Vermeesch, P; Pease, V; (2021) A genetic classification of the tholeiitic and calc-alkaline magma series. Geochemical Perspectives Letters , 19 pp. 1-6. 10.7185/geochemlet.2125.

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Abstract

The concept of the ‘magma series’ and the distinction between alkaline, calc-alkaline and tholeiitic trends has been a cornerstone in igneous petrology since the early 20th century, and encodes fundamental information about the redox state of divergent and convergent plate tectonic settings. We show that the ‘Bowen and Fenner trends’ that characterise the calc-alkaline and tholeiitic types of magmatic environments can be approximated by a simple log ratio model based on three coupled exponential decay functions, for A = Na2O + K2O, F = FeOT and M = MgO, respectively. We use this simple natural law to define a ‘Bowen-Fenner Index’ to quantify the degree to which an igneous rock belongs to either magma series. Applying our model to a data compilation of igneous rocks from Iceland and the Cascade Mountains effectively separates these into tholeiitic and calc-alkaline trends. However the simple model fails to capture the distinct dog-leg that characterises the tholeiitic log ratio evolution, which can be attributed to the switch from ferrous to ferric iron-bearing minerals. Parameterising this switch in a two stage magma evolution model results in a more accurate fit to the Icelandic data. The same two stage model can also be fitted in A–T–M space, where ‘T’ stands for TiO2. This produces a new way to identify calc-alkaline and tholeiitic rocks that does not require the conversion of FeO and Fe2O3 to FeOT. Our results demonstrate that log ratio analysis provides a natural way to parameterise physical processes that give rise to these magma series.

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