Tubes [S. B. Shirey and R. J. Walker, Anal. Chem. 67, 2136 (1995)] for 48 hours at 230°C. After dissolution, the PGEs were isolated from one another and the bulk- rock matrix by anion-exchange chromatography [M. Rehka ¨ mper and A. N. Halliday, Talanta 44, 663 (1997)]. The pure PGE fractions were then analyzed with a Plas- ma 54 multiple-collector inductively coupled plasma mass spectrometer [A. N. Halliday et al., Int. J. Mass Spectrom. Ion Processes 146/147, 21 (1995)]. Multiple analysis of Iceland basalt sample BTHO and Alexo ko- matiite sample KAL-1 demonstrate that our techniques achieve external reproducibilities of 1.5 to 9% for the PGEs in the concentration range from parts per billion to parts per trillion. Duplicate analysis of two peridotite samples (OMX-8 and C235A) indicate a somewhat low- er reproducibility (15%), probably owing to the heter- ogeneity of the coarse-grained rocks. The PGE ratios of the duplicates, however, were identical to within 1 to 8%. Blanks for the Cameroon Line samples, digested with conventional Carius Tubes, were 10 pg/g for Ru, Pd, and Ir and 100 to 200 pg/g for Pt. Northern Tanza- nian xenoliths were digested with the use of a modified Carius Tube design, and this technique achieves blanks of 15 pg/g for all analyzed PGEs (M. Rehka ¨ mper, A. N. Halliday, R. F. Wentz, Fres. J. Anal. Chem., in press). 10. D.-C. Lee et al., J. Petrol. 37, 415 (1996). 11. L. Cahen et al., The Geochronology and Evolution of Africa (Clarendon, Oxford, UK, 1984); S. F. Toteu, A. Michard, J. M. Bertrand, G. Rocci, Precambrian Res. 37, 71 (1987). 12. J. B. Dawson, D. G. Powell, A. M. Reid, J. Petrol. 11, 519 (1970); R. Hutchison and J. B. Dawson, Earth Planet. Sci. Lett. 9, 87 (1970); W. I. Ridley and J. B. Dawson, in Physics and Chemistry of the Earth, L. H. Ahrens, J. B. Dawson, A. R. Duncan, A. J. Erlank, Eds. (Pergamon, New York, 1975), pp. 559 –569. 13. R. L. Rudnick, W. F. McDonough, A. Orpin, in Pro- ceedings of the Fifth International Kimberlite Confer- ence, Araxa ´ , Brazil, 18 June to 4 July 1991, H. O. A. Meyer and O. Leonardos, Eds. (Companhia de Pes- quisa de Recursos Minerais, Rio de Janeiro, Brazil, 1991), vol. 1, pp. 336 –353. 14. R. L. Rudnick, W. F. McDonough, B. W. Chappell, Earth Plant. Sci. Lett. 114, 463 (1993). 15. P. H. Nixon, in Mantle Xenoliths, P. H. Nixon, Ed. ( Wiley, Chichester, UK, 1987), pp. 215 –240. 16. S. E. Kesson and A. E. Ringwood, Chem. Geol. 78, 97 (1989). 17. R. S. Cohen, R. K. O’Nions, J. B. Dawson, Earth Planet. Sci. Lett. 68, 209 (1984). 18. J. B. Dawson, in Mantle Metasomatism, M. A. Men- zies and C. J. Hawkesworth, Eds. (Academic Press, London, 1987), pp. 125 –144. 19. N. J. Page, J. S. Pallister, M. A. Brown, J. D. Smew- ing, J. Haffty, Can. Mineral. 20, 537 (1982); N. J. Page and R. W. Talkington, ibid. 22, 137 (1984); N. J. Page, T. Engin, D. A. Singer, J. Haffty, Econ. Geol. 79, 177 (1984); A. Cocherie, T. Auge ´, G. Meyer, Chem. Geol. 77, 27 (1989); B. McElduff and E. F. Stumpfl, Mineral. Petrol. 42, 211 (1990); M. Leblanc, Econ. Geol. 90, 2028 (1995); J. Vuollo et al., ibid., p. 445. 20. H. J. B. Dick and T. Bullen, Contrib. Mineral. Petrol. 86, 54 (1984). 21. J. A. Pearce, S. J. Lippard, S. Roberts, in Marginal Basin Geology, B. P. Kokelaar and M. F. Howells, Eds. (Blackwell, Oxford, UK, 1984), pp. 77–94; D. Elthon, Nature 354, 140 (1991). 