of primordial isotopes and implications for mantle structure

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.     

Rare gases,water, and carbon in kaersutites

Kaersutites from Kakanui, New Zealand and from three localities in the southwestern United States have been analyzed for rare gases, water and carbon to investigate the volatile signature of the sub-continental mantle. This study does not confirm the high ³He/⁴He and ²¹Ne/²²Ne ratios reported by Saito et al. [1] for the Kakanui kaersutite. Instead, a ³He/⁴He ratio of 6 R_A and atmospheric ²¹Ne/²²Ne ratios were measured which are consistent with our current knowledge of the earth's mantle. A low ⁴⁰Ar/³⁶Ar of 320 and more than 10^?8 cm³/g of ³⁶Ar confirms the argon results of Saito et al. and indicates that significant quantities of ³⁶Ar reside in this portion of the mantle. Kaersutites from the southwestern United States (Arizona) have a heterogeneous helium isotope signature, ranging from 8.8 R_A at San Carlos to 0.46 at Hoover Dam. All D/H ratios for the water in kaersutites (?56‰ to ?78‰) represent typical mantle values with no apparent correlation with ³He/⁴He. The correlation of increasing carbon content (140–400 ppm) with increasing δ¹³C (?24.5‰ to ?16.7‰) may reflect differences in the proportions of oxidized and reduced carbon in these samples.     

Radiogenic argon in glauconites as a function of mineral recrystallization

Natural glauconitic grains were subjected to hydrothermal treatment at a pressure of 2 kbars and temperatures of 200–414°C for the period of one month.The measurement of radiogenic argon before and after treatment shows that the mica-type glauconite mineral contains 92–96% of the argon initially present up to temperatures as high as 320°C.After recrystallization of the mixed-layered glauconitic minerals the new phases still contain significant amounts of argon in the structure. The mixed-layered glauconitic minerals are much more sensitive to temperature treatment at elevated pressure than glauconitic-type mica, they, however, still contain up to 84% of the initial argon of the untreated material at temperatures as high as 320°C, i.e., just below the thermal stability of glauconite mica. It is evident that low-temperature metamorphism will affect the apparent radiogenic age of glauconitic grains to an extent which depends upon their composition and the temperature of metamorphism.     

Annealing of fission tracks in zircons

Etching conditions and thermal annealing effects have been studied for fission tracks in four kinds of zircons. The observation of fission tracks under an optical microscope was made with respect to the mean pit length as well as the mean pit density. Apparent rate constants were obtained by substituting the isochronal (60 minutes) annealing data into the ordinary equation of the rate constant. The order of reaction (almost 3rd or 4th) and the activation energy (1.7–2.9 eV) of annealing were determined from the Arrhenius plot of the apparent rate constants.     

Radiogenic heat production in the lithosphere of Sulu ultrahigh-pressure metamorphic belt

The Chinese Continental Scientific Drilling (CCSD) project is located at the Sulu ultrahigh-pressure metamorphic (UHPM) belt. It offers a unique opportunity for studying the radiogenic heat production of both shallower and deeper rocks. Based on the concentrations of radiogenic elements U, Th and K on 349 samples from main hole of CCSD (CCSD MH), pilot holes and exposures, we determined radiogenic heat productions of all major rock types in the Sulu UHPM belt. Results show the mean values of orthogneiss and paragneiss are respectively 1.65 ± 0.81 and 1.24 ± 0.61 µW m^? 3. Due to different composition and grade of retrogressive metamorphism, the eclogites display significant scatter in radiogenic heat production, ranging from 0.01 to 2.85 µW m^? 3, with a mean of 0.44 ± 0.55 µW m^? 3. The radiogenic heat production in ultramafic rocks also varies within a large range of 0.02 to 1.76 µW m^? 3, and the average turns out to be 0.18 ± 0.31 µW m^? 3. Based on the measurements and crustal petrologic model, the vertical distribution model of heat production in Sulu crust is established. The resulting mean heat production (0.76 µW m^? 3) contributes 24 mW m^? 2 to the surface heat flow. 1-D thermal model indicates that the temperature at the Moho reaches above 750 °C, and the thermal thickness of the lithosphere is ~ 75 km, in good agreement with the geophysical results. The high teat flow (~ 75 mW m^? 2) together with thin lithosphere presents strong support for the extension events during the late Cretaceous and Cenozoic.     

Noble gas systematics of the Réunion mantle plume source and the origin of primordial noble gases in Earth’s mantle

New noble gas data of ultramafic xenoliths from Réunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth’s mantle plume reservoirs. The mantle source excess of nucleogenic ²¹Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic ⁴He. ⁴⁰Ar/³⁶Ar of the Réunion mantle source can be constrained to range between 8000 and 12 000, significant ¹²⁹Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Réunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and ‘atmosphere-like’ or ‘planetary’ Ar, Kr, Xe (Science 288 (2000) 1036). ²⁰Ne/²²Ne ratios extend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth’s solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI–CM–CV–Earth. In turn, CV chondrites closely match Earth’s mantle composition in ²⁰Ne/²²Ne, ³⁶Ar/²²Ne and ³⁶Ar/³⁸Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate ‘meteoritic’ building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth’s mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.