4.2       Nd isotope evolution and model ages

 

DePaolo and Wasserburg (1976a) made the first Nd isotope determinations on terrestrial igneous rocks. When they plotted the ages and initial 143Nd/144Nd ratios of these units on a diagram of Nd isotope evolution against time, they found that Archean plutons had initial ratios which were remarkably consistent with the evolution of the Chondritic Uniform Reservoir (CHUR) observed in meteorites (Fig. 4.13). The CHUR evolution path is normally drawn as a straight line, but in fact it is a very gentle curve, due to the finite half-life of 147Sm (ca. 106 Byr).

Fig. 4.13. Diagram of 143Nd/144Nd against time showing the close correspondence of early Nd isotope analyses of terrestrial igneous rocks to the chondritic growth line. BCR)1 = Columbia River basalt, USA. After DePaolo and Wasserburg (1976a).

 

            Because Sm and Nd are rare earth elements (REE) with atomic numbers only two units apart, their chemical properties are very similar, and they undergo only slight relative fractionation during crystal)liquid processes. This means that in terrestrial rocks, departures of 143Nd/144Nd from the CHUR evolution line are small relative to the steepness of the line (Fig. 4.13). DePaolo and Wasserburg therefore developed a notation whereby initial 143Nd/144Nd isotopes ratios could be represented in parts per 104 deviation from the CHUR evolution line, termed epsilon units (, Nd). Mathematically, this notation is defined as:

 

                        | (143Nd/144Nd)sample (t)              |

  , Nd (t)  =     |   )))))))))))))   ! 1   |  @  104             [4.3]

                        | (143Nd/144Nd)CHUR  (t)             |

 

where t indicates the time at which , Nd is calculated. The , notation makes it much easier to compare the initial Nd isotope ratios of bodies of different ages. Also, by normalising all data to CHUR, it removes the effects of the different fractionation corrections which have been applied for Nd analysis as the metal or as the oxide species.

 

            Using the , notation, DePaolo and Wasserburg (1976b) presented a larger data set of Nd isotope analyses on a diagram of , Nd against time (Fig. 4.14). They noted that continental igneous rocks through time had , Nd values very close to zero. Indeed, for Archean rocks the error bars overlapped with zero, suggesting that continental igneous rocks were ‘derived from a reservoir with a chondritic REE pattern, which may represent primary material remaining since the formation of the Earth.’

Fig. 4.14. Diagram of Nd isotope evolution against time in the form of deviations from the chondritic evolution line in , units. After DePaolo and Wasserburg (1976b).

 

 

4.2.1    Chondritic model ages

 

DePaolo and Wasserburg (1976b) argued that if the CHUR evolution line defines the initial ratios of continental igneous rocks through time, measurement of 143Nd/144Nd and 147Sm/144Nd ratios in any crustal rock would yield a model age for the formation of that rock (or its precursor) from the chondritic reservoir. This is true, providing that there was sufficient Nd/Sm fractionation during the process of crustal extraction from the mantle to give a reasonable divergence of crustal and mantle evolution lines (Fig. 4.15), and hence a precise intersection. The model age is then given as:

                                                                                                            [4.4]

                                    |           (143Nd )0           (143Nd )0          |

                                    |           ()))))   !      ()))))           |

                1                  |           (144Nd )sample     (144Nd )CHUR    |

  TCHUR =  ))  @  ln       | 1 +      )))))))))))))))          |

                8                  |           (147Sm )0           (147Sm)0           |

                                    |           ()))))   !      ()))))           |

                                    |           (144Nd )sample     (144Nd )CHUR    |

 

            DePaolo and Wasserburg argued that if the Sm/Nd ratio of a rock sample had not been disturbed since its separation from the chondritic reservoir (taken to be the mantle source), then TCHUR might provide a ‘crustal formation’ age for a wide variety of rocks. Elemental investigations have indicated the general immobility of REE on a whole-rock scale during the processes of weathering and low-temperature metamorphism associated with sedimentary rock formation (e.g. Haskin et al., 1966), and even during high-grade metamorphism (Green et al., 1969). This immobility is schematically illustrated by the lack of deflection in the evolution line of a crustal rock sample in Fig. 4.15 during metamorphic and sedimentary events. Hence, Nd model ages may be able to date crustal formation in rocks that have been subjected to high-grade metamorphism and even erosion)sedimentation.

Fig. 4.15. Schematic Nd isotope evolution diagram showing the theory of model ages. Tmet = age of metamorphic event; Tsed = age of erosion)sedimentation event; f = fractionation of sample Sm/Nd relative to Bulk Earth. Dashed vector shows the development of the depleted mantle as a result of crustal extraction. After McCulloch and Wasserburg (1978).