22. G. Agiorgitis and R. Wolf, Chem. Geol. 23, 267 (1978); I. O. Oshin and J. H. Crocket, Econ. Geol. 77, 1556 (1982); C. J. Copobianco, R. L. Hervig, M. J. Drake, Chem. Geol. 113, 23 (1994). 23. Spinels from fertile western Australian lherzolites, for example, are characterized by superchondritic Pd/Ir ra- tios (2), as are chromitites from stratiform chromite de- posits [C. McLaren and J. P. R. De Villiers, Econ. Geol. 77, 1348 (1982); S. J. Perry, Chem. Geol. 43, 115 (1984)]. Furthermore, a large proportion of the PGE budget of fertile peridotite xenoliths appears to be con- tained in acid-leachable, intergranular sulfide phases, whereas spinel is only a minor host for the PGEs (2) [S. R. Hart and G. E. Ravizza, in Earth Processes: Read- ing the Isotopic Code, A. Basu and S. R. Hart, Eds. (American Geophysical Union, Washington, DC, 1996), pp. 123–134]. Although chromitites from ophiolite com- plexes are characterized by high PGE abundances, a number of detailed studies demonstrate that the PGEs are not incorporated into the lattice of the chromites but are concentrated in sulfide and alloy inclusions; the chromites themselves have no bearing on the fraction- ation of the PGEs [for example, H. W. Stockmann and P. F. Hlava, Econ. Geol. 79, 491 (1984); R. W. Talking- ton, D. H. Watkinson, P. J. Whittaker, P. C. Jones, Tschermaks Mineral. Petrogr. Mitt. 32, 285 (1984); R. J. Walker, E. Hanski, J. Vuollo, J. Liipo, Earth Planet. Sci. Lett. 141, 161 (1996)]. 24. Three abyssal harzburgites from the MARK area (Mid-Atlantic Ridge, Kane Fracture Zone) with 1.0% CaO are characterized by suprachondritic Pd/Ir ratios and Pt /Ru ratios that are only marginally subchondritic (M. Rehka ¨ mper et al., in preparation). Two harzburgites from the Horomann peridotite (CaO  0.5%), generally considered to represent former suboceanic mantle lithosphere, have Pd/Ir ratios of 0.15 to 0.20 but higher-than-chondritic ra- tios of Pt /Ir and Pt /Ru [E. Takazawa, thesis, MIT (1996); M. Rehka ¨ mper et al., in preparation]. These characteristics are remarkably similar to the results for harzburgite sample N12 from the Cameroon Line (this study). Six harzburgites and dunites from the Ronda and Beni Bousera massifs with 0.3 to 1% CaO display a wide range of Pd/Ir ratios (0.24 to 1.44) and Pt /Ru ratios (0.31 to 2.40) (6). These val- ues are significantly larger, by at least a factor of 5, than the minimum ratios recorded for the northern Tanzanian xenoliths. 25. I. Kushiro, Am. J. Sci. 267-A, 269 (1969). 26. Z. Johann, H. Dunlop, L. Le Bel, J. L. Robert, M. Volfin- ger, Fortschr. Mineral. 61, 105 (1983); P. Schiano et al., Earth Planet. Sci. Lett. 146, 489 (1997). 27. S. A. Wood, Geochim. Cosmochim. Acta 51, 3041 (1987); M. E. Fleet and T.-W. Wu, ibid. 57, 3519 (1993). 28. R. R. Keays, E. H. Nickel, D. I. Groves, P. J. McGold- rick, Econ. Geol. 77, 1535 (1982); C. G. Ballhaus and E. F. Stumpfl, Contrib. Mineral. Petrol. 94, 193 (1986); A. D. Brandon, R. A. Creaser, S. B. Shirey, R. W. Carlson, Science 272, 861 (1996). 29. J. T. Wasson and G. W. Kallemeyn, Philos. Trans. R. Soc. London Ser. A 325, 535 (1988). 30. K. P. Jochum, Geochim. Cosmochim. Acta 60, 3353 (1996). 