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Abstract Viscosity of silicate melts governs magma transport and influences mantle dynamics, yet effects of pressure and water on melt viscosity remain poorly understood. Here, we report in situ falling‐sphere viscosity measurements on diopside (Di) melts with 0–3 wt.% H₂O along the liquidus up to 7 GPa and 2103 K using synchrotron X‐ray radiography. By incorporating our hydrous melt data into a previously validated model for the dry system, the effects of pressure, temperature, and H₂O contents on Di melt viscosity can be satisfactorily captured by the function: whereT*is the homologous temperature,x_H2Ois the molar % H₂O,η₀ = 8.90 (1.50) × 10⁻⁸ Pa s,b₀ = 3.02 (0.10), andH*(P) = 15.72 (0.03)−0.35 (0.01)·P + 1.07 (0.07) × 10⁻²·P²−1.19 (0.14) × 10⁻⁴ P³, ×10⁻³ GPa⁻¹. Adding 3 wt.% H₂O systematically reduces viscosity by ∼0.7 log units. For both dry and hydrous melts, viscosity along the liquidus decreases monotonically with increasing pressure, suggesting that moderate hydration may not significantly alter the compressional behavior of Di melts. Combining the Di viscosity model with models for feldspar and olivine, we simulated the viscosity of analog basaltic magmas under mantle conditions. Increasing H₂O content from 0 to 3wt.% raises mobility of basaltic magma increases by >1 order of magnitude. In hot plume settings, the mobility further increases by a factor of 30 relative to typical ambient mantle. Assuming a simple percolation model, the increased mobility corresponds to faster melt ascent in mantle plumes that could, in part, explain the voluminous magmatism of large igneous provinces. 

Abstract Silicate melts play a crucial role in planetary differentiation. The density contrast between silicate melts and the surrounding solid residue exerts a primary control on many magmatic processes. However, direct measurements of the density of silicate melts at high pressure (P) and temperature (T) conditions remain challenging, particularly for the highly viscous and reactive silica‐ and alkali‐rich melts. Here we determined the highP‐Tdensities of three sodium aluminosilicate melts with nepheline (NaAlSiO₄), jadeite (NaAlSi₂O₆), and albite (NaAlSi₃O₈) compositions, using the high‐PX‐ray microtomography technique up to 4.1 GPa and 2020 K. Our results suggest that the substitution of (NaAl)⁴⁺for Si⁴⁺along the NaAlSiO₄‐NaAlSi₃O₈join leads to higher melt density and lower melt compressibility. In addition, our new data, combined with a wide range of literature data, were employed to re‐calibrate a modified hard‐sphere equation of state (HS‐EOS) for silicate melts, which provides a unified framework for calculating the density and other compressional properties of multi‐component silicate melts in the CaO‐MgO‐Al₂O₃‐SiO₂‐FeO‐Na₂O‐K₂O (CMASFNK) system up to ∼25 GPa. The calibration also reveals that SiO₂, alkalis, and CaO are the major components contributing to the compositional dependence of melt elastic properties. The HS‐EOS was then applied to alkali basaltic melts at cratonic mantle conditions and silica‐ and alkali‐rich melts at early planetesimal melting conditions, with implications for the gravitational stability and extraction of melts in Earth's mantle and planetesimal settings. 

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. 

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 (η_min) 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. 

Significance The exothermic metamorphic reaction in orthopyroxene (Opx), a major component of oceanic lithospheric mantle, is shown to trigger brittle failure in laboratory deformation experiments under conditions where garnet exsolution takes place. The reaction product is an extremely fine-grained material, forming narrow reaction zones that are mechanically weak, thereby facilitating macroscopic faulting. Oceanic subduction zones are characterized by two separate bands of seismicity, known as the double seismic zone. The upper band of seismicity, located in the oceanic crust, is well explained by dehydration-induced mechanical instability. Our newly discovered metamorphism-induced mechanical instability provides an alternative physical mechanism for earthquakes in the lower band of seismicity (located in the oceanic lithospheric mantle), with no requirement of hydration/dehydration processes. 

Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth’s interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated V P /V S ratio of the deep upper mantle (∼180–330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated V P /V S ratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80–140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.