Six basement units have been identified at Site 1204: two were defined in Hole 1204A and four were defined in Hole 1204B. In Hole 1204A, the sequence includes one basaltic unit and one unit composed of volcaniclastic breccia (see "Physical Volcanology and Igneous Petrology"). In Hole 1204B, three lava flow units were identified, Unit 2 being subdivided into four subunits (2a, 2b, 2c, and 2d) (see "Physical Volcanology and Igneous Petrology"). The lava flow subunits overlie a fourth subunit of volcaniclastic breccia. Unit 4 is a calcareous vitric-lithic sandstone.
All lava flows and volcaniclastic sediments have undergone secondary alteration. Alteration mineralogy as well as vesicle and vein fillings were defined in rocks from Site 1204 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.
The effects of alteration in rocks from Site 1204 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 color, mineral assemblages, and vein and vesicle fillings of basaltic lava flows recovered at Site 1204 (Holes 1204A and 1204B). All basalt pieces recovered are slightly to highly altered (Figs. F31, F32). Alteration degree is higher overall in lobed units (Hole 1204A, Unit 2; Hole 1204B, Units 1, 2a, 2c, and 3) than in the massive Hole 1204B Subunit 2b diabase (see "Physical Volcanology and Igneous Petrology"). The degree of alteration also increases toward veins and glassy lobe margins, where it is high. Within the massive Hole 1204B Subunit 2b and massive interiors of lobes, alteration is slight to moderate.
Sample color is the first indication of alteration conditions. Two main alternating colors, with very sharp contacts, were observed in the sequences of Holes 1204A and 1204B (Figs. F33, F34). In the first color type, colors range from grayish brown (10YR 5/2) to light brownish gray (10YR 6/2), brownish yellow (10YR 6/8), or yellowish brown (2.5YR 5/4). In the second color type, basalt is characterized by a gray-green (5BG 4/1) color. These colors can be directly related to the oxidation state of the alteration process: oxidizing conditions in the brownish zones and reducing conditions in the greenish zones.
In the following sections, the alteration assemblages along with vesicle and vein fillings are described for each color type. Downhole alteration assemblages and vesicle and vein fillings are summarized in Figures F31 (Hole 1206A) and F32 (Hole 1204B). All information was also recorded in the alteration and vein logs (see "Site 1204 Alteration Logs" and "Site 1204 Vein Logs").
Overall, the alteration assemblages are dominated by carbonate (calcite) precipitation and clay. The latter are mainly brown to green smectite (saponite and/or nontronite). In both Holes 1204A and 1204B, zeolite minerals were also identified. These are fibrous zeolite with a small amount of analcite (see "Site 1204 Thin Sections").
Zones of oxidizing alteration were observed in Hole 1204A, Unit 2, from the top of Section 197-1204A-7R-2 (820.95 mbsf) through Section 10R-2, 93 cm (850.7 mbsf). Another smaller interval is recorded between Sections 197-1204A-10R-5, 24 cm (854.06 mbsf), and 10R-5, 47 cm (854.29 mbsf) (Fig. F31). In Hole 1204B, oxidizing zones are observed from the top of Section 197-1204B-1R-3 (814.03 mbsf) through the bottom of Section 9R-1 (887.29 mbsf). The second oxidizing zone is between Sections 197-1204B-10R-1, 37 cm (889.17 mbsf), and 13R-2, 134 cm (911.11 mbsf). Two additional oxidizing zones are recorded between Section 197-1204B-14R-1, 41 cm (916.31 mbsf), and the bottom of Subunit 2c (935.20 mbsf) and in Unit 3 from 944.90 to 947.26 mbsf (Fig. F32).
The alteration assemblage in these zones is dominated by Fe oxyhydroxide, which is sometimes well crystallized as goethite associated with carbonate (calcite), brown clay, and zeolite. The alteration assemblage is also composed of minor amounts of green clay (saponite and/or nontronite) (Hole 1204B, Subunit 2a) (Fig. F32).
Vesicles are mostly filled with carbonate (calcite). However, some display mixed fillings of carbonate (calcite), brown clay, minor amounts of green clay (saponite and/or nontronite), and zeolite (phillipsite?). When unfilled, vesicles are lined with either Fe oxyhydroxide (sometime crystallized as goethite) or zeolite (phillipsite?) (Figs. F31, F32).
