Abstract. The reaction between melt derived by mafic heterogeneities and peridotites
in an upwelling mantle may form hybrid olivine-free pyroxenites. In order to
evaluate the impact of these lithologies on the chemistry of primitive
magmas and their ability to give rise to new mantle heterogeneities, we
experimentally investigate the melting relations at 2 GPa of the model
olivine-free pyroxenite Px1 (XMg=0.81, SiO2=52.9 wt %,
Al2O3 = 11.3 wt %, CaO = 7.6 wt %). The subsolidus
assemblage consists of clinopyroxene, orthopyroxene, and garnet. At 2 GPa,
the solidus of Px1 is located between 1250 and 1280 ∘C, at a
temperature about 70 ∘C lower than the solidus of
fertile lherzolite. At increasing melt fraction, the sequence of mineral
phase disappearance is garnet–clinopyroxene–orthopyroxene. Across the
solidus, partial melting of Px1 is controlled by reaction garnet +
clinopyroxene = liquid + orthopyroxene, and above 1300 ∘C,
once garnet is completely consumed, by reaction clinopyroxene +
orthopyroxene = liquid. Orthopyroxene is the liquidus phase, and at
1480 ∘C olivine-free pyroxenite Px1 is completely molten
indicating a melting interval of about 200 ∘C. Isobaric melt
productivity is similar to garnet clinopyroxenites, and it is more than 3
times that of a fertile lherzolite at 1400 ∘C. Px1 partial melts
cover a wide range of XMg (0.57–0.84), with SiO2, Al2O3
and Na2O decreasing and Cr2O3 increasing with the degree of
melting. CaO content in partial melts increases as long as clinopyroxene is
involved in melting reactions and decreases after its exhaustion. At 2 GPa and for melting degrees higher than 10 %, Px1 produces MgO-rich basaltic
andesites matching the composition of eclogitic melts in terms of silica and
alkali contents but with significantly higher XMg values. These melts
differ from those derived from lherzolites at 2 GPa by higher SiO2 and
lower CaO contents. Their high silica activity makes them very reactive with
mantle peridotite producing hybrid orthopyroxene-rich lithologies and
residual websterites. Melt–rock reactions likely prevent direct extraction
of melts produced by olivine-free pyroxenites.