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   

Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth   总被引：2，自引：0，他引：2  

P.J Patchett  J.D Vervoort  U Söderlund 《Earth and Planetary Science Letters》2004,222(1):29-41

Sm-Nd and Lu-Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm-Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean ¹⁴⁷Sm/¹⁴⁴Nd of 0.1960±0.0005, and a mean ¹⁴³Nd/¹⁴⁴Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm-Nd reference parameters, ¹⁴⁷Sm/¹⁴⁴Nd=0.1966 and ¹⁴³Nd/¹⁴⁴Nd=0.512638 (when fractionation-corrected to ¹⁴⁶Nd/¹⁴⁴Nd=0.7219).Lu-Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of ¹⁷⁶Lu/¹⁷⁷Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10⁻¹¹ year⁻¹ is used for the decay constant of ¹⁷⁶Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311-324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with ¹⁷⁶Lu/¹⁷⁷Hf less than 0.030 are employed, no combination of the main population of chondrite Lu-Hf data yields a regression with sufficiently low error to constrain the decay constant of ¹⁷⁶Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu-Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu-Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu-Hf characteristics results in a BSE Hf-Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε_Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu-Hf.  相似文献   

Isotopic and REE studies of lunar basalt 12038: implications for petrogenesis of aluminous mare basalts     

L.E. Nyquist  J.L. Wooden  C.-Y. Shih  H. Wiesmann  B.M. Bansal 《Earth and Planetary Science Letters》1981,55(3):335-355

We report Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts. A precise internal Rb-Sr isochron yields a crystallization age of 3.35±0.09 AE and initial⁸⁷Sr/⁸⁶Sr=0.69922?2 (2σ error limits, 1AE=10⁹ years, λ(⁸⁷Rb)=0.0139AE^?1). An internal Sm-Nd isochron yields an age of 3.28±0.23AE and initial¹⁴³Nd/¹⁴⁴Nd=0.50764?28. Present-day¹⁴³Nd/¹⁴⁴Nd is less than the “chondritic” value, i.e. ?(Nd, 0)=?2.3±0.4 where ?(Nd) is the deviation of¹⁴³Nd/¹⁴⁴Nd from chondritic evolution, expressed as parts in 10⁴. At the time of crystallization ?(Nd, 3.2AE)=1.5±0.6.We have successfully modeled the evolution of the Sr and Nd isotopic compositions and the REE abundances within the framework of our earlier model for Apollo 12 olivine-pigeonite and ilmenite basalts. The isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb=2.2 for this hypothetical magma ocean pattern. A plot of I(Sr) versus ?(Nd) for the Apollo 12 basalts clearly shows the influence of varying proportions of olivine, clinopyroxene, orthopyroxene, and plagioclase in the basalt source regions. A small percentage of plagioclase (～5%) in the 12038 source apparently is responsible for low I(Sr) and ?(Nd) in this basalt. Aluminous mare basalts from Mare Crisium (Luna 24) and by inference Mare Fecunditatis (Luna 16) occupy locations on the I(Sr)-?(Nd) plot similar to that of 12038, implying that some basalts from three widely separated lunar regions came from plagioclase-bearing source regions. A summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when REE abundances in the lunar magma ocean increased from ～20× chondritic to >100× chondritic. Although there is a general trend from olivine to clinopyroxene-dominated source regions with progressive magma ocean evolution, significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution, as evidenced in particular by the three Apollo 12 magma types.  相似文献   

The surface composition of lunar soil grains: A comparison of the results of Auger and X-ray photoelectron (ESCA) spectroscopy     

R.L. Baron  E. Bilson  T. Gold  R.J. Colton  B. Hapke  M.A. Steggert 《Earth and Planetary Science Letters》1977,37(2):263-272

Sm-Nd data for ten greenstone belt volcanics from Rhodesia define an age of2.64 ± 0.14AE, which is in agreement with other geochronological data and with observed field relationships. This age and the initial¹⁴³Nd/¹⁴⁴Nd ratio of0.50919 ± 0.00018 yield a time-integrated Sm/Nd ratio of0.302 ± 0.009 in the mantle source region, similar to that in chondrites. Sm/Nd ratios of some lavas are0.31 and imply that a small fraction of their source was removed prior to or as part of the main melting event.The utility of the Sm-Nd system for dating altered Archaean volcanic rocks is amply demonstrated by these data.  相似文献   

Nd and Sr isotope ratios and rare earth element abundances in Reykjanes Peninsula basalts evidence for mantle heterogeneity beneath Iceland     

Alan Zindler  S.R. Hart  F.A. Frey  S.P. Jakobsson 《Earth and Planetary Science Letters》1979,45(2):249-262

Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in ¹⁴³Nd/¹⁴⁴Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in ¹⁴³Nd/¹⁴⁴Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in ⁸⁷Sr/⁸⁶Sr (0.7031–0.7032). These ⁸⁷Sr/⁸⁶Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and ¹⁴³Nd/¹⁴⁴Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, ⁸⁷Sr/⁸⁶Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.  相似文献   

SmNd isotopic systematics of Enderby Land granulites and evidence for the redistribution of Sm and Nd during metamorphism     

M.T. McCulloch  L.P. Black 《Earth and Planetary Science Letters》1984,71(1):46-58

Measurements of¹⁴³Nd/¹⁴⁴Nd and¹⁴⁷Sm/¹⁴⁴Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock SmNd isochron age of3060 ± 160m.y. and an initial¹⁴³Nd/¹⁴⁴Nd ratio of0.50776 ± 10 (?_Nd(3060m.y.) = ?2.0 ± 1.8). The negative ?_Nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at～ 3100m.y. by RbSr and UPb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 ± 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at～ 2450m.y.Due to the resetting of the SmNd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly3100m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir(?_Nd(T) ? 2), evolution of the negative ?_Nd value of ?2.0 with the observed Sm/Nd ratios requires a prehistory of～ 380m.y. This implies a primary age of～ 3480m.y. However, substantially older primary ages can be inferred if the source reservoirs had?_Nd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with?_Nd ≥ 0 during granulite facies metamorphism.  相似文献   

The Sm/Nd secular evolution of the continental crust and the depleted mantle     

Francis Albarde  Marc Brouxel 《Earth and Planetary Science Letters》1987,82(1-2)

The published Nd isotopic data on rocks representative of either the continental crust or the depleted mantle are used to determine the Sm/Nd evolution of each system through time making allowance for a contribution from a primitive (chondritic) mantle. Screening using the ¹⁴⁷Sm/¹⁴⁴Nd ratio permits data of doubtful significance to be discarded. Mass balance equations describing mantle-crust exchange processes are numerically integrated. They suggest that crustal growth probably occurs through the addition of strongly LREE-enriched magmas derived from the mantle either directly (andesites) or indirectly (rhyolites). If the modern mean ¹⁴⁷Sm/¹⁴⁴Nd ratio of the crust is close to the sediment average (0.11), then progressive enrichment of LREE in the crust and depletion in the depleted mantle has occurred. If this ratio is of 0.13, then it, and the probable depleted-mantle ¹⁴⁷Sm/¹⁴⁴Nd ratio (0.26) have been constant over the last 3.8 Ga. The fraction of the total Nd (exclusive of the primitive mantle) stored in the continental crust has varied from 40% to 50% over the same period.The volume of the continents can have remained constant only if the rate of sediment reinjection into the mantle is 2.5 km³ a⁻¹ or more. For lower, probably more geologically reasonable, reinjection rates, a nearly uniform continent growth rate over the past 3.8 Ga is inferred. In all cases, the depleted mantle is continuously forming from a primitive reservoir.  相似文献   

Episodic mantle differentiation: Nd and Sr isotopic evidence     

H.G. Stosch  R.W. Carlson  G.W. Lugmair 《Earth and Planetary Science Letters》1980,47(2):263-271

Clinopyroxenes separated from two hydrous and four anhydrous ultramafic nodules, selected from a suit of xenoliths from Dreiser Weiher (DW), West Germany, have been studied for Nd and Sr isotopic composition. Nd exhibits a range of ε_JUV(T) from 0 to +12.4 and ⁸⁷Sr/⁸⁶Sr varies between 0.70185 and 0.70400. T_ICE model ages for anhydrous nodules indicate that the mantle underlying DW was originally depleted ?2 AE ago. Correlation of ¹⁴³Nd/¹⁴⁴Nd with Sm/Nd in this group of samples suggests that a second partial melting event occurred about 560 m.y. ago resulting in LREE enrichment of at least part of the anhydrous mantle. During a later episode, probably contemporaneous with the eruption of the host basalt in Quaternary times, most of the spinel peridotitic upper mantle below DW was modified. This metasomatism led to hydration and incompatible element enrichment of originally anhydrous mantle. The isotopic data for the anhydrous nodules again demonstrate that oceanic-type mantle underlies at least some continental areas. It is apparent that the separation of subcontinental mantle regions from an initially chondritic reservoir may occur in several discrete episodes. However, differing histories of depletion and/or enrichment will produce isotopically distinct mantle reservoirs. Therefore, basalts extracted from these mantle reservoirs will scatter about an average Nd-Sr trend line reflecting the nature of the differentiation in their source regions.  相似文献   

