Nd isotopic systematics and chemistry of Central Australian sapphirine granulites: an example of rare earth element mobility     

D.P. Windrim  M.T. McCulloch  B.W. Chappell  W.E. Cameron 《Earth and Planetary Science Letters》1984,70(1):27-39

Lower Proterozoic sapphirine-bearing and associated granulites from Central Australia exhibit the greatest range of present-day¹⁴³Nd/¹⁴⁴Nd ratios (∈_Nd(O)= ?26.5 to +112.3) yet reported for rocks believed to be cogenetic. The Nd isotopic data and REE abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform CuPbZn sulfide deposits with which they are closely associated. Field relationships, petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite-rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism; calculations employing REE abundances of the sapphirine granulites and associated rocks, combined with bulk solid-fluid distribution coefficient data yield high fluid/rock ratios, consistent with a pre-metamorphic hydrothermal origin for the unusual REE patterns. The SmNd data for these rocks define an age of 1760±75Ma, which is significantly younger than the crust formation age of the terrain ( 2070±125Ma) but indistinguishable from the RbSr whole rock age for granulite facies metamorphism (1790±35Ma). These data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly (perhaps tens of millions of years) before granulite facies metamorphism, followed by redistribution of Nd isotopes or small fractionations of the Sm/Nd ratio during the granulite facies event, and possibly also during intense retrogression which reset RbSr whole rock and UPb zircon and monazite systematics at about 1700 Ma.  相似文献   

Sm-Nd isotopic evolution of chondrites and achondrites,II     

S.B. Jacobsen  G.J. Wasserburg 《Earth and Planetary Science Letters》1984,67(2):137-150

The¹⁴⁷Sm¹⁴³Nd and¹⁴⁶Sm¹⁴²Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of (¹⁴³Nd/¹⁴⁴Nd)_CHUR⁰ = 0.511847 and (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR⁰ = 0.1967. Most of the bulk chondrites analyzed have ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an SmNd internal isochron age of T = 4.55 ± 0.33AE and an initial ε_Nd = 0.11 ? 0.26. Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial ε_Nd values obtained for Moama and ADOR are 0.03 ? 0.25and0.14 ? 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial ε_Nd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial ε_Nd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the ε_Nd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods.¹⁴⁶Sm¹⁴²Nd systematics of ADOR and Moama are supportive of the hypothesis that¹⁴⁶Sm was present in the early solar system and suggests a ¹⁴⁶Sm/¹⁴⁴Sm ratio of about 0.01 for the solar system ～ 4.56 AE ago. This inferred high¹⁴⁶Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.  相似文献   

The effect of initial²³⁰Th disequilibrium on young UPb ages: the Makalu case,Himalaya     

Urs Schärer 《Earth and Planetary Science Letters》1984,67(2):191-204

Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic²⁰⁶Pb which originate from an excess or deficit of²³⁰Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess²⁰⁶Pb due to excess²³⁰Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of²⁰⁶Pb due to deficiency of²³⁰Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of²⁰⁶Pb were, with reference to the amount of radiogenic²⁰⁶Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic²⁰⁷Pb originating from the decay of initial excess²³¹Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected²³⁸U²⁰⁶Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.  相似文献   

Isotope geochemistry of xenoliths from East Africa: Implications for development of mantle reservoirs and their interaction     

R.S. Cohen  R.K. O&#x;Nions  J.B. Dawson 《Earth and Planetary Science Letters》1984,68(2):209-220

Pb, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenicPb(²⁰⁶Pb/²⁰⁴Pb= 15.55) and Nd(¹⁴³Nd/¹⁴⁴Nd= 0.51127; ?_Nd⁰ = ?26.7) isotope compositions recorded so far for an ultramafic xenolith, and ⁸⁷Sr/⁸⁶Sr= 0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77<²⁰⁶Pb/²⁰⁴Pb<17.50 and some show evidence for depletion of U relative to Pb up to 2.0 Ga ago. Overall the isotope results suggest that the mantle part of the continental lithosphere beneath Tanzania has components that have undergone a complex history that includes major chemical fractionations ca. 2.0 Ga ago. A phlogopite-amphibole vein from the Pello Hill sample has Sr, Nd and Pb isotope compositions similar to those of mid-ocean ridge basalts, indicating both a young emplacement age for the vein material and a source which had an isotopic signature characteristic of depleted mantle.The Sr, Nd and Pb isotope systematics of ultramafic xenoliths do not conform with those of MORB, particularly in terms of their PbSr, and NdPb relationships. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved.  相似文献   

