The textural and mineralogical characteristics of the samples show that they can be divided into three major groups (Fig. 3, Fig 4; Table 2). Group 1 samples are almost completely altered basalt from the brecciated flow top, while Group 3 samples are massive basalts from the interior of the lavas with little alteration. Group 2 samples are altered basalt in the transition zone between the massive interior and brecciated flow top, but also include altered, highly vesicular samples.
Group 1 thin sections show highly vesicular lava broken into subangular fragments of partly devitrified glass. Pseudomorphs of non-aligned tablets of plagioclase were found in the vitreous groundmass with abundant dusty hematite and/or limonite. A few 0.1-mm olivine pseudomorphs were observed. Linings of chalcedony are abundant around the subangular scoria fragments and also where occasional phenocrysts of less altered clinopyroxene and impregnations of fine-grained magnetite are found. The modal XRD analysis shows 60%-80% clay and 20%-40% iron oxide.
Group 2 thin sections consist of subhedral and anhedral pseudomorphs of plagioclase and minor clinopyroxene and olivine phenocrysts. The 0.5- to 2-mm plagioclase phenocrysts are partially altered into microcrystalline clayey aggregates. The 0.2- to 0.5-mm olivine phenocrysts are completely altered into serpentine-chlorite-iddingsite aggregates, whereas the pyroxene phenocrysts appear virtually unaltered. The groundmass is a fine-grained plagioclase-magnetite-clinopyroxene matrix. Sparsely distributed vesicles lined with chalcedony are also characteristic. The modal XRD analysis shows about 30% plagioclase, 40%-50% pyroxene, 5%-15% clay, and 10%-20% iron oxides.
Group 3 thin sections consist of subhedral phenocrysts of plagioclase with grain sizes of 0.3-3.5 mm in an equigranular, fine-grained groundmass with grain sizes of 0.05-1 mm. The groundmass is composed of mainly clinopyroxene and plagioclase in an interlocking, nonaligned texture. Main accessory minerals are opaque ore minerals, chiefly magnetite, and olivine or olivine pseudomorphs as microcrystalline serpentine-chlorite-iddingsite aggregates. The modal XRD analysis shows 40%-50% plagioclase, 30%-40% pyroxene, ~5% olivine, and, finally, 10%-20% iron oxide.
The <2-µm-fraction XRD diffractograms show that smectite is the dominant clay mineral in all samples. Traces of kaolinite are identified in four samples, whereas chlorite is identified as ~25% of the clay minerals in one sample.
The major element compositions (Table 2) show systematic variations in the lava units. There is a downward decrease in aluminum, iron, potassium, and titanium oxide content. In contrast, the calcium, sodium, and phosphor oxide content is increasing downwards. The systematic changes in major element composition generally correlate with the degree of alteration. A notable exception is the potassium oxide content-in Unit 2 and Subunit 3A, this is lower in the highly altered upper brecciated interval, containing >60% clay, than in the intermediately altered sections below, containing <10% clay.
Shipboard measurements of seismic Vp in Unit 2 and Subunit 3A varies from 2.5 km/s in the brecciated flow top to 6 km/s in the massive interior, while a smaller variation of 4 to 5 km/s is observed in Unit 9 (Fig 4). The shore-based P-wave velocities measured under atmospheric conditions show 0.5 to 1.0 km/s lower values than the shipboard data (Fig 4; Table 2). The lithostatic pressure in the drilled basement is estimated to be 10-13 MPa, and P-wave velocities measured on 13 massive samples under 10 MPa confining pressure are 0.1 to 0.7 km/s (mean 0.4 km/s) lower than the velocities measured shipboard on the same samples (Cerney and Carlson, Chap. 3, this volume). At 200 MPa confining pressure, corresponding to ~7-km burial depth, the difference between the ship- and shore-based velocities are reduced to -0.1 to -0.2 km/s, with a mean deviation of 0.04 km/s.
