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doi:10.2204/iodp.pr.305.2005
Onboard physical property data in combination with downhole logging data provide an initial means to assess which aspects of the geological characteristics of the domal core of Atlantis Massif might contribute to the regional geophysical data sets. In addition, inherent rock properties can be assessed and related to rock type and alteration.
Magnetic susceptibility is highest in the dunitic troctolites recovered from Hole U1309D, and it is quite low in most of the gabbros (Fig. F33A, F33B). Both dunitic troctolite and oxide gabbro intervals can have very high magnetic susceptibility signal (2000–10,000 x 10–5 instrument units), although only the former is consistently at these levels. The dunitic troctolites are commonly highly serpentinized (Fig. F33C), and the susceptibility reflects magnetite produced during the alteration process.
The natural remanent magnetization (NRM) of the rocks from Hole U1309D was determined onboard following removal of a drilling-induced overprint. Alternating-field (AF) demagnetization (typically 30 mT) was used to remove the overprint, and the bulk of the archive-half sections show negative inclination direction (Fig. F33D), which corresponds to a reversed magnetic polarity epoch. Minicore samples, cleaned of overprint by either AF (up to 100 mT) or thermal demagnetization (to 500°–550°C), generally show very good agreement with the half-core inclination patterns downhole. The inclination for most of the hole is somewhat shallower (~32°–38°) than would be predicted for the geomagnetic field at the site location (48°). However, the interval from ~900 to 1100 mbsf clearly has a steeper negative inclination (~42°). The standard deviation of these inclination estimates is –12°. The change to higher values is sharp, the lower boundary possibly coinciding with an inferred small fault at 1100 mbsf. Whether there is any relationship between structure or lithology and the top of this interval is not clear at this stage.
There are a number of short intervals that have positive NRM inclination. All such intervals correspond to rocks with high magnetic susceptibility and intensity—troctolite and dunitic troctolite. The steepness of some of the positive polarity determinations suggests caution, as the drilling overprint may not have been fully removed. However, many of the positive values are quite stable, so the signature must have been imparted during a normal polarity epoch. A valid working hypothesis at this point is that these intervals record the relative chronology between crystallization of the gabbro (negative polarity) and both serpentinization and intrusion of diabase intervals (positive polarity).
The physical properties that are most relevant for relating the rocks from Hole U1309D to broader scale geophysical measurements are seismic velocity and density. Onboard measurements provide an indication of the inherent properties of small samples at room temperature and pressure. Variability in the measured values can be due to a number of factors, including mineralogy, porosity and grain size, and the style and degree of alteration.
A significant change in several core sample and downhole logging properties occurs between 280 and 400 mbsf (Fig. F34). Density values have reduced scatter and slightly higher average values below 350 mbsf, increasing from 2.8 g/cm3 in the interval 280–340 mbsf to 2.9 g/cm3 in the interval 350–400 mbsf. Average compressional velocity (VP) of minicore samples in the 280–340 mbsf interval drops to 5.3 km/s (from 5.5 km/s in the overlying 200 m) before increasing to 5.7 km/s at 340–400 mbsf. Logging VP increases from ~5.5 to 6.0 km/s between 340 and 370 mbsf. These changes combine to produce a significant impedance contrast (Fig. F35), and this can be related to the seismic reflection data, as discussed below. Electrical resistivity measured by the Dual Laterolog shows a marked increase over the same interval (Fig. F34). The low but variable values of all these physical properties are associated with serpentinization of olivine-rich rock types (dunitic troctolites and, to a lesser extent, olivine gabbros that overly them) in the 280–340 mbsf interval (Fig. F34A). Overall alteration here is greater than in the overlying section, averaging 50%–75%, and it drops steadily to 20%–40% by 400 mbsf. Higher velocity and density values correspond to the underlying gabbroic interval.
Both on a local and a broader scale, the influence of crack closure with depth may contribute to the impedance contrast at 300–400 mbsf. Samples have consistently higher VP (average = 5.6 km/s) than the downhole logging data (average = 4.5–5.5 km/s) in the ~50–300 mbsf interval, but the two data sets merge near 350 mbsf with mean sample and logging values tracking rather closely for the rest of the seismically logged interval (Fig. F34). The existence of open cracks in the upper few hundred meters is expected and would explain the lower logging velocities. However, the continued small downhole increase in measured sample density suggests that closure of microcracks may continue to greater depths.
If ~340 mbsf coincides with the D reflector, the average velocity of the overlying section would need to be ~5.44 km/s locally, slightly lower than the 5.54 km/s value determined by a vertical seismic profile (VSP) shot to 345 mbsf during the expedition. Both of these values are higher than the 5.0 km/s interval velocity noted by Canales et al. (2004) for the regionally applicable interval velocity above this reflector.
A second strong gradient in borehole electrical resistivity is present from 730 to 760 mbsf. Below 760 mbsf, average sample velocities steadily decrease from ~5.9 to ~5.5 km/s at 1415 mbsf. The cause of this decrease is not clear but could be related to progressive microcracking water depth, due to unloading during the coring process.
The VSP experiment extended from 275 to 840 mbsf. Tool failure and high seas combined to preclude collection of any seismic measurements during the final logging run. Initial analysis, based on automatic arrival picks of stacked seismograms, indicates an average velocity in the upper 550 m of the footwall in Hole U1309D of 5.5–5.6 km/s. An increase is indicated for greater depths by higher average velocities (5.8 km/s for stations 580–796 mbsf). The actual gradient of velocity is uncertain at this stage. More detailed processing postcruise (filtering, selection of individual seismograms that make up the stack, and repicking) should improve the estimates.
Temperature in the borehole increases with depth as expected (Fig. F36). Because of the significant impact of drilling in the hole, the measurements made almost certainly provide minimum estimates of what the actual formation temperature is. The Temperature/Acceleration/Pressure tool recorded a temperature of 120°C at the bottom of the hole (1415 mbsf). The temperature is somewhat lower than predictions from a simple cooling plate model of a spreading ridge flank of age near 2 Ma. However, the measured temperatures are a minimum (owing to hole cooling during coring) and much more careful measurement is required before quantitative interpretation should be made. These initial results suggest that thermally driven flow in the hole is likely to occur.