ALTERATION AND WEATHERING

Ten basement units have been defined at Hole 1137A, including seven igneous units interpreted as separate subaerial lava flows (see "Igneous Petrology" and "Physical Volcanology") and three volcaniclastic sedimentary units (see "Lithostratigraphy", and "Physical Volcanology"). The three sedimentary units include siltstone interbedded with sandstone, conglomerate, and a crystal vitric tuff that has been reworked by fluvial and/or mass wasting processes. Fluid-rock interaction has variably altered all basement units after emplacement, as indicated by secondary minerals that partly replace primary minerals, partly to completely replace mesostasis, and partly to completely fill veins and vesicles. Although the alteration in Hole 1137A is predominantly manifested by limited secondary minerals (clays, zeolites, calcite, amorphous silica, and quartz), the abundance of these phases in veins, vesicles, and breccias varies distinctly downhole. In this section, we describe these patterns and other notable features related to alteration for each basement unit.

Downhole variations in alteration, including the distribution of secondary minerals, are recorded in the alteration and vein/structural logs (see the "Supplementary Materials" contents list; Fig. F63). In general, the most highly altered rocks are associated with horizons characterized by higher permeability, including brecciated or vesicular flow tops and flow bases and zones with high vein and fracture densities.

Basement Unit 1 is overlain by a highly oxidized and well-indurated sedimentary breccia consisting of highly vesicular clasts cemented by amorphous silica, calcite, and zeolite (interval 183-1137A-24R-1, 0-70 cm). The flow top of Unit 1 is not present; consequently, the first basement rocks encountered in Hole 1137A represent the massive interior of a basalt flow. The dark gray interior of Unit 1 is slightly altered with the mesostasis replaced by saponite. Sparse vesicles (<2%) are filled with saponite and zeolite. Saponite lines vesicles where both phases coexist, indicating clay formation before zeolite.

Numerous 0.1- to 8.0-mm-wide veins in Unit 1 are filled with clay and calcite in the upper part of the flow (219.51-228.40 mbsf) and clay and zeolite in the bottom part of the flow (228.45-232.22 mbsf). Similar to the paragenesis indicated for vesicles, clay minerals tend to line veins, suggesting clay formation before zeolite and calcite. Well-defined vein halos are absent.

Unit 2 has been subdivided into two subunits: a brecciated flow top (Subunit 2A) and a massive flow bottom (Subunit 2B). The brecciated flow top is highly to completely altered and consists of oxidized vesicular basalt clasts (some with chilled margins) cemented by calcite and zeolite (Fig. F64). Vesicles are numerous in the clasts (as much as 30%) and are filled with calcite, zeolite, and blue-green celadonite. The massive interior of this flow (Subunit 2B) is slightly to moderately altered with well-defined mesostasis bands altered to saponite. Vesicle abundance decreases from ~15% near the base of Subunit 2A to <1% in the flow interior, with clay, zeolite, and (to a lesser extent) calcite as vesicle fillings. Mineral zonations within vesicles suggest early formation of a celadonite lining followed by saponite and zeolite.

Numerous 0.1- to 3.0-mm-wide veins are present in Subunit 2B, and vein minerals exhibit irregular downhole distributions through the flow. Calcite and clays are the most abundant vein fillings from the top of Subunit 2B to 237.50 mbsf. From 237.50 to 240.75 mbsf, calcite is absent, and vein filling is dominated by green clays and zeolite. In addition, one vein within this interval (Sample 183-1137A-26R-2 [Piece 11, 87-95 cm]) contains as much as 10% pyrite. Finally, from 240.75 mbsf to the base of Subunit 2B, zeolite is absent, and vein filling is again dominated by clays and calcite.

A highly vesicular upper half (Subunit 3A) and massive lower half (Subunit 3B) characterize the third basalt flow. Well-indurated and laminated blue-green siltstones also occur in the upper portion of Subunit 3A (Fig. F65), and in some sections they comprise as much as 50% of the rock (e.g., Section 183-1137-27R-2). XRD analysis of this material shows celadonite in addition to quartz and orthoclase (Table T13), which are probably clastic components instead of alteration minerals. High induration of these altered siltstones contrasts with the soft, waxy green clay that fills nearby vesicles. The laminated sediments display an intricate contact with the basalt, and a possible chilled margin (~5 mm wide) suggests that the sediments could have been indurated and altered by baking. Alternatively, high induration of these sediments could have been a product of silicification long after cooling. It is noteworthy that the first occurrence of amorphous silica in the basement of Hole 1137A is <2 m below the first appearance of these indurated sediments.

