I see seas of blue (and red garnets too) : investigations into the subduction and metamorphism of the eclogite and blueschists of the Greek Cyclades

Abstract

Overall, these findings are consistent with the recent identification of overstepped garnet nucleation on the island of Sifnos. Taken together, these studies suggest that all metamorphic petrology research reliant on traditional chemical equilibrium techniques, not just work in the Cyclades, must be reevaluated for the potential of overstepped garnet nucleation.

QuiG and ZiR reveal that garnet in all six samples nucleated and grew at overstepped conditions. Additionally, it was determined that applying the equilibrium core isopleth method to overstepped garnets retrieves spurious P–T conditions. Synthesizing QuiG and ZiR results with maximum driving force (MDF) thermodynamic modeling reveal a range of possible prograde paths. The presence of lawsonite PM in LMA-00-12b and LMA-00-20a necessitate a relatively cold P–T path that allowed for nucleation, growth and decomposition of lawsonite before garnet nucleation. The lack of any petrographic evidence for lawsonite having been present in the remaining samples suggest a warmer P–T path that either entirely bypassed lawsonite stability or was not in the lawsonite field long enough to nucleate the phase. Additionally, the presence of a wide variety of peak conditions across the island suggest that exhumation was not coherent and facilitated by a rheologically weak mélange.

Similar methods were used to determine the P–T conditions of garnet nucleation and growth for six garnet-bearing blueschists from the Cycladic blueschist unit, Syros, Greece. Samples LMA-00-12b, LMA-00-16a and JWO-20a are metapelitic blueschists, while LMA-00-20a and SYR-127 are mafic blueschists. SYR-11D is a mafic eclogite. Garnet in all samples nucleated at essentially the same temperature (~520-550 °C) but at different pressures. Garnet in LMA-00-12b, LMA-00-16a and JWO-20a nucleated at 17 kbar, 11.5 kbar and 13.5 kbar, respectively. Garnet in SYR-11D, LMA-00-20a and SYR-127 nucleated at 13.5 kbar, 16.8 kbar and 19 kbar, respectively.

Modeling the modal evolution of mineral assemblages along a prograde path while allowing for the overstepped (OS) nucleation and growth of garnet shows that the bulk of devolatilization occurred post-Lws decomposition. The breakdown of chlorite (both stable and metastable) and the OS nucleation of Grt, not Lws decomposition, are responsible for the majority of fluid generation. Integrating these results with a subduction zone thermotectonic model shows that prograde devolatilization is pulsed. Lawsonite-dominated devolatilization (with some chlorite decomposition) occurs over 40–50 km depth and generates ~16-30% of the total fluid, while metastable chlorite decomposition and associated garnet nucleation and growth occurs at ~54 km depth and produces ~60–84% of the devolatilizing fluid. This study helps constrain the origin of slab generated fluids and suggests that geochemical and geophysical studies of subduction zones that rely on tradition equilibrium thermobarometry must be reevaluated.

Equilibrium thermodynamic modeling, quartz in garnet (QuiG) Raman geobarometry, and modeling of garnet nucleation at overstepped conditions were applied to three garnet-bearing blueschists from a 1.5 km long transect across the eclogite-blueschist unit in Sifnos, Greece in order to evaluate the accuracy of P–T conditions calculated via equilibrium thermodynamics. Raman inclusion barometry relies on mechanical equilibrium between the host-inclusion pair and is independent of chemical equilibrium. Garnet nucleation temperatures were obtained from Zr in rutile thermometry. These conditions were then compared to P–T conditions calculated at the equilibrium garnet isograd and the method of intersecting core isopleths.

In all three cases, P–T conditions calculated with intersecting garnet core isopleths retrieve spurious P–T conditions that are either too hot (SPH99-1a), too cold (SPH99-7) or too shallow (SPH99-5). Additionally, nucleation of garnet in all samples is shown to have occurred at overstepped conditions ranging from ~6-15 kbar above the garnet-in isograd. Nucleation of garnet in SPH99-1a at 12 kbar and ~484 °C requires overstepping of ~6 kbar and a reaction affinity of 2.2 kJ/mol O. SPH99-5 requires overstepping of ~8 kbar with garnet reaction affinities of at least 2.0 kJ/mol O at 15 kbar and ~520 °C. SPH99-7 requires overstepping of approximately 15 kbar and affinities of about 2.0-2.4 kJ/mol O at ~23 kbar and ~530 °C. The geotherms calculated from SPH99-7 (~6.7 °C/km) and SPH99-5 (9.8 °C/km) are in accordance with previous studies. The ~10 kbar pressure difference between SPH99-7 and SPH99-1a implies significant crustal imbrication and can be explained by thrusting and accretion of thin slices of underplated wedge material facilitated by slab rollback and gravitational collapse.

Quartz-in-garnet inclusion barometry, zirconium-in-rutile thermometry, thermodynamic modeling and major element stranded diffusion modeling have been applied to a metabasalt from the eclogite-blueschist unit on Sifnos, Greece. Results indicate that garnet nucleation has been overstepped relative to the equilibrium garnet isograd by ~7 kbar and 30°C. Models that incorporate abrupt garnet nucleation and growth after significant overstepping reveal substantial volatile release at maximum depth. Modeling of the diffusive relaxation of growth zoning in garnet suggests <1 myr time scales for devolatilization. These inferred conditions of garnet nucleation and growth are consistent with relatively warm (8-10 °C/km) subduction zone thermal models and suggest that the bulk of dewatering of metabasaltic rocks occurs below the mantle wedge and at the base of the seismic-aseismic transition. Dehydration of chlorite to garnet after considerable overstepping is proposed as the primary volatile producing reaction in Sifnos metabasites. This reaction may be responsible for both the seismic disaggregation of the upper surface of the subducting slab and the transfer of rocks to the exhuming subduction channel.

QuiG, ZiR, and MDF modeling were also employed to constrain the relative order and pressure–temperature (P–T) conditions of lawsonite and garnet porphyroblast growth/decomposition in two samples from the Cycladic blueschist complex, Syros, Greece. Samples LMA-00-12b and LMA-00-20a are garnet bearing blueschists that contain epidote + white mica pseudomorphs (PM) after lawsonite. Garnet in LMA-00-12b occurs in both the pseudomorph and the matrix, while garnet in LMA-00-20a occurs only in the matrix. QuiG and ZiR indicate that garnet in LMA-00-12b nucleated at ~17 kbar and 540 °C, overstepping equilibrium conditions by a maximum of ~15 °C and < 1 kbar. This is near-coincident with the lawsonite out reaction. Garnet in LMA-00-20 nucleated at 15.5 kbar and 550 °C, ~30 °C hotter than the lawsonite-out reaction.