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1.
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, 21Neexcess 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 excess131–136Xe is 244Pu, but not238U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.  相似文献   

2.
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.  相似文献   

3.
This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, 84Kr/4He and 132Xe/4He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of 3He/4He and 40Ar/36Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in 20Ne/22Ne and 21Ne/22Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess 129Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.  相似文献   

4.
In order to picture C geodynamics past and present, theC3He ratios of the relevant reservoirs are considered. Evaluation of publishedC3He ratio in conjunction with new results for MORB glasses worldwide, suggests that this ratio is unfractionated during magma outgassing, a best estimate being 2 × 109.C3He ratios from other volcanic emissions (hot spots and arcs) do not appear significantly different when the subducted component is omitted.This result permits scaling of the CO2 degassing flux to that of3He and yields a value of 2 × 1012 mol/yr which corresponds to a model degassing duration of 3.9 Gyr when recycling to the mantle is disregarded.A bulk Earth chondritic ratio of about 2 × 109 is calculated, very close to the MORB value. On the other hand the reconstructed exospheric (“Rubey inventory”) value of4 ± 1 × 107 is very different from both basaltic and chondritic values.Among the possible interpretations of these results the following two are retained: (1) CO2 was not released in the early age of the Earth because of the reducing conditions prevailing at that time in the mantle. Formation of the core changed this picture and permitted subsequent degassing of CO2. (2) Carbonates need a continental crust of significant size to become stabilized in the exosphere. Therefore accumulation in the exosphere was delayed until crustal formation.Alternatively, a similar degassing behaviour for both He and CO2 requires a massive recycling of carbonates throughout time. This possibility is in contradiction with the present-day maximum recycling rate and the severe imbalance with the observed outgassing flux on one hand and with the small fraction of carbon now present in the exosphere on the other.We conclude that carbon has never been severely degassed. The mantle acts as a buffer for C and most carbon is still retained there, possibly as graphite (or diamond?) or dissolved in minerals.  相似文献   

5.
Microscopic mechanisms for heat transport in dense minerals (phonon scattering and photon attenuation) exhibit aspects of threshold behavior, discussed qualitatively here. For all minerals examined so-far using laser-flash analysis, the lattice component of the thermal conductivity of the mantle asymptotes to a constant above a critical temperature of 1500 K. Radiative transfer calculated from absorption spectra has thresholds in both grain-size and Fe content, and a rather complex dependence on temperature. These critical phenomena impact convection of the lower mantle, because the lattice contribution tends to destabilize the cold boundary layers, whereas radiative transfer mostly promotes stability in the lower mantle, unless the grains are large and Fe-rich, which makes convection chaotic and time-dependent. The specific behavior suggests that flow in the lower mantle is sluggish, whereas flow in the upper mantle-transition zone is time-dependent. The decrease in krad as Fe/(Fe + Mg) increases beyond 0.1 may be connected with formation of lower mantle, thermo-chemical plumes through positive feedback.  相似文献   

6.
Noble gas isotopes including 3He/4He, 40Ar/36Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount.Glass and coexisting olivine crystals have similar 3He/4He ratios (2.8–3.4) × 10?5, 20 to 24 times the atmospheric ratio (RA), but different 40Ar/36Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not.The apparent high 3He/4He ratio (3 × 10?5; = 21 RA) coupled with a relatively high 40Ar/36Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma.Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component.Excess 129Xe has not been observed due to apparent large mass fractionation among Xe isotopes.  相似文献   

