Variable initial Pb isotopic compositions of rocks associated with the UG2 chromitite, eastern Bushveld Complex

An investigation of the Pb isotopic compositions of plagioclase and sulfide in a stratigraphic interval including the UG2 chromitite of the eastern Bushveld Complex has been conducted to determine the Pb isotopic composition(s) of the magma(s) that crystallized to form this part of the intrusion, gain a better understanding of why coexisting plagioclase and sulfide commonly exhibit widely different Pb isotopic compositions, and explore the use of Pb isotopes in deducing post-accumulation history. Analyses were obtained in situ with a NuPlasma multicollector ICP-MS coupled with 193 nm Excimer or 213 nm lasers.Most plagioclase compositions fall on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb geochron of 2.06 Ga, which is the solidification age of the intrusion. The measured ratios have not been affected by radiogenic ingrowth, and plagioclase generally remained closed to Pb exchange after initial cooling. The array of plagioclase compositions on the geochron is significantly larger than that defined by analytical error. This indicates that in terms of Pb at least two different magma compositions were present. The composition of the least radiogenic magma was approximated by that of the contemporaneous BSE with μ (²³⁸U/²⁰⁴Pb) and ω (²³²Th/²⁰⁴Pb) values of ≈9.0 and 35, respectively, suggesting a mantle derivation with little or no involvement of the continental crust, while the second magma possessed a Pb isotopic composition similar to the upper crust with μ ≈ 9.6.Compared to plagioclase, sulfides generally possess slightly higher ²⁰⁶Pb/²⁰⁴Pb ratios for equivalent ²⁰⁷Pb/²⁰⁴Pb ratios such that their compositions fall between the 2.06 and 1.86 Ga geochrons. The latter age is much younger than the cooling age. The data are interpreted to mean that the Bushveld Complex remained buried in the crust at temperatures of several hundred °C for about 200 Ma after solidification, and that any sulfides accessible to fluid continued to re-equilibrated during this time with more radiogenic Pb. The sulfide Pb may have been transported into the Bushveld Complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable. Alternatively, the sulfide Pb may have originated mainly from radiogenic decay of U and Th present in minerals other than the sulfides in the immediately surrounding Bushveld rocks, followed by local redistribution of Pb by whatever fluid was present. Indeed, some sulfides are characterized by ²⁰⁸Pb/²⁰⁴Pb ratios sufficiently high that an external source is unlikely. This observation and the fact that the sulfides display small-scale heterogeneity suggest that most, if not all, of the radiogenic sulfide Pb was locally derived. It also implies that during the post-solidification, re-equilibration period there was no large-scale fluid-flow through the microfracture network because otherwise the isotopic heterogeneities would not have been preserved. The minerals in 2 of the 19 samples studied contain young Pb, presumably introduced by meteoric waters that permeated the macrofracture network.