应用大地热流和地下流体氦同位素组成资料计算中国大陆地壳生热元素丰度

Radioactive heat-producing element abundance in the crust of continental China estimated from terrestrial heat flow and the helium isotopic composition of underground fluids

根据能量守恒原理和中国大陆实测热流数据,给出中国大陆地壳生热率上限值为1.3μWm-3。根据热流值和地下流体氦同位素组成资料,估算出中国大陆地壳生热率为0.58～1.12μWm-3,中位数为0.85μWm-3,相应的铀、钍、钾丰度范围分别是0.83～1.76μg/g、3.16～6.69μg/g和1.0%～2.12%。中国陆壳铀、钍、钾元素整体丰度值明显高于太古宇地壳,反映中国陆壳成分演化程度较高。同时,中国大陆地壳成分具有明显的横向非均匀性特征:东部地壳相对西北部富集铀、钍、钾等强不相容元素,褶皱带相对克拉通地区富集铀、钍、钾元素。基于大陆地壳SiO2含量与地壳生热率之间的正相关关系推断,中国东部地壳较西部富长英质组分,褶皱带地壳成分较克拉通富长英质组分。此区域性变化特征与基于地震波速资料推断的结果相符。基于中国大陆地壳生热率变化范围以及地震波速低于全球平均值的特征,推断Rudnick和Fountain(1995)、Rudnick和Gao(2003)、Weaver和Tarney(1984)、Shaw等(1986)以及Wedepohl(1995)的全球陆壳成分模型均高估了铀、钍、钾等强不相容元素丰度。

Based on the law of conservation of energy and heat flow data in continental China, the upper limit of heat production for the crust beneath continental China is determined as 1.3μWm^-3. Then, using the data of the heat flow and helium isotopic composition of underground fluids, the heat productions of various tectonic units in the crust of continental China are estimated to be in the range of 0.58-1.12μWm^-3 with a median of 0.85μWm^-3, and the corresponding U, Th and K abundances are in the ranges of 0.83-1.76μg/g, 3.16- 6.69μg/g and 1.0%-2.12% respectively. These data indicate that the abundances of the radioactive elements U, Th and K in continental crust of China are notably higher than those in the Archean crust, suggesting that the components of continental crust of China are highly evolved. In addition, the crustal composition of continental China exhibits significant lateral heterogeneity. The crust beneath eastern China is enriched in highly incompatible elements such as U, Th and K relative to that beneath western China, and the crust beneath fold belts is enriched in U, Th and K relative to that beneath cratonic areas. It is inferred on the basis of a positive correlation between the SiO2 content and heat production of continental crust that the crust beneath eastern China and fold belts are more felsic than beneath western China and cratons. This regional variation is consistent with the results of inference from the seismic wave velocity data in China. According to the fact that the seismic wave velocity and heat production range of the crust of continental China are lower than the global average values, combined with a comparison with the global crustal composition models published by previous studies, it is deduced that the abundances of highly incompatible elements such as U, Th and K in continental crust are overestimated in the average composition models of global continent crust constructed by Rudnick and Fountain （1995）, Rudnick and Gao （2003）, Weaver and Tarney （1984）, Shaw et al. （1986） and Wedepohl （1995）.