Fe-Ni-rich Silicate Aggregates Formed after Sulfides in High-pressure Serpentinites

High-pressure serpentinites from Alpine Corsica, the Piedmont Zone (Western Alps), the Tso Morari dome (Himalaya), the Dominican Republic and Cuba (Greater Antilles) contain reddish (Ni,Fe)-rich silicate aggregates, a few hundred micrometers in size, which have not been reported so far from lower-pressure serpentinites, nor found there through systematic screening. The high atomic (Ni + Fe)/Si ratio (5 +/- 4) of these aggregates, as well as their occurrence as coronas around (Ni,Fe)-sulfides, indicates their formation at the expense of sulfides and serpentine. The high (Ni + Fe)/Si ratio may suggest the presence of an Fe or Ni analogue of the rare serpentine-related minerals balangeroite or cronstedtite. However, compositional variations within and between aggregates, synchrotron X-ray diffraction data acquired on single aggregates, and transmission electron microscopy reveal that these aggregates are not a single phase but rather a mixture at the hundreds of nanometer scale of oxyhydroxides, mainly goethite (FeOOH) + (Ni,Fe)-rich serpentine +/- chlorite +/- reevesite [Ni6Fe2(OH)(16)(CO3)center dot 4H(2)O] +/- magnetite. In addition to microtexture and mineralogy, strong chemical variations at the border of the aggregates and along micro-veins suggest formation through a low-temperature alteration process. However, the occurrence of such pseudomorphic aggregates only in high-pressure serpentinites suggests that they were formed at the expense of a high-pressure silicate precursor of high M2+/Si ratio, still to be identified but which could be a main nickel carrier at depth in subduction zones.