中国东北EM-I型钾质火山岩成因的Ca同位素制约

Mantle geochemistry has been demonstrating that the potassic-ultrapotassic rocks with EM-I signature play a key role in the formation and evolution of the mantle. The potassic-ultrapotassic rocks with EM-I signature in Erkeshan-Wudalianchi-Keluo-Xiaogulihe from northeastern China can be regarded as a good example. However, we need to do more work to explore the source of the basaltic melts and the nature of EM-I signature. Among them, the materials related to the subducted slab play a crucial role in LILEs, enriched EM-I and light Mg isotopic composition of the potassic-ultrapotassic rocks. Two major views have been proposed: (1) one is that the role of recycled sedimentary carbonates derived from the stagnant slab in the mantle transition zone and (2) the other is the partial melting of subducted slab in the form of carbonated eclogites. In order to further identify the nature of materials related to the subducted slab and source melts, this project focuses on the Ca isotopes of the potassic rocks with the lowest δ26Mg values from Wudalianchi and Erkeshan area. Combined with previous geochemical data, we try to further the component and the evolution of the mantle source and finally provide the Ca isotopic constraints on the origin of the EM-I type endmember.

地幔地球化学研究表明，EM-I型钾质-超钾质火山岩是我们认识大陆地幔形成和演化的重要窗口。中国东北二克山-五大连池-科洛-小古里河出露一条EM-I型钾质-超钾质火山岩带，关于其玄武质熔体的来源、EM-I源区物质的属性等问题，是目前仍需要深入研究的问题。其中，源区中俯冲板片来源相关物质的属性对于钾质玄武岩的LILEs、富集EM-I和轻Mg同位素组成起到了关键作用。目前，一种观点认为是位于地幔过渡带的滞留板片来源的沉积碳酸盐岩的加入，另一种观点认为是俯冲洋壳自身（以碳酸盐化榴辉岩形式）在岩石圈地幔内参与熔融。为了进一步厘定源区洋壳来源物质的属性和源区熔体的性质，本项目拟对东北火山岩区的具有最低δ26Mg的五大连池和二克山玄武岩开展Ca同位素地球化学示踪手段，并结合已有的其它地球化学数据，进一步示踪其源区的物质组成和演化过程。从而为地幔EM-I型端元的成因提供新的地球化学制约。

中国东北地区出露一套典型的EM-I特征的钾质-超钾质火山岩，为进一步研究这套火山岩的成因，本项目尝试从Ca同位素的角度为火山岩形成提供新证据。本项目对东北钠质-钾质-超钾质火山岩开展了Ca同位素研究，结果表明这套火山岩具有明显的Ca同位素变化，其中钠质火山岩δ44/40Ca变化范围为0.72-0.75‰，过渡玄武岩δ44/40Ca变化范围为0.52-0.69‰，钾质-超钾质火山岩δ44/40Ca变化范围为0.23-0.65‰。这套火山岩与主量和Sr-Nd-Pb-Mg同位素都有非常好的相关性，表明其Ca同位素变化和EM-I特征很可能是由沉积碳酸盐岩交代形成地。考虑到该玄武岩的Ca同位素如此低，该碳酸盐岩应该属于古老俯冲沉积物而并不是来自太平洋俯冲板片。对于钠质火山岩，它们与MORB相似的Mg-Ca同位素表明其源区并没有受到沉积碳酸盐岩的影响。结合玄武岩其它地球化学特征，我们认为是古老板块俯冲在地幔过渡带滞留导致携带的沉积碳酸盐熔融交代上覆地幔形成的。随后，在经过岩石圈地幔过程中发生不同程度物质交换。