Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: Effects of Orthopyroxene Volume Fraction

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 of f opx ¼ 0.15, 0.26, and 0.35. For samples with larger amounts of opx, f opx ¼ 0.26 and 0.35, the value of the stress exponent decreases with increasing strain from n ≈ 3 for γ ≲ 5 to n ≈ 2 for 5 ≲ γ ≲ 25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples with f opx ¼ 0.15, the stress exponent is constant at n ≈ 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 with f opx ¼ 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 larger f opx , which is an important constraint for understanding strain localization in the upper mantle of Earth.

Evolution and demise of passive margins through grain mixing and damage

“…or through dissolution of one mineral and precipitation of it at the other mineral's grain-boundary junctions (41,42,49). These mechanisms are both limited by element diffusion and similarly driven by imposed stresses.…”

“…The grain-scale physics occurring in mylonites are important for understanding processes at the global and planetary scale and have been the subject of a number of theoretical (18,19,(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) and observational (36)(37)(38)(39)(40)(41)(42) studies. Plate boundary mylonites and ultramylonites typically form in the lithospheric mantle where there are multiple mineralogical phases, such as olivine and pyroxene in peridotite, and especially where these phases mix (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53). This observation suggests that Zener pinning (in which one mineral phase blocks the grain-boundary migration of the other phase) plays an important role in suppressing grain growth and keeping the medium in permanent diffusion creep (43-45, 51, 53).…”

Constraining Upper Mantle Viscosity Using Temperature and Water Content Inferred From Seismic and Magnetotelluric Data

“…We assume that viscosity flow laws for olivine represent the bulk upper mantle viscosity because olivine is the most abundant and well‐studied mineral phase. However, the inherent viscosity of other phases such as pyroxenes (e.g., Chen et al., 2006) and the effect of different phases on the overall rheology (e.g., Bercovici & Skemer, 2017; Hansen & Warren, 2015; Tasaka et al., 2020; Warren & Hirth, 2006; N. Zhao et al., 2019) certainly place errors in this analysis. In principle, such uncertainty could be reduced by employing a viscosity law that incorporates multiple phases but such a flow law does not yet exist.…”

Mantle mineralogy limits to rocky planet water inventories

“…The present work does not implement the extra complexity of an informed Tp, in particular because compositional effects on viscosity are potentially significant and not easy to constrain. As Spaargaren et al (2020) discuss, we would generally expect less-viscous lower mantles at higher Mg/Si because fp is weaker than pv, and the overall viscosity is controlled by the weaker phase (Girard et al 2016); higher opx fractions may also cause rheological weakening (Tasaka et al 2020). Viscosities further depend on the instantaneous water content itself, whereby a water-saturated mantle should be rheologically weaker than a dry mantle (Karato & Wu 1993).…”