摘要:

The concentrations of H2O and C in the upper mantle can be estimated as approximately 200 ppm and 50 ppm respectively from their concentrations in Mid Ocean Ridge Basalts. Estimates for the bulk silicate earth are less precise, but, from geochemical and cosmochemical arguments values of 550‐1900 ppm for H2O and 900‐3700 ppm for C are plausible. The implication, on the 2‐reservoir model, is that the (undegassed) lower mantle is enriched relative to the upper mantle in these volatile components, but that concentrations there are still only of the order of 2000 ppm. An analysis of available phase equilibrium data for hydrated peridotite shows that no known hydrate is stable in the asthenosphere and that recycling of water, by subduction, into the deeper parts of the mantle is only likely in the colder parts of rapidly subducting oceanic lithosphere. In the absence of stable hydrates, water must reside in nominally anhydrous phases such as olivine and &bgr;‐(Mg,Fe)2SiO4both of which have recently been shown to dissolve some H2O. The important question is whether or not storage of large amounts of H2O in the earth by &bgr;‐(Mg,Fe)2SiO4which has been shown to be both theoretically and experimentally feasible can be tested.It has been found that the water contents of olivine and &bgr;‐phase in the mantle can be constrained by the width of the seismic discontinuity at 410 Km, provided that the latter corresponds to the olivine to &bgr;‐phase transformation. Given reasonable models for the solution of H2O in the two phases the seismically observed width of the discontinuity constrains the water content of upper mantle olivine to be 0‐500 ppm. A similar type of constraint can probably be applied to the &ggr;‐spinel to perovskite plus magnesiowu¨stite transformation. Thus, arbitrary amounts of water cannot be assigned to the deeper parts of the mantle without considering the seismological implications. In contrast to hydrates, carbonates in peridotite are extremely refractory; they are likely to survive subduction under most conditions and are stable over a wide range of mantle P‐T conditions. Storage of carbon depends on redox relationships with Fe, however and recent experimental results indicate that the deeper parts of the upper mantle and transition zone are relatively reduced. Subducted carbonate should therefore be reduced by Fe to diamond and stored in the mantle as diamond rather than carbonate. Re‐oxidation to CO2or carbonate occurs in the shallower parts of the asthenosphere or in the lithosphere. The reduction of subducted carbonate to C is a hypothesis consistent with the ‘eclogitic suite’ of diamond inclusions, minerals included within diamond which could reasonably be the remnants of subducted basaltic crust. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48756

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Strong low velocity anomalies in regions of active seismic subduction are indicative of either fluids, partial melts or a temperature close to the melting point. Since the solidus for dry peridotite is far above the probable geotherm, low velocities imply the occurrence of volatiles to lower the solidus. Seismic evidence indicates that subducting slabs in the northwest Pacific have a regime of devolatilization that extends to 400 km depth. Observations in central Europe indicate that deeply penetrating fluids have eroded the deep roots at the edge of the Russian platform. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48732

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation after subduction. We consider the effects of the strength of the slab using two‐dimensional calculations of a slab‐like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowu¨stite phase transition and simple temperature‐dependent rheologies based on an Arrhenius law diffusion mechanism. Phase transitions can have two pronounced effects on subducting slab deformation and the resulting geotherms. First, an endothermic phase transformation can inhibit the vertical descent of the slab. If the slab is weak, this can lead to large deformation, even in the upper 200 km of the slab. Second, the phase transformation can slow the subduction velocity (and plate velocity) of the entire slab, a more pronounced effect than the slab deformation. Because the initial thermal structure of the descending lithosphere, the volatile content, and subduction velocity all affect the viscosity of the slab, it is likely that subduction zones may behave differently‐some with more pronounced pile‐up and avalanche periods and some where the subduction velocity is more uniform with time. These mechanisms create a highly uneven distribution of recycled components in the mantle within relatively short periods of time in Earth’s history. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48748

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We investigate the thermal history of an Earth‐like planet with the help of a parameterized mantle convection model including the volatile exchange. The weakening of mantle silicates by dissolved water is described by a functional relationship between creep rate and water fugacity. We use flow law parameters of diffusion creep in olivine under dry and wet conditions. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate which as well is dependent on the mantle heat flow. To calculate the spreading rate, we assume that the heat flow under the mid‐ocean ridges is double the average mantle heat flow. The rate of regassing also depends on the seafloor spreading rate as well as on other factors like the efficiency of volatile recycling through island arc volcanism. Both mechanisms (de‐and regassing) are coupled self‐consistently with the help of the parameterized convection model under implementation of a temperature and volatile‐content dependent mantle viscosity. We calculate time series for the Earth’s and Venusian evolution over 4.6 Gyr. In the case of Venus, there is the possibility that the Venusian mantle convection might have changed from oscillatory to quasi‐steady circulation, i.e., Venus changed from an Earth‐like planet to a Mars‐like planet at around 500 Myr ago as far as its tectonic style is concerned. Based on this view we also discuss the importance of our model for the investigation of the degassing history of Venus. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48749

