Exponential Decay Of Concentration Variance During Magma Mixing: Robustness Of A Volcanic Chronometer And Implications For The Homogenization Of Chemical Heterogeneities In Magmatic Systems

Stefano Rossi1*, Maurizio Petrelli1, Daniele Morgavi1, Diego González-García1, Lennart A. Fischer2, Francesco Vetere1, Diego Perugini1

1 Department of Physics and Geology, University of Perugia, Piazza Università, 06100 Perugia, Italy 2 Institute of Mineralogy, Leibniz Universität Hannover, Callinstrasse 3, 30167, Hannover, Germany

• Corresponding Author Stefano Rossi Tel. +39 0755852601 e-mail: stefano.rossi1@studenti.unipg.it

2 Abstract The mixing of magmas is a fundamental process in the Earth system causing extreme compositional variations in igneous rocks. This process can develop with different intensities both in space and time, making the interpretation of compositional patterns in igneous rocks a petrological challenge. As a time-dependent process, magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub-volcanic magma chambers and eruptions. This allowed the use of magma mixing as an additional volcanological tool to infer the mixing-to-eruption timescales. In spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established. This represents a prerequisite to apply reliably this conceptual model. Here, new chaotic magma mixing experiments were performed at different times using natural melts. The degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability. We further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples. Results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time. These empirical relationships remain valid by removing up to 80% of the analytical determinations. Experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing. The calculations show that, when the mixing dynamics generate millimetre thick filaments, homogenization timescales of the order of a few minutes are to be expected.

3 Keywords: magma mixing, chemical exchanges, homogenization time, volcanic chronometer

1. Introduction

Magma mixing is a major petrogenetic process concurring in the generation of the wide compositional diversity of igneous rocks on Earth (Bateman, 1995; Bacon, 1986; Eichelberger, 1978; Sparks et al., 1977) in both the plutonic and volcanic environment (Albert et al., 2015; Anderson, 1976, 1982; De Rosa et al., 1996; Kratzmann et al., 2009; Perugini and Poli, 2005; Wiebe, 1994). In this work, according to Perugini and Poli (2012) we refer to magma mixing as the process combining the physical dispersion of the two magmas and the development of the chemical exchanges between them.
The evidence of magma mixing processes remained recorded by a range of structural and textural features in igneous rocks, including the occurrence of mineral phases showing thermo-chemical disequilibria (Anderson, 1984; Didier and Barbarin, 1991; Hibbard, 1981; Wada, 1995; Wallace and Bergantz, 2002), magmatic enclaves dispersed into compositionally different host rocks (Bacon, 1986; Vetere et al., 2015), and bandings of different magma compositions (Flinders and Clemens, 1996; Morgavi et al., 2016). Geochemically, magma mixing is witnessed by extreme compositional variations of major and trace elements and isotopes, which can occur in the rocks even at very short length scales, of the order of millimetres or micrometres (Montagna et al., 2015; Wiesmaier et al., 2015; Laeger et al., 2017). The presence of compositionally different domains at such short length scale can cause diffusive fractionation processes of chemical elements that, due to their different mobility in magmas (Perugini et al., 2006), generate

4 volumes of melts whose compositional variation is difficult to reconcile with classical geochemical models (e.g. Fourcade and Allegre, 1981).

The use of numerical models and experimental petrology provided a powerful tool in the study of magma mixing, and several attempts have been made to capture the most relevant parameters involved during the interaction between magmas (Kouchi and Sunagawa, 1985; Laumonier et al., 2014a-b; 2015; Bergantz et al., 2015; Schleicher et al., 2016). These studies also highlighted an extreme complexity of the mixing process in space and time due to the interplay of the fluid dynamic regime and chemical exchanges between the interacting magmas.

The complexity of magma mixing processes has been

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