Chronology and petrogenesis of young achondrites,Shergotty, Zagami,and ALHA77005: late magmatism on a geologically active planet     

C.-Y. Shih  L.E. Nyquist  D.D. Bogard  G.A. McKay  J.L. Wooden  B.M. Bansal  H. Wiesmann 《Geochimica et cosmochimica acta》1982,46(11):2323-2344

A study was undertaken to determine the chronology, petrogenesis and relationships among the shergottites, Shergotty and Zagami and the unique achondrite ALHA77005. These meteorites are the product of a variety of complex processes.Petrogenesis: Chondrite-normalized abundance patterns of Shergotty and Zagami are very similar and show pronounced depletions of both the light REE (La-Nd) and heavy REE (Dy-Lu) relative to Sm-Gd. These characteristic depletions are even more pronounced for ALHA77005. The light REE depletion is qualitatively consistent with the presence of cumulus pyroxene and/or olivine in these meteorites, but trace element models show that the parental magmas of all three meteorites were probably also light REE depleted. Both trace element model calculations and combined Rb-Sr and Sm-Nd isotopic systematics show that the meteorites could not have been co-magmatic nor can ALHA77005 be representative of the source material of the shergottites. Light REE depletion of the parental magmas also implies light REE depletion of the source material. The Sm-Nd systematics of the shergottites require a time-averaged sub-chondritic (light REE enriched) Sm-Nd ratio since 4.6 AE ago. The Sm-Nd systematics of ALHA77005 permit a time-averaged super-chondritic (light REE depleted) Sm/Nd ratio if its crystallization age is less than T_ICE = 0.72 AE.Chronology. Rb-Sr internal isochrons for all three meteorites and a Sm-Nd internal isochron for Zagami are concordant at ~ 180 Myr. ³⁹Ar-⁴⁰Ar plateau ages of Shergotty and Zagami maskelynite are ~250–260 Myr. These ages apparently reflect resetting of these isotopic systems by shock metamorphism which converted the feldspar to maskelynite. The concordance of these ages suggests a single shock event during which the meteorites were in close physical proximity. The time of this event is most precisely given by the Rb-Sr age of 180 ± 4 Myr for Zagami.The crystallization ages of the meteorites were not precisely determined. Extreme upper limits are determined by Sm-Nd model ages relative to an eucrite initial ${}^{143}\text{Nd}{}^{144}\text{Nd = 0.505835}$ at 4.6 AE ago. These model ages for Shergotty, Zagami and ALHA77005 are 3600, 3500 and 2850 Myr, respectively. The Sm-Nd whole rock age of 1340 ± 60 Myr for the three meteorites gives the crystallization age if the Sm/Nd ratios of the precursor materials were always the same. We consider this 1340 Myr age as a “best estimate” upper limit. “Best estimate” lower limits for Shergotty and Zagami are taken from the average ³⁹Ar-⁴⁰Ar ages of 1200 and 900 Myr of pyroxene separates. The average ³⁹Ar-⁴⁰Ar age of a whole rock sample of ALHA77005 was 1600 Myr and can be partitioned between a low temperature (feldspar) phase and a high temperature (olivine + pyroxene + inclusions) “phase”. The average apparent ³⁹Ar-⁴⁰Ar age of the low temperature phase is ~1050 Myr, which is chosen as the “best estimate” lower limit to the age. The crystallization ages of Shergotty, Zagami and ALHA77005 probably lie within the ranges of 1200–1300, 900–1300 and 1000–1300 Myr, respectively. The Rb-Sr whole rock age of 4400 ± 400 Myr and single-stage BABI model ages of ~4800–5100 Myr are interpreted as reflecting differentiation of the parent body at ~4600 Myr ago.The complex geochemical and isotopic evolution recorded by these meteorites suggests a geologically active parent body capable of sustaining melting at two or more epochs in its history.  相似文献   

The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism     

F.A Podosek  J.C Huneke  A.J Gancarz  G.J Wasserburg 《Geochimica et cosmochimica acta》1973,37(4):887-904

Ages were determined by the ⁴⁰Ar-³⁹Ar method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 ± 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence for loss of radiogenic ⁴⁰Ar, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratio.The Luna 20 results greatly extend the area of the Moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval 4.6?3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts.  相似文献   

