LAWSONITE REVISITED: INSIGHTS FROM INTEGRATED QUIG RAMAN BAROMETRY, TRACE ELEMENT THERMOMETRY, AND OS THERMODYNAMIC MODELLING OF GARNET INCLUSIONS IN LAWSONITE PSEUDOMORPHS FROM SYROS, GREECE

The presence of Grt porphyroblasts in Lws pseudomorphs (PM) in the blueschists of Syros, Greece is a longstanding conundrum, as equilibrium calculations show that Lws should nucleate before Grt is stable. Determining the relative order and pressure-temperature (P-T) conditions of porphyroblast growth/decomposition is key to understanding the dynamics of dewatering during Cycladic subduction, as both Grt growth and Lws breakdown produce volatiles. To that end, we applied quartz in garnet (QuiG) Raman barometry and overstepped (OS) thermodynamic modelling to two of the Lws PM bearing blueschists. Calibration of the QuiG barometer was based on new piston cylinder experiments; temperature estimates were obtained via Zr in rutile trace element thermometry and Fe/Fe+Mg core isopleths. Stability of the Grt protoassemblage and Grt nucleation affinities were determined from mineral assemblage diagram calculations. The P-T conditions of Lws decomposition were determined by the intersection of the Lws-out reaction and possible prograde P-T paths.

LMA-12b is a metapelite with the assemblage Pg, Ep, Qtz, Grt, Ttn, Ap, and Gln. LMA-20a, a metabasalt, has a similar matrix assemblage with the addition of Omp and Chl. Lws PMs are composed primarily of Ep and white mica. Grt in LMA-12b occurs in both PMs and matrix. Matrix Grt is mostly chloritized; PM-Grt is up to 1 mm in diameter and may be partially chloritized. PM-Grt is typically poikiloblastic with a protoassemblage of Qtz+Gln+Chl+Cld+Ms. Grt in LMA-20a occurs only in the matrix and is largely pristine. It ranges from ~0.5-3.0 mm in diameter with a protoassemblage of Qtz+Gln+Chl+Cld+Zo+Ms. Grt in LMA-12b nucleated at 590 °C, 17.3 kbar while Grt in LMA-20a nucleated at 555 °C, 15 kbar. In both cases, garnet nucleated at temperatures ≥100 °C hotter than the equilibrium Lws-out reaction.

Results suggest Grt nucleation and growth and the bulk of devolatilization occurred post-Lws decomposition, implying major-element intergranular diffusion fast enough for garnet growth within PMs. Pristine Grt inclusions in the core of PMs suggest they were partially protected from greenschist facies alteration. This study also implies that the breakdown of Chl (both stable and metastable) and the OS nucleation of Grt, not Lws decomposition, are responsible for the bulk of fluid generation.