A Sm-Nd isotopic study of atmospheric dusts and particulates from major river systems

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.     

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

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.     

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

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.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Re-Os and Nd isotopes of black shales at the bottom of the Lower Cambrian from the northern Tarim Platform and their comparison with those from the Yangtze Platform

This paper reports Re-Os and Nd isotopes of black shales at the bottom of Lower Cambrian from the northern Tarim Basin and traces source materials of the black shales through isotopes. The average Re/Os, 187Re/188Os, and 187Os/188Os ratios of the black shales at the bottom of Lower Cambrian from the Tarim Basin are 7.18, 5.6438, and 1.9616, respectively. These isotopic ratios suggest the crustal sources of the black shales. The εNd(0) value is -13.17, the εNd(540 Ma) value is -7.32 and the Nd model ages are 1.535 Ga. These parameters in the black shales are quite consistent with those from the basement rocks. Based on the Re-Os and Nd isotopic characteristics of the black shales, we conclude that the continental crust mainly composed of basement rocks is the source material of the black shales. Through comparison of these isotopic parameters with those from the Yangtze Platform, it is clear that the Re-Os isotopic characteristics in the black shales from the Tarim and Yangtze platforms are quite different, which maybe indicates the differences in depositional settings between two platforms. These Re-Os isotopic data provide us with constraints to analyze the genetic relation between the two platforms.     

Rb–Sr and Sm–Nd isotopic studies of mafic igneous rocks from the Ryoke plutono-metamorphic belt in the Setouchi area, Southwest Japan: implications for the genesis and thermal history

Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?_Sr > 0 and ?_Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.     

Nb/Ta,Zr/Hf and REE in the depleted mantle: implications for the differentiation history of the crust–mantle system

High-precision Nb, Ta, Zr, Hf, Sm, Nd and Lu concentration data of depleted mantle rocks from the Balmuccia peridotite complex (Ivrea Zone, Italian Alps) were determined by isotope dilution using multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionisation mass spectrometry (TIMS). The Zr/Hf ratios of all investigated samples from the Balmuccia peridotite complex are significantly lower than the chondritic value of 34.2, and the most depleted samples have Zr/Hf ratios as low as 10. Correlated Zr/Hf ratios and Zr abundances of the lherzolites preserve the trend of a mantle residue that has been depleted by fractional melting. This trend confirms experimental studies that predict Hf to behave more compatibly than Zr during mantle melting. Experimentally determined partition coefficients imply that the major Zr and Hf depletion most likely occurred in the spinel stability field, with (D_Zr/D_Hf)^cpx≈0.5, and not in the garnet stability field, where (D_Zr/D_Hf)^grt is probably close to one. However, minor amounts of melting must have also occurred in a garnet facies mantle, as indicated by low Sm/Lu ratios in the Balmuccia peridotites. The Nb/Ta ratios of most lherzolites are subchondritic and vary only from 7 to 10, with the exception of three samples that have higher Nb/Ta ratios (18–24). The overall low Nb/Ta ratios of most depleted mantle rocks confirm a higher compatibility of Ta in the mantle. The uniform Nb/Ta ratios in most samples imply that even in ‘depleted’ mantle domains the budget of the highly incompatible Nb and Ta is controlled by enrichment processes. Such a model is supported by the positive correlation of Zr/Nb with the Zr concentration. However, the overall enrichment was weak and did barely affect the moderately incompatible elements Zr and Hf. The new constraints from the partitioning behaviour of Zr–Hf and Nb–Ta provide important insights into processes that formed the Earth’s major silicate reservoirs. The correlation of Zr/Hf and Sm/Nd in depleted MORB can be assigned to previous melting events in the MORB source. However, such trends were unlikely produced during continental crust formation processes, where Sm/Nd and Zr/Hf are decoupled. The different fractionation behaviour of Zr/Hf and Sm/Nd in the depleted mantle (correlated) and the crust (decoupled) indicates that crustal growth by a simple partial melting process in the mantle has little effect on the mass budget of LREE and HFSE between crust and mantle. A more complex source composition, similar to that of modern subduction rocks, is needed to fractionate the LREE, but not Zr/Hf and the HREE.     

Nd and Sr isotopic study of volcanic rocks from Japan

Two older granitic rocks and some selected Quaternary volcanic rocks from the Japanese Islands were analyzed in a reconnaissance study for the purpose of examining the relationships between Nd and Sr isotopic abundances and the megatectonic structure around the Japanese Islands. Model ages of ～0.9 AE were determined by the Nd and Sr methods on a Paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the Japanese Islands. The Nd and Sr isotopic data show that the Cretaceous granodiorite is the result of partial melting of older crust.The Nd of tholeiitic rocks from the Izu arc gives ε_Nd ranging from 8.3 to 9.3 and with the corresponding ε_Sr from ?14.5 to ?18.5. These results are identical to those found for the Mariana arc. These values are distinctly lower than typical MORB by around 1～2 εu. This difference in ε_Nd between arcs and MORB is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust. The Hakone volcano is clearly confirmed as belonging to an oceanic source by Nd isotopic results.ε_Sr-ε_Nd values of the volcanics from a section along the Fossa Magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material. The crustal component clearly increases in going from south to north. Volcanics across the Northeast Japan arc also show a distinct correlation of ε_Sr-ε_Nd related to the position relative to the active subduction zone but with the opposite trend. These relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the Japan Sea.In general there appear to be regular isotopic relationships between the Izu-Mariana oceanic island arc and the continental island arc of Japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas.     

