Previous coring at Site 801 during Leg 129 (Holes 801B and 801C) penetrated 60.2 m of alkali basalts and 63.1 m into the underlying tholeiitic basalts, which are separated from the alkali basalts by an ~9.5-m-thick silicified hydrothermal iron oxide deposit (Lancelot, Larson, et al., 1990). Thus, total basement penetration was 132.7 m. Alteration of the basalts recovered from Site 801 during Leg 129 was studied by Alt et al. (1992) and has since been re-examined by J.C. Alt and D.A.H. Teagle (unpubl. data). These authors divided alteration into several different types. Most of the alteration observed in the Leg 129 section at Site 801 is typical of that observed elsewhere in the upper oceanic crust and results from interaction of basalts with seawater fluids at temperatures of 0°-50°C (Alt et al., 1992; summary in Alt, 1999).
Dark gray rocks are the most abundant and are present throughout the Leg 129 tholeiitic section. These rocks are the least altered, generally containing 2%-20% by volume secondary minerals, and are characterized by saponite and calcite replacing olivine and filling pore space and by common disseminated pyrite. Chemical changes caused by alteration are slight. Dark gray type alteration also affects the alkali basalts, but these are more intensely altered than the tholeiites (30%-80% vs. 2%-20% altered). Celadonite and Fe oxyhydroxides are locally present in the dark gray tholeiites in millimeter- to centimeter-wide alteration halos along veins.
Brown rocks are similar to the dark gray alteration type but contain abundant disseminated iron oxyhydroxides, in some cases with celadonite present as well. These rocks reflect greater fluxes of oxygenated seawater than the dark gray pyrite-bearing rocks.
The hydrothermal deposit at 521.7-531.2 mbsf consists of silicified ocherous material and was formed on the seafloor at temperatures of 15°-60°C (Alt et al., 1992). Green alteration in the Leg 129 section is found only in the primitive tholeiites of Unit 5 at 531-535 mbsf, just beneath the hydrothermal deposit, and is probably related to upwelling hydrothermal fluids that formed the deposit. The green rocks are intensely veined and recrystallized (80%-100%) to celadonite, glauconite, montmorillonite, K-feldspar, calcite, and titanite. Buff-colored rocks are closely associated with the green rocks from 531 to 535 mbsf. The buff rocks are highly altered (60%-80%) to smectite, calcite, and titanite.
Major questions to be addressed by further coring in Hole 801C during Leg 185 were the deeper distribution of alteration types and the chemical effects of alteration on the crust. On the basis of other drilled sections, it was expected that the effects of seawater alteration would decrease with depth (Staudigel et al., 1995; Alt et al., 1996). This section documents the presence of veins, breccias, interpillow sediments, altered basalts, and chemical changes resulting from alteration of the rocks that were cored in Hole 801C during Leg 185.
Secondary phyllosilicates in rocks from Leg 185 Hole 801C were identified by color and hardness in hand specimen, by optical properties in thin section, and by analogy with well-studied minerals identified in the shallower section of Hole 801C cored during Leg 129 and in other sections of oceanic crust. The identification of phyllosilicates in the Leg 185 rocks remains tentative, however, pending further shore-based study.
Throughout the following sections we refer to volume percentages of alteration types, breccias, and veins. We assume that the surface areas of these features on the cut faces of the core, when converted to area percent, are equivalent to volume percent of the core, similar to modal analyses of a thin section.
Veins, halos, breccias, and interpillow sediments observed in the archive half of the cores were recorded in the vein log (see the "Site 801 Vein Log," also in ASCII format). The logs include the record of abundance, volume, width, and mineralogy of the veins calculated per core.
Based on the texture, shape, and filling, we defined three major types of fractures. Veins are interpreted to result from brittle failure and subsequent filling by a variety of minerals. Interpillow materials are mainly sediments filling the spaces between basalt pillows. Breccias are composed of angular to subangular clasts of altered or fresh basalt with a cement. Abundances and distributions of veins, breccias, and interpillow material are illustrated in Figures F40, F41, and F42.
Approximately 3500 veins were logged during core description, and veins make up 1%-4% by volume of the core (Fig. F40). The volume percent of veins for each core was estimated by calculating the volume of veins relative to the volume of the core recovered.
