ALTERATION AND WEATHERING

Thirty basement units were identified in Hole 1205A. In this hole, the sequence includes 25 units or subunits of basaltic composition, 7 volcaniclastic breccia units or subunits, and 12 soil horizons, defined as units or subunits. Unit 4 is a sandstone (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 1205 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. However, instrumental problems were encountered during the cruise and the XRD data should be used only as a general guide for mineral recognition.

The effects of alteration in rocks from Site 1205 are identified in the lava flow units in terms of (1) alteration assemblages and vein and vesicle filling and (2) alteration chemistry.

Alteration Assemblages and Vein and Vesicle Fillings

All lava flows recovered are slightly to highly altered basalt (Fig. F24). Overall, the degree of alteration is slight in the Hole 1205A basement sequence. However, several thin weathered flow tops were identified and present higher degrees of alteration (see "Physical Volcanology and Igneous Petrology"). The alteration degree increases slightly downhole toward flows of tholeiitic composition (see "Physical Volcanology and Igneous Petrology"), although the degree of alteration remains low.

The sample color is the first indication of the alteration conditions. Cores fall mainly in the range dark gray (N3) to medium-light gray (N6), which are characteristic of unaltered basalt. In more altered intervals, colors are variable between light brown (5YR 5/6), moderate yellowish brown (10YR 5/4), dark yellowish orange (10YR 6/6), grayish orange (10YR 7/4), or greenish gray (5G 6/1). No clear zonation was observed in the cores of Hole 1205A 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 and vein fillings, see Figure F24. All vein information was also recorded in the alteration and vein logs (see "Site 1205 Alteration Logs", and "Site 1205 Vein Logs"). The alteration assemblages and vein and vesicle fillings are generally homogeneous 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. Zeolite minerals are present in almost all of the sequence. The main zeolite mineral was identified by XRD as phillipsite. A small amount of carbonate was identified in the uppermost units (Subunits 1a and 3b) as well as some secondary pyrite in Subunit 1a. In the most altered intervals, Fe oxyhydroxide dominates and is sometimes well crystallized as goethite. From examination of thin sections (see "Site 1205 Thin Sections"), we identified small amounts of micas next to the bottom of Hole 1205A (Subunits 28a, 29b, and 30b). These minerals partly replace olivine (as phlogopite-biotite) or are incipient in the groundmass (as chlorite and stilpnomelane?).

Vesicularity is high in some intervals of the Hole 1205A sequence (see also "Physical Volcanology and Igneous Petrology"). Vesicles are variably unfilled, lined, or filled by secondary minerals. Figure F25 shows an example of vesicles filled and/or lined with celadonite. As for alteration assemblages, the dominant minerals are clay (saponite/nontronite and celadonite), Fe oxyhydroxide (sometimes as goethite), and zeolite. Zeolite is often present as well-shaped crystals. In the uppermost units of the Hole 1205A sequence, vesicles are also filled with carbonate.

The basement sequence in Hole 1205A is very sparsely veined. Veins are usually very thin (~0.5 mm wide) and do not present a preferred orientation. Veins are filled with variable proportions of green clay (saponite/nontronite), blue-green clay (celadonite), Fe oxyhydroxide, and zeolite minerals (Fig. F24). Overall, zeolites are the dominant alteration minerals.

Alteration Chemistry

ICP-AES analyses of major and trace element contents were conducted on board and are used in this section to evaluate the effects of alteration on the whole-rock chemistry. It should be noted, however, that mainly the freshest samples were analyzed and these data might not be representative of the overall effect of alteration processes. Variations in the abundances and ratios of some chemical elements (K2O and Cu/Zr, Co/Zr, and Zn/Zr ratios) and loss on ignition (LOI) vs. depth are reported in Figure F26. LOI is low in the whole sequence, varying from ~1 to 3 wt%. LOI in one sample from volcaniclastic Subunit 19b (Sample 197-1205-35R-4, 77-79 cm) is higher (5.52 wt%). LOI increases slightly downhole toward the tholeiitic basalt units, which confirms the hand-specimen observations and the visual estimations of the degree of alteration (Fig. F24).

