Basalt from Hole 1160A is slightly to moderately altered by low-temperature reactions. The uncut surfaces are weathered to a buff color, suggesting that a rubble deposit was encountered. Patchy coatings of yellowish white clay are <0.3 mm thick and usually associated with ~0.2-mm spots of Mn oxide on the uncut surfaces of some pieces (e.g., Section 2R-1 [Pieces 3 and 6]). More commonly, spots (0.2-0.5 mm) of Mn oxide alone are present on the uncut surfaces of pieces. The degree of alteration is highest within concentric oxidation halos extending 1-10 mm inward from the piece margins (Fig. F7). Within these halos, the degree of alteration increases toward the piece margin and is characterized by 5%-15% groundmass replacement by Fe oxyhydroxide and smectite (Fig. F8). Veins are not present, but rare open fractures are 1 mm wide, lined with Mn oxide and Fe oxyhydroxide, and surrounded by 1- to 9-mm alteration halos. Even within single pieces, minor vesicles (<1%, ~0.1 mm) range from unfilled to lined to partially filled to completely filled with light green clay, yellow clay, Mn oxide, Fe oxyhydroxide, cryptocrystalline bluish silica, or crystalline quartz. Sparse, fresh glass rinds range from <<1 mm (Section 187-1160A-2R-1 [Piece 2]) to 1.5-2.5 mm in thickness (Section 187-1160A-3R-1 [Pieces 2 and 5]). Orange-brown palagonite layers (0.5 mm thick) entirely cover the fresh glass or dissect the fresh glass along minute (~0.3 mm) cracks.
Based on significant differences in the degree of alteration and vein mineralization, two facies of alteration are recognized in the seven lithologic units from this hole.
This facies is characterized by moderate to high degrees of low-temperature alteration that dominates the pillow basalt units recovered from Hole 1160B (see "Igneous Petrology"). These units are moderately to highly affected by low-temperature alteration, except for fresh glass from quenched pillow margins and a significantly less altered area within Section 187-1160B-4R-2 (Piece 7C). Patchy coatings or spots of Mn oxide, white clay, Fe stains, and calcite (in places Fe stained) are present on the uncut surfaces throughout. Uncut, weathered outer surfaces on most 1- to 4-cm-sized pieces in Section 187-1160B-6R-1 indicate that at least some sections consist of rubble rather than an in situ pillow lava pile. In Units 1 and 5, all fractures are open, usually 0.1-0.4 mm wide and 1-3 cm long, and commonly lined with Mn oxide and/or Fe oxyhydroxide; most are free of alteration halos. In Unit 3, sparry calcite-filled veins 0.1-1 mm wide sometimes surrounded by 5- to 20-mm-wide alteration halos are present (Fig. F9). Within the alteration halos, the groundmass is mostly replaced by Fe oxyhydroxide and clay, resulting in a reddish brown color. The walls of the veins are commonly lined with Mn oxide and, in places, with Fe oxyhydroxide. A single 3-mm-thick vein of Fe-stained cryptocrystalline silica is present in Section 187-1160B-6R-1 (Piece 19A) (Fig. F10). In the same section, irregular calcite patches up to 5 mm across, some of which are associated with calcite veins, may represent groundmass replacement and/or vug fillings (Fig. F11).
The most distinctive alteration feature in this facies, however, is the pervasive groundmass replacement by Fe oxyhydroxide and smectite, giving the basalts a light to medium red-brown appearance and causing their overall moderately to highly altered condition. Olivine (80%-100%), clinopyroxene (15% in Fig. F12; elsewhere as much as 30%), and mesostasis (20%-30%) appear to be most affected, whereas groundmass plagioclase remains relatively fresh throughout. The percentage of groundmass replacement ranges from 10% to 20% in Unit 1 to 20%-50% in Unit 3. The outer zones of some pieces are highly (80%-95%) altered, resulting in up to 1.5-cm-wide, light brown halos (e.g., Section 187-1160B-8R-1 [Pieces 6 and 7]) that grade into a patchy orange-brown to grayish brown replacement textures toward the centers of pieces (Fig. F13). The grayish brown areas appear to contain more smectite and less Fe oxyhydroxide. Plagioclase phenocrysts in Units 3 and 5 are mostly fresh throughout, but many are Fe stained, resulting in a yellowish green color throughout the crystal or along crystal edges and faces. Some plagioclase phenocrysts of Unit 5 in the vicinity of Mn oxide-lined microcracks appear black but are transparent under the binocular microscope, suggesting that the discoloration stems from the Mn oxide that possibly lines crystal faces. Olivine phenocrysts in Units 3 (<1%) and 5 (1%-2%) are totally (90%-100%) altered throughout.
