Exponential decay of concentration variance during magma mixing: Robustness of a volcanic chronometer and implications for the homogenization of chemical heterogeneities in magmatic systems

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 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;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.

Viscosity of Pyroxenite Melt and Its Evolution During Cooling

“…Chemical compositions for the starting material and run products were determined by using the electron probe microanalyses (EPMA) installed at the Department of Mineralogy at the University of Hannover (DE), following the procedure reported in Rossi et al (2017) for glasses and in Vetere et al (2017) for crystalline phases. Table 1 shows the starting compositions presented in this study plus densities calculated according to Ochs and Lange (1999) for melts and Klöß (2000) for glasses.…”

“…Recently, experiments under constant temperature were performed in order to study the behavior of lavas in different geological settings on both Earth and other Earth‐like planets (; Chevrel et al, ; Rossi et al, ; Sehlke & Whittington, ; Vona et al, , and references therein). Indeed, volcanic eruptive styles are governed by magma rheology, and rheology governs lava emplacement mechanisms.…”

Magmatic evolution and textural development of the 1739 CE Pietre Cotte lava flow, Vulcano, Italy

“…in a time-series of high temperature experiments, Na has been shown to have a higher mobility than K and other major elements, while Rb, Sr and Ba have high diffusivities compared to the REEs, Nb, Ta, Zr, and Hf; Perugini et al, 2015). The inherent complexity of mixing systems results from the interplay of fluid dynamic regimes and chemical exchanges between interacting magmas (Rossi et al, 2017). While examples of data such as that shown in Fig.…”

“…While examples of data such as that shown in Fig. 9 can indicate system variability between two end-members, this may still be skewed with variations in chaotic mixing processes such as duration of mixing time, degree of crystallinity and the role of volatiles dissolved in a melt (Rossi et al, 2017).…”

Diffusive exchange of trace elements between alkaline melts: Implications for element fractionation and timescale estimations during magma mixing

“…Ferguson et al, 2016), as well as the use of rocks showing evidence of magma mixing as volcanic chronometers (e.g. Perugini et al, 2015, Rossi et al, 2017. For a successful application of petrological techniques based on the diffusion process, an accurate knowledge of the diffusive behaviour for the investigated chemical elements is mandatory.…”