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The age of the Stillwater complex—a comparison of U-Pb zircon and Sm-Nd isochron systematics
0016-7037 KI 101%I-03102.00~0 Copyrlphr CQ 19X1 Pergamon Press Ltd
LETTER TO THE EDITOR
The age of the Stillwater complex-a comparison of U-Pb zircon and Sm-Nd isochron systematics P. D. NUNES Jack Satterly Geochronology Laboratory. Department of Mineralogy and Geology. Royal Ontario Museum, Toronto. Ontario. Canada M5S 2C6 (jPecei& 23
February
1981; accepted in re~ise~,~orrn 22 April 1981)
zircon U-Pb age of 2713 + 3 Myr confirms the less precise age of about 2710 Myr (corrected for new decay constants of JAFFEY ef al., 1971)obtained from the same sample of zircon from the Stillwater complex chill zone (NAVESand TILTON. 1971).This age compares with Sm-Nd mineral and whole-rock isochron ages of 2701 f 8 Myr [DEPAOLO and WASSERBURG, 1979): or 2706 & 8 Myr if the LLIGMAIR ef al. (1975) normalization procedure is used. The agreement of these ages to within about 0.4’” indicates that the ij4, = 0.00654 x lo-’ yr- ’ value is less than 0.8% too large relative to the U decay constants determined by JAFFEY er al. (1971) and enables more correct geological syntheses to be made when working out detailed absolute age stratigraphies using precise data from both systems. Abstract-A
tNTRODUCTlON
IT IS currently possible in geological@ favorable cases, to obtain very precise ages with un~~inties of less than f 0.35?; using both the Sm-Nd isochron and U-Pb zircon methods. Direct determinations of the ““Sm half life (WRIGHT er al. 1961; D~NHOFFER, 1963; VALLI er al.. 1965) resulted in uncertainties of about k 20b. A subsequent more precise determination of the i47Sm isotopic abundance (Russ er al., 1971) enables a more precise &,, to be calculated. Hence the i.l,- = 0.00654 x lo-‘yr-’ used by DEPAOLO and WASSERBURG(1979) is correct to about i 0.9”, (DEPACXO, written communication, 1981) which causes, for example, an age accuracy uncertainty of & 24 Myr for a 2700 Myr old rock. In contrast to this the 23*U and 235U decay constants are correct to 0.22 and 0.280, respectively (ACT,)(JAFFEY et al.. 1971) which, for example. yields an age accuracy uncertainty of about + 7 Myr for a 2700Myr old rock. Clearly a more precise determination of the 147Sm decay constant would allow more meaningful comparisons of U-Pb zircon and Sm-Nd isochron age determinations which, in turn, would enhance our abilities to chronologically document detailed geological histories. Precise Sm-Nd and U-Pb zircon age determinations on the same igneous rock would enable the *“Sm decay constant to be geologically determined more precisely. The very precise Sm-Nd isochron reported for the Stillwater Complex by DEPAOLO and WASSERBURG (1979) (2701 t_ 8 Myr using ;.1d7 = 0.00654 x lo- 9 yr) and the availability of zircons from the StillI,.< A 45 10
I‘
water chili zone originally analysed by NUNIB and (1971), prompted a more precise documentation of the sircon U-Pb age of the comptex in order to compare the results of the U-Pb and Sm-Nd decay systems. Prior to this work. the best available data for comparison of the two decay systems were obtained on the achondrite Angra dos Reis. An internal Sm-Nd isochron age of 4.55 2 0.04 Myr (LUGMAIR and MARTI, 1977) agrees within the precision uncertainties with 207Pb/206Pb whole-rock model ages of 4.555 + 0.005 Myr (TATS~MOTOet nl., 1973) and 4.544 f 0.002 Myr (ADORABLE%1976). These data indicate that ages obtained with both decay systems agree to within about _t 1.0%. Inherent in this comparison, however. is the assumption that the Canyon Diablo troiiite formed at the same time as did Angra dos Reis. This is apparently an excellent ~sumption considering the agreement of the age data-but an assumption nevertheless, and one not used in the zircon data.