The vein materials are mostly calcite, saponite, celadonite, iron oxides or sulfides, and silica (quartz and chalcedony). The veins are commonly composed of a combination of these minerals, but several end-member types were defined based on the main mineralogy. A summary of the different types of veins is reported in Table T8. Carbonate (mostly calcite) and saponite veins are by far the most common and are present as multiple generations. Veins range in thickness from ~0.1 to ~50 mm. The orientations of the veins are subhorizontal to oblique in the cut face of the core, but vertical veins several decimeters long, are not uncommon.
Carbonate, principally as calcite, is the most abundant vein mineral. Pure carbonate veins (e.g., >90%) correspond to one-third of all veins. Carbonate in veins comprises 1% by volume of all material recovered from the volcanic section of Leg 185 Hole 801C. In general, decreases in the proportion of carbonate in veins per core correspond with a reduction of the average vein thickness for that core. Identification of coarse-grained calcite and fibrous aragonite in some veins must be confirmed by X-ray diffraction (XRD) analyses. Some unusual subvertical carbonate veins in Section 185-801C-37R-2 are associated with highly vesicular lava and may reflect a local increase in fluid pressure and related hydraulic fractures.
Saponite is the major mineral filling thin veins (~0.1 mm thick). These veins clearly represent filled fractures that allowed fluid access to the fresh rocks. Dark alteration halos and disseminated pyrite rims are ubiquitously associated with saponite veinlets but represent a very low proportion of the core.
Veins are commonly filled with both carbonate and smectite identified as saponite. The proportion of carbonate to saponite in this "end-member" type ranges from 40% to 60%. The carbonate generally occupies the central portion of the larger veins and, thus, is a later mineral. The properties of these veins, such as the average width, are intermediate between those of the saponite and the calcite-type veins.
Celadonitic phyllosilicates are locally abundant, but pure celadonite veins are scarce. Celadonitic patches are associated with large veins of carbonate and/or saponite and brecciated zones. Celadonite veins are commonly characterized by wide, dark alteration halos as wide as 2 cm (intervals 185-801C-43R-2, 125-130 cm, and 37R-5, 60-65 cm) and are locally oxidized or stained with Fe oxyhydroxide (43R-3, 48-58 cm).
Pyrite veins are generally present as minute veinlets (0.1 mm thick) and are randomly oriented. Pyrite may also be present locally in fractures filled with smectite and/or carbonate. Some alteration halos associated with veins (see "Mixed Veins") contain disseminated fine-grained pyrite.
Iron oxyhydroxides are found mostly in thin veins but may be present as disseminated minerals and as staining saponite within larger veins and in alteration halos. In some cases, iron oxides clearly replace pyrite in small saponite + pyrite veins. Enrichment in iron oxyhydroxide is recorded in the more oxidized sections of Hole 801C (i.e., between 905 and 970 mbsf) and in the upper 50 m of the section drilled during Leg 185.
Silica veins (i.e., >70% SiO2) represent <0.5% of the veins recorded in Hole 801C. However, silica is locally enriched in the veins in the upper section of the hole, between 600 and 700 mbsf. Silica-bearing veins are significantly larger than the other veins and tend to be associated with the breccias and interpillow material. In many cases, silica, like carbonate, appears to be a late phase (e.g., Samples 185-801C-27R-1, 50-55 cm, and 16R-1, 70-71 cm). The largest veins may be related to the siliceous hydrothermal deposits (e.g., Sample 185-801C-15R-1, 129 cm).
Mixed veins are generally irregular to sinuous in shape and are associated with wide, mixed brown and dark alteration halos. In some sections, the veins exhibit branching and anastamosing forms. Green clay (saponite or celadonite) and calcite probably filled the veins during several reopening events. Some thick, long vertical veins with these features are recorded in different parts of Hole 801C (e.g., intervals 185-801C-44R-2, 73-147 cm (4 mm thick); 37R-5, 57-126 cm (10 mm thick); 31R-6, 30-77 cm (0.7 mm thick); 31R-6, 91-136 cm (1.8 mm thick); 38R-1, 45-86 cm (5 mm thick); and 37R-3, 1-40 cm (4 mm thick). For the vertical veins at interval 185-801C-37R-5, 57-126 cm, carbonate is present in extensional fractures, with or without cutting the saponite vein. The associated mixed brown and dark alteration halo is 15 mm wide. The heterogeneous nature of these vein fillings is in some cases emphasized by changes in mineralogy along the length of the vein (Fig. F43C).
