ABSTRACT

In order to provide a time constraint on the ¹³C-depleted kerogen formation in silica dikes that intruded 3.5 Ga greenstone from Pilbara Craton in Western Australia, we have carried out an ion microprobe U-Pb dating and rare earth element (REE) analysis of apatite from one of the dikes. Two types of apatite were identified in the dike, based on their occurrences. One is stick-shape apatites (Type 1 apatite) in secondary silica micro-veins that cut the silica dike. The other is granular apatites (Type 2 apatite) that occur in matrix of the silica dike. Occurrence in the secondary micro-veins (Type 1), non-igneous REE patterns (Type 1 and 2), chemical zoning (some of Type 1 and 2), iron sulfide within apatite (some of Type 2), and intergrowth of apatite and sulfide (some of Type 2) suggest that both types of apatite were crystallized in the silica dike. Ion microprobe U-Pb dating of Type 1 apatite did not give a well-constrained age, while Type 2 apatite yields a Tera-Wasserburg concordia intercept age of 3214 ± 140 Ma (95% confidence level, MSWD = 0.6) in a three-dimensional ²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb-²⁰⁴Pb/²⁰⁶Pb diagram, and a ²⁰⁴Pb/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb isochron age of 3191 ± 150 Ma (95% confidence level, MSWD = 0.5). It is difficult to judge whether the U-Pb and Pb-Pb age of Type 2 apatite is crystallization age or metamorphic age, since the estimated range of closure temperature of U-Pb system in the apatite and that of metamorphic temperature is partly overlapped. In either case, it can be safely concluded that the minimum age of the dike and kerogen is 3.2 Ga. These ages might allow the interpretation that the kerogen was produced by biological carbon fixation and/or abiological reaction at least before 3.2 Ga.