PULSED GARNET GROWTH AND DEHYDRATION DURING SUBDUCTION, SIFNOS, GREECE

Coupling zoned geochronology with thermodynamic modeling of garnet elucidates the nature and scale of progressive metamorphism and dehydration during subduction of a quartzofeldspathic lithology from Sifnos, Greece. Zoned garnet geochronology is used to determine the timing and duration of garnet growth (used as a proxy for associated dehydration). Two adjacent, large garnet porphyroblasts (~5cm and ~3.5cm) were microdrilled based on major element chemical contours. Ten and nine distinct growth zones of garnet, respectively, were separated for Sm-Nd geochronology. Garnet separates were acid-cleansed, yielding very low Nd concentrations (as low as 0.02 ppm) and very high ¹⁴⁷Sm/¹⁴³Nd (as high as 9.82), and analyzed with TIMS using a NdO⁺ with Ta₂O₅ method. Acid-cleansed garnet/inclusion “powders” from each zone were also used, when appropriately fitted on an isochron. Thermodynamic modeling, along a series of prescribed P-T paths, reveals the causes and characteristics of garnet growth. Direct comparison of observed garnet growth timing with modeled growth allows for determination of the subduction P-T path. We chose eight possible P-T paths and tested mineral and fluid evolution along each. Combining multi-point garnet-powder-matrix isochron ages with thermodynamic modeling, three distinct stages of garnet growth were defined: initiation of garnet growth at 53.4 ± 2.6 Ma (~0.75 GPa and ~300°C); a period of slow to no growth until a second stage, centered at 47.27 ± 0.16 Ma; a final growth pulse, incorporating the majority of garnet growth, between 45.30 ± 1.00 Ma and 45.49 ± 0.19 Ma (2.13-2.19 GPa and 490-550°C). This amounts to a > 2 order of magnitude increase in volumetric growth rate from early core growth to the final growth pulse. This final pulse occurred rapidly (< 0.83 My) during a period of relatively isobaric heating, at a corresponding heating rate > 75°C/My. Garnet growth appears to have terminated at roughly the maximum temperature attained. Bulk rock dehydration during garnet growth amounts to 0.55 wt.% H₂O loss, with the majority of H₂O release (0.4 wt.%) occurring during the final rapid growth pulse. This represents a rapid, focused pulse of metamorphism along the otherwise continuous process of subduction, and provides strong evidence for short (< 1 My) timescales of dehydration during subduction.