31. G. W. Kallemeyn and J. T. Wasson, ibid. 50, 2153 (1986). 32. W. F. McDonough and S.-s. Sun, Chem. Geol. 120, 223 (1995). 33. We thank M. Johnson, D.-C. Lee, J. Christensen, C. Hall, D. Teagle, and the other members of the RIGL team for help in keeping the P54 running smoothly and for many fruitful discussions; C. Paslick for field assistance; D. Bugocki and W. Yi for vital assistance during sample preparation; and S. Mukasa and J. Zipfel for helpful comments. Three anonymous refer- ees provided very thorough and helpful reviews. This research was supported by NSF grants EAR 94 – 06248 and EAR 96 –14457 and by DOE grant DE- FG02–94ER14412. 2 June 1997; accepted 20 October 1997 Transitions Between Blocked and Zonal Flows in a Rotating Annulus with Topography Eric R. Weeks, Yudong Tian, J. S. Urbach,* Kayo Ide, Harry L. Swinney,† Michael Ghil The mid-latitude atmosphere is dominated by westerly, nearly zonal flow. Occasionally, this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer. Experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the Coriolis force. In the presence of two symmetric ridges at the bottom of the annulus, the resulting flows were nearly zonal at high forcing or blocked at low forcing. Intermittent switching between blocked and zonal patterns occurs because of the jet’s interaction with the topography. These results shed further light on previous atmospheric observations and numerical simulations. On short time scales (1 to 10 days), weather evolution is largely driven by three-dimen- sional, baroclinic instabilities of the prevail- ing westerlies (1) that convert the potential energy in the atmosphere’s pole-to-equator temperature difference into the kinetic en- ergy of storms (2). One to three times each Northern Hemisphere winter—and occa- sionally during other seasons—large high- pressure anticyclones form and persist for at least 10 days and sometimes longer than a month (3–5). These anticyclones block the nearly zonal flow and deflect it poleward (Fig. 1B). The prediction of blocking events has become central to improving extended- range weather prediction (6, 7). Low-frequency atmospheric variability on the time scale of 10 to 100 days, such as persistent blocking anomalies, is predomi- nantly barotropic, that is, nearly two-dimen- sional (5, 8, 9). Analytic and numerical mod- els have shown that blocked and zonal flow patterns can arise through the interaction of large-scale eastward zonal flow with idealized Northern Hemisphere topography (7, 8, 10– 12), and recent numerical simulations using a general circulation model (13) support these results. Zonal and blocked flows appear as two stable equilibria (8, 10, 11) or two separate chaotic flow regimes (12, 14–16) in simple and intermediate models. E. R. Weeks, J. S. Urbach, H. L. Swinney, Center for Nonlinear Dynamics and Department of Physics, Univer- sity of Texas at Austin, Austin, TX 78712, USA. Y. Tian, K. Ide, M. Ghil, Department of Atmospheric Sci- ences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA. *Present address: Department of Physics, Georgetown University, Washington, DC 20057, USA. †To whom correspondence should be addressed. E-mail: swinney@chaos.ph.utexas.edu SCIENCE  VOL. 278  28 NOVEMBER 1997  www.sciencemag.org 1598