Showing papers by "Michael R. Carroll published in 2020"
TL;DR: In this article, the authors present the first data on crystallization kinetics of alkali feldspar, which is the main crystal phase in peralkaline rhyolitic melts, in order to improve our understanding of the evolutionary timescales of these melts and their ability to shift between effusive and explosive activity.
Abstract: Peralkaline rhyolites, associated with extensional tectonic settings, are medium to low viscosity magmas that often produce eruptive styles ranging from effusive to highly explosive eruptions. The role of pre-eruptive conditions and crystallization kinetics in influencing the eruptive style of peralkaline rhyolitic magmas has been investigated and debated considering equilibrium conditions. However, experimental constraints on the effect of disequilibrium in crystallization in such magmas are currently lacking in the literature. Therefore, we performed isobaric cooling experiments to investigate alkali feldspar crystallization kinetics in peralkaline rhyolitic melts. Experiments were performed under water-saturated, water-undersaturated and anhydrous conditions between 25 and 100 MPa, at 670-790 °C and with experimental durations ranging from 0.5 to 420 hours. Here we present the first data on crystallization kinetics of alkali feldspar, which is the main crystal phase in peralkaline rhyolitic melts, in order to improve our understanding of the evolutionary timescales of these melts and their ability to shift between effusive and explosive activity. Our experimental results indicate that the alkali feldspar nucleation delay can range from hours to several days as a function of undercooling and H2O content in the melt. Thus, a peralkaline rhyolitic magma can be stored at the pre-eruptive conditions for days without important variations of its crystal fraction. This suggests that crystallization may not necessarily play the main role in triggering fragmentation during explosive eruptions of peralkaline rhyolitic magmas.
16 citations
TL;DR: In this article, the molar absorptivities for near-infrared (NIR) absorption bands related to molecular water (5200 cm-1) and OH groups (4500 cm -1) were determined in synthetic hydrous pantelleritic glasses, with compositions similar to natural pantellerites from the Eburru complex of the Kenya Rift Valley.
Abstract: Abstract To aid current work on the genesis of pantelleritic magmas, and the desire to use IR spectroscopy to measure water contents in natural (e.g., melt inclusions) and experimental glasses of pantelleritic composition, we have determined molar absorptivities for near-infrared (NIR) absorption bands related to molecular water (5200 cm–1) and OH groups (4500 cm–1) in synthetic hydrous pantelleritic glasses, with compositions similar to natural pantellerites from the Eburru complex of the Kenya Rift Valley. The experiments were conducted at P = 30 to 150 MPa and T = 850–900 °C using a synthetic pantelleritic starting composition with (wt%) SiO2 = 76.60, Al2O3 = 8.48, FeO* = 5.48, K2O = 3.68, Na2O = 4.72, and with molar ratio (Na+K)/Al = 1.38. The experiments were H2O undersaturated (~1.1 to 6.5 wt% H2O), and the run products were analyzed by Karl-Fischer Titration (KFT) for total dissolved H2O abundance. Different combinations of baseline types (GG or TT) and intensity measurements (peak height and peak area) were applied to measure both hydroxyl group (OH) and molecular water (H2O) in the experimental samples. For instance, evaluating the peak heights and using the TT baseline e4500 results to be equal to 0.98(4) (L mol-1cm–1) and e5200 to 1.92(2) (L mol-1 cm–1); these values differ by ~20 to 50% from published values for metaluminous rhyolitic compositions.
7 citations
01 Jan 2020
TL;DR: Experimental studies of compositions relevant to magmatism at Vesuvius and Campi Flegrei (CF) provide constraints on the pressure, temperature, and magmatic volatile activities prevailing during various phases of eruption as discussed by the authors.
Abstract: Experimental studies of compositions relevant to magmatism at Vesuvius and Campi Flegrei (CF) provide constraints on the pressure, temperature, and magmatic volatile activities prevailing during various phases of eruption. Such information helps to define pressures (depths) of origin for some well-studied eruptions and differentiation trends that link magma compositions potentially related by crystal–liquid differentiation processes. The presence of mixed magmas and zoned eruptions (magma chambers) may complicate some interpretations, but the phase equilibrium constraints are robust for magma compositions not strongly affected by mixing. Similarly, studies of volatile solubilities in the relatively alkali-rich melt composition characteristic of Vesuvius and CF magmatism can provide valuable constraints for interpreting the composition of melt inclusions (MIs) in phenocrysts of many eruptive products. These experimental data can help to estimate pressures of MI entrapment, the possible importance of hydrosaline brines in some magmas, and degassing processes or CO2-fluxing experienced by melt compositions preserved in MIs.
6 citations