The breccias in Hole 801C were divided into two types:
The average proportion of cement in breccias is 20%, but it varies from 1% to 99%. The breccia cements are dominantly carbonate (44%) and saponite (39%), with lesser amount of celadonite (11%) and minor quartz (5%). Breccia in Core 185-801C-40R extends over 77 cm (from 0 to 77 cm) and is composed of large, angular, slightly displaced blocks linked by a network of saponite and carbonate veins.
The density of veins in the basement is highly variable (Fig. F40). At 700 mbsf (Core 185-801C-25R), 850 mbsf (Core 41R), and 916 mbsf (Core 48R), the proportions of veins and fractures per core are below 1 vol%, and intervals of slightly altered, dark gray rocks are common. These zones of restricted fracturing and fluid flow exhibit only limited chemical change (see "Alteration Geochemistry") and slight recrystallization. Clay-cemented breccias are common throughout Hole 801C but are very heterogeneously distributed. The proportion of breccias is the highest in Core 185-801C-40R (i.e., ~8 vol%). Hyaloclastites make up 1-3 vol% of Cores 185-801C-19R, 24R, 28R, 32R, and 42R.
The frequency of veins per meter does not correlate with the volume percent of veins per core; rather, the latter depends on the average thickness of veins in each core. The density of veins for each core correlates neither with percentage recovery nor with the proportion of breccias; however, for cores consisting completely of massive basalt, vein frequency is generally lower than for those comprising pillows and breccias (e.g., Core 185-801C-31R). The minimum vein abundance of Core 185-801C-41R corresponds to a large brecciated unit and is not representative of the abundance of secondary minerals in that core (Fig. F40). The average density of veins in Hole 801C is 24 veins/m, which increases locally to 40 veins/m in Core 185-801C-18R (Fig. F40). This mean value is slightly lower than in other sections of upper oceanic crust, which contain 27 veins/m (Hole 896A) and 31 veins/m (Hole 504B) (Alt et al., 1996).
The maximum vein abundance in Hole 801C is in the cores drilled with the diamond bit: for Cores 185-801C-49M through 52M, the average density of veins is 39 veins/m corresponding to 2.08 m of rocks recovered. The increase of vein abundance in this section reflects the greater proportion of highly fractured pillow lavas and hyaloclastites. However, the difference in abundance of veins and breccias in different sections of Hole 801C, and particularly between Cores 185-801C-48R and 49M, may not be significant and could be an artefact of measurement caused by variable core recovery between RCB and DCB bits. The data presented in Figure F40 indicate that there is no simple relationship between core recovery, vein abundances, and lithologic units. However, it is possible that variations in recovery are related to the presence or absence of veins and fractures along which the core breaks into small pieces and is ground up during drilling.
Fe oxyhydroxide-bearing and pyrite-bearing veins tend to not be present together, and their distributions correlate with the proportion of celadonite and saponite, respectively. There is no clear overall depth trend in vein mineralogy or alteration along this profile of the upper oceanic crust because the lower section between 850 and 950 mbsf is characterized by the presence of alternations of oxidative and reducing alteration effects (Figs. F41, F42).
Based on the composition, abundance, and distribution of breccias, interpillow sediments, and veins (Figs. F40, F41, F42), however, two major units are distinguished:
Many veins are braided and branching, and crosscutting is common. In general, most of the oblique and vertical veins have very irregular shapes suggesting that they were generated during several stages. Carbonate veins tend to be late, with carbonate minerals (calcite and rare aragonite) filling reopened saponite veins and cutting saponite veins. On the other hand, thin-section observations indicate that saponite can occur locally as intergrowths with carbonate minerals. Clear crosscutting relationships between veins were observed in only a few cases. In one sample, at least three generations of carbonate and saponite veins are present (Fig. F45). Alteration halos are associated with both subhorizontal and vertical veins. The thin saponite veins are commonly accompanied by dark alteration halos, but pure carbonate veins do not have these features. Reopening of saponite veins and their filling with carbonate are apparently accompanied by formation of late alteration halos. This reactivation could be a result of late tectonic movement, as suggested by offset carbonate veins.
