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

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 concentrat ions (2.3 x 10(-9)-2.5 x 10(-7) cm(3) STP g(-1)) and large variations of He -3/He-4 ratios (1.6 x 10(-7)-15.8 x 10(-6) or R/Ra = 0.1-11). He-3/He-4, Sr -87/Sr-86 and Nd-143/Nd-144 ratios in xenoliths from Kuandian Holocene volc anic provinces are consistent with those observed in mid-oceanic ridge basa lt (MORB), indicating that the source of He-Sr-Nd in the lithospheric mantl e is chemically and isotopically identical to that of MORE reservoir. Howev er, xenoliths in Huinan Pleisteocene-Holocene volcanic province are charact erized by slightly low He-3/He-4 ratios (R/Ra = 4.3-6.5) and more radiogeni c Sr and Nd. The age-corrected He-3/He-4 ratios suggest that their time-int egrated ratio of (U + Th)/He-3 is slightly elevated compared to the deplete d upper mantle. The Hannuoba xenoliths in the Miocene volcanic province sho w much lower and wider variable He-3/He-4 ratios, which is most likely caus ed by in situ radiogenic He-4 accumulation since it erupted on to the surfa ce. The He-Sr-Nd results suggest chemical and isotopical heterogeneity in t he subcontinental mantle beneath NE China. (C) 1998 Elsevier Science Ltd. A ll rights reserved.