Geophysical Aging of Oceanic Crust: Evidence from Hole 801C

Abstract

The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, is thought to cause a corresponding evolution of geophysical properties such as velocity, density, and resistivity. This geophysical aging has been studied previously by comparing young and old crust formed at a slow spreading rate, as well as young crust formed at a moderate-to-fast spreading rate. Here we extend this analysis to the upper 131 m of volcanic crust at Hole 801C-old crust formed at a spreading rate faster than that of crust in previous studies. The geophysical aging of the upper basaltic crust at Hole 801C includes competing changes on intergranular and intraflow scales. At the intergranular scale, as seen by laboratory measurements on 2.5-cm core plugs, greater basalt alteration increases porosity and decreases matrix velocity and matrix density; alteration therefore dramatically decreases bulk velocity and bulk density. The same alteration effects on matrix properties are evident at the intraflow and interflow scale, as seen by logging tools with ~0.5-m vertical resolution, although that scale includes macroporosity unsampled by core plugs. Average velocities and densities measured in plugs and logs agree, possibly because alteration has filled cracks and interpiUow voids, thereby reducing macroporosity and increasing velocity and density. This meter-scale increase in velocity and density may dominate the plug-scale reductions, so that the average upper crustal properties at Hole 801C are as expected for very old crust: substantial alteration resulting in layer-2b velocities. Alternatively, the agreement between plugs and logs, as well as the observation of layer-2b velocities, may primarily be the result of the strong predominance of flows over pillows, because flows have much lower initial porosity than do pillows. Geochemical and geophysical logs discriminate three zones in the logged basement interval at Hole 801C: an upper sequence of altered alkalic basalts, a lower interval of tholeiites, and an intervening zone of hydrothermal precipitate and associated extensive basalt alteration. Imaging logs delineate the pervasive horizontal layering of filled fractures and of flow boundaries. A fossil hydrothermal zone, with predominantly silicon and iron composition and with extremely heterogeneous geophysical properties, includes some porosities and conductivities that are far above those expected in old oceanic crust.

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

“…CHN‐based hydration measurements were not available for the Leg 185 index property samples, so we determined relative variations in hydration with LAS. Figure 5a shows that matrix density for these basalts is only subtly correlated with LAS‐based hydration, in contrast to the much better association between matrix density and hydration for the shorter interval analyzed by Jarrard et al [1995]. Increasing hydration is associated with increasing porosity (Figure 5c), with a higher correlation ( R = 0.69) than observed by Jarrard et al [1995].…”

“…Although Hole 801C was drilled during Leg 129, no downhole experiments were undertaken in the hole at that time. ODP Leg 144 returned to this site and completed both packer [ Larson et al , 1993] and logging [ Jarrard et al , 1995] measurements. A complete suite of in situ geophysical logs was obtained through the upper 100 m of basalt, including velocity, density, neutron porosity, resistivity, three‐component magnetometer, geochemistry, and Formation MicroScanner® logs.…”

“…Physical properties of the upper hydrothermal zone and overlying alkali basalts have been presented and analyzed previously [ Busch et al , 1992; Wallick et al , 1992; Larson et al , 1993; Jarrard et al , 1995] and are not presented here. Our analyses focus on the tholeiitic section, first encountered on Leg 129 but mostly cored and logged during Leg 185.…”

“… Wallick et al [1992] also measured velocity versus pressure. Our analyses exclude the data of Wallick et al [1992], because of incomplete removal of interstitial water during drying [ Jarrard et al , 1995]. Leg 185 basalt physical properties sampling used a much closer sample spacing than at any previous deep crustal site: approximately three samples per core.…”

“…Typical H 2 O + contents of 3–6 wt % for the upper extrusives contrast with initial MORB H 2 O + of only 0.12 wt % in fluid inclusions [ Sobolev and Chaussidon , 1996]. For the portion of Hole 801C cored during Leg 129, the index property measurements of Busch et al [1992] were paired with geochemical analyses of Alt et al [1992], permitting demonstration of a close correlation between increasing hydration (greater H 2 O + ) and decreases in velocity, matrix density, and matrix velocity [ Busch et al , 1992; Jarrard et al , 1995]. An association between alteration and porosity was also evident, probably because of two factors: (1) alteration increases porosity and (2) high porosity promotes alteration because of high permeability and therefore enhanced fluid flow.…”