Temporal variations in the diversity of primitive melts supplied to the Santorini silicic magmatic system and links to lithospheric stresses
Résumé
The South Aegean Volcanic Arc overlies a slowly subducting, cool slab of oceanic-to-transitional crust, and hosts the hazardous Christiana-Santorini-Kolumbo volcanic field. In order to investigate the primitive melts feeding the volcanic field, we present major and trace element analyses of 130 olivine-hosted melt inclusions from Santorini, integrated with previously published H2O and CO2 data. Following post-entrapment corrections, we identify four endmember primitive melt types preserved in Fo ≥ 80 olivines, ranging from low-K island-arc basalts with La/Yb ~ 1.5 and 1.5-3.0 wt% H2O to andesites with La/Yb ~ 6-10 and 3.0-3.5 wt% H2O. They are consistent with melting at 1.3 to 2.3 GPa and 1350-1440 °C of variably depleted peridotitic mantle fluxed by slab-derived melts and fluids. The chemical signatures of sediment melts dominate, while those of fluids derived from the ocean crust are low compared to global datasets. This is consistent with thick sediment accumulations observed in the Hellenic trench, and with low calculated fluid fluxes from the downgoing slab. The low H2O contents estimated for the primary melts (0.8-1.8 wt%) may imply a component of decompression melting beneath the arc. Coupled with a well-constrained chronostratigraphic context, the melt inclusion archive provides a time series of mantle-derived input into the silicic crustal magmatic system over the last 530 ka. Primitive melts with La/Yb ≤ 5 have been erupted encased in olivines over the last 530 ky, without any evident time variation. Melt inclusions with La/Yb > 5 have, on the other hand, been restricted to two periods: (1) prior to the onset of major explosive volcanism at ~ 360 ka, and (2) the products of the 3.6 ka Late-Bronze-Age eruption and the 22-to-3.6 ka inter-Plinian period immediately preceding it. The observations may be explained by time-varying differential extraction of melts from deep storage zones in the mantle or lower crust, related to lithospheric rifting and caldera collapse events. Temporal variations in the supplies of slab-derived melts and fluids may also play a role.