Sr and Nd isotope geochronology,geologic history,and origin of the Adirondack Anorthosite     

Lewis D Ashwal  Joseph L Wooden 《Geochimica et cosmochimica acta》1983,47(11):1875-1885

We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ (0.7039–0.7050) and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.  相似文献   

Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body     

R.F. Dymek  A.L. Albee  A.A. Chodos  G.J. Wasserburg 《Geochimica et cosmochimica acta》1976,40(9):1115-1130

Detailed mineralogic and petrographic data are presented for four isotopically-dated basaltic rock fragments separated from the howardite Kapoeta. Clasts C and ρ have been dated at ~4.55 AE and ~ 4.60 AE respectively, and Clast ρ contains ²⁴⁴Pu and ¹²⁹I decay products. These are both igneous rocks that preserve all the features of their original crystallization from a melt. They thus provide good evidence that the Kapoeta parent body produced basaltic magmas shortly after its formation (< 100 m.y.). Clast A has yielded a Rb-Sr age of ~ 3.89 AE and a similar ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age. This sample is extensively recrystallized, and we interpret the ages as a time of recrystallization, and not the time of original crystallization from a melt. Clast B has yielded a Rb-Sr age of ~ 3.63 AE, and an ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age of ? 4.50 AE. This sample is moderately recrystallized, and the Rb-Sr age probably indicates a time of recrystallization, whereas the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age more closely approaches the time of crystallization from a melt. Thus, there is no clearcut evidence for ‘young’ magmatism on the Kapoeta parent body.Kapoeta is a ‘regolith’ meteorite, and mineral-chemical and petrographic data were obtained for numerous other rock and mineral fragments in order to characterize the surface and near-surface materials on its parent body. Rock clasts can be grouped into two broad lithologic types on the basis of modal mineralogy—basaltic (pyroxene- and plagioclase-bearing) and pyroxenitic (pyroxenebearing). Variations in the compositions of pyroxenes in rock and mineral clasts are similar to those in terrestrial mafic plutons such as the Skaergaard, and indicate the existence of a continuous range in rock compositions from Mg-rich orthopyroxenites to very iron-rich basalts. The FeO and MnO contents of all pyroxenes in Kapoeta fall near a line with FeO/MnO ~ 35, suggesting that the source rocks are fundamentally related. We interpret these observations to indicate that the Kapoeta meteorite represents the comminuted remains of differentiated igneous complexes together with ‘primary’ undifferentiated basaltic rocks. The presently available isotopic data are compatible with the interpretation that this magmatism is related to primary differentiation of the Kapoeta parent body. In addition, our observations preclude the interpretation that the Kapoeta meteorite is a simple mixture of eucrites and diogenites.The FeO/MnO value in lunar pyroxenes (~60) is distinct from that of the pyroxenes in Kapoeta. Anorthositic rocks were not observed in Kapoeta, suggesting that plagioclase was not important in the evolution of the Kapoeta parent body, in contrast to the Moon. Both objects appear to have originated in chemically-distinct portions of the solar system, and to have undergone differentiation on different time scales involving differing materials.  相似文献   

Single-grain 207Pb206Pb and U/Pb age determinations with a 10-μm spatial resolution using the ion microprobe mass analyzer (IMMA)     

J.R. Hinthorne  C.A. Andersen  R.L. Conrad  J.F. Lovering 《Chemical Geology》1979,25(4):271-303

Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics.Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained, averaging 4.60 ± 0.09 (2σ) Ga.A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples.  相似文献   

Nd and Sr isotopic crustal contamination patterns in an Archaean meta-basic dyke from northern Labrador     

Kenneth D Collerson  Malcolm T McCulloch  David Bridgwater 《Geochimica et cosmochimica acta》1984,48(1):71-83

