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Many of the ideas incorporated here were generated within the ACSOE programme funded by the U.K. National Environmental Research Council and the JONUS programme funded by the U.K. Ministry of Agriculture Fisheries and Food; the Ministry of Transport, Environment and the Regions; and the Environment Agency. The views expressed are mine and do not reflect the policy of the funding departments. I acknowledge useful discussions with all my colleagues in these projects, particularly S. Malcolm, R. Sanders, and L. Spokes. I also acknowledge the fine work of the many researchers whose papers I have drawn on for this review and the helpful comments of the referees. The Quantum Event of Oceanic Crustal Accretion: Impacts of Diking at Mid-Ocean Ridges J. R. Delaney, D. S. Kelley, M. D. Lilley, D. A. Butterfield, J. A. Baross, W. S. D. Wilcock, R. W. Embley, M. Summit REVIEW Seafloor diking-eruptive events represent the irreducible, quan- tum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy’s real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subsea- floor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically ac- tive portions of our planet, other hydrothermally active planets may harbor similar life forms. The ocean basins cover nearly 60% of Earth’s surface. They are underlain by 5 to 6 km of oceanic crust formed through solidification of molten basalt along the global spreading center network (Fig. 1). The basalt is generated by partial melting in the rising mantle beneath mid-ocean ridges. Annually, about 20 km 3 (1) of basaltic magma is buoyantly emplaced along the spreading centers where it cools to form the trailing edges of large tectonic plates. Rates of plate diver- gence vary from 10 to nearly 200 mm per year. Upon rising into the young crust bounding the zone of divergence, the melt commonly collects in crustal chambers located 1 to 2 km below the seafloor. A significant fraction of the magma solidifies slowly in the chamber to form coarse-grained gabbros, but the remaining magma episodically breaks through the crystalline roof of the chamber, rises to the seafloor through planar, ridge-parallel conduits, and produces submarine erup- tions of basaltic lava flows. The conduits are known as dikes, a term that also applies to the rapidly cooled rock that fills the conduit after magma transfer ceases. J. R. Delaney, D. S. Kelley, M. D. Lilley, J. A. Baross, W. S. D. Wilcock, and M. Summit are at the School of Oceanography, University of Washington, Seattle, WA 98195, USA. D. A. Butterfield is at the Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration (NOAA), Seattle, WA 98115, USA. R. W. Embley is at the Pacific Marine Environmental Laboratory, NOAA, Newport, OR 97365, USA. S PECIAL S ECTION C HEMISTRY AND B IOLOGY OF THE O CEANS 10 JULY 1998 VOL 281 SCIENCE www.sciencemag.org 222