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During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation. Plain Language Summary: Within volcanic arcs, magmas evolve from silica‐poor basalts to more silica‐rich compositions. This process is dominantly controlled by the minerals that crystallize from the magmas and generates characteristic changes in magma compositions that can be measured in erupted lavas. The crystallizing minerals and evolution in magma composition are sensitive to changes in the conditions at which magmas crystalize including the amount of water dissolved in the magma and the depth of crystallization within the crust. We develop two new methods that use changes in magma composition to understand how these conditions vary between different volcanic arcs. We find that arcs built on continental crust typically erupt lavas that began to crystallize at greater depths and are more water‐rich, while arcs built on oceanic crust primarily erupt water‐poor lavas that began to crystallize at shallower depths. These differences have important implications for the processes by which new continental crust is created within volcanic arcs. Key Points: The major element evolution of arc magmas is controlled by the fractionation of pressure and H2O sensitive mineral assemblagesWe use these relationships to develop two proxies that calculate early fractionation pressure and H2O contents from magma compositionsContinental arcs are dominantly H2O‐rich and deeper fractionating, while island arcs are more H2O‐poor and shallower fractionating [ABSTRACT FROM AUTHOR]

Centrifuge experiments on olivine, chromite, and plagioclase aggregates saturated in basaltic liquid show evidence of viscous compaction by grain‐boundary diffusion‐controlled creep. The experiments confirm that the exponential dependence of shear viscosity on melt fraction, observed at low porosities in earlier shear deformation experiments, extends to sedimentary porosities. The compaction profiles are inconsistent with the porosity‐dependence commonly ascribed to viscosity in macroscopic compaction models, which underestimate the effect of matrix disaggregation and consequently overestimate the viscosities of crystal mushes by 1–2 orders of magnitude. The time to halve the porosity of natural olivine igneous sediments by compaction is estimated from the centrifuge experiments to be O(103)y. Half‐times for plagioclase and chromite layers are O(104–105)y, suggesting that such layers compact on magmatic time scales only if they are loaded by additional sedimentation. At conditions relevant to melt flow in asthenospheric settings and trans‐crustal magmatic systems, the bulk and shear viscosities inferred for olivine and plagioclase are O(1017)Pa s, 4 orders of magnitude less than inferred from earlier experimental studies of the diffusion creep rheology. The reduced viscosities imply time‐ and length‐scales for compaction processes that are substantially shorter than previously anticipated. Our analysis serendipitously reveals that the oft‐neglected solidity term of the Carman‐Kozeny porosity‐permeability relation is essential to prevent non‐physical behavior in models of cumulate compaction. Plain Language Summary: The weight of the rocks overlying partially molten regions of the Earth squeezes melt toward the surface. This process is usually limited by rock viscosity and because rock viscosities are extremely high it is not easily observed. We review three sets of experiments on common crustal and mantle minerals in which melt expulsion was accelerated in a centrifuge. The results are consistent with recent theoretical models for rock viscosity and suggest that melt expulsion is substantially faster than previously anticipated. Key Points: Experimental compaction profiles are consistent with a porosity‐weakening effect predicted for grain‐boundary controlled diffusion creepBulk viscosities for plagioclase and olivine aggregates are three orders of magnitude lower than reported from earlier experimentsExtrapolation to natural conditions indicates diffusion creep remains an efficient deformation mechanism in partially molten natural rocks [ABSTRACT FROM AUTHOR]

