The Role of Chaotic Dynamics and Flow Fields in the Development of Disequilibrium Textures in Volcanic Rocks |
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| Authors: | PERUGINI D; BUSA T; POLI G; NAZZARENI S |
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| Institution: | 1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF PERUGIA, PIAZZA UNIVERSITÀ, 06100 PERUGIA, ITALY
2 DEPARTMENT OF MINERALOGY AND PETROLOGY, UNIVERSITY OF PADOVA, C. SO GARIBALDI, 37, 35127 PADOVA, ITALY |
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| Abstract: | Disequilibrium textures in minerals are often observed in igneousrocks. Their occurrence is commonly related to the variationof intensive variables (e.g. pressure, temperature, etc.) thatperturbed a pre-existing state of equilibrium. However, if thevariation of intensive variables provides a reliable explanationfor the occurrence of disequilibrium textures in minerals, itdoes not explain why, over very short length scales (<1–2cm), in the same rock, crystals of the same mineral phase oftenappear to have reacted very differently to the disequilibriumprocess. A good example of this puzzling phenomenon is givenby clinopyroxene phenocrysts occurring in the Santa Venera alkalibasalt (Mt. Etna, Italy), in which a great variety of disequilibriumtextures, coexisting on very short length scales (<1–2cm), are observed. Clinopyroxenes exhibit heterogeneously resorbedCr–Al diopside cores around which a rim of Al–Fe3+diopside, having a highly variable area, has grown. The areaof the Al–Fe3+ diopside rim is used as a discriminantparameter for the studied pyroxenes as it displays a tri-modalstatistical distribution. In addition, the chemical zoning fromthe core to the rim of pyroxenes exhibits both continuous anddiscontinuous patterns. These continuous and discontinuous patternsare associated with crystals having low and high values of therim area, respectively. To explain these zoning patterns, amixing process between magmas having different geochemical andthermodynamic properties, governed by chaotic dynamics, is proposed.In particular, the occurrence in the same system, and at shortlength scales, of regular and chaotic regions is suggested asthe basic dynamic inducing a heterogeneous distribution of themagmas involved in the mixing process; this leads to a strongcontrol on the propagation of the disequilibrium phenomena andon the crystallization of pyroxenes, even over short lengthscales. The occurrence of regular and chaotic regions withinthe same magmatic system can explain the entire spectrum offeatures observed in the studied pyroxenes, from the occurrenceof the tri-modal distribution of rim areas to the presence oftwo distinct patterns of chemical zoning, continuous and discontinuous,from the core to the rim of pyroxenes. KEY WORDS: chaotic dynamics; disequilibrium textures; heterogeneity; magma mixing; pyroxenes |
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