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Base metal sulfides (Fe–Ni–Cu–S) are ubiquitous phases in mantle and subduction-related lithologies. Sulfides in the mantle often melt incongruently, which leads to the production of a Cu–Ni-rich sulfide melt and a solid residue called monosulfide solid solution (mss). Even though peridotite-hosted sulfides, which tend to be more Ni-rich, are likely completely molten at mantle potential temperatures, the same is not true for eclogite-hosted Ni-poor, Fe-rich sulfides. Because of this, solid crystalline mss may persist at higher pressures and equilibrate with co-existing sulfide melt along colder geotherms, like those associated with subduction zones. Because highly siderophile elements (HSE—Pt, Pd, Rh, Ru, Os, Ir, and Re) are known to fractionate as a result of mss/sulfide-melt equilibrium, the persistence of an mss/sulfide-melt assemblage to higher pressures may lead to the fractionation of these elements during the subduction process. In this contribution, we carried out an experimental investigation of the partitioning behavior of the HSE, as well as Cu and Ni, between mss and sulfide melt over a pressure and temperature range relevant to equilibration between Earth's surface and transition zone depths (0.1 MPa to 14 GPa; 930–1530 ∘ C), and variable Ni contents in sulfide. Results show that at higher pressures, the HSE are considerably less fractionated as a result of mss and sulfide melt equilibrium compared to lower pressure conditions. This is exemplified by a lowering of the D i mss / melt for the more compatible HSE (Ru, Os, Ir, Rh and Re) from around 10 at 0.1 MPa to just above or below unity at 14 GPa. Moreover, the higher the Ni content of the bulk sulfide assemblage, the larger the degree of change in the magnitude of HSE fractionation seen over the pressure range studied. The exchange coefficient ( K D Ru - Pt ) between highly compatible HSE (Ru) and less compatible Pt illustrates a notable contrast. In the Ni-poor composition (E1), K D Ru - Pt changes from 27 at 0.1 MPa to 6 at 14 GPa. In contrast, the Ni-rich composition exhibits a broader range, with K D Ru - Pt ranging from 150 to 17 across the same pressure interval. Our results highlight key differences between experimental data obtained at lower and higher pressure, and how composition, namely the Ni content of sulfide, affects HSE partitioning behavior. [ABSTRACT FROM AUTHOR]

The Izu-Bonin subduction zone in the Northwest Pacific is an ideal location for understanding mantle dynamics such as cold lithosphere subduction. The slab produces a lateral thermal anomaly, inducing local topographic changes at the boundary of a post-spinel phase transformation, considered to be the origin of the '660-km discontinuity.' In this study, the short-period (1-2 Hz) S-to-P conversion phase S660P was used to obtain the fine-scale structure of the discontinuity. More than 100 earthquakes that occurred from the 1980s to the 2020s and were recorded by high-quality seismic arrays in the United States and Europe were analyzed. A discontinuity in the ambient mantle with an average depth of ~670 km was found beneath the 300-400-km event zone in the northern Bonin region near 33°N. Meanwhile, the '660-km discontinuity' has been pushed upward, away from the slab, possibly because of a hot upwelling mantle plume. In the central part of the subduction zone, the 660-km discontinuity is depressed to an average depth of (690 ± 5) km within the slab at approximately 150 km below the coldest slab core, indicating a (300 ± 100) °C cold anomaly estimated using a post-spinel transformation Clapeyron slope of (-2.0 ± 1.0) MPa/K. In southern Bonin near 28°N, the discontinuity was found to be further depressed at an average depth of (695 ± 5) km below the deepest event and with a focal depth of ~550 km. The discontinuity is located where the slab bends abruptly to become sub-horizontal toward the west-southwest. Near the zone of the isolated Bonin Super Deep Earthquake, which occurred at ~680 km on May 30, 2015, the discontinuity is depressed to ~700 km, suggesting a near-vertical penetrating slab and an S-to-P conversion in the coldest slab core, where a large low-temperature anomaly should exist. [ABSTRACT FROM AUTHOR]

We report the P-V-T equation of state measurements of B4C to 50 GPa and approximately 2500 K in laser-heated diamond anvil cells. We obtain an ambient temperature, third-order Birch–Murnaghan fit to the P-V data that yields a bulk modulus K0 of 221(2) GPa and derivative, (dK/dP)0 of 3.3(1). These were used in fits with both a Mie–Grüneisen–Debye model and a temperature-dependent, Birch–Murnaghan equation of state that includes thermal pressure estimated by thermal expansion (α) and a temperature-dependent bulk modulus (dK0/dT). The ambient pressure thermal expansion coefficient (α0 + α1T), Grüneisen γ(V) = γ0(V/V0)q and volume-dependent Debye temperature, were used as input parameters for these fits and found to be sufficient to describe the data in the whole P-T range of this study. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'. [ABSTRACT FROM AUTHOR]

