12:00 〜 12:15
[SCG54-12] Water-promoted deformation mechanisms: experiments of quartz-albite mixtures at brittle-ductile transitional condition
キーワード：上部地殻、流体－岩石相互作用、石英－長石混合物、変形実験、微細組織、脆性－延性遷移
The strength of the upper crust is estimated to become maximum around the brittle-ductile transition zone and is expected to decrease in the presence of water[1]. Laboratory experiments have shown strength reduction in wet conditions, where grain boundary migration is observed to be promoted[2]. However, experimental data on polymineralic aggregates with water are lacking, particularly for the realistic continental crust composition of quartz-feldspar mixtures. Aiming to reveal how water affects the deformation mechanisms that reduce the bulk strength, we performed shear deformation experiments on quartz-albite aggregates using a Griggs-type apparatus with solid salt assembly. To investigate the difference in the dominant deformation mechanism between different amounts of water, we varied the water contents into 0.2 wt% and 0.4 wt%, and further compared them to the sample deformed at a room-dry condition[3]. The confining pressure PC and the temperature T were set almost identical (PC: 750-760 MPa and T: 720 °C) in all the experiments so that the only difference between these experiments was the water amount. Each experiment used sample mixtures of ~ 0.1 g. The shear strain rate was repeatedly changed between ~ 10-3 /s and ~ 10-4 /s to evaluate the dependence of the strength on the strain rate. The retrieved samples underwent observations using electron microscopes. The sample deformed with the 0.4 wt% water further underwent a transmission electron microscopy (TEM).
Mechanical results show that the peak shear stress decreases with increasing water. The sample with 0.2 wt% of water reaches 1040 MPa while that deformed with 0.4 wt% of water reaches 790 MPa. Both of these values are smaller than the peak shear stress of the room-dry experiment, that is 1280 MPa[3]. In the experiment with 0.2 wt% of water, the shear stress remains almost constant towards the final shear strain. Meanwhile, in the experiment with 0.4 wt% of water, the shear stress shows a reduction from the peak value by 200 MPa towards the end of the experiment.
Microstructural observations revealed that foliations similar to an S-C’ mylonite develop in the sample deformed with the 0.2 wt% of water. Both albite domains and quartz domains are foliated. Meanwhile, in the sample deformed with the 0.4 wt% of water, the sample layer is pervasively fractured. These microstructures in the water-added samples differ from those in the as-is sample, where fractures are localized in the quartz domains and the albite domains show few pores[3]. Furthermore, TEM observation of the sample deformed with the 0.4 wt% of water confirmed nanograins.
Our results suggest a change in deformation mechanisms with different amounts of water. With the 0.2 wt% of water, the quartz domains become weaker than those in the room-dry condition and ductile flow occurs in accordance with the albite domains. With further addition of water to 0.4 wt%, deformation is accommodated by fracturing, potentially due to the water-assisted reduction of the mineral surface energy as previously suggested for quartz[4]. We conclude that a subtle increase in water changes the dominant deformation mechanism from ductile to brittle. Such a transition would potentially explain seismic observations suggesting that upwelling fluids could be the cause of earthquake swarms[5].
References
[1] Kohlstedt et al., 1995JGR.
[2] Pongrac et al., 2022JSG.
[3] Furukawa et al., 2023 WRI-17.
[4] Parks, 1984JGR.
[5] Nakajima, 2022EPS.
Mechanical results show that the peak shear stress decreases with increasing water. The sample with 0.2 wt% of water reaches 1040 MPa while that deformed with 0.4 wt% of water reaches 790 MPa. Both of these values are smaller than the peak shear stress of the room-dry experiment, that is 1280 MPa[3]. In the experiment with 0.2 wt% of water, the shear stress remains almost constant towards the final shear strain. Meanwhile, in the experiment with 0.4 wt% of water, the shear stress shows a reduction from the peak value by 200 MPa towards the end of the experiment.
Microstructural observations revealed that foliations similar to an S-C’ mylonite develop in the sample deformed with the 0.2 wt% of water. Both albite domains and quartz domains are foliated. Meanwhile, in the sample deformed with the 0.4 wt% of water, the sample layer is pervasively fractured. These microstructures in the water-added samples differ from those in the as-is sample, where fractures are localized in the quartz domains and the albite domains show few pores[3]. Furthermore, TEM observation of the sample deformed with the 0.4 wt% of water confirmed nanograins.
Our results suggest a change in deformation mechanisms with different amounts of water. With the 0.2 wt% of water, the quartz domains become weaker than those in the room-dry condition and ductile flow occurs in accordance with the albite domains. With further addition of water to 0.4 wt%, deformation is accommodated by fracturing, potentially due to the water-assisted reduction of the mineral surface energy as previously suggested for quartz[4]. We conclude that a subtle increase in water changes the dominant deformation mechanism from ductile to brittle. Such a transition would potentially explain seismic observations suggesting that upwelling fluids could be the cause of earthquake swarms[5].
References
[1] Kohlstedt et al., 1995JGR.
[2] Pongrac et al., 2022JSG.
[3] Furukawa et al., 2023 WRI-17.
[4] Parks, 1984JGR.
[5] Nakajima, 2022EPS.