Permeability of oceanic crustal rock samples from IODP Hole 1256D

Abstract

Permeability is an important parameter of oceanic crust: it controls hydrothermal circulation and influences the exchange of heat and chemicals between seawater and the crust. Using the most complete section of intact, in situ normal oceanic crust, this paper presents the first permeability measurements of samples from Integrated Ocean Drilling Program (IODP) Hole 1256D in a relatively undisturbed section through lavas, dikes, and into gabbros. At in situ pressures, saturated gabbro from Hole 1256D is about half as permeable as basalt (2.4 × 10−20 m2 and 4.0 × 10−20 m2, respectively). Although fresh basalt samples have higher permeabilities, the basalts at Hole 1256D contain saponite, an alteration mineral which drastically reduces permeability. These measurements represent an opportunity for comparison to models that predict permeability at IODP Hole 1256D. Similar to model predictions, sample permeability generally decreases with depth. However, even after applying the scaling rule, models predict higher permeabilities than exhibited by the samples, suggesting large-scale cracks still control permeability in the 15 My old crust at Hole 1256D.