K2O abundance is low in the Hole 1205A sequence except for the two first samples (e.g., Samples 197-1205A-5R-2, 31-33 cm, and 5R-2, 101-103 cm), which are clasts within a conglomerate. K2O abundance decreases slightly downhole from ~1.5 to 0.5-1 wt%. This feature is more likely related to a change in the parental composition of the magma than a change of the alteration conditions, as the uppermost units are alkali basalt and several of lowermost units are tholeiitic basalt (see "Physical Volcanology and Igneous Petrology"). The sample from Subunit 19b, which has high LOI, does not have a high K2O abundance, indicating that K2O might not be highly mobilized during alteration of the Hole 1205A sequence.

Co/Zr ratios are constant downhole, with values of ~0.5. The only exception is the sample from Subunit 19b, which displays a higher ratio (~1.2). Zn/Zr ratios are also fairly constant downhole, with values of ~0.5. Three samples have higher ratios at ~1: Subunit 5b (e.g., Sample 197-1205A-16R-2, 84-86 cm), Unit 16 (e.g., Sample 197-1205A-32R-2, 114-116 cm), and Subunit 19b (e.g., Sample 197-1205A-35R-4, 77-79 cm). Cu/Zr ratios are also fairly constant downhole, with values of ~0.5. Cu/Zr ratios slightly increase downhole toward tholeiitic units. The constant Cu/Zr, Co/Zr, and Zn/Zr ratios of the Hole 1205A sequence indicate that these trace metals were not likely mobilized during alteration processes.

Volcaniclastic Units

Subunits 3a, 8b, 19c, 24b, 26b, 28b, and 30a are described as volcaniclastic basaltic breccia (see "Physical Volcanology and Igneous Petrology"). Subunits 3a, 8b, and 30a are olivine-plagioclase-phyric basalt breccia, Subunit 19c is composed of olivine-phyric basalt breccia, and Subunits 24b and 26b are aphyric basalt breccia. Alteration features associated with these breccia are similar to those described for basaltic lava flow units. Cement is composed of carbonate in Subunit 3a and variable proportions of Fe oxyhydroxide and zeolite in the other units.

Summary

All igneous rocks recovered at Site 1205 (Hole 1205A) have undergone low-temperature alteration. Alteration features are defined in the basaltic units in terms of secondary mineral assemblage, apparent as vesicle filling, vein filling, and replacement of groundmass and primary minerals. Overall, the lava flows are slightly altered with a small increase downhole toward the units of tholeiitic basalt composition. Weathered flow tops were recognized where the degree of alteration is higher (see also "Physical Volcanology and Igneous Petrology"). The alteration assemblage is homogeneous downhole and is composed of Fe oxyhydroxide, saponite and/or nontronite, celadonite, and zeolite. Vesicle and vein fillings present the same assemblages. Veining is very sparse in the entire Hole 1205A sequence, indicating small focused fluid circulation. Most of the secondary alteration probably developed in diffuse fluid circulation systems.

All these minerals are characteristic of very low temperature conditions. The estimated temperatures for the formation of celadonite and nontronite are within the range 30-60C (Alt, 1995), and saponite can form at slightly higher temperature (from 15 to 170C) (Alt, 1995). The presence of micas replacing olivine (phlogopite-biotite?) is interpreted as high-temperature deuteric alteration during the last stages of magma cooling. The presence of incipient micas in the groundmass is more difficult to interpret because there is no evidence for high-temperature hydrothermal fluid circulation in the basement sequence of Hole 1205A, and further shore-based studies are needed to resolve their origin.

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 1205. Other trace elements (e.g., Cu, Zr, and Zn) do not show evidence for mobilization, as expected for low-temperature alteration events.

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