In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust

Detrick, R. S.; Collins, J. A.; Stephen, Ralph A.; Swift, Stephen A.

doi:10.1038/370288a0

Cited by 174 publications

(171 citation statements)

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“…Hole 504B remains the only site whether the seismic Layer 2/3 boundary has been penetrated (e.g., Detrick et al, 1994). At that location, the change in seismic gradient clearly occurs within the dikes and the Layer 2/3 transition most probably reflects The upper gabbros are more strongly hydrothermally altered than the immediately overlying sheeted dikes that have been subjected to contact metamorphic recrystallization.…”

Section: Discussionmentioning

confidence: 99%

“…A fundamental question we will address in this experiment is how velocity changes within seismic Layer 2 and the Layer 2-Layer 3 transition relate to physical, lithological, structural, and alteration variations in the volcanic rocks, dikes, and gabbros. At Site 504, in crust generated at an intermediate-rate spreading ridge, the Layer 2-Layer 3 transition lies within the 1 km thick sheeted dike complex and coincides with a metamorphic change (Detrick et al, 1994;Alt et al, 1996a), but it is unknown whether the results from Hole 504B are representative of ocean crust in general or of crust generated at different spreading rates. Is the depth to gabbros shallower in crust generated at a superfast spreading rate as predicted, and what are the relative thicknesses of volcanic and dike sections compared with crust constructed at slow or intermediate spreading rates?…”

Section: Scientific Objectives Of Expeditions 309 and 312mentioning

confidence: 99%

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Site 1256

2006

Proceedings of the IODP

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Section: Discussionmentioning

confidence: 99%

Section: Scientific Objectives Of Expeditions 309 and 312mentioning

confidence: 99%

Site 1256

2006

Proceedings of the IODP

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“…Our preferred explanation for this difference is the presence of a thicker extrusive/dike transition at 35øN than is typical of the EPR. Within the well-resolved depth range of our experiment (1-2.5 km below the seafloor), factors that may be responsible for the velocity variation with depth include a decrease in porosity due to lithostatic pore closure [Spudich and Orcutt, 1980], crack cementation by alteration products [Wilkens et al, 1991], a decrease in the proportion of high-porosity extrusive rocks relative to dikes [Herron, 1982], and an increase in metamorphic grade [e.g., Detrick et al, 1994]. Without additional information, the significance of each of these processes cannot be ascertained.…”

Section: Discussionmentioning

confidence: 99%

Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

Barclay¹,

Toomey²,

Solomon³

1998

J. Geophys. Res.

116

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Abstract. We have determined the three-dimensional P wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the Mid-Atlantic Ridge (MAR) near 35øN. The best fitting one-dimensional model is -0.8 km/s lower in velocity at a given depth than for fast spreading ridges, while the isotropic three-dimensional velocity structure shows a strong lateral heterogeneity (up to 1.4 km/s peak-to-peak variation at 1 km depth) in both the along-and cross-axis directions. An anomalous region, with velocities as much as 0.8 km/s lower than average and 3-5 km in lateral extent, is located beneath a near-axis seamount. We interpret this feature as a region of near-solidus temperatures with possibly a small degree of partial melt. From the variation of travel time residuals with azimuth we determine a best fitting P wave anisotropy model that has 4% anisotropy from 500 m to 1 km depth, 2% anisotropy in the depth range 1-1.5 km, and no anisotropy at greater depth. This finding is consistent with vertical cracks aligned parallel to the axial valley. We find no evidence for the shallow, high-velocity, ridge-parallel feature commonly observed at the axis of fast spreading ridges and attributed to high-velocity dikes beneath a thin extrusive layer. The absence of this anomaly at the MAR implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25,000 years. An unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes, which may account for the generally lower velocities beneath the MAR compared with the East Pacific Rise, and a lateral variation in upper crustal lithology.

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“…Porosities and thermal conductivities decrease with depth in the dikes, and seismic velocity and resistivity gradients flatten out at about 1500 mbsf. Detrick et al (1994) interpreted these changes as marking the transition from seismic Layer 2 to Layer 3 at Site 504. Drilling in Hole 504B was halted in a suspected fault zone at 2111 mbsf, where the drilling rate increased in the presumably pulverized rock, and in which the drill string became stuck, preventing further penetration (Alt, Kinoshita, Stokking, et al, 1993).…”

Section: Site 504mentioning

confidence: 99%

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

Alt¹,

Teagle²,

Bach³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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Leg 148 penetrated 111m farther into Hole 504B, to a total depth of 2111 meters below seafloor (mbsf), and recovered diabase dikes similar to the immediately overlying rocks. Secondary mineralogy and whole-rock oxygen and strontium isotopic compositions were determined for samples from Leg 148. In addition, strontium isotopic analyses were performed on previously recovered samples of the lower dikes (1550-2000 mbsf). Trends observed in the lower dikes (1550-2000 mbsf) continue in the Leg 148 section (2000-2111 mbsf). These include the local presence of magnesiohornblende and secondary calcic plagioclase and high Ti contents of amphiboles. Cu, Zn, and S contents of the more altered rocks are low, and whole-rock δ 18 θ values of 33%350°C) and greater extents of recrystallization than in the upper dikes. All of the data for the new Leg 148 section are consistent with the rocks being a continuation of a subsurface reaction zone, where metals and sulfur are leached from the crust by hydrothermal fluids and transported to form metal sulfide mineralizations on or within the crust or vent into seawater.Sr contents of the lower sheeted dikes (1550-2111 mbsf) are generally 48-62 ppm, and whole-rock 87 Sr/ 86 Sr ratios range from 0.70265 to 0.70304, only slightly elevated relative to fresh MORB values. The rocks were altered by seawater-derived hydrothermal fluids, with little change in the Sr concentrations of the rocks. Although amphibole is the most abundant secondary phase, the Sr isotopic composition of the rocks is probably dominated by the abundance of secondary plagioclase. The 87 Sr/ 86 Sr ratios of hydrothermal fluids were higher than measured in variably recrystallized (10%-80%) whole rocks. The seawater component of hydrothermal fluids increased through time, as documented by 87 Sr/ 86 Sr ratios of 0.7028-0.7034 for amphibole and chlorite, to 0.7035-0.7038 for vein epidotes. The latter values record the composition of upwelling black smoker-type endmember hydrothermal fluids at Site 504.The low 87 Sr/ 86 Sr ratios for the lower dikes differ from the uniformly high ratios of the thoroughly recrystallized sheeted dikes from the Troodos ophiolite. The oceanic dikes interacted with much smaller volumes of seawater than the ophiolitic rocks and apparently to a lesser extent. These differences are in some way related to the contrasting tectonic setting, mineralogy, and chemical composition of ophiolites compared to in situ ocean crust.

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In situ evidence for the nature of the seismic layer 2/3 boundary in oceanic crust

Cited by 174 publications

References 20 publications

Site 1256

Site 1256

Seismic structure and crustal magmatism at the Mid‐Atlantic Ridge, 35°N

Stable and Strontium Isotopic Profiles through Hydrothermally Altered Upper Oceanic Crust, Hole 504B

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