Trace Element Evidence for Major Contribution to Commercial Oils by Serpentinizing Mantle Peridotites
Peter Szatmari1, Tereza Cristina Oliveira Da Fonseca1, and Norbert Fritz Miekeley2
1 Petrobras Research Center (CENPES/GEOTEC) Rio de Janeiro, Brazil 21949-900
2 Pontífica Universidade Católica, Rio de Janeiro, Brazil
An organic (biogenic) contribution to petroleum is well attested by biomarkers but this need not exclude major contribution also from inorganic sources. During serpentinization, in the absence of free oxygen, oxidation of bivalent Fe to magnetite breaks up the water molecule, generating hydrogen and creating one of the most reducing environments near the Earth's surface (Janecky & Seyfried, 1986). Szatmari (1989) proposed that petroleum forms in commercial amounts by Fischer-Tropsch-type synthesis over serpentinizing peridotites, especially along plate boundaries, and suggested that Ni, an element rare in the continental crust but important in both petroleum and the mantle, is indicative of such a source. Recently, Holm and Charlou (2001) reported linear saturated hydrocarbons of 16 to 29 carbon atoms, forming by Fischer-Tropsch-type synthesis, in a hydrogen- and methane-bearing hydrothermal plume over serpentinizing peridotites of the Mid-Atlantic Ridge. The similarly serpentinite-hosted Lost City hydrothermal field lies 15 km off-ridge, and produces hydrogen and methane through reactions between exposed peridotites and seawater (Kelley et al., 2005). To test whether the abundances of trace elements other than Ni in commercial petroleum are compatible with a serpentinizing source, we analyzed by internally coupled plasma-mass spectroscopy (ICP-MS) 22 trace elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Rb, Y, Mo, Ag, La, Ce, Pr, Nd, W, Pb, U, plus Sr and Ba to indicate residual water) in 77 oils; 68 sampled in seven sedimentary basins throughout Brazil and 9 oils in major oil-producing areas outside Brazil. We found that median composition of the 22 trace element in the 77 oils analyzed correlates significantly better with chondrite (r2=0.77), serpentinized fertile mantle peridotite (r2=0.76), and the primitive mantle (r2=0.60) than with oceanic (r2=0.41) or continental crust (r2=0.35), and shows no correlation with seawater (r2=0.02). V abundances are low and V/Ni ratios range from chondritic to mantle-like values in oils from lacustrine sequences in aborted rifts, rising toward crustal and seawater values elsewhere. Normalized to chondrite or the mantle, trace elements that during serpentinization are largely retained in magnetite and other spinels (Cr, Mn, Fe, Ti) are about two orders of magnitude less abundant in the oils than those elements which during serpentinization pass into serpentine minerals, suggesting the different availability of the two groups during the formation of petroleum and/or its source rocks. Within both groups, trace metal ratios in the oils are close to mantle ratios. These correlations suggest that serpentinization of mantle peridotites, deformed and partially unroofed during rifting and continental break-up, may play a significant role in petroleum formation by contributing catalytically active trace elements, hydrogen, and hydrocarbons to the petroleum.
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