河北汉诺坝新生代玄武岩中幔源包体和高压巨晶的氦同位素地球化学研究

Helium Isotope Studies of the Mantle Xenoliths and Megacrysts from the Cenozoic Basalts in Hannuoba, Hebei Province, China

对该地区新生代玄武岩中幔源包体和高压巨晶的氦同位素进行了初步研究。幔源包体中橄榄石的3 He 4 He值为 (0 .15～ 7.4)× 10 - 6,较MORB值明显偏低 ,甚至低于大气的值 ,说明该地区曾发生过强烈的地幔交代作用。高压巨晶辉石和石榴子石的氦同位素组成与此明显不同 ,其3 He 4 He值为 (5 .7～ 2 4.3)× 10 - 6。提出幔源包体和高压巨晶不是同源的 ,二者可能与寄主玄武岩均无必然成因联系。在汉诺坝地区一件石榴子石巨晶中还发现了异常高的3 He 4 He值。

Helium isotope compositions of the mantle-derived xenoliths and megacrysts from the Cenozoic basalts in Hannuoba were measured. The 3He/ 4He ratios of olivine from xenoliths in Hannuoba vary from 0.15×10 -6 to 7.4×10 -6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios,implying that the continental mantle is strongly replaced in Hannuoba areas. The 3He/ 4He ratios of pyroxenes and garnets megacrysts vary from 5.7×10 -6 to 24.3×10 -6, and are obviously different from those of the mantle xenoliths. It is inferred that the mantle xenoliths and megacrysts are from different parts of the continental mantle. No necessary relationships in orgin exist between the mantle xenoliths, megacrysts and their host basalts. An extremely high 3He/ 4He ratio of garnet megacryst from Hannuoba was found.