TILTON
RESULTS AND DiSCUSSION Four new zircon analyses of Stillwater chill zone sample A-29-3 are presented in Table 1 and Fig. 1. The four subpopuiations were obtained from the least magnetic (noR-magnetic at 0” of tilt on the Frantz isodynamic separator) zircons originally analysed by NUNE~ and TILTON (1971). The original analyses of this material and a more magnetic subpopulation are also plotted in Fig. 1. The uncertainties plotted in Fig. 1
1961
1962
Letter to the Editor are the conservative ‘maximum precision error’ estimates for those data and propagated error uncertainties documented in NUNE~ and THURSTON(1980) for data presented in this report. Zircon subpopulations were prepared from the non-magnetic at 0” split using KROGH’S(1980) abrasion and paramagnetic techniques. For these zircons the abrading process did not significantly alter the UjPb ratios. The paramagnetic separation did, on the other hand, yield subpopulations with a significant variation in degree of discordance, although not in the manner expected (i.e. the least magnetic material. in this case, was more discordant). In any case, three of the four new analyses lie within error of a discordia line (probability of fit = 650,o) with upper and lower intercept ages of 2713 k 3 Myr and 211 + 134Myr. The age uncertainties resulting from this regression treatment are at the 95% confidence limit and assume that the data are precisely collinear within analytical error (LUDWIG, 1980). The fourth new analysis lies barely outside of analytical error off this line. Despite this departure from ideal two-stage U/Pb development, other best-fit lines obtained by regressing various combinations of all the Santa Barbara and/or the Royal Ontario Museum. Toronto data. have upper concordia intercept ages which range from 2709 to 2714 Myrreasonably consistent with the 2713 + 3 Myr age and uncertainty obtained from the three analytically collinear R.O.M., Toronto analyses. The U-Pb zircon age of 2713 fi 3 Myr is 0.44 + 0.32% higher than the Sm-Nd age of 2701 + 8 Myr. These uncertainties are precision errors at the 20 level. Hence the discrepancy between the U and Sm decay constants appears to be about 0.49; (< 0.8%). The cause of the discrepancy between these two age determinations may be owing to one or more of the following: (1) Incorrect values of the decay constants of 14’Sm (most uncertain), 23aU. and 2351_J. (2) Geological uncertainties regarding the timing of closure of these two different radiometric age-dating systems. (3) An age error in the Sm-Nd results owing to uncertainties in the Nd normalization procedure. If, for example, Nd ratios are obtained by normalizing to values used by LUGMAIRet al. (1975), the Stillwater complex age determination of DEPAOLO and WaBURG (1979) increases from 2701 + 8 Myr to 2706 f 8 Myr (DEPAOLO, written communication, 1981). Using this higher value, the difference between the U-Pb zircon age and Sm-Nd isochron age of the Stiilwater complex decreases to 0.25o/,well within the 2a precision errors. This further implies a discrepancy between the U and Sm decay constants of c 0.6%. Indeed one may use this age comparison to argue that it is more appropriate to use the LUGMAIRet al. (1975) normalization procedure for calculating Sm-Nd ages.
Letter to the Editor
1963
Fig. 1. Concordia diagram with zircon analyses from the Stillwater chill zone. Large error bars are ‘maximum precision error’ estimates of data obtained at Santa Barbara (NUNESand TILTON.1971). New R.O.M.. Toronto analyses have smaller error envelopes (Table 1).
