The water content in Earth's mantle today remains poorly constrained, but the bulk water storage capacity in the solid mantle can be quantified based on experimental data and may amount to a few times the modern surface ocean mass (OM). An appreciation of the mantle water storage capacity is indispensable to our understanding of how water may have cycled between the surface and mantle reservoirs and changed the volume of the oceans through time. In this study, we parameterized high pressure‐temperature experimental data on water storage capacities in major rock‐forming minerals to track the bulk water storage capacity in Earth's solid mantle as a function of temperature. We find that the mantle water storage capacity decreases as mantle potential temperature (T_p) increases, and its estimated value depends on the water storage capacity of bridgmanite in the lower mantle: 1.86–4.41 OM with a median of 2.29 OM for today (T_p = 1600 K), and 0.52–1.69 OM with a median of 0.72 OM for the early Earth's solid mantle (for a T_p that was 300 K higher). An increase in T_p by 200–300 K results in a decrease in the mantle water storage capacity by – OM. We explored how the volume of early oceans may have controlled sea level during the early Archean (4–3.2 Ga) with some additional assumptions about early continents. We found that more voluminous surface oceans might have existed if the actual mantle water content today is > 0.3–0.8 OM and the early Archean T_p was ≥1900 K.

We report experimental constraints on the melting curve of potassium chloride (KCl) between 3.2 and 9 GPa from in situ ionic conduction measurements using a multi-anvil apparatus. On the basis of concurrent measurements of KCl and sodium chloride (NaCl) at 1 bar using the differential thermal analysis (DTA) method and Pt sphere marker, we show that the peak rate of increase in ionic current with temperature upon heating coincides with latent heat ledge and fall of Pt sphere, thus establishing the criterion for melting detection from ionic conduction measurements. Applying this criterion to high pressures, we found that the melting point of KCl rose steeply with increasing pressure to exceed 2443 ± 100 K at 9 GPa. Fitting the results of this study together with existing data at pressures below 4 GPa and above 20 GPa, we obtained the Simon’s melting equation for KCl in the simple cubic B2 structure between 1.8 and 50 GPa: $T_m=1323{\left(\frac{P-1.87}{2.2\left(1\right)}+1\right)}^{\frac{1}{2.7\left(1\right)}}$ , where T is in K and P is in GPa. Starting at 1 bar, the melting point of KCl increases at an average rate of ~150 K/GPa to cross that of Pt near 9 GPa. The highly refractory nature of KCl makes it a sensitive pressure calibrant for the large-volume pressure at moderate pressures and a potential sample container for experiments at moderate pressures and very high temperatures.

Nitrogen, the most abundant element in Earth's atmosphere, is also a primary component of solid nitride minerals found in meteorites and on Earth's surface. If they remain stable to high pressures and temperatures, these nitrides may also be important reservoirs of nitrogen in planetary interiors. We used synchrotron X‐ray diffraction to measure the thermal equation of state and phase stability of titanium nitride (TiN) in a laser‐heated diamond anvil cell at pressures up to ∼70 GPa and temperatures up to ∼2,500 K. TiN maintains the cubic B1 (NaCl‐type) crystal structure over the entire pressure and temperature range explored. It has K₀ = 274 (4) GPa, K₀′ = 3.9 (2), and γ₀ = 1.39 (4) for a fixed V₀ = 76.516 (30) ų (based on experimental measurements), q = 1, and θ₀ = 579 K. Additionally, we collected Raman spectra of TiN up to ∼60 GPa, where we find that the transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical phonon modes exhibit mode Grüneisen parameters of 1.66(17), 0.54(15), and 0.93 (4), respectively. Based on our equation of state, TiN has a density of ∼5.6–6.4 g/cm³ at Earth's lower mantle conditions, significantly more dense than both the mantle of the Earth and the estimated densities of the mantles of other terrestrial planets, but less dense than planetary cores. We find that TiN remains stable against physical decomposition at the pressures and temperatures found within Earth's mantle, making it a plausible reservoir for deep planetary nitrogen if chemical conditions allow its formation.

Phase transitions in the mantle control its internal dynamics and structure. The post-spinel transition marks the upper–lower mantle boundary, where ringwoodite dissociates into bridgmanite plus ferropericlase, and its Clapeyron slope regulates mantle flow across it. This interaction has previously been assumed to have no lateral spatial variations, based on the assumption of a linear post-spinel boundary in pressure and temperature. Here we present laser-heated diamond anvil cell experiments with synchrotron X-ray diffraction to better constrain this boundary, especially at higher temperatures. Combining our data with results from the literature, and using a global analysis based on machine learning, we find a pronounced nonlinearity in the post-spinel boundary, with its slope ranging from –4 MPa/K at 2100 K, to –2 MPa/K at 1950 K, and to 0 MPa/K at 1600 K. Changes in temperature over time and space can therefore cause the post-spinel transition to have variable effects on mantle convection and the movement of subducting slabs and upwelling plumes.

Oceanic basalts provide an invaluable window into evolutionary processes governing mantle spatial and temporal chemical heterogeneity. Ocean island basalts (OIBs) and enriched mid-ocean ridge basalts (E-MORBs) are powerful tracers of mantle melting and crust-mantle recycling processes. Whether the elemental and isotopic variations observed in both E-MORBs and OIBs are derived from similar mechanisms, however, remains under debate. Investigating compositional differences between E-MORBs and OIBs is a simple approach to constrain their origins, a technique for which machine learning classification algorithms are optimal. Here we implemented a novel machine learning approach complemented by mantle component mixing models to highlight compositional differences between E-MORBs and OIBs and further investigate their petrogenesis (data sourced from GEOROC database and Gale et al., 2013). Considering Random Forest-based Gini indexes, elements sensitive to pressure and degree of melting (FeO, TiO ₂ , Lu, and Sr) were identified as the best discriminators between E-MORBs and OIBs. Our Gaussian process classification algorithm successfully classified OIBs and E-MORBs better than 97% of the time when considering 1) Sr & FeO and 2) TiO ₂ & Lu. The probabilistic nature of Gaussian process modeling permitted calculation of new quantitative discriminant diagrams rooted in probability (Sr vs. FeO and TiO ₂ vs. Lu). Complementary trace element modeling yielded compositionally similar E-MORB and OIB sources with moderately incompatible element enrichments in the OIB source due to the influence of recycled oceanic crust (Prytulak & Elliott, 2007). Our source compositions are consistent with a simple, joint model for E-MORB and OIB petrogenesis after Donnelley et al. (2014): low-degree partial melts of subducted slabs metasomatize the depleted mantle producing a re-fertilized mantle (RM). RM is randomly sampled at mid-ocean ridges to produce E-MORB, while upwelling plumes sample both RM and recycled oceanic crust, yielding OIB. References: Donnelly et al. (2004). Earth and Planet. Sci. Lett., 226(3–4), 347–366. Gale et al. (2013). Geochem., Geophys., Geosyst., 14(3), 489–518. Prytulak & Elliott (2007). Earth and Planet. Sci. Lett., 263(3–4), 388–403.

We present globally inverted pressure-temperature (P-T) phase diagrams up to 5,000 GPa for four fundamental planetary materials, Fe, MgO, SiO2, and MgSiO3, derived from logistic regression and supervised learning, together with an experimental phase equilibria database. These new P-T phase diagrams provide a solution to long-standing disputes about their melting curves. Their implications extend to the melting and freezing of rocky materials in the interior of giant planets and super-Earth exoplanets, contributing to the refinement of their internal structure models.