The study of the physical properties of gabbroic rocks from Hole 1105A is of significant value in understanding the evolutionary history of the Atlantis II Fracture Zone and in assessing the relative contributions of magmatic and tectonic processes to crustal evolution. Advantages of drilling along the Atlantis II Fracture Zone are that the geologic and tectonic frameworks are well established, previous drilling has been highly successful, and the water depth is shallow. Drilling operations during Leg 179 at Site 1105 were done in a water depth of ~695 m on a wave-cut platform devoid of the basaltic and diabasic carapace typical of a generic model of ocean crustal stratigraphy. Routine onboard measurements of physical properties included whole-core magnetic susceptibility, split-core magnetic intensity, inclination, and declination, as well as index properties on discrete minicore samples. These data can be examined as a function of lithology and alteration and can aid in the identification of boundaries between lithologies.
The physical properties of gabbroic rocks provide constraints for crustal model studies based on geophysical data. Measurements of the magnetic properties of these rocks are necessary to assess the contribution of lower crustal rocks to observed magnetic anomaly patterns, which form the basis of the universally accepted theory of plate tectonics advanced by Vine and Mathews (1963). Although many studies have apportioned the primary source of these anomalies to the upper crustal extrusive basalts in the generic ocean crust model (e.g., Le Pichon and Heirtzler, 1968; Mckenzie and Sclater, 1971; Klitgord et al., 1975; and Patriat and Achache, 1984), recent paleomagnetic studies suggest that gabbroic rocks may provide a significant contribution to the magnetic anomaly signature (Kikawa and Pariso, 1991; Pariso et al., 1991).
Continuous rock magnetic susceptibility measurements of 118 m of whole core were completed using the Bartington magnetic susceptibility meter with an 80-mm loop sensor. Measurements were made at 2-cm intervals. Several idiosyncrasies inherent in the data set archived in the JANUS database should be noted. First, the susceptibility meter saturates at values >10,000 SI. As a result, a value of 12,500 SI will show up in the database as 2,500 SI. Figure F73 shows a continuous record of magnetic susceptibility measurements for the entire section cored at Hole 1105A. A simple filter, which removes anomalously low values on either side of values approaching the saturation limit of measurement, has been applied to the data in this plot. Additionally, measurements for Core 179-1105A-3R, 28.70-32.42 mbsf, are not included in the database because it was initially curated incorrectly, and the whole core was run through the MST with pieces out of order. However, using off-line reapportioning of the measurements, these data were reintegrated into data files used for additional processing (see Table AT4.XLS in the "Appendix" contents list).
It should be noted that the magnetic susceptibility measurements as recorded in the database have not been volume normalized. Most hard-rock cores (including all recovered during Leg 179) have an average radius of <58 mm and have irregular shapes and sizes. Additionally, the sphere of influence of the susceptibility loop is on the order of 10 cm. Therefore any measurement on a piece <10 cm long or at a location <5 cm away from the end of a piece will necessarily have a greater error than measurements on the central parts of continuous large pieces of core. Figure F74 shows the whole-core magnetic susceptibility data after filtering for piece length. These data have been smoothed using a weighted moving average (dashed line).
As expected, the magnetic susceptibility data shows a marked correlation to macroscopic descriptions of the core. Intervals identified with abundant Fe-Ti oxide minerals in hand sample show a one-to-one correspondence with increased magnetic susceptibility. As an added benefit, using the magnetic susceptibility data as a proxy for oxide-mineral abundance, hence as an indicator of change in modal mineralogy, we can correlate and even refine locations of interval and unit boundaries in the core. The interval boundaries in the graphic lithology column on Figure F74 were established based on petrographic criteria. The same interval boundaries are marked by a sharp increase or decrease in the magnetic susceptibility profile. An example of the utility of the magnetic susceptibility data is reflected in the position of the Subunit IIA/IIB boundary. From petrologic criteria, the boundary between these two subunits is gradational, marked by a gradual grain-size change and a gradual decrease in reported abundance of oxide minerals. However, in the magnetic susceptibility profile, there is a sharp decrease in the value of the core at 92.79 mbsf. This exact location in the core is described as the uppermost contact of a downward succession from gabbro, to olivine gabbro, to oxide-bearing olivine gabbro, to oxide olivine gabbro.
