腾冲新生代火山作用流体组成及其来源——火山岩流体化学组成和碳同位素制约

Compositions and origin of volatiles in Tengchong Cenozoic volcanism from SE margin of the Tibetan Plateau: Constraints from chemical and carbon isotopic compositions of volatiles in volcanic rocks

腾冲新生代火山区是青藏高原唯一幔源挥发份大量排放的火山-地热区,大规模火山作用的流体组成的系统研究具有多方面的科学意义。对腾冲马鞍山、老龟坡和打鹰山等地新生代火山岩进行流体化学组成和碳同位素分析,结果表明腾冲新生代火山岩的流体组成中H2O占有极高的比例,CO2、N2和O2的含量较高,而且不同火山区的流体组成有所差异。CO2的δ13C值为-27.1‰^-7.5‰,位于地壳和地幔范围之间;CH4、C2H6、C3H8和C4H10等甲烷同系物的碳同位素组成随碳数增高具有整体正序、C2H6与C3H8局部反序的分布特征,显示海洋环境I型有机质热裂解成因烃类气体的特征。腾冲火山作用中存在地幔来源的CO2,岩浆存在轻微的CO2去气作用。含碳流体挥发份主要表现为俯冲大洋板片脱出流体挥发份的加入,特别是俯冲洋壳沉积有机质热裂解产物,大量的H2O可能来源于岩浆上升过程中围岩流体或再循环流体的加入,不同火山区岩浆上升演化的差异造成了流体组成的不同。

The Tengchong Cenozoic volcanic field is characterized uniquely with a large quantity of mantle-derived volatiles emitted from both volcano and geothermal fields in the Tibetan Plateau, Western China. The volatiles in the Cenozoic volcanic events have not been studied systemically for their compositions and origin. In this study, the chemical and carbon isotopic compositions of volatiles in the Cenozoic volcanic rocks were measured for their origins. The results showed H20 as the dominant component with CO2, N2 and 02 as major ones in the volatiles of the Tengchong Cenozoic volcanic rocks. The chemical compositions of volatiles varied among different volcanic fields. The 813C values of CO2 were in ranges between -27.1%r to -7. 5% that was located in the 813C range of crust and mantle, the 813C values of CH4, C2H6, C3Hs and C4H1o etc. methane homologue volatiles show a normal carbon isotopic distribution pattern among CH4 to C4H10 with partial reversal between C2H6 and C3Hs, which are the 813C features of thermogenic hydrocarbon gases by thermal decomposition of type I organic materials in oceanic sedimentary. The mantle-derived CO2 were preserved in Tengchong Cenozoic volcanic rocks, and magma undergone the slight CO2 degassing. Carboniferous volatiles were mainly derived from addition of ernst volatiles, especially from thermal decomposition of subducted oceanic sedimentary organic materials. Large quantities of H20 could be added from the volatiles of county rocks or recycled fluids during magma ascent. The differences in volatile compositions of different volcano fields could be caused by their distinct magma evolution.