构造环境对幔源岩中稀有气体同位素比值的制约

稀有气体是地球化学研究中有效的物源示踪剂。在广泛收集资料的基础上,通过对He、Ne、Ar同位素数据的统计分析,总结、对比了不同构造环境幔源物质中轻稀有气体同位素的组成特征。统计表明,大洋中脊玄武岩(MORB)中3He/4He=7 0～10 0Ra者约占统计样品的90%;洋岛幔源物质(OIB)的3He/4He值变化范围大,但>10 0Ra者占65%;消减带幔源岩的3He/4He值总体略低于MORB;大陆幔源岩的3He/4He值主要变化于6 0～10 0Ra间,但环太平洋大陆边缘低于MORB者普遍;大陆热点地幔流体具有类似OIB的3He/4He值分布特征。MORB与OIB的40Ar/36Ar值分布范围类似(主要范围为296～12000),但低于1000者在MORB中仅24 5%,而在OIB中约43 4%;板块俯冲区幔源岩的40Ar/36Ar值总体较低,<1000者约占67%;大陆各类地幔流体的40Ar/36Ar值普遍偏低,主要在4000以下。大陆、大洋幔源岩中20Ne/22Ne值主要介于9 8～12 5间,但<9 8者在各大陆地幔普遍存在。幔源物质的上述组成特征与大气、地壳等源区对其影响有关。我国东部幔源物质稀有气体同位素组成具有大气 地幔混合源区特征。

CONSTRAINTS OF TECTONIC SETTING ON NOBLE GAS ISOTOPE RATIOS IN MANTLE-SOURCE ROCKS

Based on about 1440 isotope data of the light noble gases (He, Ne and Ar) from mantle-source rocks in the world, some isotopic characters of different tectonic settings were recognized. And the noble gas characters of these tectonic settings, such as the Middle Ocean Ridge, Ocean Island, etc, which were derived from the single or mixed sources, are different. The statistics data show: the ratios of~3He/~4He in 90% of the samples of MORB are equal to 7.0～10.0 Ra; the ratios vary over a wider range in OIB but 65% of the samples have higher ratios than MORB; the ratios in basalts of the trench-arc system are lower than those of MORB; ~3He/~4He ratios in continental mantle-source rocks are within the range of 6.0～10.0 Ra; the ratios in the same samples from continental hot-spots are the same as those of OIB. The ratios of ~(40)Ar/~(36)Ar in OIB are similar to those in MORB (296-12000), but there are some differences between them; the ratios in basalts of the trench-arc system are lower than those of MORB; generally, for the continental mantle-source rocks, the ratios are lower than 4000. The ratios of ~(20)Ne/~(22)Ne are usually 9.8～12.5 for the continental and oceanic mantle-source rocks. And the data have been published and afforded in this paper, suggesting that eastern China belongs to the tectonic framework of continental margin, and is as the same as the areas of Western U.S.A. and Andes or Trench-Arc, which reflects a mantle with the composition of ancient atmosphere. Finally, we should pay more attention to the similarities and differences between continental and oceanic mantle-source noble gases.