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Abstract The continental crust is produced by the solidification of aluminosilicate‐rich magmas which are sourced from deep below the surface. Migration of the magma depends on the density (ρ) contrast to source rocks and the melt viscosity (η). At the surface, these silica‐rich melts are typically sluggish due to highη > 1,000 Pa s. Yet at their source regions, the melt properties are complexly influenced by pressure (P), temperature (T), and water contents (). In this study, we examined the combinedP‐T‐ effects on the behavior of melts with an albite stoichiometry (NaAlSi3O8). We usedfirst‐principlesmolecular dynamics simulations to examine anhydrous (0 wt % H2O) and hydrous (5 wt % H2O) melts. To constrain thePandTeffects, we exploredP ≤ 25 GPa across several isotherms between 2500 and 4000 K. The melts show anomalousP‐ρrelationships at lowP ∼ 0 GPa and highT ≥ 2500 K, consistent with vaporization. At lithospheric conditions, meltρincreases with compression and is well described by a finite‐strain formalism. Water lowers the melt density (ρhydrous < ρanhydrous) but increases the compressibility, that is, 1/Khydrous>1/KanhydrousorKhydrous < Kanhydrous. We also find that the meltηdecreases with pressure and then increases with further compression. Water decreases the viscosity (ηhydrous < ηanhydrous) by depolymerizing the melt structure. The ionic self‐diffusivities are increased by the presence of water. The decreasedρandηby H2O increase the mobility of magma at crustal conditions, which could explain the rapid eruption and migration timescales for rhyolitic magmas as observed in the Chaitén volcano in Chile.
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The Viscosity of Albitic Melt at High Pressures and Implications for the Mobility of Crust‐Forming Magmas
Abstract The earliest form of continental crust was produced by tonalite‐trondhjemite‐granodiorite (TTG) magmas. Molten albite (NaAlSi3O8) is representative of TTGs and also a major component of modern crust‐forming magma. The viscosity of the melt controls the magma ascent rate and hence influences the production of new continental crust. It is well known that the viscosity (η) of albitic melt exhibits an anomalous pressure (P) dependence. However, prior results on the meltηat high‐Pdiffer significantly which limits our ability to predict the movement of crust‐forming magma at depth. In this study, we more tightly constrained theP‐effect onηin anhydrous albitic melt via high‐Pand high‐temperature (T) falling sphere experiments. We limited undesirable drag effects by using small sphere‐to‐capsule diameter ratios (d/D) such thatd/D ≤ 0.12, and evaluated uncertainties due to such drag using a Monte Carlo approach. Our results show that meltηfirst decreases withP(i.e., ∂η/∂P < 0) and then increases with continued compression (∂η/∂P > 0) with a well‐definedηminimum (ηmin) at ∼6 GPa along a ∼2,000 K isotherm. We find that the viscosity of the melt can be described by an Arrhenius formalism with an activation volume that varies withPandT. The results indicate thatηof aluminosilicate magmas decrease with depth and temperature in the crust, thereby mobilizing the magmas to promote rapid volcanic eruptions. The results also suggest that TTG magmas relevant for the early Earth could pond during ascent due to the anomalousP‐effect onη.
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- Award ID(s):
- 2246803
- PAR ID:
- 10672634
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 130
- Issue:
- 5
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024JB030527
