Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

Abstract

[1] The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity-dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large-scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age-dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular-scale crustal alteration occurs after the first 10-15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.

Expedition 301 synthesis: hydrogeologic studies

“…F3A). This change in geophysical properties within the uppermost extrusive crust is similar to changes seen in other upper crustal sections (e.g., Bartetzko et al, 2001;Jarrard et al, 2003;Pezard et al, 1992).…”

“…Logging data help to define the lithostratigraphy of Hole U1301B and establish whether or not conditions are as expected, in comparison to measurements made on core samples and borehole and core data collected from other basement holes (Bartetzko and Fisher, 2008;Tsuji and Iturrino, 2008). Logging data show that upper basement around Hole U1301B is highly layered at a scale of meters to tens of meters, similar to what is observed in other ocean crustal holes (e.g., Bartetzko et al, 2001;Jarrard et al, 2003;Pezard et al, 1992). Rapid penetration during drilling and poor hole conditions during casing operations suggest that the upper 85 m of basement is highly brecciated and poorly cemented.…”

Site 1256

“…Figs. F178, F181), likely dominated by 40 K (Jarrard et al, 2003), indicating a high abundance of clay minerals in these samples.…”

Carbon dioxide sequestration in deep-sea basalt

“…Carbonate precipitation over time may alter in situ porosity and permeability within basalt aquifers, however, and thus progressively decrease the CO 2 -basalt reaction rate to a finite limit. Although natural weathering processes in deep-sea basalt precipitate pore-filling carbonates, fractured and permeable basalt crust extends for millions of years before its porosity has been appreciably filled (24). Land-based experiments provide some insight into these effects, but estimating the in situ rates and accelerated effects, if any, of carbonate precipitation in basalt are difficult to predict without deep-sea CO 2 injection experiments.…”