Radiogenic, Nucleogenic and Fissiogenic Noble Gas Compositions in Early Archaean Magmatic Zircons from Greenland

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. N UTMAN, VICKIE C. BENNETT and IGOR YATSEVICH

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 tem- 4 86 136 peratures (Tc) for radiogenic He* and fissiogenic Kr* and Xe* 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 INTRODUCTION Greenland zircon samples is available in Honda et al. Honda et al. (2003) analysed xenon isotope composi- (2003), and the original references are therein. tions in magmatic zircons from three early Archaean meta- About 50Ð100 mg of the zircon samples were heated granitoids from Greenland, and demonstrated that the from 400°C to 2000°C for 30 min. for each step in a re- extinct radioactive isotope 244Pu was present in the early sistively heated, double-vacuum tantalum furnace system. Earth. Elemental and isotopic compositions of the other The uncertainties of temperature measurements were es- noble gases, helium, neon, argon and krypton from the timated to be ~50°C. Blank measurements for noble gases same zircon samples are reported here; noble gases were were taken periodically at various extraction temperatures. extracted by step-heating. Our data include radiogenic Typical procedural blank levels for step-heating were (4He* and 40Ar*), nucleogenic (21Ne* and 22Ne*) and 4He = 2Ð6 × 10Ð11, 20Ne = 1Ð4 × 10Ð12, 40Ar = 4Ð9 × 10Ð9, fissiogenic (86Kr* and 136Xe*) noble gas compositions. 84Kr = 1Ð5 × 10Ð13, and 132Xe = 3Ð5 × 10Ð14 cm3STP. To our knowledge, the only other literature reporting dif- Within uncertainties noble gas isotopic ratios in blank runs fusion characteristics of noble gases in zircons are Reiners were atmospheric. et al. (2002) and Tagami et al. (2003). The results of this study provide background information for noble gas RESULTS AND DISCUSSION thermochronometry of zircons (e.g., (U-Th)/He thermochronometry (Reiners et al., 2002), Xe-Xe dating Abundances of radiogenic, nucleogenic and fissiogenic technique (Shukolyukov et al., 1974)). noble gases Radiogenic, nucleogenic and fissiogenic noble gas compositions in the Greenland zircons obtained by step- SAMPLES AND EXPERIMENTS heating experiments are presented in Appendix, where all Two samples are of granite (G97/111) and ferrogabbro/ 4He released from the samples is assumed to be radio- ferrodiorite (G97/112) from the same ca. 3.64 Ga intru- genic. In order to calculate the amounts of nucleogenic sive body (Baadsgaard, 1973; Nutman et al., 1984; 21Ne and 22Ne (21Ne* and 22Ne*), radiogenic 40Ar (40Ar*) Nutman et al., 2000). The third sample (G97/018) is a and fissiogenic 86Kr and 136Xe (86Kr* and 136Xe*), cor- well-preserved metatonalite with an age of ca. 3.81 Ga rections for atmospheric noble gas compositions were (Nutman et al., 1999). The U/Pb ages of these zircons made according to the following equation: were determined by SHRIMP (Table 1). A summary of X* = Xref × {(X/Xref)observed Ð (X/Xref)atmospheric} (1)

*Corresponding author (e-mail: [email protected]) 21 22 40 86 136 20 where X = Ne, Ne, Ar, Kr or Xe and Xref = Ne, Copyright © 2004 by The Geochemical Society of Japan. 36Ar, 82Kr or 130Xe, respectively. Note that hot blank cor-