7.
Vesicle characteristics (vesicularity, largest vesicle size, number of vesicles/cm2), CO2-δ13C and CO2-4He-40Ar-40Ar/36Ar in vesicles and CO2-δ13C in the glass have been measured in 19 tholeiitic basalt glasses from the Easter Microplate East Ridge (East Pacific Rise) collected at 3 different sites (26°S East Ridge, Pito Seamount and Pito Deep at 23°S).Carbon supersaturation values (Cmelt/Csolubility) vary from 1.3 to 4.3. Carbon supersaturation values are strongly correlated with the number of vesicles/cm2. There is also a correlation between number of vesicles/cm2 and vesicle size. At the Pito Seamount site, there is a negative correlation between carbon supersaturation values and observed carbon isotope fractionation between CO2 in vesicles and carbon dissolved in the glass (Δ13Cobserved). High 4He/40Ar* ratios in vesicles (from 49 to 190) are observed in both the most and least carbon supersaturated samples, while samples with intermediate carbon supersaturation have the lowest 4He/40Ar* ratios (16±1). These correlations show that most quenched melts record different disequilibrium to equilibrium states during closed-system degassing.The samples showing the highest carbon supersaturation (4.3) have the highest 4He/40Ar* (from 94 to 190). This observation shows for the first time that the 4He/40Ar* ratio can be kinetically fractionated during incomplete degassing of magmas from the magma chamber to the seafloor. This result implies that high 4He/40Ar* ratios are not a systematic indicator of open-system degassing (Rayleigh distillation) and that caution should be taken when using this ratio for any degassing correction.A two-stage degassing model, with the first stage being a closed-system degassing occurring between the source and the magma chamber, and the second stage of degassing (with a mode varying from open-system degassing to different degrees of kinetic closed-system degassing) taking place between the magma chamber and eruption on the seafloor, is the most appropriate to describe the degassing of MORB. Reconstructing initial carbon content of the magma prior to degassing and extrapolating the results to the entire ridge system results in a carbon flux of 1.6-0.3+0.6×1014 g/year. This value implies vigorous exchange of carbon between the mantle and the surface throughout geological times.  相似文献   

8.
We report new trace element data for an extensive suite of quench basalt glasses dredged from the southern Mid-Atlantic Ridge (MAR) between 40°S and 52.5°S. Ratios between highly incompatible trace elements are strongly correlated and indicate a systematic distribution of incompatible element enriched mid-ocean ridge basalt (MORB) (E-type: Zr/Nb=5.9-19, Y/Nb=0.9-8.4, (La/Sm)n=1.0-2.9) and incompatible element depleted MORB (N-type: Zr/Nb=30-69, Y/Nb=11-29, (La/Sm)n=0.48-0.79) along this section of the southern MAR. A notable feature of N-type MORB from the region is the higher than usual Ba/Nb (4-9), La/Nb (1.2-2.4) and primitive mantle normalised K/Nb ratios (>1). Ba/Nb ratios in E-type MORB samples from 47.5 to 49°S are especially elevated (>10). The occurrence and geographic distribution of E-type MORB along this section of the southern MAR can be correlated with the ridge-centred Shona and off-axis Discovery mantle plumes. In conjunction with published isotope data for a subset of the same sample suite [Douglass et al., J. Geophys. Res. 104 (1999) 2941], a model is developed whereby prior to the breakup of Gondwana and the opening of the South Atlantic Ocean, the underlying asthenospheric mantle was locally contaminated by fluids/melts rising from the major Mesozoic subduction zone along the south-southwest boundary of Gondwana, leaving a subduction zone geochemical imprint (elevated (K/Nb)n and 87Sr/86Sr ratios, decreased 143Nd/144Nd ratios). Subsequent impingement of three major mantle plume heads (Tristan/Gough, Discovery, Shona) resulted in heating and thermal erosion of the lowermost subcontinental lithosphere and dispersal into the convecting asthenospheric mantle. With the opening of the ocean basin, continued plume upwelling led to plume-ridge interactions and mixing between geochemically enriched mantle derived from the Shona and Discovery mantle plumes, material derived from delamination of the subcontinental lithosphere, and mildly subduction zone contaminated depleted asthenospheric mantle.  相似文献   

9.
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 (T1/2 = 16 Ma) and 244Pu (T1/2 = 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.  相似文献   

10.
The abundance and isotopic composition of noble gases were determined in samples of CO2 well gas from Harding County, New Mexico. Our results confirm the presence of radiogenic129Xe and fissiogenic131–136Xe. Relative to noble gases in air, the CO2 gas is selectively depleted in the lighter weight, nonradiogenic noble gases, except at neon. It is suggested that loss of atmospheric neon into space could account for an apparent excess of neon in juvenile gases.  相似文献   

