Primordial monazite growth in Archean “metapelites” of the Isua supracrustal belt, southern West Greenland

We are exploring the origins of some of Earth’s oldest monazite minerals found in the Isua supracrustal belt of southern West Greenland. Monazite is the primary host for light rare earth elements, Th and U, and is a predominant accessory mineral in low Ca granites and metapelites. Its occurrence is controlled by the Al/Ca ratio of the host rock, and at lower metamorphic grade it may be preceded by allanite. Monazite does not form if the Al/Ca ratio falls below a certain threshold.
Rock types with a high Al/Ca ratio suitable for monazite formation are widespread in modern geological settings. Archean rocks, however, typically have a basaltic to ultramafic composition. Even Archean felsic igneous rocks such as TTGs generally exhibit relatively low Al/Ca ratios. Consequently, the composition of most Archean rocks essentially limits the growth of monazite, with apatite, allanite, titanite or zircon being the predominant accessory phases. Nonetheless, the Al/Ca ratio of some rocks must have exceeded the critical threshold during crustal differentiation, allowing the first monazites to be formed. The exact timing of when this happened, and which rocks were involved, is not known. Here we present evidence of very early monazite formation, as discovered in the Isua supracrustal belt (ISB) of southern West Greenland.
The "metapelitic" rock used for this study contains multiphase garnet porphyroblasts and exhibits two generations of monazite. Monazite found in garnet cores yield Th-U-Pb ages of ~3.6 Ga, placing these crystals among the oldest monazites known to date. The younger monazite generation found in the more peripheral parts of the garnet grains and in the rock matrix yields Th-U-Pb ages of ~2.7 Ga. These younger ages are consistent with recently published garnet ages that have been interpreted to record the dominant tectono-metamorphic event of the ISB (Eskesen et al., 2023, Geology 51, 1017–1021). A detrital origin of either monazite generation is unlikely because there are no other known occurrences of monazite in Isua and both monazite generations appear to have formed in their current host rock via a metamorphic overprint.
Due to the rocks’ pelitic bulk composition, they are interpreted as one of the oldest preserved clastic sedimentary rocks, consistent with a provenance consisting of ultramafic, mafic and felsic igneous rocks (Bolhar et al., 2005, GCA 69, 1555–1573). Alternatively, their composition can be interpreted as a metasomatic modification of a basaltic rock (Rosing et al., 1996, Geology 24, 43–46). In either case, the observed ~3.6 Ga monazite indicates a minimum age for the process that led to the high Al/Ca composition of the host rock, which in turn eventually allowed the growth of monazite in the Archean crust of the ISB. As for the hotly-debated matter of whether or not the Isua supracrustal belt hosts late Eoarchean metamorphism, the ~3.6 Ga data appears to potentially represent the first direct-dating evidence in support of this interpretation.