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We present a global synthesis of Cu, Zn, Pb and Ga contents of mafic dike complexes and volcanic rocks associated with 259 ophiolites, ranging in age from Archaean throughout the Phanerozoic. These ophiolites are geochemically classified as subduction-unrelated and subduction-related with various sub-categories, as defined in Dilek and Furnes (2011). The subduction-unrelated ophiolites include Mid-Ocean Ridge (MOR), and Rift, Continental Margin and Plume type ophiolites, collectively grouped as the R/CM/P sub-category. The subduction-related ophiolites include Backarc (BA), Forearc (FA), Backarc to Forearc (BA-FA), and Volcanic Arc (VA) sub-categories. Compositional distribution of these elements in different ophiolite sub-categories show that Zn and Ga patterns are largely uniform and unrelated to the tectonic setting, whereas Cu and Pb patterns show significant variations. Average copper concentrations progressively increase from subduction-related ophiolites to R/CM/P and MOR. Although less pronounced, lead shows a similar increase in average concentrations from subduction zone environments to MOR, with rather irregular patterns for the R/CM/P and VA types. Mafic subunits in analysed ophiolites define similar trends for Cu and Pb. The mafic subunits, comprising alkaline basalts, mid-ocean ridge basalts (MORB), island arc tholeiites (IAT) and boninites, define a progressive shift towards increasing proportions of low concentrations of Cu and Pb in the listed order. To constrain the large variations in the contents of the given elements, we applied petrogenetic modelling of glass analyses. Petrogenetic modelling of the MgO versus Cu, Zn, Pb and Ga distributions in modern MORB show a scatter that can be explained by different degrees of fractional crystallization (20 – 80%) of primitive MORB lavas. In support of previous studies, we find that most erupted MORB lavas are sulphur saturated, whereas primitive boninitic and IAT magmas are S-undersaturated. The trends observed for IAT are in agreement with previous findings that IAT precipitate sulphide only at very high degrees of fractional crystallization, owing to crystallization of magnetite. Boninites are variable and Cu concentration in boninitic glasses indicates that a fraction of them may be S-saturated at relatively low degrees of fractional crystallization. We model two boninitic compositions and achieve S saturation at 15 and 50% fractional crystallization. The observed Pb enrichment in the R/CM/P ophiolites was likely caused by crustal contamination. Mantle sources of mafic magmas of the ophiolites were also enriched in Cu and Pb by a combination of subduction-related processes as reflected in the chalcophile element (Cu and Pb) behavior patterns of various mafic rock types in the ophiolites. Comparing with in-situ oceanic crust, we conclude that the chalcophile element distribution patterns of Cu, Zn, Pb and Ga in mafic lavas and dikes in ophiolites were ca. 80–90% magmatically controlled by their abundances in the mantle melt sources, partial melting episodes, and extents of fractional crystallisation processes. The remaining 10–20% difference we attribute mainly to alteration processes (predominantly loss), as well as types and amounts of subducted sediments, whose melt products contributed to the melt column above subducting slabs.

Exploration for diamonds in the Soviet Union started in the 1940s, however it was not until the beginning of 1950s that the government acknowledged a strong need for locally mined diamonds. In this article, based on publications from Russian literature, we recount a story of two female geologists, Larisa Popugaeva and Natalia Sarsadskhih. Natalia was the head of the mineralogical laboratory who implemented a new methodology to search for mineral indicators of primary diamond deposits. Larisa was a young geologist who joined Natalia's team in 1953. The work of these women led to the discovery in 1954 of the first diamond deposit in the country – a kimberlite pipe “Zarnitsa”. In 1954 Natalia was unable to go into the field, therefore the discovery was made by Larisa. Credit for this discovery, however, was claimed by the higher officials from the Amakinskaya expedition, one of the largest diamond exploration organisations in the country. Multiple efforts to restore justice did not succeed, with Larisa only being awarded the title of the “Discoverer” in 1970, and Natalia not until 1990. This article provides a description of Larisa's and Natalia's life up until the discovery of Zarnitsa, and a few significant events after.

Majoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.