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A steady state upper mantle model for the rare gases has been constructed which explains the available observational data of mantle He, Ne, Ar, and Xe isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, subduction of rare gases, and mantle rare gas concentrations. The model incorporates two mantle reservoirs; an undegassed lower mantle (P) and a highly degassed upper mantle (D). Chemical species are transferred into D within mass flows from P at plumes and from the atmosphere by subduction. Rare gases in D are derived from mixing of these inflows with radiogenic nuclides producedinsitu. The upper mantle is degassed at mid‐ocean ridges and hotspots. Flows of each isotope into D are balanced by flows out of D, so that upper mantle concentrations are in steady state. Rare gases with distinct3He/4He,20Ne/22Ne,129Xe/130Xe, and136Xe/130Xe are stored in P and are transferred into D. In P, isotopic shifts are due to decay of U‐ and Th‐ decay series nuclides,40K,129I, and244Pu over 4.5 Ga. Radiogenic136Xe in P is dominantly from244Pu.In the well‐outgassed D reservoir, additional isotopic shifts are due to decay of U‐ and Th‐ series nuclides and40K over a residence time of ∼1.4 Ga. Since4He,21Ne,40Ar, and136Xe are produced in proportions fixed by nuclear parameters, the resulting isotopic shifts are correlated. The model predicts that the shift in21Ne/22Ne in D relative to that in P is the same as that for4He/3He in the respective mantle reservoirs. This is compatible with the available data for MORB and hotspots. The minimum40Ar/36Ar,129Xe/130Xe, and136Xe/130Xe ratios in P are found to be substantially greater than the atmospheric ratios. The range in Ne, Ar, and Xe isotopes measured in MORB are interpreted as reflecting contamination of mantle rare gases by variable proportions of atmospheric rare gases. Subduction is not significant for He and Ne, but may account for a substantial fraction of Ar and Xe in D. The rare gas relative abundances in P are different than that of the atmosphere and are consistent with possible early solar system reservoirs as found in meteorites.The3He/22Ne and20Ne/36Ar ratios of P are within the range for meteorites with ‘solar’ Ne isotope compositions. The130Xe/36Ar ratio of the lower mantle is greater than that of the atmosphere and may be as high as the ratio found for meteoritic ‘planetary’ rare gases.In the model, atmospheric rare gas isotope compositions are distinct from those of the mantle. If the Earth originally had uniform concentrations of rare gases, degassing of the upper mantle would have provided only a small proportion of the nonradiogenic rare gases presently in the atmosphere. The remainder was derived from late‐accreted material with higher concentrations of rare gases. However, radiogenic129Xe and136Xe abundances imply a substantial loss of rare gases up to ∼108years after meteorite formation either from the early Earth or from late‐accreting protoplanetary materials. Rare gases must have been lost during accretion and the moon‐forming impact, so that nonradiogenic rare gases in the atmosphere must have been supplied by subsequently accreted material with nonradiogenic Xe, possibly from comets. Fractionation of atmospheric Xe isotopes relative to other early solar system components occurred either on late‐accreting materials or during loss from the Earth. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48750

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The isotopic composition of primitive Xe common to the Earth’s atmosphere and carbonaceous meteorites is determined using multivariate correlation analysis. Primitive Xe is defined as the intersection of the unfractionated, fission‐free Earth’s atmospheric Xe and the correlation line formed by Xe in carbonaceous meteorites in a multidimensional data space. The primitive Xe isotopic composition is similar to that of solar Xe rather than U‐Xe. The isotopic variations of Xe in carbonaceous meteorites used in the present analysis are mostly accounted for by those in chemically separated phases, whereas U‐Xe was determined using only those on bulk samples. The discrepancy between the present and previous studies may suggest that U‐Xe itself is a multi‐component mixture and is altered by chemical separation processes. However, since it is not guaranteed that bulk meteoritic samples really preserve unaltered primitive Xe, an alternative interpretation can be that unbiased primitive Xe is isotopically closer to solar Xe. The isotopic spectrum of the fission‐like component in the Earth’s atmospheric Xe, which is estimated through the procedure for determining the primitive Xe isotopic composition, does not agree satisfactorily with either of two possible parent nuclides,244Pu and238U, although it is closer to that of244Pu than238U. The amount of the fissiogenic Xe added to the Earth’s atmosphere is tentatively estimated as136Xefission/136Xetotal=2.8±1.3%. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48751

