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cooling ages and crystalli .... Colorado in low temperature (280–600°C) helium
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Geochemical Journal, Vol. 38, pp. 265 to 269, 2004
Radiogenic, nucleogenic and fissiogenic noble gas compositions in early Archaean magmatic zircons from Greenland MASAHIKO HONDA,* ALLEN P. NUTMAN, VICKIE C. BENNETT and IGOR Y ATSEVICH Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia (Received May 22, 2003; Accepted November 29, 2003) We report a full suite of radiogenic, nucleogenic and fissiogenic noble gas compositions obtained by step-heating experiments from early Archaean Greenland zircons. We estimated activation energy (Ea) of diffusion and closure temperatures (Tc) for radiogenic 4He* and fissiogenic 86Kr* and 136Xe* in zircons. These data demonstrate that the combined study of (U-Th)/He and (U-Th)/Kr-Xe can provide powerful geochronological information on cooling ages and crystallization ages from the same samples. Keywords: noble gas, diffuson, zircon, closure temperature, early Archaean
the geology and SHRIMP U/Pb zircon dating of the Greenland zircon samples is available in Honda et al. (2003), and the original references are therein. About 50–100 mg of the zircon samples were heated from 400°C to 2000°C for 30 min. for each step in a resistively heated, double-vacuum tantalum furnace system. The uncertainties of temperature measurements were estimated to be ~50°C. Blank measurements for noble gases were taken periodically at various extraction temperatures. Typical procedural blank levels for step-heating were 4 He = 2–6 × 10–11, 20Ne = 1–4 × 10 –12, 40Ar = 4–9 × 10 –9, 84 Kr = 1–5 × 10 –13, and 132Xe = 3–5 × 10 –14 cm3STP. Within uncertainties noble gas isotopic ratios in blank runs were atmospheric.
INTRODUCTION Honda et al. (2003) analysed xenon isotope compositions in magmatic zircons from three early Archaean metagranitoids from Greenland, and demonstrated that the extinct radioactive isotope 244Pu was present in the early Earth. Elemental and isotopic compositions of the other noble gases, helium, neon, argon and krypton from the same zircon samples are reported here; noble gases were extracted by step-heating. Our data include radiogenic (4He* and 40Ar*), nucleogenic (21Ne* and 22Ne*) and fissiogenic (86Kr* and 136Xe*) noble gas compositions. To our knowledge, the only other literature reporting diffusion characteristics of noble gases in zircons are Reiners et al. (2002) and Tagami et al. (2003). The results of this study provide background information for noble gas thermochronometry of zircons (e.g., (U-Th)/He thermochronometry (Reiners et al., 2002), Xe-Xe dating technique (Shukolyukov et al., 1974)).
RESULTS AND DISCUSSION Abundances of radiogenic, nucleogenic and fissiogenic noble gases Radiogenic, nucleogenic and fissiogenic noble gas compositions in the Greenland zircons obtained by stepheating experiments are presented in Appendix, where all 4 He released from the samples is assumed to be radiogenic. In order to calculate the amounts of nucleogenic 21 Ne and 22Ne (21Ne* and 22Ne*), radiogenic 40Ar (40Ar*) and fissiogenic 86Kr and 136Xe (86Kr* and 136Xe*), corrections for atmospheric noble gas compositions were made according to the following equation:
SAMPLES AND EXPERIMENTS Two samples are of granite (G97/111) and ferrogabbro/ ferrodiorite (G97/112) from the same ca. 3.64 Ga intrusive body (Baadsgaard, 1973; Nutman et al., 1984; Nutman et al., 2000). The third sample (G97/018) is a well-preserved metatonalite with an age of ca. 3.81 Ga (Nutman et al., 1999). The U/Pb ages of these zircons were determined by SHRIMP (Table 1). A summary of
X* = Xref × {(X/Xref)observed – (X/Xref)atmospheric} (1) *Corresponding author (e-mail:
[email protected])
where X = 21Ne, 22Ne, 40Ar, 86Kr or 136Xe and Xref = 20Ne, 36 Ar, 82Kr or 130Xe, respectively. Note that hot blank cor-
Copyright © 2004 by The Geochemical Society of Japan.