Fil: Assis, Omar. Instituto de Geociências, Universidade de Brasília, Laboratório de Geocronologia e geoquímica isotópica. Brasil. Fil: Zaffarana, Claudia. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina. Fil: Orts, Darío. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina. Fil: Puigdomenech, Carla. Instituto de Geociências Básicas, Aplicadas y Ambientales de Buenos Aires (IGeBA); CONICET; Universidad de Buenos Aires. Argentina. Fil: Ruiz González, Víctor. Instituto de Geociências Básicas, Aplicadas y Ambientales de Buenos Aires (IGeBA); CONICET; Universidad de Buenos Aires. Argentina. Fil: Gallastegui, Gloria. Instituto Geológico y Minero de España (IGME, CSIC). España. Fil: Hauser, Natalia. Instituto de Geociências, Universidade de Brasília, Laboratório de Geocronologia e geoquímica isotópica. Brasil. Fil: Kiseeva, Ekaterina. School of Biological, Earth and Environmental Sciences, University College. Irlanda. Fil: Molina, José. Departamento de Mineralogía y Petrología, Universidad de Granada. España. Fil: Pernich, Sebastián. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina. During the Late Cretaceous Andean orogeny, the compressive deformation associated with the shallowing of the subducting slab caused the development of the arc-related igneous rocks known as the Naunauco Belt. This study presents petrographic, mineralogical and anisotropy of magnetic susceptibility data for the Varvarco Intrusives (the Varvarco Tonalite, Butal´ on Tonalite and Radales Aplite), which crop out in the Cordillera del Viento, Neuquén Province, Argentina. The assembly of plutons was formed by mafic magma episodic injection. Amphibole and biotite compositions suggest that the Varvarco Tonalite is related to calc-alkaline, I-type magmas, typical of subduction environments. Different geothermobarometers based on amphibole and plagioclase compositions for the Varvarco Tonalite suggest shallow emplacement conditions (~2–3 kbar, equivalent to ~12 km depth). Apatite fission-track analy-ses give exhumation ages of 67.5 ± 8 Ma for the Varvarco Tonalite and 50.3 ± 5.9 Ma for the Butal´ on Tonalite. A calculated continuous fast exhumation rate of at least 330 °C Ma−1 is con-sistent with the shallow emplacement conditions, textural data and geobarometric estimations. In agreement with the thermal profile, the magmatic system was exhumed by ~12 km within c. 2.1 Ma implying a geothermal gradient of ~62.5 °C km−1. The last step of exhumation occurred between ~65.3 and 56.9 Ma. The magmatic fabrics observed in the studied plutons reflect mostly magma chamber processes. The Varvarco Intrusives represent satellite calc-alka-line plutons of the North Patagonian Batholith which were emplaced syn- to post-tectonically with respect to a major deformation stage of the Southern Central Andes.

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth's lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93. Here we use Synchrotron Mössbauer Source (SMS) spectroscopy and single-crystal X-ray diffraction to determine the iron oxidation state and structure of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. Inclusion Mg#s vary between 16.1 and 84.5. Ferropericlase inclusions contain no ferric iron within the detection limit of SMS, while both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite ((Mg,Fe)Fe

Arsenian pyrite containing above 1 wt% As plays a crucial role in deposition and deportment of Au and other chalcophile elements. The importance of arsenian pyrite led to theoretical and experimental studies that examined properties and genesis of the mineral; however, the interpretation of the phase relations between arsenian pyrite and arsenopyrite is conflicting. In this contribution, we present the results of a review of the experimental studies that have investigated the crystallisation of pyrite in As-bearing systems, a summary of As content in pyrite coexisting with arsenopyrite in 37 deposits and the composition of arsenian pyrite in deposits with little or no arsenopyrite. The review demonstrates that the previous experimental studies that attempted to achieve equilibrium between pyrite and arsenopyrite observed from