11.
High-precision Nb, Ta, Zr, Hf, Sm, Nd and Lu concentration data of depleted mantle rocks from the Balmuccia peridotite complex (Ivrea Zone, Italian Alps) were determined by isotope dilution using multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionisation mass spectrometry (TIMS). The Zr/Hf ratios of all investigated samples from the Balmuccia peridotite complex are significantly lower than the chondritic value of 34.2, and the most depleted samples have Zr/Hf ratios as low as 10. Correlated Zr/Hf ratios and Zr abundances of the lherzolites preserve the trend of a mantle residue that has been depleted by fractional melting. This trend confirms experimental studies that predict Hf to behave more compatibly than Zr during mantle melting. Experimentally determined partition coefficients imply that the major Zr and Hf depletion most likely occurred in the spinel stability field, with (DZr/DHf)cpx≈0.5, and not in the garnet stability field, where (DZr/DHf)grt is probably close to one. However, minor amounts of melting must have also occurred in a garnet facies mantle, as indicated by low Sm/Lu ratios in the Balmuccia peridotites. The Nb/Ta ratios of most lherzolites are subchondritic and vary only from 7 to 10, with the exception of three samples that have higher Nb/Ta ratios (18–24). The overall low Nb/Ta ratios of most depleted mantle rocks confirm a higher compatibility of Ta in the mantle. The uniform Nb/Ta ratios in most samples imply that even in ‘depleted’ mantle domains the budget of the highly incompatible Nb and Ta is controlled by enrichment processes. Such a model is supported by the positive correlation of Zr/Nb with the Zr concentration. However, the overall enrichment was weak and did barely affect the moderately incompatible elements Zr and Hf. The new constraints from the partitioning behaviour of Zr–Hf and Nb–Ta provide important insights into processes that formed the Earth’s major silicate reservoirs. The correlation of Zr/Hf and Sm/Nd in depleted MORB can be assigned to previous melting events in the MORB source. However, such trends were unlikely produced during continental crust formation processes, where Sm/Nd and Zr/Hf are decoupled. The different fractionation behaviour of Zr/Hf and Sm/Nd in the depleted mantle (correlated) and the crust (decoupled) indicates that crustal growth by a simple partial melting process in the mantle has little effect on the mass budget of LREE and HFSE between crust and mantle. A more complex source composition, similar to that of modern subduction rocks, is needed to fractionate the LREE, but not Zr/Hf and the HREE.  相似文献   

12.
A neutron-irradiated bulk sample of the Murray (C-2) carbonaceous chondrite was etched with H2O2 and then divided into colloidal and non-colloidal fractions. The H2O2 treatment removed ～80% of the trapped Xe and greatly increased variations in the129Xe/132Xe ratio measured in stepwise heating. The colloid showed very little excess129Xe, but the anti-colloid gave a fairly good I-Xe correlation corresponding to formation 3.7 ± 2.1 m.y. after Bjurböle.Variations in the trapped Xe component were also observed; most notably the 550°C anti-colloid fraction has large deficiencies relative to AVCC at the heavy isotopes. A tentative decomposition suggests U-Xe, a “primitive” trapped component, as the dominant component with minor contributions from H-Xe, L-Xe, and S-Xe (s-process nucleosynthesis). The identification of U-Xe rests primarily on the agreement of themeasured134Xe/136Xe ratio with U-Xe. This observation lends support to proposals for such a “primitive” trapped Xe component and demonstrates that at least some carbonaceous chondrite phases sampled a xenon reservoir nearly devoid of H-Xe.  相似文献   

13.
Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other.The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 208Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.  相似文献   

