Nanoscale Structure of Zoned Laurites from the Ojen Ultramafic Massif, Southern Spain

We report the first results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of zoned laurite (RuS2)-erlichmanite (OS2) in mantle-hosted chromitites. These platinum-group minerals form isolated inclusions (<50 mu m across) within larger crystals of unaltered chromite form the Ojen ultramafic massif (southern Spain). High-magnification electron microscopy (HMEM), high angle-annular dark field (HAADF) and precession electron diffraction (PED) data revealed that microscale normal zoning in laurite consisting of Os-poor core and Os-rich rims observed by conventional micro-analytical techniques like field emission scanning electron microscope and electron microprobe analysis (FE-SEM and EPMA) exist at the nanoscale approach in single laurite crystals. At the nanoscale, Os poor cores consist of relatively homogenous pure laurite (RuS2) lacking defects in the crystal lattice, whereas the Os-richer rim consists of homogenous laurite matrix hosting fringes (10-20 nm thickness) of almost pure erlichmanite (OsS2). Core-to-rim microscale zoning in laurite reflects a nonequilibrium during laurite crystal growth, which hampered the intra-crystalline diffusion of Os. The origin of zoning in laurite is related to the formation of the chromitites in the Earth's upper mantle but fast cooling of the chromite-laurite magmatic system associated to fast exhumation of the rocks would prevent the effective dissolution of Os in the laurite even at high temperatures (1200 degrees C), allowing the formation/preservation of nanoscale domains of erlichmanite in laurite. Our observation highlights for the first time the importance of nanoscale studies for a better understanding of the genesis of platinum-group minerals in magmatic ore-forming systems.