A large volume of dunite (56%) associated with a minor amount of harzburgite (<1%) is a peculiar characteristic of Hole 1271B relative to other holes, where dunite constitutes 5%–20% of recovered cores, except for Hole 1271A (98%). Dunite in abyssal peridotite and ophiolite has been considered to be a reaction product of harzburgite and orthopyroxene-undersaturated mid-ocean-ridge basalt (MORB) melt that dissolved orthopyroxene during melt migration in the shallow upper mantle (Quick, 1981; Kelemen, 1990; Kelemen et al., 1997). The large volume of dunite at Site 1271 indicates abundant melt migration and reaction in this region. Because of low Mg# and Ni content in whole rock, Kelemen, Kikawa, Miller, et al. (2004) concluded that the Site 1271 dunites are the product of combined olivine crystallization and reaction between residual peridotite and migrating melts with relatively low Mg#. Gabbroic material associated with the large volume of dunite indicates that the melt activity was significant in this region relative to other sites drilled during Leg 209 near the 15°20´ Fracture Zone.
Cannat et al. (1997) reported coarse-grained gabbroic dikelets in harzburgite from Site 920 in the Kane Fracture Zone (MARK) area near the Mid-Atlantic Ridge. These dikelets were considered to be cumulates or trapped melts from magmas that had undergone variable degrees of crystal fractionation and differentiation before crystallizing in the ultramafic rocks. Most dikelets are <1 cm thick; rare dikelets are >5 cm thick. A 3.5-cm-thick gabbroic dike contains spinel with Mg# = 46.8, Cr# = 49.9, and TiO2 = 1.4 wt%, similar to spinel in olivine gabbro from Hole 1271B.
The association of olivine gabbro and troctolite with dunite also resembles those reported from Hess Deep Site 895 (Arai and Matsukage, 1996; Dick and Natland, 1996). Cannat et al. (1990) also reported similar lithologies from the Garret transform fault. The difference between Hess Deep Site 895 and Hole 1271B is that gabbroic material intruded into harzburgite in the former but into dunite in the latter. Hole 1271B dunite may have formed by dissolution of orthopyroxene in harzburgite by pervasive melt flow prior to the growth of hybrid rocks as a mixture of melt and dunite within a melt flow channel located in Unit III. The occurrence of similar lithologies in both fast-spreading and slow-spreading ridges may indicate that they followed a similar cooling path in the uppermost mantle.
The dunite associated with gabbroic material recovered in Hole 1271B is similar to the Mohorovicic (Moho) transition zone of ophiolite (Boudier and Nicolas, 1995; Koga et al., 2001). The Moho transition zone of the Oman ophiolite is dominantly composed of dunite with gabbros, pyroxenites, and chromitites showing evidence of intense magmatic activity such as diffuse melt impregnations and formation of dikes and sills (Boudier and Nicolas, 1995). Dunites in Hole 1271B are also inferred to have originally formed by extensive interaction with melt in the uppermost mantle. Melt pervasively percolated through the peridotite matrix in the early stage, and as temperature decreased the melt crystallized plagioclase and clinopyroxene. Finally, the evolved melt stagnated in the peridotite matrix and formed gabbroic materials (olivine gabbro, olivine gabbronorite, and troctolite).
The Cr# of spinel is commonly restricted to <60 in abyssal peridotites (Dick and Bullen, 1984; Arai, 1987). However, the Cr# of spinel in Hole 1271B dunite reaches a maximum value of 62.7 at the center of Unit III. The formation of such high-Cr# spinel may be due to preferential extraction of the MgAl2O4 component from spinel during dissolution by migrating melt. Moreover, dunite with high-Cr# spinel is in contact with hybrid rocks in the upper and lower horizons. We interpret the occurrence of refractory dunite and hybrid rocks in Unit III to indicate that a large melt channel is located in Unit III. In the center of the melt channel the dunite became more refractory due to partial dissolution of spinel, whereas in the wall rock the melt lost heat and crystallized plagioclase and clinopyroxene. The melt evolved by fractional crystallization was finally trapped in the dunite matrix, forming hybrid rocks. The width of Unit III may indicate the scale of the melt flow channel in the uppermost mantle. Because of possible tectonic rotation of peridotite and the lack of constraints on the relative position of drilling the hole, the true thickness of the melt channel is ambiguous. In any case, the interval of Unit III may indicate that the maximum width of melt channel was ~35 m.