Origins of Primordial Monazite in Archean Metapelites from the Isua Supracrustal Belt, Greenland

We are exploring the origin of some of Earth’s oldest monazites discovered in the Isua supracrustal belt (ISB). Monazite, crucial for light rare earth elements Th and U, is widespread in low Ca granites and metapelites. Its formation depends on the Al/Ca ratio of the host rock and ceases below a critical threshold and is potentially preceded by allanite at lower metamorphic grades.

Modern geological settings often feature rocks with high Al/Ca ratios suitable for monazite formation. In contrast, Archean rocks, typically basaltic to ultramafic, generally have low Al/Ca ratios, prohibiting monazite growth. However, during crustal differentiation, some rocks exceeded this threshold, enabling initial monazite formation, though specifics remain uncertain. Our study presents evidence of early monazite formation in the ISB.

Focusing on a "metapelitic" rock with garnet porphyroblasts, hosting two generations of monazite, we find older monazites in garnet cores dating to ~3.6 Ga, placing them among the oldest monazites known to date, while younger ones in garnet rims and matrix date to ~2.7 Ga.

The absence of other monazite occurrences in Isua and the in situ formation of both generations suggest a metamorphic origin rather than detrital. These rocks, with a pelitic bulk composition, represent some of the oldest preserved clastic sedimentary rocks, sourced from ultramafic, mafic, and felsic igneous rocks or reflecting metasomatic alterations of basaltic rocks.

The presence of ~3.6 Ga monazite establishes a minimum age for the high Al/Ca composition essential for monazite growth in Isua's Archean crust, supporting interpretations of late Eoarchean metamorphism.