Acknou\lrdgemenrs-This paper is dedicated to the late LUDWIGK. R. (1980) Calculation of uncertainties of U-Pb E. D. JACK~C~N who selected sample A-29-3. data. Earrh Planet. Sci. Lett. 46. 212-220. I thank T. E. KR~GH for sharing his sample preparation LUGMAIRG. W. and MARn K. (1977) Sm-Nd-Pu Timeinnovations. Important support from R. BARLOW.Ontario pieces in the Angra DOS Reis meteorite. Earth Planet. Geological Survey, J. HODGKIN and B. PODSTAWSKYJ, SC;. Left. 35, 273-284. Royal Ontario Museum, and I. DAVIES,University of TorLUGMAIRG. W., SCHEININN. B. and Mmn K. (1975) onto, is gratefully acknowledged. Thanks to G. NELSONfor Sm-Nd age and history of Apollo 17 basalt 75075: Evihelp with manuscript preparation. dence for early differentition of the lunar interior. Proc. This research was funded by the Ontario Geological 6th Lunar Planet. Sci. ConJ Vol. 2, pp. 1419-1429. Survey. NUNESP. D. and THUR~TON P. C. (1980) Two hundred and I thank F. TERA. P. C. THURSTONand. especially. D. J. twenty million years of Archean evolution: a zircon DEPAOLOand G. R. TILTONfor their very helpful reviews. U-Pb age stratigraphic study of the Uchi-Confederation Lakes greenstone belt, northwestern Ontario. Can. J. Earrh Sci. 17, 710-721. REFERENCES NUNESP. D. and TILTONG. R. (1971) Uranium-lead ages ADORABLES (1976) Progress by the consorts of Angra DOS of minerals from the Stillwater igneous complex and associated rocks. Montana. Geol. Sot. Am. Bull. 82, Reis. In Lunar Science VII, p. 443. Lunar Sci. Inst. 223 l-2250. DEPAOLOD. J. and WASSE~BURGG. J. (1979) Sm-Nd age of the Stillwater complex and the mantle evolution Russ G. P.. BURNETTD. S.. LINGENFELTER R. E. and WAScurve for neodymium. Geochim. Cosmochim. Acta 43, SERBURG G. J. (1971) Neutron canture on ‘49Sm in lunar samples. Earth Planer. Sci. Lert.*l3, 53-60. 999-1008. DONHOFFERD. (1963) Determination of the half-lives of TATSUMOTO M., KNIGHT R. J. and ALLEGREC. J. (1973) radioactive nuchdes i4’Sm and 1’6Lu occurring in Time differences in the formation of meteorites as deternature by means of liquid scintillators. Nucl. Phgs. 50, mined from the ratio of lead-207 to lead-206. Science 489-496. 180, 1279-1283. JAFFEYA. H., FLYNN K. F.. GLENDENINL. E., BENTLEY VALLI K.. AALTONENJ., GREFFE G., NURMIA M. and W. C. and ES~LINGA. M. (1971) Precision measurements POYHONENR. (1965) Half-life of “‘Sm: comparison of of half-lives and specific activities of 235U and 138U. ionization-chamber and liquid-scintillation results. Ann. Phys. Ret;. C4, 1889-1906. Ad. Sci. Fennicae Ser. A VI, 177-197. KR~GH T. E. (1980) Reduction of zircon age discordance WRIGHT P. M.. STEINBERGE. P. and GLENDENINL. E. by high gradient separation and air abrasion technique (1961) Half-life of samarium-147. Phvs. Ret. 123, (abstr.) Eos. 61, 399. 205-208.
LETTER TO THE EDITOR
The age of the Stillwater complex-a comparison of U-Pb zircon and Sm-Nd isochron systematics P. D. NUNES Jack Satterly Geochronology Laboratory. Department of Mineralogy and Geology. Royal Ontario Museum, Toronto. Ontario. Canada M5S 2C6 (jPecei& 23
February
1981; accepted in re~ise~,~orrn 22 April 1981)
zircon U-Pb age of 2713 + 3 Myr confirms the less precise age of about 2710 Myr (corrected for new decay constants of JAFFEY ef al., 1971)obtained from the same sample of zircon from the Stillwater complex chill zone (NAVESand TILTON. 1971).This age compares with Sm-Nd mineral and whole-rock isochron ages of 2701 f 8 Myr [DEPAOLO and WASSERBURG, 1979): or 2706 & 8 Myr if the LLIGMAIR ef al. (1975) normalization procedure is used. The agreement of these ages to within about 0.4’” indicates that the ij4, = 0.00654 x lo-’ yr- ’ value is less than 0.8% too large relative to the U decay constants determined by JAFFEY er al. (1971) and enables more correct geological syntheses to be made when working out detailed absolute age stratigraphies using precise data from both systems. Abstract-A
tNTRODUCTlON
IT IS currently possible in geological@ favorable cases, to obtain very precise ages with un~~inties of less than f 0.35?; using both the Sm-Nd isochron and U-Pb zircon methods. Direct determinations of the ““Sm half life (WRIGHT er al. 1961; D~NHOFFER, 1963; VALLI er al.. 1965) resulted in uncertainties of about k 20b. A subsequent more precise determination of the i47Sm isotopic abundance (Russ er al., 1971) enables a more precise &,, to be calculated. Hence the i.l,- = 0.00654 x lo-‘yr-’ used by DEPAOLO and WASSERBURG(1979) is correct to about i 0.9”, (DEPACXO, written communication, 1981) which causes, for example, an age accuracy uncertainty of & 24 Myr for a 2700 Myr old rock. In contrast to this the 23*U and 235U decay constants are correct to 0.22 and 0.280, respectively (ACT,)(JAFFEY et al.. 1971) which, for example. yields an age accuracy uncertainty of about + 7 Myr for a 2700Myr old rock. Clearly a more precise determination of the 147Sm decay constant would allow more meaningful comparisons of U-Pb zircon and Sm-Nd isochron age determinations which, in turn, would enhance our abilities to chronologically document detailed geological histories. Precise Sm-Nd and U-Pb zircon age determinations on the same igneous rock would enable the *“Sm decay constant to be geologically determined more precisely. The very precise Sm-Nd isochron reported for the Stillwater Complex by DEPAOLO and WASSERBURG (1979) (2701 t_ 8 Myr using ;.1d7 = 0.00654 x lo- 9 yr) and the availability of zircons from the StillI,.< A 45 10
