Peridotite-basalt association at MAR between 19°42′ and 19°59′ N: Evaluation of petrogenetic conditions and material balance during hydrothermal transformation of the oceanic crust.

Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19°42′-19°59′ N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the Sr/Sr-Nd/Nd diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the crest zone of the ridge expose blocks of deep-sitting rocks that were not simultaneously brought to the surface. The MAR axial segment between 19°42′ and 19°59′ N can be viewed as prospective for searches for hydrothermal ore mineralization. Compositional parameters of basalt in our rock collection correspond to those of the most depleted MORB varieties in the Atlantic. [ABSTRACT FROM AUTHOR]