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Sm-Nd dating of Archaean basic and ultrabasic volcanics
Earth and Planetary Science Letters, 36 (1977) 263-268 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
263
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Sm-Nd DATING OF ARCHAEAN BASIC AND ULTRABASIC VOLCANICS P.J. HAMILTON, R.K. O'NIONS and N.M. EVENSEN Lamont-Doherty Geological Observatory o f Columbia University, Palisades, iV. Y. 10964 (USA)
Received April 14, 1977 Revised version received June 20, 1977
Sm-Nd data for ten greenstone belt volcanics from Rhodesia define an age of 2.64 -+0.14 AE, which is in agreement with other geochronological data and with observed field relationships. This age and the initial 143Nd/144Nd ratio of 0.50919 ± 0.00018 yield a time-integrated Sm/Nd ratio of 0.302 ± 0.009 in the mantle source region, similar to that in chondrites. Sm/Nd ratios of some lavas are >0.31 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.
1. Intoduction Rb-Sr and U-Pb isotopic studies of igneous rocks formed during the last ca. 3.8 AE have been, and continue to be important in establishing a temporal and genetic framework of crustal evolution. The Rb-Sr system has been particularly useful in this respect since it has also been possible to infer the degree o f Rb-Sr fractionation in the mantle from a consideration of initial 87Sr/a6Sr ratios. The utility of the RbSr system is often limited by post-crystallization fractionation of these two elements as a result of alteration and metamorphism. Rare earth element (REE) abundances in altered and metamorphosed basic volcanics are generally less affected than alkali and alka. line-earth element abundances [ 1 - 7 ] suggesting that the Sm-Nd system may offer some advantages over the Rb-Sr system in these cases. The difficulty o f obtaining reliable age and initial ratio data from Rb-Sr systematics of Archaean volcanics is well exemplified by recent work on the greenstone belt volcanics o f Rhodesia (see Fig. 1). Lamont-Doherty Geological Observatory Contribution No. 2534.
Hawkesworth et al. [8] obtained imperfect Rb-Sr isochrons from the Bulawayan Group at the Que Que and Bulawayo areas, whereas the data obtained from less well-preserved samples from the Shabani and Fort Victoria areas failed to yield any useful geochronological information. A selective and not entirely objective evaluation of these and new data by Jahn and Condie [9] produced an older age (3.08 AE) than suggested by Hawkesworth et al. [8]. Clearly the initial 87Sr/a6Sr ratios obtained from such uncertain data cannot be regarded with great confidence. In an attempt to overcome this particular problem, and to evaluate the usefulness of the Sm-Nd system for dating altered and metomorphosed basic volcanics, a SmNd study of these Rhodesian greenstone belt volcanics has been undertaken. Included in the study are some of the most altered samples described previously [8]. The Sm-Nd system has previously been successfully applied to the dating of lunar rocks and basaltic achondrites [ 1 0 - 1 7 ] , and recently the isotopic composition of Nd has been measured in terrestrial samples [ 7 , 1 8 - 2 0 ] . The range of Sm/Nd ratios in basaltic rocks is considerably less than the range o f Rb/Sr ratios. This fact, together with the small decay con-
264 stant of 147Sm CA= 6.54 X 10 -12 y r - l ) , makes highprecision measurements of both 147Sm/144Nd and 143Nd/144Nd a routine requirement.
2. Analytical techniques 200--300-mg powdered samples are decomposed in an HF-NHOa mixture and the residue dissolved in HC1. The Nd-isotope composition is determined on one aliquot of this solution and Sm and Nd concentrations on a second. Sm and Nd are separated using anion exchange procedures which have been described previously [7]. The separation blanks for Sm and Nd are <10 - l ° g. Details of the mass spectrometric techniques used for Ndhsotope composition analyses have been described in some detail previously [7]. In this and previous studies 143Nd/144Nd ratios have been normalized to 146Nd/144Nd --0.7219 which is the average value obtained for the L-D.G.O. NDN-1 standard [7]. A full spectral analysis of BCR-1 is given in Table 1. Sm and Nd concentrations are determined by mass spectrometric isotope dilution using 149Sm and lOaNd tracers calibrated against Johnson-Mathey specpure oxides ignited to constant weight. Nd-mass spectra were measured at masses 142, 143,144 and 146 and Ce and Sm were monitored at masses 140 and 147 respectively. The measured ratios are corrected for mass fractionation. In the case of Sm, masses 147,148 and 149 were measured and again ratios were corrected for mass fractionation. Sm/Nd ratios are determined with a precision of 0.2% (20) or better. The atomic abundance of 147Sm in normal Sm has been measured in a sample of Jolmson-Mathey specpure Sm203 at 15.00%, and this value has been used in this work. All ratios discussed in this paper are atomic ratios.
