Insights into the chemical diversity of the martian mantle from the Pb isotope systematics of shergottite Northwest Africa 8159
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Radiogenic nuclide
Pyroxene
Pyroxene
Anorthosite
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Anorthosite
Pyroxene
Ilmenite
Microprobe
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Abstract We investigated properties of frictional sliding of plagioclase, pyroxene, and their mixture gouges with a small amount of hornblende, biotite, and quartz as accessory minerals, under hydrothermal conditions with an effective normal stress of 200 MPa, pore pressure of 30 MPa, and temperatures from 100°C to 600°C. Axial loading rate was stepped between 0.001 and 0.0001 mm/s to acquire the rate dependence. Both plagioclase and pyroxene gouges showed velocity‐weakening behavior in the whole temperature range except the velocity‐strengthening behavior of pyroxene at 200°C. For temperatures above 400°C, both plagioclase and pyroxene gouges showed oscillatory slips, as a result of small d c values of 3–4 µm which make the critical stiffness rise remarkably and approach the system stiffness. Above 300°C, the direct effect of plagioclase shows an increasing trend with temperature, indicating control of the deformation process by thermally activated mechanisms. As the difference of a and b values here are only 20% at most, this trend also applies to the evolution effect. Our analytical derivation based on the theory of pressure solution shows a log‐linear contact area growth with time that corresponds to an evolution effect, and estimations based on this encompass the plagioclase data, though the identification of actual mechanisms is not easy. Finally, it is found that a little quartz (3–5%) added to the plagioclase (60–62%)‐pyroxene (35%) mixture has a strong stabilizing effect, leading to a transition from velocity weakening to velocity strengthening. These results may help constrain the depth range of seismic slips on deep faults in the lower crust of gabbroic composition.
Pyroxene
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Various dihedral angles were measured in partially molten, synthetic peridotite (Fo 60 En 35 Ab 2.5 (wt%)), pressure‐annealed at 1.5 GPa (15 kbar), 1400°C (water free) for 192 hours and 1250°C (water saturated) for 212 hours; the melt fractions in the two specimens were 8.2% and 9.2%, respectively. The orthopyroxene‐melt interface shows the greatest difference in dihedral angles between the water‐free and saturated samples. At textural equilibrium the melt phase exists stably at grain edges except at those edges which include two or three orthopyroxene grains in the water‐free specimen. The melt phase is stable in all corners for both the water‐free and saturated samples. The equilibrium melt phase distribution for a system of two solids with coexisting melt is characterized by a newly proposed stability criteria. We conclude that the network of melt phase through edges and corners becomes more connected in the water saturated partial melt. It is suggested that the connectivity of the melt phase through grain edges is likely to be interrupted by the presence of orthopyroxene grains in dry ultramafic rocks. The increase of the melt fraction and/or the water content in the melt tends to diminish this “pyroxene effect.”
Pyroxene
Saturation (graph theory)
Peridotite
Dihedral angle
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