ALTERATIONBasalts recovered during Leg 187 have all been subjected to low temperature alteration. Macroscopically, alteration is manifested most commonly as (1) alteration halos around the margins of pieces and (2) following veins and open fractures. There is a broad range of alteration intensity within and between sites, from completely fresh, to incipient iron staining, to pervasive discoloration. Alteration phases replacing phenocrysts and groundmass include abundant Fe oxyhydroxides and clay minerals, with common cryptocrystalline silica and Mn oxide encrustations. Less commonly, carbonate has replaced groundmass and precipitated along veins. At one site, greenschist facies assemblages were recovered.
Alteration halos on core pieces usually have sharp, smooth contacts with the less altered piece interiors (Fig. 18), although some are gradational and/or irregular. At most sites, several zones can be distinguished in these alteration rinds, progressing from more intensely altered rims where Fe oxyhydroxides and clay pervasively replace groundmass and phenocrysts alike, to less intensely altered areas where only groundmass phases are altered.
In at least some pieces from every site, low-temperature alteration is pervasive. This alteration is developed as brown-red discoloration of the entire piece, apparent on drill-cut exteriors of pieces as well as on the cut faces of cores. In these pieces the groundmass and most phenocrysts are altered to Fe oxyhydroxide and clay, although fresh kernels are common. In both alteration halos and pervasively altered pieces, olivine phenocrysts are much more intensely altered than plagioclase. Fe oxyhydroxide and clays commonly only highlight cleavage planes, fractures, and crystal margins in plagioclase.
The most widespread evidence of low-temperature alteration seen in thin section is partial to complete replacement of quench textured phases and mesostasis by Fe oxyhydroxide and clay (Fig. 19). Crystalline groundmass phases are variably altered through replacement of olivine, clinopyroxene, and mesostasis to smectite and Fe oxyhydroxide, visible as a pervasive or patchy discoloration and cloudy appearance. Groundmass plagioclase is usually fresh or incipiently altered.
Olivine phenocrysts are commonly completely altered, or nearly so, to clay, Fe oxyhydroxide, and magnetite. In some sections, olivine pseudomorphs show euhedral grain shapes, but only the cores of grains are preserved. In others, olivine alteration is restricted to fractures. The main alteration product of plagioclase is Fe oxyhydroxide, mostly along cleavage planes and fractures. Plagioclase is rarely completely replaced; in nearly every thin section there is more fresh than altered plagioclase.
Overall alteration characteristics vary from site to site. Sites 1152,
1153, and 1154 are only slightly altered. This is interesting in that these
are the oldest sites we drilled (~26 to 28 Ma). Alteration at these sites is
expressed as only minor replacement of groundmass and phenocrysts by
secondary phases. Sites 1161 and 1162, associated with a westward
propagating rift, are predominantly carbonate- and/or silica-cemented
breccias, and both show moderate to high degrees of alteration. Angular
basalts clasts in the breccia are commonly pervasively altered, and at
least some of the alteration occurred prior to brecciation, as evidenced by
matrix cutting through alteration halos. Site 1164, our last site, shows
the most pervasive effects of alteration and was interpreted to have
sampled only basaltic rubble (see "Igneous Petrology"). All other sites
showed a range of slight to moderate alteration, less intense overall than
Sites 1161, 1162, and 1164, although we recovered at least a few pieces
of pervasively altered basalt at every site. The effects of alteration on
bulk rock composition are discussed in "Geochemistry."
Palagonitized pillow rinds ranging in thickness from 0.5 to 10 mm were sampled at every site. Within the rinds, fresh, black, basaltic glass is common, but there is ubiquitous yellow to orange palagonite with dull surfaces in places crosscut by a dense network of anastomosing silica (Fig. 20) and, more rarely, silica and calcite veins. These veins are most commonly oriented subparallel to rind margins, although crosscutting veins are also abundant. Fresh glass and palagonite commonly occur in thin (0.5-2 mm), irregularly interlayered sheets, with palagonite being most abundant (>90%) in the outermost sections. In thin section, altered glass rims show weak parallel banding, except at the alteration front, which often displays dendritic features extending into fresh glass and believed to be related to microbial degradation (Thorseth, et al., 1995) (Fig. 21).
Veins are present in all the cores. They include both compositionally homogeneous and composite veins, as well as rare crack-seal veins. Veins filled with combinations of silica, Fe oxyhydroxide, clay, and Mn oxide are present at every site, whereas calcite-bearing veins were only observed at Sites 1153, 1155, 1156, 1157, 1160, and 1163. The most common vein assemblage where carbonate was observed includes thin (<1 mm) linings of cryptocrystalline silica along vein selvages, spotted with sporadic submillimeter Mn oxide accompanied by fillings of microcrystalline to cryptocrystalline calcite (Fig. 22). Additionally at these sites, it is not unusual to see composite veins that change from silica filling within the palagonitized glass and chilled margins to calcite filling toward the interiors of pillow fragments. Sites 1160 and 1162 were unique in that these were the only sites where chlorite is a common vein-filling phase. Dolomite veins are present only in cores from Site 1162. Sediment-filled fractures are common in breccia clasts and pillow lavas that retain adhered interpillow sediment. At Site 1156, sediment-filled fractures and the breccia matrix contain two generations of calcite. First, a drusy pink micritic calcite permeates the sediment and breccia matrix; subsequently, sparry calcite precipitated along fractures and in void spaces.
Greenschist Facies Metamorphism
At Site 1162, a number of breccia clasts have developed a distinct greenschist facies alteration mineralogy. Macroscopically, both aphyric and phyric basalts are greenish brown, strikingly different in color to any other rocks recovered during Leg 187. Olivine phenocrysts are completely replaced by Fe oxyhydroxide and a pale yellow combination of chlorite, talc, and clay. Groundmass olivine and clinopyroxene are extensively replaced by Fe oxyhydroxide and clay in most pieces, and several pieces have a chloritized groundmass. The presence of chlorite indicates that these rocks underwent metamorphism at 150°-250°C (Alt et al., 1996).
Somewhat more intense high-temperature alteration has affected gabbro and diabase breccia clasts recovered at this site. Fibrous actinolite and chlorite replace clinopyroxene, and olivine is totally replaced by concentric layers of talc, chlorite, and magnetite. Plagioclase is partially replaced by clay and chlorite and is recrystallized to albite along grain margins. Thin veins (<1 mm) are filled with chlorite and clay. The occurrence of actinolite, talc, chlorite, and albite in upper crustal basalts is consistent with lower greenschist facies metamorphism (Alt et al., 1996).
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