High-Pressure Velocity Measurements of Jurassic Basalt, Leg 129

Wallick, Brian P.; Christensen, Nikolas I.; Ballotti, Dean M

doi:10.2973/odp.proc.sr.129.140.1992

Cited by 3 publications

(14 citation statements)

References 11 publications

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“…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.…”

Section: Datamentioning

confidence: 99%

“… Busch et al [1992] and Wallick et al [1992] measured core‐based physical properties of upper Hole 801C basalts. These measurements included index properties (bulk density, matrix (or grain) density, porosity) and P wave velocity at atmospheric pressure.…”

Section: Datamentioning

confidence: 99%

“…These measurements included index properties (bulk density, matrix (or grain) density, porosity) and P wave velocity at atmospheric pressure. 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].…”

Section: Datamentioning

confidence: 99%

“… 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.…”

Section: Datamentioning

confidence: 99%

“…Comparison of the two trends indicates generally lower formation factors, for a given velocity, for log data than for core data. Velocity difference between in situ and laboratory pressures is unlikely to be responsible for this offset between trends, because rebound here is small [ Wallick et al , 1992] and reduces both velocity and formation factor.…”

Section: Site 801 Analysismentioning

confidence: 99%

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

et al. 2003

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[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.

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Section: Datamentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Section: Site 801 Analysismentioning

confidence: 99%

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

et al. 2003

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Geophysical Aging of Oceanic Crust: Evidence from Hole 801C

Jarrard¹,

Larson²,

Fisher³

et al. 1995

Proceedings of the Ocean Drilling Program, 144 Scientific Results

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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.

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Elastic-Wave Velocities in Jurassic-Age Oceanic Crust from Analysis of Sonic Full Waveform Logs in Hole 801C

Moos¹,

Schaack²,

Ito³

1995

Proceedings of the Ocean Drilling Program, 144 Scientific Results

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Using sonic full waveforms recorded by a borehole compensated logging sonde during Leg 144, we determined P-and S-wave velocities in the uppermost basement interval (from 480 to-560 mbsf) of Jurassic crust penetrated by Hole 801C. The basement consists of three principal units: an uppermost ~157-Ma alkali basalt, erupted off-axis; a zone of hydrothermal deposits; and a section of ~167-Ma mid-ocean ridge tholeiites. In the alkali basalts, V p ranges from 4.8 to 5.1 and V^ ranges from 2.4 to 2.6 km/s, yielding V p /V s between 1.9 and 2.0. In the tholeiites, V p ranges from 5.2 to 6.2 and V^ ranges from 2.8 to 3.4 km/s, yielding V p /V s between 1.75 and 1.85. We were unable to confirm the extremely low f-wave velocities reported previously within sections of the hydrothermal interval. Within the basalts, low-velocity zones mark the boundaries between individual lithostratigraphic units. A comparison of sonic log velocities within crusts of a wide variety of ages with laboratory-determined ultrasonic velocities reveals that (1) lower average in situ velocities in young crust are a consequence of the presence of macroscopic voids and fractures and of systematic sampling bias; (2) these effects are minimized in old crust by the infilling of fractures and voids and by a decrease in sampling bias in older, more altered basalts; (3) velocities within pillows and interflow zones never reach values equivalent to those in massive units; and (4) alteration of the basalt matrix can significantly reduce its velocity. Velocities, therefore, are controlled by the relative percentages of pillows and flows, the progressive increase in velocity within the pillows resulting from infilling, and an alteration-dependent decrease in velocities within the basalt matrix with increasing crustal age.

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High-Pressure Velocity Measurements of Jurassic Basalt, Leg 129

Cited by 3 publications

References 11 publications

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

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

Geophysical Aging of Oceanic Crust: Evidence from Hole 801C

Elastic-Wave Velocities in Jurassic-Age Oceanic Crust from Analysis of Sonic Full Waveform Logs in Hole 801C

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