3. General geology
The Rhodesian craton consists of a granite-gneissgreenstone association (Fig. 1) typical of other Archa. can shield areas. The major greenstone belt successions generally consist of a volcanic sequence (Bulawayan Group) composed of ultrabasic, basic and andesitic volcanics overlain by a predominantly sedimentary sequence [ 2 1 - 2 3 , 2 5 ] . The contacts between the greenstone belts and surrounding granites and gneisses are usually poorly exposed. Whereas it is possible to demonstrate the existence of postgreenstone belt intrusives [8,21-24] the relationships with the gneisses are usually equivocal. One important exception is the Belingwe belt described by Bickle et al. [25], where basal sediments of the greenstone belt succession clearly rest uncomformably on granitic gneiss. Rb-Sr whole rock dating [8,26-28] of both the basement gneisses and the better-preserved greenstone belt volcanics has demonstrated that the basement is ca 3.5 AE old, whereas the greenstone belts are at least 500 m.y. younger. Published and unpublished Rb-Sr data [8,9,28] for green-
TABLE 1 BCR-1 Nd-isotope composition 142/144 143/144 145/144 148/144 150/144
1.14189 ± 8 0.512617 ± 26 0.348407 ± 18 0.241565 ± 16 0.236394 ± 18
Normalised to 146/144 = 0.7219.
Fig. I. Geological sketch map of the Archaean craton of Rhodesia.
265 stone belts at Belingwe, Salisbury, Bulawayo and Que-Que (Fig. 1), suggest a period of formation between 2.5 and 2.7 AE. However, Jahn and Condie [9] report a "preferred" age of 3.08 AE in one instance, but this is based on a somewhat arbitrary selection of data. The timing of greenstone belt formation is constrained by Rb-Sr dates for three other igneous events. These are: (1) Intrusion of the Mashaba tonalite at 2.97 -+ 0.16 AE prior to extrusion of the Fort Victoria greenstone belt volcanics [28]. (2) Northwest of Que Que the Sesombi tonalite intruded the younger rocks of the Bulawayan at 2.69 + 0.14 AE [8]. (3) The emplacement of the Great Dyke postdates the greenstone belts and has been dated by the Rb-Sr method at 2.51 -+ 0.02 AE [29].
4. Results 147Sm/144Nd and 143Nd/144Nd ratios are presented for 10 samples in Table 2. Included in this study are one peridotitic komatiite, two basaltic komatiites and a basaltic andesite from Belingwe, two
tholeiitic greenstones from Fort Victoria, and two tholeiites, two basaltic andesites and two andesites from Que Que (Fig. 1). Major element, trace element and Rb-Sr isotopic data for all of these samples have been published previously [8,30,31]. When plotted on a Sm-Nd evolution diagram (Fig. 2) the data points define a rectilinear array. Using the method of York [32], the best fit line to the data points corresponds to an age of 2.64 -+ 0.14 AE (20) and an initial 143Nd/144Nd ratio of 0.50919 + 0.00018 (2o). As is commonly the case with whole rock studies o f terrestrial rocks, some scatter exists about the bestfit line which is in excess of experimental error. The problem of terminology i.e. isochron or errorchron, in such situations has been discussed at considerable length [40], and will not be repeated here. The fact that the age obtained on the volcanics agrees very well with the predicted age suggests that the age and error quoted are meaningful and refer to the time of the volcanic event. Whether the scatter that exists reflects small differences in initial 143Nd/t44Nd ratio, time of eruption, or minor modification of Sm/Nd ratios by alteration processes cannot be resolved at this time.
TABLE 2 Sm-Nd data Sample and rock type
Sm (ppm)
Fort Victoria Rh-73-134 basaltic greenstone Rh-73-138 basaltic greenstone
1.639 1.241
Belingwe NG-157 peridotitic komatiite Rh-73-69 basaltic greenstone NG-12 basaltic komatiite NG-220 basaltic komatiite Que Que fLower Bulawayan] Rh-73-149 basalt Rh-73-166 basaltic greenstone Que Que (Maliyami Formation} Rh-73-150 basaltic andesite Rh-73-155 basaltic andesite * Atomic ratios.
