Search

This paper reports lithium concentrations and isotopic compositions of olivines in the oceanized subcontinental lithospheric mantle (SCLM) peridotites of the Tibetan Yunzhug ophiolite. The results show systematic Li isotope changes with distance from the rim of olivine grains. δ7Li values of olivine in dunites decrease from + 10.46 to + 1.33‰ with increasing distance to olivine rim from 26.15 to 124.71 μm. A negative correlation of δ7Li and Li content in olivine from dunite and harzburgite indicates recent diffusive ingress of Li into the peridotites. The extremely heavy Li isotopic composition requires the seawater or seawater alteration endmember in the mixing model, and reveals Li diffusion from seawater into olivine. As in dunites, olivines in a harzburgite sample show similar variations in δ7Li as a function of distance from the grain rim (e.g., 6.01 to 1.73 in sample 14YZ13). We suggest that the behavior of Li in the oceanized SCLM peridotites may be controlled by Li diffusion from seawater, as Li activity in the liquid state is higher than the solid state in transporting Li through the olivines in the peridotites. This study supports that seawater Li diffusion is one of the important factors for the heterogeneity of mantle Li isotopes in ophiolites. [ABSTRACT FROM AUTHOR]

• Magnetite-ilmenite pairs occur in Holocene Tengchong volcanics from SE Tibet. • Post-collisional Tengchong magmas are rather reducing (△FMQ = −1.8 to −0.7). • Reduced Tengchong magmas may exemplify other post-collisional magmas on Tibet. • High Fe-Ti oxide and Ti-in-zircon temperatures indicate warm storage of magmas. Rapidly cooled and well-preserved magnetite and ilmenite in young volcanic rocks provide good samples for constraining the magmatic redox conditions. We report chemical compositions of magnetite and ilmenite from the youngest Tengchong volcanoes at the southeastern margin of the Tibetan Plateau, to constrain the oxygen fugacity and temperatures for the Tengchong magmas. Oxygen fugacity values (△FMQ) relative to the fayalite-magnetite-quartz (FMQ) buffer are −1.8 ∼ −0.7 (averaging −1.1 ± 0.4) for Dayingshan; −1.3 ∼ −0.7（averaging −1.1 ± 0.3） for Maanshan; and −1.3 for Heikongshan. The temperatures for magnetite-ilmenite pairs of Tengchong lavas range from 789 to 1090 °C. The relatively reducing oxygen fugacity values (△FMQ = −1.8 ∼ −0.7) of the Tengchong magmas are significantly lower than those oxidizing arc magmas (△FMQ = +0.5 ∼ +3.5). The reduced magmas at Tengchong may exemplify other Quaternary post-collisional lavas of the Tibetan Plateau. The crystallization temperatures from mineral-liquid equilibrium are 1160–1192 °C for olivine, 1049–1110 °C for plagioclase, 978–1122 °C for clinopyroxene, 789–1090 °C for Fe-Ti oxides, 743–792 °C for zircon. The high Fe-Ti oxide temperature and Ti-in-zircon temperature indicate warm magmatic conditions (>750 °C) prior to eruptions of Tengchong magmas. [ABSTRACT FROM AUTHOR]