Primordial Component Research Articles

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.

Noble gas isotopes and mineral assemblages of Antarctic micrometeorites collected at the meteorite ice field around the Yamato Mountains

Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined.AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration.High 3He/4He ratios are detected for AMMs with high 20Ne/4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean 20Ne/22Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for 36Ar and 38Ar, the abundance of the Q component is comparable to that of the SEP component. 84Kr and 132Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.

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.

Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.

We combine spatial variations of P- and S-wave speeds, 1000 fault plane solutions, and 6600 well-determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the Tonga back-arc. We show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere. The detached slab may be from a previous episode of subduction along the fossil Vitiaz trench about 5 to 8 million years ago. The juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle. Such a configuration, if common in the past, would allow the preservation of a primordial component in the lower mantle.

Diamond genesis, mantle fractionations and mantle nitrogen content: a study of δ13C–N concentrations in diamonds

A compilation of more than 1200 δ13C–N data from well-characterised diamonds show a correlation of the maximum diamond nitrogen content (i.e. a limit sector) with δ13C over the full diamond δ13C range (i.e. more than 30‰). Diamonds with low δ13C values are characterised by rather low N contents (∼0 ppm at δ13C<−30‰), whereas diamonds with high δ13C have more variable nitrogen contents, with a much higher upper limit (∼3500 ppm at δ13C=−4.5‰). This correlation defines a concave trend that is therefore incompatible with a mixing relationship, such as would be produced by the admixture of subducted and primordial components. The limit sector more likely reflects the evolution of mantle melts (or fluids) during differentiation. Nitrogen uptake is seen as a kinetic process, depending mostly on the diamond rate of growth; at a given δ13C value, as a result of slow growth conditions, diamonds with nitrogen contents lower than the maximum value are interpreted as having fractionated the N/C ratio relative to their growth medium. The limit sector is applicable to every diamond paragenesis (peridotitic, eclogitic and fibrous) suggesting that every diamond type may derive from a similar isotopic source. Assuming a mantle δ13C value of −4.5‰, we deduce that the initial C/N ratio of mantle melts (i.e. the diamond growth medium) from which diamonds crystallise ranges between 200 and 500, which is surprisingly similar to that of mid-ocean ridge basalts. Therefore, in spite of their different context and age, it appears that subcontinental and oceanic mantles give samples with similar δ13C, δ15N and C/N, suggesting an overall homogeneity of volatiles within these parts of the Earth since the Archaean. Diamonds also demonstrate that carbon and nitrogen do not behave similarly during the evolution of the diamond growth medium. Accordingly, mantle nitrogen concentration cannot be deduced in a simple way. If N behaved as an incompatible element during partial melting, a mantle nitrogen concentration of about 2 ppm could be expected, provided that the mantle carbon content is about 400 ppm. However, from several lines of evidence presented in this study, nitrogen is not regarded as a totally incompatible element, and a higher mantle nitrogen concentration (perhaps up to 40 ppm) is preferred.

The nature of pristine noble gases in mantle plumes

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

Does the plant mitochondrion integrate cellular stress and regulate programmed cell death?

Research on programmed cell death in plants is providing insight into the primordial mechanism of programmed cell death in all eukaryotes. Much of the attention in studies on animal programmed cell death has focused on determining the importance of signal proteases termed caspases. However, it has recently been shown that cell death can still occur even when the caspase cascade is blocked, revealing that there is an underlying oncotic default pathway. Many programmed plant cell deaths also appear to be oncotic. Shared features of plant and animal programmed cell death can be used to deduce the primordial components of eukaryotic programmed cell death. From this perspective, we must ask whether the mitochondrion is a common factor that can serve in plant and animal cell death as a stress sensor and as a dispatcher of programmed cell death.

Textural analysis of sidescan sonar imagery and generic seafloor characterization

Sidescan sonar imagery is a primordial component of geophysical and environmental surveys of the seafloor. Most of the data are still interpreted visually and qualitatively by experts. The method presented here uses textural analysis, and supplements the interpreter with reliable quantitative results. Indices are derived from grey-level co-occurrence matrices, whose mathematical expressions have been adapted to the specifics of sidescan sonar imagery. The indices measure the different properties of the acoustic textures and correlate them with the characteristics of the seafloor. This technique, called TexAn, has been extensively ground-truthed in complex midocean ridge terrains, using submersible and ROV observations, and in situ sampling. The frequencies used so far range from 6.5 to 500 kHz; all applications have been validated and ground-checked. Current applications are mainly concerned with the geology of midocean ridges, continental margins, and coastal waters. The technique has also been demonstrated for the detection of mines and subsurface structures. Being hardware-independent, TexAn enables quantitative assessments of sidescan sonar imagery, at all stages of processing and in all conditions. Another advantage of TexAn is the detection of details hitherto invisible to the human eye, however experienced.