Samples from the core to the margin of a 20 cm wide meta-dolerite dyke are sequentially enriched in K, Rb, Sr, and the light REE's. Rb-Sr and Sm-Nd compositional and isotopic profiles in the dyke are interpreted to be the result of selective contamination with components of country rock derivation, rather than the result of simple bulk mixing. ${}^{87}\text{Rb}{}^{86}\text{Sr}$ ratios are higher at the edge of the dyke than at its centre, although they are somewhat irregular, due probably to the effects of subsequent alteration. This profile and one shown by unsupported ⁸⁷Sr are both consistent with contamination of the dyke by a fluid phase derived by the breakdown of biotite. Common Sr shows a parallel, albeit weaker, contamination profile which is interpreted to reflect the contribution of a Sr-bearing phase such as plagioclase. ${}^{147}\text{Sm}{}^{144}$Nd ratios and ¹⁴⁴Nd concentrations increase and decrease respectively from the margin to the core of the dyke. In addition, the margin of the dyke is significantly less radiogenic than the interior. This contrasts with the relatively radiogenic character of an adjacent pegmatite vein. As this sample does not lie on an anticipated contamination profile between the Uivak gneisses and the dyke it is concluded that the REE contamination of the dyke occurred by the addition of a REE-enriched fluid phase which gained access to the dyke by flow along the dyke-pegmatite interface. If it is assumed that both the Rb-Sr and Sm-Nd contamination profiles are the result of diffusion limited processes, then the observations of scale made in this paper suggest that the rate of diffusion of Nd is an order-of-magnitude slower than that for Sr. In view of the scale and nature of these profiles, ages obtained from isotopic data for such mafic dykes must be interpreted with some care. Nevertheless, in spite of these limitations the ?_Nd values for the least contaminated specimens provide a clear indication that the Saglek dykes were derived from a depleted mantle source with ?_Nd? +2.  相似文献   

Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya,Nepal)   总被引：1，自引：0，他引：1  

Ph. Vidal  A. Cocherie  P. Le Fort 《Geochimica et cosmochimica acta》1982,46(11):2279-2292

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Geochronologic investigations of the Sudbury Nickel Irruptive and the Superior Province granites north of Sudbury     

R.W. Hurst  J. Farhat 《Geochimica et cosmochimica acta》1977,41(12):1803-1815

RbSr ($\text{λ}\text{Rb}\text{= 1.39 × 10}{}^{\mathrm{?11}}\text{yr}{}^{\mathrm{?1}}\text{)}$ and U-Pb ($\text{λ 238 = 1.54 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}\text{, λ235 = 9.72 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}$) measurements were undertaken in the Sudbury area, Sudbury, Ontario to determine the ages of the Sudbury Nickel Irruptive, Superior Province granites north of Sudbury, Sudbury Breccia and subsequent metamorphism. The Sudbury Nickel Irruptive norite whole rock Rb-Sr data yield an age of $\text{1883 ± 136}\text{Myr}$ ($\text{I.R}\text{. = 0.7071 ± 0.0005}$; all results quoted at 2π level) while the Nickel Irruptive micropegmatite Rb-Sr system has been disturbed and does not yield an isochron. A plagioclase-whole rock pair from the norite near the norite-micropegmatite transition yields an age of 1866 Myr, which when taken in conjunction with field (Stevenson and Colgrove, 1968) and geochemical (Naldrettet al., 1970, 1972) data does not support the conclusion of gibbins and McNurr (1972) that the micropegmatite is a later intrusion rather than a differentiate of the magma which produced the norite. Rb-Sr studies of the Superior Province granites north of Sudbury yield an age of $\text{2698 ± 162}\text{Myr}$ ($\text{I.R.}\text{= 0.7019 ± 0.0012}$). U-Pb zircon studies of these granites and granitic clasts within the Sudbury Breccia yield an age of $\text{2.71 ± 0.05}$ Byr and suggest the breccia granitic clasts were derived from the Superior Province granites. The granitic rocks ~150 km north of Sudbury have been undisturbed for ~ 2.6 Byr based on Rb-Sr mineral studies, whereas the granites and Sudbury Breccia within ~ 15 km of the Nickel Irruptive, as well as the Sudbury norite at the perimeter of the Irruptive have been disturbed by the Penokean Orogeny 1.7–1.75 Byr ago. The Penokean event appears to have overprinted isotopic evidence of the Sudbury impact event at least in the area studied.  相似文献   

Strontium isotope geology of the South Mountain batholith,Nova Scotia     

D.B. Clarke  A.N. Halliday 《Geochimica et cosmochimica acta》1980,44(8):1045-1058

The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}$ ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.70942 ± 35}$); porphyry (361.2 ± 1.4 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.71021 ± 119}$); using λ⁸⁷Rb = 1.42 × 10^?11yr^?1.A suite of Meguma country rock samples showed a variation of ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7113?0.7177}$ at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{372}\text{= 0.7081}$ of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known.  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   

