Combined petrography, noble gas, stable isotope and fluid inclusion chemistry of carbonatites from Uganda: Implications for the origin of the carbonatite melt in continental rift setting

Abstract

An integrated petrographic, fluid inclusion, noble gas, and stable isotope study was performed for the Late Oligocene, spatially associated Sukulu and Tororo carbonatite complexes in SE Uganda in order to understand their genesis and provide additional constraints on the evolution of the East African Rift System (EARS). Both carbonatite complexes comprise mainly ring-shaped calcio‑carbonatites with accessory primary magmatic phases, including olivine, phlogopite, aegirine and pyrochlore. Absent or low Na deficiency on the A-site and high F contents (0.4–1.2 apfu) reveal the magmatic origin of pyrochlore. In contrast, the corroded rims of olivine, Sr-enrichments of the rims of apatite, as well as the occurrence of Sr- and Mg-rich lamellae in calcite, coupled with the presence of barite, REE-witherite, siderite, pyrite, bastnäsite and galena as secondary phases imply a hydrothermal overprint of the primary magmatic mineral assemblage.Fluid inclusions found in the cores of clustered apatite crystals are primary, two-phase H2O + NaCl aqueous inclusions in Sukulu-, and two-, or multi-phase H2O + NaCl ± NaHCO3 ± other solid inclusions in Tororo Complex. In the gas phase of some inclusions small amounts of CO2 or CH4 have been detected by micro-Raman analysis. The estimated trapping temperature of 330–425 °C and pressure of 465–1330 bars using the accepted denudation rates calculated for the EARS as independent barometers, reveal subsolidus crystallization conditions for the intercrystalline clustered apatite aggregates. Stable C-O isotope compositions of both carbonatite complexes suggest ultimately a mantle origin for the carbonate phase.Helium, neon and argon isotope compositions, liberated by crushing of bulk rock, apatite and calcite samples, show a systematic variation between the analyzed phases. Relatively low 3He/4He ratios of apatite and pyroxene separates (<5.3 Ra) are accounted for by radiogenic production of 4He in these minerals, whereas the air-corrected values in calcite reach as high as 9.6 Ra, characteristic for plume sources. Whole-rock Ne isotope ratios provide evidence for a depleted MORB mantle (DMM) trend. Combined (21Ne/22Ne)EX (air-corrected 21Ne/22Ne ratio extrapolated to Ne-B composition Holland and Ballentine, 2006; Trieloff et al., 2000) versus 4He/3He are indicative of mixing of a plume-like and a DMM-like component in the source region of the parental carbonatite magmas, which is in contrast to some recent interpretations of recent/sub-recent magmas in the EARS. Based on new noble gas data from this study we argue that in the early phase of the magmatic activity of the EARS, a low-viscosity carbonatite magma may have intruded into the shallow crust rapidly, without any significant contamination by the sub-continental lithospheric mantle.Combined fluid inclusion data and noble gas results collectively imply that the carbonatites worldwide are carbothermal (hydrothermal) rather than purely magmatic products and that they crystallized at sub-solidus temperatures from dominantly magma-derived fluids.