Small-Scale Reactional Features in Abyssal Peridotites from the Mid-Atlantic Ridge at 17°04′ to 17°10′ N

Serpentinized peridotites (lherzolite to harzburgite) with relict coarse-grained protogranular and porphyroclastic matrix and locally developed fine-grained spinel-pyroxene microstructures were sampled from a previously unknown tectonic exposure of the Mid-Atlantic Ridge (17 degrees 04 '-17 degrees 10 ' N). The mineral composition of the coarse-grained relics is typical of abyssal residual peridotites and corresponds to 13-14% fractional melting. Fine-grained spinel-pyroxene (spinel-orthopyroxene and spinel-two-pyroxene) intergrowths are regarded as traces left by peridotite interaction with an interstitial melt during the transition to the lithospheric conductive cooling at temperature of 1100-1000 degrees C. The peridotite-melt interaction resulted in the partial orthopyroxene dissolution, local crystallization of spinel +/- clinopyroxene, and uneven decrease of Al and Cr contents in both pyroxenes and Cr/Al ratio in spinel. An additional signature of the reaction melt is an overall trend of enrichment in magmatic components: clinopyroxene in REE and spinel in Zn. The inferred interstitial reaction melt was significantly depleted in incompatible elements compared to MORB-type melts. Further lithospheric cooling favored "freezing" of mineral assemblages and small-scale reactional features.