Peridotite-melt interaction under transitional conditions between the spinel and plagioclase facies beneath the Mid-Atlantic Ridge: Insight from peridotites at 13°N

Abstract

Peridotites (diopside-bearing harzburgites) found at 13°N of the Mid-Atlantic Ridge fall into two compositional groups. Peridotites P1 are plagioclase-free rocks with minerals of uniform composition and Capyroxene strongly depleted in highly incompatible elements. Peridotites P2 bear evidence of interaction with basic melt: mafic veinlets; wide variations in mineral composition; enrichment of minerals in highly incompatible elements (Na, Zr, and LREE); enrichment of minerals in moderately incompatible elements (Ti, Y, and HREE) from P1 level to abundances 4–10 times higher toward the contacts with mafic aggregates; and exotic mineral assemblages Cr-spinel + rutile and Cr-spinel + ilmenite in peridotite and pentlandite + rutile in mafic veinlets. Anomalous incompatible-element enrichment of minerals from peridotites P2 occurred at the spinel-plagioclase facies boundary, which corresponds to a pressure of about 0.8–0.9 GPa. The temperature and oxygen fugacity were estimated from spinel-orthopyroxene-olivine equilibria. Peridotites P1 with an uniform mineral composition record the temperature of the last complete recrystallization at 940–1050°C and FMQ buffer oxygen fugacity within the calculation error. In peridotites P2, local assemblages have different compositions of coexisting minerals, which reflects repeated partial recrystallization during heating to magmatic temperatures (above 1200°C) and subsequent reequilibration at temperatures decreasing to 910°C and an oxygen fugacity significantly higher than FMQ buffer (Δlog \( f_{O_2 } \) = 1.3–1.9). Mafic veins are considered to be a crystallization product from basic melt enriched in Mg and Ni via interaction with peridotite. The geochemical type of the melt reconstructed by the equilibrium with Ca-pyroxene is defined as T-MORB: (La/Sm)N ≈ 1.6 and (Ce/Yb)N ≈ 2.3, which is well consistent with compositional variations of modern basaltic lavas in this segment of Mid-Atlantic Ridge, including new data on quenched basaltic glasses.