Age of the Archean Talga-Talga Subgroup, Pilbara Block, Western Australia, and early evolution of the mantle: new SmNd isotopic evidence     

G. Gruau  B.M. Jahn  A.Y. Glikson  R. Davy  A.H. Hickman  C. Chauvel 《Earth and Planetary Science Letters》1987,85(1-3)

Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal¹⁴³Nd/¹⁴⁴Nd vs.¹⁴⁷Sm/¹⁴⁴Nd isochron plot. The data give an age of 3712 ± 98 Ma and initialε_Nd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the SmNd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, withε_Nd(T) values ranging from at least 0 to +4.0.  相似文献   

Sm-Nd studies of Archaean metasediments and metavolcanics from West Greenland and their implications for the Earth's early history     

P.J. Hamilton  R.K. O&#x;Nions  D. Bridgwater  A. Nutman 《Earth and Planetary Science Letters》1983,62(2):263-272

Metasedimentary and metavolcanic rocks from the Archaean of West Greenland have been examined for evidence of crustal components greater than 3.8 Ga in age and for their compatibility with the presently adopted bulk Earth Sm-Nd parameters. Sm-Nd isotopic data have been obtained for the garbenschiefer metagabbro unit, metasediments from the Isua supracrustal belt, gneisses interior to the Isua belt and metasediments from the Malene supracrustal belt.Using estimates of emplacement age (T) of between 3.77 and 3.67 Ga for the parental volcanics to the garbenschiefer unit, initial¹⁴³Nd/¹⁴⁴Nd ratios yield positiveε_Nd^T values between +1.0 and +3.1 (relative to the CHUR parameters) for seven out of eight samples. Model Sm-Nd ages for the Isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca.+2ε_Nd relative to CHUR at those times. Similarly, model Sm-Nd ages for the Malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveε_Nd^T. Implications of these results for the early development of the Earth's mantle are discussed.  相似文献   

A geochemical study of island-arc and back-arc tholeiites from the Scotia Sea     

C.J. Hawkesworth  R.K. O&#x;Nions  R.J. Pankhurst  P.J. Hamilton  N.M. Evensen 《Earth and Planetary Science Letters》1977,36(2):253-262

⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios, REE and selected minor and trace elements are presented and compared for present-day volcanic rocks in the Scotia Sea.Tholeiitic basalts from the South Sandwich Islands show widely ranging contents of some lithophile elements, e.g. K₂O (0.09–0.55%) and Rb (1.55–14.2 ppm), but fairly constant Na₂O and Sr. Total REE contents range from about 4–20 times chondritic abundances with significant light-REE depletion and both positive and negative Eu anomalies. The variations in minor and trace element abundances are consistent with low-pressure fractional crystallization of plagioclase and clinopyroxene but only minor amounts of olivine. The⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios of the parental magmas are thought be 0.7038–0.7039 and 0.51301–0.51314 respectively, and indicate derivation of at least some⁸⁷Sr from subducted ocean crust.The back-arc tholeiites in the Scotia Sea have lower⁸⁷Sr/⁸⁶Sr ratios (0.7028–0.7033), similar¹⁴³Nd/¹⁴⁴Nd ratios (0.51305) and are variably light-REE-enriched(Ce_N/Yb_N= 1.0–1.6). Total REE contents are comparable to those of the South Sandwich Islands tholeiites.  相似文献   

A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems   总被引：29，自引：0，他引：29  

S.L. Goldstein  R.K. O''Nions  P.J. Hamilton 《Earth and Planetary Science Letters》1984,70(2):221-236

¹⁴³Nd/¹⁴⁴Nd ratios, and Sm and Nd abundances, are reported for particulates from major and minor rivers of the Earth, continental sediments, and aeolian dusts collected over the Atlantic, Pacific, and Indian Oceans. Overall, Sm/Nd ratios and Nd isotopic compositions in contemporary continental erosion products vary within the small ranges of ¹⁴⁷Sm/¹⁴⁴Nd= 0.115 ± 0.01 and¹⁴³Nd/¹⁴⁴Nd= 0.51204 ± 0.0002 (ε_Nd = −11.4 ± 4). The average period of residence in the continental crust is estimated to be1.70 ± 0.35Ga.

