He, Sr and Nd isotopes of mantle-derived xenoliths in volcanic rocks of NE China

Abstract

Abundances and isotopic ratios of He and isotopic ratios of Sr and Nd have been measured for 18 mantle-derived xenoliths from three Cenozoic volcanic provinces in NE China. The xenoliths are characterized by low He concentrations (2.3×10 −9–2.5×10 −7 cm 3 STP g −1) and large variations of 3He/ 4He ratios (1.6×10 −7–15.8×10 −6 or R/Ra=0.1–11). 3He/ 4He, 87Sr/ 86Sr and 143Nd/ 144Nd ratios in xenoliths from Kuandian Holocene volcanic provinces are consistent with those observed in mid-oceanic ridge basalt (MORB), indicating that the source of He–Sr–Nd in the lithospheric mantle is chemically and isotopically identical to that of MORB reservoir. However, xenoliths in Huinan Pleisteocene–Holocene volcanic province are characterized by slightly low 3He/ 4He ratios (R/Ra=4.3–6.5) and more radiogenic Sr and Nd. The age-corrected 3He/ 4He ratios suggest that their time-integrated ratio of (U+Th)/ 3He is slightly elevated compared to the depleted upper mantle. The Hannuoba xenoliths in the Miocene volcanic province show much lower and wider variable 3He/ 4He ratios, which is most likely caused by in situ radiogenic 4He accumulation since it erupted on to the surface. The He–Sr–Nd results suggest chemical and isotopical heterogeneity in the subcontinental mantle beneath NE China.