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Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a “brittle-to-ductile” transition (BDT) region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a BDT in marble during deformation experiments in the laboratory is accompanied by systematic increase in the frequency of acoustic emissions suggesting a profound change in the mean size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle–ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At higher pressures, signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole pressure range, in agreement with postmortem microstructural observations. Our measurements provide unique, real-time data of microscale dynamics over a broad range of pressures (10 to 200 MPa) and can inform micromechanical models for semi-brittle deformation. 

Abstract. Geological carbon sequestration provides permanentCO2 storage to mitigate the current high concentration of CO2 inthe atmosphere. CO2 mineralization in basalts has been proven to be oneof the most secure storage options. For successful implementation and futureimprovements of this technology, the time-dependent deformation behavior ofreservoir rocks in the presence of reactive fluids needs to be studied indetail. We conducted load-stepping creep experiments on basalts from theCarbFix site (Iceland) under several pore fluid conditions (dry,H2O saturated and H2O + CO2 saturated) at temperature,T≈80 ∘C and effective pressure, Peff=50 MPa,during which we collected mechanical, acoustic and pore fluid chemistrydata. We observed transient creep at stresses as low as 11 % of thefailure strength. Acoustic emissions (AEs) correlated strongly with strainaccumulation, indicating that the creep deformation was a brittle process inagreement with microstructural observations. The rate and magnitude of AEswere higher in fluid-saturated experiments than in dry conditions. We inferthat the predominant mechanism governing creep deformation is time- andstress-dependent subcritical dilatant cracking. Our results suggest thatthe presence of aqueous fluids exerts first-order control on creepdeformation of basaltic rocks, while the composition of the fluids playsonly a secondary role under the studied conditions.