At the 26^th AIRAPT conference in 2017, a task group was formed to work on an International Practical Pressure Scale (IPPS). This report summarizes the activities of the task group toward an IPPS ruby gauge. We have selected three different approaches to establishing the relation between pressure (P) and ruby R1-line shift (Δλ) with three groups of optimal reference materials for applying these approaches. Using a polynomial form of the second order, the recommended ruby gauge (referred as Ruby2020) is expressed by: $P[GPa]=1.87(\pm 0.01)\times {10}^3\left({\displaystyle \frac{\mathrm{\Delta }\lambda }{{\lambda }_0}}\right)\left[1+5.63(\pm 0.03)\left({\displaystyle \frac{\mathrm{\Delta }\lambda }{{\lambda }_0}}\right)\right]$, where λ₀ is the wavelength of the R1-line near 694.25 nm at ambient condition. In June of 2020, the Executive Committee of AIRAPT endorsed the proposed Ruby2020. We encourage high-pressure practitioners to utilize Ruby2020 within its applicable pressure range (up to 150 GPa), so that pressure data can be directly compared across laboratories and amended consistently as better scales emerge in the future.

Composition, mean pressure, mean melt fraction, and crustal thickness of model mid-ocean ridge basalts (MORBs) are calculated using MELTS. Polybaric, isentropic batch and fractional melts from ranges in source composition, potential temperature, and final melting pressure are integrated to represent idealized passive and active flow regimes. These MELTS-derived polybaric models are compared with other parameterizations; the results differ both in melt compositions, notably at small melt fractions, and in the solidus curve and melt productivity, as a result of the self-consistent energy balance in MELTS. MELTS predicts a maximum mean melt fraction (∼0·08) and a limit to crustal thickness (≤15 km) for passive flow. For melting to the base of the crust, MELTS requires an ∼200°C global potential temperature range to explain the range of oceanic crustal thickness; conversely, a global range of 60°C implies conductive cooling to ∼50 km. Low near-solidus productivity means that any given crustal thickness requires higher initial pressure in MELTS than in other models. MELTS cannot at present be used to model details of MORB chemistry because of errors in the calibration, particularly Na partitioning. Source heterogeneity cannot explain either global or local Na–Fe systematics or the FeO–K₂O/TiO₂ correlation but can confound any extent of melting signal in CaO/Al₂O₃.

We designed a plate impact shock recovery experiment to simulate the starting materials and shock conditions associated with the only known natural quasicrystals, in the Khatyrka meteorite. At the boundaries among CuAl₅, (Mg_0.75Fe²⁺_0.25)₂SiO₄ olivine, and the stainless steel chamber walls, the recovered specimen contains numerous micron-scale grains of a quasicrystalline phase displaying face-centered icosahedral symmetry and low phason strain. The compositional range of the icosahedral phase is Al_68–73Fe_11–16Cu_10–12Cr_1–4Ni_1–2 and extends toward higher Al/(Cu+Fe) and Fe/Cu ratios than those reported for natural icosahedrite or for any previously known synthetic quasicrystal in the Al-Cu-Fe system. The shock-induced synthesis demonstrated in this experiment reinforces the evidence that natural quasicrystals formed during a shock event but leaves open the question of whether this synthesis pathway is attributable to the expanded thermodynamic stability range of the quasicrystalline phase at high pressure, to a favorable kinetic pathway that exists under shock conditions, or to both thermodynamic and kinetic factors. Significance The singular occurrence, to date, of natural quasicrystals requires an explanation both of the possibility and of the rarity of their formation outside of the laboratory. Successful synthesis by an experimental shock, with starting materials similar to the exotic intermetallic alloys in the Khatyrka meteorite, demonstrates a mechanism that is feasible in space but not in any natural setting on Earth. The previously unrecognized composition of the synthesized quasicrystal, the first, to our knowledge, to be created by shocking discrete bulk starting materials, demonstrates a method for discovery of previously unknown quasicrystal compositions.

Geological and geochemical studies of a pelitic migmatite complex within the Isabella pendant of the southern Sierra Nevada batholith, California, suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent Goat Ranch pluton ca. 100 Ma. The leucosomes preserve a record of large- magnitude Nd isotope disequilibrium with respect to their pelitic source. The leucosomes have a wide range of «Nd(100 Ma) from 26.0 to 211.0, as compared to 28.7 to 211.3 for the source. They can be subdivided into two groups based on their major elements and Sr and Nd isotope geochemistry. Group I leucosomes have higher P2O5 contents and «Nd(100 Ma) values than those of group II. The «Nd(100 Ma) values of group I leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units, suggesting that group I leucosomes are in Nd isotope disequilibrium with their sources. Correlations among P2O5 contents, «Nd(100 Ma) values, and Sm/Nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating Sm from Nd and generating Nd isotope disequilibrium. Dissolution of apatite or monazite might play a critical role in regulating the behavior of the Sm-Nd isotope systems and thus the Nd isotope compositions of melts generated during crustal anatexis, especially in metasedi- mentary protoliths.