I‘
water chili zone originally analysed by NUNIB and (1971), prompted a more precise documentation of the sircon U-Pb age of the comptex in order to compare the results of the U-Pb and Sm-Nd decay systems. Prior to this work. the best available data for comparison of the two decay systems were obtained on the achondrite Angra dos Reis. An internal Sm-Nd isochron age of 4.55 2 0.04 Myr (LUGMAIR and MARTI, 1977) agrees within the precision uncertainties with 207Pb/206Pb whole-rock model ages of 4.555 + 0.005 Myr (TATS~MOTOet nl., 1973) and 4.544 f 0.002 Myr (ADORABLE%1976). These data indicate that ages obtained with both decay systems agree to within about _t 1.0%. Inherent in this comparison, however. is the assumption that the Canyon Diablo troiiite formed at the same time as did Angra dos Reis. This is apparently an excellent ~sumption considering the agreement of the age data-but an assumption nevertheless, and one not used in the zircon data.
TILTON
RESULTS AND DiSCUSSION Four new zircon analyses of Stillwater chill zone sample A-29-3 are presented in Table 1 and Fig. 1. The four subpopuiations were obtained from the least magnetic (noR-magnetic at 0” of tilt on the Frantz isodynamic separator) zircons originally analysed by NUNE~ and TILTON (1971). The original analyses of this material and a more magnetic subpopulation are also plotted in Fig. 1. The uncertainties plotted in Fig. 1
1961
1962
Letter to the Editor are the conservative ‘maximum precision error’ estimates for those data and propagated error uncertainties documented in NUNE~ and THURSTON(1980) for data presented in this report. Zircon subpopulations were prepared from the non-magnetic at 0” split using KROGH’S(1980) abrasion and paramagnetic techniques. For these zircons the abrading process did not significantly alter the UjPb ratios. The paramagnetic separation did, on the other hand, yield subpopulations with a significant variation in degree of discordance, although not in the manner expected (i.e. the least magnetic material. in this case, was more discordant). In any case, three of the four new analyses lie within error of a discordia line (probability of fit = 650,o) with upper and lower intercept ages of 2713 k 3 Myr and 211 + 134Myr. The age uncertainties resulting from this regression treatment are at the 95% confidence limit and assume that the data are precisely collinear within analytical error (LUDWIG, 1980). The fourth new analysis lies barely outside of analytical error off this line. Despite this departure from ideal two-stage U/Pb development, other best-fit lines obtained by regressing various combinations of all the Santa Barbara and/or the Royal Ontario Museum. Toronto data. have upper concordia intercept ages which range from 2709 to 2714 Myrreasonably consistent with the 2713 + 3 Myr age and uncertainty obtained from the three analytically collinear R.O.M., Toronto analyses. The U-Pb zircon age of 2713 fi 3 Myr is 0.44 + 0.32% higher than the Sm-Nd age of 2701 + 8 Myr. These uncertainties are precision errors at the 20 level. Hence the discrepancy between the U and Sm decay constants appears to be about 0.49; (< 0.8%). The cause of the discrepancy between these two age determinations may be owing to one or more of the following: (1) Incorrect values of the decay constants of 14’Sm (most uncertain), 23aU. and 2351_J. (2) Geological uncertainties regarding the timing of closure of these two different radiometric age-dating systems. (3) An age error in the Sm-Nd results owing to uncertainties in the Nd normalization procedure. If, for example, Nd ratios are obtained by normalizing to values used by LUGMAIRet al. (1975), the Stillwater complex age determination of DEPAOLO and WaBURG (1979) increases from 2701 + 8 Myr to 2706 f 8 Myr (DEPAOLO, written communication, 1981). Using this higher value, the difference between the U-Pb zircon age and Sm-Nd isochron age of the Stiilwater complex decreases to 0.25o/,well within the 2a precision errors. This further implies a discrepancy between the U and Sm decay constants of c 0.6%. Indeed one may use this age comparison to argue that it is more appropriate to use the LUGMAIRet al. (1975) normalization procedure for calculating Sm-Nd ages.