 

            These premises were applied by McCulloch and Wasserburg (1978) in a model age study aimed at measuring the crustal formation ages of several cratonic rock bodies, mainly from the Canadian Shield. McCulloch and Wasserburg found Nd model ages within the range 2.5 ) 2.7 Byr for composite samples of the Superior, Slave, and Churchill structural provinces. In the first two areas, previously determined K)Ar and Rb)Sr ages had given the same results, but the 2.7 Byr model age for the Churchill province was 0.8 Byr older than the previously determined K)Ar age, which presumably had been reset by more recent metamorphism. These data supported a model of episodic continental growth by showing the period 2.5 ) 2.7 Byr ago to be a time of remarkably widespread continental growth. In contrast, a Grenville Province composite yielded a model age of 0.8 Byr which did not reveal any Archean component, suggesting it to be an addition of more recent crust to the pre-existing shield. However, this sample was by no means representative of the Grenville Province as a whole, which also contains extensive areas of reworked Archean and Early Proterozoic crust (Dickin, 2000).

 

            Although Nd model ages are generally applied to dating the time of crustal separation from the mantle, other more specialised applications have been made. For example, Richardson et al. (1984) investigated the time of diamond formation in the South African mantle lithosphere by dating garnet inclusions in diamonds. Three samples were analysed, each consisting of a composite of several hundred sub-calcic garnet inclusions, and yielding a total of 10 ) 45 ng Nd. The unradiogenic Nd in these samples gave rise to TCHUR ages of 3.19 ) 3.41 Byr (Fig. 4.16). When this evidence is combined with evidence of sub-solidus temperatures for diamond growth (based on equilibrium garnet)olivine inclusions in diamonds), it suggests that sub-continental lithosphere has existed under the African craton since the Early Archean. This material may represent the residue from 3.5 Byr-old komatiite extraction (section 7.1).

Fig. 4.16. Nd isotope evolution diagram showing model age calculations for silicate inclusions in South African diamonds. The initial ratio of Onverwacht lavas is shown for comparison. After Richardson et al. (1984).

 

 

4.2.2    Depleted mantle model ages

 

While observing the good fit of Archean plutons to the CHUR Nd isotope evolution line, DePaolo and Wasserburg (1976b) also noted that young mid ocean ridge basalts (MORB) lay +7 to +12 , units above the CHUR evolution line (Fig. 4.14). They recognised that Archean continental igneous rocks which fall within error of the CHUR evolution line could conceivably lie on a depleted mantle evolution line characterised by progressively increasing Sm/Nd and 143Nd/144Nd. Such a source could be formed as a residue from magma extraction, as shown in Fig. 4.15. However, DePaolo and Wasserburg rejected this model in favour of a chondritic source for continental igneous rocks on the basis of a comparison with lunar Nd isotope evolution.

 

            Lunar basalts and troctolites with ages of 3.3 ) 4.3 Byr show a wide range of initial 143Nd/144Nd ratios, equivalent to a variation from +7 to !2 , units relative to CHUR (Fig. 4.17; Lugmair and Marti, 1978). This spread shows that very early Sm/Nd fractionation occurred in the Moon, and that there was no long-lived uniform magma source with a chondritic Sm/Nd ratio. In contrast, none of the Archean terrestrial rocks analysed by 1976 showed any dispersion outside error of CHUR, which led DePaolo and Wasserburg (1976b) to conclude that the Earth did not undergo early differentiation, or if it did, that this was re-mixed by convection.

Fig. 4.17. , Nd evolution diagram for lunar rocks indicating very early Sm/Nd fractionation between lunar reservoirs. After Lugmair and Marti (1978).

 

            The paucity of Nd isotope data for the Proterozoic was a serious weakness in this model, since it left a gap between the Archean CHUR data and the depleted MORB source (attributed to an elevated Sm/Nd ratio), with questions about the relationship between the two. An important stage in filling this gap was a study on Proterozoic metamorphic basement from the Colorado Front Range (DePaolo, 1981). Four meta-volcanics and two charnockitic granulites from the Idaho Springs Formation were dated by the Sm)Nd isochron method. In addition, Nd isotope and Sm/Nd determinations were made on three plutons previously dated by the Rb)Sr whole-rock method, (the Boulder Creek, Silver Plume and Pikes Peak granitoids). The initial 143Nd/144Nd ratios of all these samples are plotted on an , Nd versus time diagram in Fig. 4.18.

 

            The Idaho Springs meta-igneous rocks cluster at , Nd (t) = +3.7 " 0.3, showing them to be derived from a depleted mantle reservoir with respect to CHUR at 1.8 Byr. Boulder Creek samples also have positive , Nd (+1.7 to +3.5), whereas the Silver Plume and Pikes Peak granites have progressively lower , Nd values which lie on the 143Nd/144Nd evolution line of average Idaho Springs crust, suggesting that they contain a large fraction of re-melted 1.8 Byr-old basement.

