Fission-track ages of four meteorites     

Eugene A. Carver  Edward Anders 《Geochimica et cosmochimica acta》1976,40(4):467-477

A method for selective annealing of cosmic-ray tracks has been developed, permitting determination of fission-track ages in the presence of a large background of cosmic-ray tracks. The mesosiderite Bondoc contains 41 fission tracks/cm², of which about 75% are due to neutron-induced fission of U²³⁵ during cosmic-ray exposure. Its net fission-track age is 140 ± 40 Myr, nearly identical to its cosmic-ray exposure age of 150 Myr. The mesosiderite Mincy has a fission-track age of 1500 ± 400 Myr.Nakhla (nakhlite) contains an excess of apparent fission tracks, which may be either genuine fission tracks from Pu²⁴⁴ or etch pits mimicking fission tracks in length, thermal stability, random orientation, and other characteristics. On the assumption that they are fission tracks, the Pu²⁴⁴/U²³⁸ ratio at the onset of track retention in Nakhla was (3.1 ± 1.3) × 10^?3, nearly an order of magnitude lower than the initial solar system ratio. This may reflect a chemical fractionation of Pu and U, or a late impact or magmatic event. Different minerals of the Washougal howardite have different Pu²⁴⁴/U²³⁸ ratios, from (24 ± 7) × 10^?3 to (2.3 ± 0.7) × 10^?3. This may imply a succession of impacts over a period of time. Additionally, Pu and U may have been chemically fractionated from each other in this meteorite.Shocked meteorites show no consistent pattern in the retentivity of fission tracks and of fissiogenic or radiogenic noble gases. Some meteorites, e.g. Bondoc, Serra de Magé, and Mincy, retain gases more completely than tracks; others, e.g. Nakhla and Allende, retain them less completely.Uranium was determined in feldspar and/or pyroxene from 19 Ca-rich achondrites and mesosiderites. For most, only upper limits of 0.01–0.03 ppb were obtained. Apparently the uranium in these meteorites resides almost exclusively in minor phases, as in terrestrial and lunar rocks.  相似文献   

Xenon spallation systernatics in Angra dos Reis     

C.M. Hohenberg  B. Hudson  B.M. Kennedy  F.A. Podosek 《Geochimica et cosmochimica acta》1981,45(10):1909-1915

We have resolved literature Xe data for the Angra dos Reis meteorite into constituent spallation, fission and trapped components. The spallation Xe compositions vary over a range wider than observed in any other samples, including lunear samples. These variations are due to the mixing of spallation Xe from Ba and rare earth element targets. It is possible to infer the Ba and rare earth spallation Xe compositions. Angra dos Reis spallation Xe compositions are systematically different from those observed in lunar samples, possibly because of differences in the irradiation conditions (geometry and shielding). Thus the Angra dos Reis data appear to be superior to lunar data for predicting spallation Xe compositions in other meteorites.  相似文献   

Fission track studies of xenolithic chondrites: implications regarding brecciation and metamorphism     

Brajesh K. Kothari  R.Sundar Rajan 《Geochimica et cosmochimica acta》1982,46(10):1747-1754

