Exploring for native H2 emissions within iron-rich Precambrian rocks in Kansas and Northern Minnesota (USA)
Though native hydrogen (H2) generation has long been associated to serpentinization of ultramafic to mafic rocks at mid-oceanic ridges, it is now well established that H2 is also emitted from intracontinental settings. Here we investigate drill cores from the DR1-A well (Kansas, USA) in the iron-rich Precambrian granitoid basement, from where high concentrations of H2were reported. Further up the Mid-Rift System, in Northern Minnesota, we study gas emanations and water compositions from shallow wells.
Petrographic observations of the Kansas samples from a depth of 452 m within the basement show fractured fayalites (Fe2SiO4) distributed in a matrix of amphibole, pyroxene, feldspar, quartz, and oxides. As revealed by SEM, these Fe-rich olivines are filled with two types of phyllosilicates, associated with iron oxides. XANES spectroscopy at the Fe L-edge showed that these phases contain both ferrous and ferric iron: the external part of the veins exhibits almost 30% of ferric iron versus only 20% for the center part. Transmission electron microscopy investigations conducted on ultrathin FIB sections extracted across these veins show Fe-phyllosilicates structurally related to chlorites. Although rare in such geological context, the presence of fayalite and chlorite with a heterogeneous but significant ferric iron content strongly suggests water-rock interactions leading to the local generation of H2. The combination of µCT with XANES and electron microprobe data allow to estimate the volume fractions of those Fe(III)-rich phases, and thus to quantify the potential mass of H2 produced per kg of rock, associated to those phyllosilicates.
The investigation of shallow water wells in Northern Minnesota reveals surface emission of low amounts of geologic H2 above similar fayalite-rich rocks. The water composition in short organic compounds, and the isotopic signature of associated gases, strongly suggest that H2 consumption at depth is taking place along its migration pathways towards the surface. Those results have important implications for exploring similar Precambrian granitoids present in every continent that might correspond to potential natural H2 production sites.