Mantle-like Trace Element Composition of Petroleum – Contributions from Serpentinizing Peridotites

The origin of trace metals in petroleum is intimately connected with the origin of petroleum itself, one of the most exciting topics in chemistry and geology for the past 150 years. Because of its fluidity and consequent migration, the origin of petroleum is more difficult to interpret than that of most rocks. Biogenic and abiogenic models developed side by side, each championed by some of the greatest names in chemistry and geology. According to biogenic models, petroleum forms by thermal breakdown, in the presence of water, of complex, ultimately biogenic organic polymers (kerogen) (Tissot & Welte, 1984; Lewan, 1997). Conversely, according to abiogenic models, petroleum forms by reductive polymerization of simple carbon compounds such as CO and CO2. Experiments and calculations indicate that such polymerization is thermodynamically stable only at high temperatures and pressures (above 1300°C and 30 kbar; Kenney et al., 2002) but it takes place also at low temperatures (<400°C) and low pressures in the presence of metallic hydrogenation catalysts in the Fischer-Tropsch-type synthesis, FTTS (Fischer, 1926; Szatmari, 1989). Although the biogenic, organic model has been the one generally accepted by the petroleum industry almost since its birth, abiogenic, inorganic models recurrently emerge, proposed by geologists and, more often, chemists. Major contribution to oil deposits of hydrogen and hydrocarbon liquids formed by FTTS during serpentinization was suggested by Szatmari (1989). The process was demonstrated by the discoveries of the Rainbow (Holm and Charlou, 2001) and Lost City (Kelley et al., 2001; Fruh-Green et al, 2003; Kelley, 2005; Proskurowski et al., 2008; Bach and Fruh-Green, 2010) hydrothermal fields at the Mid-Atlantic ridge, where hydrothermal plumes rising over serpentinizing peridotites contain abundant hydrogen, methane, and aliphatic hydrocarbons in the C16-C29 range, formed by FTTS. This finding, together with the discovery of aliphatic and cyclic hydrocarbons in the Orgueuil meteorite and, more recently, in the pristine Tagish Lake meteorite (Pizzarello et al., 2001; Nakamura et al., 2003), as well as new studies on hydrocarbon stability at high P-T conditions (Kenney et al., 2002), gave renewed stimulus to the abiogenic models. The Tagish Lake meteorite is a new type of water and carbon-rich Type 2 carbonaceous chondrite that contains about 5% total carbon, of