Breakdown mechanisms of titanclinohumite in antigorite serpentinite (Cerro del Almirez massif, S. Spain): A petrological and TEM study

Veins with titanclinohumite (Ti-Chu) and olivine occur close to clinopyroxenite layers scattered through the antigorite serpentinites from the Cerro del Almirez ultramafic massif (Sierra Nevada, Betic Cordillera, S. Spain). They formed at estimated maximum P-T conditions of 1.3 GPa and 475 °C, due to simultaneous breakdown of former Ti-rich clinopyroxene and brucite in the presence of 6 vol.% of free water. Breakdown of Ti-Chu single crystals to an olivine + ilmenite framework was gradual: small relict patches of Ti-Chu appear even at some distance from the reaction front and ilmenite grains coarsen and become less frequent away from the front. Detailed TEM data support this interpretation: olivine in equilibrium with Ti-Chu is structurally and chemically homogeneous, whereas Ti-Chu shows ubiquitous intermixed (001) olivine lamellae. Product olivine close to the front shows pervasive and continuous (001) Ti-Chu lamellar faults. These become less frequent away from the reaction front, due to the completion of the Ti-Chu breakdown. This textural sequence suggests both progressive transfer of Ti to ilmenite and progressive release of H2O during the breakdown of Ti-Chu. A second hydration breakdown reaction can be observed very locally, in the contact between Ti-Chu and diopside, producing a symplectitic intergrowth of tremolite and ilmenite. Textural features of both reactions point to a very limited availability of water during Ti-Chu breakdown. This suggests that veins operated as closed systems. Complete obliteration of Ti-Chu owing to the antigorite dehydration breakdown can be observed in the Almirez massif rocks. This precludes OH-Ti-Chu from Atg-serpentinite as an efficient water or titanium carrier into deep mantle in hot subduction zones.