Analysis and numerical simulation of chaotic advection and chemical diffusion during magma mixing:petrological implications

摘要

Structures generated by magma mixing in a lava flow are studied with the aim to understand the interplay between chemical diffusion and the dynamics of the mixing process.Mesoscopic analysis of mixing structures indicates that magmas mixed intimately generating the contemporaneous occurrence of filament-like and globular regions within the same system.The extent of chemical exchange between interacting magmas has been measured by EPMA and LAM-ICP-MS analysis on a transect crossing filaments.Results indicate that elements with similar values of diffusion coefficients display linear correlations in interelemental plots whereas elements with different diffusion coefficients do not show any correlation.The mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme for several elements.Simulations show that the occurrence of chaotic flow fields is essential to explain the decoupling of the correlation at a short length scale between elements with similar and different diffusion coefficients,as observed in natural samples.In particular,the "sensitivity to initial conditions" of chaotic systems induces elements having similar values of diffusion coefficients to be linearly correlated in interelemental plots,whereas,at the same time,the correlation between elements having different diffusion coefficients is lost.The degree of correlation of the different elements in the simulations and natural data has been utilized to estimate the intensity of mixing,and results indicate intermediate mixing intensities for the natural samples,according to field evidence.It is shown that chemical diffusion processes coupled with chaotic mixing dynamics can generate strongly dishomogeneous batches of magmas coexisting at a very short length scale (of the order of a few microns) in which elements display very different degrees of correlation depending on the magnitude of their diffusion coefficients.These results open a key question about the suitability of using melt inclusions for petrogenetic purposes in magma mixing systems because they may be affected by the small-scale dishomogeneity of the magmatic system.On the contrary,it is shown that whole rock analysis is a more suitable technique to understand rock petrogenesis because it is poorly influenced by this dishomogeneity.