Nd (ppm)
147Sm/144Nd *
143Nd/144Nd + 2o *
4.720 3.470
0.2088 0.2151
0.512868 ± 40 0.512872 ± 34
0.580 3.671 1.521 1.378
1.539 14.86 5.35 4.946
0.2267 0.1485 0.1710 0.1675
0.513101 ± 30 0.511785 ± 42 0.512104± 24 0.512183 ± 38
2.434 3.140
7.19 10.08
0.2036 0.1873
0.512796 ± 30 0.512426 + 34
2.255 2.522
11.34 12.41
0.1196 0.1222
0.511221 ± 30 0.511352 + 44
266
Fig. 2. Sm-Nd evolution diagram for basic and ultrabasic volcanic rocks of Rhodesian greenstone belts. Data for the two andesites from Que Que have not been plotted (see text).
5. Discussion Probably one of the most significant facts about the whole rock Sm-Nd isochron described above is that it is in part based upon samples from Belingwe and Fort Victoria, which were not amenable to dating by the Rb-Sr whole rock method [8]. The date obtained most probably represents the time of the eruption of the volcanics. It is difficult to envisage how it could reflect a post-eruption alteration event when the samples are so variably altered. 5.1. Implications for terrestrial evolution of Sm/Nd
The 143Nd/‘44Nd ratio of the Earth at the time of its formation is not known exactly, but reasonable estimates can be made from the Sm-Nd data on basaltic achondrites [ 1O-l 31. As mentioned before the 143Nd/‘44Nd data reported here are directly com-
parable with those of Lugmair and co-workers and in this discussion the initial 143Nd/‘44Nd ratio of the Earth is taken as 0.50677, which is the initial ratio for the achondrite Juvinas [13]. If the age of the Earth is taken as 4.55 AE, the time-integrated Sm/Nd ratio of the source region of the Rhodesian volcanics for the period 4.55 to 2.64 AE is 0.302 + 0.009 as measured today. This value is within error of the Sm/Nd ratio of Juvinas (0.308) and ordinary chondrites [33-351. The only published Sm-Nd data for Archaean rocks are those of DePaolo and Wasserburg [ 19,201, who obtained single whole rock analyses of the Great Dyke (2.5 1 AE), the Preissac-Lacorne batholith (2.65 AE), the Louis Lake granodiorite (2.65 AE), the Fiskenaesset anorthosite (2.8 AE) and the Amitsoq gneiss (3.59 AE). These authors demonstrated that the initial 143Nd/‘44 Nd ratios of these samples plot close to a terrestrial growth line for Sm/Nd = 0.308, which is identical to that of Juvinas [ 191.
267
5.2. Coherence of Sm/Nd and Rb/Sr in the mantle Investigations of the Nd- and Sr-isotope compositions in a variety o f recent oceanic basalts [ 7 , 1 8 20] has demonstrated a striking anticorrelation of 87Sr/86Sr and 143Nd/144Nd ratios which has led O'Nions et al. [7] and DePaolo and Wasserburg [20] to independently infer a mean 87Sr/86Sr for the bulk Earth o f ca. 0.705 and a Rb/Sr ratio of ca. 0.03. A bulk Earth with a Rb/Sr of 0.03 which had an initial 87Sr/86Sr = BABI = 0.69898 [36] at 4.55 AE would have a 87Sr/86Sr ratio o f 0.7014 at 2.60 AE ago. This is close to the reported initial 87Sr/a6Sr ratios o f the Rhodesian greenstone belt volcanics [8], and indeed other Archaean volcanics in Ontario and Minnesota [37,38] erupted ca. 2.6 AE ago.
chronological data and probably represents the time of greenstone belt volcanism. (2) No support was found in this study for the existence of a 3.08 AE greenstone belt event as suggested by Jahn and Condie [9]. (3) The time integrated Sm/Nd atomic ratio of the source region for these volcanics is 0.302 -+ 0.009, similar to that in chondrites. (4) The Sm/Nd ratios of some of the volcanics are higher than the time-integrated Sm/Nd o f their source. This suggests loss of a small fraction of the source prior to the main melting event. (5) Many of the rocks used in this study were too altered to yield useful Rb-Sr data. The acquirement of useful Sm-Nd data emphasises the stability o f REE in an environment o f alteration and low-grade metamorphism.
5.3. Constraints on melting processes Some of the samples included in this study, notably from the Belingwe and Fort Victoria belts (NG157, Rh-73-134 and Rh-73-138), have Sm/Nd ratios which are higher than the inferred integrated value for their source in the period 4.55 AE to 2.6 AE (see also Hawkesworth and O'Nions [30], and Bickle et al. [31 ]). The implied light-REE depletion of their source region must have occurred close in time to, or possibly as part of, the melting event itself. The loss of a few percent of melt from the source region of the light-REE-depleted samples (see [7, fig. 5 ] ) w o u l d be sufficient to produce the observed effect. Comparative REE studies by Sun and Nesbitt (unpublished data, quoted in Nesbitt and Sun [39]) of other high-Mg Archaean lavas also indicate some source regions to have undergone such a depletion in light REE.
