2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 306-15
Presentation Time: 9:00 AM-6:30 PM


BROOKS, Hanna L.1, LAMADRID, Hector M.2, CADDICK, Mark J.3, DRAGOVIC, Besim1, BODNAR, Robert J.1 and BAXTER, Ethan F.4, (1)Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, (2)Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, (3)Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, (4)Department of Earth and Environment, Boston University, 675 Commonwealth Ave, Boston, MA 02215, hannab93@vt.edu

Rock–fluid interactions in subduction zone settings have important implications for the metamorphic evolution of the subducting slab and overlying mantle. Hot fluids can dissolve and transport chemical species, and initiate and sustain metamorphic reactions. Sometimes only cryptic evidence of these fluids is recorded, but a small fraction of the fluid is often trapped as primary inclusions in mineral phases. Subsequent fluids that pass through fractures are potentially trapped as secondary inclusions. Primary and secondary fluid inclusions thus provide direct samples of the fluids that interacted with the rock during metamorphism, subject to assumptions about subsequent modification of the inclusions. Analysis of fluid inclusion chemistry and determination of the conditions of entrapment, combined with thermodynamic modeling of the stability of host phases, can provide a detailed history of these interactions.

Previous petrological and geochronological work on Sifnos and surrounding Aegean islands has provided insight into the metamorphic history of the Hellenic subduction zone, but temporal and compositional constraints on syn-subduction fluids are currently limited. This study thus focuses on two blueschist samples from Sifnos, Greece. The majority of the primary fluid inclusions examined are hosted in epidote, which mineral textures and thermodynamic modeling suggest is a relatively late-stage, retrograde phase. The fluid inclusions are thus interpreted to record high-pressure metamorphic fluids trapped during early stages of exhumation. Additionally, secondary, quartz-hosted inclusions containing halite daughter crystals are present. The halite crystals indicate that the initial fluid was a high salinity brine, containing at least 30 wt% NaCl equivalent. Raman spectroscopy and microthermometry provides information to constrain the P-T conditions of fluid trapping. Combining fluid inclusion data with thermodynamic modeling of major phase stability provides a more complete understanding of the P-T-t history of the system. By building models that predict the stable mineralogy along a subduction and exhumation path, and coupling these models with constructed isochores, inferences can be made about fluid origin, trapping location, and the chemical evolution of fluids through time.