Geochemical evolution of rodingites during subduction: insights from Cerro del Almirez (southern Spain)

Abstract

Metarodingites from Cerro del Almirez (Betic Cordillera, southern Spain) experienced multiple mineralogical and compositional changes during their evolution, from rodingitization on the seafloor to dehydration of the hosting subducted antigorite serpentinites at ~ 1.7 GPa and 650 °C. Basaltic and doleritic protoliths of Grandite-metarodingites lost alkaline metals (Na, K, Cs and Rb), Si, Ba, Pb and Sr, and gained Ca through intense seafloor rodingitization by highly reducing fluids reacting with serpentinites. Rare earth elements in whole-rocks, conversely, preserved their igneous abundances during rodingitization and partitioned into the new metamorphic assemblage dominated by granditic garnet, chlorite and diopside. At the ocean floor and during subduction, the strong gradients of chemical potentials of Mg, Ca and Al at the serpentinite-rodingite interface triggered the transformation of metarodingites to metasomatic chlorite-blackwalls and of host serpentinites to chlorite (± olivine)-diopside rims. Grandite-metarodingites interacted during subduction with fluids released by the brucite breakdown in serpentinites at ~ 1.5 GPa, leading to oxidation of iron, precipitation of new andraditic garnets with high MREE/HREE ratios, and loss of Re. Strong fluxing of fluids produced by the antigorite breakdown in serpentinites at ~ 1.7 GPa promoted the transformation of Grandite- to Epidote-metarodingites. This transformation caused the release of Ca and Mn into serpentinite-derived fluids and the incorporation of Si, Sr, Pb, Ba ± Eu into metarodingites, especially in epidote. Pyralspite-metarodingites formed at peak metamorphic conditions (~ 1.8 GPa and 680 °C) by, among other reactions, epidote breakdown that liberated Ca, Sr and Pb into fluids. Finally, all metarodingite types underwent variable degrees of retrograde amphibolitization induced by external fluids derived from metasedimentary rocks, causing Ca depletion and enrichment in Mg, Ba and alkaline elements (Na, K, Cs and Rb). These geochemical variations reflect the exchange of major and trace elements between different lithologies (serpentinites, metarodingites and metasedimentary rocks) in a subducting slab or slab-mantle interface. Metarodingites may influence the redox state and trace element signature of slab fluids, especially through iron oxidation, fractionation of Sr, Pb and Ba into epidote, and release of Ca and Re in fluids that may induce metasomatism in the sub-arc mantle. Recycling of residual metarodingites in the convective asthenosphere may produce Ca-rich reservoirs in the deep mantle.