Jen Gorce

I am a PhD candidate at Virginia Tech. My work seeks to better understand subduction zone processes through a combination of fieldwork, thermodynamic modeling, geochronology, and petrography. Currently, my work broadly focuses through the exploration of several questions. 1. What do the P-T-t paths of subducted lithologies tell us about the behavior of the subduction channel and overlying mantle wedge? 2. How can we liberate carbon from the subducting lithosphere and what does this mean for the flux of carbon between the interior and exterior of the planet? And 3. Is thermodynamic disequilibrium relevant to subduction zones and how do we deal with it?


  Why is subduction important?

  P-T-t paths of subducted lithologies

  Liberation of carbon from the subducting lithosphere

  Disequilibria in subduction

Jen:    CV   email

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Abstracts

Gorce, J.S., Caddick, M.J., Baxter, E.F., Ashley, K.T., Kendall, J.A., Dragovic, B., Brooks, H.L., Ramos, E.J., 2015. P-T paths from Syros, Greece, and constraints on subduction zone fluid generation. Geological Society of America Annual Meeting, Baltimore, MD, 2015. (poster)

Kendall, J.A., Baxter, E.F., Caddick, M.J., Gorce, J.S., Ramos, E.J., Brooks, H.L., 2015. Samarium/Neodymium garnet geochronology of eclogites from Syros, Greece. Geological Society of America Annual Meeting, Baltimore, MD, 2015. (poster)

Ramos, E.J., Baxter, E.F., Caddick, M.J., Kendall, J.A., Gorce, J.S., Brooks, H.L., 2015. Thermodynamic analysis of blueschist links garnet growth to progressive subduction zone dehydration in the cycladic blueschist unit of Syros, Greece. Geological Society of America Annual Meeting, Baltimore, MD, 2015. (poster)

Gorce, J.Geoscience Student Research Symposium (GSRS) - Towards a better understanding of subduction zone processes: constraining the histories of Blueschist rocks from Syros, Greece (2014)

Why is subduction important?

Subduction zones are some of the most well studied and most important convergent plate tectonic margins. Because subduction zones are the only geological environment in which material from the exterior of the planet is transported to the earth’s interior, the processes that occur along subduction zones have important implications for elemental cycles, geodynamics, mineral phase equilibra and mass transport of materials. The cold, dense lithosphere that gets subducted into the interior of the earth experiences prograde metamorphism that results in the breakdown of volatile bearing minerals and produces anhydrous minerals plus fluid (Bebout 1991, Peacock 1993, Kerrick and Connolly 2001). Quantifying the nature of fluids in subduction zones is crucial to understanding tectonic processes such as slab metamorphism, volatile release, mantle wedge dynamics, and metasomatism. Thus, understanding the location and timing of these dehydration reactions and their subsequent interactions with the subduction system is critically important for constraining other geological processes.

P-T-t paths of subducted lithologies

Numerical models predict that material can move freely at the interface between the subducting slab and the overlying mantle wedge (mélange zone) independent of the motion of the subducting slab. The implication of this is that blocks of subducted material now exposed in adjacent outcrops could have each experienced unique and very complex Pressure-Temperature-time (P-T-t) paths, resulting from the cycling and recycling of subducted material within the mélange zone. Such behavior can affect the expulsion and retention of fluid during metamorphism, depending on the physical properties and location of the block in the mélange zone.

The island of Syros, Greece preserves rocks that experienced blueschist–eclogite grade metamorphism during subduction. Preliminary observations suggest that samples from Syros preserve compositions and textures that record evidence for complex P-T-t paths. While thermodynamic modeling has been used to constrain the pressure and temperature conditions of these rocks, Sm/Nd geochronology will be used to constrain the temporal evolution of these rocks.

Liberation of carbon from the subducting lithosphere

The presence of carbon bearing volatiles like CO2 and CH4 at subduction related volcanic arcs has been attributed to the release of carbon in subducted lithologies (Sanos and Williams 1996, Marty and Tostikhin 1998). Furthermore, the presence of CO2-H2O-NaCl bearing fluid inclusions found in subducted rocks (Manning, 2004) provides us with direct evidence for complex fluid compositions in the subducting system. However, experimental data (Yaxley and Green 1994, Molina and Poli 2000) and thermodynamic models (Baxter and Caddick, 2013) predict that the stability of carbonate minerals in the subducting slab extends deep into the inner earth.

Though carbonate dissolution is frequently cited as an importantly mechanism for carbon release, and numerous studies have attempted to quantify that release (i. e. Scambelluri & Philippot 2001, Gorman et al 2006), few studies have attempted to test the feasibility of this mechanism. In essence, how much additional fluid, and of what composition, is required to liberate carbon at some point along a subduction path?

Figure 1: XH2O-XCO2 pseudosection contoured for the value of (delta mcarb)/(delta mH2O) for a calculated bulk rock composition of a hydrated MORB at 1200K and 2.64GPa. A positive value implies that carbonate minerals will grow. A negative value implies that carbonate minerals will breakdown and carbon will be liberated from the subduction slab. A value of zero implies that there is no change in the amount of carbonate minerals along that contour. The slope of the line where (delta mcarb)/(delta mH2O) = 0 can be used to say something about the composition of the external fluid facilitating carbonate mineral breakdown.

Disequilibria in subduction

Observations made during the construction of P-T paths from Syros imply that these rocks were overstepped significantly. Differences between the location of thermodynamically-predicted garnet-in reactions and the P-T of initial garnet growth estimated by their compositions suggest reaction overstepping of 25-50 C, implying that these samples may have spent a sizable part of their history removed from sample-wide equilibrium. A similar phenomenon was noticed Sifnos, Greece. This neighboring island experienced a similar metamorphic history to Syros and garnets are reported to be overstepped by as much as 1GPa and 80C (Dragovic et al 2012). The equilibrium model for prograde metamorphism works on the assumption that rocks traveling through P-T space do so in a state of equilibrium. Essentially, the rate in which reactions in the rock occur is much faster that the rate of changing P-T conditions. However, in natural samples, a certain degree of departure from equilibrium is required for mineral reactions to process. This begs two important questions; 1. How far from equilibrium must rocks depart? 2. How does this affect how we interpret metamorphic rocks? Both questions are incredibly important to ask when studying high pressure assemblages from subducted lithologies because subduction processes have important implications for elemental cycles, geodynamics, mineral phase equilibra and mass transport of materials.