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Understanding the viscosity of mantle-derived magmas is needed to model their migration mechanisms and ascent rate from the source rock to the surface. High pressure–temperature experimental data are now available on the viscosity of synthetic melts, pure carbonatitic to carbonate–silicate compositions, anhydrous basalts, dacites and rhyolites. However, the viscosity of volatile-bearing melilititic melts, among the most plausible carriers of deep carbon, has not been investigated. In this study, we experimentally determined the viscosity of synthetic liquids with ~31 and ~39 wt% SiO2, 1.60 and 1.42 wt% CO2 and 5.7 and 1 wt% H2O, respectively, at pressures from 1 to 4.7 GPa and temperatures between 1265 and 1755 °C, using the falling-sphere technique combined with in situ X-ray radiography. Our results show viscosities between 0.1044 and 2.1221 Pa·s, with a clear dependence on temperature and SiO2 content. The atomic structure of both melt compositions was also determined at high pressure and temperature, using in situ multi-angle energy-dispersive X-ray diffraction supported by ex situ microFTIR and microRaman spectroscopic measurements. Our results yield evidence that the T–T and T–O (T = Si,Al) interatomic distances of ultrabasic melts are higher than those for basaltic melts known from similar recent studies. Based on our experimental data, melilititic melts are expected to migrate at a rate ~from 2 to 57 km·yr−1 in the present-day or the Archaean mantle, respectively.
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From flow to furnace: Low viscosity of three-phase lavas measured at Kīlauea 2018 eruption conditions
Abstract Melt composition, temperature, and crystallinity are often seen as the three most important characteristics driving lava rheology, which controls eruptive behavior. Traditional methods of measuring the viscosity of crystallizing basalts often yield different mineral characteristics to natural samples and are typically bubble-free. To quantify the viscosity of basalts inclusive of bubble and crystal cargo, we developed a new technique to measure high-temperature three-phase isothermal lava viscosity and applied it to samples from the 2018 eruption of Kīlauea. This new experimental technique begins at subliquidus temperatures, preserving original phenocrysts. A short experimental duration allows for the retention of most of the original bubble population (19%–31% vs. 36% in the original lava) and accurate replication of crystal textures from field samples, as documented in quenched postexperiment samples. The observed rheological behavior in these experiments, conducted at syneruptive temperatures (1150–1105 °C) and strain rates (0.4–18 s–1), should therefore be representative of the lava flows. We measured average viscosities of 116 Pa·s at 1150 °C to 167 Pa·s at 1115 °C (i.e., only 10%–25% higher than calculated liquid viscosities at those temperatures) and a maximum of 1800 Pa·s at 1105 °C. These results are much lower than viscosity measured in traditional bubble-free experiments, which plateaued at ~14,000 Pa·s at 1115 °C. Our results suggest the effect of bubbles in three-phase magmas may be greater than predicted by models based on two-phase bubbly liquids, and this effect must be included in realistic lava flow rheology models. The method proposed here supplies a framework for providing the necessary experimental constraints.
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- Award ID(s):
- 1928923
- PAR ID:
- 10621035
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geology
- Volume:
- 53
- Issue:
- 2
- ISSN:
- 0091-7613
- Page Range / eLocation ID:
- 135 to 139
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Most lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle‐bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid‐rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho’eho’e.more » « less
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Abstract Viscosity is a fundamental physical property that controls lava flow dynamics, runout distance, and velocity, which are critical factors in assessing and mitigating risks associated with effusive eruptions. Natural lava viscosity is driven by a dynamic interplay among melt, crystals, and bubbles in response to the emplacement conditions. These conditions are challenging to replicate in laboratory experiments, yet this remains the most common method for quantifying lava rheology. Few in situ viscosity measurements exist, but none of those constrains the spatial evolution of viscosity along an entire active lava flow field. Here, we present the first real-time, in situ viscosity map of active lava as measured in the field at Litli-Hrútur, Iceland. We precisely measured a lava viscosity increase of over two orders of magnitude, associated with a temperature decrease, crystallinity increase, and vesicularity decrease from near-vent to distal locations, crossing the pāhoehoe–‘a‘ā transition. Our data expand the limited database of three-phase lava viscosity, which is crucial for improvements and validation of the current numerical, experimental, and petrological approaches used to estimate lava viscosity. Further, this study showcases that field viscometry provides a rapid, accurate, and precise assessment of lava viscosity that can be implemented in eruptive response modeling of lava transport.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G52679.1
