Helium and argon isotopes in spinel lherzolite from Damaping, northern Zhangjiakou

Spinel lherzolite found in Damaping, northern Zhangjiakou, Hebei Province occurs as xenoliths in the Hannuoba basalts that consist of alkali basalt and tholeiite. Spinel lherzolites contain 50%-70% olivine (Fo: 90%), 10%-20% clinopyroxene (predominantly Di), 10%-30% orthopyroxene (predominantly En), and less than 5% spinel.^sup 3^He/^sup 4^He and^sup 40^Ar/^sup 38^Ar ratios in the olivine are 7.56×10^sup -7^ and 299.1, respectively.^sup 3^He/^sup 4^He and^sup 40^Ar/^sup 38^Ar ratios in the orthopyroxene (enstatite) are 9.1×10^sup -7^ and 307, respectively. Olivine grains are fractured irregularly, and pyroxene grains characterized by well developed cleavages, which would have resulted from explosion during the rapid eruption of lava from the deep interior to the surface. The lower isotope ratios of helium and argon may indicate that the spinel lherzolite xenoliths were derived from the strongly degassed and depleted upper mantle, and that the mantle is inhomogeneous.^sup 3^He losses to some extent might affect the helium isotope ratios.[PUBLICATION ABSTRACT]