Mechanical Responses to Metamorphic Fluid-Rock Reactions - Natural Examples of Weakening vs. Embrittlement

Metamorphic reactions can influence strain accommodation mechanisms by changing grain size and by releasing, consuming, or changing the composition of an equilibrium fluid phase. Different deformation mechanisms, in turn, can affect metamorphic reaction rates and approaches to equilibrium by changing grain size, dislocation density, the arrangement of mineral grain boundaries, and local bulk composition. Our general understanding of water weakening effects in silicate minerals might lead us to predict that dehydration reactions will contribute to enhanced crystal plasticity, and that water-consuming reactions will strengthen rocks. Natural examples of interactions between fluid-rock reactions and strain accommodation in samples from the Tauern Window, eastern Alps, illustrate cases that both support and refute these predictions. (1) Finely interlayered graphitic and nongraphitic schists show different mechanisms of strain accommodation at different stages in their history. During burial, ductile strain was localized into graphitic horizons. During decompression, however, closely spaced Mode I extension cracks and carbonic fluid inclusion (FI) planes developed throughout the graphitic layers. Nongraphitic layers lack cracks, contain aqueous FIs, and maintained strain compatibility via crystal plasticity during unroofing. During decompression, reaction between graphite and aqueous pore fluid produced increasingly carbonic fluid that inhibited dislocation climb, experienced >60% volume expansion, and promoted Mode I crack formation. In these rocks, H2O- consuming reactions thus led to embrittlement at mid-crustal depths. (2) Finely banded mafic eclogites show outcrop- and microscopic scale evidence for synchronous strain accommodation via both crystal plasticity and brittle failure at 2 GPa. These rocks also record significant heterogeneities in reaction-controlled aH2O. Layers with aH2O>0.6 initially accommodated strain by pressure solution, producing complexly zoned grains with high aspect ratio. Continued deformation at high P caused elongate porphyroblasts to form extension fractures that propagated across the high aH2O layers. In contrast, layers with low aH2O accommodated strain via dislocation creep. High aH2O thus promoted brittle failure rather than water weakening in these rocks. Polymineralic rocks have the potential to produce fluids of different composition at different P-T conditions. These fluids can contribute to both weakening and strengthening of the rocks. We should thus expect complex feedback effects between metamorphism and deformation during orogenesis. These interactions may play a fundamental role in controlling rheology and patterns of seismicity in the deep crust and mantle.