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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 (Fo92–93), 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 (Fo91.9–92.7), enstatite (~0.5–1.0 wt. % Al2O3 and CaO), minor clinopyroxene (diopside), and spinel (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 (187Os/188Os = 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 (~1250°C–1350°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. [ABSTRACT FROM AUTHOR]

The "wet" silicate solidus of mantle peridotite defines the initial melting temperature of Earth's mantle under water‐saturated conditions and the second critical endpoint (SCEP) marks the high P‐T end of the wet solidus. However, the location of the wet solidus has remained an outstanding issue for over 50 years and the position of the SCEP is hotly debated. Published wet solidi show a difference of 200–600°C at a given pressure while reported SCEPs range from <4 to >6 GPa. Using a large‐volume multianvil apparatus, we investigated the water‐saturated melting behavior of a fertile peridotite at 3–6 GPa, 950–1200°C, and obtained well‐preserved quenched materials. On the basis of textures and compositions of the quenched materials, we bracket the wet solidus to between 950°C and 1000°C at 3 GPa and the SCEP between 3 and 4 GPa. Combining our experimental results with seismologic and petrologic observations, we propose that the lithosphere‐asthenosphere boundary in subduction zones should be constrained by the wet solidus and emphasize the role of a deep hydrous partial‐melting zone (DHPMZ) on magma genesis within the mantle wedge. We suggest that the DHPMZ is a source of hydrous melts to the primary melting zone in the mantle wedge and that the position of the volcanic front and its magma production rate may largely be controlled by melting and melt segregation processes within the DHPMZ. Our experimental results also suggest that high‐magnesian magmas (e.g., boninite, picrite, and komatiite) could be formed at conditions representative of subduction zones. Key Points: We investigated H2O‐saturated melting behavior of peridotite with a large‐volume multianvil apparatus and obtained well‐preserved quenched materialsWe determined the water‐saturated solidus and SCEP of peridotite from the textures and compositions of the quenched materialsThe experimental results provide new constraints on the melting processes and magma genesis within the mantle wedge [ABSTRACT FROM AUTHOR]

The connectivity of molten Fe–S in peridotite has been experimentally investigated by means of in situ electrical conductivity measurements at high temperatures and 1 GPa. Starting materials were powdered mixtures of peridotite KLB-1 with various amounts (0, 3, 6, 13, 19, 24 vol.%) of the 1 GPa eutectic composition in the Fe–FeS binary system. At temperatures above the eutectic point in the Fe–FeS system (∼980 °C) and below the solidus of KLB1 (∼1200 °C), molten Fe–S in a solid silicate matrix interconnects when the volume fraction is over ∼5%. Conductivity–temperature paths indicate that in the presence of partial silicate melting the connectivity of molten Fe–S in a peridotite matrix is inhibited. Based on observations of retrieved samples, the percolation threshold of Fe–S melts in the presence of low to moderate degrees of silicate melt is estimated at 13±2 vol.%. These results indicate that if the volume fraction of Fe-alloy in a planetesimal was initially greater than 5%, and if early heating by decay of radionuclides raised the temperature of the interior above the Fe-alloy melting point, initial metal segregation was controlled by permeable flow of molten iron alloy in a solid silicate matrix. These conditions were likely met by many terrestrial objects in the early solar nebula. Efficient removal of residual Fe-alloy (5 vol.%) from silicate requires high-degree melting of silicate so that metal can segregate as droplets. Giant impacts during the final stage of accretion of large planetary objects could supply the energy required for high-degrees of melting. Alternatively, if initial metal segregation were delayed until a planetary object grew to large size (∼1000 km in diameter), release of gravitational potential energy due to metal segregation could contribute enough heat to form a magma ocean. [Copyright &y& Elsevier]

