A seismic low velocity layer (LVL) above the mantle transition zone (MTZ), often thought to be caused by volatile-induced melting, can significantly modulate planetary volatile cycles. In this work, we show that an LVL observed beneath northeast Asia is characterized by small, 0.8 +/- 0.5 vol%, average degrees of partial melting. Seismically derived P-T conditions of the LVL indicate that slab-derived CO2, possibly combined with small amounts of H2O, is necessary to induce melting. Modeling the reactive infiltration instability of the melt in a stationary mantle above a stalled slab, we demonstrate that the volatile-rich melt slowly rises above the stalled slab in the MTZ, with percolation velocities of 200-500 mu m/yr. The melt remains stable within the LVL for this geologically significant period of time, potentially transferring up to 52 Mt/yr of CO2 from the subducting slab to the mantle for an LVL similar in areal extent (3.4x106km2) to the northeast Asian LVL. Reaction between the melt channels and the LVL mantle precipitates up to 200 ppmw solid C in localized zones. Using the inferred small melt volume fraction to model trace element abundances and isotopic signatures, we show that interaction between this melt and the surrounding mantle can over the long-term produce rocks bearing a HIMU like geochemical signature.

We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where greater than or similar to 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with similar to 0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing. (c) 2022 The Author(s). Published by Elsevier B.V.

Tschauner et al. (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds.

Tschauner et al. (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds.

Harzburgites and dunites forming the base of the Late Cretaceous-Paleocene Papuan Ultramafic Belt (PUB) and Marum ophiolites of Papua New Guinea (PNG) are among the most refractory mantle peridotites on Earth. We present a new integrated dataset of major element, bulk plus mineral trace element and Re-Os isotopic analyses aimed at better understanding the genesis of these peridotites. The PUB harzburgites contain olivine (Fo(92-93)), low-Al enstatite (less than or equal to 0.5 wt. % Al2O3 and CaO), and Cr-rich spinel (Cr#= 0.90-0.95). The Marum harzburgites are less refractory with olivine (Fo(91.9)-(92.7)), enstatite (similar to 0.5-1.0 wt. % Al2O3 and CaO), minor clinopyroxene (diopside), and spine! (Cr# = 0.71-0.77). These major element characteristics reflect equivalent or greater levels of melt depletion than that experienced by Archean cratonic peridotites. Whereas bulk-rock heavy rare earth element (HREE) abundances mirror the refractory character indicated by the mineral chemistry and major elements, large-ion lithophile elements indicate a more complex melting and metasomatic history. In situ olivine and orthopyroxene REE measurements show that harzburgites and dunites have experienced distinct melt-rock interaction processes, with dunite channels/lenses, specifically, showing higher abundances of HREE in olivine. Distinctive severe inter-element fraction of platinum group elements and Re result in complex patterns that we refer to as 'M-shaped'. These fractionated highly siderophile element (HSE) patterns likely reflect the dissolution of HSE-rich phases in highly depleted peridotites by interaction with subduction-related melts/fluids, possibly high-temperature boninites. Osmium isotope compositions of the PNG peridotites are variable (Os-187/Os-188 = 0.1204 to 0.1611), but fall within the range of peridotites derived from Phanerozoic oceanic mantle, providing no support for ancient melt depletion, despite their refractory character. This provides further evidence that highly depleted peridotites can be produced in the modern Earth, in subduction zone environments. The complex geochemistry indicates a multi-stage process for the formation of the PNG mantle peridotites in a modern geodynamic environment. The first stage involves partial melting at low-pressure (<2 GPa) and high-temperature (similar to 1250 degrees C-1350 degrees C) to form low-K, low-Ti tholeiitic magmas that formed the overlying cumulate peridotite-gabbro and basalt (PUB only) sequences of the ophiolites. This is inferred to have occurred in a fore-arc setting at the initiation of subduction. Later stages involved fluxing of the residual harzburgites with hydrous fluids and melts to form replacive dunites and enstatite dykes and interaction of the residual peridotites in the overlying mantle wedge with high-temperature hydrous melts from the subducting slab to generate the extremely refractory harzburgites. This latter stage can be linked to the eruption of low-Ca boninites at Cape Vogel, and other arc-related volcanics, in a nascent oceanic island arc. Both ophiolites were emplaced shortly after when the embryonic oceanic island arc collided with the Australian continent.

