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

Non-silicate solid inclusions in garnet from ultra-deep garnet peridotites, Otroy, Western Gneiss Region, Norway, have been studied using light-optica l, scanning and analytical electron microscopic techniques. Texturally, the investigated garnets reveal protogranular, porphyroclastic and equigranula r microstructures. Protogranular and porphyroclastic garnets contain micros tructural evidence of the former existence of 'super-titanic' garnet. The m icrostructural evidence consists of exsolution textures invoking rutile and ilmenite needles //(111)(Grt) as well as interstitial rutile grains. This exsolution microstructure is similar to the relict majoritic garnet microst ructures found in the same peridotite. Some garnets contain both pyroxene a nd rutile exsolution. Other non-silicate mineral inclusions in protogranular and porphyroclastic garnet consist of nickel-iron alloys (Ni99Fe01) and Cr-rich spinel. In addi tion, some protogranular. porphyroclastic and equigranular garnets contain composite Ni-Fe-Cu sulphide inclusions. The latter represent immiscible sul phide melt trapped within cracks that have healed. The original melting temperature of Ni Fe-Cu sulphides was determined as be ing greater than or equal to 1000 degreesC and contrasts with temperatures derived from garnet-olivine-pyroxene mineral compositions using conventiona l geothermobarometry (c. 800 degreesC/3 GPa). This contradiction is explain ed by the decoupling of microstructures and mineral chemistry; the microstr uctures were formed at higher temperatures than indicated by the current mi neral chemistry. The decoupling of microstructures and current mineral chem istry has important applications for geodynamic models. Copyright (C) 2000 John Wiley & Sons, Ltd.