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This thesis aimed to obtain diffusion timescales from olivine crystals in a basaltic lava flow on the island of Flores (Azores), and determine pre-eruptive processes active beneath the island. In the process, we also add to the geochemical data available for Flores. A total of 24 samples were taken from the island. The bulk geochemistry of these samples, obtained through X-ray Fluorescence, classifies them as alkali basalt. Scanning Electron Microscopy and traditional petrography were applied for a microtextural analysis of the samples. An Electron Microprobe was used to precisely measure the Fe-Mg content (forsterite, Fo) along multiple profiles within olivine crystals. The microscopic analysis of the samples highlighted different magmatic textures in the samples, including disequilibrium textures in both olivine and plagioclase, such as resorption and overgrowths. This indicates that a mixing event between two different magmas occurred. The compositions of the olivine crystals also indicate different origins. The largest olivine crystals (type 1) had the highest Fo content in their cores, indicating an origin near the crust-mantle boundary, and lower Fo content in their rims. These rims have sharp boundaries and are mostly the result of renewed growth after mixing. A group of smaller olivine crystals (type 2) had lower Fo contents in both the cores and the rims, and grew from the newly mixed magma. A third group (type 3) showed low Fo content in the cores, and an increase in Fo at the rim. These crystals originate from the wall-rock or from the more evolved magma. The thermodynamic process of diffusion can be used to calculate timescales of magmatic processes. It is based on the gradient in chemical potential (in this case, Fo content) between the crystal and the melt / overgrowth. The Fo profiles of zoned olivine crystals are entered into a model which calculates the time necessary to produce the observed profile, based on Fick’s second law of diffusion and given diffusion constants in olivine. In our modeling, we used Monte Carlo simulations to account for the variability of thermodynamic variables (pressure, temperature, oxygen fugacity). The type 1 olivines yielded an average diffusion time of 23.5 days. Type 2 olivines were more affected by growth, and yielded very short diffusion times of 4.8 days on average. Type 3 crystals yielded diffusion times in between these values. All of these values are minimum timescales, as we did not obtain EBSD data to correctly orient the profiles in the samples. From the diffusion modeling, it is clear how diffusion is increased at higher temperatures, yielding shorter diffusion timescales. We suggest that a hot, Mg-rich magma originating from the crust-mantle boundary moved upwards in the crust and mixed with a cooler, more Si-rich magma in a mid- to upper crustal magma chamber, creating the disequilibrium textures observed in the samples. The difference in temperature may have been the main driver for creating disequilibrium, more important than the chemical potential. During mixing, convective processes in the magma chamber may have caused complex zoning in both plagioclase and type 1 olivine crystals. Magma mixing was the event that eventually triggered the eruption, and the diffusion time of 23.5 days is thus a minimum estimate for the time between the mixing event and the eruption that produced the lava flow.

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Pure sciences. Natural sciences (general)