Precise age determinations and petrogenetic studies using the KCa method     

B.D. Marshall  D.J. DePaolo 《Geochimica et cosmochimica acta》1982,46(12):2537-2545

High precision mass spectrometric determination of calcium isotope ratios allows the ⁴⁰K → ⁴⁰Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ is used to monitor enrichments in ⁴⁰Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low $\text{K}\text{Ca}$ (～0.01) that it has retained “primordial” ${}^{40}\text{Ca}{}^{42}\text{Ca}\text{= 151.016}$ ± 0.011 (normalized to ${}^{42}\text{Ca}{}^{44}\text{Ca}\text{= 0.31221}$), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ ratios can be conveniently expressed relative to this value as ?_Ca in units of 10^?4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λ_K = 0.5543 b.y.^?1, $\text{λ}{}_{\mathrm{\beta ?}}\text{λ}{}_{\mathrm{<mtext>K</mtext>}}\text{= 0.8952}$, ⁴⁰K = 0.01167%). The initial ${}^{40}\text{Ca}{}^{42}\text{Ca}$ of 151.024 ± 0.016 (?_Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of ⁴⁰K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.  相似文献   

Ca isotope fractionation on the Earth and other solar system materials   总被引：1，自引：0，他引：1  

W.A. Russell  D.A. Papanastassiou  T.A. Tombrello 《Geochimica et cosmochimica acta》1978,42(8):1075-1090

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Origin of granitic magma by crustal remobilisation: Rb-Sr and Pb/Pb geochronology and isotope geochemistry of the late Archaean Qôrqut Granite Complex of southern West Greenland     

Stephen Moorbath  Paul N. Taylor  Roy Goodwin 《Geochimica et cosmochimica acta》1981,45(7):1051-1060

Rb-Sr and Pb/Pb whole rock isochrons on the Qôrqut Granite Complex yield ages of $\text{2530 ± 30}\text{Myr}\text{(}\text{initial}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7081 ± 0.0008)}$ and 2580 ± 80 Myr respectively. A model relating initial Sr and Pb isotopic compositions of the Qôrqut granites to the Sr and Pb isotopic compositions of the Amîtsoq gneisses (ca. 3700 Myr) and Nûk gneisses (ca. 2900 Myr) at 2550 Myr ago, as well as Sr and Pb contents of the gneiss units, suggests that between 40 and 50% of the Qôrqut granite magma was generated by partial melting of Amîtsoq gneisses, and the remainder by partial melting of Nûk gneisses.  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   

⁸⁷Rb-⁸⁷Sr age of fragments and soils from the lunar sea of fertility     

Jean Louis Birck  Claude J Allegre 《Geochimica et cosmochimica acta》1973,37(9):2025-2031

The Luna 16 materials were dated by the Rb-Sr method.An internal isochron age of 3.4 ± 0.2 has been determined for a 6 mg fragment.The Luna 16 total soil is poorer in radiogenic Sr than any other analyzed soil from the Moon. Apollo 14 and 15 soils have also been studied; all of them fall nearly on a 4.65 b.y. isochron with the ADOR initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio.A comparison of the integrated $\text{Rb}\text{Sr}$ of the basalt source region and the $\text{Rb}\text{Sr}$ of the rocks suggests that these basaltic fragments have been generated with only minor $\text{Rb}\text{Sr}$ fractionation.The existence of an old Rb-rich subcrust which contaminated the basalts is also in agreement with the present results.  相似文献   

The statistical analysis and interpretation of imperfectly-fitted Rb-Sr isochrons from polymetamorphic terrains     

M. Cameron  K.D. Collerson  W. Compston  R. Morton 《Geochimica et cosmochimica acta》1981,45(7):1087-1097

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Delivery of transuranic elements by rain to the Mediterranean Sea     