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 heavier noble gases, particularly fissiogenic 86Kr* and the samples because noble gas isotopic ratios in blank 136Xe*, will be discussed later by investigating diffusion runs were atmospheric. profiles of these gases. The expected amounts of radiogenic 4He produced in On the basis of the amounts of nucleogenic 22Ne* and the zircons, calculated from U and Th concentrations and U contents in the samples, fluorine contents in samples ages of the samples (Table 1), are presented in Appendix. G97/018, 111 and 112 were estimated to be 19, 17 and 28 Similarly, the table presents the expected amounts of ppm, respectively. In contrast, fluorine contents in igne- nucleogenic 21Ne* and 22Ne* (produced from nuclear re- ous zircon are reported to be 6Ð10 ppm (Hoskin, 1999). actions 18O(α, n)21Ne, and 19F(α, n)22Na → 22Ne and Thus, apparently high fluorine contents in samples G97/ 19F(α, p)22Ne, respectively); the calculation was made by 018, 111 and 112 may reflect the existence of sub-micron using algorithms described in Yatsevich and Honda size fluorine-rich mineral inclusions such as apatite and (1997). It is noted that the production of nucleogenic possibly biotite within zircons; evidence for micro apa- 21Ne* in zircons is about 17% smaller than the produc- tite inclusions in zircons comes from coupled P, Ca, Sr tion in silicates (Yatsevich and Honda, 1997). This is be- “spikes” in LA-ICP-MS (laser ablation inductively cou- cause the mass fraction of oxygen in zircon is lower than pled plasma mass spectrometry) analyses (Nutman, un- in silicate. For the calculation of nucleogenic 22Ne* a fluo- published data). Similarly, from the amount of radiogenic rine content of 30 ppm was arbitrarily assumed. 40Ar* potassium contents in samples G97/018, 111 and A precise value of [ λ × Y136 ] (=6.8 × 10Ð18/a) has 112 were estimated to be 0.18, 0.27 and 0.54 ppm, re- Usf Usf been determined from the study of U-bearing accessory spectively, indicating the presence of potassium-bearing mineral inclusions in the Greenland zircon samples. minerals (Ragettli et al., 1994), where λ and Y136 are Usf Usf 238 the decay constant for spontaneous fission of U and Gas release the fission yield of 136Xe, respectively, and it was used, In order to examine the characteristics of radiogenic together with U contents and ages of the samples (Table 4He* and 40Ar*, nucleogenic 21Ne* and fissiogenic 86Kr* 1), to calculate the expected amounts of fissiogenic 136Xe* and 136Xe* release from the Greenland zircons, the sam- 136 86 in the samples. The Xe*/ Kr* spontaneous fission ra- ples were heated in temperature increments from 400°C tio of 6.1 was previously determined from the measure- to 2000°C. Figure 1 shows release patterns of these gases ments of uranium-bearing minerals (Eikenberg et al., by step-heating from sample G97/018. Radiogenic 4He* 1993), and this value was used to calculate the expected release increased monotonically, with maximum at 800°C, 86 amounts of fissiogenic Kr* from the expected amounts and by 1200°C the gas extraction was virtually complete. of 136Xe* in the samples. These amounts are also shown In contrast, fissiogenic 86Kr* and 136Xe* were released in Appendix. at higher temperatures than helium, beginning at 1200°C Reasonable agreement between the expected and ob- and ending at 2000°C. The gas release patterns of served amounts of nucleogenic 21Ne* and fissiogenic nucleogenic 21Ne* and radiogenic 40Ar* were not 86Kr* and 136Xe* indicates that these noble gas composi- monotonic and were somewhat more complex. This prob- tions have been retained in the zircon samples without ably reflects release of these gases from mineral inclu- significant loss since the zircon samples formed at 3.81 sions within zircons, and therefore the diffusion profiles and 3.64 Ga. In contrast, about 90% of radiogenic 4He of nucleogenic 21Ne* and radiogenic 40Ar* in these zir- appears to have been lost from the zircon samples. The cons will not be discussed. difference in diffusive characteristics between helium and

