Three Types of Olivine Crystal Size Distribution in Dunites from the Yoko-Dovyren Layered Massif as Signals of Their Different Crystallization History

Crystal size distributions (CSD) of olivine were obtained for 17 samples of plagiodunite and Pl‑bearing dunite from the central part of the Yoko-Dovyren massif, northern Baikal region, Russia. Three types of CSD were identified: loglinear, bimodal, and lognormal. Combining these data with the results of petrological reconstructions, which earlier revealed two main types of the Dovyren magmas (using the method of geochemical thermometry), we proposed a basic scenario of interaction between magmatic suspensions of different temperature to explain the diversity of the CSD. The intratelluric olivine transported by magmas of different temperature, which had not subjected to abrupt cooling or heating in the chamber, retained an original loglinear CSD. For some portions of the hottest magma (∼1290°C), it is assumed that the original olivine evolved into a bimodal CSD due to accelerated crystallization at faster cooling of the high-temperature injections contacting relatively cold crystal mush (∼1190°C). An interpretation of the lognormal CSD suggests that part of the olivine crystals composing the protocumulate systems efficiently interacted with the pore melt infiltrating upward during the compaction of the underlying crystal mush. This led to cycles of partial dissolution and regrowth of the olivine grains resulting in a final lognormal CSD. The infiltrating hot melt, which was undersaturated with immiscible sulfide liquid, could dissolve sulfides preexisting in the low-temperature mush. This produced dunites with lognormal CSD relatively depleted in sulfur and chalcophile elements. The lognormal CSD is considered to be a marker of crystal mush regions through which the focused infiltration of the pore melt proceeded.