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 high η > 1,000 Pa s. Yet at their source regions, the melt properties are complexly influenced by pressure (P), temperature (T), and water contents (XH2O ${X}_{{\mathrm{H}}_{2}\mathrm{O}}$). In this study, we examined the combined P‐T‐XH2O ${X}_{{\mathrm{H}}_{2}\mathrm{O}}$ effects on the behavior of melts with an albite stoichiometry (NaAlSi3O8). We used first‐principles molecular dynamics simulations to examine anhydrous (0 wt % H2O) and hydrous (5 wt % H2O) melts. To constrain the P and T effects, we explored P ≤ 25 GPa across several isotherms between 2500 and 4000 K. The melts show anomalous P‐ρ relationships at low P ∼ 0 GPa and high T ≥ 2500 K, consistent with vaporization. At lithospheric conditions, melt ρ increases 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/Kanhydrous or Khydrous < Kanhydrous. We also find that the melt η decreases 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 decreased ρ and η by 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.
Plain Language Summary
The continental crust is produced by solidifying aluminosilicate‐rich magmas. Such magmas are known to be highly viscous at the surface. It is expected that the magmas will become more viscous due to increasing pressure at the deep crustal depths near their sources. However, observations contrast the expectations. Some volcanic eruptions indicate rapid movement of the aluminosilicate‐rich magmas before the eruption. The movement of magma is influenced by its density and viscosity. These properties are influenced by pressure, temperature, and the water contents of the magma. To better understand how these parameters affect magmas in the crust, we performed computer simulations on molten albite with and without water. The albite chemistry mimics the chemistry of aluminosilicate‐rich magmas in the crust. At conditions of the deep crust where magmas originate, the densities of the magmas increase with compression. The magma viscosities also decrease under the same pressure. Our results provide, in part, a plausible explanation for a surprisingly rapid eruption of the Chaitén volcano in Chile. Water lowers both the magma density and viscosity which helps to explain the rapid eruption of the hydrous aluminosilicate‐rich lavas.
Key Points
We constrain the combined effects of pressure, temperature, and water contents on aluminosilicate melts with an albitic composition
In the crust, relatively low temperatures should increase melt viscosity. Yet pressure and H2O lower the viscosity at these conditions
Our results provide a plausible explanation for rapid eruption of lavas which are typically very viscous and should hence be sluggish
The viscosity of magma plays a crucial role in the dynamics of the Earth: from the crystallization of a magma ocean during its initial stages to modern-day volcanic processes. However, the ...pressure-dependence behavior of viscosity at high pressure remains controversial. In this study, we report the results of first-principles molecular dynamics simulations of basaltic melt to show that the melt viscosity increases upon compression along each isotherm for the entire lower mantle after showing minima at ~6 GPa. However, elevated temperatures of the magma ocean translate to a narrow range of viscosity, i.e., 0.01-0.03 Pa.s. This low viscosity implies that the crystallization of the magma ocean could be complete within a few million years. These results also suggest that the crystallization of the magma ocean is likely to be fractional, thus supporting the hypothesis that present-day mantle heterogeneities could have been generated during the early crystallization of the primitive mantle.
Abstract
Water influences geodynamic processes such as melting, deformation and rheology, yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and ...glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savai‘i and two dunite xenoliths from Ta‘ū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), and major and trace element concentrations. Clinopyroxenes from the Ta‘ū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savai‘i peridotites are highly depleted harzburgites (melt depletion ≥17 %). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element enrichments. However, despite metasomatism the H2O concentrations in olivines (0 − 4 ppm H2O) and orthopyroxenes (17 − 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than those of other incompatible trace elements in these samples. Transects across mineral grains show generally homogeneous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai‘i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere.
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 geofluid η at 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 of η. 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 overcorrect η. We also suggest the uncertainty in η is best captured by the correction rather than propagated errors from experimental parameters. We develop an overlying view of Stokes' Law viscometry at high pressures.
Plain Language Summary
Liquids and vapors, collectively known as fluids, occur throughout the Earth and other planets. Compared to solid rock, fluids can move rapidly due to their lower viscosities and hence influence and promote important geologic processes. The large span of pressures and temperatures inside planets influences the viscosities of fluids. Measuring a fluid viscosity at relevant pressures is most often done by tracking a sphere that sinks in the fluid. The sinking speed of the sphere is converted to the fluid viscosity by balancing forces which cause and oppose the sinking, known as Stokes' Law. To create the right pressures, unique devices are used which require small chambers to house the fluid and sphere. The small chambers affect the sinking speeds and hence Stokes' Law becomes inaccurate. There are several corrections for the chamber effects on the sphere. However, there is no consensus on which correction should be used for high‐pressure measurements. We examined the chamber effects on the sinking speeds in high‐pressure experiments. We calculated the fluid viscosity using each correction and considered the uncertainties. We find that the corrections produce unique values of viscosity and are not equal. Future work should carefully consider the choice in correction.