Eleven isobaric experimental series simulate the fractional crystallization of 1,150 km deep lunar magma ocean. Crystallization begins at 1,850°C with olivine (to 32 per cent solidified, pcs), followed at 1,600°C by olivine + opx ± Cr‐spinel (to 62 pcs), at 1,210°C cpx + plagioclase ± olivine ± Ti‐spinel (to 97 pcs) and at 1,060°C quartz + cpx + plagioclase + Ti‐spinel, leaving 1.8 wt% residual magma that crystallizes minor K‐feldspar and apatite in addition. Melt compositions remain near 45 wt% SiO2, while FeO increases from 11 to 26 wt%, TiO2 peaks at 4 wt% at Ti‐spinel saturation. The available experimental liquid lines of descent yield an overall fractional crystallization sequence of olivine→opx→cpx + plagioclase→quartz→FeTi‐oxide. Plagioclase appears concomitantly with cpx, a result of the low magma ocean floor pressures (≤1 GPa) after 66%–76% of olivine + opx‐fractionation. A few wt% of FeTi‐oxides form mostly once the quartz + plagioclase + cpx‐cotectic is reached, cumulate densities remain ≤3,740 kg/m3. Scaled to a full magma ocean, plagioclase appears at 210–120 km depth, mainly as a function of bulk Al2O3. As buoyancy driven plagioclase‐cpx separation is likely limited, these depths may correspond to the primordial lunar crustal thickness. Allowing for complete plagioclase flotation to the quartz + plagioclase + cpx + FeTi‐oxide ± olivine cotectic yields 95–70 km primordial crust of anorthosite and quartz‐gabbro, far in excess of the 35–50 km observed. This supports an overturn of primordial layers, remelting of dense gabbroic cumulates in the harzburgitic cumulate mantle leading to further mixing and differentiation. We posit that such complex density induced convection led to a lunar marble cake mantle with primitive and fairly evolved reprocessed cumulates next to each other. Plain Language Summary: Upon its accretion the Moon was likely completely molten, forming a global magma ocean. This study experimentally simulates the crystallization of this magma ocean and constrains the resulting mineral cumulate layers, the initial temperature distribution, and the nature of the original lunar crust. Overall this crust was 2–5 times thicker than observed nowadays, supporting that post‐magma ocean convection has removed the larger part of this in some layers quite dense crust. The down‐slumping crust remelts in the hot deeper lunar mantle, leading to a second generation of magmatism explaining part of the available lunar samples. Key Points: Crystallization of a full Moon magma ocean defines the initial selenothermOur and previous results yield >95–70 km primordial crust, at least twice the modern value, requiring reprocessing through lunar overturnSlumping dense crust remelts in the cumulative harzburgitic mantle, adding further diversification to a lunar marble‐cake mantle [ABSTRACT FROM AUTHOR]

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits. [ABSTRACT FROM AUTHOR]

Centrifuge-assisted piston cylinder experiments were conducted on plagioclase in basaltic melt at 1140–1250 °C, 0.42–0.84 GPa and mostly 1000 g. One set of experiments assesses the settling velocity of a dilute plagioclase suspension; a second sinks or floats plagioclase in a MORB-type melt exploring conditions of neutral buoyancy; and a third set examines floatation of plagioclase from an evolved lunar magma ocean composition. A compaction rate for plagioclase cumulates is established. The experiments demonstrate that neutral density of plagioclase An74 in a MOR-type tholeiitic basalt occurs at 0.59 ± 0.04 GPa (1200 °C), contrasting predictions by present models on melt density which yield a density inversion pressure at 0.10–0.15 GPa. In nature, the level of neutral buoyancy depends on melt composition; nevertheless, for the onset of plagioclase crystallization in dry tholeiitic basalts, our result is robust. As the molar volume of plagioclase is well known, the experimentally determined pressure of neutral buoyancy indicates a correction of -1.6% to previous density models for silicate melts. It follows that for (tholeiitic) layered mafic intrusions, plagioclase is negatively buoyant for early, relatively primitive, parent melts. In contrast, the extreme Fe enrichment of a fractionating lunar magma ocean leads to melt densities that let anorthite always float. Compaction φ/φ0 of experimental plagioclase cumulates is quantified to φ/φ0 = − 0.0582 log (Δρ·h·a·t) + 1.284, where φ0 is the porosity after settling (67 ± 2%), h the cumulate pile height, a acceleration and φ porosity as a function of time t. Gravitational-driven compaction in tens of m-thick plagioclase cumulate in basaltic magmas reaches down to ~ 40% porosity within hundreds of years, a timescales competing with characteristic cooling times of cumulate layers of mafic intrusions. To achieve plagioclase modes > 80% due to compaction, an additional overload of ~ 100 m (layers) of mafic minerals would be required. Compaction of a lunar anorthosite crust of 35 km to 20% porosity (i.e. ~ 90% plagioclase after crystallization of the interstitial melt) would require 30 kyrs. [ABSTRACT FROM AUTHOR]