Fig. 4.18. Plot of , Nd against time showing Colorado data relative to a model depleted mantle evolution curve. After DePaolo (1981).

 

            DePaolo was able to fit a quadratic curve to Idaho Springs and modern island-arc data (Fig. 4.18), representing the Nd isotope evolution of a progressively depleted reservoir which was the source area for calc-alkaline (subduction related) magmatism. This curve begins on the CHUR evolution line in the Early Archean, but diverges progressively to the present day. The composition of the depleted reservoir, relative to CHUR, at time T, is given as:

 

                , Nd (T)  =  0.25 T2  !  3 T  +  8.5                          [4.5]

 

Sm)Nd model ages calculated using this depleted mantle curve are denoted TDM. DePaolo argued that TDM model ages would be a more accurate indication of ‘crustal formation ages’ than TCHUR ages for studies of continental evolution. For example, an anomalously low TCHUR age of 0.8 Byr for McCulloch and Wasserburg’s Grenville composite (section 4.2.1) is revised to a TDM age of 1.3 Byr, within the range expected for juvenile crust formation during the Grenville orogeny.

 

            Subsequent to the discovery of Proterozoic depleted mantle by DePaolo (1981), new analyses have prompted several re-interpretations of the evolution of the depleted mantle reservoir. An important alternative to DePaolo’s (1981) model was proposed by Goldstein et al. in 1984 (Fig. 4.19). This model assumes linear depletion of the mantle from , Nd = 0 at 4560 Myr to , Nd = +10 at 0 Myr (the MORB composition). It also provides a good fit to Early Proterozoic greenstones from the SW United States and Greenland (Nelson and DePaolo, 1984; Patchett and Arndt, 1986). The most depleted , Nd values in these suites may represent flood basalts erupted in rifting environments that suffered little crustal contamination. Goldstein et al. denoted Nd model ages based on this mantle model as TCR (crustal residence). However, this is not the most appropriate mantle model for calculating crustal extraction ages of tonalitic crust-forming rocks generated in arc settings, which at the present day have less depleted Nd isotope signatures than spreading ridges.

Fig. 4.19. Plot of , Nd against time showing two of the most widely used depleted mantle evolution models. Dashed curve: DePaolo (1981); Solid line: Goldstein et al. (1984).

 

            More recently, Nagler and Kramers (1998) reverted to a model involving essentially chrondritic mantle evolution until 3 Byr ago, followed by more-or-less linear evolution to , = 8 at the present day. The resulting evolution curve was fairly close to that of DePaolo (1981) along some of its length. However, the model of Nagler and Kramers was constrained to fit the average , Nd values of a wide variety of different rock types, some of which (e.g. the Stillwater Complex) have been shown to exhibit crustal contamination. The result is a model which does not seem to be in tune with recent thinking about depleted mantle evolution in the Archean (section 4.4.3).

 

            There has been a tendency towards a proliferation of depleted mantle models as new data for various geographical areas become available. However, an examination of the literature suggests that the models of DePaolo (1981) and Goldstein et al. (1984) have had the widest application by other workers. This is illustrated in Fig. 4.20 by a comparison of citation rates for these two studies, compared with two control papers: Nelson and DePaolo (1984), discussed above, and Allegre and Rousseau (1984), who proposed a curved mantle evolution line similar to that of DePaolo (1981). The durability of citations for DePaolo (1981) and Goldstein et al. (1984) indicates the wide usefulness of these mantle models, which therefore provide an important basis for the comparison of different magma suites, even if the absolute values of the model ages are slightly in error. Hence it is desirable that the TDM and TCR notations should be restricted to the models of DePaolo (1981) and Goldstein et al. (1984), while other acronyms can be used to denote different models.

Fig. 4.20. Plot of annual citation rates for four papers that introduced new depleted mantle evolution models for Nd. Data from the Science Citation Index, averaged over two-year intervals.

 

 

References

 

Albarede, F. and Goldstein, S. L. (1992). World map of Nd isotopes in sea-floor ferromanganese deposits. Geology 20, 761–3.

 

Allegre, C. J. and Rousseau, D. (1984). The growth of the continents through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Lett. 67, 19)34.

 

Armstrong, R. L. (1981). Radiogenic isotopes: the case for crustal recycling on a near steady-state no-continental-growth Earth. Phil. Trans. Roy. Soc. Lond. A301, 443)72.

 

Armstrong, R. L. (1991). The persistent myth of crustal growth. Aust. J. Earth Sci. 38, 613)30.

 

Arndt, N. T. and Goldstein, S. L. (1987). Use and abuse of crust-formation ages. Geology 15, 893)5.

 

Awwiller, D. N. and Mack, L. E. (1991). Diagenetic modification of Sm)Nd model ages in Tertiary sandstones and shales, Texas Gulf Coast. Geology 19, 311)14.

