Geochemistry and Melt Inclusion study of Peridotite Xenoliths from Sinsanri, Jeju, South Korea

The Sinsanri region of Jeju Island, South Korea, is renowned for its abundant xenoliths, making it one of the prime locations for their occurrence. Jeju Island itself is a Cenozoic intraplate volcano situated in East Asia. A variety of xenolith types such as Spinel Lherzolite, Spinel Harzburgite to Wehrlite are entrapped in Quaternary intraplate alkali basalts on Sinsanri, Jeju Island, South Korea, during the first and second eruption episodes of the Quaternary Volcanic Activity.  Here we report the major element composition of the xenoliths, including the first Raman spectroscopy analysis of the melt inclusion inside olivine to constrain the post-entrapment depth and the latest recrystallizing depth of the melt inclusion itself.  The prominent xenoliths are spinel lherzolite, showing transitions between proto-granular textures and porphyroclastic textures. Mg# ranges from 73.4 to 93.8 in the Sinsanri xenolith’s samples. The spinel lherzolite xenoliths originated at pressure of 15kb with an equilibrium temperature ranging from 922°C.  The size of the CO₂ rich melt inclusion entrapped inside the mantle peridotite xenoliths ranges from 20 to 40 µm.  The densities of CO₂ melt inclusion were measured to be between 302.8516 kg/m³ and 972.4025 kg/m³.  Most of the melt inclusions inside the xenoliths may have formed while they are ascending to the surface together with the host basalt.  The post-entrapment depth is calculated using the equilibrium temperature of the olivine mineral, which caused the most melt inclusion.  It can be estimated that the post-entrapment depth of the CO₂ melt inclusion of the Sinsanri is 86 MPa. The Sinsanri xenoliths are predominantly formed at relatively lower temperatures and pressures.

Acknowledgement 
This research was funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant numbers 2019R1I1A2A01057002 and 2019R1A6A1A03033167); and Korea Ministry of Environment as “The SS (Surface Soil conservation and management) projects; 2019002820004”.