Thirty-one basement units have been identified at Site 1203 (Hole 1203A). The sequence includes up to 18 basaltic units interpreted as pillow lava, compound pahoehoe, and pahoehoe sheet lobes (see "Physical Volcanology and Igneous Petrology") and 13 units of volcaniclastic sediment or sediment interbeds consisting of tuff, lapilli tuff (lapillistone), and breccia. They represent either primary pyroclastic (tephra) and hyaloclastite deposits or resedimented tuff that are intercalated with vitric siltstone and sandstone and calcareous siltstone to mudstone (see "Physical Volcanology and Igneous Petrology").
All basaltic rocks and volcaniclastic sediments have undergone secondary alteration and/or weathering. Alteration mineralogy was defined in rocks from Site 1203 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. Although X-ray diffraction measurements were conducted on samples from Site 1203, instrumental problems did not allow precise identification of the clay and zeolite minerals. The identification of secondary minerals in Hole 1203A therefore remains tentative, pending further shore-based studies.
The effects of alteration in rocks from Site 1203 are defined in the basaltic units in terms of (1) alteration assemblages and vein and vesicle filling and (2) alteration chemistry.
In this section, we describe the alteration paragenesis for each basaltic unit as recorded during core description and following thin section examination. For a summary of alteration assemblages downhole, see Figure F48. We also include the vein and vesicle filling as recorded during hand specimen observation (Figs. F49, F50). Additional data on vein fillings were recorded during thin section observation (Fig. F51; see also "Site 1203 Thin Sections"). All vein information was recorded in the alteration and vein logs (see "Site 1203 Alteration Logs" and "Site 1203 Vein Logs"). Vesicle and vein fillings at Site 1203 indicate complex multistage alteration events. No preferred vein orientation was evident in the basement units. Veins mostly indicate complex patterns, occasionally reflecting ancient cooling joints.
Basement Unit 1 consists of lobes, interpreted as pillow lava. All the lobes are slightly to moderately altered. The extent of alteration increases toward veins, vesicles, and fine-grained lobe margins. This is mainly in the form of Fe oxyhydroxide formation as highlighted by changes in color from medium light gray-light gray (N6-N7) to light brownish gray (5YR 6/1). Olivine phenocrysts are completely replaced by white carbonate minerals (magnesite or calcite), brown amorphous clay or oxides (iddingsite), and, sometimes, green clay (likely saponite).
Veining is sparse throughout Unit 1. Veins and vesicles are filled with white carbonate (calcite), brown clay, and black to brown Fe oxyhydroxide.
As in basement Unit 1, Unit 3 is composed of slightly to highly altered lava lobes. The extent of alteration increases toward veins, vesicles, and glassy lobe margins. Alteration is usually high <2 cm from veins and vesicles and moderate >2 cm away from veins. Color throughout the unit varies from medium light gray-light gray (N6-N7) to light brownish gray (5YR 5/3 and 6/1) in the interior of lobes and dark, moderate, and pale yellowish brown (10YR 4/4, 5/4, and 6/2) at altered fine-grained lobe margins, around veins, and next to vesicles. The only exception is in interval 197-1203A-20R-3, 20-25 cm, where the color is medium gray (N5) with a light bluish green tint (5BG 6/6).
As highlighted by color, Fe oxyhydroxide formation is pervasive through the unit and is most prominent at the lobe margins. Most of the olivine phenocrysts are replaced by either white carbonate (magnesite or calcite) or reddish brown Fe oxyhydroxide (iddingsite), generally near fractures, and green clay (saponite) away from fractures. Plagioclase is moderately altered to green/white phyllosilicates (illite/sericite) near fractures and is slightly altered elsewhere. Glass is unaltered to moderately altered. Groundmass is also moderately altered near fractures, being replaced by brown clay and Fe oxyhydroxide, and is slightly altered elsewhere. In Section 197-1203A-20R-3, color reflects changes in alteration minerals and style: Fe oxyhydroxide is succeeded by dark blue-green clay (celadonite), which likely reflects a change from highly oxidizing to more reducing conditions.