Letter to the Editor
1963
Fig. 1. Concordia diagram with zircon analyses from the Stillwater chill zone. Large error bars are ‘maximum precision error’ estimates of data obtained at Santa Barbara (NUNESand TILTON.1971). New R.O.M.. Toronto analyses have smaller error envelopes (Table 1).
Acknou\lrdgemenrs-This paper is dedicated to the late LUDWIGK. R. (1980) Calculation of uncertainties of U-Pb E. D. JACK~C~N who selected sample A-29-3. data. Earrh Planet. Sci. Lett. 46. 212-220. I thank T. E. KR~GH for sharing his sample preparation LUGMAIRG. W. and MARn K. (1977) Sm-Nd-Pu Timeinnovations. Important support from R. BARLOW.Ontario pieces in the Angra DOS Reis meteorite. Earth Planet. Geological Survey, J. HODGKIN and B. PODSTAWSKYJ, SC;. Left. 35, 273-284. Royal Ontario Museum, and I. DAVIES,University of TorLUGMAIRG. W., SCHEININN. B. and Mmn K. (1975) onto, is gratefully acknowledged. Thanks to G. NELSONfor Sm-Nd age and history of Apollo 17 basalt 75075: Evihelp with manuscript preparation. dence for early differentition of the lunar interior. Proc. This research was funded by the Ontario Geological 6th Lunar Planet. Sci. ConJ Vol. 2, pp. 1419-1429. Survey. NUNESP. D. and THUR~TON P. C. (1980) Two hundred and I thank F. TERA. P. C. THURSTONand. especially. D. J. twenty million years of Archean evolution: a zircon DEPAOLOand G. R. TILTONfor their very helpful reviews. U-Pb age stratigraphic study of the Uchi-Confederation Lakes greenstone belt, northwestern Ontario. Can. J. Earrh Sci. 17, 710-721. REFERENCES NUNESP. D. and TILTONG. R. (1971) Uranium-lead ages ADORABLES (1976) Progress by the consorts of Angra DOS of minerals from the Stillwater igneous complex and associated rocks. Montana. Geol. Sot. Am. Bull. 82, Reis. In Lunar Science VII, p. 443. Lunar Sci. Inst. 223 l-2250. DEPAOLOD. J. and WASSE~BURGG. J. (1979) Sm-Nd age of the Stillwater complex and the mantle evolution Russ G. P.. BURNETTD. S.. LINGENFELTER R. E. and WAScurve for neodymium. Geochim. Cosmochim. Acta 43, SERBURG G. J. (1971) Neutron canture on ‘49Sm in lunar samples. Earth Planer. Sci. Lert.*l3, 53-60. 999-1008. DONHOFFERD. (1963) Determination of the half-lives of TATSUMOTO M., KNIGHT R. J. and ALLEGREC. J. (1973) radioactive nuchdes i4’Sm and 1’6Lu occurring in Time differences in the formation of meteorites as deternature by means of liquid scintillators. Nucl. Phgs. 50, mined from the ratio of lead-207 to lead-206. Science 489-496. 180, 1279-1283. JAFFEYA. H., FLYNN K. F.. GLENDENINL. E., BENTLEY VALLI K.. AALTONENJ., GREFFE G., NURMIA M. and W. C. and ES~LINGA. M. (1971) Precision measurements POYHONENR. (1965) Half-life of “‘Sm: comparison of of half-lives and specific activities of 235U and 138U. ionization-chamber and liquid-scintillation results. Ann. Phys. Ret;. C4, 1889-1906. Ad. Sci. Fennicae Ser. A VI, 177-197. KR~GH T. E. (1980) Reduction of zircon age discordance WRIGHT P. M.. STEINBERGE. P. and GLENDENINL. E. by high gradient separation and air abrasion technique (1961) Half-life of samarium-147. Phvs. Ret. 123, (abstr.) Eos. 61, 399. 205-208.