Compared to elements of similar volatility, such as Rb, Ga and K, the bulk silicate Moon (BSM) exhibits significant enrichment in the heavy isotopes of Zn and Cu. However, both elements display a greater affinity for lunar sulfide phases than the other volatiles, suggesting their isotopic abundance in the BSM may also reflect their sequestration to the lunar core. Experimentally determined Cu and Zn isotopic fractionation between liquid metal, sulfide and silicates reveals carbon-bearing iron melts to be isotopically heavier than the silicate melt, and sulfide melts the lightest. During sulfide sequestration from a cooling lunar magma ocean, Cu, unlike Zn, partitions strongly into sulfides (100 < DCuSulfide/Melt < 200), leaving the BSM both elementally depleted in Cu, and isotopically heavier. Sulfide sequestration therefore explains the larger offsets in the Cu isotope compositions of lunar rocks and the silicate Earth relative to other moderately volatile elements. The lunar Zn isotopic inventory is best explained by volatility driven surface processes. Irrespective of the elemental loss mechanism, the Cu isotopic content of the BSM rules out carbon as a significant light element of the lunar core.

This chapter reviews the systematics of partial melting of mantle lithologies – like peridotite and eclogite – in the presence of carbon dioxide. It discusses the composition of mantle-derived magmas generated in the presence of carbon dioxide and whether magmas erupted on Earth’s surface resemble carbonated magmas from the mantle. It reviews how the production of carbon dioxide-rich magma in the mantle varies as a function of tectonic settings – beneath continents and oceans and in subduction zones – and time.

Sulfide-silicate melt partitioning controls the behavior of gold in magmas, which is critical for understanding the Earth's deep gold cycle and formation of gold deposits. However, the mechanisms that control the sulfide-silicate melt partitioning of gold remain largely unknown. Here we present constraints from laboratory experiments on the partition coefficient of gold between monosulfide-solid-solution (MSS) and silicate melt ( D A u M S S / S M ) under conditions relevant for magmatism at subduction settings. Thirty-five experiments were performed in Au capsules to determine D A u M S S / S M at 950-1050 °C, 0.5-3 GPa, oxygen fugacity (fO2) of ∼FMQ-1.7 to FMQ+2.7 (FMQ refers to the fayalite-magnetite-quartz buffer), and sulfur fugacity (fS2) of −2.2 to 2.1, using a piston cylinder apparatus. The silicate melt composition changes from dry to hydrous andesite to rhyolite. The results obtained from electron microprobe and laser-ablation ICP-MS analyses show that the gold solubility in silicate melts ranges from 0.01 to 55.3 ppm and is strongly correlated with the melt sulfur content [S]melt at fO2 of ∼FMQ-1.7 to FMQ+1.6, which can be explained by the formation of complex Au-S species in the silicate melts. The gold solubility in MSS ranges from 130 to 2800 ppm, which is mainly controlled by fS2. D A u M S S / S M ranges from 10 to 14000 at fO2 of ∼FMQ-1.7 to FMQ+1.6, the large variation of which can be fully explained by combined [S]melt and fS2. Therefore, all of the parameters that can directly affect [S]melt and fS2, such as alkali metals, water, FeO, and fO2, can indirectly affect D A u M S S / S M . The mechanisms that control the sulfide-silicate melt partitioning of gold and the other chalcophile elements, such as Ni, Re, and Mo, differ significantly. This is because gold is dissolved mainly as Au-S species in the silicate melts, while the other chalcophile elements are dissolved mainly as metal oxides in the silicate melts. Applying the correlation between D A u M S S / S M and [S]melt to slab melting and arc magmatic differentiation under different redox conditions, we find that ancient to modern slab melts carry negligible to less than 25% of the slab gold to the subarc mantle; however, gold-enrichment can occur in MSS-saturated arc magmas that have differentiated under moderately oxidized conditions with fO2 between FMQ and FMQ+1.6, in particular if the magmatic crystallization follows a fractional crystallization model. We conclude that moderately oxidized magmas with high contents of alkali metals, sulfur, and water, owing to their low D A u M S S / S M and efficient magma-to-fluid transfer of gold and sulfur, have a high potential to form gold deposits.

Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.