Hydrothermal Mn-deposits of the Franciscan Assemblage,II. Isotope and trace element geochemistry,and implications for hydrothermal convection at spreading centers     

M.S. Chyi  D.A. Crerar  R.W. Carlson  R.F. Stallard 《Earth and Planetary Science Letters》1984,71(1):31-45

Major element, trace element and Sr, Nd, Pb and O isotopic data for a Franciscan Mn-deposit suggest an origin by seafloor hydrothermal circulation. Based onQ-mode factor analysis the cherts and Mn-lenses of the Blue Jay mine formed from a combination of 4 components representing 1 biogenic, 1 hydrothermal, and 2 detrital sources. RbSr, UThPb and O isotopic systematics in the Mn-lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts. Nd isotopic compositions in both cherts and Mn-lenses are identical and within the range measured for Pacific Ocean water suggesting the REE were not mobilized by hydrothermal activity. Correlation of δ¹⁸O with SiO₂ and MnO₂ in the Mn-lenses implies the lenses formed by simple mixing of hydrothermally derived Mn-oxides with seawater and biogenic silica. δ¹⁸O of the cherts is both uniform and depleted relative to DSDP Jurassic cherts but similar to microquartz-bearing cherts of the Monterey Formation: this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism. Finally, a well defined RbSr isochron of158 ± 5Myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record.  相似文献   

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution     

S.B. Jacobsen  J.E. Quick  G.J. Wasserburg 《Earth and Planetary Science Letters》1984,68(3):361-378

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A SmNd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initialε_Nd = + 10.4 ? 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzoliteP-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The SmNd model age of the plagioclase lherzolite totalrock isT_CHUR^Nd = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a SmNd mineral isochron age ofT = 435 ± 21 Ma andε_Nd = + 6.7 ? 0.3. A pyroxenite dike yields an initialε_Nd = + 7.3 ± 0.4. The initialε_Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. RbSr data do not give any coherent pattern. However, some bounds can be put on initial Sr values ofε_Sr ? ?21 for the plagioclase lherzolite andε_Sr ? ?8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and SmNd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ～ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (～ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ～ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.  相似文献   

SmNd study of Archean alkalic rocks from the Superior Province of the Canadian Shield     

Asish R. Basu  Alan M. Goodwin  Mitsunobu Tatsumoto 《Earth and Planetary Science Letters》1984,70(1):40-46

The Abitibi Volcanic Belt in eastern Superior Province of the Canadian Shield is the largest continuous greenstone belt in the world and is a key example of late Archean crust. This belt has, in general, suffered a low intensity of metamorphism and deformation, and, as a result, the stratigraphy and geology are well established. Tholeiitic and calc-alkaline series of igneous rocks are present in this belt in about equal proportions. However, the undersaturated potassic and leucitic volcanics of the Timiskaming Group are a unique feature of this belt.SmNd systematics were determined for twelve Timiskaming volcanic rocks. These rocks show nepheline, diopside and/or olivine plus leucite in the norm and a highly fractionated REE pattern. Sm and Nd concentrations range from 25 to 160 and 45 to 300 times the chondritic abundance, respectively. The Sm and Nd isotopic data yield an isochron age of 2702±105Ma for these volcanic rocks with an initial ε_Nd of +1.9±1.6. This age establishes the Timiskaming alkalic rock to be one of the oldest of their kind. From stratigraphic relations, 2705 Ma is an upper limit for the age and the ε_Nd values of +1.8 to +2.2 at this age for the twelve rocks are also upper limits. Further, this small but positive ε_Nd value for the isochron, when compared to other mantle-derived Archean rocks in the Superior Province, indicates that the Archean mantle was heterogeneous beneath the Canadian Shield and that the Timiskaming alkalic lavas were derived from a depleted mantle.  相似文献   

