Formation of redox gradients during magma-magma mixing

Abstract:

Magma-mixing is a key process that controls mass transfer in magmatic systems. The variations in melt compositions near the magma-magma interface potentially change the Fe oxidation state [1] and, thus, affect the solubility and transport of metals. To test this hypothesis, diffusion-couple experiments were performed at 1000 °C, 150 MPa and QFM+4. Synthesized crystal-bearing cylinders of hydrous dacite and hydrous basaltic andesite were equilibrated for up to 80 h.

The run products show that mafic components (Fe, Mg, etc.) were transported from the andesite into the dacite, while Si, Na and K diffused from the dacite into the andesite. A crystal dissolution sequence in the order of cpx, opx, plag, and spl/il was observed for the andesite.

We combined µ-XANES spectroscopy at Fe K-edge [2] with two-oxide oxybarometry [3] to measure redox profiles within our experiments. Here, fO₂ decreased towards the interface within the dacite and increased towards the interface within the andesite. This discontinuous fO₂ evolution, with a sharp redox gradient of ~1.8 log fO₂ units at the interface was maintained throughout the time-series despite the externally imposed fO₂ of the vessel. We propose a combination of two mechanisms that create and sustain this redox gradient: 1) The dissolution of cpx and opx in the andesite mainly introduced Fe²⁺ into the melt, which diffused towards the dacite, lowering Fe³⁺/SFe near the interface. 2) Charge balance calculations in the melt during diffusive exchange suggest net positive charge excess in the andesite near the interface (i.e., oxidation) and net negative charge excess in the dacite near the interface (i.e., reduction).

We suggest that this (metastable) redox layer can help to explain the contrasting Au/Cu ratios observed for arc-related porphyry-type ore deposits.

[1] Moretti (2005), Ann. Geophys. 48, 583-608. [2] Cottrell et al. (2009), Chem. Geol. 268, 167-179. [3] Ghiorso and Evans (2008), Am. J. Sci. 308, 957-1039.