Interpillow sediment represents material that was originally deposited on the ocean floor and has subsequently been recrystallized and possibly altered by basement fluids. Interpillow sediment contains calcite, chalcedony, quartz, smectite/celadonite, and iron oxyhydroxide in varying proportions (Fig. F46) and is distinguished from breccia and hyaloclastite by the scarcity of fresh or altered clasts of basalt and/or glass. Interpillow sediment makes up 2.5% of core recovered from Leg 185 Hole 801C (Fig. F40). Interpillow sediments were observed in Cores 185-801C-14R through 18R, 20R through 22R, 25R through 27R, and 32R. Below Core 185-801C-32R interpillow sediment was not observed. Overall, the volume percent of interpillow material decreases downhole (Fig. F40).
Some of the mineralogical and microscopic morphological details of interpillow sediments are displayed as photomicrographs in Figure F47. The occurrences of radiolarian tests provide evidence for the original deposition of interpillow sediment as pelagic sediment.
Two types of contacts were observed between lavas and interpillow sediment: (1) pillow rim/sediment contacts and (2) flat basalt/sediment contacts (Fig. F48). Pillow rim/sediment contacts are characterized by a chilled pillow margin at the sediment/lava interface. In many cases flat basalt/sediment contacts appear to be lava flows that were injected into interpillow sediment. Some of these contain clear examples of centimeter-scale patches of sediment included within fine-grained basalt. Because of incomplete core recovery the actual thickness of individual units is uncertain, but observed thickness of interpillow sediment units range from 1-2 cm (Fig. F47H) to 39 cm for the cherty hydrothermal interpillow unit of interval 185-801C-16R-3, 0-39 cm. Overall, the thickness of individual interpillow sediment horizons decreases with depth.
The distribution of interpillow material is reflected in downhole variations in the proportions of SiO2 and celadonite, which decrease from ~18% and 3%, respectively, in Core 185-801C-14R to values of <1% below Core 17R (Figs. F42, F45). Silica in the interpillow sediments is typically fine chalcedony and/or quartz. However, as seen in Figure F47F, there are some fibroradial occurrences of chalcedony, and the conversion of chalcedony to finely crystalline quartz is also observed (Fig. F47F). In thin section celadonite is present as massive groundmass (Fig. F47B), as discrete crystals (Fig. F47C), and as pore-filling material (Fig. F47E).
Within interpillow material, the abundance of carbonate ranges from 4 to 90 vol% and saponite ranges from 2 to 80 vol%. Saponite is shown as a pore-filling material (Fig. F47A, F47E) and intergrown with calcite (Fig. F47C).
The occurrence of calcite (Fig. F47A, F47B, F47C, F47E, F47F, F47G, F47H) suggests wholesale recrystallization. The radiolarian depicted in Figure F47A sits in a matrix of calcite and has itself been replaced by calcite. Calcite in Figure F47B shows concentric growth zones, indicating its introduction as a secondary phase. Calcite in Figure F47E is subhedral and appears to have crystallized around small opaque crystals. Figure F47H shows a calcisphere (~0.75-mm diameter) near the calcite/basalt interface that is probably of biologic origin.
The presence of Fe oxyhydroxides in interpillow material was recognized in hand specimen by red or yellow staining and in thin section by spherulitic and disseminated iron oxides (Fig. F47D, F47G). Interpillow units containing significant amounts of Fe oxyhydroxides are found in Cores 185-801C-14R through 17R, 22R, and 27R. In Core 185-801C-16R, Fe oxyhydroxide represents 21 vol% of the interpillow material. This is related to the presence of a second silica- and iron-rich hydrothermal unit (lithologic Unit 48; interval 185-801C-16R3, 0-43 cm), which consists of 5- to 100-mm spherules and filaments of iron oxyhydroxide in a matrix of granular quartz (<1-50 mm grain size).
Siliceous interpillow material, which was probably originally pelagic radiolarian ooze that has been recrystallized and possibly hydrothermally altered, represents periods of sedimentation and/or channelled sediment flows during hiatuses between periods of volcanic activity. Carbonate material may represent original calcareous sedimentation or a later authigenic precipitate. The presence of Fe oxyhydroxides may result from precipitation from hydrothermal fluids within interpillow spaces beneath the seafloor, or as hydrothermal sediment at the seafloor. Variation in the abundance of interpillow sediment with depth may reflect the rate of production of lava flows; the paucity of interpillow sediments below Core 185-801C-32R may be a result of rapid lava production and accumulation rates at a fast-spreading ridge, whereas slower, off-axis lava accumulation at the top of the section would possibly have allowed greater accumulation of interpillow sediments.
