微量元素地球化学 大三教学课件.ppt

* * * * * * * * * * * * * Can also apply the Rayleigh equation to Rayleigh fractional melting * Incremental batch melting in reverse Equation 9-5 would still apply Batch FX: F = proportion of liquid remaining * Incremental batch melting in reverse Equation 9-5 would still apply Batch FX: F = proportion of liquid remaining * * Note that 85% Ol + Opx, but 5% Grt raises bulk D to 0.366 * Suppose that a source rock with a mode of 51% plagioclase, 33% clinopyroxene, and 18% olivine undergoes batch melting * Rb is incompatible and Sr only slightly so, but near unity * * * * * * * * D = 1.0 No fractionation so CL/CO = 1 for all values of F * Values of F 0.4 unlikely for batch melting since greater amounts should separate and rise * 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. * * * 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 * * therefore some TEs will follow similar major E Periodic Table is next slide * therefore some TEs will fol