Unit 3 is sparsely to highly veined. Veins are 2-4 mm wide in lobe interiors to 1-2 mm wide near lobe margins. They are filled with white carbonate (calcite), brown clay, green clay (saponite), and brown and black Fe oxyhydroxide. In Section 197-1203A-20R-3, veins are additionally filled with blue-green clay (celadonite) and sulfide (pyrite). Vesicles are filled with white carbonate (calcite), brown clay, and green clay (saponite).
Units 5 and 6 are massive lava flows and are slightly altered. Color varies from medium dark gray (N4) to medium gray (N5) and medium light gray (N6). Olivine phenocrysts are replaced by white carbonate (magnesite or calcite) or reddish brown Fe oxyhydroxide (iddingsite). Part of the groundmass is replaced by brown clay and Fe oxyhydroxide. Small amounts of sulfide (pyrite) are present in the groundmass. These two units are sparsely veined, with veins being 0.1-3 mm wide. They are filled with white carbonate (calcite), followed by sulfide (pyrite). Vesicles are filled with white carbonate (calcite), green clay (saponite), and sulfide (pyrite).
Unit 8 consists of slightly altered lava lobes 
30 cm in diameter. Color varies in the unit from medium dark gray (N7) in lobe interiors to pale yellowish brown (10YR 6/2) near glassy lobe margins and veins because of Fe oxyhydroxide formation. Olivine is replaced by reddish brown Fe oxyhydroxide (iddingsite) and white carbonate (magnesite or calcite). Groundmass is altered to brown clay and Fe oxyhydroxide.
Unit 8 is sparsely to highly veined, mostly near glassy lobe margins. Most veins are <1-3 mm wide. They are mostly filled with white carbonate (calcite), brown and black Fe oxyhydroxide, and green clay (saponite). Additionally, celadonite and chlorite were identified in thin section. Vesicles are filled with white carbonate (calcite), brown clay, green clay (saponite), and sulfide (pyrite).
This massive lava flow unit is slightly altered. No variation in color was noted (gray black [N2]). In the most altered part of the unit, olivine phenocrysts are replaced by white carbonate (magnesite or calcite) and reddish brown Fe oxyhydroxide (iddingsite). The groundmass is altered to brown clay, Fe oxyhydroxide, and green clay (saponite). Zeolite and sulfide (pyrite) are present occasionally throughout the groundmass.
Unit 11 is sparsely veined, and veins are preferentially filled with white carbonate (calcite). Minor saponite, chlorite, and sulfide (pyrite) are also present. Vesicles are mainly filled with white carbonate (calcite) associated with minor amounts of green clay (saponite), brown clay, and sulfide (pyrite). One megavesicle, present at the top of Section 197-1203A-32R-1, is shown in Figure F52. It is round and partially filled with coarse calcite; the irregular central vug remains unfilled.
Unit 14 is characterized by alternating slightly altered massive basalt and moderately altered pillowed intervals. Color varies from medium gray (N5) to medium light gray (N6) and dark yellowish orange (10YR 6/6) in more altered patches. Fe oxyhydroxide has formed throughout the unit but mostly near veins and within 2 cm of the upper margin of the flow. Plagioclase phenocrysts are slightly altered to sericite and/or illite. Blue-green clay (celadonite), green clay (saponite), and brown clay replace the groundmass mostly near veins and within 2 cm of the glassy margins.
Unit 14 is sparsely to moderately veined. Veins are <0.1-5 mm wide. They show complex, sequential fillings, as shown in Figures F53 and F54. White carbonate (calcite), black Fe oxyhydroxide, green clay (saponite), blue-green clay (celadonite), and sulfide (pyrite) were identified in hand specimen. Chlorite and zeolite were further identified in thin section. Vesicles also display multiple fillings composed of white carbonate (calcite), brown clay, green clay (saponite), blue-green clay (celadonite), zeolite, and sulfide (pyrite).
Unit 16 is massive and interpreted as a classic pahoehoe sheet lobe. Alteration is slight, and the whole unit is characterized by its gray-black color (N2). Olivine phenocrysts are partially altered to white carbonate (magnesite or calcite) or talc. Some brown clay is observed in the groundmass. Veins are sparse in Unit 16 and filled mostly with white carbonate (calcite) and green clay (saponite) with minor amounts of Fe oxyhydroxide.