The degree of alteration in Subunit 3A progressively decreases from high to slight with depth through the unit. A general decrease in oxidation is suggested by a progressive change in color from red near the top of Subunit 3A to pink-gray and, finally, to gray near the bottom. This decrease in alteration with depth also coincides with a progressive decrease in vesicularity (Fig. F63). Vesicles are filled with green clays, calcite, and zeolite from 247.60 to 250.60 mbsf. Vesicle mineralogy changes markedly at 250.43 mbsf with the first appearance of amorphous silica as well as clays and zeolite, a mineral assemblage that is present to the base of Subunit 3A. Moreover, geopetal structures are common in this interval and are characterized by a subhorizontal boundary with agate filling below and zeolite or green clay filling above (Fig. F66). Calcite is absent in vesicles within this lower portion of Subunit 3A.

Vein mineralogy mimics changes in vesicle mineralogy with depth through Subunit 3A. Calcite- and clay-filled veins as much as 3 mm wide are common from the top of Subunit 3A (247.60 mbsf) to 248.75 mbsf. In contrast, from 251.15 mbsf to the bottom of Subunit 3A (253.00 mbsf), veins are 1 mm wide and contain green clay, amorphous silica, and zeolite (with calcite absent).

Subunit 3B exhibits only slight to moderate alteration, with well-defined bands of mesostasis that have been altered to saponite (Fig. F67). Vesicles are sparse and are lined with light green saponite and filled with zeolite. Veins are as much as 5 mm wide and also contain clay, zeolite, and (more rarely) amorphous silica. Distinct green alteration halos are common around veins from 258.00 to 258.45 mbsf (e.g., Sample 183-1137-29R-1 [Piece 1]).

The contact between Units 3 and 4 is complicated by a narrow interval of highly altered volcaniclastic debris (Fig. F68). Within this interval, the basalt clasts are variably oxidized and highly vesicular. Well-indurated blue-green sediments are present along with green clays (saponite and nontronite), zeolite (clinoptilolite), and calcite (e.g., Sample 183-1137-29R-2 [Pieces 3-4]), similar to the sediments observed in Subunit 3A.

Basement Unit 4 (259.10-286.72 mbsf), a basalt flow, has a thick, highly vesicular upper section and three highly vesicular horizons within the flow that are more highly altered than the massive portions. Vesicles comprise ~20%-30% of Unit 4 from 259.10 to 267.75 mbsf. The color within this upper interval varies from red near the top to pink gray and, finally, to gray near the bottom, suggesting an overall decrease in oxidation with depth. The most abundant alteration phases in this portion of Unit 4 include blue-green clay (celadonite), light green clay (saponite), and zeolites. These minerals replace groundmass and also fill vesicles and veins.

Vesicle mineralogy displays irregular paragenetic sequences within the highly vesicular upper portion of Unit 4. Some vesicles are progressively filled from rim to core with blue-green clay (celadonite), light green clay (saponite), and zeolite (clinoptilolite and heulandite), suggesting that clay formation preceded zeolite formation (e.g., Sample 183-1137A-29R-4 [Pieces 1-7]). In other cases, zeolite lines vesicles with clay interiors, indicating an opposite paragenetic sequence (e.g., Sample 183-1137A-30R-4 [Pieces 1-9]). In rare cases, multiple generations of zeolite minerals are present in a single vesicle. Geopetal structures are also common in some vesicles, with light green saponite below and zeolite or amorphous silica above a subhorizontal boundary (Sample 183-1137A-30R-2 [Piece 1]).