These results combined with data from the literature have implications for the age, history, and composition of the sedimentary mass and the continental crust: (1) The average “crustal residence age” of the whole sedimentary mass is about 1.9 Ga. (2) The range of Nd isotope compositions in the continent derived particulate input to the oceans is the same as Atlantic sediments and seawater, but lower than those of the Pacific, demonstrating the importance of Pacific volcanism to Pacific Nd chemistry. (3) The average ratio of Sm/Nd is about 0.19 in the upper continental crust, and has remained so since the early Archean. This precludes the likelihood of major mafic to felsic or felsic to mafic trends in the overall composition of the upper continental crust through Earth history. (4) Sediments appear to be formed primarily by erosion of continental crust having similar Sm/Nd ratios, rather than by mixing of mafic and felsic compositions. (5) The average ratio of ¹⁴³Nd/¹⁴⁴Nd≈ 0.5117 (ε_Nd ≈ −17) in the upper continental crust, assuming its mean age is about 2 Ga. (6) The uniformity of the SmNd isotopic systematics in river and aeolian particulates primarily reflects efficient recycling of old sediment by sedimentary processes on a short time scale compared to the amount of time the material has resided in the crust.  相似文献   

Implications of correlated Nd and Sr isotopic variations for the chemical evolution of the crust and mantle   总被引：1，自引：0，他引：1  

Donald J. DePaolo 《Earth and Planetary Science Letters》1979,43(2):201-211

Published data showing a linear correlation between initial Nd and Sr isotope compositions in young basalts indicate the existence of a spectrum of isotopically distinct reservoirs in the mantle which represent either (1) mixtures of two homogeneous endmember reservoirs, one of which may be undifferentiated material or (2) fractionated reservoirs all derived from a homogeneous initial reservoir with the same ratio of enrichment factors for Sm/Nd and Rb/Sr. The slope of the correlation, which can be described approximately by (⁸⁷Sr/⁸⁶Sr) = ?3.74114 (¹⁴³Nd/¹⁴⁴Nd) + 2.61935orε_Nd = ?2.7 ε_Sr, places constraints on the origin of these reservoirs and hence on the chemical evolution of the crust-mantle system. The reservoirs could be residual regions of the mantle left after ancient partial melting events. If so, the requirement of constant relative fractionation of Sm/Nd and Rb/Sr in refractory residues is a strong constraint on partial melting models. Calculations suggest that batch melting models are more compatible with this constraint than are fractional melting models, but models incorporating currently accepted distribution coefficients and residual phase assemblages cannot reproduce the observed isotope effects except under highly specific conditions. The slope of the correlation is not consistent with the hypotheses that chemical structure in the mantle is due to accretional heterogeneity or variable loss of elements to the core. If the mantle reservoirs are complementary in composition to the continental crust, and if the crust + mantle has ε_Nd = 0andε_Sr = 0 and chondritic Sr/Nd, then Rb/Sr in the crust is calculated to be less than 0.10, suggesting that the crust may be more mafic in composition and contain a smaller proportion of the earth's Rb and heat-producing elements than previously estimated.  相似文献   

Mariana Trough basalts (MTB): trace element and SrNd isotopic evidence for mixing between MORB-like and Arc-like melts     

Alan M. Volpe  J. Douglas Macdougall  James W. Hawkins 《Earth and Planetary Science Letters》1987,82(3-4)

Mariana Trough basalt (MTB) glasses from zones of of active seafloor volcanism have incompatible trace element compositions which are intermediate between normal MORB and basaltic rocks from the active northern Mariana Island Arc (MIAB). The chemical variation is observed in trace elemental abundances and ratios such as LIL/LIL and LIL/HFS. MTB glasses with high LIL/HFS and Ba/Sm ratios, and low K/Rb, K/Ba, and Sm/Nd ratios have more enriched Sr and Nd isotopic compositions.Comparison of the SrNd isotopic compositions of MTB and MIAB suggests that the source region within the mantle wedge is heterogeneous. The diverse trace element and isotopic compositions of MTB glasses both within and between dredge sites near 18°N imply small-scale source heterogeneity. Correlation between Sm/Nd and¹⁴³Nd/¹⁴⁴Nd of the MTB glasses is interpreted as due to recent binary mixing, rather than closed system evolution of a common homogeneous source. Mixing of melts at or near the source region between a mantle component with long-term LREE and LIL element depletion (MORB-like) and a relatively enriched component with lower integrated¹⁴³Nd/¹⁴⁴Nd (Arc-like) is suggested by trends of the MTB data on ratio-ratio, ratio-element and element-element plots.  相似文献   

Sr and Nd isotope composition of the metamorphic,sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources     

Alfredo Aparicio  Colombo C.G. Tassinari  Roberto García  Vicente Araña 《Journal of Volcanology and Geothermal Research》2010,189(1-2):143-150