Units 18 to 21 consist of a sequence of lava flows, each with multiple lobes that can be highly vesicular and up to 2 m thick. Depending on the position from the veins (within 1-2 cm) or in the lobes (interior or margins), basalt is, respectively, slightly to highly altered. Color reflects the degree of alteration and varies from medium-light gray (N6), medium gray (N5), medium-dark gray (N4), and dark gray (N3) to moderate brown (5YR 4/4), light brown (5YR 6/4 and 5/6), pale brown (5YR 5/2), pale yellowish brown (10YR 6/2), or dark yellowish orange (10YR 6/6) in more altered regions. Altered glass rims are dark greenish gray (5G 4/1). Fe oxyhydroxide is pervasive in alteration halos adjacent to glassy margins and around veins (up to 8 cm wide). Similar alteration halos are present around vesicle cylinders, although these are narrower. Olivine phenocrysts are replaced by white carbonate (magnesite or calcite), reddish brown Fe oxyhydroxide (iddingsite), and green clay (saponite). When present (e.g., Unit 18), plagioclase phenocrysts are sometimes slightly altered to sericite and/or replaced by illite or calcite. The groundmass is variably altered to brown clay, green clay (saponite), and Fe oxyhydroxide. Alteration to saponite is, however, minor in Unit 19. Glassy margins are usually completely devitrified and replaced by brown clay and Fe oxyhydroxide. In Unit 20, however, glass and groundmass are mainly replaced by blue-green clay (celadonite). Zeolite minerals were also identified in the Unit 21 matrix.
Units 18 to 21 are sparsely to moderately veined. Veins are most abundant near pillow margins and brecciated tops. They are <0.1-6 mm wide in the interior of lobes and larger (up to 15 mm wide) next to lobe margins. Voids are present in several of the larger veins. Veins are mostly filled with white carbonate (calcite), brown to black Fe oxyhydroxide, green clay (saponite), and sulfide (pyrite, mainly in massive sections). Zeolite minerals were identified in thin section. In brecciated areas, veins can contain angular glass fragments derived from adjacent glassy margins. Brown alteration halos (1-5 mm in size), rich in Fe oxyhydroxide, are present around veins. Vesicles are filled with mainly white carbonate (calcite), green clay (saponite), blue-green clay (celadonite) (Unit 20), and pyrite (Unit 21).
Units 23 and 24 consist of a sequence of highly vesicular lava lobes. The degree of alteration is slight in the interior of lobes to moderate/high toward the veins and lobe margins and complete in the glassy margins. Color varies from dark gray (N3), medium-dark gray (N4), medium gray (N5), and medium-light gray (N6) to moderate reddish brown (10R 4/6) and moderate yellowish brown (10YR 5/4) around veins, in alteration halos, and toward lobe margins. Altered glass is dark greenish gray (5G 4/1). Olivine microphenocrysts are replaced by white carbonate (magnesite or calcite) and reddish brown Fe oxyhydroxide (iddingsite). Glass is completely devitrified, and the mesostasis is altered to green clay (saponite), brown clay minerals, or Fe oxyhydroxide. Sulfide (pyrite) is present in the mesostasis, mostly adjacent to veins and vesicles.
Units 23 and 24 are sparsely veined. Veins are 0.1-4 mm wide and are filled with white carbonate (calcite), Fe oxyhydroxide, green clay (saponite), and sulfide (pyrite). Zeolite and chlorite were identified in thin section. Vesicles are partially to totally filled with white carbonate (calcite), green clay (saponite), brown clay, zeolite, and pyrite, but are also unfilled and lined with green clay (saponite).
Unit 26 is a succession of slightly (interiors) to moderately (next to margins) altered lobes. Color varies from dark gray (N3), medium-dark gray (N4), medium gray (N5), and medium-light gray (N6) to moderate yellowish brown (10YR 5/4) and grayish orange (10R 7/4) adjacent to veins and in alteration halos. Medium bluish gray (5B 5/1) colors are present at the base of the unit. Sparse olivines are altered to reddish brown Fe oxyhydroxide (iddingsite). The mesostasis is altered to brown clay, green clay (saponite), and Fe oxyhydroxide (goethite). Glass is completely altered to green clay (saponite). Minor sulfide (pyrite) and zeolite were identified in the upper part of the unit. Blue-green clay (celadonite) is present at the base of the unit.
