Twenty-two units and additional associated subunits have been identified in the basement section recovered from Hole 1206A. The entire sequence is characterized by alternating basaltic flow units, volcaniclastic sediment units, and biogenic-lithic sediment units (see "Physical Volcanology and Igneous Petrology").
All lava flows and volcaniclastic sediment have undergone secondary alteration. Alteration mineralogy and vesicle and vein fillings were defined in rocks from Site 1206 by color, habit, and hardness in hand specimen, by optical properties in thin section, and by analogy with well-studied minerals identified during previous legs. Some X-ray diffraction (XRD) analyses were conducted to confirm identification of secondary minerals. Because of instrumental problems encountered on board, these data should, however, be used with caution and only as a general guide for mineral identification.
The effects of alteration in rocks from Site 1206 are identified in the lava flow units in terms of (1) alteration assemblages and vein and vesicle filling and (2) alteration chemistry.
All lava flows recovered are slightly to highly altered basalt (Fig. F47). Overall, the degree of alteration is slight to moderate in the Hole 1206A basement sequence, although some units show higher degrees of alteration (e.g., Unit 1, parts of Unit 6, and the top of Unit 17). The alteration degree is fairly constant downhole.
The core sample color is the first indication of the alteration conditions. Cores fall mainly within the range of dark gray (N3) to medium-light gray (N6), which are characteristic of unaltered basalt. In more altered intervals, colors are variable between light brownish gray (5YR 6/1), grayish green (10GY 5/2), grayish red (5R 4/2), moderate yellowish brown (10YR 5/4), or grayish brown (5YR 3/2). No clear zonation was observed in the cores of Hole 1206A that could be related to different redox conditions in the sequence.
In the following sections, the alteration assemblages and vesicle and vein fillings are described. For a summary of downhole alteration assemblages and vesicle fillings, see Figure F47. Alteration assemblage and vein information were also recorded in the alteration and vein logs (see "Site 1206 Alteration Logs", and "Site 1206 Vein Logs"). The alteration assemblages and vein and vesicle fillings show only slight variations downhole.
The alteration assemblages are dominated by Fe oxyhydroxide, clay, and zeolite. Green clay such as smectite (saponite and nontronite) and blue-green clay such as celadonite are the main phases. Some brown clay is also present as well as white-pale green expanding smectite. The latter was recognized by XRD as aliettite (alternating talc- and saponite-layered smectite). Zeolite is present in almost all of the sequence. The main zeolite mineral was identified by XRD as phillipsite. Some carbonate (calcite) is present in the most altered part of the sequence (e.g., Units 1, 6, and 17).
Vesicularity is high in some intervals of the Hole 1206A sequence (see also "Physical Volcanology and Igneous Petrology"). Vesicles are variably unfilled, lined, or filled by secondary minerals. Figures F48 and F49 show examples of vesicles filled with brown clay, saponite (pale green), and celadonite (blue green). As for alteration assemblages, the dominant minerals are clay (brown clay, aliettite, saponite/nontronite, and celadonite), Fe oxyhydroxide, and zeolite. Zeolite is often present as small well-shaped crystals. Carbonates (mainly calcite) are also present in most of the sequence (Fig. F47).
The sequence in Hole 1206A is sparsely veined. Veins are usually very thin (~0.5 mm wide) and do not present a preferred orientation. Veins are mostly filled with carbonate associated with variable proportions of green clay (saponite/nontronite) and blue-green clay (celadonite). Zeolite is also present in the veins but in lesser amounts than in the vesicles (Fig. F47).
