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Title: Low-pressure–temperature stability of pyrope + quartz relative to orthopyroxene + kyanite: a new model for aluminous orthopyroxene with vacancies
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Contributions to Mineralogy and Petrology
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    To understand the effects of secondary minerals on changes in the mechanical properties of upper mantle rocks due to phase mixing, we conducted high‐strain torsion experiments on aggregates of iron‐rich olivine + orthopyroxene (opx) with opx volume fractions offopx = 0.15, 0.26, and 0.35. For samples with larger amounts of opx,fopx = 0.26 and 0.35, the value of the stress exponent decreases with increasing strain fromn ≈ 3 for γ  5 ton ≈ 2 for 5  γ  25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples withfopx = 0.15, the stress exponent is constant atn ≈ 3.3 for 1  γ  25, suggesting that no change in deformation mechanism occurs with increasing strain for samples with smaller amounts of opx. The microstructures of samples with larger amounts of opx provide insight into the change in deformation mechanism derived from the mechanical data. Elongated grains align subparallel to the shear direction for samples of all three compositions deformed to lower strains. However, strain weakening with grain size reduction and the formation of a thoroughly mixed, fine‐grained texture only develops in samples withfopx = 0.26 and 0.35 deformed to higher strains of γ  16. These mechanical and associated microstructural properties imply that rheological weakening due to phase mixing only occurs in the samples with largerfopx, which is an important constraint for understanding strain localization in the upper mantle of Earth.

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  2. Abstract As a major nominally anhydrous mineral (NAM) in the Earth’s upper mantle, orthopyroxene could host up to several hundred parts per million H2O in its crystal structure and transport the H2O to the deep Earth. To study the effect of structural H2O on the elasticity of orthopyroxene, we have measured the single-crystal elasticity of Mg1.991Al0.065Si1.951O6 with 842–900 ppm H2O and 1.64 ± 0.20 wt% Al2O3 at ambient conditions using Brillouin spectroscopy. The best-fit single-crystal elastic moduli (Cijs), bulk (KS0), and shear (G0) modulus of the hydrous Al-bearing orthopyroxene were determined as: C11 = 235(2) GPa, C22 = 173(2) GPa, C33 = 222(2) GPa, C44 = 86(1) GPa, C55 = 82(1) GPa, C66 = 82(1) GPa, C12 = 75(3) GPa, C13 = 67(2) GPa, and C23 = 49(2) GPa, KS0 = 111(2) GPa, and G0 = 78(1) GPa. Systematic analysis based on the results presented in this and previous studies suggests that the incorporation of 842–900 ppm H2O would increase C13 by 12.0(7)% and decrease C23 by 8.6(8)%. The effects on C11, C22, C33, C44, C66, KS0, and VP are subtle if not negligible when considering the uncertainties. The C55, C12, G0, and VS are not affected by the presence of structural H2O. Although laboratory experiments show that Fe,Al-bearing orthopyroxenes can host up to 0.8 wt% H2O in its structure, future high-pressure-temperature elasticity measurements on orthopyroxene with higher H2O content are needed to help better quantify this effect. 
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  3. Abstract

    Hydrogen solubility was determined in olivine and orthopyroxene under water‐saturated conditions atP = 3–5 GPa andT = 1373–1573 K. For olivine, polycrystalline samples were prepared from San Carlos olivine, and for orthopyroxene synthetic samples were prepared from oxide mixture containing 1.5–5 wt% of Al2O3. Olivine and orthopyroxene were placed next to each other and annealed under various pressure and temperature conditions for 3–5 hr. Hydrogen content was measured across each sample by FTIR spectroscopy. Under the water‐saturated conditions, the hydrogen solubility in olivine increases with pressure and temperature similar to previous results. Hydrogen solubility in Al2O3‐bearing orthopyroxene also increases with temperature and pressure for a fixed Al2O3content. Based on these observations we calculated the partition coefficients of hydrogen between orthopyroxene and olivine assuming the fugacity dependence of hydrogen solubility in olivine and Al2O3‐bearing orthopyroxene reported by previous studies. We find that the partition coefficient depends weakly on temperature but strongly on pressure and water fugacity. Our results are extended to an open system where Al2O3content in orthopyroxene changes with pressure and temperature. At relatively low pressures and low water fugacity (in the lithosphere (shallower than ∼50 km)), the partition coefficient is high and a majority of hydrogen is present in orthopyroxene. Consequently, the influence of water on the bulk physical properties is small. In contrast, at higher pressures and higher water fugacity (in the asthenosphere), the partition coefficient is smaller and a substantial amount of hydrogen is present in olivine. Consequently, hydrogen has a strong effect on the bulk properties of the asthenosphere reducing viscosity and increasing electrical conductivity.

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