Phase equilibrium data for melting of lherzolite in simplified and natural systems are used to show that the `polybaric melting reaction' for mid-ocean ridge basalt proposed by Niu (1997, Journal of Petrology 38, 1047-1074) is erroneous for two reasons. First, the modal abundances of minerals in abyssal peridotites that have equilibrated at low pressures and temperatures cannot be used to calculate high-pressure and -temperature melting reactions. This is because the observed low-temperature mineral modes do not faithfully reproduce the changing abundances of clinopyroxene and orthopyroxene as a function of temperature and pressure that occur at the conditions of melting. Using abyssal peridotite mineral modes to calculate melting reactions results in a severe overestimation in the amount of orthopyroxene, and an underestimation in the amount of clinopyroxene, that dissolves into the melt in polybaric, near-fractional melting paths. The magnitude of these errors can be quantitatively significant for trace element melting models. Second, experimental data show that the complexity of phase relations as a function of pressure and temperature leads to a rich variety of melting reactions in polybaric melting paths. A single `polybaric melting reaction' is not realistic. Only high-pressure and -temperature phase relations can yield reliable melting reactions. [ABSTRACT FROM PUBLISHER]

Melting experiments on fertile peridotite KR4003, a ‘pyrolitic’ composition, were made from 3 to 7 GPa in piston-cylinder and multi-anvil apparatus. Temperature gradients across the sample were minimized (<25°C), and the compositions of all phases were determined. Modal abundances of coexisting phases were calculated by mass balance, and the results were used to determine phase relations. Orthopyroxene is not stable at the solidus of garnet peridotite above ∼3.3 GPa, but crystallizes above the solidus by incongruent melting of cpx. Melt compositions from 3 to 7 GPa (gt;10% melting) are picritic, komatiitic, and peridotitic. The Al2O3 content of partial melts decreases with increase in pressure because of an increase in garnet stability, providing a barometer for melting. The Al2O3 contents of komatiites indicate secular variation in the average pressure of melt segregation from residues, with early Archean komatiites and Cretaceous komatiites generated at the highest and lowest average pressures, respectively. The high CaO/Al2O3 ratios of Archean alumina undepleted komatiites ( 0.9–1.5) require residual garnet if their sources were pyrolitic. Paradoxically, chondrite-normalized Gd/Yb of about unity in these komatiites precludes garnet involvement. Archean komatiite source regions may have had CaO/Al2O3 values of about 1.4 and 1.0 in the early and late Archean, respectively, significantly greater than the pyrolitic ratio of 0.8, whereas the source of Cretaceous komatiites may have had pyrolitic CaO/Al2O3. Thus, secular variations in this ratio are indicated. Chemical differences between coeval alumina undepleted and alumina depleted komatiites can be explained by melting at similar pressures, with alumina undepleted komatiites segregating from a garnet-free residue, and alumina depleted komatiites segregating from a garnet-bearing residue. Depleted, high-temperature peridotites from cratons, and oceanic peridotites, can be melting residues of pyrolitic mantle at low pressures (<3 GPa). Average low-temperature peridotite from the Siberian craton can be generated as a residue of komatiite melt extraction from a near-pyrolitic mantle at ∼6 GPa and 40% melting. Average southern African low-temperature peridotite cannot be a melting residue of pyrolitic mantle. However it can be a residue of komatiite melt extraction at >7 GPa from a mantle enriched in SiO2 relative to pyrolite. [ABSTRACT FROM AUTHOR]

Eutectic melting curves in the system MgO–SiO 2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO–MgSiO 3 binary, and melting of bridgmanite plus stishovite in the MgSiO 3 –SiO 2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower d T / d P slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ∼25 GPa ( Liebske and Frost, 2012 ; de Koker et al., 2013 ). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO 2 endmember, and indicate a maximum in short-range ordering in MgO–SiO 2 melts close to Mg 2 SiO 4 composition. The curvature of the model peridotite eutectic relative to an MgSiO 3 melt adiabat indicates that crystallization in a global magma ocean would begin at ∼100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ∼ 500 K above the mantle geotherm at the core–mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected. [ABSTRACT FROM AUTHOR]