Meter-Scale Chemical Interaction between Pyroxenite-Derived Melts and Mantle Peridotites in the Northern Apennine Ophiolites (Italy)

Mantle peridotites from the Northern Apennine ophiolites are characterized by the occurrence of cm-thick pyroxenite layers originated by rather deep segregation of MORB-type melts. Our recent work documents that during the pyroxenite emplacement portions of the country peridotite have been significantly modified in their modal, chemical and isotopic compositions by reaction with melts percolating from pyroxenite veins [1]. Here we report the results of detailed bulk and mineral major and trace element profiles carried out through pyroxenite-peridotite boundaries to investigate the physico-chemical parameters governing the melt-rock reaction process. Relative to the peridotites far (> 2 m) from the pyroxenite veins, wall-rock peridotites show i) modal orthopyroxene enrichment at the expense of olivine, ii) higher Al, Ca, Si contents and slightly lower Mg# iii) Al-richer spinel and lower-Mg# pyroxenes. Moreover, clinopyroxenes in the wall-rock peridotites have LREE-depleted patterns and initial %Nd(430 Ma) = +4.7-+7.6, pointing to a reaction with an enriched tholeiitic silica-saturated melt. Along the pyroxenite peridotite traverses, clinopyroxenes record a trace element gradient: at the pyroxenite-peridotite contact they have the lowest MREE-HREE abundances, with lower Sm/Nd ratios than the distal pyroxenite-free peridotites. The overall REE abundances progressively increase away from the pyroxenite peridotite boundary up to about 20 cm as a result of percolative reactive flow at decreasing melt mass. Beyond 20 cm from the contact, the HREE content decreases with distance from the pyroxenite, while the LREEs remain at nearly constant level, pointing to a more efficient chemical buffering of the host peridotite on the HREE composition of the percolating melt through ion exchange chromatographic type processes.