The tension between CO 2 dissolved at relatively high atmospheric pressure in the Hadean ocean, and H 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust, was resolved by the onset of life. We suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean floor. Exothermic reaction between hydrothermal H 2 , HCOO − and CH 3 S − with CO 2 was catalyzed in inorganic membranes near the mound's surface by mackinawite (FeS) nanocrysts and “ready-made” clusters corresponding to the greigite (Fe 5 NiS 8) structure. Such clusters were precursors to the active centers (e.g., the C-cluster, Fe 4 NiS 5) of a metalloenzyme that today catalyzes acetate synthesis, viz., the bifunctional dehydrogenase enzyme (ACS/CODH). The water, and some of the acetate (H 3 C.COO −), produced in this way were exhaled into the ocean together as fluid waste. Glycine (+ H 3 N.CH 2 .COO −) and other amino acids, as well as tiny quantities of RNA, generated in the same milieu were trapped within tiny iron sulfide cavities. Energy from the acetate reaction, augmented by a proton gradient operating through the membrane, was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface. In turn these peptides sequestered, and thereby protected, the catalytically and electrochemically active pyrophosphate and iron/nickel sulfide clusters, from dissolution or crystallization. Intervention of RNA as a polymerizing agent for amino acids also led to an adventitious, though crude, process of regulating metabolism—a process that was also to provide genetic information to offspring. The fluxes of energy and nutrient available in the hydrothermal mound—commensurate with the requirements of life—encouraged differentiation of the first microbes into two separate domains. At the bifurcation the Bacteria were to specialize in acetogenesis and the Archaea into methanogenesis. Representatives of both these domains left the mound by way of the ocean floor and crust to colonize the deep biosphere. Once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids, light could have been used directly as an energy source for biosynthesis. Certain bacteria may have been able to do this, where protected from hard UV by a thin coating of chemical sediment produced at a sub-aerial hot spring operating in an obducted and uplifted portion of the deep biosphere. Embedded in fresh manganiferous exhalites, early photosynthetic bacteria could further protect themselves from radiation by adsorbing manganese on the membrane. Organization of the manganese with calcium, within a membrane protein, happened to result in a CaMn 3 O 4 cluster. In Mn(IV) mode this structure could oxidize two molecules of water, evolve waste oxygen, and gain four electrons and four protons in the process to fix CO 2 for biosynthesis. All these biosynthetic pathways had probably evolved before 3.7 Ga, though the reduced nature of the planet prevented a buildup of free atmospheric oxygen until the early Proterozoic.

ABSTRACT Blocks of variably serpentinized oceanic mantle peridotite (harzburgite, olivine orthopyroxenite, and dunite) are entrained within a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust. Correspondingly, the Cemetery Ridge peridotite contains four serpentinization-related sulfide or intermetallic minerals quite unusual for an inland, continental tectonic setting: pentlandite, cobalt pentlandite, heazlewoodite, and awaruite. The peridotite also contains two Ni-arsenide minerals, orcélite and maucherite; and, less commonly, the sulfides pyrrhotite, bismuthinite, bornite, and parkerite(?). These minerals form tiny to small (~3–100 μm) grains sparsely scattered amongst the profusion of serpentine and magnetite produced by serpentinization of olivine; many grains are enclosed in magnetite. The three principal sulfide minerals at Cemetery Ridge are pentlandite [(Ni,Fe) 8.93 S 8 ] in all samples studied, and cobalt pentlandite [(Co,Ni,Fe) 9.01 S 8 ] and heazlewoodite (Ni 2.98 S 2) in many or most. Pentlandite and cobalt pentlandite form a discontinuous series to 74 molar percent of the Co end member. High-Co pentlandite has systematically elevated Ni/Fe. Pyrrhotite (Fe 7.88 S 8) occurs only in one high-S sample. Although orcélite (Ni 4.71 As 2) and maucherite (Ni 11.06 As 8) are volumetrically rare, one or both are found in most samples. Awaruite (Ni 5 Fe) is extremely rare, a few minute blebs in two samples. Sulfide assemblages at Cemetery Ridge indicate the highly reduced pore fluid typical of serpentinization. Sulfur was introduced into Cemetery Ridge peridotite during the early stages of serpentinization, as one aspect of a general enrichment in fluid-mobile elements in a suprasubduction (mantle-wedge) environment. Further serpentinization was accompanied by progressive desulfurization, with concomitant transformation from minerals of higher to lower sulfidation, and partial transfer of first Fe then Ni from sulfide to oxide phases. Five Ni or Ni-Co sulfide or arsenide minerals at Cemetery Ridge are new and unexpected for Arizona, where serpentinite is quite rare and Ni or Co deposits unknown. Sulfide minerals and assemblages at Cemetery Ridge are comparable to those of serpentinites in such places as the California and Oregon Coast Ranges and Mid-Atlantic Ridge, emphasizing the uniqueness of the tectonic setting of the subducted oceanic peridotite at Cemetery Ridge, and enhancing the status of the Orocopia and related schists as the world’s type (first-recognized, best-known) low-angle paleosubduction complex.