Petrology of titanian clinohumite and olivine at the high-pressure breakdown of antigorite serpentinite to chlorite harzburgite (Almirez Massif, S. Spain)

Abstract

Rocks of the Cerro del Almirez ultramafic massif (Sierra Nevada, Betic Cordillera, S. Spain) record the high-pressure dehydration of antigorite–olivine serpentinite to form chlorite harzburgite (ol + opx + chl). In the field, these two rock types are separated by a well-defined isograd. Titanian clinohumite (TiCl) and olivine show textural and compositional differences depending on the rock type. OH–TiCl occurs in the serpentinite as disseminated grains and in veins. F–OH–TiCl is observed exclusively in the chlorite harzburgite, where it occurs as porphyroblastic grains and within prograde olivine as irregular and lamellar, planar intergrowths at microscopic and submicroscopic scales. Petrological evidence of partial to complete breakdown of TiCl to olivine + ilmenite is preserved in both rock types. Chlorite harzburgite is characterized by a brown pleochroic olivine with abundantally oriented microscopic to submicroscopic oxide particles. The mean Ti-content of the brown olivine is 144 ppm. The brown olivine preserves TiCl lamellae that sometimes grade into ghost lamellae outlined by the oxide trails. This observation suggests that some of the oxide inclusions in the brown olivine are derived from the breakdown of TiCl intergrowths. Thermodynamic modelling of selected Almirez bulk rock compositions indicates a temperature increase from 635°C to 695°C, at pressures ranging from 1.7 GPa to 2.0 GPa, as the cause for the compositional adjustment of TiCl between the Almirez antigorite serpentinite and chlorite harzburgite. These P–T estimates are in good agreement with the sequence of phase relations observed in the field. The computed phase diagrams in conjunction with the geothermal conditions envisaged for different subduction settings indicate that TiCl is stable in the vicinity of the antigorite serpentinite/chlorite harzburgite phase boundary in some subduction settings. In these circumstances, clinohumite–olivine intergrowths in chlorite harzburgite may act as a sink for high field strength elements, and probably other elements, that are present in the mantle–wedge fluids.