All the samples examined in this study were collected from Hole 1271B. Hydrothermal alteration of recovered cores from Hole 1271B was fully described in the Leg 209 Initial Reports volume (Kelemen, Kikawa, Miller, et al., 2004). Principal features are described for each lithology below.
Dunite and harzburgite in Hole 1271B are heavily altered. Olivine is completely replaced by serpentine and magnetite, whereas orthopyoxene is altered to bastite pseudomorphs. Reddish brown spinel of rounded shape is present in most samples that have survived alteration. Pseudomorphs after olivine and orthopyroxene are recognized by shape because magnetites frequently form trails along grain boundaries of these pseudomorphs. On the basis of magnetite trails, the primary texture of dunite is inferred to be equigranular. Some dunites in Units II–IV contain veinlets and patches composed of fine-grained talc, chlorite, and amphibole along grain boundaries. We interpret this alteration phase as hydrothermal alteration of plagioclase and clinopyroxene crystallized from melts migrating along grain boundaries in dunite (Kelemen, Kikawa, Miller, et al., 2004). Here we call dunite with interstitial gabbroic material "impregnated dunite."
The shape and size of spinel grains in dunite vary with location in Hole 1271B (Fig. F3). The size of spinel grains decreases from Unit I to Unit III. Dunite in Unit I contains spinel grains 1–2 mm in size. Only a few grains occur in one thin section. Spinel grains in impregnated dunite in Unit II are much smaller (<1 mm) and form symplectite with amphibole on the rims. Unit III dunites contain a large number of spinel grains with smaller euhedral shape (<0.5 mm).
Varying amounts of gabbroic material within the dunite matrix cause various lithologies in Hole 1271B, such as troctolite, olivine gabbro, and olivine gabbronorite. Gabbroic material in impregnated dunite is present along olivine grain boundaries and/or forms disaggregate olivine matrix in centimeter-sized fragments. Gabbroic materials also fill microfractures crosscutting the olivine matrix. Alteration also severely affects gabbroic materials; only olivine and spinel are relict minerals in these rocks, whereas gabbroic materials are replaced by hornblendic amphibole, tremolite-actinolite, talc, chlorite, sericite, metamorphic plagioclase, and minor quartz. On the basis of proportion of primary and alteration minerals, we inferred the primary lithologies and classified them into troctolite, olivine gabbro, and olivine gabbronorite. Gabbroic materials are interpreted as crystallization products of melt migrating through peridotite (Kelemen, Kikawa, Miller, et al., 2004). The amount of gabbroic materials increases in the following order: impregnated dunite, troctolite, and olivine gabbro/gabbronorite.
Brown amphibole gabbro consists of brown amphibole and plagioclase with minor amounts of ilmenite, rutile, titanite, and zircon. This rock type is distinguished from olivine gabbro by the presence of brown amphibole and the absence of olivine. Brown amphiboles are partially altered to low-temperature amphiboles such as tremolite. Plagioclases mostly become sericite except for a few grains.