 

Barovich, K. M. and Patchett, P. J. (1992). Behaviour of isotopic systematics during deformation and metamorphism: a Hf, Nd and Sr isotopic study of mylonitized granite. Contrib. Mineral. Petrol. 109, 386)93.

 

Bennett, V. C. and DePaolo, D. J. (1987). Proterozoic crustal history of the western United States as determined by neodymium isotope mapping. Geol. Soc. Amer. Bull. 99, 674–85.

 

Bennett, V. C. and Nutman, A. P. (1998). Extreme Nd-isotope heterogeneity in the early Archean-- fact or fiction? Case histories from northern Canada and West Greenland– Comment. Chem. Geol. 148, 213–7.

 

Bennett, V. C., Nutman, A. P. and McCulloch, M. T. (1993). Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth. Earth Planet. Sci. Lett. 119, 299)317.

 

Bertram, C. J. and Elderfield, H. (1993). The geochemical balance of the rare earth elements and Nd isotopes in the oceans. Geochim. Cosmochim. Acta 57, 1957–86.

 

Bock, B., McLennan, S. M. and Hanson, G. N. (1994). Rare earth element redistribution and its effects on the neodymium isotope system in the Austin Glen Member of the Normanskill Formation, New York, USA. Geochim. Cosmochim. Acta 58, 5245)53.

 

Bowring, S. A. and Housh, T. (1995). The Earth’s early evolution: Science 269, 1535–40.

 

Bowring, S. A. and Housh, T. (1996). Sm–Nd isotope data and Earth’s evolution: Reply. Science 273, 1878–9.

 

Bowring, S. A., King, J. E., Housh, T. B., Isachsen, C. E. and Podosek, F. A. (1989). Neodymium and lead isotope evidence for enriched early Archean crust in North America. Nature 340, 222)5.

 

Bros, R., Stille, P., Gauthier-Lafaye, F., Weber, F. and Clauer, N. (1992). Sm)Nd isotopic dating of Proterozoic clay material: an example from the Francevillian sedimentary series, Gabon. Earth Planet. Sci. Lett. 113, 207)18.

 

Burton, K. W., Lee, D.-C., Christensen, J. N., Halliday, A. N. and Hein, J. R. (1999). Actual timing of neodymium isotopic variations recorded by Fe–Mn crusts in the western North Atlantic. Earth Planet. Sci. Lett. 171, 149)56.

 

Burton, K. W., Ling, H.-F. and O’Nions, R. K. (1997). Closure of the Central American Isthmus and its effect on deep-water formation in the North Atlantic. Nature 386, 382–5.

 

Burton, K. W. and O’Nions, R. K. (1991). High-resolution garnet chronometry and the rates of metamorphic processes. Earth Planet. Sci. Lett. 107, 649)71.

 

Burton, K. W. and Vance, D. (2000). Glacial–interglacial variations in the neodymium isotope composition of seawater in the Bay of Bengal recorded by planktonic foraminifera. Earth Planet. Sci. Lett. 176, 425)41.

 

Cattell, A., Krogh, T. E. and Arndt, N. T. (1984). Conflicting Sm)Nd whole rock and U)Pb zircon ages for Archean lavas from Newton Township, Abitibi Belt, Ontario. Earth Planet. Sci. Lett. 70, 280)90.

 

Chapman, H. J. and Moorbath, S. (1977). Lead isotope measurements from the oldest recognised Lewisian gneisses of north-west Scotland. Nature 268, 41)2.

 

Chase, C. G. and Patchett, P. J. (1988). Stored mafic/ultramafic crust and early Archean mantle depletion. Earth Planet. Sci. Lett. 91, 66)72.

 

Chauvel, C., Dupre, B. and Jenner, G. A. (1985). The Sm)Nd age of Kambalda volcanics is 500 Ma too old! Earth Planet. Sci. Lett. 74, 315)324.

 

Chauvel C., Hofmann, A. W. and Arndt, N. T. (1983). New evidence for early mantle depletion from Nd isotopes in greenstones. Terra Cognita 3, 129 (abstract).

 

Chester, R., Griffiths, A. G. and Hirst, J. M. (1979). The influence of soil-sized atmospheric particulates on the elemental chemistry of deep sea sediments of the northeastern Atlantic. Marine Geol. 32, 141–54.

 

Claoue-Long, J. C., Thirlwall, M. F. and Nesbitt, R. W. (1984). Revised Sm-Nd systematics of Kambalda greenstones, Western Australia. Nature 307, 697)701.

 

Compston, W., Williams, I. S., Campbell, I. H. and Gresham, J. J. (1985). Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones. Earth Planet. Sci. Lett. 76, 299)311.

 

Cullers, R. L., Bock, B. and Guidotti, C. (1997). Elemental distributions and neodymium isotopic compositions of Silurian metasediments, western Maine, USA: Redistribution of rare earth elements. Geochim. Cosmochim. Acta 61, 1847)61.

