Major element evolution of basaltic magmas: a comparison of the information in CMAS and ALFE projections |
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Authors: | John B. Reid Jr. Eric Steig Wilfred B. Bryan |
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Affiliation: | (1) School of Natural Science, Hampshire College, 01002 Amherst, MA, USA;(2) Woods Hole Oceanographic Institution, 02543 Woods Hole, MA, USA |
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Abstract: | Basalt petrologists disagree as to whether the commonly used projection, forsterite-diopside-silica, in the system CMAS (CaO-MgO-Al2O3-SiO2), can adequately resolve differences in basaltic glass compositions for purposes of petrogenetic modelling. Here, we suggest than an analogous plot, the aluminium-factor diagram (ALFE) of Nesbitt and Cramer (1981), has greater diagnostic value than Fo-Di-Sil. A plot of molar (Al2O2-CaO-Na2O-K2O)/(FeO + MgO) vs FeO/(FeO + MgO), it produces more coherent patterns both for experimental basalt glasses, and for natural lavas. It is, like Fo-Di-Sil, a projection through plagioclase, but has the advantage that it monitors changes in Fe/(Fe + Mg) in melts and associated crystalline phases, and is particularly useful in assessing the timing of clinopyroxene crystallization in a suite of lavas. The diagram owes its greater resolving power to the fact that the computation of its coordinates is less sensitive to analytical uncertainty than for Fo-Di-Sil. In the latter diagram, normative quartz is calculated as a residual and thus manifests the uncertainties in all the major elements. |
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