6. Conclusions The observations and inferences made from the study of the Sm-Nd systematics of Rhodesian greenstone belt volcanics are as follows: (1) t43Nd/144Nd and 147Sm/144Nd ratios of ten whole rocks yield a rectilinear array on a Sm-Nd evolution diagram. A best fit line to these data corresponds to an age of 2.64 -+ 0.14 AE and an initial 143Nd]144Nd ratio of 0.50919 -+ 0.00018. The age is in agreement with field evidence and other geo-
Acknowledgements This work has been supported by NSF grants EAR-75-20840 and EAR-76-03802. Genevieve Wesselman is thanked for skilled technical help. Dr. J. Wilson and reviewers of the journal are thanked for their helpful comments.
References 1 A.G. Herrman, M.J. Potts and D. Knake, Geochemistry of the rare earth elements in spilites from the oceanic and continental crust, Contrib. Mineral. Petrol. 44 (1974) 1. 2 R. Kay, N.J. Hubbard and P.W. Gast, Chemical charactedstics and origins of oceanic ridge volcanics, J. Geophys. Res. 75 (1970) 1585. 3 F.A. Frey, M.A. Haskin, J.A. Poetz and L.A. Haskin, Rare earth abundances in some basic rocks, J. Geophys. Res. 73 (1968) 6085. 4 J.D. Sinewing and P.J. Potts, Rare earth abundances in basalts and metabasalts from the Troodos Massif, Cyprus, Contrib. Mineral. Petrol. 57 (1976) 245. 5 R.K. O'Nions and R.J.Pankhurst, Sr isotope and rare earth element geochemistry of DSDP Leg 37 basalts, Earth Planet. Sci. Lett. 31 (1976) 255. 7 R.K. O'Nions, P.J. Hamilton and N.M. Evensen, Variations in 143Nd/144Nd and 87Sr/86Sr ratios in oceanic basalts, Earth Planet. Sci. Lett. 34 (1977) 13. 8 C.J. Hawkesworth, S. Moorbath, R.K. O'Nions and J.F. Wilson, Age relationships between greenstone belts and "granites" in the Rhodesian Archaean craton, Earth Planet. Sci. Lett. 25 (1975) 251.
268 9 B.-M. Jahn and K.C. Condie, On the age of the Rhodesian grcenstone belts, Contrib. Mineral. Petrol. 57 (1976) 317. 10 K. Notsu, H. Mabuchi, O. Yoshioka, J. Matsuda and M. Ozima, Evidence of the extinct nuclide 146Sm in the Juvinas achondrite, Earth Planet. Sci. Lett. 19 (1973) 29. 11 N. Nakamura, M. Tatsumoto and D.M. Unruh, Rb-Sr, Sm-Nd and U-Th-Pb systematics of the Pasamonte meteorite, Meteorit. Soc. 39th Annu. Meet. (1976) 125. 12 G.W. Lugmair and N.B. Scheinin, Sm-Nd systematics of Angra dos Reis, Meteorit. Soc. 39th Annu. Meet. (1976) 108. 13 G.W. Lugmair, K. Marti, J.P. Kurtz and N.B. Scheinin, History and genesis of lunar troctolite 76535, or: how old is old?, Proc. 7th Lunar Sci. Conf. (1976) 2009. 14 G.W. Lugmair, N.B. Scheinin and K. Marti, Sm-Nd age of Apollo 17 basalt 75075: two-stage igneous processes in mare basalt genesis, 6th Lunar Sci. Conf. Abstr. (1975) 531. 15 G.W. Lugmair, N.B. Scheinin and K. Marti, Search for extinct 146Sm, 1. The isotopic abundance of 142Nd in the Juvinas meteorite, Earth Planet. Sci. Lett. 27 (1975) 79. 16 G.W. Lugmair and K. Marti, Evolution of the lunar interior: Sm-Nd systematics of A15 green glass and the question of lunar initial 143Nd/144Nd, Lunar Sci. Conf. Abstr., (1977) 597. 17 D.A. Papanastassiou, D.J. DePaolo, F. Tera and G.J. Wasserburg, An isotopic triptych on mare basalts: Rb-Sr, Sm-Nd, U-Pb, 8th Lunar Sci. Conf. Abstr. (1977) 750. 18 P. Richard, N. Shimizu and C.J. All~gre, 143Nd/146Nd, a natural tracer: an application to oceanic basalts, Earth Planet. Sci. Lett. 31 (1976) 269. 19 D.J. DePaolo and G.J. Wasserburg, Nd isotopic variations and petrogenetic models, Geophy s. Res. Lett. 3 (1976) 249. 20 D.J. DePaolo and G.J. Wasserburg, Inferences about magma sources and mantle structure from variations of 143Nd/144Nd, Geophys. Res. Lett. 3 (1976) 743. 21 J.F. Wilson, The Rhodesian Archaean craton - an essay in eratonic evolution, Philos. Trans. R. Soc. Lond., Ser. A, 273 (1973) 389. 22 N.M. Harrison, The geology of the country around Que Que, Bull. Geol. Surv. Rhodesia 67 (1970) 125 pp. 23 C.W. Stowe, Summary of the tectonics of the Rhodesian Archaean craton, Spec. Publ. Geol. Soc. Aust. 3 (1971) 377. 24 J.F. Wilson, Granites and gneisses of the area around Mashaba, Rhodesia, Spec. Publ. Geol. Soc. S. Afr. 3 (1973) 79.