 

DePaolo, D. J. (1981). Neodymium isotopes in the Colorado Front Range and crust)mantle evolution in the Proterozoic. Nature 291, 193)7.

 

DePaolo, D. J. and Wasserburg, G. J. (1976a). Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249)52.

 

DePaolo, D. J. and Wasserburg, G. J. (1976b). Inferences about magma sources and mantle structure from variations of 143Nd/144Nd. Geophys. Res. Lett. 3, 743)6.

 

DePaolo, D. J. and Wasserburg, G. J. (1979). Sm)Nd age of the Stillwater complex and the mantle evolution curve for neodymium. Geochim. Cosmochim. Acta 43, 999)1008.

 

Dia, A., Allegre, C. J. and Erlank, A. J. (1990). The development of continental crust through geological time: the South African case. Earth Planet. Sci. Lett. 98, 74)89.

 

Dickin, A. P. (2000). Crustal formation in the Grenville Province: Nd-isotope evidence. Can. J. Earth Sci. 37, 165–81.

 

Frank, M. (2002). Radiogenic isotopes: tracers of past ocean circulation and erosional input. Rev. Geophys. 40 (1), 1)38

 

Frank, M., O’Nions, R. K., Hein, J. R. and Banakar, V. K. (1999a). 60 Myr records of major elements and Pb–Nd isotopes from hydrogenous ferromanganese crusts: Reconstruction of seawater paleochemistry. Geochim. Cosmochim. Acta 63, 1689)1708.

 

Frank, M., Reynolds, B. C. and O’Nions, R. K. (1999b). Nd and Pb isotopes in Atlantic and pacific water masses before and after closure of the Panama gateway. Geology 27, 1147)50.

 

Galer, S. J. G. and Goldstein, S. L. (1991). Early mantle differentiation and its thermal consequences. Geochim. Cosmochim. Acta 55, 227)39.

 

Goldberg, E. D., Koide, M., Schmidt, R. A. and Smith, R. H. (1963). Rare earth distributions in the marine environment. J. Geophys. Res. 68, 4209)17.

 

Goldstein, S. L. and Jacobsen, S. B. (1987). The Nd and Sr isotopic systematics of river-water dissolved material: implications for the sources of Nd and Sr in seawater. Chem. Geol. (Isot. Geosci. Section) 66, 245)72.

 

Goldstein, S. L. and Jacobsen, S. B. (1988). Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution. Earth Planet. Sci. Lett. 87, 249)65.

 

Goldstein, S. L., O’Nions, R. K. and Hamilton, P. J. (1984). A Sm)Nd isotopic study of atmospheric dusts and particulates from major river systems. Earth Planet. Sci. Lett. 70, 221)36.

 

Green, T. H., Brunfeldt, A. O. and Heier, K. S. (1969). Rare earth element distribution in anorthosites and associated high grade metamorphic rocks, Lofoten-Vesteraalen, Norway. Earth Planet. Sci. Lett. 7, 93)8.

 

Griffin, W. L. and Brueckner, H. K. (1980). Caledonian Sm)Nd ages and a crustal origin for Norwegian eclogites. Nature 285, 319)20.

 

Hamilton, P. J., Evensen, N. M., O’Nions, R. K. and Tarney, J. (1979). Sm)Nd systematics of Lewisian gneisses : Implications for the origin of granulites. Nature 277, 25)8.

 

Hamilton, P. J., O’Nions, R. K., Bridgwater, D. and Nutman, A. (1983). Sm)Nd studies of Archean metasediments and metavolcanics from West Greenland and their implications for the Earth’s early history. Earth Planet. Sci. Lett. 62, 263)72.

 

Hanson, G. N. (1978). The application of trace elements to the petrogenesis of igneous rocks of granitic composition. Earth Planet. Sci. Lett. 38, 26)43.

 

Harper, C. L. and Jacobsen, S. B. (1992). Evidence from coupled 147Sm)143Nd and 146Sm)142Nd systematics for very early (4.5-Gyr) differentiation of the Earth’s mantle. Nature 360, 728)32.

 

Haskin, L. A., Frey, F. A., Schmidt, P. A. and Smith, R. H. (1966). Meteoritic, solar and terrestrial rare-earth distributions. Phys. Chem. Earth 7, 167)321.

 

Hurley, P. M., Hughes, H., Faure, G., Fairbairn, H. W. and Pinson, W. H. (1962). Radiogenic strontium-87 model of continent formation. J. Geophys. Res. 67, 5315)34.

 

Hurley, P. M. and Rand, J. R. (1969). Pre-drift continental nuclei. Science 164, 1229)42.

