The Influence of Water on the Strength of Olivine Dislocation Slip Systems

Abstract

The nature of lattice-preferred orientation (LPO) in olivine-rich rocks strongly influences many important physical properties of Earth's upper mantle. Different LPO types have been observed to develop in deformation experiments on olivine-rich rocks carried out at different water fugacity conditions. The development of the different LPO types has been attributed to dislocation slip systems in olivine having different sensitivities to water fugacity, but this hypothesis has not been directly tested. To measure the influence of water fugacity on the relative strengths of olivine dislocation slip systems, we carried out a series of deformation experiments on olivine single crystals under either anhydrous or hydrous conditions. The crystals were oriented to activate either the (010)[100], (001)[100], or (100)[001] dislocation slip systems using a direct shear geometry, which allows for isolation of single slip systems, in contrast to the multiple systems activated in experiments carried out in compression. Post-deformation electron backscatter diffraction analyses reveal orientation gradients consistent with deformation occurring via the motion of dislocations on the activated slip systems. Crystals in all of the investigated orientations exhibit hydrolytic weakening, but crystals oriented to activate the (001)[100] slip system exhibit the largest degree of weakening. These results are consistent with a water-induced change in LPO in olivine-rich rocks deforming by dislocation creep. The rheological data obtained from the experiments can be used to improve models of LPO evolution in Earth's mantle, which is critical for imaging the structure of Earth's interior and predicting the movement of Earth's tectonic plates.

The distribution of hydrogen/water in Al and Ti-containing forsterite: A thermodynamic model

“…This "hydrolytic weakening" has been observed in both diffusion and dislocation creep regimes (Mei and Kohlstedt, 2000a, Mei and Kohlstedt, 2000b, Karato et al, 1986, Demouchy et al, 2012, Girard et al, 2013a, Tielke et al, 2018, Jung and Karato, 2001, Hirth and Kohlstedt, 2003, Mackwell et al, 1985, as has an increase in the silicon diffusion rate (Costa andChakraborty, 2008, Fei et al, 2013) with these two mechanisms hypothesised as linked (Fei et al, 2016). The strain rate of forsterite has been found to increase in the presence of water from around half an order of magnitude (Mei and Kohlstedt, 2000a, Mei and Kohlstedt, 2000b, Fei et al, 2013, Girard et al, 2013b, Tielke et al, 2018, Demouchy et al, 2012, Umemoto et al, 2011 to a couple of orders of magnitude (Costa and Chakraborty, 2008, Karato et al, 1986, Jung and Karato, 2001) with these differences often explained by experimental differences in strain rates, compositions and water content with grain boundary water likely an important confounder of results. The two works at the highest pressures (4-8 GPa (Fei et al, 2013, Girard et al, 2013a) where [HSi] should be the largest found the lowest weakening which could be evidence that either [HSi] is not important in Si diffusion or that Si diffusion is not important in the rheological strength of forsterite (which would also explain the difference between direct strength measurements and those from Si diffusion measurements) but may be evidence that other studies have overestimated the weakening effects.…”

Dislocation Creep of Olivine and Amphibole in Amphibole Peridotites from Åheim, Norway

“…The subgrain boundaries in olivine observed from the Åheim amphibole peridotites (Figures 2, 4 and 7) May represent a secondary stage of deformation after the deformation of samples during amphibolite facies. As discussed above, one possible mechanism of subgrain boundary generation is deformation under moderate water content conditions induced by fluid infiltration [3,11,20,73]. The samples were deformed under the same conditions during the exhumation stage, and as a result, an identical slip system was activated when the olivine subgrain boundaries were formed.…”

“…The (001)[100] slip system has been identified from the olivine subgrain boundaries in Many other natural peridotites [21,61,62,65]. Previous experimental studies showed that the Etype LPO of olivine, i.e., the activation of the (001)[100] slip system, was obtained under conditions of moderate water content [3,11,20,73]. The Åheim amphibole peridotites have undergone multiple stages of aqueous fluid infiltration during the Late Caledonian uplift [55] or subsequent exhumation after orogenic collapse.…”

Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

“…The similar CPO and deformation mechanisms in the peridotites and the pyroxenite layers suggest that solid-state ductile deformation affected uniformly the peridotite-pyroxenite assemblages. The dominant slip systems proposed to be responsible for the olivine and pyroxene CPO in the EL ophiolites are commonly observed in mantle rocks deformed at high-temperature, spinel-to plagioclase facies conditions in a fluid-poor environment (e.g., Bascou et al, 2002;Frets et al, 2012;Karato and Wu, 1993;Mainprice et al, 2005;Tielke et al, 2019;Tommasi and Vauchez, 2015).…”