ICP-AES analyses performed on board are used in this section to evaluate the effects of alteration on whole-rock chemistry. It should be noted, however, that only the freshest samples were analyzed, and the data reported here might not be representative of the overall alteration effects. Variations in the abundance (K2O) and ratios (Cu/Zr, Co/Zr, and Zn/Zr) of some chemical elements and loss on ignition (LOI) vs. depth are reported in Figure F50. LOI is generally low in the whole sequence, varying from -0.32 to ~3 wt%. Six samples have LOI >3 wt% (Fig. F50) and are in Units 4, 6, 7, 11, and 17. These higher values confirm hand specimen observations. K2O abundances are low in the Site 1206 sequence, with values ~0.5 wt%. Three samples display slightly higher K2O content in Units 4 and 18 (Fig. F50). As these units are composed of alkali basalt, we believe that the higher K2O abundances are related to the parental magma composition. K2O abundance is mostly well correlated with LOI (Fig. F50), but the overall amounts remain very low, suggesting very slight chemical mobilization during alteration processes. The exception is the sample from Unit 18, which has high K2O content and low LOI. As Unit 18 is alkali basalt, this feature is likely related to the primary composition of the magma.
Co/Zr ratios are very constant downhole, with values of ~0.4. Zn/Zr ratios are also fairly constant downhole, with values of ~0.8. Three samples from Subunit 2c and Units 6 and 11 display higher ratios (>1), compatible with their higher LOI and K2O abundances. One sample from Unit 21 displays a high Zn/Zr ratio (>2) associated with low LOI. This feature is therefore more likely related to the primary composition of the magma. Two groups of samples can be identified with Cu/Zr ratios: a low Cu/Zr group with values of ~0.5 and a high Cu/Zr group with values of 1.1-1.2. The latter is composed of samples from Units 4 and 5 and one sample from Unit 6 (e.g., Sample 197-1206A-18R-1, 80-83 cm). No differences were noted between the alteration assemblages of both groups, and further studies are needed to determine whether the high Cu/Zr ratios are related to alteration processes or to the parental magma composition. The fairly constant Co/Zr and Zn/Zr ratios of the Hole 1206A sequence suggest that mobilization of these trace metals was relatively minor during alteration processes.
Subunits 2a and 2c and parts of Units 8, 11, and 14 are described as hyaloclastite olivine basalt breccia. Unit 19 is described as a volcaniclastic basalt breccia (see "Physical Volcanology and Igneous Petrology"). Alteration features associated with this breccia are similar to those described for basaltic lava flow units; olivine phenocrysts are variably altered to serpentine, iddingsite, or carbonate, and the groundmass is variably altered to clay, Fe oxyhydroxide, or zeolite. Cement is mostly composed of variable proportions of zeolite, green clay, and Fe oxyhydroxide (see also "Physical Volcanology and Igneous Petrology").
All igneous rocks recovered at Site 1206 (Hole 1206A) have undergone low-temperature alteration. Alteration features are defined in the lava flow units in terms of the secondary mineral assemblage, apparent as vesicle filling, vein filling, and replacement of groundmass and primary minerals. Overall, the lava flows are slightly altered, with some exceptions in Units 1, 6, and 17, where the degree of alteration is higher. The alteration assemblage is homogeneous downhole and is composed of Fe oxyhydroxide, saponite and/or nontronite, celadonite, and zeolite. In some specific intervals (e.g., Unit 6 [Core 197-1206A-18R] and Unit 17 [Core 197-1206A-37R]) the occurrence of white-pale green expanding smectite (aliettite) produced a complete disaggregation of portions of the rock as soon as the core came in contact with water at ambient pressure. This phenomenon can be related to the expandability of smectite, replacing the groundmass in large proportions. Despite this very high level of fracturing, part of these intervals are almost unaltered and contain fresh olivine (see also "Physical Volcanology and Igneous Petrology", and "Site 1206 Thin Sections"). Vesicle and vein fillings present the same mineralogical features. The entire Hole 1206A sequence is sparsely veined, indicating only small-scale fluid circulation. Most of the secondary alteration probably developed in diffuse fluid circulation systems.
All the identified alteration minerals are characteristic of very low temperature conditions. The estimated temperature for the formation of celadonite and nontronite are within the range of 30°-60°C (Alt, 1995), and saponite can form at slightly higher temperature (from 15° to 170°C) (Alt, 1995). In contrast to the first two sites of Leg 197 (Sites 1203 and 1204), K2O was not greatly mobilized during alteration event(s) at Site 1206. Trace elements also do not show evidence for mobilization, as expected for low-temperature fluid flow.