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In order to investigate the behavior of slab‐derived volatiles in the subduction environment, helium isotope ratios have been measured in geothermal gases from the Tabar‐Lihir‐Tanga‐Feni (TLTF) chain in the Bismarck Archipelago of Papua New Guinea. As recorded by several geochemical tracers, these volcanos carry an exceptionally large slab‐derived component, and therefore may provide new insights to the old question of volatiles in subduction zones. Geothermal gases from Lihir Island have homogeneous3He/4He ratios of 7.18±0.07 times the atmospheric ratio (RA), while those from Ambitle Island (Feni Group) have lower ratios of 6.61±0.13 RA. These3He/4He ratios are within the range defined by more‐typical arc volcanos, but lie at the low end of the spectrum observed in arc volcanos erupted through purely oceanic crust. Although a small slab‐derived signature (3He/4He ratio lower than depleted mantle) exists in the TLTF gases, these data demonstrate that even in volcanos with a comparatively large slab component, He is overwhelmingly derived from the depleted mantle wedge. This observation further confirms the relative insensitivity of He isotopes to the presence of slab fluids. He isotope ratios of 6.25 RAwere measured in geothermal gases from the Rabaul Caldera on New Britain Island. Coincidentally, these samples were taken six months prior to the major 1994 eruption at Rabaul. In conjunction with samples taken from the same locality 8 years earlier, these data allow us to test whether increasing He isotope ratios associated with fresh ascending magmas precede volcanic eruptions. Although some of the 1986 samples had much lower3He/4He ratios (5 RA) than observed in 1994, one did not. We thus find no strong evidence for a systematic rise in the He isotope ratio of the Rabaul fluids between 1986 and 1994. If a3He/4He increase did precede the Rabaul eruption, then it occurred either prior to 1986 or sometime between our 1994 sampling and the eruption. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48752

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Argon and halogens (Cl, Br and I) have been measured, using40Ar‐39Ar stepped heating method, in diamonds from Jwaneng, Orapa (both in Botswana) and Zaire. The samples analysed included cubic (coated) stones and polycrystalline diamonds of eclogitic association. Both these types of diamond contain H2O, CO2, carbonate and silicate inclusions. Coated stones have relatively constant40Ar*/Cl and Br/Cl, show limited variation in I/Cl, and have normal mantle &dgr;13C values (−5 to −7%). This contrasts with polycrystalline diamonds which, although having similar Br/Cl values to coated stones, possess significantly higher and more variable40Ar*/Cl and I/Cl values coupled with lower &dgr;13C values (≤−20%). The origin of polycrystalline diamonds with high I/Cl‐low &dgr;13C is tentatively considered in terms of the subduction of organic carbon and iodine in pelagic sediment. Coated stones have Br/Cl, I/Cl and &dgr;13C values that are similar to depleted upper mantle (MORB source). Mantle fluid trapped in the coated stones is enriched in halogens40Ar by about a factor of 5000 relative to present‐day upper mantle values. However, the estimated halogen content of the source from which the fluid derived is 7 ppm Cl, 25 ppb Br and 0.1‐2.5 ppb I. These values are strikingly similar to those estimated previously for the source of MORB and therefore indicates that the halogens, like other volatile elements (e.g., noble gases, C and N), are homogeneously distributed throughout large portions of the upper mantle. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48753

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The origin and evolution of the terrestrial alkali element inventory is investigated in the framework of the accretion and differentiation history of the Earth. We predict that a significant percentage of the Earth’s bulk alkali element inventory is in the core (30% for Na, 52% for K, 74% for Rb, and 92% for Cs). These predictions agree with independent estimates from nebular volatility trends and (for K) from terrestrial heat flow data. Vaporization and thermal escape during planetary accretion are unlikely to produce the observed alkali element depletion pattern. However, loss during the putative giant impact which formed the Moon cannot be ruled out. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48754

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experiments on C(graphite)+basaltic melt and C(graphite)+Mg2SiO4+melt, C(graphite)+Mg2SiO4+(CO‐CO2) vapor+melt at 15‐40 kbar, 1400‐1700 °C have shown that carbon may be soluble in forsterite and pyroxene in concentrations of 10‐100 ppm, and reactions with graphite lead to formation in the melt of 100‐1000 ppm CO2concentrations. Carbon is an incompatible element in the melt+crystals system. The redox state of the mantle and interactions among primary carbon, melt and crystals are suggested to play an important role in the formation of carbon species in terrestrial basalts. ©1995 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.48734

出版商:AIP    

年代:1995

数据来源: AIP