266 M. Honda et al. Fig. 2. Arrhenius plots of radiogenic 4He* and fissiogenic 86Kr* and 136Xe* for zircon sample G97/018. The 4He*, 86Kr* or 136Xe* data points obtained from step-heating experiments (first 4 40 21 Fig. 1. Radiogenic He* and Ar*, nucleogenic Ne* and three points; 400Ð800°C for 4He*, and five points; 1200Ð 86 136 fissiogenic Kr* and Xe* patterns for zircon sample G97/ 1700°C for 86Kr* and 136Xe*) lie on straight lines, from which 018. Radiogenic 4He* release was virtually complete at 1200 C, 2 ° activation energies (Ea) and D0/a , where D0 is the frequency whereas fissiogenic 86Kr* and 136Xe* were released at higher factor were calculated (Table 2). 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. Table 2. Diffusion data and closure temperatures calculated for the Greenland zircons 4 86 Diffusion of radiogenic He* and fissiogenic Kr* and 2 136 Sample Ea D0/a Tc Xe* (kJ/mol) (sÐ1)(¡C) Figure 2 presents Arrhenius plots for radiogenic 4He* and fissiogenic 86Kr* and 136Xe* obtained by step-heat- G97/018 4 ing experiments for sample G97/018, where the x-axis He* 138 6.7E + 02 138 86Kr* 302 3.7E + 03 562 indicates a reciprocal of temperature (in Kelvin) for step- 136Xe* 365 9.9E + 04 663 2 heating experiments and the y-axis denotes ln(D/a ) in G97/111 which D and a are the diffusion coefficients and a char- 4He* 101 3.3E Ð 02 116 acteristic diffusion length, respectively, for 4He*, 86Kr* 86Kr* 314 9.6E + 03 582 136 or 136Xe*. In calculations a cylindrical geometry for the Xe* 335 1.8E + 03 669 G97/112 diffusion domain was assumed and used the method de- 4He* 109 2.9E Ð 02 147 4 86 scribed in Fechtig and Kalbitzer (1966). The He*, Kr* 86Kr* — — — or 136Xe* data points obtained from step-heating experi- 136Xe* 406 2.7E + 05 750 4 ments (first three points: 400Ð800°C for He*, and five Diffusion data were calculated from data obtained by step-heating ex- 86 136 points: 1200Ð1700°C for Kr* and Xe*) lie on straight periments (400Ð600°C fractions for 4He*, and 1200Ð1700°C fractions lines. The slopes and y-intercepts of the lines allow cal- for 86Kr* and 136Xe*). 2 Closure temperatures (T ) were calculated assuming a 10 C/Myr cool- culation of activation energies (Ea) and D0/a , where D0 c ° is the frequency factor, and these values are listed in Ta- ing rate. 2 ble 2. Based on Ea and D0/a estimated for each of the gases from the zircon samples, closure temperatures (Tc) (Dodson, 1973) were calculated assuming a 10°C/Myr 138Ð101 kJ/mol and 116Ð147°C, respectively. Reiners et cooling rate. The diffusion data and closure temperatures al. (2002) determined activation energies of helium dif- for the other zircon samples (G97/111 and G97/112) were fusion in 28 Ma zircons from Fish Canyon Tuff, calculated in the same manner and they are also listed in Colorado in low temperature (280Ð600°C) helium diffu- Table 2. sion experiments, where they found Ea and Tc of 184 kJ/ 4 86 136 Both Ea and Tc calculated for He*, Kr* and Xe* mol and 190°C. Because radiation damage associated with in the Greenland zircon samples show mass-dependent alpha recoil and fission products generally increase dif- characteristics. The Ea and Tc calculated for helium range fusivity and decrease activation energy, the difference