Sparse veins, 0.1-3 mm wide, are present in Unit 26. They are filled with white carbonate (calcite), Fe oxyhydroxide (goethite), and minor amounts of green clay (saponite). Minor chlorite is also present and was identified in thin section. Most vesicles are filled with white carbonate (calcite), green clay (saponite), brown clay, and, rarely, sulfide (pyrite) and blue-green clay (celadonite).
Units 29 to 31 are composed of lobes with brecciated tops and massive interiors. They are slightly altered (interiors) to moderately or highly altered (brecciated tops or glassy margins). Color varies from medium-dark gray (N4) and medium gray (N5) to moderate brown (5YR 3/4) and light brown (5YR 6/4) in highly altered areas, grayish orange (10R 7/4) to pale reddish brown (10R 5/4) adjacent to veins, and dark greenish gray (5G 4/1), grayish green (10GY 5/2), or grayish blue green (5GB 5/2) at altered glassy lobe margins. When present (e.g., Units 30 and 31), plagioclase phenocrysts are slightly altered to sericite and/or illite. The mesostasis is altered to green clay (saponite), brown clay, white carbonate (calcite), blue-green clay (celadonite), or zeolite. Fe oxyhydroxide (goethite) alteration is pervasive throughout these units. Occasional sulfide (pyrite) was identified in the groudmass and adjacent to veins. Glassy clasts in the brecciated flow top and in lobe margins are completely altered to green clay (saponite) and brown clay.
Units 29 to 31 are sparsely to moderately veined. Veins are 0.1-5 mm wide and are principally filled with white carbonate (calcite) and subordinate amounts of green clay (saponite) and Fe oxyhydroxide (goethite). Vesicles are filled with minor amounts of white carbonate (calcite) and brown clay. The main filling minerals are green clay (saponite), blue-green clay (celadonite), zeolite, and sulfide (pyrite).
Chemical analyses were performed on board by ICP-AES (see "Physical Volcanology and Igneous Petrology" in the "Explanatory Notes" chapter). These data were used to estimate the effects of alteration throughout the basement section. However, only the freshest pieces were sampled. The chemical variations reported here could therefore underrepresent the overall alteration-related chemical effects downhole. Further shore-based studies will be undertaken to address this issue.
Variations in the abundances of some chemical elements and LOI vs. depth are reported in Figures F55 and F56. LOI is a proxy for the degree of alteration. LOI values vary downhole from ~1 to ~7 wt%, with a significant increase in Unit 19 downward (the sample with 11.73 wt% LOI was taken from volcaniclastic Unit 4). This trend confirms macroscopic and petrographic observation of the samples. Igneous rocks from the lower units of Hole 1203A show higher degrees of groundmass alteration compared to upper units. Zeolite abundances increase, whereas carbonate replacement decreases downhole. As shown in Figure F55, CaO decreases downhole, with a significant decrease from the top of Unit 23. In the same interval, K2O increases, although K2O abundances are more scattered overall. Na2O, Ba, and Sr are approximately constant with depth.
Figure F56 illustrates the variability of Zn/Zr, Co/Zr, and Cu/Zr ratios with depth. The normalization to the incompatible and immobile element Zr is used here to minimize the effects of magma evolution and to reveal the effects of low-temperature alteration. The Zn/Zr ratio is fairly constant in the upper units of Site 1203 and decreases slightly from Unit 19 downward. The highest Zn/Zr ratios are reported for the two olivine-rich samples (e.g., Sample 197-1203A-32R-3, 85-87 cm, and 37R-3, 10-13 cm) and are more likely a primary feature than an alteration effect, given the observation of primary sulfide inclusions in olivine (see "Physical Volcanology and Igneous Petrology"). Likewise, the Co/Zr ratio remains constant in the upper part of Hole 1203A and decreases slightly from Unit 20 downward. High Co/Zr ratios are also displayed by the same olivine-rich samples. Although Cu/Zr ratios are more scattered, the same overall trend is seen in Hole 1203A, with higher ratios within the upper units compared with Unit 20 downward. Some samples from Units 11, 14, 16, and 24 show higher Cu/Zr ratios.