Nature geoscience 11(2), 144 - 147 (2018). doi:10.1038/s41561-017-0055-7, The oxidation state of iron in Earth’s mantle is well known to depths of approximately 200 km, but has not been characterized in samples from the lowermost upper mantle (200–410 km depth) or the transition zone (410–660 km depth). Natural samples from the deep (>200 km) mantle are extremely rare, and are usually only found as inclusions in diamonds. Here we use synchrotron Mössbauer source spectroscopy complemented by single-crystal X-ray diffraction to measure the oxidation state of Fe in inclusions of ultra-high pressure majoritic garnet in diamond. The garnets show a pronounced increase in oxidation state with depth, with Fe$^{3+}$/(Fe$^{3+}$+ Fe$^{2+}$) increasing from 0.08 at approximately 240 km depth to 0.30 at approximately 500 km depth. The latter majorites, which come from pyroxenitic bulk compositions, are twice as rich in Fe$^{3+}$ as the most oxidized garnets from the shallow mantle. Corresponding oxygen fugacities are above the upper stability limit of Fe metal. This implies that the increase in oxidation state is unconnected to disproportionation of Fe$^{2+}$ to Fe$^{3+}$ plus Fe$^0$. Instead, the Fe$^{3+}$ increase with depth is consistent with the hypothesis that carbonated fluids or melts are the oxidizing agents responsible for the high Fe$^{3+}$ contents of the inclusions., Published by Nature Publ. Group, London

The oxidation response and microstructural evolution of an Inconel 625 alloy exhaust manifold exposed to an automobile racing environment has been examined using a range of advanced electron microscopy-based techniques, atom probe tomography and high-sensitivity laser ablation mass spectrometry. The dynamic, corrosive gas conditions result in accelerated oxidation, with the inner exhaust surface also heavily contaminated by multiple species including Zn, P, K and Na. Nb carbides and Ti nitrides identified in stock control samples evolve into mixed (Ti, Nb)N species during exposure, decorated by smaller Mo, Si-rich precipitates. The exposed alloy component therefore reveals unique surface and subsurface features following in-service use.

Sulfides are among the most important petrogenetic agents in magmatic systems. They are ubiquitous in most upper-mantle rock types, common as inclusions in diamonds and they host significant amounts of geochemically and economically important chalcophile (‘sulfur-loving’) elements, such as Cu, Ni, Pb, In, Au and the platinum-group elements. Despite their low abundance (<< 1% of the bulk rock), residual sulfides have a disproportionate control over the chalcophile element budget in upper mantle lithologies, as well as that of melts derived from the Earth's mantle.

The accretion of the Earth was marked by the high-pressure segregation of the core, accompanied by dissolution of about 10% of one or more “light” elements into the metal. Cosmochemical data suggest that, of these 10% “light” elements, the core contains ∼1.7% S (Dreibus and Palme, 1996) and there is evidence that volatile elements such as S accreted to the Earth late in planetary growth, plausibly as a sulfide “matte” (O’Neill, 1991). Given that metallurgical data indicate that dissolution of even small amounts of sulfur in liquid Fe can have profound effects on the activities of some trace components, we have undertaken a study of the affect of S on the metal–silicate partitioning of a number of the most important chalcophile and siderophile elements. We performed experiments at 1.5 GPa and 1460–1650 °C on metal–silicate partitioning of W, Mo, Ni, Co, Cu, Ag, Mn, Cd, Tl, Cr, Sb, In, Pb, Ga, Ge, V and Zn under conditions where the S content of the metal was varied from 0 to 37 wt%. Mn and Ag were found to exhibit the highest ratio of chalcophile to siderophile behaviour while W, Ga and Sb are the most “chalcophobic” of the elements studied. In terms of the 1-parameter epsilon model (Wagner, 1962) we derived values for each element at 1600 °C as follows (negative values indicate chalcophile behaviour): e Cu S = - 2.57 ; e Mn S = - 6.41 ; e Ag S = - 4.15 ; e Sb S = 4.36 ; e Cd S = - 3.78 ; e In S = - 0.24 ; e Tl S = - 6.21 ; e Ga S = 6.54 ; e Pb S = - 3.73 ; e Cr S = - 3.70 ; e Ge S = 7.03 ; e V S = - 3.14 ; e Zn S = - 1.68 ; e Mo S = 2.27 ; e W S = 6.47 ; e Ni S = 2.17 ; e Co S = 2.40 . We use these new data in conjunction with published pressure–temperature dependences of metal–silicate partitioning to test the effects of accreted S on the calculated trace element concentrations in bulk silicate Earth. The approach employs a continuous accretion model in which the oxidation state of the Earth and pressure of core segregation both increase during accretion. We find that, without addition of S in the latter stages of accretion, the Mo/W ratio of silicate Earth would be several times larger than that observed. Addition of ∼2% S accompanied by small amounts of carbon in the last 15% of accretion, however, enables us to match the observed concentrations of these elements in silicate Earth. This confirms an earlier conclusion that the Mo/W ratio of silicate Earth requires late sulfide addition to the core (Wade et al., 2012). Further support for late sulfide addition to the core comes from the depletion factors of volatile chalcophile elements Cu, Ga, Sb, Ag, Zn, Pb, Cd, In and Tl in silicate Earth relative to lithophile elements of similar volatility. We find that depletions of these elements are well correlated with their partition coefficients into sulfide (FeS) liquids at 1.5 GPa and temperatures of 1460–1650 °C. In contrast there is essentially no correlation between their depletion factors and sulfur-free liquid Fe–silicate partition coefficients.

