The edge of time: Dating young volcanic ash layers with the 40Ar- 39Ar laser probe

Science

Volumn 274, Issue 5290, 1996, Pages 1176-1178

  Chen, Yanshao ^a   Smith, Patrick E ^a   Evensen, Norman M ^a   York, Derek ^a   Lajoie, Kenneth R ^b  

^a UNIVERSITY OF TORONTO   (Canada)

^b U S GEOLOGICAL SURVEY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ARGON;

ARGON LASER; ARTICLE; EVOLUTION; GEOLOGY; PRIORITY JOURNAL; VOLCANIC ASH;

ARGON DATING; ISOCHRON; TEPHROCHRONOLOGY; VOLCANIC ASH;

USA, CALIFORNIA, MONO CRATERS;

EID: 0030427787     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.274.5290.1176     Document Type: Article

References (33)

1. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
2. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
3. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
4. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
5. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
6. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
7. 39Ar dating method [C. M. Merrihue and G. Turner, J. Geophys. Res. 71, 2852 (1966)], enhanced by the laser-heated single-crystal technique [D. York et al., Geophys. Res. Lett. 8, 1136 (1981)], has recently been extended into latest Quaternary time [(3); P. v. d. Bogaard, Earth Planet. Sci. Lett. 133, 163 (1995)].
8. J. F. Evernden and G. H. Curtis, Curr. Anthropol. 6, 343 (1965); G. B. Dalrymple, Earth Planet. Sci. Lett. 3, 289 (1967).
9. J. F. Evernden and G. H. Curtis, Curr. Anthropol. 6, 343 (1965); G. B. Dalrymple, Earth Planet. Sci. Lett. 3, 289 (1967).
10. Q. Hu, P. E. Smith, N. M. Evensen, D. York, Earth Planet. Sci. Lett. 123, 331 (1994).
11. We use the terminology of G. B. Dalrymple and M. A. Lanphere [Potassium-Argon Dating (Freeman, San Francisco, 1969)] for excess argon, that is, that present from nonatmospheric sources at the time of argon closure; and inherited argon, that produced by in situ radioactive decay before the event being dated.
12. K. R. Lajoie, thesis, University of California, Berkeley (1968); in Quaternary Stratotypes of North America, vol. 1 (revised edition), P. F. Karrow, Ed., (Publ. 8, Quaternary Sciences Institute, Waterloo, Ontario, 1993), pp. 64-69.
13. K. R. Lajoie, thesis, University of California, Berkeley (1968); in Quaternary Stratotypes of North America, vol. 1 (revised edition), P. F. Karrow, Ed., (Publ. 8, Quaternary Sciences Institute, Waterloo, Ontario, 1993), pp. 64-69.
14. Complete stratigraphic sections of the WCF occur at two localities: in the narrow gorge along lower Wilson Creek on the northwest shore of Mono Lake, and in the wave-cut cliffs on the southeast shore of the lake. The most distinctive feature of the WCF is the occurrence of 19 ash layers, which provide stratigraphic control throughout Mono Basin and correlation with sediments in nearby basins. Eighteen of the ash layers are rhyolitic and were erupted, as indicated by chemical data, from the Mono Craters (5). Crooked Meadow, underlain mostly by the Bishop Tuff, lies about 15 km east of the Mono Craters, and shows a 3.3-m-thick sequence of peat layers interbedded with 21 rhyolitic ash layers, ranging from 7 to 170 mm in thickness.
15. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
16. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
17. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
18. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
19. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
20. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
21. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
22. 14C dates on peat samples from 13 horizons date the Crooked Meadow section at 11.1 to 1.1 ka (Lajoie, unpublished data). Age estimates of ash layers are obtained by simple interpolation between dated horizons, calibrated using the tree ring record [M. Stuiver and B, Becker, Radiocarbon 35, 1 (1993); B. Kromer and B. Becker, ibid., p. 125]. Therefore, for ash layer CMA-13, the estimated time of its eruption (deposition) is 4.6 ± 0.1 ka.
23. Ash layers WCA-8 and WCA-15 are from the south shore section of the WCF, downwind from Mono Craters, which has thicker and coarser ash layers than the dated type section at Wilson Creek. Hand-picking of 1-to 2-mm sanidine crystals, in which we avoided crystals of nonvolcanic origin, yielded mostly clear crystals, with varying amounts of adhering glass, which was removed by carefully breaking it loose. All crystals, together with flux monitors of Fish Canyon Tuff sanidine (assumed age = 27.84 million years ago), were irradiated in Cd-shielded capsules for 30 min (1 MWh) in position 5C of the McMaster Nuclear Reactor, Hamilton, Ontario, and analyzed for Ar isotope ratios in the University of Toronto Geochronology Laboratory (3). Isochron calculations are based on the algorithm of D. York [Earth Planet. Sci. Lett. 5, 320 (1969)]. All uncertainties are given at 1σ.
24. 40Ar intercept = 1/295.5 for modern atmosphere).
25. Our method explores all possible subdivisions of the sphenochron into smaller wedges, for each sub-wedge, computing the MSWD of the corresponding isochron constrained to pass through modern atmosphere (11). It then chooses that subdivision with the smallest number of sub-wedges for which the MSWD of each sub-wedge is less than 1 and the total MSWD is a minimum.
26. The mean square weighted deviate, or MSWD, is S, the sum of the squares of the weighted deviations from the best fit isochron, divided by the degrees of freedom, n-2, where n is the number of data points. Its expected value is 1.
27. P. E. Damon, A. W. Laughlin, J. K. Percious. in Radioactive Dating and Methods of Low-Level Counting (International Atomic Energy Agency, Vienna, 1967), pp. 463-481.
28. According to recent studies on the nearby Bishop Tuff [A. N. Halliday et al., Earth Planet. Sci. Lett. 94, 274 (1990); (14)], that magma chamber could have been in existence for more than 1 million years.
29. P. v. d. Bogaard and C. Schirnick, Geology 23, 759 (1995).
30. Using the diffusion constants measured by P. Zeitler [Chem. Geol. 65, 167 (1987)] for sanidine from the Fish Canyon Tuff, and an estimated magma temperature of 750°C, we calculate that a 1-mm crystal would lose 90% of its argon in 8.5 days, or 99.99% in 41 days. A 25°C change in temperature would change these times by about a factor of two. About 12 hours in a 750°C magma would cause enough argon loss to change the apparent age of a 760 ka Bishop Tuff sanidine to 500 ka.
31. P. v. d. Bogaard, C. M. Hall, H.-U. Schmincke, D. York, Nature 342, 523 (1989).
32. The sphenochron appears only where the range of xenocryst ages is large compared with the sample age. The absolute spread in ages represented by the sphenochrons shown here would virtually disappear into the analytical error in analyzing a sample of a million years or more in age. Sphenochrons are therefore more likely to appear in the dating of young material.
33. We thank R. C. Walter for discussions and suggestions and M. Lanphere, A. M. Sarna-Wojcicki and two anonymous reviewers for constructive comments. Supported by the University of Toronto Connaught Fund and the Natural Sciences and Engineering Research Council of Canada. Y.C. was supported by a National Research Council of Canada Postdoctoral Fellowship.