中国活动火山区甲烷的碳同位素研究

Carbon isotopic compositions of the methane derived from magma at the active volcanic regions in China.

中国长白山天池、腾冲、五大连池等主要活动火山区逸出气体中甲烷的碳同位素组成测试结果显示,腾冲、长白山(不包括天池火山口内湖滨)火山区岩浆来源气体中甲烷的δ^(13)C值的变化范围与国际上地热区甲烷大致相同,平均值分别为-19．0‰、-32．6‰;五大连池火山区与天池火山口内湖滨强气体释放带逸出甲烷的δ^(13)C值较低,平均值分别为- 45．8‰和-47．9‰,类似于东非裂谷带基伍(Kivu)湖的甲烷。研究认为,这些低δ^(13)C值甲烷可能直接来自上地幔;岩浆来源甲烷在火山喷发过程中的动力学分馏导致了其δ^(13)C值的降低,岩浆源区越深,其δ^(13)C值越低;岩浆源区深度(d)与甲烷δ^(13)C值有如下关系:d(km)=0．0107(δ^(13)C_1)~2+1．14。岩浆来源CO_2和CH_4之间的碳同位素分馏温度可指示气体最后源区的深度。

The mean δ^13C values of methane for 22 springs from the Tengchong volcanic area and for 11 springs from the Changbaishan Tianchi volcanic area （except Hubin springs） in China are respectively -19.0‰ and -32.6‰, similar to that from world’s geothermal area. But the mean 813C values of methane at the Wudalianchi volcanic area and the Hubin strong gas discharge region located within the Tianchi crater lake are respectively- 45.8‰ and -47.9‰, much more lower than that of other volcanic area, and similar to that from the lake Kivu in the Eastern Africa. We suppose that such methane could be directly derived from the upper mantle, and the main cause for the occurrence of methane with lower δ^13C value is dynamically isotope fractionation during the ascending migration of methane derived from the deep magma chamber. The more deeply the magma chamber, the lower the δ^13C value of methane. The correlation formula between the magma chamber depth and the methane δ^13C value is： d （km） = 0.0107 （813 C1 ）^2 ＋ 1.14 （where d expresses the depth （km） of magma chamber, the δ^13C is the δ^13C value of methane）. The authors suggest that the carbon isotope fractionation temperature between the carbon dioxide and methane derived from the mantle magma may be only the last reservoir depth of the gases, indefinitely the initial magma chamber depth.