Basalt chilled margins containing fresh glass in 3- to 14-mm-wide zones are present in all three units (see "Igneous Petrology"), with the outermost glass rim commonly covered with the thickest (0.3-1 mm) layer of orange-brown palagonite (Fig. F14). Smaller palagonite-filled veins (<0.5 mm) are mostly aligned subparallel to the quenched margin or dissect the fresh glass subperpendicular to the quenched margin, thereby splitting the glass zone into alteration cells. Plagioclase phenocrysts associated with the glass zones in Units 3 and 5 are fresh throughout. Some palagonite veins in Unit 5 are associated with cryptocrystalline silica and Mn oxide. In all three units, the zone of discrete spherulites is 1-3 mm wide and altered to Fe oxyhydroxide and clay.
Alteration Facies II, which occurs exclusively in the massive basalts of this hole (see "Igneous Petrology"), is characterized by fresh to slightly altered rocks with chlorite-bearing fractures and veins. The low degree of alteration in the massive basalts (Fig. F3) contrasts with the more severe alteration seen in the pillow basalts of Units 1, 3, and 5 of this hole. An exception is the massive basalt of Unit 4, which is slightly to moderately altered in Sections 187-1160B-7R-2 and 8R-1 (Piece 1). As with Facies I, alteration occurred at low temperature.
The massive basalts are cut by numerous randomly oriented fractures or very thin veins (<<0.5 mm). The fracture surfaces are covered with (1) pale green chlorite ± silica (Fig. F15), chlorite ± calcite, or (2) chlorite + clay + Fe oxyhydroxide. Chlorite was not observed at any previous Leg 187 site. In most places there are no substantial alteration halos associated with the chlorite-lined fractures or small veins. An exception occurs in Unit 6 in Section 9R-2, where a narrow (0.5 cm) zoned alteration halo has developed around a chlorite vein (<<0.5 mm). Closest to the vein the groundmass of the halo appears virtually unaltered, but the outer edge of the halo is darker, separating the halo from the fresh basalt. This halo differs macroscopically from alteration halos associated with calcite veins seen so far in this leg, but the origin of these color differences is unclear. Patches of chlorite + white clay are present in the vicinity of chlorite-bearing fractures and veins in Section 187-1160B-4R-1 (Piece 8) (Fig. F16) and in Section 9R-2 (Pieces 1 and 3-7). In some places where it is associated with chlorite, calcite is present as radiaxial clusters of elongate crystals as long as 5 mm. Minute fractures lined with Mn oxide in Section 187-1160B-7R-2 of Unit 4 lack alteration halos and are closely associated with calcite-filled veins. Fractures lined with cryptocrystalline silica are present only in Section 187-1160B-9R-1 (Piece 21).
Veins filled with sparry calcite + Fe oxyhydroxide ± Mn oxide are usually 0.2-2 mm wide and generally surrounded by 1- to 4-cm-wide alteration halos (Fig. F17), except for a calcite vein in Section 187-1160B-7R-2 (Piece 7C) (Unit 4) that has no alteration halo. Clinopyroxene and mesostasis are the most commonly replaced groundmass phases (by Fe oxyhydroxide and smectite), whereas groundmass plagioclase is fresh throughout. Alteration related to calcite + Fe oxyhydroxide-bearing veins is most abundant in Sections 187-1160B-7R-2 and 8R-1, corresponding to the most altered sections of Unit 4. Notably, the outer edge of the alteration halo is characterized by a greater abundance of Fe oxyhydroxides than areas closer to the vein. The inner walls of the calcite veins are commonly lined with Mn oxide.
In Section 187-1160B-4R-2 (Pieces 1G and 1H), an Fe oxyhydroxide + calcite vein terminates in a chlorite-lined vein/fracture. A similar spatial sequence in vein mineralogy is observed for Unit 4 in Section 187-1160B-7R-1 (Piece 10) where, in addition to numerous chlorite-lined fractures, the center of a 5-mm-wide vein in Piece 10F is filled by sparry calcite, lined by Fe oxyhydroxide/Mn oxide, and surrounded by chlorite toward the wall rock.
Except for alteration halos, the groundmass is exceptionally fresh throughout all units of this alteration facies (Fig. F18). Plagioclase phenocrysts are fresh throughout; rare olivine phenocrysts at the base of Unit 4 (Section 187-1160B-7R-3) and olivine microphenocrysts of Unit 7 are totally altered to Fe oxyhydroxide and clay. Rare botryoidal, radiaxial, or dogtooth calcite mineralization is present in Sections 187-1160B-4R-1 (Piece 8) and 7R-2 (Piece 2) (Fig. F16). Vesicles are only present in Unit 7 (<1%, 0.2 mm); they are filled with yellow and dark green smectite within 3- to 10-mm-wide alteration halos (Fig. F19) that are aligned subparallel to the piece margins. Within these halos, as much as 15% of the groundmass is replaced by smectite (Fig. F19).