Three other highly vesicular intervals are within Unit 4, all with vesicle abundances generally exceeding 10% of the rock. The color of these intervals tends to be slightly more red to pink compared to more massive adjacent intervals, which suggests higher degrees of oxidation. Vesicle mineralogy is virtually identical to that described above for the upper portion of Unit 4, although we observed calcite in small amounts (<5%) within vesicles in one section (Sample 183-1137A-31R-7 [Pieces 14-23]). In addition, one large vesicle (Fig. F69) is lined with clinoptilolite and filled with barite (confirmed by XRD).

Less vesicular portions of Unit 4 are gray to dark gray in color and generally show similar vesicle fillings, except for an anomalous 4-cm-wide vesicle completely filled with quartz and surrounded by a 2-m-wide alteration halo (interval 183-1137A-31R-2 [Piece 11, 142-145 cm]). As a consequence of such a large vesicle, quartz represents ~3% of this section of core. In addition, small amounts of calcite (<8%) fill vesicles in one less-vesicular portion of the flow (Sample 183-1137A-32R-5 [Pieces 1-6]).

Numerous veins from 0.1 to 3.0 mm wide in Unit 4 are filled with iron-rich saponite/nontronite, celadonite, zeolites, calcite, and quartz (Table T13). Calcite is notably absent from veins within most of Unit 4, but calcite increases abruptly within veins near the base of the unit from 285.50 to 286.72 mbsf (Sample 183-1137A-33R-1 [Piece 1]). Within this interval, numerous veins as much as 1 mm wide are filled with calcite and green saponites, but zeolites are absent.

The interbedded siltstones and sandstones of Unit 5 are very dark brown to black immediately below the contact with Unit 4, suggesting a baked zone (Sample 183-1137A-33R-1 [Piece 3]). Below this possible baked zone, Unit 5 is light green and is cut by a number of veins as wide as 2 mm, filled with green clays (saponite?) and calcite and identical to the calcite-bearing veins at the base of Unit 4.

The conglomerate (Unit 6) is composed of variably altered volcanic clasts set within a green clay (saponite?) and blue-green clay (celadonite?) matrix along with minor calcite. The clasts are well rounded and display continuous oxidation rims as much as 1 cm wide. Saponite replaces primary minerals in the clasts, and in rare circumstances calcite replaces plagioclase laths (e.g., Sample 183-1137A-36R-2 [Piece 1]). Vesicles within clasts are filled with saponite, and we observed some geopetal structures. Veins are sparse (as much as 10 mm wide) and generally crosscut the matrix but not the clasts. Calcite was the only vein mineral observed.

Basement Unit 7 represents another subaerial basalt flow with a brecciated flow top (Subunit 7A) and a more massive flow bottom (Subunit 7B). The flow-top breccia (Fig. F70) is highly to completely altered and is composed of angular clasts that exhibit different degrees of oxidation, as suggested by the highly variable color of this unit (brick red, brown red, pink gray, and gray). Secondary minerals within the matrix, vesicles, and veins of this breccia include calcite, blue-green clays, red-brown clays, and zeolites. In addition, calcite replaces plagioclase laths locally (e.g., Sample 183-1137A-37R-1 [Piece 1]). Variably oxidized, well-laminated, and highly indurated sediments within the breccia resemble those sediments described from other basalt flows at higher stratigraphic levels within Hole 1137A.

The more massive flow bottom (Subunit 7B) is slightly to moderately altered, with well-defined subhorizontal bands of mesostasis altered to saponite. Overall, the color of Subunit 7B varies from pink gray near the top to dark gray in the middle of the unit to pink gray near the base, close to the highly vesicular flow top of Unit 8 below. This emphasizes that flow contacts represent more permeable horizons with higher degrees of alteration. Vesicles are not abundant (<2%) and are filled with calcite, zeolite, amorphous silica, and saponite. Numerous veins and thin fractures between 0.1 and 1.0 mm wide in Subunit 7B contain the same minerals filling vesicles, with saponite most abundant. Similar to the alteration patterns observed in other units within Hole 1137A, calcite veins and vesicles are not randomly distributed and are only found near the top of Subunit 7B from 327.73 to 328.15 mbsf (Sample 183-1137A-38R-1 [Piece 3]).

