Abstract
Soil above a platinum-group element (PGE)-bearing horizon within the Freetown Layered Intrusion, Sierra Leone, contains anomalous concentrations of n-alkanes (CnH2n+2) in the range C14 to C22 not readily attributable to an algal or lacustrine origin. Longer chain n-alkanes (C23 to C31) in the soil were derived from the breakdown of leaf litter beneath the closed canopy humid tropical forest. Spontaneous breakdown of the longer chain n-alkanes to form C14–22 n-alkanes without biogenic or abiogenic catalysts is unlikely as the n-alkanes are stable. In the Freetown soil, the catalytic properties of the PGE (Pt in particular) may lower the temperature at which oxidation of the longer chain n-alkanes can occur. Reaction between these n-alkanes and Pt species, such as Pt2+(H2O)2(OH)2 and Pt4+(H2O)2(OH)4 can bend and twist the alkanes, and significantly lower the Heat of Formation. Microbial catalysis is a possibility. Since a direct organic geochemical source of the lighter n-alkanes has not yet been identified, this paper explores the theoretical potential for abiogenic Pt species catalysis as a mechanism of breakdown of the longer n-alkanes to form C14–22 alkanes. This novel mechanism could offer additional evidence for the presence of the PGE in solution, as predicted by soil geochemistry.
Highlights
The Freetown Peninsula, Sierra Leone consists of a thick (7 km) sequence of layered gabbroic rocks.Platinum-group minerals (PGM) have been located in these rocks and have shown to be altered by superficial weathering [1,2]
Spontaneous breakdown of the longer chain n-alkanes to form C14–22 n-alkanes without biogenic or abiogenic catalysts is unlikely as the n-alkanes are stable
Since a direct organic geochemical source of the lighter n-alkanes has not yet been identified, this paper explores the theoretical potential for abiogenic Pt species catalysis as a mechanism of breakdown of the longer n-alkanes to form C14–22 alkanes
Summary
The Freetown Peninsula, Sierra Leone consists of a thick (7 km) sequence of layered gabbroic rocks.Platinum-group minerals (PGM) have been located in these rocks and have shown to be altered by superficial weathering [1,2]. Alluvial PGM have been recovered from the streams draining the intrusion and they are notably different in mineral assemblage, composition, and size from the PGM in the unaltered host rocks [3,4] This difference has created debate concerning whether supergene processes could be responsible for the observed mineralogical differences. It has been suggested [5,6,7] that some of the PGM from the host rocks were altered during weathering and the platinum-group elements (PGE) products transported in solution to a regime of changed Eh and pH where growth of a new PGM suite occurred. There is textural evidence in the alluvial PGM, that is consistent with low-temperature growth and re-solution and re-growth
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