Decoding the Origins of Eoarchean Monazite in the Isua Supracrustal Belt: A Machine Learning Approach to Crustal Evolution

Monazite is a critical petrochronometer for deciphering metamorphic histories, yet its formation in Archean terranes is restricted by the predominantly basaltic to ultramafic nature of early crust. These lithologies typically lack the bulk chemical compositions required for monazite stability, favoring allanite instead. We investigate rare monazite-bearing metapelites from the Isua Supracrustal Belt (ISB) to determine the geochemical drivers that enabled these rocks to host some of Earth’s oldest preserved monazite.
We utilized a supervised machine learning workflow to objectively identify the elemental ratios controlling monazite formation. By ranking an automated ratio library using Random Forest and optimizing feature selection by maximizing the Silhouette Score through iterative Linear Discriminant Analysis (LDA), we determined the geochemical drivers of group separation. This analysis highlights CaO/Al₂O₃, CaO/SiO₂, CaO/Y₂O₃, CaO/Ce₂O₃, CaO/MnO, MgO/SiO₂ and FeO/MgO as some of the most critical discriminants distinguishing monazite-bearing lithologies from typical Archean crust.
The LDA reveals that monazite-bearing rocks from Isua chemically overlap with modern monazite-bearing metasediments and S-type orthogneisses. This suggests that the weathering of mixed mafic-felsic sources to form clastic sediments, or the metasomatic alteration of a basaltic precursor, allowed specific Archean lithologies to evolve into compositions indistinguishable from modern crustal rocks. In-situ U-Th-Pb dating links this chemical evolution to the Eoarchean, yielding two monazite generations: ancient grains in garnet cores at ~3.6 Ga and younger grains in garnet rims and the matrix at ~2.7 Ga. The absence of other monazite occurrences in Isua and the in-situ formation of both generations suggest a metamorphic rather than detrital origin, indicating at least two metamorphic events affected the ISB. The ~3.6 Ga population represents one of the oldest monazite occurrences ever discovered on Earth. This finding establishes a minimum age for the emergence of "modern" high-Al/Ca crustal compositions, demonstrating that geological processes capable of stabilizing monazite were active in the ISB by 3.6 Ga.