14.
In an attempt to constrain the origin of polycrystalline diamond, combined analyses of rare gases and carbon and nitrogen isotopes were performed on six such diamonds from Orapa (Botswana). Helium shows radiogenic isotopic ratios of R/Ra = 0.14–1.29, while the neon ratios (21Ne/22Ne of up to 0.0534) reflect a component from mantle, nucleogenic and atmospheric sources. 40Ar/36Ar ratios of between 477 and 6056 are consistent with this interpretation. The (129Xe/130Xe) isotopic ratios range between 6.54 and 6.91 and the lower values indicate an atmospheric component. The He, Ne, Ar and Xe isotopic compositions and the Xe isotopic pattern are clear evidence for a mantle component rather than a crustal one in the source of the polycrystalline diamonds from Orapa. The δ13C and δ15N isotopic values of − 1.04 to − 9.79‰ and + 4.5 to + 15.5‰ respectively, lie within the range of values obtained from the monocrystalline diamonds at that mine. Additionally, this work reveals that polycrystalline diamonds may not be the most appropriate samples to study if the aim is to consider the compositional evolution of rare gases through time. Our data shows that after crystallization, the polycrystalline diamonds undergo both gas loss (that is more significant for the lighter rare gases such as He and Ne) and secondary processes (such as radiogenic, nucleogenic and fissiogenic, as well as atmospheric contamination). Finally, if polycrystalline diamonds sampled an old mantle (1–3.2 Ga), the determined Xe isotopic signatures, which are similar to present MORB mantle – no fissiogenic Xe from fission of 238U being detectable – imply either that Xe isotopic ratios have not evolved within the convective mantle since diamond crystallization, or that these diamonds are actually much younger.  相似文献   

15.
The Deccan flows at Mahabaleshwar are divisible into a lower and an upper group, based on Nd and Sr isotopic ratios, which define two correlated trends. This distinction is supported by incompatible element ratios and bulk compositions. The data reflect contamination in a dynamic system of magmas from an LIL-depleted,εJUV ≥ +8 mantle by two different negative εJUV endmembers, one undoubtedly continental crust, the other either continental crust or enriched mantle. The depleted mantle source, anomalously high in (87Sr/86Sr), may have been in the subcontinental lithosphere or a region of rising Indian Ocean MORB mantle.  相似文献   

16.
The vesicle size distribution (VSD) and rare gas abundances in popping rocks from 14°N on the Mid-Atlantic Ridge provide constraints on the behavior of volatiles during ridge crest volcanism. These popping rocks, which contain 16–18 volume percent vesicles, are rare mid-ocean ridge basalt (MORB) magmas which appear to have retained much of their volatile inventory. The logarithm of vesicle population density displays the same linear correlation with decreasing size in two of the samples studied. This implies that continuous and simultaneous nucleation and bubble growth have occurred during magma ascent, with no significant perturbations due to accumulation, coalescence or loss of bubbles. In contrast, most MORB magmas display low vesicularities and we suggest that they have suffered some degree of pre-eruptive vesicle loss. We tentatively propose that large vesicles are produced by coalescence when MORB melt is at rest in chambers and conduits, and may be lost during early gas-rich episodes. Most MORB would represent residual liquids which erupt after vesicle loss has occurred, whereas popping rocks would represent a rare case where physical sorting of vesicles from melt did not occur, because storage in a magma chamber did not occur.The rare gas concentrations in the studied popping rocks are the highest yet measured in glassy ridge basalts ([He] > 50 μccSTP/g). The rare gas abundance pattern of these popping rocks probably resembles the pattern for non-vesiculated MORB magma and potentially reflects that of the depleted mantle source. This pattern is similar to the “mean MORB” pattern (computed from MORB glasses with40Ar/36Ar > 10,000) although a higher enrichment in He (and possibly Ne) compared to the heavier rare gases is observed in MORB. The overall similarity in abundance patterns for MORB and popping rocks indicates that vesiculation and vesicle loss do not fractionate the ArKrXe relative abundances from those in non-vesiculated magma, and that the modern flux ratios of these gases at ridges are similar to their elemental ratios in the depleted mantle. The degassing flux of He at ridge crests estimated from the MORB He deficit relative to popping rocks is comparable to the flux derived from the3He budget for the abyssal ocean. This suggests that degassing at ridges may be strongly influenced by the dynamics and style of submarine volcanism.  相似文献   

17.
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…  相似文献   

18.
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.  相似文献   

19.
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.  相似文献   

20.
Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of129Xe, where we infer 129/Xe132Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of129Xe from decay of129I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low129Xe seems to be to suppose that before decay to129Xe r-process production at mass 129 condensed into dust as129I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped129Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.  相似文献   

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