Veins do not exhibit a preferred orientation in either hole. Veins are 1-30 mm wide and are filled with assemblages of carbonate (calcite) and Fe oxyhydroxide (sometimes as goethite) in various proportions (Figs. F31, F32). Minor amounts of green clay (saponite and/or nontronite) were recognized in veins from Hole 1204B (Fig. F32).
Zones of reducing conditions were identified in both Site 1204 holes (1204A and 1204B). Two reducing zones were identified next to the bottom of Hole 1204A, between Sections 197-1204A-10R-2, 93 cm (850.7 mbsf), and 10R-5, 24 cm (854.06 mbsf), and between Section 10R-5, 47 cm (854.29 mbsf), and the bottom of the recovered section of Hole 1204A (856.56 mbsf). Two other reducing zones were identified in Hole 1204B, between the top of Sections 197-1204B-9R-2 (88.29 mbsf) and 10R-1, 37 cm (889.17 mbsf), and between Sections 13R-2, 134 cm (911.11 mbsf), and 14R-1, 41 cm (916.31 mbsf).
The characteristic alteration assemblage in these zones consists of green clay (saponite and/or nontronite) and secondary sulfide (pyrite and rare chalcopyrite). This assemblage also contains carbonate (calcite) and zeolite. Minor amounts of Fe oxyhydroxide are sometimes present in these zones, which could represent the last stages of seafloor weathering.
In Hole 1204A, vesicles are filled mostly with green clay (saponite) and secondary pyrite (Fig. F35). When unfilled, vesicles are lined with brown clay and/or zeolite. In the Hole 1204B sequence, vesicles are filled mainly with an assemblage of carbonate (calcite), green clay (saponite), and secondary pyrite. When unfilled, vesicles are lined with zeolite.
Veins are 0.1-5 mm wide, thinner than in the oxidizing zones. They are filled with carbonate (calcite), green clay (saponite), and pyrite in various proportions. Minor amounts of Fe oxyhydroxide are sometimes present and could represent later stages of seafloor weathering.
As noted for Hole 1203A (see "Alteration and Weathering" in the "Site 1203" chapter), only the freshest samples were analyzed for major and trace element contents. Therefore, the chemical variations reported here might not be representative of the overall alteration-related chemical effects downhole. Variations in the abundances and ratios of some chemical elements (K2O, Cu/Zr, Co/Zr, and Zn/Zr) and LOI vs. depth are reported in Figures F36 (Hole 1204A) and F37 (Hole 1204B).
In Hole 1204A, LOI decreases downhole from ~10 wt% in the upper part of the oxidizing zone to ~4 wt% at the bottom of the first oxidizing zone and 3 wt% in the first reducing zone. This trend confirms macroscopic and petrographic observation of the samples. K2O displays a similar pattern, in that abundances are high in oxidizing zones (~1 wt%) and lower in the reducing zone (lowered to 0.5 wt%). In contrast to Site 1203 (see "Alteration and Weathering" in the "Site 1203" chapter), Cu/Zr, Co/Zr, and Zn/Zr ratios are constant downhole (Fig. F36). The incompatible element Zr is used as a normalization element to diminish the effect of crystal fractionation and to highlight any variations that might be ascribed to alteration processes.
LOI varies downhole in the Hole 1204B sequence. This trend confirms macroscopic and petrographic observation of the samples. LOI values are ~3-4 wt% in Unit 1 and the top part of Subunit 2a, reaching a maximum at the bottom of Subunit 2a (e.g., 7.81 wt% [Sample 197-1204B-6R-4, 21-24 cm]) and decreasing to 3-4 wt% in Subunits 2b, 2c, and Unit 3. Overall, LOI values are slightly lower (~3 wt%) in the massive Subunit 2b (diabase). As previously noted in Hole 1204A, K2O abundances are different between oxidizing and reducing zones. In oxidizing zones, K2O values are high (1-1.5 wt%). On the contrary, K2O is low in reducing zones (
0.5 wt%). As in Hole 1204A, Cu/Zr and Co/Zr ratios are constant with depth in Hole 1204B. However, some variations were noted in the Zn/Zr ratios. Zn/Zr ratios are high for Sample 197-1204B-8R-3, 53-55 cm, and for the four samples from Subunit 2c and Unit 3 (Fig. F37).
