Petrochemical Study of Mantle Xenoliths in Alkali Basalts from South Korea: P/T Regime of the Upper Mantle

Abstract

Mantle xenoliths in alkali basalt at three locations in South Korea—Boun, the Gansung area, and Baegryung Island—are spinel lherzolites composed of olivine, orthopyroxene, clinopyroxene, and spinel. The xenoliths generally display triple junctions between grains, kink-banding in olivine and pyroxenes, and protogranular and equigranular textures, with no preferred crystal orientation. Anhedral brown spinels occur interstitially. Minerals in lherzolites from each of the three localities are compositionally homogeneous. Olivine compositions have Fo89.0 to Fo90.2, low CaO (.03 to 0.12 wt%), and NiO of 0.34 to 0.40 wt%; the orthopyroxene is enstatite with En89.0 to En90.0 Al2O3 of 4 to 5 wt%; the clinopyroxene is diopside with En47.2 to En49.1 and Al2O3 of 7.42 to 7.64 wt% from Boun and 4.70 to 4.91 wt% from Baegryung. Spinel chemistry shows a distinct negative trend, with increasing Al corresponding with decreasing Cr, and Mg# and Cr# of 75.1 to 81.9 and 8.5 to 12.6, respectively. Temperatures and pressures o...