7. Williams, A. W. S., Lightowlers, E. C.& Collins, A. T. Impurity conduction in synthetic semiconducting diamond. J. Phys. C 3, 1727–1735 (1970). 8. Vishnevskii, A. S., Gontar’, A. G., Torishnii, V. I. & Shul’zhenko, A. A. Electrical conductivity of heavily doped p-type diamond. Sov. Phys. Semicond. 15, 659–661 (1981). 9. Werner, M. et al. Charge transport in heavily B-doped, polycrystalline diamond films. Appl. Phys. Lett. 64, 595–597 (1994). 10. Borst, T. H. & Weis, O. Boron-doped homoepitaxial diamond layers: fabrication, characterization and electronic applications. Phys. Status Solidi A 154, 423–444 (1996). 11. Zhang, R. J., Lee, S. T. & Lam, Y. W. Characterization of heavily boron-dopeddiamond films. Diamond Relat. Mater. 5, 1288–1294 (1996). 12. Eremets, M. I., Struzhkin, V. V., Mao, H.-K. & Hemley, R. J. Superconductivity in boron. Science 293, 272–274 (2001). 13. Werthamer, N. R., Helfand,E. & Hohenberg, P. C. Temperatureand purity dependence of the superconducting critical field Hc 2 . III. Electron spin and spin-orbit effects. Phys. Rev. 147, 295–302 (1966). 14. McMillan, W. L. Transition temperature of strongly coupled superconductors. Phys. Rev. 167, 331–344 (1968). 15. Gurevich, V. L., Larkin, A. I.& Firsov, Yu. A. On the possibility of superconductivity in semiconductors. Sov. Phys. Solid State 4, 131–135 (1962). 16. Cohen, M. L. Superconductivity in many-valley semiconductors and in semimetals. Phys. Rev. 134, A511–A521 (1964). 17. Cohen, M. L. in Superconductivity (ed. Parks, R. D.) Vol. 1, 615–644 (Marcel Dekker, New York, 1969). 18. Kohmoto, M. & Takada, Y. Superconductivity from an insulator. J. Phys. Soc. Jpn 59, 1541–1544 (1990). 19. Nozie `res, P. & Pistolesi, F. From semiconductors to superconductors: a simple model for pseudogaps. Eur. Phys. J. B 10, 649–662 (1999). 20. Hulm, J. K., Ashkin, M., Deis, D. W. & Jones, C. K. in Progress in Low Temperature Physics (ed. Gorter, C. J.) Vol. VI, 205–242 (North-Holland, Amsterdam, 1970). 21. Kawaji, H., Horie, H., Yamanaka, S. & Ishikawa, M. Superconductivity in the silicon clathrate compound (Na,Ba)xSi46. Phys. Rev. Lett. 74, 1427–1429 (1995). 22. Grosche, F. M. et al. Superconductivity in the filled cage compounds Ba6Ge25 and Ba4Na2Ge25. Phys. Rev. Lett. 87, 247003 (2001). 23. Khvostantsev, L. G., Vereshchagin, L. F. & Novikov, A. P. Device of toroid type for high pressure generation. High Temp. High Press. 9, 637–639 (1977). 24. Voronov, O. A. & Rakhmanina, A. V. Parameter of the cubic cell of diamond doped with boron. Inorg. Mater. 29, 707–710 (1993). 25. Brunet, F., Deneuville, A., Germi, P., Pernet, M. & Gheeraert, E. Variation of the cell parameter of polycrystalline boron doped diamond films. J. Appl. Phys. 81, 1120–1125 (1997). 26. Bean, C. P. Magnetization of hard superconductors. Phys. Rev. Lett. 8, 250–253 (1962). Acknowledgements We thank D. Wayne for mass spectrometry measurements of the B content of our samples, A. Presz for SEM images and S. Gierlotka for help in sample analysis. This work was supported by the Russian Foundation for Basic Research and by the Strongly Correlated Electrons Program of the Department of Physical Sciences, Russian Academy of Sciences. Work at Los Alamos was performed under the auspices of the US DOE. Authors’ contributions Boron-doped diamond samples were synthesized by E.A.E., and their physical properties measured by V.A.S., E.D.B., N.N.M., N.J.C., J.D.T. and S.M.S. Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to V.A.S. (sidorov@hppi.troitsk.ru) .............................................................. Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change Haemyeong Jung 1 , Harry W. Green II 1,2 & Larissa F. Dobrzhinetskaya 2 1 Institute of Geophysics and Planetary Physics, 2 Department of Earth Sciences, University of California, Riverside, California 92521, USA ............................................................................................................................................................................. Earthquakes are observed to occur in subduction zones to depths of approximately 680 km, even though unassisted brittle failure is inhibited at depths greater than about 50 km, owing to the high pressures and temperatures 1–3 . It is thought that such earth- quakes (particularly those at intermediate depths of 50–300 km) may instead be triggered by embrittlement accompanying dehy- dration of hydrous minerals, principally serpentine 1–3 . A problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2–4 GPa (60–120 km depth), above which brittle fracture mecha- nics predicts that the instability should be quenched 4,5 . Here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine-grained solid reaction products formed during dehydration. This phenomenon was observed under all conditions tested (pressures of 1–6 GPa; temperatures of 650–820 8C), independent of the sign of the volume change of reaction. Although this result contradicts expectations from fracture mechanics, it can be explained by separation of fluid from solid residue before and during faulting, a hypothesis supported by our observations. These observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth, as long as there are hydrous minerals breaking down under a differential stress. A popular hypothesis for overcoming brittle fracture inhibition, especially for earthquakes at intermediate depths (,300 km), is assistance of brittle fracture by generation of a free fluid as a result of dehydration of serpentine or other hydrous minerals 1–3,6–11 . The phenomenon of dehydration embrittlement was discovered almost 40 years ago 1 but has been studied only sporadically since that time 4–8 . In particular, this phenomenon has not been addressed by controlled deformation experiments at pressures greater than 700 MPa, equivalent to only ,20 km depth in Earth. Studies of acoustic emission at elevated pressures 5,8 , however, have impli- cations for deeper earthquakes. The latter studies, although lacking control of differential stress, strain, or strain rate, recorded acoustic emissions at much higher pressures and inferred that faulting had occurred. The fracture mechanics explanation of how dehydration embrittlement can enable brittle shear failure at elevated pressure is based upon production of a pore pressure as a consequence of a positive volume change, DV , of the dehydration reaction and consequent decrease in the effective pressure on existing or potential planes of weakness. Thus, as conventionally understood, the theory predicts that if the DV of the reaction were to become negative, failure would become more difficult and the shearing instability would vanish 4,5 . Because hydrous fluid is much more compressible than solid silicates, the total DV of dehydration of the common hydrous phases of primary interest (for example, serpentine and chlorite) is progressively reduced as pressure increases and becomes negative at pressures of 2–4 GPa, equivalent to pressures of 60–120 km in Earth. One consequence of this prediction is that earthquakes should not be possible by dehydration embrittlement at greater depths 5 . However, dehydration embrittlement is the only earthquake nucleation mechanism known to be viable for depths less than 300 km (ref. 2). Thus, it is important to determine whether this mechanism can function under conditions where DV is negative. We report here the results of deformation experiments at press- ures of 1–6 GPa and temperatures of 550–820 8C using an antigorite serpentinite from Val Malenco, Italy, for which the phase diagram has been measured 12 . Figure 1a shows the experimental conditions investigated; the slope of the high-temperature limit of antigorite stability (Fig. 1a) is negative above ,2.2 GPa, reflecting negative DV of reaction above that pressure. Figure 1b–f shows microstructures of the starting material and results of annealing without deformation outside antigorite sta- bility. The layering, strong foliation and proportions of antigorite and relict olivine shown are typical of our starting material. Figure 1f shows breakdown of antigorite along boundaries with relict olivine and healed cracks of several orientations outlined by fluid inclusions; such inclusion trails are rare in the starting material, letters to nature NATURE | VOL 428 | 1 APRIL 2004 | www.nature.com/nature 545 ©2004 Nature Publishing Group