Link between melt-impregnation and metamorphism of Atlantis Massif peridotite (IODP Expedition 357)

Abstract IODP Expedition 357 drilled 17 shallow sites scattered over ~ 10 km in the west to east spreading direction across Atlantis Massif oceanic core complex (OCC, MAR, 30 ºN). Mantle exposed footwall of Atlantic OCC is nearly wholly serpentinized (80–100%) harzburgite and subordinate dunite. A recent whole-rock chemistry study by Whattam et al. (Chemical Geology 594. 10.1016/j.chemgeo.2021.120681, 2022) subdivides peridotites into: Type I fluid–rock dominated serpentinite, which exhibits almost nil evidence melt-impregnation or silica metasomatism; II melt–rock dominated, mafic melt-impregnated serpentinite; III Si-metasomatized serpentinite. In this study, on basis EPMA, three kinds Cr–spinel are distinguished serpentinite: (I) primary, unmetamorphosed mantle array, (II) low-Ti metamorphosed, (III) high-Ti melt reacted. All western site serpentinite ( n = 34) comprises 68% all Cr–spinel. Metamorphosed 100) most abundant occur central eastern whereas melt-reacted chromite restricted predominantly Estimates degree extraction using F 10ln(spinel Cr#) + 24 9–17%. Fugacity calculations yield Δlog( f O 2 ) FMQ − 1.7 1.0 olivine–spinel Mg–Fe exchange thermometry a mean closure temperature 808 ± 39 °C. Mafic resulted with anomalously high TiO 0.27–0.68 wt.% production Ti-rich (up 1.23 ). Greenschist facies metamorphism (< 500 °C) 2+ between forsterite low Mg#; higher degrees amphibolite (~ 500–700 also Cr# due Al–Cr exchange. As has been previously established, significant Al loss from cores above 550 °C result equilibration fluids equilibrium chlorite, may be valid for our samples. On vs. compositions, clear relationship exists we postulate heat associated magma injection subsequent localized contact metamorphism. To knowledge, such relation peridotite (of peridotite) not established general specifically peridotite. Closure temperatures 440–731 metamorphosed approximate greenschist metamorphic conditions.