The Role of Magmatic Volatiles and Saturation States in Plinian and Inter-Plinian Eruptions at Santorini, Greece

Volatiles play a crucial role in magmatic systems and in conditioning a magma reservoir for remobilisation and eruption. Understanding the concentrations of volatiles in a melt, how these change with time, and identifying the magma’s saturation state are essential for establishing the conditions necessary for explosive eruptions of varying magnitudes and for probing the dynamics of complex crystal-rich mushy systems. Santorini (Greece) offers an excellent natural laboratory to study the variations in processes governing the development of magmatic bodies in the run-up to large Plinian eruptions and smaller inter-Plinian events over the last 250 kyr. Here, we present new volatile data from apatite Ca5(PO4)3[F, Cl, OH] for a range of Plinian and inter-Plinian explosive eruptions from Santorini, which—when combined with an iterative forward model to reproduce apatite volatile trends—allows us to estimate the melt’s initial volatile concentrations and the saturation conditions under which the magmas fractionated. Our data demonstrate that while magmas from the Minoan and Cape Riva Plinian eruptions were likely water-saturated prior to eruption, this is not a consistent feature of large-scale Plinian eruptions at Santorini. Apatite volatile modelling indicates water-undersaturated conditions in the Lower Pumice 1 and Lower Pumice 2 eruptions and several other compositionally intermediate Plinian eruptions. Modelling of inter-Plinian magmas (M8-MSF, M7a, M6a, M4 and M1) shows a complex picture with both water-saturated and undersaturated conditions able to produce good fits to our apatite data. All magmas experienced significant crystal fractionation compared with the erupted magma crystallinity, indicating that melt was segregated and preferentially extracted relative to crystals to form the erupted magma. We suggest that the H2O saturation state is controlled primarily by magma intrusion or storage pressure, rather than initial volatile concentration or magma composition. We discuss the nature of the magmatic plumbing system at Santorini, and the relative control of magma compressibility and crustal relaxation vs external magma inputs in triggering eruptions.