Gabbroic rocks and peridotites are exposed on the seafloor on the western median valley wall of the Mid-Atlantic Ridge, south of the Kane Transform (MARK). The gabbroic rocks occupy an uplifted massif directly south of the transform-ridge intersection, whereas the peridotites extend 20 km along a median valley parallel ridge just south of the gabbro massif. Acoustic velocity measurements have been made at elevated confining pressures for a suite of samples extracted from drill cores collected during Ocean Drilling Program Leg 153. Drilling operations at Site 920 produced the deepest penetration and most substantial recovery to date in a coherent block of serpentinized peridotite from any ocean basin. Site 923, in the gabbro massif, yielded nearly 75% recovery of fresh troctolite, olivine gabbro, and gabbro. A sample suite was selected from these drill cores to be representative of the primary lithologies recovered.
Evaluation of physical properties measurements from the serpentinized peridotites suggests that serpentinization is an ongoing and rapid process such that we can see evidence of changes in these properties over a time span of a few months and potentially as quickly as a few days. Samples showed a broad range of degree of serpentinization, even between samples only a few centimeters apart. A strong negative correlation exists between degree of serpentinization and density, as well as compressional- (Vp) and shear- (Vs) wave velocity, for samples from ophiolitic peridotites, and the serpentinized samples from MARK mimic this correlation. Published data indicate that Vp, Vs, and density of intensely serpentinized peridotites and fresh peridotites plot in separate and distinct fields as compared to values derived from gabbroic rocks. Physical properties data from moderately serpentinized peridotites from MARK, however, as well as published data from samples exhibiting partial serpentinization, are virtually indistinguishable from the values obtained from gabbroic rocks.
Physical properties data are in accord with petrographic observations, indicating that the gabbroic samples collected at MARK are considerably less altered than gabbroic rocks sampled from near Hess Deep. Data from gabbroic samples suggest that, given reliable densities, velocity data from remote geophysical surveys may be useful in estimating oceanic crustal modal composition. Elastic constants derived from physical properties measurements suggest that, although drilling in the gabbroic massif at MARK may be more difficult in terms of bit life than in other gabbroic exposures on the seafloor, the holes may well be more stable and conducive to extended drilling operations.
Date of initial receipt: 28 July 1995 Date of acceptance: 19 December 1995
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