Extreme fractionation of 234U238U and 230Th234U in spring waters,sediments, and fossils at the Pomme de Terre Valley,southwestern Missouri     

Barney J Szabo 《Geochimica et cosmochimica acta》1982,46(9):1675-1679

Isotopic fractionation as great as 1600% exists between ²³⁴U and ²³⁸U in spring waters, sediments, and fossils in the Pomme de Terre Valley, southwestern Missouri. The activity ratios of ${}^{234}\text{U}{}^{238}\text{U}$ in five springs range from 7.2 to 16 in water which has been discharged for at least the past 30,000 years. The anomalies in ${}^{234}\text{U}{}^{238}\text{U}$ ratio in deep water have potential usefulness in hydrologic investigations in southern Missouri. Clayey units overlying the spring bog sediments of Trolinger Spring are enriched in ²³⁰Th relative to their parent ²³⁴U by as much as 720%. The results indicate that both preferential displacement via alpha recoil ejection and the preferential emplacement via recoiling and physical entrapment are significant processes that are occurring in the geologic environment.  相似文献   

Uranium in submarine metalliferous deposits     

Harold S Rydell  Enrico Bonatti 《Geochimica et cosmochimica acta》1973,37(12):2557-2565

Hydrothermal submarine metalliferous deposits, common in areas of the ocean floor with high heat flow, contain generally about 10 ppm U as an order of magnitude. The $\text{U}{}^{234}\text{U}{}^{238}$ ratio is in the majority of cases close to that of seawater; only in a few cases is it anomalously high. Anomalous $\text{U}{}^{234}\text{U}{}^{238}$ ratios are coupled with low U concentrations. These data are explained by a model where thermal water (essentially heated seawater) in its sub-bottom circulation often is unable to leach U from the basaltic oceanic crust; in fact, these thermal waters may in some cases lose U. When leaching of U from the basalt does take place, probably during shallow stages of the sub-bottom circulation, the resulting anomalous $\text{U}{}^{234}\text{U}{}^{238}$ ratio can be preserved in the hydrothermal deposit only if mixing with ‘seawater’ U is prevented.  相似文献   

The distribution of U and Pu in the St. Severin chondrite     

J.H. Jones  D.S. Burnett 《Geochimica et cosmochimica acta》1979,43(12):1895-1905

A detailed study of the U distribution of the St. Severin chondrite has been made by fission track radiography in order to clarify the interpretation of fission Xe thermal release data in terms of the mineralogical location of the fission Xe within the meteorite. This is of importance because the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin has been widely adopted as the average solar system value. The U contents of the constituent minerals cannot account for the total rock U which, instead, appears to be primarily localized on grain boundaries. The greatest localizations of U are in olivine-poor, orthopyroxene-rich ‘clasts’. Our data coupled with those of Podosek (1970a) show that ²⁴⁴Pu in St. Severin was also located on grain boundaries and that the bulk of Pu and U are unfractionated within this meteorite. Due to recoil, the ²⁴⁴Pu fission Xe is found in 10 micron surface layers on major phases. Assuming that the grain boundaries (on which the Pu was located) was formed during metamorphism, the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin applies to a time subsequent to the textural recrystallization of the meteorite. Our data support the interpretation of Podosek and our best estimate of the solar system ${}^{244}\text{Pu}{}^{238}\text{U}$ is 0.015.  相似文献   

234Th238U radioactive disequilibrium in the surface layer of the ocean     

Eiji Matsumoto 《Geochimica et cosmochimica acta》1975,39(2):205-212

²³⁴Th produced from ²³⁸U within sea water was found to be in radioactive disequilibrium with respect to its progenitor nuclide ²³⁸U in the surface layer of the ocean. The median value for ${}^{234}\text{Th}{}^{238}\text{U}$ activity ratio is 0.80 in the upper 200 m layer. A box-model calculation gives a removal residence time of thorium of about 0.38 yr. This suggests that ²³⁴Th is scavenged from the surface layer by the uptake of thorium by biota.  相似文献   

$\text{U}{}^{234}\text{U}{}^{238}$ Ratios in limestone cave seepage waters and speleothem from West Virginia     

Peter Thompson  Derek C Ford  Henry P Schwarcz 《Geochimica et cosmochimica acta》1975,39(5):661-669

