中国东部新生代碱性玄武岩橄榄石斑晶中含CH_4熔体包裹体的发现及其地质意义

Methane-bearing melt inclusion in olivine phenocryst in Cenozoic alkaline basalt from Eastern China and its geological significance

碱性玄武质岩浆起源于上地幔或者更深处,是了解地幔物质组成的重要窗户.熔体包裹体是主矿物在结晶过程中被捕获的,不易受到后期地质作用的影响,最早结晶的矿物橄榄石所捕获的熔体可以有效地记录原始岩浆的物理化学信息.在此次研究中,利用激光拉曼光谱(LA-Raman)对华北克拉通东部山东杨庄新生代碱性玄武岩橄榄石中,73个熔体包裹体内的气体组分进行了原位分析,并利用激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)对寄主橄榄石的主微量元素进行了测试.结果显示,熔体包裹体中含有高含量的CH4等烃类,而且含有石墨和碳酸盐等矿物,这为CH4在上地幔高温高压条件下能够稳定存在提供了直接证据,也表明中国东部地幔存在低氧逸度的区域.橄榄石斑晶具有高Ni,高Fe/Mn比值的特点,指示源区可能存在辉石岩组分.结合其他地球化学和地球物理资料,西太平洋板块对中国东部岩石圈地幔的俯冲效应可以很好地解释玄武岩源区存在辉石岩组分和CH4富集特征的现象.CH4是深部碳的重要存在形式之一,太平洋板块俯冲作用可能是中国东部深部碳循环重要的动力学机制.

Alkaline basalts originate from the partial melting of the upper mantle or deeper mantle region, and their study can contribute important knowledge of mantle composition. Melt inclusions in these basalts are small natural droplets trapped during the formation of olivine, and can be isolated from later alterations. Therefore, they can effectively record information from the early stages of magma formation and evolution. In this study, the volatile composition of 73 melt inclusions in olivine phenocrysts of alkaline basalts from Yangzhuang in Shandong Province were analyzed by Laser Raman Spectroscopy. The major and trace element compositions of some host olivines were measured by LA-ICP-MS. The results show that these melt inclusions contain amounts of methane （CH4） and other hydrocarbons, and some carbonaceous minerals such as graphite and carbonate. These findings prove that CH4 can stably exist in the upper-mantle environment of high pressure and temperature, and indicate that such a mantle region with low oxygen fugacity does exist below Eastern China. The host olivine phenocrysts are characterized by high Ni content and a high Fe/Mn ratio, implying the presence of a pyroxenite component in the mantle source. The subduction of the Western Pacific Plate beneath the lithospheric mantle of Eastern China is a possible source for the pyroxenite, combined with other geochemical and geophysical data. CH4 is an important form of deep carbon and the subduction of oceanic crust is responsible for the recycling of deep carbon.