Continental basalts in the accretionary complexes of the South-west Japan Arc: Constraints from geochemical and Sr and Nd isotopic data of metadiabase

Abstract The Ryoke Belt is one of the important terranes in the South‐west Japan Arc (SJA). It consists mainly of late Cretaceous granitoid rocks, meta‐sedimentary rocks (Jurassic accretionary complexes) and mafic rocks (gabbros, metadiabases; late Permian–early Jurassic). Initial ?_Sr (+ 25– + 59) and ?_Nd (? 2.1–?5.9) values of the metadiabases cannot be explained by crustal contamination but reflect the values of the source material. These values coincide with those of island arc basalt (IAB), active continental margin basalt (ACMB) and continental flood basalt (CFB). Spiderdiagrams and trace element chemistries of the metadiabases have CFB‐signature, rather than those of either IAB or ACMB. The Sr–Nd isotope data, trace element and rare earth element chemistries of the metadiabases indicate that they result from partial melting of continental‐type lithospheric mantle. Mafic granulite xenoliths in middle Miocene volcanic rocks distributed throughout the Ryoke Belt were probably derived from relatively deep crust. Their geochemical and Sr–Nd isotopic characteristics are similar to the metadiabases. This suggests that rocks, equivalent geochemically to the metadiabases, must be widely distributed at relatively deep crustal levels beneath a part of the Ryoke Belt. The geochemical and isotopic features of the metadiabases and mafic granulites from the Ryoke Belt are quite different from those of mafic rocks from other terranes in the SJA. These results imply that the Ryoke mafic rocks (metadiabase, mafic granulite) were not transported from other terranes by crustal movement but formed in situ. Sr–Nd isotopic features of late Cretaceous granitoid rocks occurring in the western part of the Japanese Islands are coincident with those of the Ryoke mafic rocks. Such an isotopic relation between these two rocks suggests that a continental‐type lithosphere is widely represented beneath the western part of the Japanese Islands.     

Recycling of the continental crust

In order to understand the evolution of the crust-mantle system, it is important to recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal recycling is the turnover of crustal material by processes taking place wholly within the crust and includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment composition through time. It also explains differences found in crustal area-age patterns measured by different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three levels. The first is recycling with only short periods of time in the mantle (<10 m.y.). This may be important in explaining the origin of island-arc and related igneous rocks; there is growing agreement that 1–3% recycled sediment is involved in their origin. Components of recycled crustal material, with long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation. A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of recycling is required for steady-state models of crustal evolution where the mass of the continents remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km³/yr over the past 1–2 Ga.Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed this age. It is likely that the intense meteorite bombardment, which affected the earth during the period 4.56–3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused the destruction and probable recycling into the mantle of any early formed crust.Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal recycling.     

The growth of the continent through geological time studied by Nd isotope analysis of shales

We determined the Nd isotopic composition and the Sm/Nd ratios in a series of Australian shales ranging from 0.2 Gy to 3.3 Gy. The first result of this study is the constancy of the Sm/Nd ratio in these shales, as in granitoids. Secondly, the initial (¹⁴³Nd/¹⁴⁴Nd) ratio gives a regular curve decreasing through geological time. Both results confirm that shales are representative samples of the continental crust, when insoluble elements, like REE, are studied.We calculated their Nd model ages of crustal differentiation. The model ages regularly decrease with the stratigraphic ages and after 2 Gy, the curve flattens and tends to an asymptotic value at around 1.8 Gy. The significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin.From these results, we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas. We propose that the results obtained here are representative of the whole Gondwana continent.Having studied the case of the Australian shales, we try to extend our study. First we applied our quantitative approach to the recent results obtained by O'Nions et al. on North Atlantic provinces. The continental growth curves obtained by our inversion procedure are quite distinct from the Australian shales showing the regional character of shales.With these two cases studied we try to compare our results with the already developed model for continental growth. The consideration of the surface conservative versus surface non-conservative models clearly show the non-uniqueness of the geological interpretation of the growth curve. On the other hand, we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the Hurley-Rand province age curve. Such recycling increases with time in agreement with the data obtained on Nd and Sr initial ratios on granitoids.     

A Nd isotopic study of the Kerguelen Islands: Inferences on enriched oceanic mantle sources

The origin of the highly differentiated igneous rocks of the Kerguelen Islands and the nature of their source regions have been investigated by a Nd isotopic study. The Nd isotopic compositions of syenites and granites are identical to those of gabbros and basalts and indicate a common source. The isotopic data preclude the involvement ofold continental crustal material in the genesis of these granitic and alkalic rocks. The data from the Kerguelen samples greatly extend the Nd-Sr isotopic correlation observed for uncontaminated basalts from the oceanic mantle. The large Nd isotopic variations in the Kerguelen samples could be explained by mixing of deep mantle material brought up by a plume and the upper oceanic mantle or by heterogeneities in the lower mantle. An important finding of this study is that there are enriched mantle sources under the oceanic regions. These enriched sources may be ancient in age and are compatible with the 2-b.y. age inferred from the Pb isotope data of these samples. Earth models in future must incorporate this feature of the oceanic mantle in a consideration of mantle-crust evolutionary relationships.     