Speleothem from West Virginia, ranging in age from 2000 to 200,000 yr B.P. contains uranium with $\text{U}{}^{234}\text{U}{}^{238}$ ratios significantly greater than unity. This ratio varies from one speleothem to another, as does average U content. Initial ratios, corrected for age, are remarkably constant for a given speleothem. By contrast, $\text{U}{}^{234}\text{U}{}^{238}$ ratios in seepage waters vary significantly from month to month at a given drip site, and their average values differ from that of the speleothem which they are depositing. This discrepancy is attributed either to long-term averaging-out of fluctuations, or fractional precipitation on the speleothem of a chemical species of uranium with a more constant ratio. Constancy of initial $\text{U}{}^{234}\text{U}{}^{238}$ ratios from $\text{Th}{}^{230}\text{U}{}^{234}$. datable portions of speleothems should permit $\text{U}{}^{234}\text{U}{}^{238}$-dating of older portions of the same speleothem, back to about 10⁶ yr B.P., with estimated precision of ±5 per cent.  相似文献   

State of disequilibrium between ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn in groundwater from bedrock     

Matti Asikainen 《Geochimica et cosmochimica acta》1981,45(2):201-206

Approximately one thousand drilled wells were investigated for their natural radioactivity. The determinations of ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water. The ${}^{238}\text{U}{}^{226}\text{Ra}$ activity ratio usually fell in the range 1–20 and the ${}^{238}\text{U}{}^{222}\text{Rn}$ activity ratio in the range 1–20 × 10^?4, the highest activity ratios being from samples with an elevated uranium content. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio varied between 0.76 and 4.67, the most frequent values showing a 60% excess of ²³⁴U in the samples. Most of the ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios near unity were found in samples with a high uranium content. Several drilled wells with anomalously high uranium contents were found in southern Finland. The average ²²⁶Ra and ²²²Rn contents of these wells were not exceptionally high, which suggests high mobility of uranium in groundwater from the areas involved.  相似文献   

Comparison of 230Th-238U disequilibrium systematics in lavas from three hot spot regions: Hawaii,Prince Edward and Samoa     

S Newman  R.C Finkel  J.D Macdougall 《Geochimica et cosmochimica acta》1984,48(2):315-324

²³⁰Th-²³⁸U disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of Hawaii, Marion Island (Prince Edward hot spot), and Samoa. The (${}^{230}\text{Th}{}^{232}\text{Th}$) activity ratios of lavas from these three hot spots (1.06 ± 0.07, 1.04 ± 0.08, and 0.81 ± 0.06, respectively) imply that the source regions are each nearly homogeneous with $\text{Th}\text{U}$ weight ratios of 2.9, 3.0, and 3.8. For Marion Island and Mauna Kea, Hawaii, negligible secular variation occurs in the (${}^{230}\text{Th}{}^{232}\text{Th}$) initial ratios. This supports other evidence for very short transfer time between source and surface. Significant residence time at depth prior to eruption cannot be ruled out for the Samoan lavas we have studied; however, the data for one of these flows deviate from the proposed (${}^{230}\text{Th}{}^{232}\text{Th}$)-${}^{87}\text{Sr}{}^{86}\text{Sr}$ correlation (Condomineset al., 1981a) in the opposite sense from that expected for such residence. If it is assumed that the measured (${}^{230}\text{Th}{}^{232}\text{Th}$) ratios of the young lavas reflect $\text{Th}\text{U}$ in their mantle sources, then the observed variations among these three hot spots, combined with those reported by other workers for Iceland, the Azores and Tristan de Cunha, suggest that these sources are characterized by $\text{Th}\text{U}$ ratios ranging from values similar to that of MORB source (~2.5) to values similar to those of bulk earth (~3.8). Mixing of different proportions of depleted and enriched mantle may be responsible for the observed range.  相似文献   

UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead     

G Manhes  C.J Allegre  A Provost 《Geochimica et cosmochimica acta》1984,48(11):2247-2264

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high $\text{U}\text{Pb}$ initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component (${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 13.0}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 13.5}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 32.71}$) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.  相似文献   

Radioelements,radiogenic helium and age relationships for groundwaters from the granites at Stripa,Sweden   总被引：1，自引：0，他引：1  

J.N Andrews  I.S Giles  R.L.F Kay  D.J Lee  J.K Osmond  J.B Cowart  P Fritz  J.F Barker  J Gale 《Geochimica et cosmochimica acta》1982,46(9):1533-1543

The solution of radioelements and radiogenic ⁴He by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock-water interface. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio and the dissolved U, Rn and He contents of such groundwaters respond to changes in the flow regime with time. Although ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios change with groundwater residence time as a consequence of ²³⁴Th-recoil induced solution of ²³⁴U, the activity ratio is strongly influenced by the U distribution within fractures, by the extent of the rock-water interface and by the amount of ²³⁸U in solution. A model for the quantitative evaluation of these effects is presented.Groundwaters from depths up to 880 m in the Stripa granite have variable dissolved uranium contents and ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios. The uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age.Dissolved radiogenic ⁴He in the groundwaters increases with their depth of origin, and is dependent upon the U content of the granite and upon its fracture porosity. It increases with groundwater residence time but movement of ⁴He by diffusion and transport processes make the actual groundwater age indeterminate.  相似文献   

