Emplacement temperature determinations of proximal pyroclastic deposits on Santorini, Greece, and their implications

This paper presents the results of a study undertaken to determine emplacement temperatures of volcanic deposits on the volcano of Santorini, Greece, using palaeomagnetic methods. Two types of breccia have been identified on Santorini, lithic lag breccias and ground breccias; they have been interpreted as deposits from pyroclastic flows. However, where the breccia deposits are not intimately associated with ignimbrite, their origin is ambiguous. Emplacement-temperature determinations were carried out on lithic clasts within these deposits to assess the origin and to place constraints on their emplacement mechanisms. The results of thermal demagnetization show that all the clasts display at present Earth's field component of magnetization. The Earth's field component indicates that these clasts have cooled from elevated temperatures within the deposits. Therefore, these breccia deposits were emplaced hot, verifying their origin as primary products of pyroclastic eruption. Many of the deposits display a nide range of emplacement temperatures, and the lowest temperature at any one location probably represents the equilibrium temperature of the deposit. An emplacement temperature determination records the highest temperature experienced by the clast, before cooling in situ. These clasts may have been heated before, during or after the eruption. In estimating the emplacement temperature, it is important to understand at what stage the clasts were heated. Thermal modelling was carried out to assess the effects of heating of the clasts (a) during the eruption and subsequent transport by hot gases and juvenile pyroclasts within pyroclastic flows, and (b) by an overlying hot deposit such as a pyroclastic deposit or lava now. Results from this study indicate that many of the clasts were hot before incorporation took place. This indicates that clasts were either derived from depth within the volcano or that there were abundant rocks at elevated temperature near or at the surface.