Devolatilization of Subducting Slabs, Part I: Thermodynamic Parameterization and Open System Effects

Abstract

The amount of H 2 O and CO 2 that is carried into deep mantle by subduction beyond subarc depths is of fundamental importance to the deep volatile cycle but remains debated. Given the large uncertainties surrounding the spatio-temporal pattern of fluid flow and the equilibrium state within subducting slabs, a model of H 2 O and CO 2 transport in slabs should be balanced between model simplicity and capability. We construct such a model in a two-part contribution. In this Part I of our contribution, thermodynamic parameterization is performed for the devolatilization of representative subducting materials-sediments, basalts, gabbros, peridotites. The parameterization avoids reproducing the details of specific devolatilization reactions, but instead captures the overall behaviors of coupled (de)hydration and (de)carbonation. Two general, leading-order features of devolatilization are captured: (1) the released volatiles are H 2 O-rich near the onset of devolatilization; (2) increase of the ratio of bulk CO 2 over H 2 O inhibits overall devolatilization and thus lessens decarbonation. These two features play an important role in simulation of volatile fractionation and infiltration in thermodynamically open systems. When constructing the reactive fluid flow model of slab H 2 O and CO 2 transport in the companion paper Part II, this parameterization can be incorporated to efficiently account for the open-system effects of H 2 O and CO 2 transport.

Devolatilization of Subducting Slabs, Part II: Volatile Fluxes and Storage

“…Moreover, elevated porosity levels reflect higher extent of devolatilization. According to our parameterization in the Tian et al (2019) Part I, higher extent of devolatilization corresponds to higher CO 2 and lower H 2 O concentrations in the coexisting liquid phase, which are also evident at ∼2.5 km depth in Figures 4c and 4d. Upward infiltration of these fluids, coupled with the fractionation effect, gives rise to the diminished volatile loss immediately above the region of enhanced decarbonation (blue stripes ∼2 km deep in Figure 5).…”

“…which is used as input for the thermodynamic module from the Tian et al (2019) Part I (the subscript "th" indicates input variables for the thermodynamic module):…”

“…The solid phase in the slab is lithologically layered, as shown in Figure b and consists of sediments, MORBs, gabbros, and peridotites from top to bottom. In the Tian et al () Part I, we parameterize the equilibrium partitioning of H 2 O and CO 2 between the liquid and solid for each of the four representative lithologies; this parameterization serves as a thermodynamic module coupled to the fluid flow model detailed below.…”

“…Since fluid movement constantly changes the bulk composition of each subdomain within the modeled slab, the computational cost may be prohibitively high if a traditional phase diagram calculation is applied to each subdomain repeatedly throughout the model evolution. In the Tian et al () Part I, we have parameterized the coupled dehydration and decarbonation processes for representative subducting lithologies (i.e., sediments, mid‐ocean ridge basalt [MORB], gabbro, and peridotite). This light‐weight thermodynamic module focuses on the behaviors of H 2 O and CO 2 and can readily capture the fractionation and infiltration effects typical in open systems.…”

“…We note here that the liquid volatile phase in the model is a molecular fluid residing in the H 2 O–CO 2 binary, so it excludes other carbon species from consideration. The limitation of this assumption is discussed in section and in the Tian et al () Part 1.…”