Geochemical, Isotopic and Geochronological Investigation of the Magnet Cove Carbonatite Complex, Arkansas, USA

The Magnet Cove alkaline igneous complex, which was emplaced ~95 to 99 million years ago, is located within the northeastern region of Hot Spring County (Arkansas), and consists of alkaline silicate rocks such as nepheline syenite, ijolite, and jacupirangite along with associated carbonatite. This thesis presents new geochemical, radiogenic (Sr, Nd, Pb) and stable (C, O) isotope data, and first-time reported stable B isotope compositions (δ¹¹B values) for carbonatites and associated silicate rocks from the Magnet Cove complex. The δ¹³C_PDB (~ -5 ‰) and δ¹⁸O_SMOW (~8 to ~12‰) values for most of the carbonatites plot within or proximal to the primary igneous carbonatite (PIC) field and are therefore indicative of pristine mantle values. The radiogenic Nd, Sr, and Pb isotope compositions for both the carbonatites and alkaline silicate rocks are relatively constant, and contrast with the highly variable δ¹¹B values, in particular for the alkaline silicate rocks which range from -5.7 to ~ +26 ‰. The combined results indicate that the petrogenetic history of the Magnet Cove complex does not reflect melt differentiation in a closed-system involving a unique parental magma, and several of the alkaline silicate rocks and one carbonatite sample experienced low-temperature alteration and/or crustal contamination. The δ¹¹B values and radiogenic isotope data from the non-contaminated carbonatites and silicate samples indicate derivation from a mantle source characterized by a mixed HIMU-EMI signature, which is consistent with relatively young (<200 Ma old) carbonatite occurrences worldwide. The δ¹¹B values for pristine samples indicate that the mantle source region for the Magnet Cove complex contains recycled (subducted) crustal material, and prove their greater effectiveness in detecting open-system processes compared to their radiogenic and stable isotope counterparts.