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equipped with photodiode-array detection. The fused silica capillaries used in the experiments were 53 cm in total length (45-cm effective length) with a 50- m inner diameter and a 320- m outer diameter. Running conditions were as follows: voltage, 20 kV; current, 17.6 A; electrolytes, 5.5 mM 4-hydroxybenzoate and 0.5 mM tetradecyltrimethylammonium bromide, pH 11.6. R. G. Kemp, J. Biol. Chem. 246, 245 (1971). E. V. Schaftingen, M. F. Jett, L. Hue, H. G. Hers, Proc. Natl. Acad. Sci. U.S.A. 78, 3483 (1981). G. Colombo, P. W. Tate, A. W. Girotti, R. G. Kemp, J. Biol. Chem. 250, 9404 (1975). E. V. Schaftingen and H. G. Hers, Biochem. Biophys. Res. Commun. 101, 1078 (1981). The enzyme activities were not constant during the entire 12-hour sampling period. Six separate experiments of 2 to 3 hours each were conducted, with the same amount of enzymes (by weight) in each experiment. Nonetheless, the enzyme activities differed slightly between experiments, as indicated by a change in the unperturbed steady-state concentrations of the measured species. In order to make the time series between experiments match, we multiplied the concentrations from the time series in each experiment by the appropriate ratio of the reference concentration (as defined above) to the measured unperturbed steady-state concentration. In order to repeat the last time point of a previous experiment, each experiment repeated the last two to three sets of input from the previous experiment. 16. D. A. Card, D. E. Folmer, S. Sato, S. A. Buzza, J. A. W. Castleman, J. Phys. Chem. 101, 3417 (1997). 17. K. Uyeda, E. Furuya, L. J. Luby, J. Biol. Chem. 256, 8394 (1981). 18. We thank P. Oefner (Department of Biochemistry, Stanford University) for the use of the capillary electrophoresis apparatus and for helpful discussions, and S. Pilkis (deceased) and D. Okar (Department of Biochemistry, University of Minnesota) for providing us with PFK2::F26BPase and for helpful discussions. This work was supported in part by NSF. A.A. was supported in part by Office of Naval Research grant N00014-96-1-0564. 17 April 1997; accepted 7 July 1997

11. 12. 13. 14. 15.

40

Ar/39Ar Da ting in to the Historical Realm: Calibra tion Against Pliny the Younger

P. R. Renne,* W. D. Sharp, A. L. Deino, G. Orsi, L. Civetta
Laser incremental heating of sanidine from the pumice deposited by the Plinian eruption of Vesuvius in 79 A.D. yielded a 40Ar/39Ar isochron age of 1925 94 years ago. Close agreement with the Gregorian calendar­ based age of 1918 years ago demonstrates that the 40Ar/39Ar method can be reliably extended into the temporal range of recorded history. Excess 40Ar is present in the sanidine in concentrations that would cause significant errors if ignored in dating Holocene samples.

The ability to date geological events in the recent past is increasingly important in many areas of earth science. Applied to volcanogenic materials of high potassium content, 40 the Ar/40K method of dating has been used to date samples into the Holocene, that is, less than 12,000 years old (1), although the precision is lower than is commonly reported for 14C, uranium series, or other methods. The 40Ar/39Ar variant of this method, in which 40K is measured by proxy through 39 Ar in irradiated samples (2), potentially allows recognition of xenocrystic contamination, nonatmospheric initial argon, and argon loss. For example, a young 40Ar/39Ar date of 5.3 0.3 thousand years ago (Ka) has been reported (3) for sanidine from a tephra derived from the Mono Craters, California, whose age had been previously constrained by 14C at 4.6 0.1 Ka. Determination of this 40Ar/39Ar model age relied on the assumption that the significant scatter in apparent ages of single crystals was a result
P. R. Renne, Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA, and Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA. W. D. Sharp and A. L. Deino, Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA. G. Orsi, Dipartamento di Geofisica e Vulcanologia, Universita di Napoli, Largo Santo Marcellino 10, Napoli, Italy. ` L. Civetta, Dipartamento di Geofisica e Vulcanologia Universita di Napoli, Largo Santo Marcellino 10, Napoli, Italy, ` and Vesuvian Volcanological Observatory, 249 Via Manzoni, Napoli, Italy. * To whom correspondence should be addressed at Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.

