Melting relations of anhydrous olivine-free pyroxenite Px1 at 2 GPa

Borghini, Giulio; Fumagalli, Patrizia

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 (inline-formulaXMg=0.81, inline-formulaSiO2=52.9 wt %, inline-formulaAl2O3 = 11.3 wt %, CaO inline-formula= 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 inline-formulaC, at a temperature about 70 inline-formulaC 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 inline-formula+ clinopyroxene inline-formula= liquid inline-formula+ orthopyroxene, and above 1300 inline-formulaC, once garnet is completely consumed, by reaction clinopyroxene inline-formula+ orthopyroxene inline-formula= liquid. Orthopyroxene is the liquidus phase, and at 1480 inline-formulaC olivine-free pyroxenite Px1 is completely molten indicating a melting interval of about 200 inline-formulaC. Isobaric melt productivity is similar to garnet clinopyroxenites, and it is more than 3 times that of a fertile lherzolite at 1400 inline-formulaC. Px1 partial melts cover a wide range of inline-formulaXMg (0.57–0.84), with inline-formulaSiO2, inline-formulaAl2O3 and inline-formulaNa2O decreasing and inline-formulaCr2O3 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 inline-formulaXMg values. These melts differ from those derived from lherzolites at 2 GPa by higher inline-formulaSiO2 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.

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Borghini, Giulio / Fumagalli, Patrizia: Melting relations of anhydrous olivine-free pyroxenite Px1 at 2 GPa. 2020. Copernicus Publications.

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