Various types of alteration were encountered in Hole 801C basement rocks drilled during Leg 185. The alteration halos adjacent to veins or exposed surfaces are described in the following section.
Pervasive but slight background alteration affects most of the basalt cores starting with the first core (i.e., Section 185-801C-13R-1, at the depth of 594.3 mbsf) down to the bottom of the hole at 935.7 mbsf (Fig. F49). As a result of this alteration, the basalts display a dark gray color when wet that becomes a lighter gray or brownish gray when dry. This alteration is pervasive and not specifically focused along veins or exposed surfaces. Rocks affected by this dark gray background alteration typically contain 2%-15% secondary phases by volume.
This background alteration results from the complete replacement of rare olivine phenocrysts by smectite and, much more rarely, the partial replacement of plagioclase and augite phenocrysts by smectite. On the other hand, plagioclase microlites and intergranular plumose or dendritic augite are generally unaltered. Titanomagnetite is unaltered to slightly altered to titanomaghemite, exhibiting minor shrinkage cracks. In the groundmass, the rare interstitial glass is totally replaced by smectite. Smectite and minor calcite fill vesicles and miarolitic voids, which are generally not abundant. In thin sections, smectite varies in color from dark brown to pale greenish to yellowish brown, but it is typically black in hand specimen. Based on previous work at Site 801 and elsewhere, this widespread smectite is most likely saponite (Alt et al., 1992; Alt, 1999). Recrystallized igneous sulfide globules are locally common, as are disseminated secondary pyrite and minor marcasite.
Pale green color is restricted to intense pervasive alteration (up to 80% by volume) of basalts locally in Sections 185-801C-15R-1 to 16R-3 of Units 39 to 49 (i.e., from 613 to 626 mbsf) (Fig. F50). This type of alteration is due to the total replacement of the rare olivine phenocrysts by a tan to green smectite and intense replacement of plagioclase phenocrysts and microlites by abundant calcite and tan smectite. Plagioclase phenocrysts are less altered than the microlites. Both secondary minerals also fill vesicles and miarolitic voids and pervasively replace the groundmass. Preliminary XRD results indicate that the smectite is dioctahedral, probably a montmorillonite. Apatite was also identified by XRD. Celadonite and green nontronite are occasionally observed filling voids in halos along veins. Fine-grained pyrite is disseminated through the groundmass and particularly in bands outside alteration halos along veins. Pale green alteration progressively grades through a gray-green alteration zone (see below) into dark gray adjacent rocks within the same core section (e.g., Section 185-801C-15R-7) (see Fig. F50).
Gray-green alteration is present in Sections 185-801C-15R-1,15R-2, and 15R-5, in which it can be the dominant alteration type (up to 95% of the section), and also in Sections 16R-1 and 17R-2, where it is much less important (2 and 20 vol%, respectively). Gray-green altered rocks represent the intermediate alteration type between intensely altered pale green rocks and the typical pervasive dark gray background alteration.
Calcite is the most abundant secondary phase in gray-green altered rocks. It locally replaces plagioclase and olivine phenocrysts and fills vesicles and miarolitic voids, where it is sometimes associated with chalcedony. It is also the main vein mineral. Clinopyroxene appears to be unaltered. The groundmass is partly (20%) replaced by calcite and smectite. Titanomagnetite is intensely altered to titanite, and abundant disseminated pyrite is present in the groundmass.
Even though fresh basaltic glass is found nearly down to the bottom of the hole, glass from glassy pillow rims or hyaloclastites is generally strongly altered to smectite. Hyaloclastite glass shards are altered to smectite, whereas the cement is commonly calcite or smectite. It is remarkable that no zeolite has been observed in any of the hyaloclastite or glassy pillow rims of Hole 801C. Phillipsite is typically a very common result of "palagonitization" (i.e., the low-temperature alteration of basaltic glass by seawater).
