Boron Isotope Signatures of Carbonatites: Novel Insights into Magmatic Carbonate Processes and Deep Mantle Carbon Flux

This thesis explores the origin of carbonatites, unique mantle-derived, carbonate-rich rocks, using a relatively novel approach based on combining boron isotope signatures (δ¹¹B ‰) with other radiogenic and stable isotope tracers, geochemical indices, and petrographic evidences. This work attempts to investigate three controversial issues regarding carbonatite melt formation, which are: 1. Depth of melt generation (i.e., lithospheric or asthenospheric), 2. Possible influence of late-stage hydrothermal activity giving rise to associated mineral deposits, or secondary post-emplacement alteration (or crustal involvement), on their original (upper mantle-inherited) geochemical and isotopic signatures, 3. Origin of carbon present within their upper mantle (metasomatized) sources (i.e., primordial or recycled), and whether this flux has changed through geologic time.

By documenting the first-reported δ¹¹B values for the world’s largest rare earth element deposit, the Bayan Obo carbonatite complex, the behavior of boron during multiple hydrothermal events was investigated. The boron isotopic compositions reported for pristine samples from Bayan Obo indicate an upper mantle origin despite significant hydrothermal activity. This result demonstrates the robust nature and effective use of δ¹¹B values in deciphering the composition of their mantle source region.

Boron isotopic ratios were determined for carbonate-rich rocks of contentious origin found within the Grenville Province and Sri Lanka. The Grenville Province samples yielded B isotope compositions compatible with high-temperature regional metamorphism of limestone, whereas the Sri Lankan samples were formed from carbonate-rich and ¹¹B-poor fluids derived from a crustal source. The combined Sr-B isotope approach used was effective in establishing distinct isotope fields that can be applied to future carbonatite-related studies.

A comprehensive B investigation of worldwide carbonatites ranging in age from 2.0 Ga to 40 Ma was conducted so as to provide insights into the origin and flux of deep mantle carbon within carbonatite melts over time. The results indicate an enhanced variability in δ¹⁸O_V-SMOW, δ¹³C_V-PDB, and δ¹¹B signatures of carbonatites with increasing geologic time. The total range (up to ~16‰) in δ¹¹B values for carbonatites examined suggests either the presence of enriched recycled subducted material in their mantle source region, or reflects the interaction between continental lithosphere and upwelling asthenospheric mantle.