Re-definition of "El Taco Xe" based on 132Xe-normalized data. Multiple primordial components in IAB irons.

The “El Taco Xe” trapped in graphite fractions of the El Taco IAB iron meteorite proposed by Mathew and Begemann (1995) was reexamined on the basis of the data normalized to 132Xe. On the Xe three-isotope diagrams, their data did not form a clear linear trend between atmospheric Xe and “El Taco Xe” due to the existence of Xe-HL in the samples. The Xe-HL released at high-temperature demands a slight modification of “El Taco Xe” by subtracting Xe-HL. The modified El Taco Xe proposed in this study can be explained by a mass-fractionation from solar Xe or U-Xe. High temperature data could be explained by the addition of a Q-HL mixture with a constant Q/HL ratio of ∼5 to the modified El Taco Xe.

Nitrogen and Argon Isotopes in the Archaean Continental Crust: Investigating the Evolution of the Early Earth

Tracing the origin and fate of major volatiles, such as nitrogen, it is important to understand the evolution Of the early earth. However, the geochemical behaviour of N is still poorly constrained and the resulting N evolution model of the earth is rather complicated. The general view supposes a mantle source with N being depleted in heavy isotopes (8~5N -25%0) and its degassed product (atmosphericcrustal N) and the residual mantle both enriched in heavy isotopes (815N from 0%0 to +12?tf=Pi2>%). To reconCiliate this paradoxical view, two diverging models Of the earth evolution have been proposed: (1) an original N source from enstatite chondrite (81SN -25%0) mixed with a tardive veneer (815N +42%o) recycled in the mantle by subduction (Javoy, 1997); or (2) the fractionation, due to atmospheric escape, of an enstatite chondrite original source (Marty and Humbert, 1997). These hypotheses need to be verified by measuring the N isotopic composition in the early earth, particularly in the Archaean crust. Until now, sparse data were available (e.g. Sano and Pillinger, 1990). The aim of this ongoing study is to obtain precise isotopic N data from the Archaean crust. We selected (1) 3.5-2.4 Ga cherts from Pilbara Craton and Hamersley Basin, Western Australia, rocks that have been neVer metamorphosed above the pumpellyte-prehnite facies (~250~ and (2) 3.8 Ga cherts and BIF (Banded Iron Formation) from Isua, West Greenland, metamorphosed to amphibolite facies (-600~ The choice of measuring N isotopes in the lsua samples was mainly dictated by the need to investigate: (1) the efficiency of N retention in Subduction-related recycling; and (2) whether N primordial compOnents have been preserved or not from high grade metamorphism processes. Nitrogen was extracted using a stepwise combustion technique, whose isotopic ratio was measured by a static type mass spectrometer, together with argon isotopes and carbon amount. Calibration of N relative to argon isotopes has been proven to be effective in deconvoluting N sources in rocks (Yamamoto et at., 1998). First results showed two volatile fractions in the Archaean sediments, released at 500-900~ and at 1000-1200~ respectively. The N amount was low (0.05-1.7 ppm) and it was accompanied by tow 36Ar contents (0.2-2 x 10 -14 mole/g), but high C contents, from 11 up to 3t00 ppm. The 615N values ranged from -5%o to +19%o. The 4~ ratios ranged from 300 to t9,000. Electron microprobe analysis and literature data gave K contents of 20-2000 ppm, low, but theoretically sufficient to produce the high 4~ ratios. This point is to be confirmed by ICP-MS analysis of K to decipher if the

Nuclear reaction rates and primordial6Li

We examine the possibility that big-bang nucleosynthesis (BBN) may produce nontrivial amounts of ${}^{6}\mathrm{Li}$. If a primordial component of this isotope could be observed, it would provide a new fundamental test of big-bang cosmology, as well as new constraints on the baryon density of the universe. At present, however, theoretical predictions of the primordial ${}^{6}\mathrm{Li}$ abundance are extremely uncertain due to difficulties in both theoretical estimates and experimental determinations of the ${}^{2}\mathrm{H}(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{6}\mathrm{Li}$ radiative capture reaction cross section. We also argue that present observational capabilities do not yet allow the detection of primeval ${}^{6}$Li in very metal-poor stars of the galactic halo. However, if the critical cross section is very high in its plausible range and the baryon density is relatively low, then improvements in ${}^{6}\mathrm{Li}$ detection capabilities may allow the establishment of ${}^{6}\mathrm{Li}$ as another product of BBN. It is also noted that a primordial ${}^{6}\mathrm{Li}$ detection could help resolve current concerns about the extragalactic D/H determination.