Alteration halos along veins are common in Hole 801C core, and their occurrences are recorded in the vein logs (see the "Site 801 Vein Log," also in ASCII format). Widths of alteration halos range from ~1 to 18 mm. Several types of halos were recognized in hand specimen, and their mineralogy was determined in thin section:
Dark alteration halos were commonly difficult to detect in hand specimens. We discovered that thorough washing of the surface and partial drying enabled better detection of such halos. Narrow pyrite bands were in some cases logged in the vein log. Many of these pyrite bands may border dark alteration halos that were unrecognized because of the lack of a significant color difference in hand specimen. It is likely, therefore, that the abundance of dark halos described above is underestimated in the vein and alteration logs (see the "Site 801 Vein Log" and "Site 801 Alteration Log;" both also in ASCII format).
Alteration halos have a sporadic distribution in the cores but are most abundant in coarser grained massive units in Cores 185-801C-34R, 36R, 37R, and 43R through 46R (Fig. F51). The abundance of halos in these units must reflect local permeability and fluid flow, but formation of halos may be enhanced by coarser grain size and intergranular porosity and permeability of the rocks.
The typical basalts from Leg 185 that are slightly altered to the pervasive dark gray background alteration exhibit little or no chemical changes, although there is scatter to slightly elevated K2O, Rb, H2O, and CO2 contents, and increased LOI as the result of alteration and formation of secondary smectite, celadonite, and calcite (Fig. F52; Table T9). Several samples were selected specifically to study alteration effects, including one altered interpillow hyaloclastite, a pale green intensely altered rock, and two alteration halos along veins. Altered rock compositions were compared to the mean least-altered rock composition from the same igneous chemical unit, with samples having clearly elevated LOI, K2O, Rb, and/or CaO contents excluded from the calculations of least-altered rocks. It is assumed that Al, Ti, and Zr are immobile during alteration and that any change in the concentrations of these elements reflect density changes or dilution effects during alteration. Altered rock compositions are thus normalized to constant Ti, Al, and/or Zr, and the ratio of normalized altered rock to mean fresh rock has been plotted in Figures F53, F54, F55, and F56.
The intensely altered pale green rock from geochemical Unit 11 (Core 185-801C-15R) has undergone a 22% decrease in density. Losses of Mg, Fe, Mn, and Ni (Fig. F53) are related to the breakdown of olivine, pyroxene, and titanomagnetite. Loss of Zn reflects the complete replacement of titanomagnetite by titanite, and Cu depletion results from breakdown and loss of igneous sulfide minerals. Contents of K2O, Ce, and Ba are significantly elevated, but Rb is apparently not enriched.
An interpillow hyaloclastite from geochemical Unit 13 (Core 185-801C-24R) exhibiting typical dark gray background alteration was analyzed (Fig. F54). This sample comprises 30% slightly altered basalt, 50% glass totally altered to smectite, and 20% matrix (95% saponite and 5% calcite). The sample exhibits gains of MgO, K2O, and Rb, elevated LOI, and losses of MnO, CaO, Ni, Cu, and Ba. The significant gains of K and Rb suggest that celadonite or other K-rich phase(s) may be present but not detected in hand specimen.
Two alteration halos along veins in geochemical Unit 16 were analyzed (Figs. F55, F56). Both comprise mixed brown plus dark halos along saponite + calcite veins. Minor amounts of dark gray host rock and vein material may be included in these samples. A thin section taken adjacent to Sample 185-801C-37R-6, 33-35 cm, contains saponite, abundant disseminated Fe oxyhydroxides, and two small (0.4 mm) calcite veinlets. Compared to fresh rock, this halo is enriched in K2O, Rb, CaO, and total Fe and has a high LOI (Fig. F56). Ni and Zn exhibit slight depletions related to breakdown of olivine and titanomagnetite.
The chemical composition of the alteration halo of Sample 185-801C-36R-4, 112-114 cm (Fig. F55), is generally similar to that analyzed from Core 37R, but the former exhibits only gains of alkalis and a slight loss of Cu. Apparent changes in other elements are not significant.
As a very rough estimate, the total section cored during Leg 185 has experienced an increase of ~17% in K2O and Rb contents as the result of seawater alteration (see Table T10) (assuming that (1) all breccias are altered as the analyzed hyaloclastite, (2) all halos are represented by those analyzed, (3) celadonite veins contain 8 wt% K2O, and (4) unaltered rocks contain 0.08 wt% K2O). In this crude estimate, ~13% of the total alkali budget is contained in breccias, ~4% in alteration halos along veins, ~4% in celadonite veins, and the remainder (~83%) in essentially unaltered basalt. If 2.3 vol% interpillow sediment containing 1.5 wt% K2O is added to this estimate, then the K2O content of the Leg 185 section is 60% greater than in fresh basalt alone, with 27% of the total alkali budget residing in interpillow sediment.