A narrow interval contains highly vesicular and variably brecciated basalt from 334.11 to 334.85 mbsf near the base of Subunit 7B. Although the rocks in this interval contrast sharply with the relatively massive Subunit 7B above, the morphology of the vesicles in the lower unit suggests that a portion of this interval (down to 334.14 mbsf) may represent the brecciated bottom of Unit 7. The remainder represents the highly vesicular flow top of Unit 8 (see "Physical Volcanology"). The rocks in this interval are highly altered, with matrix and vesicle filling characterized by abundant calcite, celadonite, saponite, and smaller quantities of zeolite (clinoptilolite and chabazite). Fairly well indurated blue-green sediments are similar to those described in other units within Hole 1137A.

Subunit 8B, a more massive flow interior, exhibits slight to moderate alteration with a generally uniform dark gray color. The mesostasis is clearly visible and altered to saponite. Vesicles are not abundant (<3%) and are filled with saponite, zeolite, and amorphous silica. Minor calcite (<1%) is present in some vesicles in the lower portion of the flow from 342.29 to 344.03 mbsf. The character of veins in Subunit 8B, including frequency and mineralogy, is very similar to that of veins in Subunit 7B. Numerous 0.1- to 1.0-mm-wide veins are filled with green saponite, zeolite, and rare amorphous silica. Calcite is present only in veins near the base of Subunit 8B (Sample 183-1137A-41R-1 [Piece 1]).

Basement Unit 9 in Hole 1137A is a thick and relatively homogeneous crystal vitric tuff. The tuff consists of 40% sanidine phenocrysts that show little, if any, alteration in a matrix that has been uniformly altered to green clay (saponite/nontronite) and minor lithic fragments (Fig. F71). The color of the tuff changes progressively with depth from red brown near the top to green in the middle to a deep blue green near the base, suggesting an overall decrease in oxidation with depth. Dark green alteration halos that surround lithic fragments contain rare specks of native copper. Pyrite is present within narrow (<1 cm wide) oxidation bands (e.g., Sample 183-1137A-41R-2 [Piece 1]). Pyrite was also locally abundant within two fault planes characterized by distinct slickensides and by alteration to celadonite and saponite/nontronite (e.g., Sample 183-1137A- 41R-2 [Piece 1]). Numerous lighter olive-green bands throughout the unit possibly represent recrystallized (shear?) zones.

The lowermost basement unit in Hole 1137A is a basaltic flow with a brecciated flow top (Subunit 10A) and a massive interior (Subunit 10B). The flow top is highly altered and contains intercalated sediments (see "Physical Volcanology") and other delicate clast structures that suggest either intrusion of lava into wet sediment or reworking of an autobrecciated flow top. Partially indurated blue-green silts are abundant, and the extent of oxidation appears to be variable, as indicated by both red and green basalt clasts. In addition to the indurated sediments, the breccia matrix contains calcite, which locally replaces plagioclase phenocrysts, zeolite, and green saponite. Clasts are highly vesicular with calcite, zeolite, and saponite fillings.

The more massive flow interior (Subunit 10B) is the least-altered unit encountered in Hole 1137A. It is uniformly dark gray with relatively fresh plagioclase phenocrysts aligned subhorizontally. Vesicle abundance decreases with depth from ~2% near the top of the unit to <0.5%. Vesicles are filled with blue-green celadonite, zeolite, and amorphous silica; calcite-filled vesicles are present only near the top of the subunit (Sample 183-1137A-45R-3 [Piece 1]). Calcite veins as wide as 2 mm are present near the top of Subunit 10B but are absent below 364.26 mbsf, where sparse, narrow (<1 mm) clay-filled veins are present. We observed a trace of native copper in some veins and locally in the groundmass. The bottom of Hole 1137A is at a depth of 371.20 mbsf within Subunit 10B.

The patterns of alteration observed in Hole 1137A likely result from both weathering and low-temperature alteration. Both processes are most developed in the more permeable brecciated and/or vesicular contacts between flows. Unlike alteration in typical oceanic crust, the alteration history of this portion of the Kerguelen Plateau is potentially complex. There is the possibility of submarine weathering and alteration superimposed on subaerial weathering as well as the low-temperature interaction of basement units with groundwater.