《地球化学》第5章岩浆形成和演化过程中的微量元素地球化学模型.ppt

* Highly incompatible elements are greatly concentrated in the initial small fraction of melt that is produced by partial melting, and subsequently get diluted as F increases * Highly incompatible elements are greatly concentrated in the initial small fraction of melt that is produced by partial melting, and subsequently get diluted as F increases * All - 1.0 because all of the source is melted * If know (CL) for magma derived by a small degree of batch melting, and we know D, we can estimate the concentration of that element in the source region (CO). This can provide very valuable information in constraining and characterizing the source region. * Rb is incompatible and Sr only slightly so, but near unity * Results: Incompatible element Rb (no K minerals) strongly concentrated in the early small melt proportions (low F) Thus ? a sensitive measure of the progress of fractional crystallization (at least until rock half melted) As melting proceeds, the incompatible element is gradually diluted by more compatible ones Since D(Sr) is close to 1.0, the ratio Rb/Sr vs. F is nearly the same as Rb alone Any ratio of incompatible to compatible element should then be sensitive to the degree of partial melting (at least in the initial stages). Important for Rb/Sr isotopic systems Note that can create a series of melts from a single source each with diff Rb/Sr 第五章 微量元素地球化学 Geochemistry of Trace Elements 5、岩浆形成和演化过程中的微量元素地球化学模型 利用微量元素分配模型可以定量研究岩浆形成和演化作用过程： 部分熔融作用 岩浆结晶过程 混合作用 5.1 平衡部分熔融 （Equilibrium partial melting) 在整个部分熔融过程中，熔体与残留固体始终保持平衡，直到熔体离去，这种熔融称平衡部分熔融，或称批次熔融(Batch melting)。 为了能够推导出微量元素在产生的熔体中的浓度与部分熔融程度的关系，需作如下假定： （1）在整个部分熔融过程中微量元素在固相和液相之间的总分配系数始终保持不变； （2）整个熔融过程中残余固相中各矿物相对形成熔体的贡献比例保持不变。 基于以上假设，在平衡部分熔融过程中，微量元素i在固相和熔体相中的浓度可由质量守恒的原则有以下公式： Wo=Wl+Ws CioWo=CilWl+CisWs F=Wl/W0 式中: F是熔体比例，即形成的熔体占母岩的重量百分数，它反映部分熔融的程度。F值由0→1表明熔融程度不断增大。 Cio是微量元素i在初始固相母体物质中的浓度，Cil是微量元素i在熔体中的浓度 Cis是微量元素i在熔融后残余固相中的浓度