Mineralogical and Spectroscopic Characteristics of Olivine in Mantle- Derived Xenoliths from Hebi, Henan Province, China

This study focuses on olivine in spinel harzburgite xenoliths from the Shangyu volcanic cone in Hebi, Henan Province, China. Using polarizing microscopy, infrared spectroscopy, laser Raman spectroscopy, and electron probe microanalysis, the spectroscopic, inclusion, and compositional characteristics of the olivine samples from Hebi were systematically investigated. The results show that the olivine samples are light yellowish-green to yellowish-green in colour, with a vitreous luster, and are transparent to translucent. The olivine exhibits kink band structures and is identified as forsterite, coexisting with enstatite, diopside, and Cr-spinel. The major element compositions of the olivine samples are relatively homogeneous, with MgO contents ranging from 49.80% to 51.08%, SiO₂ from 41.01% to 41.97%, and FeO from 7.14% to 8.07%. The high Mg^# values (91.76-92.70) indicate that the lithospheric mantle beneath the Hebi area is predominantly refractory with minor transitional characteristics, with olivine-spinel equilibrium temperatures ranging from 1 110 ℃ to 1 122 ℃. Comparison with olivines from Jilin and Hebei provinces, the olivine from Hebi shows the highest Mg^# values, suggesting its formation in a more ancient and refractory mantle environment. The Mg^# values of Hebi olivine fall well within the compositional range of high-quality gem-grade olivine, indicating its potential as a gem material. However, the primary reasons it has not reached gem-grade are high-temperature plastic deformation within the deep mantle and subsequent tectonic compression during rapid ascent, which have resulted in crystal fragmentation, extensive fissure development, and reduced transparency.