Publication details

Breakdown of mantle minerals leading to crystallization of pure magnetite in ultrabasic rocks: An implication for mobility of iron during serpentinization



Year of publication 2019
Type Conference abstract
MU Faculty or unit

Faculty of Science

Attached files
Description The ultrabasic rocks of the Moldanubian Zone (Czech Republic) are represented by various lithological types, such as spinel serpentinites, garnet serpentinites, eclogites and pyroxenites. The individual ultrabasic varieties differ from each other by petrography, petrophysical properties, whole-rock chemistry, mineral composition and the prevailing metasomatic alterations affecting ultrabasic rocks themselves. This research was focused on the garnet and spinel serpentinites showing extremely high magnetic susceptibility (reaching up max. values 98 × 10-2 SI units) which is significantly influenced by the formation of pure magnetite (without TiO2) due to serpentinization of peridotites and also retrograde alterations. Both classified varieties mainly consist of serpentine (chrysotile-like composition), relics of the original olivine (MgO ~ 50 wt. %) and orthopyroxene (MgO 33–35 wt. %) along with garnet (Prp72-68 Alm19-14 Grs14-10) with a variable modal amount. As a result of retrograde transformations affecting the studied mantle rocks, olivine usually broke down to chrysotile, whereas orthopyroxene has been gradually replaced by amphibole of tsemarkite-like composition together with chrysotile. Porphyroblasts of garnet are commonly surrounded by kelyfitic rim formed by secondary mineral association: Ca-tsemarkite + Mg-clinochlore + magnetite. Preserved original mineral assemblages and their relationship with newly formed mineral phases observed in serpentinized peridotites demonstrate three different ways of crystallization of younger pure magnetite: (1) hydration of olivine (2) replacing of Cr-spinel and Fe-Ni sulfides (3) breakdown of garnet. The most significant process leading to the formation of magnetite represents serpentinization which caused releasing of Fe2+ from olivine and subsequent precipitation of exsolved Fe2+ as fine discrete magnetite particles filling intergranular spaces between main rock-forming minerals. On the other hand, replacing of Cr-spinels and breakdown of garnet to magnetite along with other silicates reflect the intensity of retrograde metamorphic processes.

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