The legacy of crystal-plastic deformation in olivine: high-diffusivity pathways during serpentinization

Abstract

Crystal-plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence of asthenospheric mantle flow. Although crystal-plastic deformation and serpentinization are spatio-temporally decoupled, we identified compositional readjustments expressed on the micrometric level as a striped Fe-enriched ( " X Fe = 0.24 ± 0.02 (zones); 0.12 ± 0.02 (bulk)) or Fe-depleted ( " X Fe = 0.10 ± 0.01 (zones); 0.13 ± 0.01 (bulk)) zoning in partly serpentinized olivine grains from two upper mantle sections in Norway. Focused ion beam sample preparation combined with transmission electron microscopy (TEM) and aberrationcorrected scanning TEM, enabling atomic-level resolved electron energy-loss spectroscopic line profiling, reveals that every zone is immediately associated with a subgrain boundary. We infer that the zonings are a result of the environmental Fe 2? Mg -1 exchange potential during antigorite serpentinization of olivine and the drive toward element exchange equilibrium. This is facilitated by enhanced solid-state diffusion along subgrain boundaries in a system, which otherwise re-equilibrates via dissolutionreprecipitation. Fe enrichment or depletion is controlled by the silica activity imposed on the system by the local olivine/orthopyroxene mass ratio, temperature and the effect of magnetite stability. The Fe-Mg exchange coefficients K Atg=Ol D between both types of zoning and antigorite display coalescence toward exchange equilibrium. With both types of zoning, Mn is enriched and Ni depleted compared with the unaffected bulk composition. Nanometer-sized, heterogeneously distributed antigorite precipitates along olivine subgrain boundaries suggest that water was able to ingress along them. Crystallographic orientation relationships gained via electron backscatter diffraction between olivine grain domains and different serpentine vein generations support the hypothesis that serpentinization was initiated along olivine subgrain boundaries.

Surface-specific measurements of olivine dissolution by phase-shift interferometry

“…The in situ study also shows that olivine reactivity with respect to dissolution can vary from one crystallographic surface to another as well as in different parts of the same surface with similar characteristics, such as terraces. Finally, the observation of precipitate formation on surfaces associated with outcropping defects supports the natural observations of preferential serpentinization along these defects (Plümper et al 2012b) implying that it will be the reactivity of these surfaces that will control olivine reactivity in nature. A recent discussion of the general implications of variable dissolution rates of different crystal surfaces and their evolution with time, to conventional flow-through dissolution experiments on powders can be found in Lüttge et al (2013).…”

Evidence for silicate dissolution on Mars from the Nakhla meteorite

“…; Plümper et al. ). We have also examined the rough surfaces of olivine grains that have been freshly broken from the meteorite to obtain a new perspective on the microstructure of the veins and their secondary minerals.…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…Morales et al (2013) demonstrate the transformation of fabric patterns from olivine to antigorite based on the topotactic relationships. Plümper et al (2012) described an olivine-bearing antigorite serpentinite from the Leka Ophiolite Complex, Norway, and concluded that antigorite serpentinization is initiated along sub-grain boundaries in olivine, supporting the topotactic model.…”