Exsolutions of Diopside and Magnetite in Olivine from Mantle Dunite, Luobusa Ophiolite, Tibet, China

Abstract: The exsolutions of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine (>95%) + chromite (1%‐4%) + diopside (<1%). Two types of olivine are found in thin sections: one (Fo=94) is coarse‐grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other (Fo=96) is fine‐grained and poly‐angled. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron‐size inclusions are also discovered. Analyzed through energy dispersive system (EDS) with unitary analytical method, the average compositions of the inclusions are: Na2O, 3.12%‐3.84%; MgO, 19.51%‐23.79%; Al2O3, 9.33%‐11.31%; SiO2, 44.89%‐46.29%; CaO, 11.46%‐12.90%; Cr2O3, 0.74%‐2.29%; FeO, 4.26%‐5.27%, which is quite similar to those of amphibole. Diopside is anhedral filling between olivines, or as micro‐inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and Al and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAl2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820°C‐960°C for olivine and nodular chromite, 630°C‐770°C for olivine and euhedral chromite, and 350°C‐550°C for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na+, Al3+, Ca2+, Fe3+, Cr3+. With temperature decreasing, the olivine solid solution decomposed; and Fe3+, Cr3+ diffused into magnetite and Ca2+ and Na+ into clinopyroxene, both of which formed intergrowth textures. A few Fe3+ and Cr3+ entered interstitial chromite. Through later tectonism, the peridotite recrystallized and formed deformational coarse grained olivine, fine grained and poly‐angled olivine, and euhedral grained chromite. Due to the fast cooling rate of the rock or rapid tectonic emplacement, the exsolution textures in olivine and compositional zones of chromite are preserved.