25 M.J. Bickle, A. Martin and E.G. Nesbet, Basaltic and peridotitic komatiites and stromatolites above a basal unconformity in the Belingwe greenstone belt, Rhodesia, Earth Planet. Sci. Lett. 27 (1975) 155. 26 M.H. Hickman, 3500 m.y. old granite in Southern Africa, Nature 251 (1974) 296. 27 S. Moorbath, J.F. Wilson and P. CotteriU, Early Archaean age for the Sebakwian Group at Selukwe, Rhodesia, Nature 264 (1976) 536. 28 C.J. Hawkesworth and M.J. Bickle, Rhodesian Rb/Sr geochronology from 3.6-2.0 b.a.: a brief review, Annu. Res. Rep., Dept. Earth Sci., Univ. Leeds (1977) preprint. 29 P.J. Hamilton, Sr isotope and trace element studies of the Great Dyke and Bushveld Mafic Phase and their relation to early Proterozoie magma genesis in Southern Africa, J. Petrol. 18 (1977) 24. 30 CA. Hawkesworth and R.K. O'Nions, The petrogenesis of some.Archaean volcanic rocks from Southern Africa, J. Petrol. (1977) in press. 31 M.J. Bickle, C.J. Hawkesworth, A. Martin, E.G. Nesbet and R.K. O'Nions, Mantle composition derived from the chemistry of ultramafic lavas, Nature 263 (1976) 577. 32 D. York, Least squares fitting of a straight line, Can. J. Phys. 44 (1966) 1079. 33 Lamont-Doherty Geological Observatory, unpublished data. 34 A. Masuda, N. Nakamura and T. Tanaka, Fine structures of mutually normalized rare earth patterns of chondrites, Geochim. Cosmochim. Acta 37 (1973) 239. 35 N. Nakamura, Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites, Geochim. Cosmoehim. Acta 38 91974) 757. 36 D.A. Papanastassiou and G.J. Wasserhurg, Initial Sr isotopic abundances and the resolution of small time differences in the formation of planetary objects, Earth Planet. Sci. Lett. 5 (1969) 361. 37 S.R. Hart and C. Brooks, The geochemistry of the early Precambrian mantle, Carnegie Inst. Washington Yearb. 73 (1974) 967. 38 B.-M. Jahn and V.R. Murthy, Rb-Sr ages of the Archaean rocks from the Vermillion district of NE Minnesota, Geochim. Cosmochim. Acta 39 (1975) 1679. 39 R.W. Nesbitt and S.-S. Sun, Geochemistry of Archaean spinifex-textured periodotites and magnesian and lowmagnesian tholeiites, Earth Planet. Sci. Lett. 31 (1976) 433. 40 C. Brooks, S. Hart and I. Wendt, Realistic use of error regression treatments as applied to Rb-Sr data, Rev. Geophys. Space Phys. 10 (1972) 551.
263
[41
Sm-Nd DATING OF ARCHAEAN BASIC AND ULTRABASIC VOLCANICS P.J. HAMILTON, R.K. O'NIONS and N.M. EVENSEN Lamont-Doherty Geological Observatory o f Columbia University, Palisades, iV. Y. 10964 (USA)
Received April 14, 1977 Revised version received June 20, 1977
Sm-Nd data for ten greenstone belt volcanics from Rhodesia define an age of 2.64 -+0.14 AE, which is in agreement with other geochronological data and with observed field relationships. This age and the initial 143Nd/144Nd ratio of 0.50919 ± 0.00018 yield a time-integrated Sm/Nd ratio of 0.302 ± 0.009 in the mantle source region, similar to that in chondrites. Sm/Nd ratios of some lavas are >0.31 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.