 

Ingram, B. L., Hein, J. R. and Farmer, G. L. (1990). Age determinations and growth rates of Pacific ferromanganese deposits using strontium isotopes. Geochim. Cosmochim. Acta 54, 1709)1721.

 

Jacobsen, S. B. (1988). Isotopic constraints on crustal growth and recycling. Earth Planet. Sci. Lett. 90, 315)29.

 

Jacobsen, S. B. and Pimentel-Klose, M. R. (1988). Nd isotopic variations in Precambrian banded iron formations. Geophys. Res. Lett. 15, 393)6.

 

Jacobsen, S. B. and Wasserburg, G. J. (1980). Sm)Nd isotopic evolution of chondrites. Earth Planet. Sci. Lett. 50, 139)55.

 

Kamber, B. S. and Moorbath, S. Initial Pb of the Amitsoq gneiss revisited: implication for the timing of early Archean crustal evolution in west Greenland. Chem. Geol. 150, 19)41.

 

Kamber, B. S., Moorbath, S. and Whitehouse, M. J.  (1998). Extreme Nd-isotope heterogeneity in the early Archean- fact or fiction? Case histories from northern Canada and West Greenland- Reply. Chem. Geol. 148, 219)24.

 

Keto, L. S. and Jacobsen, S. B. (1987). Nd and Sr isotopic variations of Early Paleozoic oceans. Earth Planet. Sci. Lett. 84, 27)41.

 

Keto, L. S. and Jacobsen, S. B. (1988). Nd isotopic variations of Phanerozoic paleo-oceans. Earth Planet. Sci. Lett. 90, 395)410.

 

Lahaye, Y., Arndt, N., Byerly, G., Chauvel, C., Fourcade, S. and Gruau, G. (1995). The influence of alteration on the trace-element and nd isotopic composition of komatiites. Chem. Geol. 126, 43–64.

 

Lambert, D. D., Morgan, J. W., Walker, R. J., Shirey, S. B., Carlson, R. W., Zientek, M. L. and Koski, M. S. (1989). Rhenium)osmium and samarium)neodymium isotopic systematics of the Stillwater Complex. Science 244, 1169)74.

 

Ling, H. F., Burton, K. W., O’Nions, R. K., Kamber, B. S., von Blanckenburg, F., Gibb, A. J. and Hein, J. R. (1997). Evolution of Nd and Pb isotopes in Central Pacific seawater from ferromanganese crusts. Earth Planet. Sci. Lett. 146, 1)12.

 

Lugmair, G. W. and Marti, K. (1977). Sm)Nd)Pu timepieces in the Angra dos Reis meteorite. Earth Planet. Sci. Lett. 35, 273)84.

 

Lugmair, G. W. and Marti, K. (1978). Lunar initial 143Nd/144Nd : differential evolution of the lunar crust and mantle. Earth Planet. Sci. Lett. 39, 349)57.

 

Lugmair, G. W. and Scheinin, N. B. (1975). Sm)Nd systematics of the Stannern meteorite. Meteoritics 10, 447)8 (abstract).

 

Lugmair, G. W., Scheinin, N. B., and Marti, K. (1975). Search for extinct 146Sm, I. The isotopic abundance of 142Nd in the Juvinas meteorite. Earth Planet. Sci. Lett. 27, 79)84.

 

McCulloch, M. T. and Compston, W. (1981). Sm)Nd age of Kambalda and Kanowna greenstones and heterogeneity in the Archean mantle. Nature 294, 322)7.

 

McCulloch, M. T. and Wasserburg, G. J. (1978). Sm)Nd and Rb)Sr chronology of continental crust formation. Science 200, 1003)11.

 

McLennan, S. M., McCulloch, M. T., Taylor, S. R. and Maynard, J. B. (1989). Effects of sedimentary sorting on neodymium isotopes in deep-sea turbidites. Nature 337, 547)9.

 

Michard, A., Albarede, F., Michard, G., Minster, J. F. and Charlou, J. L. (1983). Rare-earth elements and uranium in high-temperature solutions from East Pacific Rise hydrothermal vent field (13 oN). Nature 303, 795–7.

 

Mildowski, A. E. and Zalasiewicz, J. A. (1991). Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudstone sequences of Llandovery age from central Wales. In: Morton, A. C. et al. (Eds), Developments in Sedimentary Provenance Studies. Geol. Soc. Spec. Pap. 56, pp. 789–95.

 

Moorbath, S. (1976). Age and isotope constraints for the evolution of Archaean crust. In: Windley, B. F. (Ed.), The Early History of the Earth, Wiley, 351)60.

 

Moorbath, S., Powell, J. L. and Taylor, P. N. (1975). Isotopic evidence for the age and origin of the grey gneiss complex of the southern Outer Hebrides, Scotland. J. Geol. Soc. Lond. 131, 213)22.