The melting and phase relations of carbonated MORB eclogite have been investigated using the multi-anvil technique at 9–21 GPa and 1100–1900°C. The starting compositions were two synthetic mixes, GA1 and Volga, with the CO2 component added as CaCO3 (cc): GA1 + 10%cc (GA1cc) models altered oceanic crust recycled into the convecting mantle via subduction, and Volga + 10%cc (Volga-cc) models subducted oceanic crust that has lost some of its siliceous component in the sub-arc regime (GA1 minus 6·5 wt % SiO2). The subsolidus mineral assemblage at 9 and 13 GPa includes garnet, clinopyroxene, magnesite, aragonite, a high-pressure polymorph of TiO2 (only at 9 GPa) and stishovite (only at 13 GPa). At 17–21 GPa clinopyroxene is no longer stable; the mineral assemblage consists predominantly of garnet with subordinate magnesite (only at 17 GPa), Na-rich aragonite, stishovite, Ca-perovskite (mostly at 21 GPa), and K-hollandite (mostly at 17 GPa). Na-carbonate with an inferred composition (Na,K)2(Ca,Mg,Fe)(CO3)2 was present in Volga-cc at 21 GPa and 1200°C. Diamond (or graphite) crystallized in most runs in the GA1cc composition, but it was absent in experiments with the Volga-cc composition. In Volga-cc, the solidus temperatures are nearly constant between 1200 and 1300°C over the entire pressure range investigated. In GA1cc, the solidus is located at similar temperatures at 9–13 GPa, but at higher temperatures of 1300–1500°C at 17–21 GPa. The difference in solidi between the GA1cc and Volga-cc compositions can be explained by a change in Na compatibility between 13 and 17 GPa as omphacitic clinopyroxene disappears, resulting in the formation of Na-carbonate or Na-rich melt in Volga-cc. The solidus temperature in GA1cc also increases with increasing pressure as a consequence of carbonate reduction and diamond precipitation, possibly brought on either via progressive Fe2+–Fe3+ transition in garnet at higher pressures or by a decrease of the activity of the diopside component in clinopyroxene. The low-degree melts are highly alkalic (K-rich at 9–13 GPa and Na-rich at 17–21 GPa) carbonatites, changing towards SiO2-rich melts with increasing temperature at constant pressure. The solidi of both compositions remain higher than typical subduction pressure–temperature (P–T) profiles at 5–10 GPa; however, at higher pressures the flat solidus curve of carbonated eclogite may intersect the subduction P–T profile in the Transition Zone, where carbonated eclogite can produce alkali- and carbonate-rich melts. Such subduction-related alkali-rich melts can be potential analogues of kimberlite and carbonatite melt compositions and important agents of mantle metasomatism and diamond formation in the Transition Zone and in cratonic roots. Melting of carbonated eclogite produces a garnet-bearing refractory residue, which could be stored in the Transition Zone or lower mantle.

