Abstract
Superfast spread (22 cm/a) upper oceanic crust drilled at Ocean Drilling Program site 1256 comprises a thick sequence of extrusive lavas underlain by a thin region of sheeted dikes. The sheeted dikes at this site are characterized by the intimate association of in situ hyaloclastic fragmentation and hydrothermal alteration. Lithostatic and magmastatic pressure estimates reveal that there is no effective level of neutral buoyancy within the extrusive layer to trap dikes in the crust. This results in a situation that favors the extrusion rather than intrusion of magma. Fractures created by magmas rising in dikes were the loci of intense hydrothermal circulation and provided the pathways for subsequent dike intrusions. Magma‐rich conditions expected for fast spreading ridge segments give rise to a rapid increase in magmatic pressure, which can result in dike intrusion even under only small deviatoric stress. This allows for the emplacement of narrow dikes in the upper crust, with magmas more likely to extrude, and hence the high extrusive/intrusive ratio for Site 1256.
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