Key Points
We performed high‐pressure experiments to evaluate drag acting on falling spheres in Stokes' Law viscometry
We evaluated different correction schemes for Stokes' Law and used Monte Carlo simulations to evaluate the uncertainties in each scheme
The best correction may be for drag due to the chamber walls, while other corrections may be ineffective or over‐correct the viscosity
Water influences geodynamic processes such as melting, deformation and rheology. Yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of ...erupted mantle melts (i.e. MORB and OIB). To constrain the mechanisms influencing the distribution of water in the mantle, we analyzed 15 peridotite xenoliths from Savaiʻi and 2 dunite xenoliths from Taʻū (Samoa) for water concentrations (by polarized FTIR) and major/trace elements. Savaiʻi peridotites are among the most melt depleted oceanic xenoliths recovered (degrees of melt depletion ≥15%). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and LREE enrichments down to the grain scale, but which are up to an order of magnitude more depleted than expected for metasomatism. LA-ICP-MS analyses of inclusion-rich and -poor domains within single mineral grains reveal significant heterogeneity and patterns with strong carbonatite and silicate characteristics. Despite this, water concentrations of olivines (up to 4 ppm H2O) and orthopyroxene (17 to 89 ppm H2O) are among the lowest reported in oceanic xenoliths. Orthopyroxene H2O correlate with indicators of melt depletion (i.e. Al, Mg#) and not metasomatism (i.e. LREE, Ti). Yet, they are higher than expected for melt depletion, but also up to two orders of magnitude lower than expected for metasomatism by presumably water-rich melts. FTIR transects across mineral grains show generally flat water profiles, indicating no significant water loss during ascent, and show no significant difference between inclusion-rich and -poor domains. Also, Raman Spectroscopy on single inclusions shows the presence of CO2 but lack H2O. If the melt-inclusion trails had several weight % water, as estimated for carbonatites, diffusion of water from the inclusions into the host phase would lower their water contents and simultaneously enrich water across the grain. This mechanism, but on a larger scale, likely also affected the Taʻū dunites. Their clinopyroxenes show matching trace element systematics to their host lavas, but with water contents several times lower than expected. Thus, melt-rock interactions in the oceanic lithosphere are complex processes that result in variable hydration of the lithosphere.
With rapid advances in our understanding of cancer, there is an expanding number of potential novel combination therapies, including novel–novel combinations. Identifying which combinations are ...appropriate and in which subpopulations are among the most difficult questions in medical research. We conducted a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-guided systematic review of trials of novel–novel combination therapies involving immunotherapies or molecular targeted therapies in advanced solid tumors. A MEDLINE search was conducted using a modified Cochrane Highly Sensitive Search Strategy for published clinical trials between July 1, 2017, and June 30, 2020, in the top-ranked medical and oncology journals. Trials were evaluated according to a criterion adapted from previously published Food and Drug Administration guidance and other key considerations in designing trials of combinations. This included the presence of a strong biological rationale, the use of a new established or emerging predictive biomarker prospectively incorporated into the clinical trial design, appropriate comparator arms of monotherapy or supportive external data sources and a primary endpoint demonstrating a clinically meaningful benefit. Of 32 identified trials, there were 11 (34%) trials of the novel–novel combination of anti-programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) and anti-cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) therapy, and 10 (31%) trials of anti-PD-1/PD-L1 and anti-vascular endothelial growth factor (VEGF) combination therapy. 20 (62.5%) trials were phase II trials, while 12 (37.5%) were phase III trials. Most (72%) trials lacked significant preclinical evidence supporting the development of the combination in the given indication. A majority of trials (69%) were conducted in biomarker unselected populations or used pre-existing biomarkers within the given indication for patient selection. Most studies (66%) were considered to have appropriate comparator arms or had supportive external data sources such as prior studies of monotherapy. All studies were evaluated as selecting a clinically meaningful primary endpoint. In conclusion, designing trials to evaluate novel–novel combination therapies presents numerous challenges to demonstrate efficacy in a comprehensive manner. A greater understanding of biological rationale for combinations and incorporating predictive biomarkers may improve effective evaluation of combination therapies. Innovative statistical methods and increasing use of external data to support combination approaches are potential strategies that may improve the efficiency of trial design. Designing trials to evaluate novel–novel combination therapies presents numerous challenges to demonstrate efficacy in a comprehensive manner. A greater understanding of biological rationale for combinations and incorporating predictive biomarkers may improve effective evaluation of combination therapies. Innovative statistical methods and increasing use of external data to support combination approaches are potential strategies that may improve the efficiency of trial design.
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