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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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Viscosity Measurements at High Pressures: A Critical Appraisal of Corrections to Stokes' Law
Abstract Fluids and melts in planetary interiors significantly influence geodynamic processes from volcanism to global‐scale differentiation. The roles of these geofluids depend on their viscosities (η). Constraining geofluidηat relevant pressures and temperatures relies on laboratory‐based measurements and is most widely done using Stokes' Law viscometry with falling spheres. Yet small sample chambers required by high‐pressure experiments introduce significant drag on the spheres. Several correction schemes are available for Stokes' Law but there is no consensus on the best scheme(s) for high‐pressure experiments. We completed high‐pressure experiments to test the effects of (a) the relative size of the sphere diameter to the chamber diameter and (b) the top and bottom of the chamber, that is, the ends, on the sphere velocities. We examined the influence of current correction schemes on the estimated viscosity using Monte Carlo simulations. We also compared previous viscometry work on various geofluids in different experimental setups/geometries. We find the common schemes for Stokes' Law produce statistically distinct values ofη. When inertia of the sphere is negligible, the most appropriate scheme may be the Faxén correction for the chamber walls. Correction for drag due to the chamber ends depends on the precision in the sinking distance and may be ineffective with decreasing sphere size. Combining the wall and end corrections may overcorrectη. We also suggest the uncertainty inηis best captured by the correction rather than propagated errors from experimental parameters. We develop an overlying view of Stokes' Law viscometry at high pressures.
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- PAR ID:
- 10507580
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Edition / Version:
- 1.0
- Volume:
- 129
- Issue:
- 5
- ISSN:
- 2169-9313
- Page Range / eLocation ID:
- 1-30
- Subject(s) / Keyword(s):
- Silicate melt viscosity high pressure Stoke's law synchrotron
- Format(s):
- Medium: X Size: 5.1MB Other: pdf
- Size(s):
- 5.1MB
- 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/10.1029/2023JB028489
