The Jacupiranga carbonatite (Brazil) was investigated to better understand how modal proportions and mineral chemistry control the fO2 recorded by a natural carbonatite rock.
Mineral compositions were determined using SEM-EDS, EPMA-WDS and Raman microspectroscopy building upon detailed petrography.
Based on magnetite-ilmenite oxythermobarometry, rock-forming temperatures range from 505 to 732 °C, with a wide span of fO2 values (∆FMQ –1.21 to +4.56). In each sample, ilmenite grains and lamellae are distinguishable, with the latter recording higher temperature–fO2 conditions. Consistently, independently determined homogenization temperatures of apatite-hosted fluid inclusions show a similar range between 550 and 685 °C. From the wall-rock contact toward the carbonatite body, samples exhibit a decrease in the modal abundance of olivine, phlogopite, magnetite, ilmenite, and dolomite, alongside an increase in Mg-calcite. Over the same spatial gradient, temperature, fO2, and the magnesioferrite (in magnetite) and geikielite (in ilmenite) components decrease. During the antiskarn reaction, silica and mafic components (Mg, FeT, Al, and Ti) derived from the ultramafic wall-rock were incorporated into olivine, phlogopite, magnetite-ilmenite, and dolomite. Calcite-rich samples, which were less influenced by the wall-rock, preserved ephemeral alkali components of the carbonatite melts within burbankite. Apatite-hosted fluid inclusions represent coeval fluid modified by antiskarn reactions. Adjacent to equilibrium magnetite-ilmenite pairs, olivine and phlogopite exhibit high mg# (93–97 and 88–95, respectively).
Despite the broad span of ∆FMQ values recorded by the oxide pairs, we found only a weak positive correlation between the recorded fO2 and mg#. Consequently, we suggest that the high mg# (>88–90) of carbonatitic olivine is primarily controlled by the low (<0.2) 〖K_D〗_(Fe^(2+)-Mg)^(olivine-carbonatite melt) (Fe2+/Mg exchange partition coefficient between olivine and the carbonatite melt) at crustal conditions. The exceptionally high mg# of olivine and phlogopite, compared to the adjacent dunite cumulates (mg# 84–88), combined with their negligible Ni and Cr contents, indicate an in situ antiskarn process.
This study demonstrates that carbonatite systems can be characterized by dynamic redox conditions, spanning the range from Fe3O4 ̶ Fe2O3 to Fe2SiO4 ̶ Fe3O4 ̶ SiO2 during their evolution and emplacement.
Major and trace element concentrations and Sr, Nd, Hf, Pb isotope ratios of global mid ocean ridge and ocean island basalt whole-rock compositions from the GEOROC and PetDB databases (2021-2022).
Key publications:
Stracke, A., Willig, M., Genske, F., Béguelin, P., & Todd, E. (2022). Chemical Geodynamics Insights From a Machine Learning Approach. In Geochemistry, Geophysics, Geosystems (Vol. 23, Issue 10). https://doi.org/10.1029/2022GC010606
Stracke, A., Willig, M., Genske, F., Béguelin, P., & Todd, E. (2022). Chemical and radiogenic isotope data of ocean island basalts from Tristan da Cunha, Gough, St. Helena, and the Cook-Austral Islands [dataset]. GRO.data. https://doi.org/10.25625/BQENGN
Global database of isotopic and major element compositions of kimberlites and carbonatites as compiled in:
Tappe, Sebastian; Romer, Rolf L.; Stracke, Andreas; Steenfelt, Agnete; Smart, Katie A.; Muehlenbachs, Karlis; et al. (2017): Sources and mobility of carbonate melts beneath cratons, with implications for deep carbon cycling, metasomatism and rift initiation. Earth and Planetary Science Letters. https://doi.org/10.1016/j.epsl.2017.03.011
Global geochemistry database for cratonic / anorogenic lamproites and related potassic rocks (compiled and screened from GEOROC in April 2020).
Related key publication: Ngwenya, Ntando S.; Tappe, Sebastian (2021): Diamondiferous lamproites of the Luangwa Rift in central Africa and links to remobilized cratonic lithosphere. Chemical Geology. https://doi.org/10.1016/j.chemgeo.2020.120019