Dirac constraint quantization of a dilatonic model of gravitational collapse

We present an anomaly-free Dirac constraint quantization of the string-inspired dilatonic gravity (the CGHS model) in an open two-dimensional spacetime. We show that the quantum theory has the same degrees of freedom as the classical theory, namely, all the modes of the scalar field on an auxiliary flat background, supplemented by a single additional variable corresponding to the primordial component of the black hole mass. The functional Heisenberg equations of motion for these dynamical variables and their canonical conjugates are linear, and they have exactly the same form as the corresponding classical equations. A canonical transformation brings us back to the physical geometry and induces its quantization.

Terrestrial-type xenon in sulfides of the Allende meteorite.

Two FeS-rich samples from the Allende (C3V) chondrite were analyzed for Xe and Kr. One sample was irradiated in a neutron flux to generate a tracer isotope of Xe by the 130Te(n, γ2β–)131*Xe reaction. The experiment was designed to use this tracer isotope to identify the temperature at which gases were released from within the sulfide. When the sulfide melted at 950°C, the Xe released was terrestrial in isotopic composition, except for enrichments from spallogenic and radiogenic components. It is concluded that terrestrial-type Xe, Xe-T, was a primordial component that was dominant in the inner Fe, S-rich region of the solar nebula.

Aesthetics of Liposuction

Liposuction as such has the purpose of creating beauty by reducing the deformity of a human body through this scientific technique; therefore, it is essential to be aesthetically prepared to discriminate zones that deform or disfigure the human body. It is important to have a clear concept of what beauty is as such, through time and at present; to recognize the sense of proportion, cultural perspective, historical approach, customs, social objectivity, and artistic knowledge. The obligation of incorporating into this specialty the art concept as a primordial component of the scientific training creates liposculpture of the human body and the knowledge that the final objective is to sculpt a human body with a scientific–artistic eye.

LIGHT ELEMENT ABUNDANCES IN POPULATION II DWARFS

ABSTRACT This paper reviews the abundances of the light elements lithium, beryllium and boron in main sequence and subgiant, Population II stars. Li is important to cosmology because it is synthesized in the Big Bang, but is also used to study stellar structure. Beryllium is useful for studying galactic chemical evolution because its formation in the interstellar medium involves different physics to stellar nucleosynthesis, and it thus provides independent data on the evolution of the halo. Some (though not all) inhomogeneous Big Bang nucelosynthesis codes predict a significant primordial component to this element, so its observed abundance may constrain such models. Boron, observations of which became feasible with the operation of the Hubble Space Telescope, provides complementary data to Be, helping check the element ratios predicted by calculations of spallation reactions in the interstellar medium, and will indicate whether the observed Be abundance has an excess over the expected spallation component, indicating a possible primordial component.

Big-Bang nucleosynthesis—observational aspects

Extrapolation of observational data on the abundances of D, 3He, 4He and 7Li in various astrophysical objects to derive their primordial values leads to results in good accordance with calculations from Standard Big Bang nucleosynthesis theory over 9 orders of magnitude in abundance and has led to the following predictions: there are not more than 3 light neutrino species or other particles contributing relativistic degrees of freedom at temperatures of a few MeV; the neutron half-life is less than 10.4 min; and baryonic dark matter exists, but not in sufficient quantities to close the universe. (The first two of these predictions have been confirmed by laboratory experiments). Searches for a primordial component in the abundance of any other element heavier than hydrogen – such as might have resulted from inhomogeneities due to phase transitions in the early universe, notably the quark-hadron transition – have so far proved completely negative. The primordial helium abundance is found from observations of extragalactic ionized hydrogen clouds to be close to 0.230 by mass, a little lower than predicted, but the difference does not exceed likely errors.