Uranium-series dating of lacustrine limestones from pan deposits with final Acheulian assemblage at Rooidam,Kimberley district,South Africa     

B.J. Szabo  K.W. Butzer 《Quaternary Research》1979,11(2):257-260

Lacustrine limestone samples from sedimentary pan deposits at Rooidam, near Kimberley, South Africa, that contain late Acheulian (Fauresmith) artifacts have been dated by ${}^{230}\text{Th}{}^{234}\text{U}$ and ${}^{231}\text{Pa}{}^{235}\text{U}$ methods. Results indicate a minimum age of about 200,000 yr B.P. for the terminal Acheulian in the interior of South Africa.  相似文献   

Anomalously high concentrations of uranium,radium and radon in water from drilled wells in the Helsinki region     

M Asikainen  H Kahlos 《Geochimica et cosmochimica acta》1979,43(10):1681-1686

The concentrations of uranium. ²²⁶Ra and ²²²Rn were determined in 308 drilled and 58 dug wells in the Helsinki region. The study area was about 400 km² and geologically highly variable, with granites, amphibolites and migmatites the dominant rocks. The radioactivity of water in the dug wells was on a “normal” level, but in numerous drilled wells it was anomalously high. In 14 drilled wells the concentration of uranium exceeded 1000 μg/l, the highest concentration being 14,870 μg/l. For ²²²Rn the maximum concentration was 880,000 pCi/l. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ and ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios showed the isotopes of the uranium series to be dominant in the study area. A state of disequilibrium between ²³⁸U and ²³⁴U was very common in the samples. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios varied in the range 1.0–4.0 regardless of the amount of uranium in the water. The conclusion can be drawn from the isotopic data that the high radioactivity of water is in some cases caused by primary uranium mineralizations, but mostly by uranium deposited in fissures of the bedrock. The paper includes a summary of the results of two studies carried out between 1967 and 1977.  相似文献   

The geochemical coherence of Pu and Nd and the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio of the early solar system     

John H. Jones 《Geochimica et cosmochimica acta》1982,46(10):1793-1804

Two very different sets of ${}^{244}\text{Pu}{}^{238}\text{U}$ ratios have been reported for early solar system materials. One group of samples yields high (0.015–0.016) ratios (Podosek, 1970a, 1972; Drozd et al., 1977) and calculations based on another group of analyses yield low ratios (~0.004) (Marti et al., 1977). Recently measured partition coefficients for Pu and Sm are used to evaluate the data of Marti et al. and $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratios from other sources are also considered. A low $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratio (~0.005) is favored, and some implications of this low ratio to galactic nucleosynthesis and meteorite age dating are briefly discussed.  相似文献   

Initial Pb of the Amîtsoq gneiss,West Greenland,and implications for the age of the Earth     

A.J. Gancarz  G.J. Wasserburg 《Geochimica et cosmochimica acta》1977,41(9):1283-1301

Pb isotopic abundances and U-Th-Pb concentrations are reported for feldspar megacrysts from the 3.59 AE old Amîtsoq gneisses, Godthaab District, West Greenland. The distinctive Pb in the feldspars is the most primitive terrestrial Pb so far observed. It is observed in feldspars which are from different geographic localities and which exhibit varying degrees of deformation and recrystallization. This appear to be either the initial Pb in the Amîtsoq gneiss or the initial Pb only slightly modified by subsequent metamorphism in a low ${}^{238}\text{U}{}^{204}\text{Pb}$ environment. ${}^{238}\text{U}{}^{204}\text{Pb}$ in the feldspars is low and the corrections for in situ produced Pb are only 0.4% for ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and 0.6% for ${}^{204}\text{Pb}{}^{206}\text{Pb}$. The mean corrected isotopic abundances are ${}^{204}\text{Pb}{}^{206}\text{Pb}\text{= 0.08720}$, ${}^{207}\text{Pb}{}^{206}\text{Pb}\text{= 1.1513}$, and ${}^{208}\text{Pb}{}^{206}\text{Pb}\text{= 2.7350}$. The feldspars contain a very small amount of easily leachable radiogenic Pb which is correlated with U and which indicates the formation of U-rich phases at about 2.7 AE. The matrix surrounding the feldspar megacrysts contains Pb which is much evolved relative to the megacrysts and this matrix does not appear to have behaved as a simple closed system. Element redistribution and open system behavior at about 2.7 AE is also suggested by Pb in feldspar from a dike cutting across the gneiss. Assuming that the Amîtsoq gneiss feldspar Pb corrected for in situ U decay was the initial Pb in the gneiss at 3.59 AE (Baadsgaard, 1973), a single-stage “age of the earth” is determined as 4.47 ± 0.05 AE and μ is 8.5. This is indistinguishable from the single-stage age for modern rocks and is distinctly younger than the 4.55 AE age of some meteorites. If the feldspar Pb was modified by metamorphism at 2.7 AE the model age of the earth is calculated as 4.53 AE which is similar to the 4.55 AE age of some meteorites. Two-stage models using the nominal 3.59 AE initial Pb indicate that if the earth is ~4.55 AE old then μ values were low in the early Earth's history and differentiation occurred within a few hundred million years after the planet formed.  相似文献   

