15:45 〜 16:00
[SCG47-08] Early stages of serpentinization processes in the oceanic mantle inferred from the Salahi mantle section, the northern Oman Ophiolite
キーワード：蛇紋岩化作用、上部マントル、かんらん岩、オマーンオフィオライト、海洋リソスフェア
We examined the occurrence of serpentines and accessory minerals in the peridotites from the Salahi mantle section in order to understand the early stages of serpentinization processes in the oceanic mantle. The peridotite of Salahi mantle section contains various proportions of meshed and veined low-temperature serpentines such as lizardite and chrysotile in the entire area. Antigorite, a high-temperature serpentine, also occurs pervasively, but its frequency tends to decrease toward the northwestern part of the mantle section. Most of the antigorite occurs in veins with widths ranging from 0.1 mm to 3.0 mm. Low-temperature serpentine cuts the central part of the antigorite vein parallel to or crossing the vein. In addition, some antigorite veins cut the low-temperature serpentines.
In the serpentines (especially in the antigorite veins), magnetite occurs in patches or strings. It occurs in almost all rock samples, but its quantity and distribution are not uniform. Talc may replace the rim or the entirety of orthopyroxene to form bastite with serpentine. Tremolite and chlorite often occur near antigorite veins. They may aggregate with other minerals such as talc and antigorite. In addition, carbonate minerals (calcite, aragonite, and magnesite) occur throughout the area.
Brucite is rarely present in veins and is difficult to be identified under microscope. In a graph plotting Si versus Mg+Fe molecular ratios, the compositions of brucite, olivine, and serpentine minerals are arranged nearly in a straight line. Mesh serpentine has a wider compositional range than veined serpentine and tends to be enriched in Mg+Fe rather than Si, suggesting mixing with brucite. On the other hand, the veined antigorite is more enriched in Si than the ideal composition and may also contain Al. In serpentine in contact with olivine, the chemical composition suggests mixing of brucite and serpentine. In addition, chrysotile is formed from brucite and antigorite, and magnetite is formed from brucite and Si, suggesting that much of the brucite formed in the early stages of serpentinization may have been consumed by reactions.
Iron-rich olivine with veins of 0.02~0.3 mm is present in the olivine of the Salahi mantle section. In particular, it is often associated with antigorite veins. The Fo content of common olivine is about 90, whereas that of iron-rich olivine is 71-88. The antigorite veins are Mg-rich and Fe-poor in contact with Fe-rich olivine, suggesting interdiffusion of Fe and Mg across the interface. The Mg-Fe interdiffusion rate is higher at higher temperature, suggesting that the antigorite veins may have formed at high temperature or may have been heated after the formation of the antigorite veins.
The occurrence of antigorite and talc throughout the Salahi mantle section suggests that hydrothermal reactions occurred at temperatures around 300-700°C. The coexistence of olivine, antigorite, and tremolite suggests a possible hydrothermal reaction at 500-600°C. Furthermore, the presence of lizardite and chrysotile in all samples suggests that water infiltration occurred extensively at temperatures lower than 300°C. The cross cutting relation observed in the antigorite veins suggests a formation sequence of olivine, antigorite, chrysotile, magnetite, and carbonate minerals. Since the stable temperature decreases in this order, serpentinization is considered to have progressed with a gradual decrease in temperature. On the other hand, there are also antigorite veins that cut the lizardite mesh structure. Antigorite has higher silica activity than lizardite, suggesting the possibility of inflow of silica-rich fluids or additional heating processes.
Antigorite and talc occur less frequently in the northwestern parts of the Salahi mantle section. The boundary of paleo-ridge segment structure has been inferred at the southern end of the Salahi block, and the center of the ridge segment is estimated to have been located near the southern end of the Fizh block, north of the Salahi block, suggesting a relationship between the frequency of antigorite and talc and paleo-ridge segment structure. Near the end of the ridge segment, antigorite formed in the early stage of cooling due to seawater penetration deep into the mantle. On the other hand, the northwestern part of the Salahi body, which is closer to the segment center and deeper than the Moho, may have been unsuitable for antigorite formation due to high temperature.
In the serpentines (especially in the antigorite veins), magnetite occurs in patches or strings. It occurs in almost all rock samples, but its quantity and distribution are not uniform. Talc may replace the rim or the entirety of orthopyroxene to form bastite with serpentine. Tremolite and chlorite often occur near antigorite veins. They may aggregate with other minerals such as talc and antigorite. In addition, carbonate minerals (calcite, aragonite, and magnesite) occur throughout the area.
Brucite is rarely present in veins and is difficult to be identified under microscope. In a graph plotting Si versus Mg+Fe molecular ratios, the compositions of brucite, olivine, and serpentine minerals are arranged nearly in a straight line. Mesh serpentine has a wider compositional range than veined serpentine and tends to be enriched in Mg+Fe rather than Si, suggesting mixing with brucite. On the other hand, the veined antigorite is more enriched in Si than the ideal composition and may also contain Al. In serpentine in contact with olivine, the chemical composition suggests mixing of brucite and serpentine. In addition, chrysotile is formed from brucite and antigorite, and magnetite is formed from brucite and Si, suggesting that much of the brucite formed in the early stages of serpentinization may have been consumed by reactions.
Iron-rich olivine with veins of 0.02~0.3 mm is present in the olivine of the Salahi mantle section. In particular, it is often associated with antigorite veins. The Fo content of common olivine is about 90, whereas that of iron-rich olivine is 71-88. The antigorite veins are Mg-rich and Fe-poor in contact with Fe-rich olivine, suggesting interdiffusion of Fe and Mg across the interface. The Mg-Fe interdiffusion rate is higher at higher temperature, suggesting that the antigorite veins may have formed at high temperature or may have been heated after the formation of the antigorite veins.
The occurrence of antigorite and talc throughout the Salahi mantle section suggests that hydrothermal reactions occurred at temperatures around 300-700°C. The coexistence of olivine, antigorite, and tremolite suggests a possible hydrothermal reaction at 500-600°C. Furthermore, the presence of lizardite and chrysotile in all samples suggests that water infiltration occurred extensively at temperatures lower than 300°C. The cross cutting relation observed in the antigorite veins suggests a formation sequence of olivine, antigorite, chrysotile, magnetite, and carbonate minerals. Since the stable temperature decreases in this order, serpentinization is considered to have progressed with a gradual decrease in temperature. On the other hand, there are also antigorite veins that cut the lizardite mesh structure. Antigorite has higher silica activity than lizardite, suggesting the possibility of inflow of silica-rich fluids or additional heating processes.
Antigorite and talc occur less frequently in the northwestern parts of the Salahi mantle section. The boundary of paleo-ridge segment structure has been inferred at the southern end of the Salahi block, and the center of the ridge segment is estimated to have been located near the southern end of the Fizh block, north of the Salahi block, suggesting a relationship between the frequency of antigorite and talc and paleo-ridge segment structure. Near the end of the ridge segment, antigorite formed in the early stage of cooling due to seawater penetration deep into the mantle. On the other hand, the northwestern part of the Salahi body, which is closer to the segment center and deeper than the Moho, may have been unsuitable for antigorite formation due to high temperature.