ALBERT

Notes: Abstract  The platinum-group elements (PGE) in base metal sulfides (BMS) of the Merensky reef are mostly close to the detection limit of the proton microprobe. The only phase that accommodates appreciable PGE is pentlandite. Total average PGE plus Au grades of the sulfide fraction of the Merensky reef are about 500 ppm. We estimate the modal proportions of the major BMS to be around 53 percent pyrrhotite, 25 percent pentlandite, and 22 percent chalcopyrite (ignoring minor phases). Using this estimate, we calculate by how much the sulfides are oversaturated with respect to individual PGE. With respect to Pt, the sulfides are many times oversaturated, i.e., nearly all Pt occurs as discrete PGE phases. With regard to Pd the sulfides are oversaturated by about a factor of two. The Ru and Rh levels are at and below saturation levels. Available experiments suggest that the entire PGE content of the sulfide fraction can easily be accommodated in solid solution in BMS at temperatures as low as 500°C. The fact that the BMS are oversaturated with most PGE thus indicates that the sulfides have continued to exsolve PGE below that temperature. Calculated sulfur fugacities indicate that f S2 is controlled by silica activity, as expected in high-temperature ores, suggesting that metal/sulfur ratios of the ore may not have changed much since complete solidification of the intercumulus silicate melt of the Merensky reef. All sulfides investigated have cooled below the maximum temperature of pentlandite-pyrite coexistence, which experiments place at 250±30°C. Final closure temperatures of the sulfide-PGE mineral assemblages, approximated by extrapolating the pentlandite-pyrrhotite solvus beyond its experimentally determined range, are possibly as low as 80 to 90°C.

Notes: [Auszug] The ferric iron content in spinel coexisting with olivine and orthopyroxene forms the basis for several recent oxygen barometers5'6'9'10 applicable to mantle-derived rocks. O'Neill and Wall5 derived a solution model for the activity of magnetite in Cr-Al spinel from published thermodynamic data. ...

Notes: Abstract Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under controlled oxygen fugacity (f O 2). f O 2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO2-C, Ni−NiO, and Fe3O4−Fe2O3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe3+/ΣFe in spinel is a linear function of f O 2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog(f O 2), but decreases slightly with increasing Cr content in spinel. The Fe3+/ΣFe ratio falls with increasing pressure at given Δlog(f O 2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing f O 2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from the fayalite-magnetite-quartz (FMQ) buffer: $$\Delta log (f_{O_2 } )^{FMQ} = 0.27 + 2505/T - 400P/T - 6 log(X_{Fe}^{olv} ) - 3200(1 - X_{Fe}^{olv} )^2 /T + 2 log(X_{Fe^{2 + } }^{sp} ) + 4 log(X_{Fe^{3 + } }^{sp} ) + 2630(X_{Al}^{sp} )^2 /T.$$ The oxygen barometer is applicable to the entire spectrum of spinel compositions occurring in mantle rocks and mantle-derived melts, and gives reasonable results to temperatures as low as 800° C.

Notes: Abstract The oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities ( $$f_{O_2 } $$ ) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative $$f_{O_2 } $$ is predominantly controlled by sliding Fe3+-Fe2+ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing $$f_{O_2 } $$ with increasing refractoriness, to a relative $$f_{O_2 } $$ well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for $$f_{O_2 } $$ control. The solubility of carbon in H2O-rich fluid (and presumably in H2O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum $$f_{O_2 } $$ in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower $$f_{O_2 } $$ limit may be given by the H2O +C=CH4+O2 (C-H2O) equilibrium. Below C −H2O a fluid becomes CH4 rather than CO2-bearing and carbonatitic melt presumably unstable. The actual $$f_{O_2 } $$ in deep Archaean lithosphere is then a function of the activities of CO2 and MgCO3. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate $$f_{O_2 } $$ probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt $$f_{O_2 } $$ relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe3+/Fe2+ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform $$f_{O_2 } $$ shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along $$f_{O_2 } $$ -pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative $$f_{O_2 } $$ will be constrained to below the CCO equilibrium and will be a function of $$a_{CO_2 } $$ . Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along $$f_{O_2 } $$ -decompression paths controlled by continuous Fe3+-Fe2+ solid-melt equilibria. These equilibria will involve increases in $$f_{O_2 } $$ relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.