 

Moorbath, S. and Whitehouse, M. J. (1996). Sm–Nd isotope data and Earth’s evolution: Comment. 273, 1878.

 

Moorbath, S., Whitehouse, M. J. and Kamber, B. S. (1997). Extreme Nd-isotope heterogeneity in the early Archean– fact or fiction? Case histories from northern Canada and West Greenland. Chem. Geol. 135, 213)31.

 

Mork, M. B. E. and Mearns, E. W. (1986). Sm)Nd isotopic systematics of a gabbro)eclogite transition. Lithos 19, 255)67.

 

Nagler, Th. F. and Kramers, J. D. (1998). Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both. Precamb. Res. 91 233)252.

 

Nelson, B. K. and DePaolo, D. J. (1984). 1,700-Myr greenstone volcanic successions in southwestern North America and isotopic evolution of Proterozoic mantle. Nature 312, 143–6.

 

Nelson, B. K. and DePaolo, D. J. (1985). Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent. Geol. Soc. Amer. Bull. 96, 746)54.

 

Nelson, B. K. and DePaolo, D. J. (1988). Application of Sm)Nd and Rb)Sr isotope systematics to studies of provenance and basin analysis. J. Sed. Petrol. 58, 348)57.

 

Notsu, K., Mabuchi, H., Yoshioka, O., Matsuda, J. and Ozima, M. (1973). Evidence of the extinct nuclide 146Sm in ‘Juvinas’ achondrite. Earth Planet. Sci. Lett. 19, 29)36.

 

Nunes, P. D. (1981). The age of the Stillwater complex: a comparison of U)Pb zircon and Sm)Nd isochron systematics. Geochim. Cosmochim. Acta 45, 1961)3.

 

Nutman, A. P., Bennett, V. C., Friend, C. R. L. and McGregor, V. R. (2000). The early Archean Itsaq Gneiss Complex of southern West Greenland: The importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution. Geochim. Cosmochim. Acta 64, 3035)60.

Ohlander, B., Ingri, J., Land, M. and Schoberg, H. (2000). Change of Sm–Nd isotope composition during weathering of till. Geochim. Cosmochim. Acta 64, 813–20.

 

O’Nions, R. K. (1984). Isotopic abundances relevant to the identification of magma sources. Phil. Trans. Roy. Soc. Lond. A 310, 591)603.

 

O’Nions, R. K., Carter, S. R., Cohen, R. S., Evensen, N. M. and Hamilton, P. J. (1978). Pb, Nd and Sr isotopes in oceanic ferromanganese deposits and ocean floor basalts. Nature 273, 435)8.

 

O’Nions, R. K., Frank, M., von Blanckenburg, F. and Ling, H.-F. (1998). Secular variation of Nd and Pb isotopes in ferromanganese crusts from the Atlantic, Indian and Pacific Oceans. Earth Planet. Sci. Lett. 155, 15)28.

 

O’Nions, R. K., Hamilton, P. J. and Evensen, N. M. (1977). Variations in 143Nd/144Nd and 87Sr/86Sr in oceanic basalts. Earth Planet. Sci. Lett. 34, 13)22.

 

O’Nions, R. K., Hamilton, P. J. and Hooker, P. J. (1983). A Nd isotope investigation of sediments related to crustal development in the British Isles. Earth Planet. Sci. Lett. 63, 229)40.

 

Palmer, M. R. and Elderfield, H. (1986). Rare earth elements and neodymium isotopes in ferromanganese oxide coatings of Cenozoic foraminifera from the Atlantic Ocean. Geochim. Cosmochim. Acta 50, 409–17.

 

Patchett, P. J. and Arndt, N. T. (1986). Nd isotopes and tectonics of 1.9 ) 1.7 Ga crustal genesis. Earth Planet. Sci. Lett. 78, 329)38.

 

Pidgeon, R. T. and Bowes, D. R. (1972). Zircon U/Pb ages of granulites from the central region of the Lewisian, north western Scotland. Geol. Mag. 109, 247)58.

 

Piepgras, D. J. and Wasserburg, G. J. (1980). Neodymium isotopic variations in seawater. Earth Planet. Sci. Lett. 50, 128)38.

 

Piepgras, D. J. and Wasserburg, G. J. (1983). Influence of the Mediterranean Outflow on the isotopic composition of neodymium in waters of the North Atlantic. J. Geophys. Res. 88, 5997)6006.

 

Piepgras, D. J. and Wasserburg, G. J. (1987). Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations. Geochim. Cosmochim. Acta 51, 1257)71.

 

Piepgras, D. J., Wasserburg, G. J. and Dasch, E. J. (1979). The isotopic composition of Nd in different ocean masses. Earth Planet. Sci. Lett. 45, 223)36.

 

Reynolds, B. C., Frank, M. and O’Nions, R. K. (1999). Nd- and Pb-isotope time series from Atlantic ferromanganese crusts: implications for changes in provenance and paleocirculation over the last 8 Myr. Earth Planet. Sci. Lett. 173, 381)96.