The S-wave velocities are less reliable than the P-wave velocities because of the greater difficulties involved in picking the first arrivals. S-wave velocities measured under atmospheric conditions are in the range 2.0 to 2.9 km/s, with Vp/Vs values of 1.6 to 2.2 (mean 1.77). S-wave velocities measured on massive samples under 200 MPa confining pressure are in the range 2.7-3.5 km/s, with Vp/Vs values of 1.72-1.87 (Cerney and Carlson, Chap. 3, this volume).
The Vp data show that thermal and pressure relaxation and drying of the samples permanently change their elastic properties. The shipboard Vp measurements appear to be the most reliable, although microcracks may have already formed during drilling and recovery of the core. At later times the same samples have to be submitted to unrealistically high pressures (~200 MPa) to obtain results comparable with those measured immediately after the recovery.
The shipboard GRAPE bulk density varies systematically within the flow units in a pattern similar to the P- and S-wave velocities (Fig 4). However, there is a large scatter in the unedited GRAPE density data, and very low values are related to gaps between individual core pieces and variations in core diameter as well as to mineralogical and porosity changes. The GRAPE data were calibrated for cores within a lining. A recalibration and editing is required for hard-rock samples measured on the MST without a lining (Shipboard Scientific Party, 1996; Bücker et al., Chap. 5, this volume).
The shipboard bulk density minicore measurements are 0.3-0.4 g/cm3 higher than the GRAPE bulk density data (Fig 4). The subsequent shore-based density measurements provide intermediate values, typically 0.2-0.3 g/cm3 higher than the shipboard GRAPE density values. The difference is attributed to variations in core diameter and that the GRAPE was calibrated for quartz (grain density = 2.67 g/cm3) rather than for plagioclase and pyroxene (grain density ~3.0 g/cm3).
The porosity values are generally low (2%-5%) in the massive lava interior, but increase to 10% just above the massive interval (Fig 4). The porosity values in the vesicular Unit 9 are particularly high, decreasing downhole from 10% to 2%-6%.
There is an overall good fit between the MST data and the minicore magnetic susceptibility values (Fig. 2, Fig 4). The raw MST data are noisy, partly related to gaps between core pieces. The running-average smoothed susceptibility curve shows a distinctly different trend than the velocity and density data, a trend particularly well developed in Unit 2 and Subunit 3A. A susceptibility peak of ~30,000 10-6 SI is found in the transition zone between the brecciated flow top and the massive interior. Low susceptibility values (<10,000 10-6 SI) are recorded above the peak in the brecciated flow top, while intermediate susceptibility values of 10,000-25,000 10-6 SI are found in the massive basalt interior. In addition, less well-defined peaks are sometimes apparent near the brecciated base of the flow units.
Figure 5 shows the smoothed Vp plotted against selected physical and geochemical rock-sample parameters (Table 2). Data are further coded according to the petrological group and unit in which they belong. The Vp-porosity crossplot (Fig. 5B) shows an expected trend, with a clear negative correlation between porosity and velocity. Similarly, the Vp-LOI (loss on ignition, an alteration indicator) also shows an expected negative correlation (Fig. 5C).
Shales are typically highly anisotropic because of preferential alignment of clay minerals (Johnston and Christensen, 1995; Vernik and Liu, 1997). No strong preferential orientation of clay minerals is observed in Zone I or Zone II thin sections. In fact, the clays often are pseudomorph assemblages replacing primary phenocrysts. The Vp/Vs value is generally decreasing with increasing alteration and decreasing Vp (Fig. 5A). A small decrease in the Vp/Vs value is observed for slightly altered samples from Zone III, whereas the Vp/Vs value for the same samples increases as the confining pressure is increased from 10 to 200 MPa (Cerney and Carlson, Chap. 3, this volume). Alteration will soften the rock framework and decrease the proportion of low-aspect ratio voids (Cerney and Carlson, Chap. 3, this volume). The decrease in the Vp/Vs value with increasing alteration and crack closure may be contributed to the hypothesis that the Vs is more sensitive to changes in fracture density than the Vp (Iturrino et al., 1991).