only of xenocrystic contamination. Dating minerals in volcanic ejecta such as sanidine, which is potassium-rich and is thought to efficiently exclude initial argon at magmatic temperatures, is the most promising approach, although the possible effects of xenocrystic contamination (4) can be problematic. Analysis of single crystals, for example by laser fusion, can obviate xenocrystic contamination (5), but single crystals are seldom large enough to yield reliably measurable quantities of 40Ar* through radiogenic ingrowth in the Holocene. A relatively large spherical sanidine crystal 2 mm in diameter with 10 weight % K produces only about 2 10-18 moles of 40Ar* in 5000 years, and this quantity is one or two orders of magnitude smaller than the best attainable procedural background levels of 40Ar. We sought to investigate the limits of the 40 Ar/39Ar method by application to ejecta from the infamous eruption of Vesuvius, which destroyed Pompeii and other Roman cities, as documented in the writings of Pliny the Younger (6). Vesuvius erupted pumice clasts up to 36 cm long beginning in the early afternoon of 24 August 79 A.D., 1918 calendar years ago. The white pumice that erupted initially is highly alkaline and contains up to 5% sanidine phenocrysts with 13.7 to 15.7 weight % K2O (7), which are up to 8 mm in diameter. A sample of this pumice was collected at the Villa of Poppea, which was buried in the eruption and has only recently been excavated. We divided irradiated sanidine from the
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pumice (8) into 12 samples totaling 0.430 g and analyzed them by incremental heating with a CO2 laser (9). The samples yielded imprecise and dispersed apparent ages (10) of 24 to 521 Ka in the lower (1 to 2 W) laser power steps, which decreased and became more uniform (2 to 26 Ka) at 4 to 6 W, and became uniformly 2 to 3 Ka in the 7- to 20-W steps. The 1- to 2-W initial steps, each of which contained less than 0.1% of the total 39 Ar released, yielded 40Ar/36Ar values of 298 to 323, which are statistically indistinguishable at 95% conf idence (critical value test) from the atmospheric ratio of 296 (11). A table of the argon isotopic data is available to Science Online subscribers at www.sciencemag.org. Cast on an isotope correlation diagram (Fig. 1), the data define an isochron (12) that yields an intercept corresponding to 40Ar*/ 39 ArK 0.07551 0.0037, equivalent to an age of 1925 94 years. The initial 40Ar/36Ar ratio defined by the isochron is 306.9 1.3, significantly higher than the atmospheric value and indicative of a trapped component of extraneous 40Ar. Excluding the 1- to 2-W steps, which are most likely to contain a significant proportion of adsorbed surficial argon (Fig. 1, inset), the remaining 33 steps yielded an isochron with indistinguishable age (1927 102 years) and initial 40Ar/36Ar (306.9 1.5) compared with results from the entire data set. The mean squared weighted deviation (MSWD) values of 0.51 (N 46) and 0.55 (N 33) indicate that observed scatter about the isochron is less than would be expected from the errors in the individual analyses; therefore, the errors on individual analyses are likely to have been slightly overestimated, or some error correlations are unaccounted for in the regression used. Because there is no objective basis for excluding the lowest temperature steps, we prefer the isochron obtained from all 46 analyses as the best estimate of the age of this sample. The presence of extraneous 40Ar is substantiated by the total gas results; the 40Ar/ 39 Ar apparent age (10) calculated from the sum of all gas released is 3300 500 years, clearly distinct within error from the known calendar age. With the known age, the con1279

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Fig. 1. Isotope correlation diagram showing isochron obtained by regression of 46 analyses. The inset shows detail of 13 analyses with the lowest 39Ar/40Ar. The mean of five olivine analyses is shown on the 36 Ar/40Ar axis of the inset with error bars, labeled OL. OL data are not included in the regression. Trapped component [(40Ar/36Ar)i ] is shown by bold arrow.

