242-9 From Picrite to Rhyolite - Modeling Namibian Magmatism using the Magma Chamber Simulator and easyMelts

Abstract:

The Magma Chamber Simulator (MCS) is a thermodynamic model for computing the open-system, polybaric evolution of magma storage regions that includes crystallization, stoping, assimilation of crustal melts, mixing of magmas, mush entrainment and rejuvenation, and magma ascent and eruption [1]. easyMelts is a modern graphical user interface to the MELTS family of algorithms by Mark Ghiorso and co-workers that runs natively on all Mac, Windows and Linux platforms. easyMelts was designed by Einari Suikkanen (who also created the MCS Visualizer), implements built-in plotting, and exports results to Excel, making it an excellent introduction to thermodynamic modeling for users without coding experience. Recently, we have incorporated easyMelts into the open-source codebase used to maintain the MELTS component of MCS [2]. Upgrades to easyMelts simplify the workflow, increase interoperability with other MELTS software, and add support for mantle melting simulations. Here, we use easyMelts and MCS to remodel the early Cretaceous tholeiitic suite of picrite-to-rhyolite dykes, Spitzkoppe, Namibia, which proved central to our earlier work [3].

For most major oxides and trace elements (Al₂O₃ being an exception), observed data from the dyke swarm fall between trends previously modeled with (p)MELTS for (a) closed-system fractional crystallization, dominated by conductive heat loss, and (b) heat-balanced assimilation and fractional crystallization, with local granitic crust as the contaminant. Unlike the original MELTS liquid calibration, the rhyolite-MELTS and H₂O-CO₂ mixed fluid model successfully predicts the major element compositions of the more evolved Spitzkoppe dykes. Fractional crystallization models (without assimilation) that match the Al₂O₃ trend slightly overestimate Fe₂O₃^T at low MgO, and vice versa, but the overall fit to the major elements is significantly improved compared to [3]. We use easyMelts to evaluate the effects of oxygen fugacity and volatile content on modeled liquid lines of descent and MCS 1.0 to interrogate the model space between scenarios (a) and (b), iterating to generate a family of models that simultaneously explain major and trace element trends, and isotopic signatures of the Spitzkoppe suite. With MCS 2.0 we explore assimilation coupled with crystallization that may be fractional, equilibrium, or concurrent (some crystals removed and others remaining in equilibrium with melt) while ensuring thermal equilibrium between magma and cumulate.

[1] mcs.geol.ucsb.edu

[2] github.com/magmasource/MAGMA

[3] doi.org/10.1093/petrology/egm012

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-19759

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