Richardson, S. H., Gurney, J. J., Erlank, A. J. and Harris, J. W. (1984). Origin of diamonds in old enriched mantle. Nature 310, 198)202.

 

Rutberg, R. L., Hemming, S. R. and Goldstein, S. L. (2000). Reduced North Atlantic Deep Water flux to the glacial Southern Ocean inferred from neodymium isotope ratios. Nature 405, 935–8.

 

Samson, S. D., McClelland, W. C., Patchett, P. J., Gehrels, G. E. and Anderson, R. G. (1989). Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera. Nature 337, 705)9.

 

Shaw, H. F. and Wasserburg, G. J. (1985). Sm)Nd in marine carbonates and phosphates: implications for Nd isotopes in seawater and crustal ages. Geochim. Cosmochim. Acta 49, 503)18.

 

Smith, A. D. and Ludden, J. N. (1989). Nd isotopic evolution of the Precambrian mantle. Earth Planet. Sci. Lett. 93, 14)22.

 

Staudigel, H., Doyle, P. and Zindler, A. (1985). Sr and Nd isotope systematics in fish teeth. Earth Planet. Sci. Lett. 76, 45)56.

 

Stille, P. and Clauer, N. (1986). Sm)Nd isochron-age and provenance of the argillites of the Gunflint Iron Formation in Ontario, Canada. Geochim. Cosmochim. Acta 50, 1141)6.

 

Stordal, M. C. and Wasserburg, G. J. (1986). Neodymium isotopic study of Baffin Bay water: sources of REE from very old terranes. Earth Planet. Sci. Lett. 77, 259)72.

 

Tachikawa, K., Jeandel, C. and Roy-Barman, M. (1999). A new approach to the Nd residence time in the ocean: the role of atmospheric inputs. Earth Planet. Sci. Lett. 170, 433)46.

 

Taylor, S. R., McLennan, S. M. and McCulloch, M. T. (1983). Geochemistry of loess, continental crustal composition and crustal model ages. Geochim. Cosmochim. Acta 47, 1897)1905.

 

Thurston, P. C., Osmani, I. A. and Stone, D. (1991). Northwest Superior province: review and terrane analysis. In: Thurston, P. C., Williams, H. R., Sutcliffe, R. H. and Stott, G. M. (Eds), Geology of Ontario. Ontario Geol. Surv. Spec. Vol. 4, 81)139.

 

Tugarinov, A. I. and Bibikova, Y. V. (1976). Evolution of the chemical composition of the Earth’s crust. Geokhimiya 1976, (8) 1151)9.

 

Vance, D. and Burton, K. (1999). Neodymium isotopes in planktonic foraminifera: a record of the response of continental weathering and ocean circulation rates to climate change. Earth Planet. Sci. Lett. 173, 365)79.

 

Vance, D. and O’Nions, R. K. (1990). Isotopic chronometry of zoned garnets: growth kinetics and metamorphic histories. Earth Planet. Sci. Lett. 97, 227)40.

VonderHaar, D. L., Mahoney, J. J. and McMurtry, G. M. (1995). An evaluation of strontium isotopic dating of ferromanganese oxides in a marine hydrogeneous ferromanganese crust. Geochim. Cosmochim. Acta 59, 4267)77.

 

Wasserburg, G. J., Jacobsen, S. B., DePaolo, D. J., McCulloch, M. T. and Wen, T. (1981). Precise determination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim. Cosmochim. Acta 45, 2311)23.

 

West, G. F. (1980). Formation of continental crust. In: Strangway, D. W. (Ed.), The Continental Crust and its Mineral Deposits. Geol. Assoc. Canada Spec. Pap. 8, 117)48.

 

Whitehouse, M. J. (1988). Granulite facies Nd-isotopic homogenisation in the Lewisian complex of northwest Scotland. Nature 331, 705)7.

 

Whitehouse, M. J., Kamber, B. S. and Moorbath, S. M. (1999). Age significance of U–Th–Pb zircon data from early Archean rocks of west Greenland– a reassessment based on combined ion-microprobe and imaging studies. Chem. Geol. 160, 201–24.

 

Williams, H. R., Stott, G. M., Thurston, P. C., Sutcliffe, R. H., Bennett, G., Easton, R. M. and Armstrong, D. K. (1992). Tectonic evolution of Ontario: summary and synthesis. In: Thurston, P. C., Williams, H. R., Sutcliffe, R. H. and Stott, G. M. (Eds), Geology of Ontario. Ontario Geol. Surv. Spec. Vol. 4, 1255)1332.

 

Wust, G. (1924). Florida und Antillenstrom. Veroffentl. Inst. Meeresh. Univ. Berlin 12, 1)48.