After splitting the cores, the archive section was passed through the shipboard cryogenic magnetometer. Continuous natural remanent magnetization (NRM) intensities, inclination, and declination were measured at 2-cm intervals. As with the whole-core data, some caution should be exercised in application of the split-core measurements. The response function of the superconducting quantum interface device (SQUID) sensors is subject to similar edge effects as the magnetic susceptibility loop. For these measurements, non-oriented pieces <10-cm long were removed from the archive half before measurement of NRM. The data have not, however, been filtered for the effects of piece ends or the gaps between pieces. Additionally, recent work on gabbroic rocks from the Atlantis II Fracture Zone (J. Gee, pers. comm., 1998) indicates the 20-mT demagnetization is insufficient to remove the drilling-induced radially oriented magnetic overprint. Table AT5.XLS (see the "Appendix" contents list) is a compilation of the NRM data from split-core measurements. Note that the only filtering applied to these data is removal of overlaps from cores with >100% curated recovery. Figure F75 shows the relationship between declination, inclination, and intensity with depth. As with measurements from Leg 118 (Hole 735B), inclinations are mostly positive and steep. The small number of negative inclinations is almost certainly caused by unoriented pieces or small pieces, which were inadvertently flipped upside down. The simple arithmetic average of inclination values after 20-mT demagnetization is 67.4º (±19.8º), comparable to the 65º (±7º) calculated from shipboard data during Leg 118 (Robinson, Von Herzen, et al., 1989). The theoretical inclination of the site at 33ºS is -52º, and the age of the oceanic crust is 11.75 Ma (magnetic Chron 5 -r2n; Dick et al., 1991c). The observed inclinations are reverse in accordance with Chron 5, but steeper than the theoretical value. For a discussion of the steepening inclinations see "Structural Geology". Declination data should be examined with caution because the true azimuth of the cores is not fixed.
Compressional wave velocities (Vp) at ambient pressure and temperature were measured on 62 minicores from the Hole 1105A core. The measured velocities (see Table AT6 in the "Appendix" contents list) averaged 6.64 km/s and are comparable to the values for gabbroic rocks measured during Leg 118 (Shipboard Scientific Party, 1989) and are representative of seismic Layer 3. Figure F76 shows the velocities measured relative to depth for the entire section sampled during Leg 179. The four major lithologies identified by macroscopic core description are all represented in the sample suite and have slightly different average Vp. Rocks characterized as gabbro and olivine gabbro have similar average Vp (6.74 ± 0.04 and 6.70 ± 0.07 km/s, respectively). The amount of olivine is quite variable in these lithologies, although commonly 10% or less, and we would expect that because the plagioclase and clinopyroxene modal proportions are much higher and relatively constant that there is no significant variation in the Vp of these lithologies. In contrast, the oxide gabbro and oxide olivine gabbro samples have markedly lower Vp values (6.53 ± 0.76 and 6.59 ± 0.07 km/s, respectively). The lower velocity values can be attributed to the abundance of Fe-Ti oxide minerals, and the higher variability of values from the oxide gabbros is likely caused by the highly variable degree of alteration and variation in modal oxide abundance in these samples.
Mass and volumetric measurements were made on 56 minicores representative of the various lithologies sampled from Hole 1105A (see Table AT7 in the "Appendix" contents list). The mean bulk density of all samples is 2.93 ± 0.15 g/cm. This compares with a mean bulk density of 2.97 ± 0.12 g/cm for samples from Leg 118. The mean porosity of the Leg 179 gabbroic rocks is 1.05 ± 0.49%, which is somewhat higher than the average porosity of samples from Leg 118 (0.64 ± 0.73%, Shipboard Scientific Party, 1989).