5:15 PM - 6:45 PM
[SMP24-P01] Deformation and melt-rock interaction in the upper mantle: Insight from Horoman Peridotite Complex, Hokkaido, Japan
Keywords:mantle, peridotite, Horoman peridotite, deformation, crystallographic preferred orientation , EBSD
Deformation enhances the melt migration and controls the melt distribution. The previous experimental studies on partially molten peridotite show that shear deformation produces melt-rich bands due to stress gradients. They also demonstrate that the crystallographic preferred orientation (CPO) of olivine crystals derived from deformed olivine aggregates in melt-bearing systems differ from that formed in melt-free systems.
The upper zone of the Horoman peridotite complex has a layering structure of peridotite and mafic rocks. The width of the layering structure varies from a few millimeters to a few meters. We have studied an outcrop, which is located on the northern ridge of Mt. Apoi and composed of three types of rocks: spinel harzburgite-lherzolite, plagioclase lherzolite, and mafic rocks. We aim to clarify the deformation and melt-rock interaction in the upper mantle based on structural petrological features of this outcrop.
Microstructural analysis suggests that these rocks are fromed by deformation mainly by dislocation creep. Electron backscattered diffraction (EBSD) analysis shows that olivine has a strong shape preferred orientation (SPO), and the value of J-index, which represents the concentration of crystallographic orientation, is low compared to other peridotite massifs. Two types of olivine CPO fabrics are observed. One is the A-type CPO fabric with [100] parallel to the lineation and [010] normal to the foliation. The other is the AG-type CPO fabric with [010] normal to the foliation, and [100] and [001] on the foliation, showing girdle distribution. The olivine CPO fabrics of peridotite and mafic rocks vary depending on the modal composition of the minerals, suggesting that the concentrations of melts affect the development of CPO. The grains of orthopyroxene, plagioclase, and pargasite have an interstitial shape with distinct CPO and SPO. These features suggest that the melt migration and the deformation occur simultaneously. Micro-XRF analysis shows two types of clinopyroxene with different chemical zonal structures, suggesting chemical interaction between melts and rocks. Based on these results, we propose that the layering structure of the studied outcrop records reactions between a basaltic melt and a mantle peridotite possibly beneath a mid-ocean ridge. Parallelism observed at the boundaries of the rock layers, foliation of the deformed peridotite, SPO and CPO of the minerals indicate that these layered rocks with peridotite and mafic rocks were formed by the identical deformation event. Considering the stability field of pargasite and spinel, solidus temperature of hydrous peridotite, we propose that these rocks record deformation at temperatures of about 1050–1150°C and pressures of about 0.7–1.5 GPa.
The upper zone of the Horoman peridotite complex has a layering structure of peridotite and mafic rocks. The width of the layering structure varies from a few millimeters to a few meters. We have studied an outcrop, which is located on the northern ridge of Mt. Apoi and composed of three types of rocks: spinel harzburgite-lherzolite, plagioclase lherzolite, and mafic rocks. We aim to clarify the deformation and melt-rock interaction in the upper mantle based on structural petrological features of this outcrop.
Microstructural analysis suggests that these rocks are fromed by deformation mainly by dislocation creep. Electron backscattered diffraction (EBSD) analysis shows that olivine has a strong shape preferred orientation (SPO), and the value of J-index, which represents the concentration of crystallographic orientation, is low compared to other peridotite massifs. Two types of olivine CPO fabrics are observed. One is the A-type CPO fabric with [100] parallel to the lineation and [010] normal to the foliation. The other is the AG-type CPO fabric with [010] normal to the foliation, and [100] and [001] on the foliation, showing girdle distribution. The olivine CPO fabrics of peridotite and mafic rocks vary depending on the modal composition of the minerals, suggesting that the concentrations of melts affect the development of CPO. The grains of orthopyroxene, plagioclase, and pargasite have an interstitial shape with distinct CPO and SPO. These features suggest that the melt migration and the deformation occur simultaneously. Micro-XRF analysis shows two types of clinopyroxene with different chemical zonal structures, suggesting chemical interaction between melts and rocks. Based on these results, we propose that the layering structure of the studied outcrop records reactions between a basaltic melt and a mantle peridotite possibly beneath a mid-ocean ridge. Parallelism observed at the boundaries of the rock layers, foliation of the deformed peridotite, SPO and CPO of the minerals indicate that these layered rocks with peridotite and mafic rocks were formed by the identical deformation event. Considering the stability field of pargasite and spinel, solidus temperature of hydrous peridotite, we propose that these rocks record deformation at temperatures of about 1050–1150°C and pressures of about 0.7–1.5 GPa.