We have studied fission tracks in phosphates from one gas-poor chondrite and three gas-rich chondrites to determine their thermal history and brecciation time scales. More than 70 percent of the tracks in whitlockites in these meteorites are due to the decay of extinct Pu²⁴⁴.Whitlockites separated from Bhola, a gas-poor chondrite, have $\text{ρ}{}_{\mathrm{Pu}}\text{ρ}{}_{\mathrm{U}}\text{= 2.6–5.2}$ and a model fission track age of 4.0 Gyr for a $\text{(}\text{Pu}\text{U}\text{)}{}_{\mathrm{4.55Gyr}}\text{= 0.045}$. Brecciation of the Bhola meteorite must have occurred at ?4.3 Gyr to account for the metal data (Scott and Rajan, 1981). A minimum cooling rate of $\text{0.9}{}_{\mathrm{–0.2}}{}^{0.3}\text{K}\text{Myr}$ in the temperature interval 800 to 300 K obtained from the track data is a factor of seven higher than the metallographic cooling rate ($\text{0.1}\text{K}\text{Myr}$).For the gas-rich chondrites, the $\text{ρ}{}_{\mathrm{Pu}}\text{ρ}{}_{\mathrm{U}}$ in whitlockites are: Weston, 32–148; Fayetteville, 21–227; and St. Mesmin, 26–137. Whitlockites from all these meteorites give model fission track ages of 4.4 Gyr assuming a $\text{(}\text{Pu}\text{U}\text{)}{}_{\mathrm{4.55\; Gyr}}\text{= 0.045}$. The final brecciation event definitely did not reset the track clock in phosphates of St. Mesmin. Our data suggest that it is also true for Weston and Fayetteville. We conclude that our observed fission track ages date the end of metamorphic cooling in the meteorite parent bodies and support the planetesimal model for the formation of xenolithic chondrites.  相似文献   

The fission track records of the Estherville,Nakhla and Odessa meteorites     

P.F. Green  R.K. Bull  S.A. Durrani 《Geochimica et cosmochimica acta》1978,42(9):1359-1366

Investigation of fossil charged-particle track densities in various mineral phases of three meteorites—Estherville, Nakhla and Odessa—coupled with U content determination, has led to the evaluation of various contributions to the total fossil track density, including those due to the spontaneous fission of ²³⁸U and ²⁴⁴Pu. A fission-track age for Estherville of around 4 × 10⁹ yr is found, which is thought to reflect slow cooling of the parent body. A Pu track excess of (106 ± 9): 1 over the spontaneous fission of ²³⁸U is found in Odessa diopside, which is larger than may be allówed on a simple ‘continuous synthesis’ model for the production of heavy elements prior to solar system formation. Possible explanations for this value are discussed, including fractionation of Pu relative to U. No detectable U was found in Nakhla diopside, and a measurable contribution of track densities from the fission of superheavy elements is ruled out on the basis of track-length measurements and laboratory calibration with Fe ions.  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   

He,Ne and Ar isotopes in inclusions of some iron meteorites     

L.K Levsky  A.N Komarov 《Geochimica et cosmochimica acta》1975,39(3):275-284

He³, He⁴, Ne²¹ and Ar³⁸ contents were determined in 18 metal, troilite, sehreibersite and graphite inclusions of 9 iron meteorites, by total outgassing and stepwise heating. The $\text{He}{}^4\text{He}{}^3$ ratio in metal phase ranges from 3.85 to 4.65, but in non-metallic samples, from 6.70 to 30.5. The results for cosmogenic isotopes of helium, neon and argon disagree appreciably with data on accelerator-irradiated targets. It should be noted, however, that some inclusions have lost considerable amounts of gas by diffusion.Uranium contents of 22 troilite and sehreibersite samples were determined by the fission track technique. The average uranium content of troilite is 0.4-0.7 ppb. Excess He⁴ of unknown origin was observed in troilite inclusions. If one assumes that the excess He⁴ was produced by uranium decay in situ, then the apparent U-He⁴ age is at least 5.9 × 10⁹ yr.  相似文献   

The spontaneous fission rate of U-238 and fission track dating     

G.A Wagner  G.M Reimer  B.S Carpenter  H Faul  R Van Der Linden  R Gijbels 《Geochimica et cosmochimica acta》1975,39(9):1279-1286

The rate of spontaneous fission decay of uranium-238 (λ_f²³⁸) was determined in 4π-geometry by the fission track method. Uranium glasses of known age of manufacture were used. Spontaneous tracks have accumulated since the time of manufacture and induced tracks to determine the uranium content were produced by thermal neutron irradiations. Spontaneous tracks in all glasses were found to be partially annealed. By correcting for this annealing effect, a (λ_f²³⁸) = 8.7 ± 0.6 × 10^?17yr^?1 was obtained. Uncertainty in the neutron dose is the largest source of error.  相似文献   

