The Role of Chaotic Dynamics and Flow Fields in the Development of Disequilibrium Textures in Volcanic Rocks


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Abstract

Disequilibrium textures in minerals are often observed in igneous rocks. Their occurrence is commonly related to the variation of intensive variables (e.g. pressure, temperature, etc.) that perturbed a pre-existing state of equilibrium. However, if the variation of intensive variables provides a reliable explanation for the occurrence of disequilibrium textures in minerals, it does not explain why, over very short length scales (<1–2 cm), in the same rock, crystals of the same mineral phase often appear to have reacted very differently to the disequilibrium process. A good example of this puzzling phenomenon is given by clinopyroxene phenocrysts occurring in the Santa Venera alkali basalt (Mt. Etna, Italy), in which a great variety of disequilibrium textures, coexisting on very short length scales (<1–2 cm), are observed. Clinopyroxenes exhibit heterogeneously resorbed Cr–Al diopside cores around which a rim of Al–Fe3+ diopside, having a highly variable area, has grown. The area of the Al–Fe3+ diopside rim is used as a discriminant parameter for the studied pyroxenes as it displays a tri-modal statistical distribution. In addition, the chemical zoning from the core to the rim of pyroxenes exhibits both continuous and discontinuous patterns. These continuous and discontinuous patterns are associated with crystals having low and high values of the rim area, respectively. To explain these zoning patterns, a mixing process between magmas having different geochemical and thermodynamic properties, governed by chaotic dynamics, is proposed. In particular, the occurrence in the same system, and at short length scales, of regular and chaotic regions is suggested as the basic dynamic inducing a heterogeneous distribution of the magmas involved in the mixing process; this leads to a strong control on the propagation of the disequilibrium phenomena and on the crystallization of pyroxenes, even over short length scales. The occurrence of regular and chaotic regions within the same magmatic system can explain the entire spectrum of features observed in the studied pyroxenes, from the occurrence of the tri-modal distribution of rim areas to the presence of two distinct patterns of chemical zoning, continuous and discontinuous, from the core to the rim of pyroxenes.

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