講演情報
[SMP31-P15]Deep deformation environment at the 25°S OCC inferred from phlogopite-bearing peridotite
*曽田 祐介1、永冶 方敬1、澤口 隆2、水上 知行3、森下 知晃3、熊谷 英憲4、ウォリス サイモン5 (1.早稲田大学、2.東洋大学、3.金沢大学、4.学習院女子大学、5.東京大学)
キーワード：
OCC、フィロゴパイトかんらん岩、かんらん石、CPO
Peridotite from an oceanic core complex (OCC) can provide important information on fault strength and fluid circulation at depth. Phlogopite-bearing peridotite was collected from the 25°S OCC (Prince Albert II Sea Mount), located near the southern termination of the Central Indian Ridge near the Rodrigues Triple Junction, during the YK05-16 research cruise undertaken by the RV Yokosuka in January 2006 (Kumagai et al., 2008; Morishita et al, 2009).
The sample of phlogopite-bearing peridotite is small (<10 × 10 cm) and comprises peridotite cut by a gabbroic vein that is completely altered to amphibole-rich rock composed of tremolite–actinolite and chlorite. The shape-preferred orientation of these acicular minerals defines a foliation and lineation. The studied thin-sections were oriented normal and parallel to the foliation and lineation, respectively.
Primary minerals in the phlogopite-bearing peridotite are olivine, orthopyroxene, chromite, and phlogopite. Olivine has a range of occurrences and is divided into coarse- and fine-grained olivine. The coarse-grained olivine (1.0–0.1 mm) has a shape-preferred orientation that defines a mylonitic foliation together with orthopyroxene, which is commonly observed in mantle peridotite. The mylonitic foliation is oblique to the foliation in the altered gabbroic vein. The fine-grained olivine (<20 µm) resulted from grain-size reduction occurs at the boundaries of coarse-grained olivine or in the necks of boudinaged grains. Electron Backscatter Diffraction (EBSD) measurements show the coarse-grained olivine has a D type fabric (Jung et al., 2006, Karato et al., 2008) and AG type fabric (Mainprice, 2015), whereas the fine-grained olivine has an A type fabric (Jung et al., 2006). The relationship between foliation and lineation development, and the deformation of coarse- and fine-grained olivine is unclear. However, the two types of olivine have distinct fabrics.
The phlogopite in the central part of the peridotite has a random orientation of (001), whereas the phlogopite located near the gabbroic vein has (001) oriented parallel to the foliation in the vein. Some phlogopite fills fractures in the coarse-grained olivine; however, the phlogopite does not occur as “mica-fish” within aggregates of recrystallized olivine. These observations indicate that the phlogopite crystallized after the plastic deformation of olivine.
Based on the occurrences of the olivine and phlogopite, we propose that the coarse-grained olivine records deformation in the mantle and the fine-grained olivine records deformation during the formation of the OCC.
Fluid circulation occurs along the detachment fault in an OCC (e.g., Boschi et al., 2006). This fluid can transport the silica and potassium needed for the growth of phlogopite. The A type fabric of olivine, as observed in this study, indicates the deformation occurred under dry conditions. Therefore, the studied sample indicates that fluid did not reach the root zone of the OCC detachment fault.
The sample of phlogopite-bearing peridotite is small (<10 × 10 cm) and comprises peridotite cut by a gabbroic vein that is completely altered to amphibole-rich rock composed of tremolite–actinolite and chlorite. The shape-preferred orientation of these acicular minerals defines a foliation and lineation. The studied thin-sections were oriented normal and parallel to the foliation and lineation, respectively.
Primary minerals in the phlogopite-bearing peridotite are olivine, orthopyroxene, chromite, and phlogopite. Olivine has a range of occurrences and is divided into coarse- and fine-grained olivine. The coarse-grained olivine (1.0–0.1 mm) has a shape-preferred orientation that defines a mylonitic foliation together with orthopyroxene, which is commonly observed in mantle peridotite. The mylonitic foliation is oblique to the foliation in the altered gabbroic vein. The fine-grained olivine (<20 µm) resulted from grain-size reduction occurs at the boundaries of coarse-grained olivine or in the necks of boudinaged grains. Electron Backscatter Diffraction (EBSD) measurements show the coarse-grained olivine has a D type fabric (Jung et al., 2006, Karato et al., 2008) and AG type fabric (Mainprice, 2015), whereas the fine-grained olivine has an A type fabric (Jung et al., 2006). The relationship between foliation and lineation development, and the deformation of coarse- and fine-grained olivine is unclear. However, the two types of olivine have distinct fabrics.
The phlogopite in the central part of the peridotite has a random orientation of (001), whereas the phlogopite located near the gabbroic vein has (001) oriented parallel to the foliation in the vein. Some phlogopite fills fractures in the coarse-grained olivine; however, the phlogopite does not occur as “mica-fish” within aggregates of recrystallized olivine. These observations indicate that the phlogopite crystallized after the plastic deformation of olivine.
Based on the occurrences of the olivine and phlogopite, we propose that the coarse-grained olivine records deformation in the mantle and the fine-grained olivine records deformation during the formation of the OCC.
Fluid circulation occurs along the detachment fault in an OCC (e.g., Boschi et al., 2006). This fluid can transport the silica and potassium needed for the growth of phlogopite. The A type fabric of olivine, as observed in this study, indicates the deformation occurred under dry conditions. Therefore, the studied sample indicates that fluid did not reach the root zone of the OCC detachment fault.