Actinide microdistributions in the enstatite meteorites     

M.T. Murrell  D.S. Burnett 《Geochimica et cosmochimica acta》1982,46(12):2453-2460

Oldhamite is a major Th and U bearing phase in the enstatite meteorites. Oldhamite from E-6 chondrites has mean Th and U abundances of 1550 ± 80 ppb Th and 410 ± 20 ppb U, with $\text{Th}\text{U}\text{= 3.8 ± .2}$. With the exception of ferroan alabandite which contains 25 ± 1 ppb U, no other Th or U enriched phases were located in the E-6 chondrites, and nearly all of the total rock Th and U can be accounted for by oldhamite. In Khairpur (E6), excess fossil fission tracks were observed in enstatite grains in contact with oldhamite which indicates the presence of ²⁴⁴Pu in oldhamite. Oldhamite from St. Mark's (E5) and Abee (E4) also shows actinide enrichments but at levels about half the E-6 results. Niningerite in Abee contains 45 ± 5 ppb U and due to its high reported modal abundance is an important U reservoir in Abee. The U content of oldhamite from the aubrite Peña Blanca Spring is 1920 ± 100 ppb. All $\text{Th}\text{U}$ values measured in this study cluster tightly around a value of 4 which indicates a lack of ThU fractionation in both oldhamite and in the enstatite meteorites, themselves. This lack of fractionation, along with the presence of ²⁴⁴Pu in oldhamite and reported rare earth enrichments also in oldhamite, suggests that the enstatite chondrites may be well-suited for PuU chronology and for providing the initial PuU value in the early solar system.  相似文献   

I-Xe and ⁴⁰Ar-³⁹Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon     

S Niemeyer 《Geochimica et cosmochimica acta》1983,47(6):1007-1012

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and ⁴⁰Ar-³⁹Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic ⁴⁰Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred $\text{Pu}\text{U}$ ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most ¹⁶O-rich meteorite known, gives a complex ⁴⁰Ar-³⁹Ar age pattern which suggests a recent (?0.85 Ga) severe thermal disturbance. The absence of excess ¹²⁹Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between ⁴⁰Ar-³⁹Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.  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   

Fission tracks and cooling rates of meteorites     

Ghislaine Crozaz 《Journal of Earth System Science》1981,90(3):383-388

Recent work on fission track studies of meteorite samples to obtain cooling rates of metetorite parent bodies is reviewed. The cooling rates of chondrites are in excess of 1^oK/10⁶ yr. Fission track studies of phosphate grains in mesosiderites do not support the extremely slow cooling rates of 0°1^oK/10⁸ yr for these meteorites, inferred from metallographic studies. The accumulating evidence from fission track studies indicates a gross underestimation of the cooling rates of meteorites as determined by the metallographic techniques.  相似文献   

U-Pb ages of angrites     

Yuri Amelin 《Geochimica et cosmochimica acta》2008,72(1):221-232

Precise U-Pb ages, determined with double spike (²⁰²Pb-²⁰⁵Pb) thermal ionization m1ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D’Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D’Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured ²⁰⁶Pb/²⁰⁴Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160-4500 for LEW, and 608-8500 for D’Orbigny. Blank-corrected ²⁰⁶Pb/²⁰⁴Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible ²⁰⁷Pb^∗/²⁰⁶Pb^∗ dates with the average values of 4557.65 ± 0.13 Ma for AdoR, 4558.55 ± 0.15 Ma for LEW, and 4564.42 ± 0.12 Ma for D’Orbigny. Pb-Pb isochrons including data with slightly elevated common Pb, and U-Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopic ages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of “slowly cooled” angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of ⁵³Mn-⁵³Cr “absolute” ages by 0.7 Ma. Third, a precise age of D’Orbigny allows consistent linking of the ⁵³Mn-⁵³Cr and ²⁶Al-²⁶Mg extinct nuclide chronometers to the absolute lime scale.  相似文献   

The behavior of actinides,phosphorus, and rare earth elements during chondrite metamorphism     

