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 degrees C to 695 degrees 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.