Basalts recovered from Hole 1154A all show evidence of a minor degree of low-temperature alteration. The intensity of alteration varies from piece to piece. A few small pieces show no evidence of alteration, but virtually all pieces more than a few centimeters long show as much as 10% overall alteration. This alteration is predominantly concentrated on the surfaces of pieces and along open and partially filled fractures, penetrating up to a couple of centimeters into the interiors of pieces. The most obvious manifestation of low-temperature alteration in hand sample is a light gray to light red-brown discoloration halo visible on the cut face of the core that is in stark contrast to the darker gray, fresh interior of pieces. This halo is most commonly discontinuous and present only adjacent to weathered surfaces of the core and has a well-defined, sharp boundary with fresh basalt. Within this halo, phenocryst and vesicle abundances are masked by the alteration, but close inspection under binocular stereoscope bears out no change in either across the boundary from fresh interior to altered margin. An exception to this observation is when the alteration halo is adjacent to a glassy rim, in which case alteration is imposed upon a primary reduction in both abundance and size of vesicles and phenocrysts. Also common, particularly in alteration halos near fractures, is iron staining and alteration of phenocryst phases. Plagioclase is commonly altered to clay and Fe oxyhydroxide, and olivine is altered to iddingsite with some clay. In some pieces, the margins of olivines have been plucked as a result of the drilling or core splitting process and appear to have a thin coating of silica and/or pale yellowish clay within the cavity that remains, suggesting pervasive penetration of fluid perhaps along grain boundaries. Manganese oxide nodules (1-2 mm) and encrustations are common on weathered surfaces, particularly those on which sediment has adhered. Multiple, planar surfaces of many pieces are weathered light gray-green, indicating that these are pieces of talus that were picked up by the drill bit. Weathered surfaces commonly have patchy encrustations of microcrystalline quartz. In most cases, these weathered surfaces are <1 mm thick, and, where they have been chipped away, fresh basalt can be seen. Vesicles are commonly lined and less often filled with any or all of the following: cryptocrystalline silica, Fe oxyhydroxide, smectite, and rare zeolites.
Open fractures are much more common than veins, but the surfaces of fractures where visible are ubiquitously discolored to a dark green-gray and commonly have iron staining, although phenocrysts exposed on fracture surfaces are commonly fresh. These fracture surfaces are distinct in appearance relative to the weathered surfaces described above in that they lack the light gray-green color and dull luster. In some pieces vein halos decrease in width from the exterior margins of pieces toward the interior (Fig. F7). Vein fillings are most commonly silica, with less Fe oxyhydroxide and clay. Where fractures are partially filled, they contain variable amounts of the same alteration phases as seen in vesicles, with Fe oxyhydroxide and clay predominating.
Basalt glass was rarely recovered except as thin (a few millimeters thick) rinds on several pieces (see "Igneous Petrology"). Where present, basalt glass is always fractured and partially palagonitized. As at previous sites, symmetric alteration halos with silica cores mantled by palagonite, mantled in turn by fresh glass, are common. In Section 187-1154A-6R-1 (Piece 2), minute (±1 mm long) euhedral quartz crystals coat the outer surface of a glassy pillow rind.
In thin section, olivine phenocrysts are seen to be predominantly altered to smectite, whereas the main alteration product of plagioclase is Fe oxyhydroxide. 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. Similarly, plagioclase is rarely completely replaced, and in any section there is more fresh plagioclase than altered. The groundmass shows little alteration, except in the vicinity of altered phenocrysts, where it is altered to smectite and Fe oxyhydroxide.