We have experimentally investigated a K-bearing altered mid-ocean ridge basalt (MORB) composition to which 10% CaCO3 was added (GA1+10%cc), at temperatures of 1050-1400oC and pressures of 3•5-5•5 GPa. Experiments were conducted in piston-cylinder apparatus in Pt-Gr (Pt with inner graphite) and Au-Pd capsules. Sub-solidus assemblages for both sets of experiments contain clinopyroxene, garnet, carbonate, rutile, coesite and K-feldspar. Apatite was observed only in the Pt-Gr experiments. Melting behaviour in experiments using different capsule materials contrasted markedly. Experiments in Pt-Gr capsules showed the silicate solidus to be at temperatures less than 1100oC at 3•5GPa and less than 1050oC at 4•5-5•0 GPa.These are similar (3•5GPa) or lower (4•5-5•0 GPa) temperatures compared with the carbonate solidus (1075-1125oC at 3•5-5•0 GPa). Melts in the Pt-Gr runs evolve with increasing degree of melting from K-rich silicate melts at the lowest degree of melting to carbonate-silicate immiscible liquids and silicate-carbonate melts at intermediate degrees of melting, and finally to silicate melts at the highest degrees of melting. Experiments in Au-Pd capsules were performed only at 5•0GPa. The carbonate solidus is between 1200 and 1225oC (at least 100oC higher than in the experiments in Pt-Gr capsules at the same pressure-temperature conditions).The first melts to be produced are carbonatitic and exhibit increasing SiO2 content with increasing temperature.This contrast in melting behaviour is explained by the relatively rapid diffusion of H through the Pt-Gr capsules, resulting in formation of H2O, and thus dramatically depressing both the silicate and the carbonate solidi in the Pt-Gr experiments compared with those in the Au-Pd experiments. This presumably reflects the lower permeability of Au-Pd to H, resulting in a much lower H2O/CO2 ratio in the Au-Pd encapsulated experiments. The presence of water in the melt was demonstrated by Fourier transform infrared (FTIR) spectroscopic analysis of one Pt-Gr experiment, indicating ~0•5wt % H2O in the bulk composition. Further confirmation that H2O plays such a role in the Pt-Gr experiments was provided by an additional experiment performed in a Au-Pd capsule with ~10 wt % H2Ospecifically added. In this experiment immiscible carbonate and silicate melts were observed. Carbonate- silicate liquid immiscibility is considered to occur as a result of the H2O present in the system. These results can be applied to natural systems in several ways. First, the presence of a small amount of either silicate melt or H2O-fluid in the system will act as a ‘flux’, depressing the carbonate solidus to much lower temperatures than inanhydrous systems. Second, the full trend in melt evolution from silicate-rich to carbonate-rich melts, which is also observed in inclusions in diamonds, can be explained by melting of K- and CO2-bearing, water-undersaturated MORB compositions. In cratonic environments low-degree silicate and immiscible silicate and carbonate melts will metasomatize the overlying mantle in different ways, producing, in the first instance, Si enrichment and crystallization of additional orthopyroxene, phlogopite, pyrope-rich garnet and consuming olivine, and, in the second case, carbonate metasomatism, with additional magnesite-dolomite, clinopyroxene and apatite. Both metasomatic styles have been described in natural peridotite xenoliths from the cratonic lithosphere.

Partial melting experiments at 40, 50 and 60 kbar pressure on three peridotite compositions with 0.5–0.63 wt.% H2O and 2.0–3.2 wt.% CO2 added indicate melting temperatures only marginally above continental geotherms. Most experiments were performed on a composition with 1.5 wt.% K2O added, which causes a further decrease of about 40 °C in melting temperature. Melts progress gradually from carbonate-rich to carbonated silicate in composition: near-solidus melts have Ca/(Ca + Mg) of 0.46–0.53, which fall to 30 wt.%) and Al2O3 ( 9 wt.%) and concomitant decrease in CaO (> 20 to Partial melts of peridotite with CO2 and H2O are too low in MgO to resemble kimberlites, but will act as effective metasomatic agents enriching the lithosphere in K and carbonates, of relevance for ultramafic lamprophyres and kamafugites. Higher-degree melts (15–35%) have > 20 wt.% SiO2, are only mildly enriched in trace elements, and will not cause large time-integrated isotopic in-growth except over long geological time intervals. The temperature interval over which melts are carbonatitic with SiO2 contents

