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

摘要: Crystal-plastic olivine deformation to produce subgrain boundaries composed of edge dislocations is an inevitable consequence asthenospheric mantle flow. Although crystal-plastic and serpentinization are spatio-temporally decoupled, we identified compositional readjustments expressed on the micrometric level as a striped Fe-enriched ( $$ \bar{X}_{\text{Fe}} $$  = 0.24 ± 0.02 (zones); 0.12 ± 0.02 (bulk)) or Fe-depleted  = 0.10 ± 0.01 0.13 ± 0.01 zoning in partly serpentinized grains from two upper sections Norway. Focused ion beam sample preparation combined with transmission electron microscopy (TEM) aberration-corrected scanning TEM, enabling atomic-level resolved energy-loss spectroscopic line profiling, reveals that every zone immediately associated boundary. We infer zonings result environmental Fe2+Mg−1 exchange potential during antigorite drive toward element equilibrium. This facilitated by enhanced solid-state diffusion along system, which otherwise re-equilibrates via dissolution-reprecipitation. Fe enrichment depletion controlled silica activity imposed system local olivine/orthopyroxene mass ratio, temperature effect magnetite stability. The Fe-Mg coefficients K_{\text{D}}^{{{\text{Atg}}/{\text{Ol}}}} between both types display coalescence With zoning, Mn enriched Ni depleted compared unaffected bulk composition. Nanometer-sized, heterogeneously distributed precipitates suggest water was able ingress them. Crystallographic orientation relationships gained backscatter diffraction grain domains different serpentine vein generations support hypothesis initiated boundaries.