Development of continental lithospheric mantle as reflected in the chemistry of the Mesozoic Appalachian Tholeiites, U.S.A.

Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial ε_Nd = +3.8 to −5.7; initial ⁸⁷Sr/⁸⁶Sr= 0.7044−0.7072; ²⁰⁶Pb/²⁰⁴Pb= 17.49−19.14; ²⁰⁷Pb/²⁰⁴Pb= 15.55−15.65; ²⁰⁸Pb/²⁰⁴Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ₁ = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO₂ (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).     

Age and isotope geochemistry of the Archaean Pongola and Usushwana suites in Swaziland, southern Africa: a case for crustal contamination of mantle-derived magma

The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland.

U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial ε_Nd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age.

An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial ¹⁴³Nd/¹⁴⁴Nd that lies off the CHUR growth curve by ε_Nd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS.

The negative deviations of initialε_Nd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial ⁸⁷Sr/⁸⁶Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust.

Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.     

Evolution of continental crust in southern Africa

Nd isotopic data from the Zimbabwe and Kaapvaal cratons and the Limpopo, Kalahari, Namaqualand and Damara mobile belts imply that over 50% of present-day continental crust in this region had separated from the mantle by the end of the Archaean and that< 10% of continental crust of southern Africa has formed in the last 1.0 Ga. Such a growth rate implies that average erosion rates through geological time were high and that evolution of continental crust has been dominated by crustal growth prior to 1.4 Ga, and crustal reworking since that time. The evolution of average crust is not represented directly by clastic sediment samples but may be determined from sediment analyses if both the time of orogeneses and the average erosion rate are known. Both trace element data from southern Africa granitoids and the high erosion rates implied by the isotopic study suggest that growth of continental crust in the Archaean was by underplating rather than lateral accretion, but arc accretion was the dominant mechanism after 2.0 Ga.     

Episodic mantle differentiation: Nd and Sr isotopic evidence

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.     

Sr–Nd isotope ratios of gabbroic and dioritic rocks in a Cretaceous-Paleogene granite terrain, Southwest Japan

Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial ⁸⁷Sr/⁸⁶Sr (0.7063–0.7076) and low initial Nd isotope ratios (?_Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial ⁸⁷Sr/⁸⁶Sr (usually less than 0.7060) and higher Nd isotope ratios (?_Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.     

The Shabogamo Intrusive Suite,Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y.Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr.Initial ratios for¹⁴³Nd/¹⁴⁴Nd and⁸⁷Sr/⁸⁶Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.     

A Nd isotope investigation of sediments related to crustal development in the British Isles

The times of original fractionation of the Sm and Nd component of clastic sediments from a mantle source (≡ crustal residence age) have been estimated from Sm-Nd model ages calculated relative to a depleted mantle evolution. In this way the provenance and evolution of selected Precambrian and Phanerozoic sediments and metasediments from the British Isles have been examined. Whereas some Archaean and early Proterozoic sediments have Sm-Nd model ages that are close to their stratigraphic age, the Phanerozoic sediments analysed have model ages as much as 2.0 Ga in excess of their stratigraphic age.A more detailed study of Lower Palaeozoic sediments deposited on the northern margin of the Iapetus Ocean provides evidence for a marked change of provenance in the Ordovician after the deposition of the Dalradian Supergroup. A component with comparatively high¹⁴³Nd/¹⁴⁴Nd and Sm/Nd ratio (presumably basaltic) is present in the sediments throughout the accretionary prism. Crustal residence age estimates average about 1.5 Ga for both these Lower Palaeozoic sediments, and modern pelagic clays, and collectively fail to provide any evidence for significant continental growth during the Phanerozoic.     

Time and duration of lunar highlands crust formation

All radiometric systems indicate that crust-mantle differentiation on the moon is dominated by events which occurred very early in lunar history. However, due to remaining uncertainties in model parameters and assumptions in the calculation of model ages, it is not yet possible to resolve the precise times of occurrence of these events nor the duration of the formation of the highlands crust. The strongest time constraints are offered by direct radiometric ages of samples formed during this earliest period. Two possible candidates for this material, norites 78236 and 73255,27,45, were examined utilizing the Sm-Nd radiometric system. Sm-Nd systematics of 78236 show post-crystallization disturbance but indicate that this norite crystallized in the lunar crust about 4.34 AE ago. Data for 73255,27,45 define an isochron and yield a crystallization age of 4.23 ± 0.05 AE. The initial Nd isotopic composition of both norites is within uncertainty of a “chondritic” reference reservoir at the time of their respective crystallizations. The implications for lunar crustal formation persisting over a time span of close to 350 m.y. are discussed.