265
Table 1. Greenland zircons for noble gas analyses
Data from Honda et al. (2003). (a) Uncertainties are two standard deviation. (b) Uncertainties are one standard deviation. Large uncertainties in U and Th contents are owing to spot analyses within single zircon grains by SHRIMP.
rections were not applied for the noble gases released from the samples because noble gas isotopic ratios in blank runs were atmospheric. The expected amounts of radiogenic 4He produced in the zircons, calculated from U and Th concentrations and ages of the samples (Table 1), are presented in Appendix. Similarly, the table presents the expected amounts of nucleogenic 21Ne* and 22Ne* (produced from nuclear reactions 18O(α, n)21Ne, and 19F(α , n) 22Na → 22Ne and 19 F(α, p)22Ne, respectively); the calculation was made by using algorithms described in Yatsevich and Honda (1997). It is noted that the production of nucleogenic 21 Ne* in zircons is about 17% smaller than the production in silicates (Yatsevich and Honda, 1997). This is because the mass fraction of oxygen in zircon is lower than in silicate. For the calculation of nucleogenic 22Ne* a fluorine content of 30 ppm was arbitrarily assumed. A precise value of [ λ U sf × YU136 ] (=6.8 × 10–18/a) has sf been determined from the study of U-bearing accessory minerals (Ragettli et al., 1994), where λ U sf and YU136 are sf the decay constant for spontaneous fission of 238 U and the fission yield of 136Xe, respectively, and it was used, together with U contents and ages of the samples (Table 1), to calculate the expected amounts of fissiogenic 136Xe* in the samples. The 136Xe*/ 86Kr* spontaneous fission ratio of 6.1 was previously determined from the measurements of uranium-bearing minerals (Eikenberg et al., 1993), and this value was used to calculate the expected amounts of fissiogenic 86Kr* from the expected amounts of 136Xe* in the samples. These amounts are also shown in Appendix. Reasonable agreement between the expected and observed amounts of nucleogenic 21 Ne* and fissiogenic 86 Kr* and 136Xe* indicates that these noble gas compositions have been retained in the zircon samples without significant loss since the zircon samples formed at 3.81 and 3.64 Ga. In contrast, about 90% of radiogenic 4He appears to have been lost from the zircon samples. The difference in diffusive characteristics between helium and
266 M. Honda et al.
the heavier noble gases, particularly fissiogenic 86Kr* and 136 Xe*, will be discussed later by investigating diffusion profiles of these gases. On the basis of the amounts of nucleogenic 22Ne* and U contents in the samples, fluorine contents in samples G97/018, 111 and 112 were estimated to be 19, 17 and 28 ppm, respectively. In contrast, fluorine contents in igneous zircon are reported to be 6–10 ppm (Hoskin, 1999). Thus, apparently high fluorine contents in samples G97/ 018, 111 and 112 may reflect the existence of sub-micron size fluorine-rich mineral inclusions such as apatite and possibly biotite within zircons; evidence for micro apatite inclusions in zircons comes from coupled P, Ca, Sr “spikes” in LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) analyses (Nutman, unpublished data). Similarly, from the amount of radiogenic 40 Ar* potassium contents in samples G97/018, 111 and 112 were estimated to be 0.18, 0.27 and 0.54 ppm, respectively, indicating the presence of potassium-bearing mineral inclusions in the Greenland zircon samples. Gas release In order to examine the characteristics of radiogenic 4 He* and 40Ar*, nucleogenic 21Ne* and fissiogenic 86Kr* and 136Xe* release from the Greenland zircons, the samples were heated in temperature increments from 400°C to 2000°C. Figure 1 shows release patterns of these gases by step-heating from sample G97/018. Radiogenic 4He* release increased monotonically, with maximum at 800°C, and by 1200°C the gas extraction was virtually complete. In contrast, fissiogenic 86Kr* and 136Xe* were released at higher temperatures than helium, beginning at 1200°C and ending at 2000°C. The gas release patterns of nucleogenic 21 Ne* and radiogenic 40 Ar* were not monotonic and were somewhat more complex. This probably reflects release of these gases from mineral inclusions within zircons, and therefore the diffusion profiles of nucleogenic 21Ne* and radiogenic 40Ar* in these zircons will not be discussed.
Fig. 1. Radiogenic 4He* and 40Ar*, nucleogenic 21Ne* and fissiogenic 86Kr* and 136Xe* patterns for zircon sample G97/ 018. Radiogenic 4He* release was virtually complete at 1200°C, whereas fissiogenic 86Kr* and 136 Xe* were released at higher temperatures, beginning at 1200°C and ending at 2000°C. The gas release patterns of nucleogenic 21Ne* and radiogenic 40Ar* were not monotonic and were somewhat more complex.
