Evolution of the crustal magma plumbing system during the build-up to the 22-ka caldera-forming eruption of Santorini (Greece)

• Published in: Bulletin of volcanology Vol. 75; no. 12; pp. 1 - 22
• Main Authors: Fabbro, G. N., Druitt, T. H., Scaillet, S.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.12.2013; Springer; Springer Nature B.V; Springer Verlag
• Subjects: Accumulation; Basalt; Calderas; Crystalline rocks; Deep sea; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Sciences; Exact sciences and technology; Fractionation; Geology; Geophysics/Geodesy; Global Changes; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Lava; Magma; Mineralogy; Natural hazards: prediction, damages, etc; Plumbing; Research Article; Reservoirs; Sciences of the Universe; Sedimentology; Trace elements; Volcanic eruptions; Volcanoes; Volcanology; Cyclades; Greece; Southern Europe; Greek Aegean Islands; Santorin; Europe; Melt accumulation; Calderas; Santorini; Magma reservoirs; Crystal residence timescales; pumice; domes; dacites; mixing; reservoirs; magma chambers; lava; basalts; segregation; flow; high resolution; volcanic ash; crystals; andesites; accumulation; buffers; volcanic rocks; explosive eruptions; igneous rocks; magmas; upper crust; pyroclastics; calderas; olivine basalt; residence time
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-013-0767-5

The formation of shallow, caldera-sized reservoirs of crystal-poor silicic magma requires the generation of large volumes of silicic melt, followed by the segregation of that melt and its accumulation in the upper crust. The 21.8 ± 0.4-ka Cape Riva eruption of Santorini discharged >10 km 3 of crystal-poor dacitic magma, along with <<1 km 3 of hybrid andesite, and collapsed a pre-existing lava shield. We have carried out a field, petrological, chemical, and high-resolution 40 Ar/ 39 Ar chronological study of a sequence of lavas discharged prior to the Cape Riva eruption to constrain the crustal residence time of the Cape Riva magma reservoir. The lavas were erupted between 39 and 25 ka, forming a ∼2-km 3 complex of dacitic flows, coulées and domes up to 200 m thick (Therasia dome complex). The Therasia dacites show little chemical variation with time, suggesting derivation from one or more thermally buffered reservoirs. Minor pyroclastic layers occur intercalated within the lava succession, particularly near the top. A prominent pumice fall deposit correlates with the 26-ka Y-4 ash layer found in deep-sea sediments SE of Santorini. One of the last Therasia lavas to be discharged was a hybrid andesite formed by the mixing of dacite and basalt. The Cape Riva eruption occurred no more than 2,800 ± 1,400 years after the final Therasia activity. The Cape Riva dacite is similar in major element composition to the Therasia dacites, but is poorer in K and most incompatible trace elements (e.g. Rb, Zr and LREE). The same chemical differences are observed between the Cape Riva and Therasia hybrid andesites, and between the calculated basaltic mixing end-members of each series. The Therasia and Cape Riva dacites are distinct silicic magma batches and are not related by shallow processes of crystal fractionation or assimilation. The Therasia lavas were therefore not simply precursory leaks from the growing Cape Riva magma reservoir. The change 21.8 ky ago from a magma series richer in incompatible elements to one poorer in those elements is one step in the well documented decrease with time of incompatibles in Santorini magmas over the last 530 ky. The two dacitic magma batches are interpreted to have been emplaced sequentially into the upper crust beneath the summit of the volcano, the first (Therasia) then being partially, or wholly, flushed out by the arrival of the second (Cape Riva). This constrains the upper-crustal residence time of the Cape Riva reservoir to less than 2,800 ± 1,400 years, and the associated time-averaged magma accumulation rate to >0.004 km 3 year -1 . Rapid ascent and accumulation of the Cape Riva dacite may have been caused by an increased flux of mantle-derived basalt into the crust, explaining the occurrence of hybrid andesites (formed by the mixing of olivine basalt and dacite in approximately equal proportions) in the Cape Riva and late Therasia products. Pressurisation of the upper crustal plumbing system by sustained, high-flux injection of dacite and basalt may have triggered the transition from prolonged, largely effusive activity to explosive eruption and caldera collapse.