In both Hole 1204A and 1204B sequences, the constant Cu/Zr, Co/Zr, and Zn/Zr ratios indicate that these trace metals were not mobilized during alteration processes, with the exception of the previously noted variations of Zn/Zr ratios. No secondary phases containing Zn (such as sphalerite, [Zn, Fe]S) were identified during thin section examination, and further shore-based studies are needed to investigate this feature. Variations of K2O abundances downhole indicate a different mobilization of alkalis between oxidizing and reducing zones. By comparison with unaltered rocks from Hawaiian volcanoes (Fig. F38), K2O abundances are likely at or slightly lower than the original unaltered rock in reducing zones and are much higher in oxidizing zones. This is a common feature seen on Figure F38, where K2O against Zr abundances are reported for Site 1204 (Holes 1204A and 1204B) together with data from Site 1203, ODP Leg 145 Sites 883 and 884 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993; Keller et al., 1995), and unaltered Hawaiian lavas from Mauna Kea Volcano and Loihi Seamount (Frey et al., 1990, 1991; Rhodes, 1996). Two groups are identified—one with low K2O values and another with high values. Samples from the first group were taken from reducing zones, and samples from the second group were taken from oxidizing zones. K2O was therefore likely taken up by oxidized rocks and released in reduced zones.
Subunit 1a in Hole 1204A consists of a medium-grained volcaniclastic breccia, containing highly altered, aphyric vesicular basalt clasts in a calcareous matrix. This passes downward into the upper part of Subunit 1b, which is composed of volcaniclastic sandstone and breccia containing fragments of highly altered vesicular basalt and completely devitrified brown glass in a calcareous matrix. Subunit 2d in Hole 1204B consists of angular basalt lapilli to breccia fragments in a matrix of fine lapilli. Some alteration features associated with these deposits are described in "Physical Volcanology and Igneous Petrology".
All igneous rocks recovered at Site 1204 (Holes 1204A and 1204B) have undergone low-temperature alteration. 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 altered in the massive units (Hole 1204B, Subunit 2b) and in interiors of lobes (Holes 1204A, Unit 2, and Hole 1204B, Units 1, 2a, 2c, and 3) (see also "Physical Volcanology and Igneous Petrology"). Toward the margins of lobes, the degree of alteration is high. The alteration sequence is dominated by carbonate (calcite) formation, brown and green clay (saponite and/or nontronite), and zeolite minerals. Alteration conditions change in both sequences (Holes 1204A and 1204B) from oxidizing to reducing conditions, as highlighted by the change in color from brown to gray green. Oxidizing zones are characterized by Fe oxyhydroxide and goethite formation, and reducing zones are characterized by green clay (saponite) and secondary pyrite precipitation. Vesicle and vein fillings present the same features. Contacts between the different zones are very sharp and indicate an abrupt change in the oxidation state of circulating fluids. Reducing zones are mainly recorded in the more massive units, where fluid circulation is more restricted and water/rock ratios are likely to be lower because of lower permeability of the rock. This suggests that oxidizing and reducing alteration regimes were coeval, although Fe oxyhydroxide is sometimes superposed in reducing zones. These could correspond to later episodes of seafloor weathering at low temperature and with high water/rock ratios. The occurrence of saponite and zeolite in both regimes indicates that the temperature remained similar for both types of alteration, which is a further argument for coeval oxidizing and reducing alteration episodes. Temperatures estimated for saponite formation are in the range of 15°-170°C (Alt, 1995).
K2O was mobilized during alteration event(s) in a different manner depending on the alteration type. K2O is high in oxidizing zones and low in reducing zones. Other trace elements and trace metals apparently have not been mobilized, as expected for low-temperature alteration events. Seawater-derived fluids probably reacted at low temperature with the upper lava sequences, leading to Fe oxyhydroxide formation, and became more reduced farther down in the sequence (alkali and oxygen depleted), leading to sulfide precipitation and alkali leaching. All sequences from Site 1204 were likely altered in the so-called "seafloor weathering and alkali fixation" zone of Alt (1995).
