Geothermal gradient of the upper mantle beneath Jeju Island, Korea: Evidence from mantle xenoliths

Ultramafic xenoliths found in alkali basalts from Jeju Island, Korea are mostly spinel lherzolites accompanied by subordinate amount of spinel harzburgites and pyroxenites. The combination of results from a two-pyroxene geothermometer and Ca-in-olivine geobarometer yields temperature-pressure (T-P) estimates for spinel peridotites that fall in experimentally determined spinel lherzolite field in CaO-Fe-MgO-Al2O3-SiO2-Cr2O3 (CFMASCr) system. These T-P data sets have been used to construct the Quaternary Jeju Island geotherm, which defines a locus from about 13 kbar at 880 degreesC to 26 kbar at 1040 degreesC. The geothermal gradient of Jeju Island is greater than that of the conventional conductive models, and may be as a result of a thermal perturbation by the heat input into the lithospheric mantle via the passage and emplacement of magma. Spinel-lherzolite is the main constituent rock-type of the lithospheric mantle beneath Jeju Island. Pyroxenites may be intercalated in peridotites at similar depth and temperature as re-equilibrated veins or lenses.