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1.
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   

2.
The results of a fission track study of uranium-rich merrillite crystals in a number of mesosiderites are clearly incompatible with cooling rates as low as ~ 0.1°C/Myr which have previously been inferred for these meteorites. Classical metallographic techniques thus appear to give erroneous results when applied to this type of meteorites. There appears to be no reason to advocate exotic scenarios to explain the thermal history of mesosiderites.  相似文献   

3.
Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of26A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J s,R o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×105 to 2×106 years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (53Mn,28Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and22Ne/21Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.22Ne/21Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from60Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.  相似文献   

4.
Fossil fission tracks in a stibiotantalite were revealed by etching and used for fission track dating. The fission track age of this specimen (2.3·108 yr.) is much lower than Pb/Pb and Rb/Sr ages of other minerals from the same area. The explanation for this discrepancy could be the only moderate track retentivity of stibiotantalite which is similiar to muscovite.  相似文献   

5.
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 ρPuρU = 2.6–5.2 and a model fission track age of 4.0 Gyr for a (PuU)4.55Gyr = 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 0.9–0.20.3KMyr in the temperature interval 800 to 300 K obtained from the track data is a factor of seven higher than the metallographic cooling rate (0.1 KMyr).For the gas-rich chondrites, the ρPuρU 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 (PuU)4.55 Gyr = 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   

6.
In this study kamacite was experimentally grown in taenite grains of Fe-Ni-P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P. Both isothermal heat treatments and non-isothermal heat treatments at cooling rates of 2 to 5°C/day were carried out. Analytical electron microscopy was used to examine the orientation and chemical composition of the kamacite and the surrounding taenite matrix. The kamacite so produced is spindle or rod shaped and has a Widmanstätten pattern orientation. The presence of heterogeneous sites such as phosphides is necessary for the nucleation of the intergranular kamacite. During kamacite growth both Ni and P partition between kamacite and taenite with chemical equilibrium at the two phase interface. The growth kinetics are limited by the diffusion of Ni in taenite. Additional diffusion experiments showed that the volume diffusion coefficient of Ni in taenite is raised by a factor of 10 at 750°C in the presence of only 0.15 wt% P.A numerical model to simulate the growth of kamacite in Fe-Ni-P alloys, based on our experimental results, was developed and applied to estimate the cooling rates of the iron meteorites. The cooling rates predicted by the new model are two orders of magnitude greater than those of previous studies. For example the cooling rates of chemical groups I, IIIAB and IVA are 400–4000°C/106years, 150–1400°C/ 106 years and 750–6000°C/106years respectively. Previous models gave 1–4°C/106 years, 1–10°C/106 years and 3–200°C/106 years. Such fast cooling rates can be interpreted to indicate that meteorite parent bodies need only be a few kilometers in diameter or that iron meteorites can be formed near the surface of larger asteroidal bodies.  相似文献   

7.
Charged particle tracks were studied in the Angra dos Reis and Moore County meteorites, both of which contain an unexplained excess of He4. A selective annealing method was used to resolve cosmic-ray tracks from fission tracks. It gave the following cosmic-ray and fission-track densities, in units of 106cm?2: Angra dos Reis 1.3–4.4 and 7.8; Moore Co. feldspar 1.9–3.0 and 0.51; Moore Co. pigeonite 2.0–2.9 and 0.078–0.35. The fission-track densities are 10–100 times higher than expected from U238; the excess is probably due to extinct Pu238. The Pu244/U238 ratios at the start of track retention were 0.003 for Angra dos Reis and 0.002–0.03 for Moore Co. No evidence was found for fission tracks attributable to the unknown progenitor of excess He4 in these meteorites; the fission branch of this progenitor comprises less than 10?5 the α-branch. A search for pleochroic halos also gave negative results.The preatmospheric radii of the two meteorites are ≥13 and ≥7cm. According to meteor theory, this implies geocentric velocities of ≥ 19 and ≥ 6 km/sec.The etching behavior of Angra dos Reis augite is highly anomalous, giving rise to spurious angular anisotropies and skewed length distributions. This confirms similar observation by Fleischer et al. (1970) on lunar augite.  相似文献   

8.
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   

9.
Anorthositic series apatites of the Duluth Complex, Minnesota, USA, have high spontaneous fission‐track densities of up to ~107 cm–2 and a homogeneous age of ~900 Ma, allowing high‐precision fission‐track dating based on LA–ICP–MS U analysis. Absolute fission‐track dating, track‐length measurement and chemical composition analysis were performed to evaluate a cooling history, which is essential for age reference materials. Preliminary inverse modelling for a sample with a shortened track‐length distribution yielded a monotonic cooling history from ~100°C at 925 Ma. The apatites incur an over‐etching problem when employing the commonly used etching protocol involving 5.5 M HNO3.  相似文献   