centration of extraneous 40Ar can be estimated from the weight of the sample (0.430 g), the total air-corrected 40Ar released (2.9 10 14 mol), and the K content (12.5%) of the sanidine, yielding (2.6 0.3) 10-14 mol/g. Uncertainties in mass spectrometer sensitivity calibration, sample weight, and K content limit the uncertainty to about 10%. This concentration would translate, if unrecognized, to 1200 years of excess age during the Quaternary, and 10% error through the Holocene, a bias that is considerably larger than uncertainties for high-quality analyses. Five unirradiated samples (20 to 75 mg each) of olivine from the pumice were degassed with a Nd­yttrium-aluminum-garnet (Nd-YAG) laser, yielding 40Ar/36Ar ratios (322 10 to 385 32) whose mean value (349 24) is distinct at 95% confidence from the atmospheric ratio. The trapped component in the olivine is not distinguishable from that in the sanidine at 95% confidence, although it probably reflects an earlier stage in the isotopic evolution of excess argon in the magma. The extraneous 40Ar could be inherited (13), having been derived from incompletely degassed xenocrysts entrained from the magma conduit during eruption. The consistent results from 12 different samples, however, would require an improbably uniform admixture of such xenocrysts. More likely, the extraneous 40Ar component is excess Ar entrapped in submicroscopic inclusions or defects within the sanidine during residence in the magma chamber (14). Whether the initial 40 Ar/36Ar ratio of 307 determined by isochron regression reflects a magmatic argon isotopic composition or is biased by argon adsorbed after the eruption cannot be determined from our data. Several aspects of laboratory procedure are most important in the ability to precisely resolve an 40Ar/36Ar date in Roman times: First, the broad-beam CO2 laser system provides
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progressive, uniform heating with low backgrounds that appears to release nonradiogenic argon at lower temperatures than it releases 40 Ar*, which helps create spread on the isotope correlation diagram. Second, the mass discrimination and background corrections must be based on well-characterized and reproducible values. The short irradiation time (3 min) and use of cadmium shielding to reduce thermal neutron f luence (15) minimized the importance of the (40Ar/39Ar)K correction, which in this case would add only 20 years to the calculated age if ignored. Finally, the use of an appropriately aged (Quaternary) neutron f luence monitor reduced the dynamic range of isotopic ratios to be measured, which minimizes potential problems resulting from peak-tailing or detector nonlinearity. Thus despite the presence of excess 40Ar, a sample less than 2000 years old can be dated with better than 5% precision, validating 40 Ar/39Ar dating as a reliable geochronometer into the late Holocene. These results also demonstrate that excess 40Ar can be identified in volcanic sanidine, and that while perhaps negligible in pre-Holocene rocks, it has important consequences for samples at the limit of the method's applicability. Further improvement in precision of 40Ar/39Ar analysis of historically dated samples may lead to welcome refinements in the ages of neutron fluence monitors, currently a limitation on the accuracy of the 40Ar/39Ar method (16). Our results also substantiate validity of the 40 Ar/39Ar method in establishing the eruptive histories of populated active volcanic regions, where such information is vital to volcanic hazard assessment (17).
REFERENCES AND NOTES ___________________________
1. G. B. Dalrymple, Earth Planet. Sci. Lett. 3, 289 (1968); P.-Y. Gillot and Y. Cornette, Chem. Geol. (Isot. Geosci. Sect.) 59, 205 (1986). 2. C. Merrihue and G. Turner, J. Geophys. Res. 71,