Interpretation of Nd,Sr and Pb isotope data from Archean migmatites in Lofoten-Vesterålen,Norway     

Stein B. Jacobsen  G.J. Wasserburg 《Earth and Planetary Science Letters》1978,41(3):245-253

Sm-Nd data for the Archean granulite and amphibolite facies migmatites of Langøy and Hinnøy in Vesterålen are presented which indicate that their protoliths formed ～2.6 AE ago. Rubidium and U loss during a granulite facies metamorphism at ～1.8 AE caused serious disturbance of total-rock U-Pb and Rb-Sr systems. Therefore these systems do not provide any precise age information for the granulite facies migmatites. For the amphibolite facies migmatites of Vesterålen both SmNd, RbSr and PbPb total-rock systems give model ages of ～2.6 AE. The results on both granulite and amphibolite facies rocks are thus in agreement. Previous interpretations based on PbPb data, which indicated an age of 3.41 AE for the Archean terrane of Vesterålen, are not valid.One SmNd model age from the granulite facies migmatites at Moskenesøy in Lofoten indicates that the protoliths of these migmatites formed ～2.0 AE ago and are thus not related to the Vesterålen migmatites.  相似文献   

RbSr dating of the Yatsushiro granite and gneiss,Kyushu, Japan     

Susumu Nohda 《Earth and Planetary Science Letters》1973,20(1):140-144

RbSr measurements on the Yatsushiro granite and gneiss, which had been considered stratigraphically to be of possible Precambrian age, are reported. The whole rock isochron for the granite gives an age of 352 ± 8 my with a low initial⁸⁷Sr/⁸⁶Sr ratio of 0.7037 ± 0.0006. Data for constituent minerals of the granites are dispersed irregularly around the whole rock isochron (possibly by later tectonic events). For the gneiss, a metamorphic event around 410 my is indicated by the muscovite RbSr ages. The present results do not support the possibility that the Yatsushiro granite and gneiss are Precambrian in age.  相似文献   

Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian Series of Scotland     

R.J. Pankhurst  R.T. Pidgeon 《Earth and Planetary Science Letters》1976,31(1):55-68

RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material.UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514_?7⁺⁶ m.y. and 1316_?25⁺²⁶ m.y. Geological constraints suggest that the lower intersection records the post-F₂, pre-M₃ emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom.RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial⁸⁷Sr/⁸⁶Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial⁸⁷Sr/⁸⁶Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.  相似文献   

Correlated Nd,Sr and Pb isotope variation in Walvis Ridge basalts and implications for the evolution of their mantle source     

S.H. Richardson  A.J. Erlank  A.R. Duncan  D.L. Reid 《Earth and Planetary Science Letters》1982,59(2):327-342

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge [13, 14]. The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and consistent with formation at the paleo mid-ocean ridge [14]. The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other.The low ¹⁴³Nd/¹⁴⁴Nd (0.51238), ²⁰⁶Pb/²⁰⁴Pb (17.54), ²⁰⁸Pb/²⁰⁴Pb (15.47), ²⁰⁸Pb/²⁰⁴Pb (38.14) and high⁸⁷Sr/⁸⁶Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan de Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher¹⁴³Nd/¹⁴⁴Nd (0.51270), ²⁰⁶Pb/²⁰⁴Pb (18.32), ²⁰⁷Pb/²⁰⁴Pb (15.52), ²⁰⁸Pb/²⁰⁴Pb (38.77) and lower ⁸⁷Sr/⁸⁶Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing od depleted and enriched end member melts or partial melting of an inhomogenous, variably enriched mantle source. However, observe ZrBaNbY interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.  相似文献   

Composition of the core,II. Effect of high pressure on solubility of FeO in molten iron     