Tschauner et al. (Reports, 11 November 2021, p. 891) present evidence that diamond GRR-1507 formed in the lower mantle. Instead, the data support a much shallower origin in cold, subcratonic lithospheric mantle. X-ray diffraction data are well matched to phases common in microinclusion-bearing lithospheric diamonds. The calculated bulk inclusion composition is too imprecise to uniquely confirm CaSiO3 stoichiometry and is equally consistent with inclusions observed in other lithospheric diamonds.

The Earth could have experienced sulfide segregation during its differentiation due to sulfur (S) saturation within a magma ocean. The relative timing of sulfide saturation during magma ocean crystallisation is strongly dependent on the solubility of S at sulfide saturation (SCSS). Here, we present SCSS data directly relevant for a deep terrestrial magma ocean obtained from laser heated diamond anvil cell experiments. Our new data, along with existing SCSS data obtained for similar compositions, was parameterised to derive a new predictive equation. Our parameterisation predicts that existing models strongly underestimate the SCSS over the P-T range of a deep magma ocean. Our SCSS models provide the S abundances required at any given stage of terrestrial accretion, and imply that sulfide saturation is much less efficient at stripping the Earth's mantle of S during accretion than previously predicted. Applying our results to the most recent mantle S evolution models shows that the SCSS would be far too high to achieve sulfide saturation, until only perhaps the final stages of magma ocean crystallisation. To satisfy highly siderophile element systematics, either the S content of the magma ocean was considerably higher than currently assumed, or highly siderophile element abundances were affected by other processes, such as iron disproportionation.

How and when Earth's volatile content was established is controversial with several mechanisms postulated, including planetesimal evaporation, core formation and the late delivery of undifferentiated chondrite-like materials. The isotopes of volatile elements such as sulfur can be fractionated during planetary accretion and differentiation and thus are potential tracers of these processes. Using first-principles calculations, we examine sulfur isotope fractionation during core formation and planetesimal evaporation. We find no measurable sulfur isotope fractionation between silicate and metallic melts at core-forming conditions, indicating that the observed light sulfur isotope composition of the bulk silicate Earth relative to chondrites cannot be explained by metal-silicate fractionation. Our thermodynamic calculations show that sulfur evaporates mostly as H2S during planetesimal evaporation when nebular H-2 is present. The observed bulk Earth sulfur isotope signature and abundance can be reproduced by evaporative loss of about 90% sulfur mainly as H2S from molten planetesimals before nebular H-2 is dissipated. The heavy sulfur isotope composition of the Moon relative to the Earth is consistent with evaporative sulfur loss under 94-98% saturation condition during the Moon-forming giant impact. In summary, volatile evaporation from molten planetesimals before Earth's formation probably played a key role in establishing Earth's volatile element content.

An Excel spreadsheet compiling published major and trace element data for all important sublithospheric (upper mantle, transition zone and lower mantle) inclusion phases in diamond. Major element data are obtained by EPMA, trace element data by SIMS (ion microprobe) and LA-ICPMS. For additional details, please refer to Chapter 7: Geochemistry of Silicate and Oxide Inclusions in Sublithospheric Diamonds by Walter et al. in the RiMG volume "Diamond - Genesis, Mineralogy, and Geochemistry ", https://doi.org/10.2138/rmg.2022.88.07

The HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) of the Magellanic Clouds will use dust emission to investigate the life cycle of matter in both the Large and Small Magellanic Clouds (LMC and SMC). Using the Herschel Space Observatory's PACS and SPIRE photometry cameras, we imaged a 2 degrees x 8 degrees strip through the LMC, at a position angle of similar to 22.5 degrees as part of the science demonstration phase of the Herschel mission. We present the data in all 5 Herschel bands: PACS 100 and 160 mu m and SPIRE 250, 350 and 500 mu m. We present two dust models that both adequately fit the spectral energy distribution for the entire strip and both reveal that the SPIRE 500 mu m emission is in excess of the models by similar to 6 to 17%. The SPIRE emission follows the distribution of the dust mass, which is derived from the model. The PAH-to-dust mass (f(PAH)) image of the strip reveals a possible enhancement in the LMC bar in agreement with previous work. We compare the gas mass distribution derived from the HI 21 cm and CO J = 1-0 line emission maps to the dust mass map from the models and derive gas-to-dust mass ratios (GDRs). The dust model, which uses the standard graphite and silicate optical properties for Galactic dust, has a very low GDR = 65(-18)(+15) making it an unrealistic dust model for the LMC. Our second dust model, which uses amorphous carbon instead of graphite, has a flatter emissivity index in the submillimeter and results in a GDR = 287(-42)(+25) that is more consistent with a GDR inferred from extinction.