10.
Zircon fission track dating and track length analysis in the high‐grade part of the Asemigawa region of the Sanbagawa belt demonstrates a simple cooling history passing through the partial annealing zone at 63.2 ± 5.8 (2 σ) Ma. Combining this age with previous results of phengite and amphibole K–Ar and 40Ar/39Ar dating gives a cooling rate of between 6 and 13 °C Myr?1, which can be converted to a maximum exhumation rate of 0.7 mm year?1 using the known shape of the P–T path. This is an order of magnitude lower than the early part of the exhumation history. In contrast, zircon fission track analyses in the low‐grade Oboke region show that this area has undergone a complex thermal history probably related to post‐orogenic secondary reheating younger than c. 30 Ma. This event may correlate with the widespread igneous activity in south‐west Japan around 15 Ma. The age of subduction‐related metamorphism in the Oboke area is probably considerably older than the generally accepted range of 77–70 Ma.  相似文献   

11.
Carboniferous‐Permian volcanic complexes and isolated patches of Upper Jurassic — Lower Cretaceous sedimentary units provide a means to qualitatively assess the exhumation history of the Georgetown Inlier since ca 350 Ma. However, it is difficult to quantify its exhumation and tectonic history for earlier times. Thermochronological methods provide a means for assessing this problem. Biotite and alkali feldspar 40Ar/39Ar and apatite fission track data from the inlier record a protracted and non‐linear cooling history since ca 750 Ma. 40Ar/39Ar ages vary from 380 to 735 Ma, apatite fission track ages vary between 132 and 258 Ma and mean track lengths vary between 10.89 and 13.11 μm. These results record up to four periods of localised accelerated cooling within the temperature range of ～320–60°C and up to ～14 km of crustal exhumation in parts of the inlier since the Neoproterozoic, depending on how the geotherm varied with time. Accelerated cooling and exhumation rates (0.19–0.05 km/106 years) are observed to have occurred during the Devonian, late Carboniferous‐Permian and mid‐Cretaceous — Holocene periods. A more poorly defined Neoproterozoic cooling event was possibly a response to the separation of Laurentia and Gondwana. The inlier may also have been reactivated in response to Delamerian‐age orogenesis. The Late Palaeozoic events were associated with tectonic accretion of terranes east of the Proterozoic basement. Post mid‐Cretaceous exhumation may be a far‐field response to extensional tectonism at the southern and eastern margins of the Australian plate. The spatial variation in data from the present‐day erosion surface suggests small‐scale fault‐bounded blocks experienced variable cooling histories. This is attributed to vertical displacement of up to ～2 km on faults, including sections of the Delaney Fault, during Late Palaeozoic and mid‐Cretaceous times.  相似文献   

12.
Group IVA is a large magmatic group of iron meteorites. The mean Δ17O (=δ17O − 0.52·δ18O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ18O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta60, 3103-3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 1012 K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 1010 can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.  相似文献   

13.
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 He4He3 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   

14.
The Late Tertiary shallow subduction of the Cocos ridge under the Caribbean plate controlled the evolution of the Cordillera de Talamanca in southeast Costa Rica, which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting. Fission track thermochronology using zircon and apatite, as well as 40Ar–39Ar and Rb–Sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the Cordillera de Talamanca and the age of onset of subduction of the Cocos ridge. Shallow intrusion of granitoid melts resulted in fast and isobaric cooling. A weighted mean zircon fission track age (13 analyses) and Rb–Sr biotite ages of about 10 Ma suggest rapid cooling and give minimum ages for granitoid emplacement. In some cases 40Ar–39Ar and Rb–Sr apparent ages of amphibole and biotite are younger than the zircon fission track ages, which can be attributed to partial resetting by hydrothermal alteration. Apatite fission track ages range from 4.8 to 1.7 Ma but show no correlation with the 3090-m elevation span over which they were sampled. The apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes. The combination of the apatite fission track ages with subduction parameters of the Cocos plate such as subduction angle, plate convergence rate and distance of the Cordillera de Talamanca to the trench implies that the Cocos ridge entered the Middle America Trench between 5.5 and 3.5 Ma.  相似文献   

15.
Fission track annealing experiments for vermiculite mineral have been performed under optimised etching conditions and a correction curve translating track length reduction to track density reduction has been constructed. The blocking/closing temperature of the fission track system in the mineral has been calculated to be 125°±30° C. The corrected fission track age of vermiculite from Kasipatnam (Visakhapatnam), South India, has been calculated as 544±14 Ma. The activation energy and average uranium concentration of the mineral are 1.7 eV and 9.9×10?8 gg?1 respectively.  相似文献   