E. Ohtani  A.E. Ringwood  W. Hibberson 《Earth and Planetary Science Letters》1984,71(1):94-103

An experimental and theoretical investigation of the effect of pressure on the solubility of FeO in molten iron has been carried out. Analyses of shock-wave compression data on iron oxides combined with measurements of the FeO bond length in “metallic” oxides suggest that the partial molar volume of FeO(V^*) dissolved in molten iron is substantially smaller than that of molten wüstite. Hence the effect of high pressure should be to increase the solubility of FeO in molten iron at a given temperature. This inference is confirmed by an experimental investigation of the effect of pressure on the position of the FeFeO eutectic. Thermodynamic calculations based on these experiments yield an estimate forV^* which is in reasonable agreement with the theoretical estimates. The experimental value ofV^* is used to calculate the effect of high pressure upon the FeFeO phase diagram. Solubility of FeO in molten iron increases sharply with pressure, the liquid immiscibility region contracts and disappears around 20 GPa and it is predicted that the FeFeO phase diagram should resemble a simple eutectic system above about 20 GPa. Analogous calculations predict that the solubility of FeO in molten iron in equilibrium with magnesiowüstite (Mg_0.8Fe_0.2)O at 2500°C increase from 14 mol.%(P = 0) to above 25 mol.% at 20 GPa. If the core formed by segregation of metallic iron originally dispersed throughout the earth, it seems inevitable that it would dissolved large amounts of FeO, thereby accounting for the observation that the density of the outer core is substantially smaller than that of pure iron under correspondingP, T conditions.  相似文献   

Mineral age patterns in ca. 3700 my old rocks from West Greenland     

R.J. Pankhurst  S. Moorbath  D.C. Rex  G. Turner 《Earth and Planetary Science Letters》1973,20(2):157-170

KAr,⁴⁰Ar³⁹Ar and RbSr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. KAr dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high KAr date of 4940 my.⁴⁰Ar³⁹Ar mineral dates are in close agreement with conventional KAr dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated.Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic⁸⁷Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south.The evidence suggests that those KAr biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event.KAr mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qo?rqut granite, indicate an age of emplacement of 2580 ± 30 my.  相似文献   

U–Th–Pb and Rb–Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow Valley     

P.D. Nunes  M. Tatsumoto  D.M. Unruh 《Earth and Planetary Science Letters》1974,23(3):445-452

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using UThPb and RbSr systematics. A RbSr internal isochron age of3.89 ± 0.08b.y. with an initial⁸⁷Sr/⁸⁶Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a UPb internal isochron of3.8 ± 0.2b.y. and an initial²⁰⁶Pb/²⁰⁷Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset RbSr and UPb mineral systems of older rocks.Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and²³⁸U/²⁰⁴Pb values which are higher than those of group B. Group A rocks have typical²³²Th/²³⁸U ratios of ～ 3.85, whereas-group B rocks have unusually high Th/U values of ～ 4.1.Whole-rock UPb and PbPb ages are nearly concordant. Two events appear to be reflected in these data — one at ～ 4.4 b.y. and one at ～ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ～ 4.4 b.y. and ～ 4.5 b.y. may, like the younger ～ 4.0 b.y. ages, be related to basin excavation events.  相似文献   

Interpretation of a discordant KAr age pattern (Capo Vaticano,Calabria)     

L. Civetta  M. Cortini  P. Gasparini 《Earth and Planetary Science Letters》1973,20(1):113-118

KAr age determination on whole rocks, biotites, quartz-feldspar separates and pegmatitic muscovites from a small quartz dioritic stock give a complex discordant age pattern. KAr dates from whole rocks and mineral separates define a single 116 my isochron with positive intercept, whereas muscovites from pegmatites fit a 180 my isochron with a probable negative intercept.Both ages are younger than the probable crystallization age of the stock (around 300 my), indicating a complex post-crystallization history. The fit of different mineral phases and whole rocks to a single isochron with positive intercept suggests that a thermal event caused rehomogenization of Ar among different mineral phases.  相似文献   

RbSr isotopic characteristics and chemistry of the 3.6-b.y. Hebron gneiss,Labrador     

