Apatite in early Archean Isua supracrustal rocks, southern West Greenland: its origin, association with graphite and potential as a biomarker

Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.