Hole 896A was drilled in a 6.83-m.y.-old crust located 1 km away from Hole 504B, that is, about 200 km south of the Costa Rica Ridge, in the equatorial east Pacific. It penetrated 290 m into basement formed by variably altered pillow basalts, with frequent pillow breccias and hyaloclastites, and a few massive lava flows. The average recovery was 28%. Thirty-five samples were collected for magnetic properties measurements, petrographic and mineralogical observations, and bulk-rock chemical analyses. The study shows that all but two of the study samples have been altered by seawater at low temperature, that is, probably at less than 50°C, as indicated by the rock Curie temperatures averaging 326°C, high K2O contents up to 0.26%, and high Fe oxidation ratio ranging from 0.33 to 0.65. The samples contain homogeneous Ti-maghemite as the only ferrimagnetic mineral with a size that ranges from 0.5 to 300 µm, but is generally comprised between 1 and <10 µm. In such rocks olivine is often completely replaced by saponite with or without iddingsite and rare celadonitic minerals, whereas plagioclase is replaced by saponite in the most altered samples containing saponite with minor phillipsite veins. Void spaces are filled with saponite, iddingsite, and, more rarely, celadonitic minerals. Calcite is only present in a few samples. Two samples that have holohyaline textures corresponding to a pillow lava rim and a hyaloclastite do not contain ferrimagnetic minerals and are paramagnetic.
Curie temperatures are fairly invariable throughout the hole (Tc = 326 ± 31°C with a slight increase below 380 mbsf. Saturation magnetization increases downhole, with average values of 0.37 and 0.67 Am2/kg above and below 330 mbsf, respectively. This suggests that the basalts contain almost twice as much ferrimagnetic phases below than above this depth. The saturation magnetization increase with depth is coherent with an increase in magnetic susceptibility and in contrast to a strong decrease in NRM intensity occurring below 364 mbsf (Allerton et al., this volume). The saturation remanence to saturation magnetization ratios of the study samples are <0.2 below 330 mbsf, whereas they range from 0.2 to 0.45 above this depth, indicating that the upper basalts contain pseudo-single domain grains and, hence, have very stable magnetic properties.
The two unaltered samples are coarse, subophitic basalts with essentially fresh olivine showing only traces of replacement by saponite along cracks and with a relatively large Ti-magnetite content (i.e., 10–200 µm in diameter). These samples have low Curie temperatures (209°–250°C), a low K2O content (0.02%), and a low oxidation ratio (0.23). They also have low porosities.
Bulk-rock Curie temperatures, K2O contents, and oxidation ratio are three parameters whose high values indicate low-temperature alteration of the MORBs by seawater. The presence of titanomaghemite as the sole ferrimagnetic phase in all but two study samples, as confirmed by their small range of Curie temperature values, implies a homogeneous alteration process of the primary titanomagnetite of the basalts. This is in contrast to the extreme variability of the alteration mineralogy and chemistry of the basaltic silicates, which do not show any systematic variation. Therefore, two different low-temperature alteration processes appear to have occurred.
Date of initial receipt: 19 August 1994
Date of acceptance: 10 August 1995
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