In Figure F57, K2O, Co, Cu, and Zn are reported against Zr, together with data from Hawaiian volcanoes Mauna Kea and Loihi and results from ODP Leg 145, Sites 883 and 884. Two groups are easily distinguished at Site 1203, based on Zr abundances. As previously noted, K2O data are quite scattered and trend away from the highly correlated data from unaltered Hawaiian lavas. The increase in K2O abundances is likely an effect of low-temperature alteration, K being fixed in celadonite and smectites. Zn abundances of samples from Site 1203 plot within the field reported for Hawaiian lavas, and ODP Leg 145 Sites 883 and 884 and are consistent with magmatic evolution and minimal secondary mobilization. Co, which behaves as a compatible element, exhibits distinctly higher abundances in the lower units at Site 1203 (Units 23, 26, 29, and 30) at a given Zr than Hawaiian lavas and may reflect enrichment by alteration processes. Most samples from Site 1203 have lower Cu abundances than reported for the unaltered Hawaiian lavas. The lower abundances in Cu could reflect loss during hydrothermal alteration. Cu and Co abundances in samples from Site 1203 are difficult to interpret at this stage and will require further shore-based analyses. Mobilization of these two elements would suggest circulation of high-temperature hydrothermal fluids (T = >350°C). However, no high-temperature minerals or assemblages were observed in samples from Site 1203 (e.g., quartz or amphibole).
The volcaniclastic succession is dominantly basaltic in composition but consists of a range of sediment types comprising tuff, lapilli tuff (lapillistone), and breccia, representing primary pyroclastic (tephra) and hyaloclastite deposits as well as resedimented tuff intercalated with vitric siltstone and sandstone and calcareous siltstone to mudstone. Alteration features associated with these deposits are described in "Physical Volcanology and Igneous Petrology".
Igneous rocks recovered at Site 1203 (Hole 1203A) have undergone low-temperature alteration and/or weathering to varying extents. Alteration features are defined in the basaltic units in terms of secondary mineral paragenesis, apparent as vesicle filling, vein filling, and replacement of groundmass and primary minerals. Overall, the basalt flows are slightly to moderately altered (Cores 197-1203A-17R to 41R) with an increase downhole (from Core 197-1203A-42R to 68R) to moderately to highly altered. In the upper sequence, the alteration paragenesis is clearly dominated by carbonate, Fe oxyhydroxide, and brown and green clay (saponite) formation. The minor associated minerals are sulfide (pyrite), blue-green clay (celadonite), and zeolite. A change in the alteration conditions is seen in the lower sequence, where zeolite minerals increase and carbonates decrease. The same change is seen downhole in the vesicle-filling parageneses. Most vesicles are filled with calcite and/or saponite. Near the bottom of the hole (from Unit 29), they are mostly filled with zeolite and Fe oxyhydroxide (sometimes crystallized as goethite). The recovered basalt is sparsely to highly veined. The major constituent of veins is calcite, which is frequently associated with saponite, brown clay, and Fe oxyhydroxide. Less frequently, chlorite, celadonite, zeolite minerals, and pyrite were identified.
Igneous rocks from Site 1203 show complex multistage alteration features. Mineral associations indicative of low-temperature interactions between rocks and fluids dominate throughout the sequence. The abundant occurrence of Fe oxyhydroxide, smectites (saponite and brown clay), and associated celadonite suggests that most of the sequence was altered at low temperature. This paragenesis is typical of the two lowest temperature stages of oceanic crust alteration (also known as "seafloor weathering" and "alkali fixation" zone) (Alt, 1995). Temperature estimates for smectite and celadonite formation are in the range 30° to 150°C. Fe oxyhydroxide, smectite (nontronite? and saponite), and celadonite suggest circulation of large volumes of oxidizing fluids in the lava pile. As the fluid penetrated deeper in the sequence, more reducing conditions developed and secondary sulfides were formed (pyrite in Hole 1203A). Carbonate and zeolite are formed in both environments. Despite the clear dominance of low-temperature fluid circulation, minerals such as chlorite indicate that at least part of the volcanic pile has reacted with higher-temperature fluids (T = 250°-300°C). Although it is difficult to extract the succession of these different events, some sulfides are likely coeval with minerals such as chlorite, suggesting a reducing environment during at least some of these higher-temperature fluid circulations.
