Rare-Earth Element, Lead, Carbon, and Nitrogen Geochemistry of Apatite-Bearing Metasediments from the ∼3.8 Ga Isua Supracrustal Belt, West Greenland

Abstract

We performed rare-earth element (REE) geochemistry and U-Pb geochronology on apatites in metasediments from the ∼3.8 Ga Isua supracrustal belt (ISB) and Akilia Island, West Greenland, together with stepwise combustion isotopic investigation of carbon and nitrogen for the apatite-bearing quartz-magnetite BIF of uncontested sedimentary origin from northeastern ISB. Ion microprobe analyses reveal that apatites in psammitic schist from the ISB show a U-Pb isochron of 1.5 ± 0.3 Ga. This age is similar to those of Akilia apatite and the Rb-Sr age of 1.6 Ga for the pegmatitic gneiss in the Isukasia area in literature, suggesting a late (∼1.5 Ga) metamorphic event (≤400°C). Pb isotopic ratios of apatite in the quartz-magnetite BIF are also affected by the late metamorphic event around 1.5 Ga. Chondrite-normalized REE patterns of apatites in the BIF show flat patterns with a significant positive Eu anomaly, suggesting hydrothermal influence; this is consistent with a primary depositional origin. In contrast with the quartz-magnetite BIF, apatites in the psammitic schist from the ISB and those in the Akilia BIF show different REE patterns, which resemble those of apatites from secondary mafic and felsic rocks, respectively. Carbon isotopic ratios for the quartz-magnetite BIF by stepwise combustion suggest that two components of reduced carbon are present. One is released below 1000°C (mainly 200-400°C; lowtemperature carbon = LTC), and the other above 1000°C (high-temperature carbon = HTC). δ13C values of the LTC are about -24‰. The LTC is clearly contaminant incorporated after metamorphism, because such a low-temperature component could not have survived the ≤400°C metamorphic event. On the other hand, δ13C values of the HTC are -30‰ for one aliquot and -19‰ for another. The HTC is probably sequestered within magnetite in the BIF, because the decrepitation temperature of magnetite is ∼1200°C. The HTC could not exist within quartz and apatite (decrepitation temperatures: 400-600°C and 600-800°C, respectively), or along grain boundaries. Because the magnetite is concordant with bedding surfaces, it is plausible that the HTC was incorporated in the magnetite during diagenesis. Thus, HTC is the most important candidate for primary carbon preserved in the BIF. δ13C values of HTC cannot be explained as those of Isua carbonate. On the other hand, that the very low δ13C values (-30‰), negative δ15N values (-3‰), and low C/N elemental ratios (86) for the >1000°C fraction of one aliquot are comparable to those of kerogen in Archean metasediments. Therefore, despite the presence of secondary carbon (i.e., LTC), the BIF is suggested to possibly contain highly 13C-depleted kerogenous material, which is unlikely to have been incorporated after metamorphism. Although carbon isotopic change of the kerogenous material due to metamorphic effects cannot be precisely determined from the present data, this study shows that further analysis of magnetite from the Isua BIF is a key to the search for the early life.