The origin and evolution of eclogite xenoliths and associated diamonds from the Jericho kimberlite, northern Slave craton, Canada: an integrated petrological, geochemical and isotopic study

• Main Author: Smart, Kathleen Alexandra
• Format: Dissertation
• Language: English
• Subjects: geochemistry; geology; petrology
• ISBN: 0494894660; 9780494894668

Diamond-bearing eclogite xenoliths are relatively rare but provide insight into the composition and evolution of the cratonic lithospheric mantle and also insight into the diamond formation process. The major-, trace-element and Sr-Pb-O-isotope compositions of both high- and low-MgO diamond eclogites from the Jericho kimberlite, Nunavut, Canada, indicate that each group had different geneses. Both eclogite groups formed as parts of ancient oceanic lithosphere; positive Sr anomalies and δ18O values of 6.5-6.6‰ of the low MgO eclogites indicate seawater-altered gabbroic protoliths, while the high Mg and Cr, fractionated HREEs, and δ 18O values of 5.3-5.5‰ of the high-MgO eclogites indicate a mantle origin. High-MgO eclogite crystallization probably occurred at 2-3 GPa where mantle-derived melts are generally basaltic, as melts in the diamond stability field are far too Mg-rich. As such, eclogite crystallization likely occurred as pyroxenite veins at the underside of thick Archean oceanic lithosphere. Although there are no age constraints for the diamond eclogites, both groups could have been imbricated into the cratonic mantle by subduction stacking of lithosphere during putative Neoarchean or Paleoproterozoic subduction events in the Slave craton. The petrogenesis of the high-MgO eclogites is further complicated by diamond inclusions with lower Cr2O3 and Mg-numbers but higher CaO, Na2O and Al2O3; element exchange with surrounding peridotite after diamond formation altered the eclogite composition.
Diamonds from each eclogite group have widely different carbon isotope compositions and nitrogen contents and thus the diamond forming fluids/melts were derived from disparate sources. Diamonds from the high-MgO eclogites have extremely low δ13C values of -40‰ which require derivation from subducted organic carbon; diamonds from the low-MgO eclogites have δ13C values from - 4.75 to -3.5 and could have been sourced from either fractionated mantle-derived carbon or subducted carbonate sediments. The reducing nature of the deep CLM favors the latter carbon source. Detailed SIMS investigations of both diamond groups revealed coupled increases in δ13C and decreases in N in growth zones towards diamond rims. The coupled δ13C-N changes indicate diamond growth occurred from oxidized fluids or melts, where nitrogen behaved compatibly in diamond compared to the source fluid/melt.