Helium, volatile fluxes and the development of continental crust

Mantle-derived helium has a substantial primordial component and is readily distinguished from radiogenic “crustal” He by its isotopic composition. For some years it has been known to be escaping at mid-ocean ridges and more recently it has been shown to be escaping through the continental lithosphere in tectonically active areas, particularly those undergoing extension or volcanism. The C/ 3He value observed in ocean ridge basalts and continental gases that contain only mantle He, is close to 10 9. This is believed to be a typical value for the upper mantle. Other continental gases have ratios that vary widely and are diluted with crustal carbon. The ratio C/ 4He decreases with time through the production of radiogenic 4He, and depends on the C/(U + Th) value. Departures from the average may result from exceptional concentrations of U and Th or from C/He fractionation. There is circumstantial evidence for a steady-state flux of He through the continents that may be estimated from He accumulations in lakes and aquifers. The mantle component of such fluxes is calculated from their 3He content. If the mantle component is accompanied by C in the proportion indicated above, and extensional areas make up as little as 10% of the crust at any one time, then about 10% of the present inventory of crustal C would have been added to the crust every Ga by this means. C/K values for the crust and mantle are today very similar, and K may therefore scale as C. K/U and K/Th vary within narrow limits and they may scale with C also. The most plausible means of scavenging He from the mantle is by partial melting: He is expected to enter the first few percent of liquid formed, and the loss of mantle He and C at the surface is associated with the emplacement of basaltic bodies in the lower crust carrying K, U and Th. Some limits are placed on the thickness of basalt added in extensional areas. Mantle-derived CO 2 has often been invoked as a means of dehydrating continental crust to produce granulites. However, the amounts of CO 2, estimated from mantle He fluxes, entering the crust in those active tectonic areas studied so far appears too small to produce dehydration on a regional scale. The addition of mantle-derived material to the crust in extensional zones is a first-order crustal growth process the importance of which has previously been underestimated.

The Oklo natural reactors: cumulative fission yields and nuclear characteristics of Reactor Zone 9

The isotopic composition of molybdenum, ruthenium, palladium, silver, cadmium, tin, tellurium, neodymium and uranium have been measured by solid source mass spectrometry in eight uraninite samples from Reactor Zone 9 at the Oklo natural reactors. Cumulative fission yields have been derived for most of these elements, after correcting where necessary for the primordial component of the element concerned. Neutron capture reactions on a number of nuclides with significant thermal cross sections, and fission chains in which one of the precursor nuclides has a lengthy half-life, have also been examined to provide information on the relative mobilities of the elements involved. The Pd fission yields have been utilised to calculate the proportions of 235U, 238U and 239Pu fission to be approximately 88%, 8% and 4% respectively in Reactor Zone 9. It has also been shown that nearly 50% of the fissioning 235U nuclides were produced from the α-decay of 239Pu. The integrated neutron flux in this zone was calculated to be approximately 3.6 × 10 20 n cm −2. Estimates of the spectral index show that approximately two-thirds of the neutrons in Zone 9 were thermalised. Although it is unlikely that the Oklo reactors operated continuously, this study has shown that the duration of criticality in Zone 9 was approximately 2.2 × 10 5 years. During this period the average fission density was 0.92 × 10 20 fissions cm −3, which represent a total energy output of 4 × 10 8 J g −1 of sample with an average power output of 8.1 × 10 −5 W g −1 of sample. The accurate analysis of the isotopic abundances of nine elements, has enabled the nuclear characteristics of Reactor Zone 9 to be established, thus providing a base for estimating the nuclear inventory of the various fission products. This will allow a comparison to be made with the present-day elemental abundances and hence an evaluation of the mobility of these elements in the geological environment at Oklo.

Abundance of light elements and lepton asymmetry

By reanalysing the observations of the4He abundance, it is shown that the primordial component by mass fraction should have an upper bound as 0.23 and that it is in discrepancy with the predictions of the standard hot-big-bang model. A small degeneracy of the electron type of neutrinos can, however, remove the inconsistency. Various implications of such a solution are also discussed.

TERRESTRIAL INERT GASES: Isotope Tracer Studies and Clues to Primordial Components in the Mantle

The discovery of primitive rare-gas isotopes in the interior of the Earth, namely 3He and 129Xe, has initiated an important set of inquiries into the noble gas content of the Earth's mantle, crust, and atmosphere, and the relationship between these terrestrial rare-gas components and those observed in extraterrestrial samples such as meteorites and the solar wind. These studies have been guided by 3He, since excess 3He has become the unambiguous trademark of primordial components from the Earth's mantle. For this reason, this paper focuses on helium isotope variations in terrestrial systems, with occasional excursions into the discussions of heavier rare-gas isotopes. The primary purpose of this paper is twofold: first, to identify the basic components or end members that, when mixed together, fractionated, and perturbed, produce the rare-gas isotope variations that we observe in terrestrial systems ; and second, to discuss a few specific applications of rare-gas isotopes to studies of ocean circulation, mid-ocean ridge tectonic and hydrothermal processes, and continental zones of geothermal activity. This article emphasizes isotopic signatures-rare-gas abundance patterns and Earth degassing models are not treated here. The complexities of heavy rare-gas isotopes are discussed only insofar as they help to identify basic terrestrial components, especially primordial mantle reservoirs. The reader is referred to review articles by Ozima & Alexander (1976), Bernatowicz & Podosek ( 1978), Tolstikhin ( 1978), Craig & Lupton (1981), Fisher (1981), and to two forthcoming books (Mamyrin & Tolstikhin 1 983, Ozima & Podosek 1983) for additional discussions of terrestrial inert gases.