Studying magmatic systems through chemical analyses on clinopyroxene - a look into the history of the Teno ankaramites, Tenerife

University essay from Lunds universitet/Geologiska institutionen

Abstract: Clinopyroxenes have the ability to crystallize over and respond to a large range of temperatures, pressures and chemical compositions, and thus, they are ideal for studies regarding the history of magmatic systems. In this thesis, the chemistry and zoning of four clinopyroxene crystals from an ankaramite sample from the Teno massif, Tenerife, were analysed. The ankaramites in Teno massif were, according to previous studies, erupted following mass wasting events, and at least two mass wasting events have occurred in the history of the massif. The aim of the thesis was to link the chemistry and crystal features to possible processes affecting the magmatic system during their growth, to evaluate possible open system processes and eruption triggers related to the mass wasting events, and to gain an understanding for how clinopyroxenes can be used to study magmatic systems. The crystals were analysed using optical microscopy, scanning electron microscopy (SEM), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results show that the four clinopyroxene crystals can be divided into two groups: sector zoned crystals displaying different compositions in different faces of the crystal, and concentrically zoned crystals. The widely different crystal features, with the concentrically zoned crystals being inferred to have experienced a longer and more complex growth history, indicate that they belong to different generations or origins, indicating that magma mixing occurred in the system. On the other hand, the rims of all four crystals share enough similarities to be interpreted to have grown simultaneously, reflecting the eruption event. Furthermore, a band of deviant chemical composition is present inside the rim of all crystals, perhaps related to the mass wasting event and eruption trigger. Open system processes are inferred by Cr spikes in all four crystals, interpreted to represent events of recharge of primitive magma. These Cr spikes are located in the rims of the sector zoned crystals and in the interior of the concentrically zoned crystals. If the mass wasting event is to be considered the eruption trigger, the location of the Cr spikes close to the rim in the sector zoned crystals points to the possibility that the magma recharge and the mass wasting event are related, most likely that the magma recharge destabilized the system and caused the mass wasting event to occur. However, further work needs to be done to confirm this hypothesis. To acquire a better understanding for the magmatic history of these clinopyroxene crystals and the magmatic system in which they grew, more chemical analyses have to be made on a larger number of samples; preferably sampled systematically in order to know which ankaramite formation is being studied. Furthermore, geothermobarometry should be carried out in order to constrain the crystallization depth and temperature of the crystals and determine their respective origin.

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