Thermal effects of variable material properties and metamorphic reactions in a three-component subducting slab

TY - JOUR

T1 - Thermal effects of variable material properties and metamorphic reactions in a three-component subducting slab

AU - Chemia, Zurab

AU - Dolejš, David

AU - Steinle-Neumann, Gerd

PY - 2015/10/1

Y1 - 2015/10/1

N2 - We explore the effects of variable material properties, phase transformations, and metamorphic devolatilization reactions on the thermal structure of a subducting slab using thermodynamic phase equilibrium calculations combined with a thermal evolution model. The subducting slab is divided into three layers consisting of oceanic sediments, altered oceanic crust, and partially serpentinized or anhydrous harzburgite. Solid-fluid equilibria and material properties are computed for each layer individually to illustrate distinct thermal consequences when chemical and mechanical homogenization within the slab is limited. Two extreme scenarios are considered for a newly forming fluid phase: complete retention in the rock pore space or instantaneous fluid escape due to porosity collapse. Internal heat generation or consumption due to variable heat capacity, compressional work, and energetics of progressive metamorphic and devolatilization reactions contribute to the thermal evolution of the slab in addition to the dominating heat flux from the surrounding mantle. They can be considered as a perturbation on the temperature profile obtained in dynamic or kinematic subduction models. Our calculations indicate that subducting sediments and oceanic crust warm by 40 and 70°C, respectively, before the effect of wedge convection and heating is encountered at 1.7 GPa. Retention of fluid in the slab pore space plays a negligible role in oceanic crust and serpentinized peridotites. By contrast, the large volatile budget of oceanic sediments causes early fluid saturation and fluid-retaining sediments cool by up to 150°C compared to their fluid-free counterparts. Key Points Definition of three-component slab: sediments, oceanic crust, and harzburgite Energy budget of subducting slab components Effects of variable material properties and metamorphic reactions in subduction.

AB - We explore the effects of variable material properties, phase transformations, and metamorphic devolatilization reactions on the thermal structure of a subducting slab using thermodynamic phase equilibrium calculations combined with a thermal evolution model. The subducting slab is divided into three layers consisting of oceanic sediments, altered oceanic crust, and partially serpentinized or anhydrous harzburgite. Solid-fluid equilibria and material properties are computed for each layer individually to illustrate distinct thermal consequences when chemical and mechanical homogenization within the slab is limited. Two extreme scenarios are considered for a newly forming fluid phase: complete retention in the rock pore space or instantaneous fluid escape due to porosity collapse. Internal heat generation or consumption due to variable heat capacity, compressional work, and energetics of progressive metamorphic and devolatilization reactions contribute to the thermal evolution of the slab in addition to the dominating heat flux from the surrounding mantle. They can be considered as a perturbation on the temperature profile obtained in dynamic or kinematic subduction models. Our calculations indicate that subducting sediments and oceanic crust warm by 40 and 70°C, respectively, before the effect of wedge convection and heating is encountered at 1.7 GPa. Retention of fluid in the slab pore space plays a negligible role in oceanic crust and serpentinized peridotites. By contrast, the large volatile budget of oceanic sediments causes early fluid saturation and fluid-retaining sediments cool by up to 150°C compared to their fluid-free counterparts. Key Points Definition of three-component slab: sediments, oceanic crust, and harzburgite Energy budget of subducting slab components Effects of variable material properties and metamorphic reactions in subduction.

KW - subduction paths

KW - slab lithology

KW - metamorphic reactions

KW - thermal state of slab

KW - devolatilization

U2 - 10.1002/2015jb012080

DO - 10.1002/2015jb012080

M3 - Journal article

SN - 0148-0227

VL - 120

SP - 6823

EP - 6845

JO - Journal of Geophysical Research

JF - Journal of Geophysical Research

IS - 10

ER -