Deformation of forsterite polycrystals at mantle pressure: Comparison with Fe-bearing olivine and the effect of iron on its plasticity

Abstract

a b s t r a c tThe rheology of polycrystalline forsterite was investigated in the Deformation-DIA (D-DIA) using in situ X-ray diffraction at pressure between 3.1 and 8.1 GPa, temperature in the 1373-1673 K range, and at steady-state strain rate ranging from 0.5 Â 10 À5 to 5.5 Â 10 À5 s À1 . Microscopic observations of the run products show characteristic microstructures of the so-called ''dislocation creep regime'' in wet conditions. Based on the present data at 1473 K, the pressure effect on forsterite plasticity is quantified using an activation volume V Ã Fo ¼ 12:1 AE 3:0 cm 3 mol À1 . A comparison between the strain rates of San Carlos olivine and forsterite specimens deformed together indicates that, at the experimental conditions, they compare with each other within less than half an order of magnitude. This comparison also allows for the determination of the stress exponent of forsterite of n Fo = 2.3 ± 0.6. Our results, combined with data from the literature, indicate a clear trend of increasing stress exponent with Fe content in olivine.

Textures in deforming forsterite aggregates up to 8 GPa and 1673 K

“…At 1673 K, in the whole tested pressure range, no clear texture can be defined, neither in dry (<200 ppm H/Si, see Kohlstedt et al 1996) nor wet (>200 ppm H/Si) conditions. Grain growth and/or recrystallization do not allow the measurement of consistent textures with the capabilities of the present setup at this high P, T. This last observation is consistent with TEM observations of evidences of recovery at temperatures above 1600 K (Bollinger et al 2015). As shown in Fig.…”

“…More details about samples, experiments and rheology analysis are available in previous papers (Bollinger et al 2012(Bollinger et al , 2014(Bollinger et al , 2015. Here, we present the final analysis of the dataset, focusing on LPO development in the forsterite aggregates.…”

“…A total of 13 D-DIA experiments were performed, resulting in 26 steady-state deformation conditions in axial compression (see Bollinger et al 2014Bollinger et al , 2015. Most experiments were performed following the same protocol.…”

“…Although the water content was not controlled during our experiments, FTIR analyses show that the deformation occurred in "wet" conditions (details in Bollinger et al 2015) with more than 200 ppm H/Si for most of the forsterite samples. Our observations at 1373 K and 4.6 GPa indicate a dominant [100](010) slip system, consistent with a A-type LPO (Jung et al 2006).…”

An Experimental Investigation of the Relative Strength of the Silica Polymorphs Quartz, Coesite, and Stishovite

“…However, the formation of bridgmanite convolves other effects with the increase in silicon coordination, for example, changes in cation coordination and network connectivity of the silica units in the magnesium silicate crystal structures. Cation coordination and valence play a role in the strength of phases, for example, the substitution of 2Al 3+ by Si 4+ + Mg 2+ in pyrope garnet weakens the mineral (Hunt et al, ) as does the substitution of Fe 2+ into forsterite (Bollinger et al, ). In general substitution of multivalent and less strongly charged cations reduces the strength of minerals.…”