MANTLE TO MOLECULES: A ROUTE TO ABIOTIC ORGANIC SYNTHESIS

Version:1.0 StartHTML:0000000196 EndHTML:0000005056 StartFragment:0000002432 EndFragment:0000005020 SourceURL:file://localhost/Users/johnholloway/Desktop/GSA_Portland/GSA_AbstractMagMol.doc

CO₂ in mid-ocean ridge (MOR) glassy rims on basalts shows oversaturation at seafloor pressures. The fO₂ from primative glasses and mantle nodules require a graphite-diamond C source. Model calculations yield primary magma CO₂ of 1800 ppm and predict  ≤ 80 ppm of C in the source mantle is consumed. Mass balance for melt fractions of 15 wt. % shows the graphite required to generate 1800 ppm CO₂ in primary MORB magma is 74 ppm. The 1800 ppm value for CO₂ content of primary MORB magma erupted at present-day rates for 3.3 AE equals estimates for Earth's global crustal, oceanic and atmospheric carbon content. In MORB systems there is a significant change in the REDOX state of the system during emplacement, crystallization and degassing. fO₂ effects speciation of volatiles in evolved fluids. Concentrations of CO₂ and H₂O in MORB glass show that the magma was fluid saturated before and during eruption. H₂O is more soluble in basalt melt than CO₂, so CO₂ is the predominant degassing species. Magnetite crystallization generates H₂ by reduction of H₂O in the melt, resulting in evolution of fluids containing H₂ and H₂S.  In MOR settings this fluid mixes with CO₂ exsolving from the magma during transient diking-eruptive events so the fluids are initially in a grossly disequilibrium redox state. 

Rapid flow experiments at seafloor hydrothermal system (SFHS) conditions show CO₂ and H₂ react in the presence of magnetite in crystalline MORB to form methanol.  Static experiments show MORB glass reacts to smectite clay and that aqueous methanol solutions in the presence of smectite clay at SFHS conditions form complex organic compounds, and the time dependence of synthesis correlates with the collapse of the smectite interlayers. The most abundant species is hexamethyl benzene (HMB), with smaller amounts of other organic compounds.  We hypothesize that the abundance of HMB could be due to a similarity between the size hexagonal rings of SiO₄ units in clay layers and the HMB ring size.