Application of the ${}^{227}\text{Th}{}^{230}\text{Th}$ method to dating Pleistocene carbonates and comparison with other dating methods     

M. Gascoyne 《Geochimica et cosmochimica acta》1985,49(5):1165-1171

The ${}^{227}\text{Th}{}^{230}\text{Th}$ dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the ${}^{231}\text{Pa}{}^{235}\text{U}$ and ${}^{230}\text{Th}{}^{234}\text{U}$ methods. The ${}^{227}\text{Th}{}^{230}\text{Th}$ ages are found to be critically dependent on corrections for decay of ²²⁷Th prior to alpha counting and ingrowth of daughter isotopes of ²³²Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess ²²⁷Th in the calcite of samples younger than ~50 ky, possibly due to the coprecipitation of ²³¹Pa during formation. Calculated “negative” ${}^{227}\text{Th}{}^{230}\text{Th}$ ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the ${}^{227}\text{Th}{}^{230}\text{Th}$ is found to be a useful alternative dating technique for carbonates which are between ~50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods.  相似文献   

Unscrambling the lead model ages     

Francis Albarede 《Geochimica et cosmochimica acta》1984,48(1):207-212

A linear relation is derived for the secular evolution of ${}^{206}\text{Pb}{}^{204}\text{Pb}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios, that permits tests to be made for open system evolution on each system independently. Application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3.8 b.y. ${}^{238}\text{U}{}^{204}\text{Pb}$ and ${}^{232}\text{Th}{}^{204}\text{Pb}$ of 9.66 ± 0.15 and 37.65 ± 1.14 respectively fit best the data for this time interval.A single stage evolution since 4.5 b.y. is unlikely, however, and the major events of continent formation postdate the Earth accretion by at least 400 m.y. The larger scatter of ${}^{207}\text{Pb}{}^{204}\text{Pb}$ data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3.85 ± 0.15 b.y.  相似文献   

Micromanganese nodules in deep-sea sediments: Uranium-isotopic evidence for post-depositional origin     

Robert Immel  J.K. Osmond 《Chemical Geology》1976,18(4):263-272

Micromanganese nodules from three deep-sea cores are found to contain less U than average nodules dredged from the sea floor. The ${}^{234}\text{U}{}^{238}\text{U}$ ratio in these micronodules is higher than any previously reported in deep-sea sediments. We interpret these data to mean that at least some micronodules form well after deposition of the enclosing sediments, in particular where conditions are less oxidizing than average.  相似文献   

Chemical and isotopic evidence for mixing between depleted and enriched mantle,northwestern U.S.A.     

William K. Hart 《Geochimica et cosmochimica acta》1985,49(1):131-144

Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions.The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7030}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51305}$) and old, chemically heterogeneous, isotopically enriched subcontinental mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7078}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51233}$). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.  相似文献   

Radium content and the ${}^{226}\text{Ra}{}^{228}\text{Ra}$ activity ratio in groundwater from bedrock     