Diffusion of radiogenic 4He* and fissiogenic 86Kr* and 136 Xe* Figure 2 presents Arrhenius plots for radiogenic 4He* and fissiogenic 86Kr* and 136Xe* obtained by step-heating experiments for sample G97/018, where the x-axis indicates a reciprocal of temperature (in Kelvin) for stepheating experiments and the y-axis denotes ln(D/a2) in which D and a are the diffusion coefficients and a characteristic diffusion length, respectively, for 4He*, 86Kr* or 136Xe*. In calculations a cylindrical geometry for the diffusion domain was assumed and used the method described in Fechtig and Kalbitzer (1966). The 4He*, 86Kr* or 136Xe* data points obtained from step-heating experiments (first three points: 400–800°C for 4He*, and five points: 1200–1700°C for 86Kr* and 136Xe*) lie on straight lines. The slopes and y-intercepts of the lines allow calculation of activation energies (Ea) and D 0/a2, where D0 is the frequency factor, and these values are listed in Table 2. Based on Ea and D0/a2 estimated for each of the gases from the zircon samples, closure temperatures (Tc) (Dodson, 1973) were calculated assuming a 10°C/Myr cooling rate. The diffusion data and closure temperatures for the other zircon samples (G97/111 and G97/112) were calculated in the same manner and they are also listed in Table 2. Both Ea and Tc calculated for 4He*, 86Kr* and 136Xe* in the Greenland zircon samples show mass-dependent characteristics. The Ea and Tc calculated for helium range
Fig. 2. Arrhenius plots of radiogenic 4He* and fissiogenic 86Kr* and 136Xe* for zircon sample G97/018. The 4He*, 86Kr* or 136 Xe* data points obtained from step-heating experiments (first three points; 400–800 ° C for 4 He*, and five points; 1200– 1700°C for 86Kr* and 136Xe*) lie on straight lines, from which activation energies (Ea) and D 0/a2, where D0 is the frequency factor were calculated (Table 2).
Table 2. Diffusion data and closure temperatures calculated for the Greenland zircons Sample
G97/018 4 He* 86 Kr* 136 Xe* G97/111 4 He* 86 Kr* 136 Xe* G97/112 4 He* 86 Kr* 136 Xe*
Ea (kJ/mol)
D0 /a2 (s– 1 )
Tc (°C)
138 302 365
6.7E + 02 3.7E + 03 9.9E + 04
138 562 663
101 314 335
3.3E – 02 9.6E + 03 1.8E + 03
116 582 669
109 — 406
2.9E – 02 — 2.7E + 05
147 — 750
Diffusion data were calculated from data obtained by step-heating experiments (400–600° C fractions for 4He*, and 1200–1700° C fractions for 86Kr* and 136Xe*). Closure temperatures (Tc) were calculated assuming a 10 °C/Myr cooling rate.
138–101 kJ/mol and 116–147°C, respectively. Reiners et al. (2002) determined activation energies of helium diffusion in 28 Ma zircons from Fish Canyon Tuff, Colorado in low temperature (280–600°C) helium diffusion experiments, where they found Ea and T c of 184 kJ/ mol and 190°C. Because radiation damage associated with alpha recoil and fission products generally increase diffusivity and decrease activation energy, the difference
Diffusion of noble gases in zircons 267
between our data and Reiners’ measurements is consistent with the considerably older age of the Greenland samples. For the same reason, our estimates of Ea and Tc are likely to be the lower boundary estimates. Despite the complexity of helium diffusion in zircons associated with radiation damage, (U-Th)/He ages of zircons in most of the cases can provide geologically meaningful information (Reiners et al., 2002; Tagami et al., 2003). In the case of the Greenland zircons, relatively young (U-Th)/ He ages (300–500 Ma determined from the U and Th contents and 4He* concentrations) probably represent the last low grade metamorphic event, recorded from fission track studies of other zircon samples in the area (Gleadow, 1978). Our West Greenland results might indicate effects originating in the western foreland of the Caledonian Orogen which lies in East Greenland. E a and T c calculated for 86 Kr* and 136Xe* in the Greenland zircon samples are higher than those for helium (Table 2). From diffusion data for these isotopes we may conclude that fissiogenic 86Kr* and 136Xe* were accumulated in the zircon samples since they formed without any subsequent loss, and (U-Th)/Kr-Xe ages of zircons provide information on crystallization. In conclusion, the diffusion data for 4He*, 86Kr* and 136 Xe* demonstrate that the combined study of (U-Th)/ He and (U-Th)/Kr-Xe can provide additional geochronological information from the same samples. For example, our data imply a late Palaeozoic metamorphic event and provide additional constraints on the temperature range attained during this event. Acknowledgments—We thank Sujoy Mukhopadhyay and Samuel Niedermann for their helpful comments and suggestions.