In the basalt cored during Leg 185 in Hole 801C we logged ~3500 veins, which comprise 1.8 vol% of the recovered core. The main vein types are calcite, saponite, and calcite plus saponite, with lesser amounts of celadonite, iron oxyhydroxide, pyrite, and silica (quartz and chalcedony).
The recovered cores include an estimated 1.5 vol% breccias and hyaloclastite and 2.5 vol% interpillow sediments. Breccias contain an average of 20% cement, comprising mainly saponite (39%) and calcite (44%), but also minor celadonite and silica. Some hyaloclastites include fresh glass, but most are highly altered to smectite and calcite and have undergone an order of magnitude enrichment in K2O and Rb, and exhibit slight loss of Ca in exchange for Mg from seawater. Interpillow sediments comprise mainly chert and limestone but also contain iron oxides, smectite, and celadonite-nontronite and may include a low-temperature hydrothermal component. Alkali contents are high (1.5 wt% K2O and 30 ppm Rb).
Most (~95%) of the basalts are slightly altered and contain <15 vol% secondary minerals (saponite, calcite, and pyrite) replacing olivine and interstitial material, and filling pore space. Chemical changes are small, including slightly elevated K2O and Rb contents, losses on ignition, and sporadic CaO enrichments.
Intensely altered pale green basalts occur at 613-626 mbsf (Cores 185-801C-15R to 16R) and are probably related to upflow of low-temperature hydrothermal fluids feeding the hydrothermal iron-silica deposits at 520-531 and 625 mbsf. The pale green rocks are intensely altered (80%) to calcite, smectite, and celadonite (plus minor titanite, pyrite, and iron oxyhydroxide). Chemical changes are significant, including losses of Mg, Fe, Mn, Ni, Zn, and Cu, and gains of K2O, Ce, and Ba.
Alteration halos, 1-18 mm wide, are common along veins in the slightly altered basalts. These halos include dark, brown, and mixed brown and dark types, and are characterized by the presence of celadonite-nontronite filling pores and replacing olivine and interstitial material, by abundant disseminated iron oxyhydroxides, and commonly by disseminated pyrite in the host rock just outside the halo. Alteration halos comprise 1.7 vol% of recovered core and are present locally throughout the section, but are most common in massive units below 780 mbsf (Cores 185-801C-34R, 36R, 37R, and 43R through 46R)
The Leg 185 section can be divided into two general zones. The upper portion, from 600 to 700 mbsf (Cores 185-801C-13R to 24R) has a higher abundance of veins (mean = 27 veins/m), contains almost all of the interpillow sediment intervals, includes common breccias throughout, and contains common silica-bearing veins. This interval also includes the small interval of hydrothermal iron-silica material at 625 mbsf and the associated intensely altered pale green rocks, as well as the zone of variable hole diameter from the caliper log (~625-715 mbsf) (see "Borehole Characteristics").
The lower portion of the hole, from ~700 to ~900 mbsf, has a lower abundance of veins (mean = 20 veins/m), a general absence of interpillow sediment and silica-bearing veins, and limited occurrences of breccias. These all coincide with a zone of constant hole diameter from the caliper log (see "Borehole Characteristics"). This portion of the core also contains the most abundant oxidized alteration halos (780-900 mbsf), as well as peaks in abundances of veins of celadonite and iron oxyhydroxide.
The lowermost few meters of core (Cores 185-801C-48R through 52M) are characterized by an increase in pyrite veins and a peak in vein abundance of 39 veins/m, which occurs in the 2.08 m of basalt in Cores 185-801C-49M through 52M. The latter may be related to diamond coring in this interval, but a similar maximum in vein abundance (40 veins/m) also is found in Core 185-801C-18R, which was cored by standard RCB.
The major secondary minerals present in the Site 801 section drilled during Leg 185 (saponite, calcite, celadonite, and pyrite) are generally similar to those of the Leg 129 section and other upper crustal sections and result from interaction of basalts with seawater fluids at temperatures of ~0°-50°C (Honnorez, 1981; Alt et al., 1992; summary in Alt, 1999). Relatively low seawater fluid fluxes result in the dominant pyrite-bearing dark gray alteration, whereas locally greater fluxes of oxygenated seawater cause the oxidation observed in the brown rocks and alteration halos. Celadonitic alteration along fractures may be related to diffuse flow of distal, cooled hydrothermal fluids at the spreading axis (see paragraph immediately below) or precipitation from cold seawater-derived basement fluids (Alt, 1999).