1. Intoduction Rb-Sr and U-Pb isotopic studies of igneous rocks formed during the last ca. 3.8 AE have been, and continue to be important in establishing a temporal and genetic framework of crustal evolution. The Rb-Sr system has been particularly useful in this respect since it has also been possible to infer the degree o f Rb-Sr fractionation in the mantle from a consideration of initial 87Sr/a6Sr ratios. The utility of the RbSr system is often limited by post-crystallization fractionation of these two elements as a result of alteration and metamorphism. Rare earth element (REE) abundances in altered and metamorphosed basic volcanics are generally less affected than alkali and alka. line-earth element abundances [ 1 - 7 ] suggesting that the Sm-Nd system may offer some advantages over the Rb-Sr system in these cases. The difficulty o f obtaining reliable age and initial ratio data from Rb-Sr systematics of Archaean volcanics is well exemplified by recent work on the greenstone belt volcanics o f Rhodesia (see Fig. 1). Lamont-Doherty Geological Observatory Contribution No. 2534.
Hawkesworth et al. [8] obtained imperfect Rb-Sr isochrons from the Bulawayan Group at the Que Que and Bulawayo areas, whereas the data obtained from less well-preserved samples from the Shabani and Fort Victoria areas failed to yield any useful geochronological information. A selective and not entirely objective evaluation of these and new data by Jahn and Condie [9] produced an older age (3.08 AE) than suggested by Hawkesworth et al. [8]. Clearly the initial 87Sr/a6Sr ratios obtained from such uncertain data cannot be regarded with great confidence. In an attempt to overcome this particular problem, and to evaluate the usefulness of the Sm-Nd system for dating altered and metomorphosed basic volcanics, a SmNd study of these Rhodesian greenstone belt volcanics has been undertaken. Included in the study are some of the most altered samples described previously [8]. The Sm-Nd system has previously been successfully applied to the dating of lunar rocks and basaltic achondrites [ 1 0 - 1 7 ] , and recently the isotopic composition of Nd has been measured in terrestrial samples [ 7 , 1 8 - 2 0 ] . The range of Sm/Nd ratios in basaltic rocks is considerably less than the range o f Rb/Sr ratios. This fact, together with the small decay con-
264 stant of 147Sm CA= 6.54 X 10 -12 y r - l ) , makes highprecision measurements of both 147Sm/144Nd and 143Nd/144Nd a routine requirement.
2. Analytical techniques 200--300-mg powdered samples are decomposed in an HF-NHOa mixture and the residue dissolved in HC1. The Nd-isotope composition is determined on one aliquot of this solution and Sm and Nd concentrations on a second. Sm and Nd are separated using anion exchange procedures which have been described previously [7]. The separation blanks for Sm and Nd are <10 - l ° g. Details of the mass spectrometric techniques used for Ndhsotope composition analyses have been described in some detail previously [7]. In this and previous studies 143Nd/144Nd ratios have been normalized to 146Nd/144Nd --0.7219 which is the average value obtained for the L-D.G.O. NDN-1 standard [7]. A full spectral analysis of BCR-1 is given in Table 1. Sm and Nd concentrations are determined by mass spectrometric isotope dilution using 149Sm and lOaNd tracers calibrated against Johnson-Mathey specpure oxides ignited to constant weight. Nd-mass spectra were measured at masses 142, 143,144 and 146 and Ce and Sm were monitored at masses 140 and 147 respectively. The measured ratios are corrected for mass fractionation. In the case of Sm, masses 147,148 and 149 were measured and again ratios were corrected for mass fractionation. Sm/Nd ratios are determined with a precision of 0.2% (20) or better. The atomic abundance of 147Sm in normal Sm has been measured in a sample of Jolmson-Mathey specpure Sm203 at 15.00%, and this value has been used in this work. All ratios discussed in this paper are atomic ratios.
3. General geology
The Rhodesian craton consists of a granite-gneissgreenstone association (Fig. 1) typical of other Archa. can shield areas. The major greenstone belt successions generally consist of a volcanic sequence (Bulawayan Group) composed of ultrabasic, basic and andesitic volcanics overlain by a predominantly sedimentary sequence [ 2 1 - 2 3 , 2 5 ] . The contacts between the greenstone belts and surrounding granites and gneisses are usually poorly exposed. Whereas it is possible to demonstrate the existence of postgreenstone belt intrusives [8,21-24] the relationships with the gneisses are usually equivocal. One important exception is the Belingwe belt described by Bickle et al. [25], where basal sediments of the greenstone belt succession clearly rest uncomformably on granitic gneiss. Rb-Sr whole rock dating [8,26-28] of both the basement gneisses and the better-preserved greenstone belt volcanics has demonstrated that the basement is ca 3.5 AE old, whereas the greenstone belts are at least 500 m.y. younger. Published and unpublished Rb-Sr data [8,9,28] for green-
TABLE 1 BCR-1 Nd-isotope composition 142/144 143/144 145/144 148/144 150/144
1.14189 ± 8 0.512617 ± 26 0.348407 ± 18 0.241565 ± 16 0.236394 ± 18
Normalised to 146/144 = 0.7219.
