Hugh O’Brien

Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.

Craton Mantle Formation and Structure of Eastern Finland Mantle: Evidence from Kimberlite-Derived Mantle Xenoliths, Xenocrysts and Diamonds

The mantle that underlies Archean cratons is a poorly understood fundamental feature of Earth; consequently its genesis is an active topic of discussion and research. This ancient mantle has several unique characteristics, including low density relative to average mantle, concomitant melt-depleted compositions, highly magnesian olivine and consequently high seismic velocities. The buoyancy and rigidity of this type of mantle has allowed Archean lithosphere to remain stable and “float” in a way that keeps it mostly from being reworked back into the convecting mantle. The Karelian craton is one such ancient block of coupled crust and mantle that has survived since at least 3.5 Ga. Kimberlites and lamproites intruded into this region have carried fragments of the mantle and crust to the surface, providing unique information on the composition and structure of the lithosphere down to 250 km below the Earth’s surface. Due to the value of diamond, companies have undertaken exploration all over the world and provided materials for study of this otherwise inaccessible part of our planet.

Equilibria of Gold and Silver between Molten Copper and FeOx-SiO2-Al2O3 Slag in WEEE Smelting at 1300 °C

Waste Electrical and Electronic Equipment (WEEE) offers a significant resource for precious metals such as gold and silver. To maximize precious metal recoveries and sustainable use their behavior during WEEE smelting with copper as the collector metal needs to be characterized. This study experimentally determines the distributions of gold and silver between metallic copper and FeOx-SiO2-Al2O3 slag (LCu/s[Me] = [Me]Copper/[Me]Slag) in alumina-saturation over the oxygen potential range of 10-5–10-10 atm at 1300 °C. The experiments were conducted employing equilibration / quenching followed by major element analysis by Electron Probe Micro-Analysis (EPMA) and trace element analysis by Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) techniques. Our results show silver distribution increased exponentially from 30 to 1000 as a function of decreasing oxygen partial pressure. Gold distribution was 105 at pO2 = 10-5 atm and >106 at pO2 = 10-6–10-10 atm.

Properties of Na2O–SiO2 slags in Doré smelting

ABSTRACT Solubilities of metals in sodium silicate slags have been studied at 1000–1300°C in oxygen pressures of P(O2) = 10−4 to 0.5 atm, in Doré alloy smelting of copper anode slimes. The boundary conditions for the slags were silica saturation, magnesia saturation with about 10 wt% and without BaO. The trace elements studied were copper, gold, palladium, rhodium, and tellurium. The analytical methods used in the phase composition analyses of equilibrated samples, quenched in brine, were electron probe X-ray microanalysis and laser ablation-inductively coupled plasma-mass spectrometry. Thermodynamic features of the slags, trace element solubilities with the given constraints, and their oxidation mechanisms were determined. The data allow optimizing the anode slime-smelting for the metals’ maximum recovery in sodium silicate fluxing.

In situ Sr Isotope Compositions of Plagioclase from a Complete Stratigraphic Profile of the Bushveld Complex, South Africa: Evidence for Extensive Magma Mixing and Percolation

The cumulates of the Bushveld Complex, which form the largest layered intrusion on Earth, are known to have crystallized from several isotopically distinct magma pulses. Here, we present in situ Sr isotope compositions combined with the corresponding mineral chemistry of plagioclase from all lithological zones, covering > 6 km of stratigraphy, to constrain the petrogenesis of the complex. The in situ data coupled with high-resolution elemental maps of individual plagioclase grains reveal complex zonation patterns with respect to mineral chemistry and Sr isotope composition. This suggests that interstitial plagioclase in the Bushveld Complex crystallized from multiple, isotopically distinct influxes of melt percolating through a mafic cumulate framework and displacing the resident melt. Similarly, cumulus plagioclase grains are the result of continual ingress of a distinct magma, which picked up previously formed plagioclase crystals. Sr isotope compositions across the layered sequence seem to be largely decoupled from differentiation indices, such as Mg#, anorthite content or trace element ratios. As these observations cannot be explained by bulk assimilation, we propose that the elevated Sr isotopic signature of the Bushveld cumulates may have resulted from the interaction of the parental magma with a fluid derived from the up to 2 km thick dolomitic footwall, which caused a major shift in Sr isotope composition without significantly affecting the degree of differentiation or trace element signature. The decarbonation and/or assimilation of dolomite during the intrusion of the Bushveld Complex may be of major importance, not only for the petrogenesis but also for the emplacement of the layered intrusion, because devolatilization is directly linked to space creation owing to volume loss, thus producing a lopolith.