16.
Subsolidus Mg-Fe2+ exchange between olivine and spinel is governed by Mg-Fe2+ interdiffusion. Incomplete exchange results in Mg-Fe2+ heterogeneity in both olivine and spinel, which provides information on the thermal histories of the host rocks. A composite sphere model has been developed to obtain quantitative cooling rates or heating duration from the Mg-Fe2+ heterogeneity. The model assumes that a spherical core of spinel and a surrounding semi-infinite spherical shell of olivine interact by diffusion-controlled exchange of Mg and Fe2+. The differential equations describing the model are solved numerically by finite difference approximations. The numerical solution reveals that cooling rates or heating duration can be estimated from the relationship between the grain size of spinel and temperature calculated from the chemical compositions of the core of a spinel grain and of olivine far away from it. The calculated temperature is employed in place of Mg(Mg + Fe2+) at the center of spinel to obtain the absolute temperature of thermal events.This olivine-spinel geospeedometer has been applied to peridotites. gabbro, and picrites from some ophiolite complexes in Japan to estimate their cooling rates. The estimated cooling rates for the peridotites range from 10?4 to 10?1 °C/yr, and those for the picrites from 103 to 104 °C/yr. The geospeedometer has been extended to estimate the heating duration of lherzolite xenoliths in basalt from the Ichinomegata crater, northeast Japan. The estimated heating duration of the xenoliths is less than one day.  相似文献   

17.
The fluorite of Santa Catarina that occurs in veins cutting Precambrian granitic rocks of coastal Brazil has been difficult to date by Rb/Sr, K/Ar and Sm/Nd methods. New fission track dating of apatite in granites next to the veins yields ages of 144–76 Ma, which are related to the opening of the South Atlantic Ocean. Four groups of fission track ages were identified: the ca 145 Ma group is a hydrothermal event that preceded fluorite mineralization; the second group of ages, 131–107 Ma, records the first hydrothermal mineralizing event; the third group, 98–93 Ma, represents the second hydrothermal mineralizing event; and the fourth group, 89–76 Ma, dates the last hydrothermal mineralizing event. As shown by previous studies, the temperatures of these events varied from 170° to 70°C, but the last hydrothermal event occurred during a gradual cooling. The smaller lengths of the confined fission tracks from the fourth event support this interpretation. These results are based on sixteen carefully selected samples from four veins ranging from 1 to 4 m in thickness. The ages of these samples were established using the standard methods of fission track dating. Our study clearly demonstrates the value of apatite fission track dating for deposits whose mineralization occurred over a long time span at a wide range of temperatures.  相似文献   

18.
The noble gases He, Ne, Ar, Kr and Xe and also K and Ba were measured in the Apollo 11 igneous rocks 10017 and 10071, and in an ilmenite and two feldspar concentrates separated from rock 10071. Whole rock K/Ar ages of rocks 10017 and 10071 are (2350 ± 60) × 106 yr and (2880 ± 60) × 106 yr, respectively. The two feldspar concentrates of rock 10071 have distinctly higher ages: (3260 ± 60) × 106 yr and (3350 ± 70) × 106 yr. These ages are still 10 per cent lower than the Rb/Sr age obtained by Papanastassiouet al. (1970) and some Ar40 diffusion loss must have occurred even in the relatively coarse-grained feldspar.The relative abundance patterns of spallation Ne, Ar, Kr and Xe are in agreement with the ratios predicted from meteoritic production rates. However, diffusion loss of spallation He3 is evident in the whole rock samples, and even more in the feldspar concentrates. The ilmenite shows little or no diffusion loss. The isotopic composition of spallation Kr and Xe is similar to the one observed in meteorites. Small, systematic differences in the spallation Kr spectra of rocks 10017 and 10071 are due to variations in the irradiation hardness (shielding). The Kr spallation spectra in the mineral concentrates are different from the whole rock spectra and also show individual variations, reflecting the differences in target element composition. The relative abundance of cosmic ray produced Xe131 differs by nearly 50 per cent in the two rocks. The other Xe isotopes show no variations of similar magnitude. The origin of the Xe131 yield variability is discussed.Kr81 was measured in all the samples investigated. The Kr81/Kr exposure ages of rocks 10017 and 10071 are (480 ± 25) × 106 yr and (350 ± 15) × 106 yr, respectively. Exposure ages derived from spallation Ne21, Ar38, Kr83 and Xe126 are essentially in agreement with the Kr81/Kr ages. The age of rock 10071 might be somewhat low because of a possible recent exposure of our sample to solar flare particles.  相似文献   

19.
We present a multi‐chronometric approach for reconstructing deep‐time thermal histories using southern Baffin Island as a case study. This continuous thermal history begins with the Palaeoproterozoic Trans‐Hudson Orogeny and is derived from inverse and forward models that integrate thermochronometers spanning some 500°C: new apatite U–Pb ages and K‐feldspar 40Ar/39Ar multi‐diffusion domain data, published (U–Th)/He zircon ages and new multi‐kinetic fission‐track results. Integration of data from a wider temperature range reduces ambiguities in thermal‐history modelling and permits us to constrain the timing of geological processes including, extended post‐orogenic cooling, enhanced later Proterozoic cooling, and then episodic burial and exhumation in the Palaeozoic–Mesozoic.  相似文献   

20.
Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 106 yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic 3He, 21Ne, and 38Ar in chondrites to an adopted ‘average’ shielding: 22Ne21Ne = 1.114. The measured unsaturated 26Al concentrations and the calculated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10?8 cm3/g 106 yr) are: 3He = 2.45,21Ne = 0.47, and 38Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.  相似文献   

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