2852 (1966). 3. Y. Chen, P. E. Smith, N. E. Evensen, D. York, K. R. Lajoie, Science 274, 1176 (1996). 4. G. H. Curtis, in Potassium Argon Dating, O. A. Shaeffer and J. Zahringer, Eds. (Springer-Verlag, New Ё York, 1966), pp. 151­162. 5. P. Lo Bello et al., Chem. Geol. (Isot. Geosci. Sect.) 66, 61 (1987); A. L. Deino and R. Potts, J. Geophys. Res. 95, 8453 (1990). 6. H. Sigurdsson, S. Cashdollar, R. S. J. Sparks, Am. J. Archaeol. 86, 39 (1982); R. Cioni, P. Marianelli, A. Sbrana, Acta Vulcanologica 2, 109 (1992). 7. R. Cioni et al., J. Petrol. 36, 739 (1995). 8. The pumice clasts were washed vigorously in water before any further treatment to remove soil and any possible fragments of unrelated material that might have infiltrated into the porous deposit. We selected sanidine fragments 0.6 to 1.6 mm in diameter from crushed pumice that were free of microscopically visible inclusions of glass and other phases and loaded them into an aluminum vessel along with crystals of the Alder Creek sanidine (ACs), a 1.19 million-year-old sanidine [B. D. Turrin, J. M. Donnelly-Nolan, B. C. Hearn Jr., Geology 22, 251 (1994)] used as a neutron fluence monitor, and irradiated for 3 min in the Oregon State University (Corvallis) reactor in the cadmiumlined in-core (CLICIT ) facility. The ACs monitor was placed centrally in the irradiation packet, with all of the unknown sanidine grains within 3 mm of it. Five separate fusions of three to five grains of the monitor yielded an inverse variance-weighted mean J value (2) of (1.413 0.004) 10 5. The correction for 40Ar produced from 40K by thermal neutrons was made by using a value of (8 3) 10 4 determined as the mean and standard deviation of measurements from nine previous irradiations in the CLICIT facility. 9. The samples weighed 48 mg each and were loaded into 5-mm-diameter by 3-mm-deep wells in a copper disc. We degassed each sample in three to five steps up to fusion with a CO2 laser using an integrator lens to provide uniform heating over the wells. The laser system is described by W. D. Sharp and A. L. Deino (18). The CO2 laser allows controlled and progressive degassing of optically transmissive materials such as sanidine with minimal contribution to background levels of Ar [ W. C. McIntosh and M. T. Heizler, U.S. Geol. Surv. Circ. 1107, 212 (1994); (18)]. Background levels (blanks) of argon isotopes were measured between every one or two samples, and corrections were based on regression of the values over time. Mass discrimination was monitored by analysis of 45 samples of purified air interspersed with analysis of the unknowns, yielding a mean value of 1.0026 0.0002 per atomic mass unit based on a power law relation. 10. Apparent ages are calculated by assuming that all nonradiogenic 40Ar is atmospheric argon, using 36Ar to make a standard air correction. 11. A. O. Nier, Phys. Rev. 77, 789 (1950). 12. For isochron calculation we used the method of D. York [Earth Planet. Sci. Lett. 5, 320 (1969)]. 13. The terminology used here and throughout is that of G. B. Dalrymple and M. A. Lanphere [Potassiumargon Dating (Freeman, San Francisco, 1969)]. 14. I. M. Villa, Schweiz. Mineral. Petrogr. Mitt. 71, 211 (1991); R. P. Esser, W. C. McIntosh, M. T. Heizler, P. R. Kyle, Eos 77 (no. 17), S91 (1996); W. C. McIntosh, R. P. Esser, T. I. Wilch, ibid. 15. Now commonly used to reduce production of 40Ar from 40K by thermal neutrons, the use of cadmium shielding was first documented by N. Tetley, I. Mc Dougall, and H. R. Heydegger [J. Geophys. Res. 85, 7201 (1980)]. 16. P. R. Renne et al., Geology 22, 783 (1994). 17. G. Orsi, S. de Vita, M. de Vito, J. Volcanol. Geotherm. Res. 74, 179 (1996). 18. W. D. Sharp and A. L. Deino, Eos 77 (no. 46), F773 (1996). 19. We thank T. Becker, G. Curtis, G. Heiken, K. Ludwig, E. Piccirillo, M. Piochi, A. Necco, and J. Sime for discussion and assistance. Supported by NSF (grants SBR94-08181, EAR94-05309, and EAR9405347), the Ann and Gordon Getty Foundation, and the Osservatorio Vesuviano. 27 May 1997; accepted 17 July 1997

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