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2 years ago in Geochemical Modeling , Igneous Petrology By Walterwig
How to model the fractionation crystallization process?
I have a suite of co-genetic volcanic rocks that show a clear differentiation trend. I want to use a model like MELTS or simple Rayleigh fractionation to simulate the process. I'm unsure how to robustly choose the parent magma composition (c0) and which minerals to include in the fractionating assemblage at each step. How do I avoid non-unique solutions?
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By Varun Answered 1 year ago
I typically start not with a model, but with the rocks. Use the most primitive sample (highest Mg#, Cr, Ni) as your initial parent (c0). Then, I run simple mass balance (least-squares) to find the mineral assemblage that best fits the transition from your parent to the next most evolved sample. Use this assemblage as your first step in a Rayleigh fractionation model. I would recommend using the rhyolite-MELTS software for more robust results, as it calculates stable phases thermodynamically. Be prepared to iterate; the real value is in the mismatch between model and data, which often points to crustal assimilation or changing physical conditions.
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