We report petrographic, major and trace element data for xenoliths from the andesitic Avacha volcano (Kamchatka), which host orthopyroxene (opx)-rich veins of mantle origin formed either by rapid crystallization of intruded melts or by their interaction with the host harzburgite. Studies of such veins may give better insights into sub-arc mantle processes (in particular on a millimetre to centimetre scale) than those of (1) arc xenoliths that do not preserve solidified initial metasomatizing agents, (2) massif peridotites, probably modified during their emplacement, or (3) arc magmatic rocks, which provide indirect information. We seek to trace the evolution of these agents as they react with the host peridotite and to assess their impact on the wall-rocks. The veins cross-cut spinel harzburgite and consist mainly of opx with minor olivine, clinopyroxene (cpx) and/or amphibole. We identify 'rapidly crystallized' veins that cut wall-rock olivine without petrographic evidence of reaction, and 'reactive' veins subdivided into 'thick' (0*5-1 mm) and 'thin' (