5:15 PM - 6:45 PM
[SCG46-P24] Formation of spinifex-like textured olivine during antigorite dehydration: an experimental study at 200–700 MPa
Keywords:spinifex-like texture, olivine, antigorite serpentine, EBSD, internally heated pressure vessel
Dehydration of antigorite serpentinite could play a key role in arc magma genesis and intermediate depth seismicity in subduction zones. Fluid liberation from the dehydrating rock is a complex phenomenon in which chemical reaction, fluid migration and deformation mutually interact. Mineral textures recorded in the exhumed dehydrated serpentinites may provide the important information about the mode of fluid liberation.
Spinifex-like textured olivine with an elongated tabular shape has been reported in exhumed antigorite serpentinites dehydrated both at low pressures (Clément et al., 2019) and high pressures (above the invariant point at ~1.6 GPa, Trommsdorff et al., 1998; Nishiyama et al., 2020). Due to its resemblance to the spinifex texture observed in komatiite and high Cr2O3 content in newly grown olivine, Evans and Cowan (2012) emphasized that elongated tabular olivine can be formed by frictional melting of antigorite at high pressures. Recent petrological and structural studies, however, rather prefer the dehydration origin at conditions far from equilibrium which can be caused by the sudden fluid pressure drop associated with the brittle fracturing and possibly focused fluid flow. Unusual crystal habit with short a-axis and long c- and b-axes seen in the spinifex-like textured olivine was attributed to the presence of surfactants acting on (100) faces in the fluid (Dilissen et al., 2021). Yet, the detailed conditions of spinifex-like textured olivine formation are poorly understood due to the lack of systematic experimental studies.
In this study, we conducted dehydration experiments on a natural gem-quality antigorite aggregate from India by using internally heated pressure vessels with an argon gas pressure medium at the Geological Survey of Japan. The prism shaped antigorite aggregate was place in a gold capsule with an outer diameter of 2.3 mm along with distilled water and the capsule was welded shut. Experiments were conducted at a constant temperature of 630°C and different pressures of 200 MPa, 400 MPa and 700 MPa to examine the effect of reaction affinity caused by fluid pressure drop on reaction textures. In nature, fluid pressure can be different from total confining pressure, which causes non-zero effective pressure. However, previous experimental study showed that water pressure plays a primary role in dehydration kinetics regardless of total confining pressure (Llana-Fúnez et al., 2007). Thus, our experiments were performed at conditions where fluid pressure equals total confining pressure. Experimental duration is around 210 hours.
In all the experiments, olivine and talc were mainly grown inward from the rim part of the antigorite prism and always associated with fluid pores. Extents of reaction estimated from the image analysis are 23%, 16%, and 1% at 200 MPa, 400 MPa, 700 MPa, respectively. High reaction affinity allowed fast reaction progress at low pressures. Olivine at 700 MPa showed a granular shape, while olivine at 200 MPa and 400 MPa often showed an acicular elongated shape on cross-sections. Olivine aspect ratio defined by long-axis length/short-axis length becomes larger at lower pressures and its maximum value exceeds 10 at 200 MPa. Electron backscattered diffraction analysis revealed that the short axis of elongated olivine is mainly directed toward a-axis, while the long axis is distributed between b- and c-axes. Olivine crystals having aspect ratio larger than 10 is characterized by the long axis subparallel to c-axis.
Our experiments demonstrated that dehydration of antigorite at high reaction affinity can lead to the formation of spinifex-like textured olivine with short a-axis and long c- and b-axes. We do not have to invoke frictional melting to explain elongated tabular olivine in dehydrated antigorite. In addition, the surfactant supplied from the external fluid is not always required to inhibit the growth on (100) faces.
Spinifex-like textured olivine with an elongated tabular shape has been reported in exhumed antigorite serpentinites dehydrated both at low pressures (Clément et al., 2019) and high pressures (above the invariant point at ~1.6 GPa, Trommsdorff et al., 1998; Nishiyama et al., 2020). Due to its resemblance to the spinifex texture observed in komatiite and high Cr2O3 content in newly grown olivine, Evans and Cowan (2012) emphasized that elongated tabular olivine can be formed by frictional melting of antigorite at high pressures. Recent petrological and structural studies, however, rather prefer the dehydration origin at conditions far from equilibrium which can be caused by the sudden fluid pressure drop associated with the brittle fracturing and possibly focused fluid flow. Unusual crystal habit with short a-axis and long c- and b-axes seen in the spinifex-like textured olivine was attributed to the presence of surfactants acting on (100) faces in the fluid (Dilissen et al., 2021). Yet, the detailed conditions of spinifex-like textured olivine formation are poorly understood due to the lack of systematic experimental studies.
In this study, we conducted dehydration experiments on a natural gem-quality antigorite aggregate from India by using internally heated pressure vessels with an argon gas pressure medium at the Geological Survey of Japan. The prism shaped antigorite aggregate was place in a gold capsule with an outer diameter of 2.3 mm along with distilled water and the capsule was welded shut. Experiments were conducted at a constant temperature of 630°C and different pressures of 200 MPa, 400 MPa and 700 MPa to examine the effect of reaction affinity caused by fluid pressure drop on reaction textures. In nature, fluid pressure can be different from total confining pressure, which causes non-zero effective pressure. However, previous experimental study showed that water pressure plays a primary role in dehydration kinetics regardless of total confining pressure (Llana-Fúnez et al., 2007). Thus, our experiments were performed at conditions where fluid pressure equals total confining pressure. Experimental duration is around 210 hours.
In all the experiments, olivine and talc were mainly grown inward from the rim part of the antigorite prism and always associated with fluid pores. Extents of reaction estimated from the image analysis are 23%, 16%, and 1% at 200 MPa, 400 MPa, 700 MPa, respectively. High reaction affinity allowed fast reaction progress at low pressures. Olivine at 700 MPa showed a granular shape, while olivine at 200 MPa and 400 MPa often showed an acicular elongated shape on cross-sections. Olivine aspect ratio defined by long-axis length/short-axis length becomes larger at lower pressures and its maximum value exceeds 10 at 200 MPa. Electron backscattered diffraction analysis revealed that the short axis of elongated olivine is mainly directed toward a-axis, while the long axis is distributed between b- and c-axes. Olivine crystals having aspect ratio larger than 10 is characterized by the long axis subparallel to c-axis.
Our experiments demonstrated that dehydration of antigorite at high reaction affinity can lead to the formation of spinifex-like textured olivine with short a-axis and long c- and b-axes. We do not have to invoke frictional melting to explain elongated tabular olivine in dehydrated antigorite. In addition, the surfactant supplied from the external fluid is not always required to inhibit the growth on (100) faces.