M.T Murrell  D.S Burnett 《Geochimica et cosmochimica acta》1983,47(11):1999-2014

New data on the U, Pu, and P distributions in less metamorphosed H-chondrites (type 3–5), coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism. Preferential associations of phosphates with metals and/or sulndes in all chondrites strongly indicate an “initially” siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metal-silicate fractionation. Uranium is initially concentrated in chondrule glass at ~ 100 ppb levels with phosphates (primarily merrillite) in H-3 chondrites being essentially U-free (<20 ppb). As chondrule glass devitrified during metamorphism, U migrated into phosphates reaching ~ 50 ppb in Nadiabondi (H-5) merrillite and 200–300 ppb in merrillite from equilibrated chondrites but “froze out” before total concentration in phosphates occurred. Relative ²⁴⁴Pu fission track densities in the outer 5 μm of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in Bremervörde (H-3) give Pu(mesostasis)/Pu(merrillite) <0.01, implying total concentration of Pu in phosphates. Similarly, no detectable Pu (<0.1 ppb) was found in chondrule mesostasis in Tieschitz and Sharps; whereas, direct measurements of tracks in phosphates in H-3 chondrites are consistent with high (?10 ppb) Pu concentrations. Thus, a strong Pu-P correlation is indicated for ordinary chondrites. There is variable Pu/U fractionation in all chondritic phosphates reaching an extreme degree in the unequilibrated chondrites; therefore, the Pu/U ratio in phosphates appears relatively useless for relative meteorite chronology. Literature data indicate that the REE are located in chondrules in unequilibrated chondrites, most likely in glass; thus there may also be strong Pu/Nd fractionation within these meteorites. Like U, the REE migrate into phosphates during metamorphism but, unlike U, appear to be quantitatively concentrated in phosphates in equilibrated chondrites. Thus relative ages, based on Pu/Nd, may be possible for equilibrated chondrites, but the same chronological conclusions are probably obtainable from Pu concentrations in phosphates, i.e., on the Pu/P ratio. However, Pu/P chronology is possible only for ordinary chondrites; so there appears to be no universal reference element to cancel the effects of Pu chemical fractionation in all meteorites. Available data are consistent with — but certainly do not prove-that variations in Pu/P represent age differences, but if these age differences do not exist, then it is conceivable that the solar system ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio, important for cosmochronology, is still lower than the presently accepted value of 0.007.  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   

Age and Thermal History of the East Ore Section,Bayan,Obo Deposit—A Fission Track Study     

张峰  刘铁庚等 《中国地球化学学报》1997,16(1):80-85

Considerable attention has been paid in recent years to the study of geothermal history by using spontaneous fission tracks of 238U recorded in minerals.Apatite and zircon were used for fission track study in this paper because apatitie has been widely used as a natural geothermometer(Wang Shicheng et al., 1994) to reveal the thermal evolution of sedimentary rocks based on its low annealing temperature of fission tracks and zircon is characterized by a closing temperature above 700℃,The samples were collected from ferruginous,siliceous slate wall rock at the upper levels of the orebody and Nb-REE-Fe ores from deep tunnels.The age and thermal evolution of the orebody were discussed in terms of fission track characters and their length variations observed in the coexisting apatite and zircon in the same specimen.  相似文献   

Alpha-recoil damage and solution effects in minerals: uranium isotopic disequilibrium and radon release     

Robert L. Fleischer 《Geochimica et cosmochimica acta》1982,46(11):2191-2201

  相似文献   

26AI in stony meteorites with gas losses     

G.F. Herzog 《Geochimica et cosmochimica acta》1977,41(10):1526-1529

²⁶Al contents of 16 meteorites with ³He losses have the same average values as all chondrites indicating that gas losses did not occur in a recent, prolonged exposure to unusual cosmic-ray activity.  相似文献   

Siderophile elements in Martian meteorites and implications for core formation in Mars     

