Experimental settling, floatation and compaction of plagioclase in basaltic melt and a revision of melt density

Centrifuge-assisted piston cylinder experiments were conducted on plagioclase in basaltic melt at 1140–1250 °C, 0.42–0.84 GPa and mostly 1000 g. One set of experiments assesses the settling velocity of a dilute plagioclase suspension; a second sinks or floats plagioclase in a MORB-type melt exploring conditions of neutral buoyancy; and a third set examines floatation of plagioclase from an evolved lunar magma ocean composition. A compaction rate for plagioclase cumulates is established. The experiments demonstrate that neutral density of plagioclase An74 in a MOR-type tholeiitic basalt occurs at 0.59 ± 0.04 GPa (1200 °C), contrasting predictions by present models on melt density which yield a density inversion pressure at 0.10–0.15 GPa. In nature, the level of neutral buoyancy depends on melt composition; nevertheless, for the onset of plagioclase crystallization in dry tholeiitic basalts, our result is robust. As the molar volume of plagioclase is well known, the experimentally determined pressure of neutral buoyancy indicates a correction of -1.6% to previous density models for silicate melts. It follows that for (tholeiitic) layered mafic intrusions, plagioclase is negatively buoyant for early, relatively primitive, parent melts. In contrast, the extreme Fe enrichment of a fractionating lunar magma ocean leads to melt densities that let anorthite always float. Compaction φ/φ0 of experimental plagioclase cumulates is quantified to φ/φ0 = − 0.0582 log (Δρ·h·a·t) + 1.284, where φ0 is the porosity after settling (67 ± 2%), h the cumulate pile height, a acceleration and φ porosity as a function of time t. Gravitational-driven compaction in tens of m-thick plagioclase cumulate in basaltic magmas reaches down to ~ 40% porosity within hundreds of years, a timescales competing with characteristic cooling times of cumulate layers of mafic intrusions. To achieve plagioclase modes > 80% due to compaction, an additional overload of ~ 100 m (layers) of mafic minerals would be required. Compaction of a lunar anorthosite crust of 35 km to 20% porosity (i.e. ~ 90% plagioclase after crystallization of the interstitial melt) would require 30 kyrs.

Microstructures and Late-Stage Magmatic Processes in Layered Mafic Intrusions: Symplectites from the Sept Iles Intrusion, Quebec, Canada

Replacive symplectites (vermicular intergrowths of two or more minerals) are an important feature of layered igneous intrusions, recording evidence of late-stage reactions between interstitial liquid and crystals. They are common throughout the Layered Series of the 564 Ma Sept Iles layered intrusion in Quebec, Canada, and fall into three types: oxy-symplectites, ‘Type I’ symplectites, and ‘Type II’ symplectites. Oxy-symplectites are comprised of magnetite and orthopyroxene, nucleate on olivine primocrysts, and form via the reaction Olivine + O2 → Orthopyroxene + Magnetite; Type I symplectites (of which there are 3 distinct categories) are comprised of anorthitic plagioclase with pyroxene, amphibole, or olivine vermicules, grow from primocryst oxide grains, and replace primocryst plagioclase; and Type II symplectites (of which there are 2 distinct categories) are comprised of anorthitic plagioclase with orthopyroxene ± amphibole vermicules, grow from primocryst olivine grains, and replace primocryst plagioclase. Rare symplectites composed of biotite and plagioclase are also present. Symplectite growth occurred at 700–1030°C with pressure constraints of 1–2 kbar. We propose that Type I symplectites, and some Type II symplectites, formed from the interaction of primocrysts with residual Fe-rich liquid as a consequence of differential loss of an immiscible Si-rich liquid conjugate from the crystal mush. However, redistribution and concentration of hydrous fluids in incompletely solidified rock, or an increase in water activity of the interstitial melt, may be more plausible processes responsible for the formation of replacive symplectites comprising abundant hydrous mineral assemblages.

Rapid cooling of the Rustenburg Layered Suite of the Bushveld Complex (South Africa): Insights from biotite 40Ar/39Ar geochronology

Despite their importance to understanding magma chamber processes and the formation of economically viable precious metal deposits, the cooling histories of layered mafic intrusions remain enigmatic due to limited geochronologic constraints. We provide a comprehensive 40Ar/39Ar study of biotite throughout the Rustenburg Layered Suite (RLS) of the Bushveld Complex, South Africa. Analyses of individual biotite grains from 10 samples, encompassing ∼5.5 km of RLS stratigraphy, yielded weighted mean plateau ages that all overlap at 2σ (α-95% confidence level) and range from 2056.3 ± 3.2 Ma to 2052.0 ± 7.6 Ma (2σ). A weighted mean of all biotite plateau ages yielded an age of 2054.47 ± 0.84 Ma (2σ, n = 30, mean square of weighted deviates = 0.23, P = 1.00; ±21 Ma fully propagated). The overlap between our 40Ar/39Ar biotite and published U-Pb zircon ages suggests that the RLS cooled rapidly to the closure temperature of biotite, with cooling rates on the order of 1000 °C m.y.–1 throughout the stratigraphy. Thermal modeling requires enhanced heat loss, due to the hydrothermal system associated with the emplacement of the RLS, to produce the inferred rapid cooling rates. Previously reported young 40Ar/39Ar biotite ages from the UG-2 and MG-1 chromitite seams and the Merensky Reef are likely a product of localized late-stage circulation of hydrothermal fluids. The lack of similarly young 40Ar/39Ar biotite ages from the remainder of the stratigraphy suggests that late-stage hydrothermal events were potentially localized to chromitites and the Merensky Reef.