The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The ⁸⁷Sr/⁸⁶Sr (around 0.703) and ¹⁴³Nd/¹⁴⁴Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the ¹⁴⁷Sm/¹⁴⁴Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange (⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.  相似文献   

Neodymium isotopic variations in seawater     

Donald J. Piepgras  G.J. Wasserburg 《Earth and Planetary Science Letters》1980,50(1):128-138

New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average ε_Nd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean,ε_Nd(0) ? ?12 ± 2; Indian Ocean,ε_Nd(0) ? ?8 ± 2; Pacific Ocean,ε_Nd(0) ? ?3 ± 2. These values are considerably less than ε_Nd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of¹⁴³Nd between the Pacific and Atlantic Oceans corresponds to ～10⁶ atoms¹⁴³Nd per gram of seawater. The correspondence between the¹⁴³Nd/¹⁴⁴Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography.Distinctive differences in ε_Nd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition.The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans.  相似文献   

An estimate of the Nd isotopic composition of Iapetus seawater from ca. 490 Ma metalliferous sediments     

P.J. Hooker  P.J. Hamilton  R.K. O&#x;Nions 《Earth and Planetary Science Letters》1981

Well-preserved metalliferous sediments and pillow basalts of Lower Ordovician age (ca. 490 Ma) have been studied in an attempt to specify the Nd isotopic composition of Iapetus seawater. Initial¹⁴³Nd/¹⁴⁴Nd ratios of the pillow basalts are indistinguishable from published initial ratios for the 505-Ma Bay of Islands ophiolite complex and are within the anticipated range for MORB-type basalts 500 Ma ago. Metalliferous sediments occur both interstitial to basalt pillows and as well-developed sedimentary accumulations. The initial¹⁴³Nd/¹⁴⁴Nd ratios for the non-interstitial variety range from 0.511851 to 0.511712 (ε_Nd = ?2.7to?5.4) and are considered to provide an estimate of¹⁴³Nd/¹⁴⁴Nd in Iapetus seawater. The interstitial metalliferous sediments show evidence for a significant basalt-derived Nd component. Although volcanic activity occurred at the margin of Iapetus essentially contemporaneous with the formation of the metalliferous sediments, it is clear that arc-type volcanic material was not a major source of Nd in Iapetus seawater. Rather the source of Nd was from continental regions with a similar average age to those supplying material to the present-day Atlantic Ocean.  相似文献   

Crustal accretion in the Pan African: Nd and Sr isotope evidence from the Arabian Shield     

Henk J. Duyverman  Nigel B.W. Harris  Chris J. Hawkesworth 《Earth and Planetary Science Letters》1982,59(2):315-326

Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770?590 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7038?0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial ¹⁴³Nd/¹⁴⁴Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (ε_Nd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.  相似文献   

A neodymium,strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust     

Malcolm T. McCulloch  Robert T. Gregory  G.J. Wasserburg  Hugh P. Taylor 《Earth and Planetary Science Letters》1980,46(2):201-211

In the Samail ophiolite,¹⁴⁷Sm-¹⁴³Nd,⁸⁷Rb-⁸⁷Sr, and¹⁸O/¹⁶O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and¹⁸O/¹⁶O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the¹⁴⁷Sm-¹⁴³Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial¹⁴³Nd/¹⁴⁴Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε_Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The⁸⁷Sr/⁸⁶Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε_Sr = ?19.7 to +30.5) and the δ¹⁸O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the¹⁸O/¹⁶O water-rock fractionation.  相似文献   

SmNd constraints on early lunar differentiation and the evolution of KREEP     

R.W. Carlson  G.W. Lugmair 《Earth and Planetary Science Letters》1979,45(1):123-132

The Sm-Nd systematics of lunar KREEP basalt 15386 reflects two chronologically distinct events in the development of the incompatible element-rich materials of the moon. The measured Sm-Nd mineral isochron of 15386 indicates an age of 3.85 ± 0.08 AE which is consistent with the reported Rb-Sr and³⁹Ar-⁴⁰Ar ages of many other KREEP-rich samples. This age is interpreted as the time at which 15386 crystallized from a liquid on or near the lunar surface. The frequent occurrence of this age for KREEP-dominated samples, as well as the restricted location of KREEP near major lunar near-side impact basins, suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the postulated final bombardment phase of the surface of the moon. However, the lower than chrondritic initial¹⁴³Nd/¹⁴⁴Nd of 15386 and the essentially identical Sm-Nd evolution of other KREEP-rich samples require that the light REE enrichment which characterizes KREEP was established considerably before 3.85 AE. Within the limits imposed by model assumptions in the various radiometric systems, it is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.  相似文献