Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex

Abstract

The Bushveld Complex (South Africa) is the largest layered intrusion on Earth and plays a considerable role in our understanding of magmatic differentiation and ore-forming processes. In this study, we present new geochemical data for apatite-hosted multiphase inclusions in gabbroic cumulates from the Bushveld Upper Zone. Inclusions re-homogenized at high-temperature (1060-1100 • C) display a range of compositions in each rock sample, from iron-rich (35 wt.% FeO tot ; 28 wt.% SiO 2 ) to silica-rich (5 wt.% FeO tot ; 65 wt.% SiO 2 ). This trend is best explained by an immiscible process and trapping of contrasted melts in apatite crystals during progressive cooling along the binodal of a two-liquid field. The coexistence of both Si-rich and Fe-rich immiscible melts in single apatite grains is used to discuss the ability of immiscible melts to segregate from each other, and the implications for mineral and bulk cumulate compositions. We argue that complete separation of immiscible liquids did not occur, resulting in crystallization of similar phases from both melts but in different proportions. However, partial segregation in a crystal mush and the production of contrasting phase proportions from the Fe-rich melt and the Si-rich melt can be responsible for the cyclic evolution from melanocratic (Fe-Ti-P-rich) to leucocratic (plagioclase-rich) gabbros which is commonly observed in the Upper Zone of the Bushveld Complex where it occurs at a vertical scale of 50 to 200 m.

Iron isotopic evolution during fractional crystallization of the uppermost Bushveld Complex layered mafic intrusion

“…Notably, the top portion (>4000 m above the MR, <35% liquid remaining) of the analyzed stratigraphy displays scattered measured values not mirrored by our estimations (Figure 6). VanTongeren and Mathez [2012] proposed that the uppermost 625 m of the Upper Zone underwent large-scale silicate liquid immiscibility, and the study of Fischer et al [2016] identified the presence of both Fe-rich and Si-rich melt inclusions trapped in apatite grains within the Upper Zone. Our data within the potential liquid immiscibility portion do follow markedly different trends than the data below it ( Figure 5).…”

Microstructural evolution of silicate immiscible liquids in ferrobasalts

“…Co-existing Fe-and Si-rich melt inclusions (e.g., Fischer et al 2016;Jakobsen et al 2005Jakobsen et al , 2011Luais 1987;Roedder and Weiblen 1970) and Fe-rich droplets dispersed in Si-rich glassy mesostasis in rapidly cooled natural examples (e.g., De 1974;Philpotts 1982;Philpotts and Doyle 1980) provide evidence of immiscibility in ferrobasaltic liquid. The physical behaviour of these immiscible liquids in a crystal mush may play an important role in the evolution of mafic intrusions, particularly if there is relative movement of the two liquids (e.g., Holness et al 2011;VanTongeren and Mathez 2012;Zhou et al 2013).…”

Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms

“…Other processes, such as inducing spinel oversaturation by magma oxidation, are similarly rather difficult to assess and have not been much tested directly, but have been proposed for other intrusions based on stable isotopic evidence for carbonate assimilation which has stimulated either chromite and sulphide saturation in the Uitkomst Complex (Gauert 2001) or magnetite oversaturation in the Panzhihua layered mafic intrusion (China) (Ganino et al 2013). The concept of immiscible metal-rich liquids, proposed but discarded early on in evaluations of evolved Bushveld rocks (Reynolds 1985), has reemerged in light of studies of thick Fe-Ti-V oxide deposits in China over the past decade or so (Zhou et al 2013;Fischer et al 2016;Nielsen et al 2017).…”