The distribution and generation of carbonatites

• Main Author: Gibson, Sally
• Format: Article in Journal/Newspaper
• Language: English
• Published: Geological Society of America 2024
• Subjects: Greenland; Indian
• Online Access: https://www.repository.cam.ac.uk/handle/1810/369239; https://doi.org/10.17863/CAM.109124

The physio-chemical framework that generates carbonatites and, ultimately, the associated rare-earth element deposits remains contentious. This primarily reflects the diverse tectonic settings in which carbonatites occur: large igneous provinces; continental rifts and major extensional terranes; syn-to post-collisional settings; or ocean islands. There is, however, a broad consensus that carbonatites (or their parental melts) originate in the mantle. Additionally, these exotic melts have small volumes that make them ideal probes of conditions in their underlying source regions. We innovate by combining the locations of carbonatites emplaced on the continents with global maps of lithospheric thickness, derived from seismic tomography. We show from these that Neoproterzoic to Recent carbonatites occur preferentially above the margins of thick cratonic lithosphere (e.g. adjacent to the S. Atlantic and Indian Oceans or in N. America, Greenland and Asia), and also in regions where once thick lithosphere has undergone stretching (e.g. eastern Asia). Our thermal modelling shows that lateral and vertical heat conduction on rifted craton margins, or rapid stretching of cratonic lithosphere, can mobilise carbonated peridotite at the temperatures (950 to 1250 oC) and pressures (2 to 3 GPa) required to form primary carbonatites or their parental alkali silicate melts. Importantly, our models show that heat conduction from upwelling mantle plumes or ambient mantle on rifted cratonic margins may sufficiently modify the temperature of the lithospheric mantle to cause melting of carbonated peridotite, settling the long-standing debate on the role of rifting and heating in the generation of carbonatites.