Orthopyroxene breakdown at lherzolite–melt contacts

The Cerro de Agrás volcanic cone (Cofrentes, Spain) is a ~2 Ma monogenetic effusive edifice, approximately 1 km wide and ~100 m high. It is dominated by pyroclastic deposits with subordinate meter-sized fragments of alkali basaltic lava, such as spatter flows, suggestive of a Strombolian eruptive style. The alkali basalts are aphanitic and display a porphyritic texture, with prevailing olivine as phenocrysts partially altered to iddingsite. The alkali basalts host small (0.5-4 cm) rounded-to-irregularly shaped ultramafic xenoliths of medium-grained spinel lherzolites with a protogranular texture, characterized by coarse olivine and orthopyroxene crystals (2–3 mm) and finer clinopyroxene and spinel grains (250–300 µm). Olivine shows homogeneously high Mg# [(Mg/Mg+Fe) = 0.90 to 0.94], whereas clinopyroxene diopside display slightly lower Mg# (0.91 to 0.92) and low Al (0.16-0.22 apfu) but noticeable Ca (0.85 to 0.95 apfu). Orthopyroxenes are enstatites with Mg# varying from 0.90 to 0.94. Spinels are Al- and Mg-rich, with Al# (Al/(Al+Cr)) ranging from 0.77 to 0.79 and Mg# ranging from 0.69-0.77. Thermobarometric calculation using the mineral compositions suggests temperatures between 1100 to 1150 °C and pressures ranging 15 to 18 kbar; very likely related with partial melting at ca. 50 km depth. Typically, the rims of the xenoliths, exhibit spongy textures where orthopyroxene is partially replaced by olivine + clinopyroxene. Here, newly-formed olivine grains yield lower Mg# 0.77-0.88 wheras clinopyroxene is augite with lower Ca (0.56 -0.83 apfu) and Mg# (0.95 to 1.00). These features seem to suggest the reaction of preexisting orthopyroxene with a non-equilibrium incoming host alkali basalt during xenolith ascent to surface.