Magma mixing is common in the Earth. Understanding the dynamics of the mixing process is necessary for dealing with the likely consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers. Here, a new appara- tus has been developed in order to perform chaotic mixing experiments in systems of melts with high viscosity con- trast. The apparatus consists of an outer and an inner cyl- inder, which can be independently rotated at finite strains to generate chaotic streamlines. The two cylinder axes are offset. Experiments have been performed for ca. 2 h, at 1,400°C under laminar fluid dynamic conditions (Re * 10-7). Two end-member silicate melt compositions were synthesized: (1) a peralkaline haplogranite and (2) a haplobasalt. The viscosity ratio between these two melts was of the order of 103. Optical analysis of post-experi- mental samples reveals a complex pattern of mingled filaments forming a scale-invariant (i.e. fractal) distribution down to the lm-scale, as commonly observed in natural samples. This is due to the development in space and time of stretching and folding of the two melts. Chemical analysis shows strong non-linear correlations in inter- elemental plots. The original end-member compositions have nearly entirely disappeared from the filaments. The generation of thin layers of widely compositionally con- trasting interfaces strongly enhances chemical diffusion producing a remarkable modulation of compositional fields over a short-length scale. Notably, diffusive fractionation generates highly heterogeneous pockets of melt, in which depletion or enrichment of chemical elements occur, depending on their potential to spread via chemical diffu- sion within the magma mixing system. Results presented in this work offer new insights into the complexity of pro- cesses expected to be operating during magma mixing and may have important petrological implications. In parti- cular: (1) it is shown that, in contrast with current thinking, rheologically contrasting magmas can mix (i.e. with large proportions of felsic magmas and high viscosity ratios), thus extending significantly the spectrum of geological conditions under which magma mixing processes can occur efficiently; (2) the mixing process cannot be modeled using the classical linear two-end-member mixing model; and (3) the chemical compositions on short-length scales represent snapshots within the process of mixing and therefore may not reflect the final composition of the magmatic system. This study implies that microanalysis on short-length scales may provide misleading information on the parental composition of magmas.
Enhancement of Magma Mixing Efficiency by Chaotic Dynamics: an Experimental Study
PERUGINI, Diego;POLI, Giampiero
2011
Abstract
Magma mixing is common in the Earth. Understanding the dynamics of the mixing process is necessary for dealing with the likely consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers. Here, a new appara- tus has been developed in order to perform chaotic mixing experiments in systems of melts with high viscosity con- trast. The apparatus consists of an outer and an inner cyl- inder, which can be independently rotated at finite strains to generate chaotic streamlines. The two cylinder axes are offset. Experiments have been performed for ca. 2 h, at 1,400°C under laminar fluid dynamic conditions (Re * 10-7). Two end-member silicate melt compositions were synthesized: (1) a peralkaline haplogranite and (2) a haplobasalt. The viscosity ratio between these two melts was of the order of 103. Optical analysis of post-experi- mental samples reveals a complex pattern of mingled filaments forming a scale-invariant (i.e. fractal) distribution down to the lm-scale, as commonly observed in natural samples. This is due to the development in space and time of stretching and folding of the two melts. Chemical analysis shows strong non-linear correlations in inter- elemental plots. The original end-member compositions have nearly entirely disappeared from the filaments. The generation of thin layers of widely compositionally con- trasting interfaces strongly enhances chemical diffusion producing a remarkable modulation of compositional fields over a short-length scale. Notably, diffusive fractionation generates highly heterogeneous pockets of melt, in which depletion or enrichment of chemical elements occur, depending on their potential to spread via chemical diffu- sion within the magma mixing system. Results presented in this work offer new insights into the complexity of pro- cesses expected to be operating during magma mixing and may have important petrological implications. In parti- cular: (1) it is shown that, in contrast with current thinking, rheologically contrasting magmas can mix (i.e. with large proportions of felsic magmas and high viscosity ratios), thus extending significantly the spectrum of geological conditions under which magma mixing processes can occur efficiently; (2) the mixing process cannot be modeled using the classical linear two-end-member mixing model; and (3) the chemical compositions on short-length scales represent snapshots within the process of mixing and therefore may not reflect the final composition of the magmatic system. This study implies that microanalysis on short-length scales may provide misleading information on the parental composition of magmas.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
