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

Abstract

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°04′–17°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°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.