Here, we investigated high-pressure behaviors of four end-members of K-Na-Ca-Mg alkali-bearing double carbonates (K2Mg(CO3)2, K2Ca(CO3)2, Na2Mg(CO3)2, and Na2Ca(CO3)2) using first-principles calculations up to ~ 25 GPa. For K2Mg, K2Ca, and Na2Mg double carbonates, the transitions from rhombohedral structures (R 3 - m or R 3 - ) to monoclinic (C2/m) or triclinic (P 1 - ) structures are predicted. While for Na2Ca(CO3)2, the P21ca structure remains stable across the calculated pressure range. But the high-pressure behavior of Na2Ca double carbonate has changed over 8 GPa: the b-axis becomes more compressible than a-axis; [CO3] –I groups tilt out of the a-b plane upon compression and reverse the direction of rotation at 8 GPa. The parameters for the equations of state of these minerals and their high-pressure phases were all theoretically determined. The predicted transformation is driven by the differences in the compressibility of structural units. The K+ and Na+ coordination polyhedra are more compressible in the structure, compared with the high axial rigidity of C–O bonds in the [CO3] triangle along the a-b plane. Our results provide projections of the high-pressure behaviors of trigonal double carbonates, in part by helping to clarify the relation among the average metallic ionic radius (Ravg), the bulk modulus (K0), and the transition pressure (PT). The transition pressure (PT) is anticorrelated to the average metallic ionic radius (Ravg), and a larger Ravg results in a lower bulk modulus (K0) for the trigonal double carbonates. Furthermore, alkali-bearing double carbonates found as inclusions in the natural diamond may indicate a hydrous parental medium composition and a deeper genesis mechanism. [ABSTRACT FROM AUTHOR]

We have derived a new equation of state (EOS) based on the Eulerian finite scheme utilising the 3 / 2 th power of edge length by pressure. By truncating the order of finite strains in the expression of the Helmholtz free energy, we have derived new expressions for second- and third-order EOSs, as well as expressions for bulk modulus and its first pressure derivative. We have considered four materials, viz. CaO, Bi, NaCl and Fe 2 SiO 4 to test the applicability and validity of the derived EOS. The feature of this EOS shows more validity than other prominent EOSs and corresponds well with the available experimental results. [ABSTRACT FROM AUTHOR]

Volcanic eruptions are shallow phenomena that represent the final stage of density- and viscosity- driven processes of melt migration from source rocks at upper mantle depths. In this experimental study, we investigated the effect of pressure (0.7–7.0 GPa) and temperature (1335–2000 °C) on the viscosity and the atomic melt structure of a synthetic anhydrous primitive alkaline basalt, an analogue of the pre-eruptive magma that likely feeds the Campi Flegrei Volcanic District at present day. Obtained viscosities (0.5–3.0 Pa s), mobility (0.1–0.4 g cm3 Pa−1 s−1) and ascent velocity (1.5–6.0 m yr−1) are presented to support geochemical and geophysical observations of Campi Flegrei as a critical volcanic district currently undergoing gradual magma recharge at depth. [ABSTRACT FROM AUTHOR]

A thermal expansion coefficient (αp) is an essential thermophysical quantity for high pressure research. The thermal expansion coefficient is the volume change over temperature in an isobaric heating process. Although isobaric heating processes for diamond anvil cell (DAC) have been conducted by keeping the pressure constant during heating, pressure determination at high temperature high pressure is debatable for decades long. In this paper, a revertible heating/cooling approach is presented, while its pressure determination at high temperature high pressure is not required. A pressure-dependent thermal expansion coefficient of MgO at 9.5 GPa by revertible heating/cooling was determined by a DAC and its result matches with the one collected by large volume press, whose isobaric heating was conducted by manually adjusting its load/pressure. [ABSTRACT FROM AUTHOR]

Jeffbenite, ideally Mg3Al2Si3O12, has been identified as inclusions in super-deep diamonds originating from depths that exceed 300 km. Although Mg-end member jeffbenite has limited stability at upper-mantle conditions, iron-bearing jeffbenite may have broader P–T stability that extends to the transition zone or uppermost lower mantle, incorporating significant amounts of ferric iron. Using synchrotron-based, single-crystal X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) at pressures up to 29 GPa, we report the crystal structure, compressibility, and likely spin transition of iron in ferromagnesian jeffbenite (Mg2.32Al0.03Fe2+1.28Fe3+1.77Si2.85O12). High-pressure structure refinements reveal that Fe3+ substitution for Si in the T2 site, which shares edges with the M2 octahedron, likely stabilizes jeffbenite at high pressure, because it increases the cation-to-cation distance between these sites. Although ferromagnesian jeffbenite does not undergo a structural phase transition below 30 GPa, SMS hyperfine parameters suggest the onset of an electronic spin transition of iron from high-spin (HS) to low-spin (LS) at around 22 GPa, which may increase its stability at high pressures. Pressure–volume data were fit to a third order Birch–Murnaghan equation of state, resulting in V0 = 816.54(9), KT0 = 181.54(1.39), and K T 0 ′ = 2.76(14). These equation of state parameters are applicable to evaluating the encapsulation pressures of super-deep diamonds. The density and bulk modulus of ferromagnesian jeffbenite are similar to or higher than pyrope–almandine, pyrope–majorite, and skiagite–majorite solid solution garnets, further suggesting that jeffbenite may be an important ferric–iron silicate in the deeper parts of the mantle transition zone and uppermost lower mantle. However, future studies on the influence of temperature and oxidation state on the stability and equations of state of iron-bearing jeffbennite are still needed to determine what role, if any, jeffbenite plays in transition-zone mineralogy. [ABSTRACT FROM AUTHOR]

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