《Geochimica et cosmochimica acta》1999,63(11-12):1865-1875

Noble metals, Mo, W, and 24 other elements were determined in six SNC meteorites of presumably Martian origin. Based on element correlations, representative siderophile element concentrations for the silicate mantle of Mars were inferred. From a comparison with experimentally determined metal/silicate partition coefficients of the moderately siderophile elements: Fe, Ni, Co, W, Mo, and Ga, it is concluded that equilibrium between core forming metal and silicates in Mars has occurred at high temperatures (around 2200°C) and low pressures (<1 GPa). This suggests that metal segregation occurred concurrently with rapid accretion of Mars, which is consistent with the inference from excess ¹⁸²W in Martian meteorites (Lee and Halliday, 1997). Concentrations of Ir, Os, Ru, Pt, and Au in the analyzed Martian meteorites, except ALH84001, are at a level of approximately 10⁻²–10⁻³ × CI. The comparatively high abundances of noble metals in Martian meteorites require the addition of chondritic material after core formation. The similarity in Au/La and Pt/Ca ratios between ALH84001 and the other Martian meteorites suggests crystallization of ALH84001 after complete accretion of Mars.  相似文献   

Long-term galactic cosmic ray variations over the last billion years based on the cosmic-ray exposure ages of iron meteorites     

V. A. Alexeev 《Geochemistry International》2016,54(1):78-84

The distribution of the cosmic-ray exposure ages (T) of iron meteorites was analyzed to establish the possible variations in the intensity of the galactic cosmic ray (GCR) over the last billion years. The analysis was made for the entire data set containing ~80 age values from the literature (Voshage et al., 1983) and the corrected set after the exclusions of paired meteorites (using the Akaike information criterion). The dependence of the criterion χ² in the distribution of the phase values Ph = T/t–int(T/t) on the values of the assumed period (t) of GCR variations was analyzed for both sets of meteorites. The significant deviations of these parameters from the respective average values were found for t ~ 400–500 Myr and, in part, for t ~ 150 Myr. These deviations were interpreted by numerical modeling using the values of ages randomly distributed in the range of 0–1000 Ma. It was found that for variations with a period of 450 Myr, the distribution of the phase values and cosmic-ray exposure ages in the model data set is similar to that of iron meteorites. These results testify to the existence of the GCR variations with a period of ~400–500 Myr during the last 1 Gyr. The variations in the GCR flux can be explained by periodic galactic spiral arm crossings of the solar system. The GCR variations with a period of ~150 Myr discussed in the previous studies (Shaviv, 2002; 2003; Scherer et al., 2006) appears to be less certain.  相似文献   

Uranium and thorium in achondrites     

J.W. Morgak  J.F. Lovering 《Geochimica et cosmochimica acta》1973,37(7):1697-1707

The abundances of U and Th in 19 achondrites and two pallasite olivines have been measured by radiochemical neutron activation analysis. Brecciated eucrites are enriched relative to chondrites in both elements by factors between 10 and 20, perhaps as a result of a magmatic differentiation process. Two unbrecciated eucrites are far less enriched, possibly due to their origin as igneous cumulates. The diogenites Johnstown and Shalka contain approximately chondritic levels of U and Th, but Ellemeet is 10 times lower. The abundances in three howardites are in good agreement with those expected from major element data for a mixing model with eucrite and diogenite end members. The high O¹⁸ basaltic achondrites Nakhla, Shergotty and Angra dos Reis have a range of U and Th abundances similar to the brecciated eucrites and howardites, but have systematically higher Th/U ratios. The Bishopville aubrite has U and Th abundances and Th/U ratios similar to those of several enstatite chondrites, suggesting a genetic relationship. The Norton County aubrite has a low Th/U, similar to that observed in recrystallized and metamorphosed terrestrial ultrabasic rocks, indicating a more complex history. Pallasite olivines have low U and Th contents (0.5.4 ppb and 1.4.3 ppb, respectively) similar to those in terrestrial dunites. The Goalpara ureilite has very low U (<0–6 ppb) and Th (2.7 ppb) abundance consistent with an origin from carbonaceous chondrites by partial melting.  相似文献