Alteration of the Site 801 section can be interpreted based on previous work on the Leg 129 section and by analogy with other studied oceanic basement sections (Staudigel et al., 1995; Alt et al., 1996; Alt, 1999). The tholeiitic section of Legs 185 and 129 was formed at a fast-spreading mid-ocean ridge at ~165 Ma (Pringle, 1992). The hydrothermal iron-silica deposit formed on the seafloor at the spreading axis, at temperatures probably <50°C (Alt et al., 1992). Hydrothermal fluids were most likely derived from high-temperature reactions at depth, which produced acid fluids enriched in alkalis, metals, silica, and sulfide (Edmond et al., 1979). It was cooled, distal hydrothermal fluids, however, that resulted in the formation of the hydrothermal iron deposit at the seafloor. The intensely altered green rocks immediately beneath the deposit represent the feeder zone for the hydrothermal deposit, with the buff-colored rocks transitional to less altered dark gray host rocks. The much less intense alteration and formation of celadonitic minerals and chalcedony in veins and alteration halos along fractures at greater depths may be related to circulation of similar distal hydrothermal fluids at the ridge axis (Alt et al., 1992; summary in Alt, 1999). The intense pale green alteration observed in Cores 185-801C-15R and 16R is also probably related to the feeder zones for the hydrothermal deposit in Core 129-801C-4R and the smaller hydrothermal sediment interval in Core 185-801C-16R. The pervasive dark gray background alteration and local brown oxidizing alteration resulted from subsequent circulation of seawater fluids at low temperatures, beginning at the spreading center but continuing on ridge flanks and farther off axis. Carbonate veins form relatively late in the sequence, and the multiple generations of carbonate veins in the uppermost Leg 129 section are suggested to result from continued formation of carbonates as the crust ages (Alt and Teagle, 1999). Rb/Sr dating of celadonites from Site 801 gives an isochron age of 132 Ma, suggesting that celadonite remained open to exchange for ~30 m.y. or that the isotopic system was reset somehow at 132 Ma (Bourasseau, 1996).
The alkali basalt section at Site 801 formed ~7 m.y. after the tholeiites (Pringle, 1992), and re-examination of the rocks and overlying sediments during Leg 185 suggests that the alkali basalts were intruded as sills between the hydrothermal deposit and the overlying sediments. The reason for the more intense alteration of the alkali basalts compared to the tholeiites is uncertain but may be related to greater primary volatile concentration or simply to differences in permeability (because of a larger grain size) and greater fluid fluxes through the alkali basalts. It is not clear whether magmatic or tectonic activity related to the formation of the alkali basalts influenced fracturing, fluid flow, and alteration in the underlying tholeiitic section cored during Leg 185.
Perhaps a surprising feature of the Leg 185 section is that alteration, at least as shown by the presence of dark, brown, and mixed alteration halos along veins, is greater in the lower portion of the section than at the top. Evidence from other well-documented deep basement sites suggests that the effects of cold seawater alteration, although clearly heterogeneous, generally decrease with depth (Staudigel et al., 1995; Alt et al., 1996). Oxidation effects (brown altered rocks) are abundant in the alkali basalts at Site 801 and make up a significant proportion of alteration in the Leg 129 tholeiitic section (~10%) (J.C. Alt and D.A.H. Teagle, unpubl. data), but the dark gray, pyrite-bearing background alteration is by far the dominant alteration type below 550 mbsf. Alteration of the uppermost crust at Site 801 was controlled by local hydrological conditions, with circulation of larger volumes of oxygenated seawater focused at the very top of the tholeiite section and at depth through massive units in the lower part of the section (below 780 mbsf).
A preliminary estimate indicates that the total section cored during Leg 185 has experienced an ~17% increase in K2O and Rb contents as the result of alteration with seawater. Adding interpillow sediment to this estimate increases the bulk K2O content of the Leg 185 section to 60% greater than in fresh basalt alone, with 27% of the total alkali budget residing in interpillow sediment.