Multiple timescale constraints for high-flux magma chamber assembly prior to the Late Bronze Age eruption of Santorini (Greece)

Abstract

The rhyodacitic magma discharged during the 30-80 km 3 DRE (dense rock equivalent) Late Bronze Age (LBA; also called 'Minoan') eruption of Santorini caldera is known from previous studies to have had a complex history of polybaric ascent and storage prior to eruption. We refine the timescales of these processes by modelling Mg-Fe diffusion profiles in orthopyroxene and clinopyroxene crystals. The data are integrated with previously published information on the LBA eruption (phase equilibria studies, melt inclusion volatile barometry, Mg-in-plagioclase diffusion chronometry), as well as new plagioclase crystal size distributions and the established pre-LBA history of the volcano, to reconstruct the events that led up to the assembly and discharge of the LBA magma chamber. Orthopyroxene, clinopyroxene and plagioclase crystals in the rhyodacite have compositionally distinct rims, overgrowing relict, probably source-derived, more magnesian (or calcic) cores, and record one or more crystallization (plag>>opx>cpx) events during the few centuries to years prior to eruption. The crystallization event(s) can be explained by the rapid transfer of rhyodacitic melt from a dioritic/gabbroic region of the subcaldera pluton (mostly in the 8-12 km depth range), followed by injection, cooling and mixing in a large contiguous melt lens at 4-6 km depth (the preeruptive magma chamber). Since crystals from all eruptive phases yield similar timescales, the melt transfer event(s), the last of which took place less than two years before the eruption, must have involved most of the magma subsequently erupted. The data are consistent with a model in which prolonged generation, storage and segregation of silicic melts was followed by gravitational instability in the subcaldera pluton, causing the rapid interconnection and amalgamation of melt-rich domains. The melts then drained to the top of the pluton, at fluxes of up to 0.1-1 km 3 .y-1 , where steep vertical gradients of density and rheology probably caused them to inject laterally, forming a short-lived holding chamber prior to eruption. This interpretation is consistent with growing evidence that some large silicic magma chambers are transient features on geological timescales. A similar process preceded at least one earlier caldera-forming eruption on Santorini, suggesting that it may be a general feature of this rift-hosted magmatic system.

Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge

“…Current models for silicic caldera-forming systems involve trans-crustal magmatic systems that evolve significantly over time with rapid final stages of amalgamation preceding the Plinian caldera-forming eruption (Cashman et al, 2017). At Santorini, melt diffusion profiles in orthopyroxene and clinopyroxene crystals from the LBA rhyodacites indicate prolonged storage and segregation of melts in a sub-caldera pluton (8-12 km depth) prior to the LBA eruption (Flaherty, 2018). Since crystals from all eruptive phases yield similar timescales, the authors infer that, on the timescale of a few centuries to years, a shallow (4-6 km) short-lived chamber formed that held most of the magma erupted in the LBA eruption.…”

Magma Chamber Growth During Intercaldera Periods: Insights From Thermo‐Mechanical Modeling With Applications to Laguna del Maule, Campi Flegrei, Santorini, and Aso

“…The volume of the Minoan eruption ~3,600 ka indicates that the chamber grew to volumes of ~40–80 km 3 over ~18 kyr since the previous caldera‐forming eruption, implying a long‐term average magma supply rate of ~0.002–0.004 km 3 /year. However, approximately 7–15% of the erupted material is thought to be magma that recharged the system ~10–100 years prior (Druitt et al, ; Flaherty et al, ), which would imply a more episodic recharge at a rate of ~0.05 km 3 /year. Episodic recharge also is supported by the recent inflation and seismic unrest in 2011–2012, which suggested the arrival of ~0.01 km 3 of magma to a chamber at ~3‐ to 6‐km depth (Parks et al, ), giving a recharge rate of ~0.01 km 3 /year.…”

Rapid Magma Generation or Shared Magmatic Reservoir? Petrology and Geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

“…Globally, a similar record of cumulate "cannibalization" can be seen through bulk rock or mineral chemistry in zoned and unzoned ignimbrites, including the Ammonia Tanks Tuff (Deering et al, 2011), Peach Springs Tuff (Pamukcu et al, 2013), Tenerife (Sliwinski et al, 2015), Bishop Tuff (Evans and Bachmann, 2013;Chamberlain et al, 2014), Bandelier Tuff (Wilcock et al, 2012;Wolff et al, 2015), Kidnappers/Rocky Hill Ignimbrites (Cooper et al, 2017) and Campi Flegrei and Lipari (Forni et al, 2015(Forni et al, , 2018a. Elsewhere, evidence for high-temperature rejuvenation is seen in chemical diffusion profiles (e.g., Fe-Mg interdiffusion in orthopyroxene, Ti-in-quartz, Sr-in-plagioclase), where sub-decadal to millennial timescales are calculated (Druitt et al, 2012;Cooper et al, 2017;Flaherty et al, 2018).…”

“…Most agree that such magmas are dominantly derived from extensive, fractional-crystallization-driven, long-lived "mushy" reservoirs (Hildreth, 2004), as demonstrated by thermal models (Dufek and Bachmann, 2010;Gelman et al, 2013;Karakas et al, 2017), geochronology (Schmitt et al, 2010;Schoene et al, 2012;Wotzlaw et al, 2013;Barboni et al, 2015;Szymanowski et al, 2017) and petrology/geochemistry (Deering and Bachmann, 2010;Pamukcu et al, 2013;Ellis et al, 2014;Wolff et al, 2015;Holness et al, 2019). Such reservoirs, in addition to generating large-volume, crystal-rich ignimbrites, produce crystal-poor eruptible melt batches through slow extraction over 10's to 100's of kyr (Bachmann and Bergantz, 2004;Bachmann and Huber, 2018;Jackson et al, 2018), though many contend that such melt-dominant reservoirs are ephemeral, and the production of large volumes therefore necessitates rapid assembly from smaller melt lenses (Annen, 2009;Wilson and Charlier, 2009;Druitt et al, 2012;Allan et al, 2013;Caricchi et al, 2014;Wotzlaw et al, 2014;Cooper et al, 2017;Flaherty et al, 2018;Shamloo and Till, 2019). Such reservoirs may be prone to rapid cooling and storage in a sub-solidus state (Cooper and Kent, 2014) but the generation of hundreds to thousands of cubic kilometers of eruptible magma would then necessitate high magmatic fluxes (as opposed to incubation; see Mills and Coleman, 2013;Caricchi et al, 2014) in contradiction with results of thermo-mechanical models of magma chambers (Jellinek and DePaolo, 2003;Degruyter et al, 2016).…”