Non-silicate inclusions in garnet from an ultra-deep orogenic peridotite

Abstract

Non-silicate solid inclusions in garnet from ultra-deep garnet peridotites, Otrøy, Western Gneiss Region, Norway, have been studied using light-optical, scanning and analytical electron microscopic techniques. Texturally, the investigated garnets reveal protogranular, porphyroclastic and equigranular microstructures. Protogranular and porphyroclastic garnets contain microstructural evidence of the former existence of ‘super-titanic’ garnet. The microstructural evidence consists of exsolution textures involving rutile and ilmenite needles //<111>Grt as well as interstitial rutile grains. This exsolution microstructure is similar to the relict majoritic garnet microstructures found in the same peridotite. Some garnets contain both pyroxene and rutile exsolution. Other non-silicate mineral inclusions in protogranular and porphyroclastic garnet consist of nickel-iron alloys (Ni99Fe01) and Cr-rich spinel. In addition, some protogranular, porphyroclastic and equigranular garnets contain composite Ni-Fe-Cu sulphide inclusions. The latter represent immiscible sulphide melt trapped within cracks that have healed. The original melting temperature of Ni-Fe-Cu sulphides was determined as being ≥ 1000°C and contrasts with temperatures derived from garnet–olivine–pyroxene mineral compositions using conventional geothermobarometry (c. 800°C/3 GPa). This contradiction is explained by the decoupling of microstructures and mineral chemistry; the microstructures were formed at higher temperatures than indicated by the current mineral chemistry. The decoupling of microstructures and current mineral chemistry has important applications for geodynamic models.