M. Thein  S. Ballestra  A. Yamato  R. Fukai 《Geochimica et cosmochimica acta》1980,44(8):1091-1097

The concentrations of ²³⁸Pu, ^{239 + 240}Pu, ²⁴¹Am and ¹³⁷Cs were determined in rain samples collected at Monaco in the course of 1978–1979. Based on these data, the annual deliveries of these radionuclides to the Mediterranean by rain are computed to be 0.18 ± 0.01 pCim^?2 for ²³⁸Pu, 8.1 ± 0.1 pCim^?2 for ^{239 + 240}Pu, 0.58 ± 0.02 pCim^?2 for ²⁴¹Am and 351 ± 4 pCim^?2 for ¹³⁷Cs.Comparing the delivery data with the mixed layer inventories of ^{239 + 240}Pu and ²⁴¹Am in the Mediterranean, the upper limits of the mean residence time of these radionuclides in the mixed layer were estimated to be 12.3 yr for ^{239 + 240}pu and 2.9 yr for ²⁴¹Am. These values are consistent with the conclusion deduced from the vertical distribution pattern of these transuranic elements in the Mediterranean.Based on delivery values, the annual activity ratios for ${}^{238}\text{pu}{}^{\mathrm{239\; +\; 240}}\text{Pu}$, ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ and ${}^{\mathrm{239\; +\; 240}}\text{pu}{}^{137}\text{Cs}$ are found to be 0,022, 0.072 and 0.023 respectively. The ${}^{238}\text{pu}{}^{\mathrm{239\; +\; 240}}\text{pu}$ and ${}^{\mathrm{239\; +\; 240}}\text{Pu}{}^{137}\text{Cs}$ activity ratios vary within relatively narrow ranges with time, while a much wider variation was observed for the ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ activity ratio. The cause of the wider variation of the ${}^{241}\text{Am}{}^{\mathrm{239\; +\; 240}}\text{Pu}$ ratio may be related to the difference in the mean age of fallout brought down in different seasons.  相似文献   

Rb-Sr dating of the Lackner Lake Complex,northern Ontario,Canada     

L.B. Owen  G. Faure 《Precambrian Research》1977,5(3):299-303

The Lackner Lake Complex of northern Ontario consists of nepheline syenite and ijolite with associated bodies of apatite—magnetite rock and carbonatite. A whole-rock Rb-Sr isochron indicates that these rocks crystallized 1078 ± 7 Ma ago ($\text{λ}{}^{87}\text{Rb = 1.39·10}{}^{\mathrm{?11}}\text{a}{}^{\mathrm{?1}}$) and had an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.070282 ± 0.00011. This date is slightly lower than a K-Ar date of 1090 Ma for biotite reported previously, but is in agreement with K-Ar dates of several nearby syenite-carbonatite complexes.  相似文献   

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution     

P.Jonathan Patchett 《Geochimica et cosmochimica acta》1983,47(1):81-91

The ¹⁷⁶Lu-¹⁷⁶Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes ${}^{176}\text{Hf}{}^{177}\text{Hf}$ a powerful geochemical tracer. In general, proportional variations of ${}^{176}\text{Hf}{}^{177}\text{Hf}$ exceed those of ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}$ by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in ${}^{176}\text{Hf}{}^{177}\text{Hf}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ diagrams, ${}^{176}\text{Hf}{}^{177}\text{Hf}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ display a single, linear isotopic variation in the suboceanic mantle. These discordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased ${}^{176}\text{Hf}{}^{177}\text{Hf}$, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial ${}^{176}\text{Hf}{}^{177}\text{Hf}$. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.  相似文献   

Geochemistry of loess,continental crustal composition and crustal model ages   总被引：3，自引：0，他引：3  

S.R Taylor  S.M McLennan  M.T McCulloch 《Geochimica et cosmochimica acta》1983,47(11):1897-1905

Isotopic, major and trace element studies of loess deposits from America, China, Europe and New Zealand show general uniformity of composition. Silica, Zr and Hf are enriched relative to estimates of bulk composition of the upper continental crust. The REE data are indistinguishable from those of average shales, confirming the concept that these REE patterns ($\text{La}{}_{\mathrm{N}}\text{/}\text{Yb}{}_{\mathrm{N}}\text{= 9.5}\text{Eu}\text{/}\text{Eu}{}^1\text{= 0.66}$) represent the upper crustal average. Sm-Nd model ages are variable but <1700 m.y. They reflect derivation from younger elevated erogenic areas subject to Pleistocene glaciation. Although Sm-Nd model ages vary by a factor of two, the REE patterns remain constant. This indicates that processes responsible for formation of the upper crust have produced no secular change in composition since the mid-Proterozoic.  相似文献