Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

Abstract

Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sr i rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sr i . Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87 Sr/ 86 Sr (Sr i ) and a less radiogenic modeled 238 U/ 204 Pb composition (l 2 ) than coexisting orthopyroxene. The range of calculated Sr i for plagioclase and orthopyroxene is 0.70506-0.70662(34) and 0.70290-0.70654(36), respectively. The average difference in Sr i between mineral pairs was 0.00095. The range of calculated l 2 for plagioclase and orthopyroxene is 9.42-10.30 (average 9.7) and 9.83-15.75 (average 10.1), respectively. The range of measured 208 Pb/ 206 Pb for plagioclase and orthopyroxene is 34. 757-36.439(33) and 36.669-41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sr i = 0.70654 (36) with calculated l 2 of 10.32 for larger grains as compared with Sr i = 0.70290 (32) and calculated l 2 of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal-liquidvapor mush system.

Geochronology of Layered Intrusions

“…1.3c) and where the Rb-Sr isochron is coherent, suggested that isotopic heterogeneity between minerals in cumulates may need to be considered (e.g., incomplete equilibration between cumulus crystals and interstitial melt derived from another part of the intrusion). Evidence for mineral-scale isotopic heterogeneity in the Skaergaard intrusion was confirmed by McBirney and Creaser (2003) and there is now an increasing number of studies from the Bushveld Complex that document initial Sr-Nd isotopic differences between plagioclase and orthopyroxene (Prevec et al 2005;Chutas et al 2012;Roelofse and Ashwal 2012). A promising approach to dating layered intrusions involves utilizing the 176 Lu − 176 Hf isotopic systematics of apatite, which is a relatively common accessory mineral in mafic-ultramafic rocks.…”

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

“…Disequilibrium Sr isotope compositions between plagioclase and coexisting orthopyroxene have been shown for the CZ and lower MZ (Eales et al 1990;Seabrook et al 2005;Chutas et al 2012;Roelofse and Ashwal 2012;Yang et al 2013;Mangwegape et al 2016). These results were further confirmed by Prevec et al (2005), who measured Nd isotope compositions of the same two minerals in Merensky Reef samples.…”

“…Eales et al 1990;Prevec et al 2005;Chutas et al 2012;Yang et al 2013). Modern high-precision dating of layered intrusions relies on the availability of zircon in cumulates.…”

“…In light of the evidence presented in our study, it is rather unlikely that zircon and baddeleyite crystallised from the same magma as that which produced the cumulus minerals. Instead, migration of compositionally diverse residual melts through the crystal pile, and associated mixing, seems to be a common feature in parts of the Bushveld Complex, and thus is further substantiated by common isotopic disequilibrium between cumulus and intercumulus minerals (Eales et al 1990;Mathez and Waight 2003;Seabrook et al 2005;Prevec et al 2005;Mathez and Kent 2007;Chutas et al 2012;Roelofse and Ashwal 2012;Yang et al 2013;Mangwegape et al 2016). As a consequence, interstitial zircon may have crystallised from any pulse of residual melt that migrated through the cumulus crystal framework, and potentially mixed with resident melt.…”

Multiple, isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

“…The blending of semi-consolidated crystal mushes as a result of subsidence during cooling was proposed by Yang et al (2013) to explain both inter-and intracrystalline Sr-isotopic disequilibrium in plagioclase from the Upper Critical Zone of the Bushveld Complex (see also Maier et al, 2013), whereas Prevec et al (2005) chose to explain Nd-isotopic disequilibrium between plagioclase and orthopyroxene in the Upper Critical Zone as being the result of the density-driven mixing of minerals from isotopically distinct magma pulses. Chutas et al (2012) using micro-drilling and progressive leaching experiments proposed that variable infiltration of contaminants into a partially solidified crystal mush was responsible for Sr-and Pb-isotopic disequilibrium in plagioclase and orthopyroxene from the Lower, Critical and Upper Zones of the Bushveld Complex. The intrusion of variably contaminated crystal mushes from deeper, crustal staging chambers was proposed by Roelofse and Ashwal (2012) to explain significant Sr-isotopic disequilibrium between plagioclase and orthopyroxene in the Main Zone of the Bushveld Complex.…”