Jackson M. Barton 《Earth and Planetary Science Letters》1975,27(3):427-435

RbSr isotopic analyses of the intensely deformed Hebron gneiss, Labrador, yield an isochron of approximately 3.6 b.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7044, and chemical analyses show these rocks to be granodioritic in composition. It is believed that the isochron reflects a metamorphic event and that the Hebron gneiss was either derived from a compositionally anomalous zone in the mantle or from previously existing sialic crustal material. The Hebron gneiss is compositionally similar to some of the oldest rocks in the Archean cratons of Labrador, West Greenland, Rhodesia, South Africa and Minnesota (U.S.A.).  相似文献   

Paleomagnetism of samples from the axial zone of the Afar depression     

C.G.A. Harrison  L. Stieltjes  E. Tarasiewicz 《Earth and Planetary Science Letters》1977,34(2):273-283

We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT₂ = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.¹⁴²Nd/¹⁴⁴Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 10⁵. This variation is tentatively assigned to a radiogenic contribution from extinct¹⁴⁶Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of²⁴⁴Pu with the light REE. Computed ratios²⁴⁴Pu/Nd are the same in pyroxene and phosphate separates, but²⁴⁴Pu/²³⁸U and²⁴⁴Pu/²³²Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio²⁴⁴Pu/Nd= 1.5 × 10^?4 (or an atomic ratio²⁴⁴Pu/¹⁵⁰Nd= 1.6 × 10^?3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the⁸¹Kr-⁸³Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio¹⁵⁰Sm/¹⁴⁹Sm in Angra dos Reis by 6 parts in 10⁴.  相似文献   

The provenance and crustal residence ages of British sediments in relation to palaeogeographic reconstructions     

R.G. Miller  R.K. O&#x;Nions 《Earth and Planetary Science Letters》1984,68(3):459-470

24 SmNd isotope analyses of fine-grained Phanerozoic and modern clastic sediments from Britain and Quebec are presented. In combination with published data, they have been used to calculate “crustal residence ages” (t_CR) and to assess the provenance of the British sedimentary mass. Sediments now preserved on either side of the suture formed by the closure of the Iapetus Ocean ca. 400 Ma ago were derived from isotopically distinct source regions. Sediments north of the suture are characterised by1.7 < t_CR < 2.8 Ga, whereas those to the south exhibit a smaller range and an average value of 1.6 Ga. The northern and southern source regions were most probably Laurentia and Gondwana respectively. It seems likely that a third source, perhaps Baltica, provided the lower Palaeozoic Southern Uplands succession. Sediments deposited in southern Britain after the closure of Iapetus were derived mainly from the recycling of older sediments. The tectonic rearrangements which occurred during the Phanerozoic are not reflected in the SmNd isotopic structure of the southern British sedimentary mass, suggesting that the Caledonian and Hercynian orogenies, and even the Grenville orogeny, involved minimal accretion of new mantle-derived material into the British and adjacent continental crust. SmNd analyses of fine-grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography, clastic source areas, sediment recycling and maturity, and some aspects of sediment transport.  相似文献   

The Palung granite (Himalaya); high-resolution UPb systematics in zircon and monazite     

Urs Schärer  Claude J. Allègre 《Earth and Planetary Science Letters》1983,63(3):423-432

UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.  相似文献   

KAr isochron study of the Tudor Gabbro,Grenville Province,Ontario     

A. Hayatsu  H.C. Palmer 《Earth and Planetary Science Letters》1975,25(2):208-212

Ten whole-rock samples from the Tudor Gabbro, Grenville Province, Ontario, Canada have been dated by the KAr method. The ages calculated by the conventional method range from 900 m.y. to 2040 m.y. On an isochron plot, three samples from a sampling site near the northern border of the gabbro lie along a 670-m.y. isochron with an initial⁴⁰Ar/³⁶Ar ratio of about 17,300 whereas all other samples lie along another 670-m.y. isochron with an initial ratio of about 5000. Although it is not certain yet as to what geological event the isochron age represents, the results clearly demonstrate that the effect of initial argon can be significant even on old samples such as these.  相似文献