2:15 PM - 2:30 PM
[SMP28-15] Deformation and melt-rock interactions in the forearc mantle as evidenced by banded-dunite harzburgite in the Horoman peridotite, Japan
Keywords:peridotite, Deformation, melt-rock interactions
Fluids and melts derived from the subducting slab modify the rheological properties, microstructure, and chemical composition of the mantle wedge. To understand these processes, we conducted microstructural, mineralogical, and petrographic analysis using deformed, depleted peridotites composed of alternated dunite and harzburgite, which is named banded-dunite harzburgite (BDH), from the Horoman peridotite complex, Japan.
Our findings suggest that SiO2-rich melts migrate along pre-existing deformation textures in depleted forearc dunite and modify the deformation mechanism. Key observations include: (1) Orientation of lithological boundary between dunite and pyroxene-bearing layers are parallel to the foliation, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the deformed dunite, (2) a diffusive trend in pyroxene/olivine volume fraction and whole-rock SiO2 wt.% across the layers, (3) olivine CPOs change from an initial A-type CPO in a melt-free system to a later weak AG-type CPO in a melt-bearing system, (4) cuspate pyroxene and pargasite formed during the melt-rock interaction, and (5) Grain size reduction due to the melt-rock reaction, leading to a change in the deformation mechanism from dislocation creep in the melt-free system to diffusion creep (dissolution-precipitation creep) in the melt-bearing system.
The viscosity of the fine-grained pyroxene-bearing layers resulting from the melt-rock reaction is significantly lower than that of the initial dunite layer, which causes strain to be localized in these reactive layers. It is reasonable to hypothesize that such process occurs multiple times within the forearc mantle wedge. Consequently, substantial rheological contrasts are the result of such melt-rock interactions due to spatial variations in the melt.
Our findings suggest that SiO2-rich melts migrate along pre-existing deformation textures in depleted forearc dunite and modify the deformation mechanism. Key observations include: (1) Orientation of lithological boundary between dunite and pyroxene-bearing layers are parallel to the foliation, and the shape and crystallographic preferred orientations (SPOs and CPOs) of olivine in the deformed dunite, (2) a diffusive trend in pyroxene/olivine volume fraction and whole-rock SiO2 wt.% across the layers, (3) olivine CPOs change from an initial A-type CPO in a melt-free system to a later weak AG-type CPO in a melt-bearing system, (4) cuspate pyroxene and pargasite formed during the melt-rock interaction, and (5) Grain size reduction due to the melt-rock reaction, leading to a change in the deformation mechanism from dislocation creep in the melt-free system to diffusion creep (dissolution-precipitation creep) in the melt-bearing system.
The viscosity of the fine-grained pyroxene-bearing layers resulting from the melt-rock reaction is significantly lower than that of the initial dunite layer, which causes strain to be localized in these reactive layers. It is reasonable to hypothesize that such process occurs multiple times within the forearc mantle wedge. Consequently, substantial rheological contrasts are the result of such melt-rock interactions due to spatial variations in the melt.