Fig. I. Geological sketch map of the Archaean craton of Rhodesia.
265 stone belts at Belingwe, Salisbury, Bulawayo and Que-Que (Fig. 1), suggest a period of formation between 2.5 and 2.7 AE. However, Jahn and Condie [9] report a "preferred" age of 3.08 AE in one instance, but this is based on a somewhat arbitrary selection of data. The timing of greenstone belt formation is constrained by Rb-Sr dates for three other igneous events. These are: (1) Intrusion of the Mashaba tonalite at 2.97 -+ 0.16 AE prior to extrusion of the Fort Victoria greenstone belt volcanics [28]. (2) Northwest of Que Que the Sesombi tonalite intruded the younger rocks of the Bulawayan at 2.69 + 0.14 AE [8]. (3) The emplacement of the Great Dyke postdates the greenstone belts and has been dated by the Rb-Sr method at 2.51 -+ 0.02 AE [29].
4. Results 147Sm/144Nd and 143Nd/144Nd ratios are presented for 10 samples in Table 2. Included in this study are one peridotitic komatiite, two basaltic komatiites and a basaltic andesite from Belingwe, two
tholeiitic greenstones from Fort Victoria, and two tholeiites, two basaltic andesites and two andesites from Que Que (Fig. 1). Major element, trace element and Rb-Sr isotopic data for all of these samples have been published previously [8,30,31]. When plotted on a Sm-Nd evolution diagram (Fig. 2) the data points define a rectilinear array. Using the method of York [32], the best fit line to the data points corresponds to an age of 2.64 -+ 0.14 AE (20) and an initial 143Nd/144Nd ratio of 0.50919 + 0.00018 (2o). As is commonly the case with whole rock studies o f terrestrial rocks, some scatter exists about the bestfit line which is in excess of experimental error. The problem of terminology i.e. isochron or errorchron, in such situations has been discussed at considerable length [40], and will not be repeated here. The fact that the age obtained on the volcanics agrees very well with the predicted age suggests that the age and error quoted are meaningful and refer to the time of the volcanic event. Whether the scatter that exists reflects small differences in initial 143Nd/t44Nd ratio, time of eruption, or minor modification of Sm/Nd ratios by alteration processes cannot be resolved at this time.
TABLE 2 Sm-Nd data Sample and rock type
Sm (ppm)
Fort Victoria Rh-73-134 basaltic greenstone Rh-73-138 basaltic greenstone
1.639 1.241
Belingwe NG-157 peridotitic komatiite Rh-73-69 basaltic greenstone NG-12 basaltic komatiite NG-220 basaltic komatiite Que Que fLower Bulawayan] Rh-73-149 basalt Rh-73-166 basaltic greenstone Que Que (Maliyami Formation} Rh-73-150 basaltic andesite Rh-73-155 basaltic andesite * Atomic ratios.
Nd (ppm)
147Sm/144Nd *
143Nd/144Nd + 2o *
4.720 3.470
0.2088 0.2151
0.512868 ± 40 0.512872 ± 34
0.580 3.671 1.521 1.378
1.539 14.86 5.35 4.946
0.2267 0.1485 0.1710 0.1675
0.513101 ± 30 0.511785 ± 42 0.512104± 24 0.512183 ± 38
2.434 3.140
7.19 10.08
0.2036 0.1873
0.512796 ± 30 0.512426 + 34
2.255 2.522
11.34 12.41
0.1196 0.1222
0.511221 ± 30 0.511352 + 44
266
Fig. 2. Sm-Nd evolution diagram for basic and ultrabasic volcanic rocks of Rhodesian greenstone belts. Data for the two andesites from Que Que have not been plotted (see text).