REFERENCES Baadsgaard, H. (1973) U-Th-Pb dates on zircons from the early Precambrian Amîtsoq gneisses, Godthaab district, West Greenland. Earth Planet. Sci. Lett. 19, 22–28. Dodson, M. H. (1973) Closure temperature in cooling geochronological and petrological systems. Contrib. Mineral. Petrol. 40, 259–274. Eikenberg, J., Signer, P. and Wieler, R. (1993) U-Xe, U-Kr, and U-Pb systematics for dating uranium minerals and investigations of the production of nucleogenic neon and argon. Geochim. Cosmochim. Acta 57, 1053–1069. Fechtig, H. and Kalbitzer, S. (1966) The diffusion of argon in
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potassium-bearing solids. Potassium-Argon Dating (Schaeffer, O. A. and Zähringer, J., eds.), 68–106, Springer. Gleadow, A. J. W. (1978) Fission-track evidence for the evolution of rifted continental margins. USGS Open File Report 78, No. 701, 146–148. Honda, M., Nutman, A. P. and Bennett, V. C. (2003) Xenon compositions of magmatic zircons in 3.64 and 3.81 Ga metagranitoids from Greenland—a search for extinct 244Pu in ancient terrestrial rocks. Earth Planet. Sci. Lett. 207, 69– 82. Hoskin, P. W. O. (1999) SIMS determination of µ g g–1-level fluorine in geological samples and its concentration in NIST SRM 610. Geostandards Newsletter 23, 69–76. Nutman, A. P., Bridgwater, D. and Fryer, B. (1984) The iron rich suite from the Amîtsoq gneisses of southern West Greenland: Early Archaean plutonic rocks of mixed crustal and mantle origin. Contrib. Mineral. Petrol. 87, 24–34. Nutman, A. P., Bennett, V. C., Friend, C. R. L. and Norman, M. D. (1999) Meta-igneous (non-gneissic) tonalites and quartzdiorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation. Contrib. Mineral. Petrol. 137, 364– 388. Nutman, A. P., Friend, C. R. L., Bennett, V. C. and McGregor, V. R. (2000) The early Archaean Itsaq Gneiss Complex of southern West Greenland: The importance of field observations in interpreting dates and isotopic data constraining early terrestrial evolution. Geochim. Cosmochim. Acta 64, 3035–3060. Ragettli, R. A., Hebeda, E. H., Signer, P. and Wieler, R. (1994) Uranium-xenon chronology: precise determination of λ sf ∗ 136 Ysf for spontaneous fission of 238 U. Earth Planet. Sci. Lett. 128, 653–670. Reiners, P. W., Farley, K. A. and Hickes, H. J. (2002) He diffusion and (U-Th)/He thermochronometry of zircons: Initial results from Fish Canyon Tuff and Gold Butte, Nevada. Tectonophysics 349, 297–308. Shukolyukov, Y. A., Kirsten, T. and Jessberger, E. K. (1974) The Xe s-Xe n spectrum technique, a new dating method. Earth Planet. Sci. Lett. 24, 271–281. Tagami, T., Farley, K. A. and Stockli, D. F. (2003) (U-Th)/He geochronology of single zircon grains of known Tertiary eruption age. Earth Planet. Sci. Lett. 207, 57–67. Yatsevich, I. and Honda, M. (1997) Production of nucleogenic neon in the Earth from natural radioactive decay. J. Geophys. Res. 102, 10291–10298.
A PPENDIX (see p. 269)
Appendix. Radiogenic, nucleogenic and fissiogenic compositions in Greenland zircon samples (in cm3STP/g)
Radiogenic, nucleogenic and fissiogenic amounts in the samples are calculated by subtracting atmospheric components. Quoted errors (one standard deviation) include the uncertainties in the correction factors for mass discrimination and sensitivity determined by ten repeated analysis of the standard gases. a: Ion signals were below detection limit. b: Interference correction for 22Ne from double-charged CO2 exceeded 90% of measured mass 22 signals, and the results are not listed. c: Measured ratios were slightly less than the atmospheric values, but they were atmospheric within uncertainties. !: Because the ion gauge in the gas handling system was left on during the gas extraction from the sample, noble gas measurements were aborted.
Diffusion of noble gases in zircons 269
