Distinct Fracture Mineralogy That is Out of Equilibrium With Modern Groundwaters Provides Important Context for Subsurface Life
Rock fracture surfaces in the crust are essential habitat for microorganisms. Fracture‐groundwater interfaces provide physical substrates for biofilm growth and are sources of carbon, nutrients, and electron donors and acceptors. To better understand geochemical processes impacting fracture surfaces and the subsurface microbiome, we identified fractures in archived rock cores from the Soudan formation, which is known to host saline groundwaters and isolated microbial communities dependent on rock‐water interactions. Cores with open fractures were thin sectioned and studied via electron microprobe and synchrotron X‐ray fluorescence microprobe. Most fracture surfaces had mineralogy distinct from that of the bulk rock. Chlorite minerals were abundant on fracture surfaces and had elemental compositions suggesting deposition during late‐stage hydrothermal alteration. Fracture‐lining chlorites likely limit access to iron oxide and sulfide minerals that are active in subsurface biogeochemical cycles. Calcium‐rich rinds were also observed along fracture edges. These rinds were too thin and poorly ordered to be identified via light microscopy or X‐ray diffraction; however, Ca K‐edge micro‐X‐ray absorption near‐edge structure spectroscopy identified them as carbonates, minerals not observed in the bulk rock. Thermodynamic modeling shows that carbonate precipitation is largely unfavorable in Soudan groundwaters, indicating that fracture edge conditions differed from those in modern water samples. Because of the low carbon concentrations in Soudan groundwaters, carbonate rinds likely play an important role in subsurface carbon cycling and may mark fracture surfaces that once hosted biofilms. Overall, this study suggests that fracture alteration can both play an active role in and suppress rock‐water interactions essential to subsurface life.