5. Discussion Probably one of the most significant facts about the whole rock Sm-Nd isochron described above is that it is in part based upon samples from Belingwe and Fort Victoria, which were not amenable to dating by the Rb-Sr whole rock method [8]. The date obtained most probably represents the time of the eruption of the volcanics. It is difficult to envisage how it could reflect a post-eruption alteration event when the samples are so variably altered. 5.1. Implications for terrestrial evolution of Sm/Nd
The 143Nd/‘44Nd ratio of the Earth at the time of its formation is not known exactly, but reasonable estimates can be made from the Sm-Nd data on basaltic achondrites [ 1O-l 31. As mentioned before the 143Nd/‘44Nd data reported here are directly com-
parable with those of Lugmair and co-workers and in this discussion the initial 143Nd/‘44Nd ratio of the Earth is taken as 0.50677, which is the initial ratio for the achondrite Juvinas [13]. If the age of the Earth is taken as 4.55 AE, the time-integrated Sm/Nd ratio of the source region of the Rhodesian volcanics for the period 4.55 to 2.64 AE is 0.302 + 0.009 as measured today. This value is within error of the Sm/Nd ratio of Juvinas (0.308) and ordinary chondrites [33-351. The only published Sm-Nd data for Archaean rocks are those of DePaolo and Wasserburg [ 19,201, who obtained single whole rock analyses of the Great Dyke (2.5 1 AE), the Preissac-Lacorne batholith (2.65 AE), the Louis Lake granodiorite (2.65 AE), the Fiskenaesset anorthosite (2.8 AE) and the Amitsoq gneiss (3.59 AE). These authors demonstrated that the initial 143Nd/‘44 Nd ratios of these samples plot close to a terrestrial growth line for Sm/Nd = 0.308, which is identical to that of Juvinas [ 191.
267
5.2. Coherence of Sm/Nd and Rb/Sr in the mantle Investigations of the Nd- and Sr-isotope compositions in a variety o f recent oceanic basalts [ 7 , 1 8 20] has demonstrated a striking anticorrelation of 87Sr/86Sr and 143Nd/144Nd ratios which has led O'Nions et al. [7] and DePaolo and Wasserburg [20] to independently infer a mean 87Sr/86Sr for the bulk Earth o f ca. 0.705 and a Rb/Sr ratio of ca. 0.03. A bulk Earth with a Rb/Sr of 0.03 which had an initial 87Sr/86Sr = BABI = 0.69898 [36] at 4.55 AE would have a 87Sr/86Sr ratio o f 0.7014 at 2.60 AE ago. This is close to the reported initial 87Sr/a6Sr ratios o f the Rhodesian greenstone belt volcanics [8], and indeed other Archaean volcanics in Ontario and Minnesota [37,38] erupted ca. 2.6 AE ago.
chronological data and probably represents the time of greenstone belt volcanism. (2) No support was found in this study for the existence of a 3.08 AE greenstone belt event as suggested by Jahn and Condie [9]. (3) The time integrated Sm/Nd atomic ratio of the source region for these volcanics is 0.302 -+ 0.009, similar to that in chondrites. (4) The Sm/Nd ratios of some of the volcanics are higher than the time-integrated Sm/Nd o f their source. This suggests loss of a small fraction of the source prior to the main melting event. (5) Many of the rocks used in this study were too altered to yield useful Rb-Sr data. The acquirement of useful Sm-Nd data emphasises the stability o f REE in an environment o f alteration and low-grade metamorphism.
5.3. Constraints on melting processes Some of the samples included in this study, notably from the Belingwe and Fort Victoria belts (NG157, Rh-73-134 and Rh-73-138), have Sm/Nd ratios which are higher than the inferred integrated value for their source in the period 4.55 AE to 2.6 AE (see also Hawkesworth and O'Nions [30], and Bickle et al. [31 ]). The implied light-REE depletion of their source region must have occurred close in time to, or possibly as part of, the melting event itself. The loss of a few percent of melt from the source region of the light-REE-depleted samples (see [7, fig. 5 ] ) w o u l d be sufficient to produce the observed effect. Comparative REE studies by Sun and Nesbitt (unpublished data, quoted in Nesbitt and Sun [39]) of other high-Mg Archaean lavas also indicate some source regions to have undergone such a depletion in light REE.
6. Conclusions The observations and inferences made from the study of the Sm-Nd systematics of Rhodesian greenstone belt volcanics are as follows: (1) t43Nd/144Nd and 147Sm/144Nd ratios of ten whole rocks yield a rectilinear array on a Sm-Nd evolution diagram. A best fit line to these data corresponds to an age of 2.64 -+ 0.14 AE and an initial 143Nd]144Nd ratio of 0.50919 -+ 0.00018. The age is in agreement with field evidence and other geo-
Acknowledgements This work has been supported by NSF grants EAR-75-20840 and EAR-76-03802. Genevieve Wesselman is thanked for skilled technical help. Dr. J. Wilson and reviewers of the journal are thanked for their helpful comments.
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