Abstract
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.
Highlights
Several lines of evidence indicate that the upper mantle contains a significant fraction of mafic lithologies (e.g., Hofmann, 2007, and references therein), likely included to depth via subduction, which are expected to form eclogites at pressures above 2.0 GPa (Ringwood and Green, 1966; Allègre and Turcotte, 1986; Yasuda et al, 1994; Hirschmann and Stolper, 1996; Kogiso et al, 2004a)
There is a growing consensus that some mid-ocean ridge basalts (MORBs) derive from a heterogeneous mantle source including peridotite mixed to olivine-poor pyroxenites (Lambart et al, 2012, 2013, 2016; Borghini et al, 2017; Lambart, 2017; Brunelli et al, 2018)
We present the results of partial melting experiments performed on the secondary-type SE pyroxenite Px1 previously studied by Sobolev et al (2007)
Summary
Several lines of evidence indicate that the upper mantle contains a significant fraction of mafic lithologies (e.g., Hofmann, 2007, and references therein), likely included to depth via subduction, which are expected to form eclogites at pressures above 2.0 GPa (Ringwood and Green, 1966; Allègre and Turcotte, 1986; Yasuda et al, 1994; Hirschmann and Stolper, 1996; Kogiso et al, 2004a). Crustal-derived mafic rocks represent low-solidus mantle lithologies and start melting at higher pressure than “dry” peridotites
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