APATITE FROM THE SUBDUCTION SEISMOGENIC ZONE RECORDS A RANGE OF CHEMICAL AND MECHANICAL PROCESSES

Subduction zone deformation, is intimately linked to dehydration reactions, fluid circulation, and chemical transformations. Apatite petrochronology is an excellent tool to date the timing of fluid infiltration and resultant deformation in the seismogenic zone since it deforms and recrystallizes under a specific range of P/T conditions, it has a closure temperature for Pb consistent with temperatures at the base of the subduction seismogenic zone, and it has the ability to record chemical processes. We leveraged these characteristics for apatite and conducted microstructural analyses from blueschist and greenschist samples from a subduction complex on Andros Island, Greece. Samples show discrete deformation events and deformation within apatite appears to be controlled by the surrounding mineral phases. The blueschist contains elongated apatite grains aligned parallel to the main foliation regardless of the textural setting. However, apatite within the mica-rich matrix is elongated with the c-axis parallel to foliation and preserve minimal internal lattice bending that we interpret as evidence of dissolution-reprecipitation creep. In contrast, apatite grains surrounded by radial chlorite and glaucophane deform through dislocation accommodated deformation with lattice bending and low-angle subgrain boundaries. Random crystallographic orientations might indicate micro-faulting, which could be related to strain hardening and localization of deformation. In the greenschist, syn-kinematic apatite within chlorite matrix display little or no internal deformation, likely deforming by dissolution-reprecipitation creep, suggesting that chlorite facilitated fluid flow. Apatite between clinozoisite and albite preserve multiple deformation mechanisms, with evidence of dislocation creep in portions of the grains in contact with the harder phases and strain free potions of the grains in contact with the matrix. Microstructural analyses provide the framework for future geochronologic and geochemical analyses of structural domains formed during multiple episodes of deformation and fluid pulses. This work highlights that apatite may record different phases of fluid flow and deformation during subduction and we may be able to get chronologic and petrogenic constraints on these processes.