Reduction of nitrogen compounds in oceanic basement and its implications for HCN formation and abiotic organic synthesis

Geochemically plausible abiotic synthesis pathways and concentration mechanisms for nitrogen-containing molecules must eventually be found since nitrogen-based life is likely to have existed on Earth from early Archean onwards [1]. High ammonium contents (54-95 ppm) have been found in authigenic clays of the Isua supracrustal rocks of Western Greenland, suggesting that clays were major sinks of NH4+ or other nitrogen compounds on the Earth's surface already at 3800 Ma [2]. Ward and Brownlee have argued that plate tectonics is necessary for the origin of life on terrestrial planets and have listed a number of reasons in support of their opinion [3]. However, one argument that they have never mentioned is the connection between plate tectonics, hydrothermal geochemistry and reduction of simple carbon and nitrogen compounds suitable for abiotic organic chemistry. In our opinion, the best location where such processes could occur would be at convergent margins during the early phases of subduction of oceanic plates.Incipient alteration of mafic volcanic rocks (basalt; 45-52% SiO2) entails the palagonitization of glass with concomitant crystallization of authigenic layer silicates (e.g. smectites, double layer hydroxides) and zeolites [4-6]. Zeolites like phillipsite coexist with smectite and almost always with mafic glass [7]. The term palagonite is normally used in reference to a bulk sample of metabasite which contains a mixture of palagonitized glass, authigenic minerals like smectite, corrensite, zeolites, carbonates and Fe-Ti oxides and phosphates, as well as primary minerals like plagioclase feldspars, clinopyroxene and olivine [4]. Minerals with expanding-contracting sheet structures like double layer hydroxides (DLH) are capable of accommodating molecules of virtually any size and clamping the layer of sorbed reactant ions, and are found to have particularly high catalytic activity [1]. DLH may be formed by replacing a fraction of the divalent Mg2+ in single lay