Diffusion of noble gases in zircons 267 between our data and Reiners’ measurements is consist- potassium-bearing solids. Potassium-Argon Dating ent with the considerably older age of the Greenland sam- (Schaeffer, O. A. and Zähringer, J., eds.), 68Ð106, Springer. Gleadow, A. J. W. (1978) Fission-track evidence for the evolu- ples. For the same reason, our estimates of Ea and Tc are likely to be the lower boundary estimates. Despite the tion of rifted continental margins. USGS Open File Report complexity of helium diffusion in zircons associated with 78, No. 701, 146Ð148. Honda, M., Nutman, A. P. and Bennett, V. C. (2003) Xenon radiation damage, (U-Th)/He ages of zircons in most of compositions of magmatic zircons in 3.64 and 3.81 Ga meta- the cases can provide geologically meaningful informa- granitoids from Greenland—a search for extinct 244Pu in tion (Reiners et al., 2002; Tagami et al., 2003). In the ancient terrestrial rocks. Earth Planet. Sci. Lett. 207, 69Ð case of the Greenland zircons, relatively young (U-Th)/ 82. He ages (300Ð500 Ma determined from the U and Th con- Hoskin, P. W. O. (1999) SIMS determination of µg gÐ1-level tents and 4He* concentrations) probably represent the last fluorine in geological samples and its concentration in NIST low grade metamorphic event, recorded from fission track SRM 610. Geostandards Newsletter 23, 69Ð76. studies of other zircon samples in the area (Gleadow, Nutman, A. P., Bridgwater, D. and Fryer, B. (1984) The iron 1978). Our West Greenland results might indicate effects rich suite from the Amîtsoq gneisses of southern West originating in the western foreland of the Caledonian Greenland: Early Archaean plutonic rocks of mixed crustal Orogen which lies in East Greenland. and mantle origin. Contrib. Mineral. Petrol. 87, 24Ð34. Nutman, A. P., Bennett, V. C., Friend, C. R. L. and Norman, M. E and T calculated for 86Kr* and 136Xe* in the a c D. (1999) Meta-igneous (non-gneissic) tonalites and quartz- Greenland zircon samples are higher than those for he- diorites from an extensive ca. 3800 Ma terrain south of the lium (Table 2). From diffusion data for these isotopes we Isua supracrustal belt, southern West Greenland: constraints 86 136 may conclude that fissiogenic Kr* and Xe* were ac- on early crust formation. Contrib. Mineral. Petrol. 137, 364Ð cumulated in the zircon samples since they formed with- 388. out any subsequent loss, and (U-Th)/Kr-Xe ages of zir- Nutman, A. P., Friend, C. R. L., Bennett, V. C. and McGregor, cons provide information on crystallization. V. R. (2000) The early Archaean Itsaq Gneiss Complex of In conclusion, the diffusion data for 4He*, 86Kr* and southern West Greenland: The importance of field observa- 136Xe* demonstrate that the combined study of (U-Th)/ tions in interpreting dates and isotopic data constraining He and (U-Th)/Kr-Xe can provide additional geochrono- early terrestrial evolution. Geochim. Cosmochim. Acta 64, logical information from the same samples. For exam- 3035Ð3060. Ragettli, R. A., Hebeda, E. H., Signer, P. and Wieler, R. (1994) ple, our data imply a late Palaeozoic metamorphic event Uranium-xenon chronology: precise determination of λ ∗ and provide additional constraints on the temperature sf 136 238 range attained during this event. Ysf for spontaneous fission of U. Earth Planet. Sci. Lett. 128, 653Ð670. Reiners, P. W., Farley, K. A. and Hickes, H. J. (2002) He diffu- Acknowledgments—We thank Sujoy Mukhopadhyay and sion and (U-Th)/He thermochronometry of zircons: Initial Samuel Niedermann for their helpful comments and sugges- results from Fish Canyon Tuff and Gold Butte, Nevada. tions. Tectonophysics 349, 297Ð308. Shukolyukov, Y. A., Kirsten, T. and Jessberger, E. K. (1974) REFERENCES The Xes-Xen spectrum technique, a new dating method. Earth Planet. Sci. Lett. 24, 271Ð281. Baadsgaard, H. (1973) U-Th-Pb dates on zircons from the early Tagami, T., Farley, K. A. and Stockli, D. F. (2003) (U-Th)/He Precambrian Amîtsoq gneisses, Godthaab district, West geochronology of single zircon grains of known Tertiary Greenland. Earth Planet. Sci. Lett. 19, 22Ð28. eruption age. Earth Planet. Sci. Lett. 207, 57Ð67. Dodson, M. H. (1973) Closure temperature in cooling Yatsevich, I. and Honda, M. (1997) Production of nucleogenic geochronological and petrological systems. Contrib. Min- neon in the Earth from natural radioactive decay. J. Geophys. eral. Petrol. 40, 259Ð274. Res. 102, 10291Ð10298. Eikenberg, J., Signer, P. and Wieler, R. (1993) U-Xe, U-Kr, and U-Pb systematics for dating uranium minerals and investi- gations of the production of nucleogenic neon and argon. APPENDIX Geochim. Cosmochim. Acta 57, 1053Ð1069. (see p. 269) Fechtig, H. and Kalbitzer, S. (1966) The diffusion of argon in

268 M. Honda et al. 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. 22 b: Interference correction for Ne 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