Matti Asikainen 《Geochimica et cosmochimica acta》1981,45(8):1375-1381

The relative abundance of ²²⁶Ra and ²²⁸Ra were determined in the groundwater from 125 drilled wells containing from < 0.1 to 51.3 pCi/l of ²²⁶Ra. The determination of ²²⁸Ra was carried out with a liquid scintillation counter by measuring only the weakly energetic β particles emitted from ²²⁸Ra. Thus the interference from the daughter nuclides of ²²⁶Ra was avoided, without specific separation of ²²⁸Ac. The direct measurement of ²²⁸Ra made the method decisively simpler and faster in terms of the chemistry involved.The concentration of ²²⁸Ra was found to be independent of the amount of ²²⁶Ra present in the samples. The concentrations of ²²⁸Ra were nearly the same over the whole range of ²²⁶Ra concentrations and the average $\text{sol}{}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio sharply increased as the ²²⁶Ra content of water increased. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio in the drilled wells varied from 0.3 to 26. Abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations. The abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratios present in groundwater suggest that the radioactivity anomaly is caused by uranium deposits and not by common rocks. In samples with a low radioactivity level the average ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio was slightly below unity, corresponding to the typical U/Th ratio of granite, the most common kind of rock in the study area. The samples from the rapakivi area proved to be exceptional in that they had a low ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio independent of the concentration of ²²⁶Ra.  相似文献   

Helium-uranium dating of corals     

Michael L. Bender 《Geochimica et cosmochimica acta》1973,37(5):1229-1247

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Intensive sampling of noble gases in fluids at Yellowstone: I. Early overview of the data; regional patterns     

B.M. Kennedy  M.A. Lynch  J.H. Reynolds  S.P. Smith 《Geochimica et cosmochimica acta》1985,49(5):1251-1261

The Roving Automated Rare Gas Analysis (RARGA) lab of Berkeley's Physics Department was deployed in Yellowstone National Park for a 19 week period commencing in June, 1983. During this time 66 gas and water samples representing 19 different regions of hydrothermal activity within and around the Yellowstone caldera were analyzed on site. Routinely, the abundances of five stable noble gases and the isotopic compositions of He, Ne, and Ar were determined for each sample. In a few cases the isotopes of Kr and Xe were also determined and found to be of normal atmospheric constitution.Correlated variations in the isotopic compositions of He and Ar can be explained within the precision of the measurements by mixing of only three distinct components. The first component is of magmatic origin and is enriched in the primordial isotope ³He with ${}^3\text{He}{}^4\text{He}\text{≥ 16}$ times the air value. This component also contains radiogenic ⁴⁰Ar and possible ³⁶Ar with ${}^{40}\text{Ar}{}^{36}\text{Ar}\text{≥ 500}$, resulting in a ${}^3\text{He}{}^{36}\text{Ar}$ ratio ≥ 41,000 times the air value. The second component is assumed to be purely radiogenic ⁴He and ⁴⁰Ar (${}^{41}\text{He}{}^{401}\text{Ar}\text{= 4.08 ± .33}$). This component is the probable carrier of observed excesses of ²¹¹Ne, attributed to the α,n reaction on ¹⁸O. Its radiogenic character implies a crustal origin in U. Th, and Krich aquifer rocks. The third component, except for possible mass fractionation, is isotopically indistinguishable from the noble gases in the atmosphere. This component originates largely from infiltrating run-off water saturated with atmospheric gases.In addition to exhibiting nucleogenic ²¹¹Ne, Ne data show anomalies in the ratio ${}^{20}\text{Ne}{}^{20}\text{Ne}$, which correlate roughly with the ${}^{21}\text{Ne}{}^{22}\text{Ne}$ anomalies for the most part, but not as would occur from simple mass fractionation. Some exaggerated instances of the ${}^{20}\text{Ne}{}^{22}\text{Ne}$ anomaly occur which could be explained by combined mass fractionation of Ne and Ar isotopes to a severe degree coupled with remixing with normally isotopic gases. Otherwise exotic processes have to be invoked to explain the ²⁰Ne data.Relative abundances of the non-radiogenic and non-nucleogenic noble gases (²²Ne, ³⁶Ar, ⁸⁴Kr, and ¹³²Xe) are highly variable but strongly correlated. High Xe/Ar ratios are always accompanied by low Ne/ Ar ratios and vice versa. Except for water from the few cold (T < 20°C) springs analyzed, none of the samples have relative abundances consistent with air saturated water and the observed variations are not readily explained by the distillation of air saturated water.In characterizing each area of hydrothermal activity by the highest ${}^3\text{He}{}^4\text{He}$ ratio found for that area, we find that within the caldera this parameter is somewhat uniform at ~7 ± 1 times the air value. There are exceptions, most notably at Mud Volcano, an area located along a crest of recent and rapid uplift. Here the maximum ${}^3\text{He}{}^4\text{He}$ ratio is ~ 16 times the air value. Also noteworthy is Gibbon Basin which is in the vicinity of the most recent rhyolitic volcanism and exhibits a ${}^3\text{He}{}^4\text{He}$ ratio ~ 13 times the air value. Immediately outside the caldera the maximum $\text{sol}{}^3\text{He}{}^4\text{He}$ ratio decreases rapidly to values < ~3 times the air value.  相似文献