See schematic following for example samples which have

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Unformatted text preview: cation) relative to the least altered will have lower than normal concentrations of immobile elements. • See schematic following for example. Samples which have lost mass (e.g., chlorite-sericite alteration) relative to the least altered will have higher than normal concentrations of immobile elements. Wednesday, 15 August, 12 h e di io lut ic nr en m t n From Barrett and MacLean (1999) Wednesday, 15 August, 12 MacLean’s (1990) Method • In single precursor the displacement of the immobile element in an altered rock from the least altered rock is proportional to the mass change. • • To calculate this mass change we must calculate and enrichment factor. We calculate the enrichment factor (EF) as follows: • • • RC = EF * wt% or ppm of component in altered sample To calculate the mass change we do the following: • • • M = some immobile, incompatible element, typically Zr, but can be any of the HFSE, Al2O3, etc. We then calculate the reconstituted composition (RC) of the rock • • EF = Mprecursor / Maltered MC = RC – precursor ; where RC and precursor are the wt% or ppm of given element in both altered and fresh rocks, respectively, We then plot mass change plots and bar graphs….see below but farther on…. Wednesday, 15 August, 12 MacLean’s (1990) Method • In multiple precursor systems there are rocks that form a range of conditions due to fractional crystallization, partial melting, magma mixing, etc. • In these cases we have to define a fractionation line that explains all of these processes and is constrained by the least altered samples. • Typically we use a TiO2-Zr or Al2O3-Zr plot. Along these plots the least altered samples will form the fractionation line. • Furthermore, at different Zr/TiO2 and Al2O3/Zr ratios will lie the different rock types within specific groups – i.e., basalts, andesites, dacites, rhyolites, etc. • Variations at a given Zr/Ti and Al/Zr from the fractionation line represent mass gains (lying below line) and mass losses (lying above the line). • After defining the least altered fractionation, and the different groups, the Zr content of the precursor is found by finding the intersection of the alteration line with the fractionation curve. Wednesday, 15 August, 12 Mass Gain From Barrett and MacLean (1999) Wednesday, 15 August, 12 Mass Loss 25 Ma c Intermediate Felsic Al2O3 20 Mass Loss 15 10 fractionation curve y = -0.15x+16.47 Mass Gain 5 (A) 0 0 100 200 300 400 500 600 Zr Fractionation curve: Al2O3precursor = -0.15Zrprecursor+16.47 Alteration lines: m = Al2O3altered/Zraltered Where alteration line intersects fractionation curve then: Al2O3altered/Zraltered = Al2O3precursor/Zrprecursor. Substitution and rearranging yields: Zrprecursor = 16.47/[Al2O3altered/Zraltered - 0.15] The above is then used to compare to Zraltered to get the enrichment factor (EF) like in single precursor method to calculate the Reconstructed Composition Wednesday, 15 August, 12 Calculating Precursor Compo...
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