The Vp-K2O crossplot reveals a more complicated relationship. Potassium is an alteration-dependent element, and the velocity generally decreases with increasing K content for Group 2 and 3 samples (Fig. 5D). In contrast, the completely altered Group 1 samples have intermediate K values. The natural gamma-ray log mainly responds to variations in the K content in basaltic terrains (e.g., Planke, 1994), but Figure 5D shows that the natural gamma-ray log cannot be uniquely inverted to obtain the clay proportion in flood basalt provinces.
Unit 2 and Subunit 3A may be divided into four distinct zones with different petrophysical and morphological characteristics (Fig. 4; Table 1, Table 2). Zone I is the upper brecciated zone, Zone II is the upper transitional zone, Zone III is the massive interior zone, while Zone IV is the basal transitional and brecciated zone. Zones I-III further correspond in detail to the three groups identified by the petrological and geochemical analysis.
Zone I: The upper brecciated zone is 2-7 m thick. P-wave velocities, densities, and magnetic susceptibilities are low and fairly constant (Vp = 2-3 km/s; = 1.6-1.8 g/cm3; magnetic susceptibility = 0-7.500 10-6 SI). It is characterized by almost complete alteration to clay and iron hydroxides. The vesicularity is 0 to 20%, with fractures and veins rarely identified.
Zone II: The upper transitional zone is 3-5 m thick and characterized by a gradual increase of Vp from 2.5 to 5.5-6 km/s, although there is a scatter of the data (Fig. 4). Similarly, the density increases from 1.7 to 2.8 g/cm3. In contrast, the magnetic susceptibility is high, with peak values of 20,000-40,000 10-6 SI. Alteration is intermediate, with 5%-15% modal clay and significant plagioclase alteration with modal pyroxene content greater than the plagioclase content. Porosity is relatively high, typically 5%-10%, while the vesicularity varies between 0 and 20%.
Zone III: The massive interior zone is 10-15 m thick. Petrophysical properties are fairly constant, with high velocities and densities and intermediate magnetic susceptibilities (Vp = 5.5-6 km/s; = 2.8-2.9 g/cm3; magnetic susceptibility = 5,000-20,000 10-6 SI) and characterized by massive, largely unaltered basalt. Total porosity is generally low (<5%). Similarly, vesicularity is low (<2%), but 1-cm-thick, highly vesicular bands are identified locally. Sparsely distributed, dominantly subvertical fractures are found in this zone.
Zones IV: The basal transitional and brecciated zone exhibits similar properties as the combined upper transitional and brecciated zones but are only 1-2 m thick. No samples were collected for shore-based studies from this basal zone.
The four-zone structure is less well developed in the very vesicular pahoehoe basalt Unit 9. Only a 10-cm, very fragmented section was recovered from the flow top (Zone I). The thickness is poorly constrained, as 4.25 m of section is missing in Core 21R (Fig. 2). Sample 163-990A-21R-2, 30-32 cm, collected 50 cm from the flow top, has similar characteristics as the Zone II samples in Unit 2 and Subunit 3A (Table 2). Samples from deeper parts of the unit are relatively unaltered, although they show geochemical and petrological characteristics similar to the samples from Zone III in Unit 2 and Subunit 3A (Table 2; Fig. 4). The magnitude and trend of the velocity and density data differs in Unit 9. Here, the Vp increases gradually from 4.5 to 5.0 km/s, while density increases similarly from 2.4 to 2.7 g/cm3. The lower velocity and density in the interior of Unit 9, compared to Unit 2 and Subunit 3A, are clearly related to the high vesicularity of 15%-25% in the majority of this lava flow. The basal part of the flow is poorly recovered, and Zone IV is not identified.