We have determined the partitioning of the elements Cu, In, Tl, Pb, Ag, Mn, Zn, Cr, Co, Ni, Sb and Cd between FeS-rich sulphide liquids and anhydrous basaltic melts at high pressures and temperatures. The sulphide liquids were found to have oxygen contents which are linearly related to the FeO contents of the silicate melts. We also found simple relationships between the FeO contents of the silicate melts and the sulphide–silicate partition coefficients D M sulph / sil for the individual trace elements. These relationships can be generally represented as follows: log D M sulph / sil ≈ A + n 2 log [ FeO ] where A is a constant related to the free energy of Fe–M exchange, n is a constant related to the valence of the element and [FeO] is the FeO content of the silicate melt in mole fraction or weight %. This simple relationship effectively removes the need to define the fugacity ratio f O 2 / f S 2 when considering partitioning and hence greatly simplifies application of partitioning data to natural systems. In theory n should approximate −1 for 1+ ions, −2 for 2+ ions and so on. Regressed values of n are generally close to those expected, although deviations occur for some elements. The deviations can be understood in terms of the relative chalcophile and lithophile characteristics of the element of concern. For cases in which the sulphide is an FeS–NiS–Cu2S liquid we obtain excellent agreement with results for pure FeS by correcting the FeO content of the silicate melt as follows: [ FeO ] corrected = [ FeO ] silicate [ Fe / ( Fe + Ni + Cu ) ] sulphide We tested our model on data from the literature in which sulphide–silicate partition coefficients for Cu, Co, Ni and Mn were determined. Literature data for these elements follow the predicted linear dependence of log D M sulph / sil on log [ wt % FeO ] . Furthermore, differences between the absolute values of D M sulph / sil obtained by us and those in the literature are quantifiable in terms of temperature and matrix effects such as the Ni/Fe ratio of the sulphide. We used our results for Pb partitioning to calculate Ce/Pb and Nd/Pb ratios of basalts generated by partial melting of the mantle followed by fractional crystallization. Calculated Nd/Pb is essentially constant over wide ranges of partial melting and fractional crystallization with a value of ∼18.6 if we assume that depleted mantle contains 65 ppb of Pb. Calculated Ce/Pb varies slightly during batch partial melting from 21 to 29 with the canonical value of 25 being achieved at ∼ 10 % partial melting. These trends are in excellent agreement with measurements of oceanic basalt glasses. Finally, we used our partitioning relationships to calculate the concentrations of a number of the incompatible chalcophile trace elements in depleted mantle. These are as follows: 32 ppm Cu, 65 ppb Pb, 7.6 ppb Ag, 12 ppb In, 23 ppb Cd, 1.7 ppb Sb and 1.3 ppb Tl.

The transition zone of the Earth’s mantle (the depth interval between two major seismic discontinuities at 410 km and 660 km) is critical to understanding our planet’s evolution. Some diamonds are thought to have originated in the transition zone and the inclusions found in them are the only samples of material directly extracted from this depth range. By comparing natural majorite garnet inclusions in diamonds with the compositions of experimentally crystallized majorite garnets, we determine two major compositional trends, the pure metabasitic (or eclogitic) trend and the combined metaperidotitic and metapyroxenitic trend, that are strongly correlated with their preferred substitution mechanisms during majorite formation. Based on these trends, we demonstrate that the majority of the reported majorite inclusions in natural diamonds formed neither in a pure metabasite nor in a metaperidotite lithology, but in fact crystallized from a wide range of compositions intermediate between conventional basaltic and peridotitic, referred to here as metapyroxenitic. Given the dominance of metapyroxenite-type majorite diamond inclusions and their inferred syngenetic origin, we argue that a significant fraction of metapyroxenite rock is present within Earth’s transition zone and is important in the diamond-forming process. This is in agreement with recent self-consistent seismological and/or mineral physics studies that support models of a lithologically heterogeneous transition zone. From trace element and carbon isotope features, we infer a crustal origin for these rocks.