Bulk and clay mineralogy of ODP Leg 110 holes

The mineralogy of both bulk- and clay-sized fractions of samples from Sites 671, 672, and 674 of ODP Leg 110 was determined by X-ray diffraction. The major minerals include quartz, calcite, plagioclase feldspar, and the clay minerals smectite, illite, and kaolinite. The smectite is a dioctahedral montmorillonite and is derived primarily from degradation of volcanic ash. Percentage of smectite varies with sediment age; Miocene and Eocene sediments are the most smectite-rich. High smectite content tends to correlate with elevated porosity, presumably because of the ability of smectite clays to absorb significant amounts of interlayer water. Because of a change in physical properties, the decollement zone at Site 671 formed in sediments immediately subjacent to a section of smectite-rich, high-porosity, Miocene-age sediments. Sediments above the decollement at Site 671, as well as all sediments analyzed from Sites 672 and 674, contain nearly pure smectite characteristic of the alteration of volcanic ash. Within the decollement zone and underthrust sequence, however, the smectite contains up to 65% illite interlayers. Although the illite/smectite could be interpreted as detrital clay derived from South America, its absence in the sediments stratigraphically equivalent to the decollement and underthrust sequences at Sites 672 and 674 favors the interpretation that it originated by diagenetic alteration of pre-existing smectite similar to that in the overlying sediments. A significant percentage of the freshening of the pore waters observed in these zones could be due to the water released during smectite dehydration.

(Table 1) Bulk and grain density, porosity, and p- and s-wave velocity characteristics of ODP Hole 163-990A sediments

Seismic velocities in rocks are influenced by the properties of the solid, the pore fluid, and the pore space. Cracks dramatically affect seismic velocities in rocks; their influence on the effective elastic moduli of rocks depends on their shape and concentration. Thin cracks (or fractures) substantially lower the moduli of a rock relative to the effect of spherical voids (or vesicles), and lower moduli are reflected by lower P- and S-wave velocities. The objective of this research is to determine the types and concentrations of cracks and their influence on the seismic properties of subaerially erupted basalts drilled from Hole 990A on the Southeast Greenland margin during Ocean Drilling Program Leg 163. Ellipsoidal cracks are used to model the voids in the rocks. The elastic moduli of the solid (grains) are also free parameters in the inverse modeling procedure. The apparent grain moduli reflect a weighted average of the moduli of the constituent minerals (e.g., plagioclase, augite, and clay minerals). The results indicate that (1) there is a strong relationship between P-wave velocity and porosity, suggesting a similarity of pore shape distributions, (2) the distribution of crack types within the massive, central region of aa flows from Hole 990A is independent of total porosity, (3) thin cracks are the first to be effectively sealed by alteration products, and (4) grain densities (an alteration index) and apparent grain moduli of the basalt samples are directly related.

Bulk mineralogy of Cretaceous to Quaternary sediments from the Kerguelen Plateau

Carbonate-free portions of Upper Cretaceous to Holocene sediment samples from the Kerguelen Plateau in the southern Indian Ocean were investigated by X-ray diffraction. Downhole variations in the content of opal-A, opal-CT, quartz, feldspar, barite, and clinoptilolite were studied at Site 737 on the northern Kerguelen Plateau and at Sites 744 and 738 on the southern Kerguelen Plateau. The variation of these components reflects temporal changes in the depositional history of the Kerguelen Plateau as well as major differences in the sedimentary evolution between the northern plateau and the southern plateau. Carbonate is the dominant component in the pelagic sediments on the Kerguelen Plateau. In addition, biogenic opal sedimentation plays an important role throughout most of the sequence. A major increase in opal accumulation is documented at all sites in late Miocene time, which is in accordance with the well-known increase in silica productivity probably caused by a major cooling step. Because of its position near the Polar Frontal Zone, sediments from Site 737 show a more extensive opal deposition than at Sites 744 and 738. An earlier productivity pulse is documented at Site 744 on the southern plateau within the early Oligocene, following the initial phase of intense East Antarctic glaciation. This cooling event resulted in higher amounts of ice-rafted terrigenous quartz and, to a lesser extent, feldspar. With the exception of the Site 744 sediments, opal deposition in Paleogene and older sediments can be reconstructed only from the diagenetic transformation products of opal-CT and probably clinoptilolite. In contrast to the southern sequence, on the northern Kerguelen Plateau higher amounts of clinoptilolite and no opal-CT were found. These major differences in the diagenetic environments may be due to extensive volcanism in the northern area. The volcanic influence at Site 737 is well recorded by the higher feldspar content and higher amounts of volcanic glass shards.

Grain-size and bulk and clay mineralogy of ODP Leg 204 sediments

We present grain size, granulometric statistical parameters, and calcium carbonate content of sediment samples from the summit and east and west flanks of southern Hydrate Ridge (Sites 1244-1250). These data are compared with magnetic susceptibility measurements from the same intervals. Bulk and clay mineralogy from Sites 1244 (east flank), 1247 (west flank), and 1250 (summit) are also presented. The integration of these data allows us to characterize the main sedimentary facies and composition of the Quaternary age sediments from southern Hydrate Ridge.