The crust–mantle transition of the Khantaishir ophiolite in western Mongolia is well exposed. The mantle section shows an up to 4 km thick refractory harzburgitic mantle with local dunite channels and lenses. Towards its top, the mantle is increasingly replaced by discrete zones of pyroxenite, which form a kilometre-wide and hundreds of metres-thick horizon at the contact with the overlying crustal section. The plutonic crustal section is composed of gabbros, gabbronorites, tonalites and minor plagiogranites. The lower part of the crustal section is intercalated with pyroxenite lenses, forming a layered sequence, whereas the upper part is cut by volcanic dykes associated with the overlying basalt–andesitic volcanic section. Most of the ultramafic rocks and gabbronorites show a depletion in high field strength elements and positive anomalies for Sr and Pb, whereas gabbros, tonalites and plagiogranites are enriched in large ion lithophile elements and have slightly enriched rare earth element patterns. Non-modal fractional melting models indicate that the most depleted harzburgites of the ophiolite originated after 20–25% of melt extraction from the mantle. Leached minerals and whole-rocks from the crust–mantle transition of the Khantaishir ophiolite define a Sm–Nd isochron at 540 ± 12 Ma, which is interpreted as the formation age of the crust–mantle transition. Additionally, minerals and whole-rocks display a restricted εNd( t =540 Ma) composition (+3·5 to +7·0) and a large scatter in εSr( t =540 Ma) (–19·8 to +14·2). Clinopyroxenes in the crust–mantle transition rocks indicate that they were in equilibrium with a boninitic-like melt, consistent with the lavas observed in the volcanic section of the ophiolite. It is therefore inferred that the Khantaishir ophiolite represents a slice of an incipient oceanic island-arc formed in a suprasubduction environment. [ABSTRACT FROM AUTHOR]

Abstract: During accretion, nitrogen was distributed between metal melt, silicate melt, and the atmosphere and today's N deficit of bulk silicate Earth with respect to chondrites may be due to segregation into the core and/or atmospheric losses. To examine the former, we experimentally determined N solubilities in Fe‐dominated metal melts at 1200–1800 °C, 0.4–9.0 GPa. Results show that pressure has a strong positive influence on N solubility, increasing from 1.0 to 7.4 wt% at 1–9 GPa (1400 °C) while temperature has the inverse effect, N solubility decreasing from 1.3 to 0.6 wt% at 1200–1800 °C (1 GPa). The solubility data are parameterized as function of pressure and temperature. Our results suggest that core‐forming metal melts can dissolve large quantities of N, and the potential N contribution to the Earth's core density deficit could hence be much larger than previously assumed. Most importantly, N in the deep reduced mantle should be stored in the small metal fractions and not in silicates. [ABSTRACT FROM AUTHOR]

To constrain the thermodynamics and melting relations of the siderite–magnesite (FeCO3–MgCO3) system, 27 piston cylinder experiments were conducted at 3.5 GPa and 1170–1575 °C. Fe-rich compositions were also investigated with 13 multi-anvil experiments at 10, 13.6 and 20 GPa, 1500–1890 °C. At 3.5 GPa, the solid solution siderite–magnesite coexists with melt over a compositional range of XMg (=Mg/(Mg + Fetot)) = 0.38–1.0, while at ≥10 GPa solid solution appears to be complete. At 3.5 GPa, the system is pseudo-binary because of the limited stability of siderite or liquid FeCO3, Fe-rich carbonates decomposing at subsolidus conditions to magnetite–magnesioferrite solid solution, graphite and CO2. Similar reactions also occur with liquid FeCO3 resulting in melt species with ferric iron components, but the decomposition of the liquid decreases in importance with pressure. At 3.5 GPa, the metastable melting temperature of pure siderite is located at 1264 °C, whereas pure magnesite melts at 1629 °C. The melting loop is non-ideal on the Fe side where the dissociation reaction resulting in Fe3+ in the melt depresses melting temperatures and causes a minimum. Over the pressure range of 3.5–20 GPa, this minimum is 20–35 °C lower than the (metastable) siderite melting temperature. By merging all present and previous experimental data, standard state (298.15 K, 1 bar) thermodynamic properties of the magnesite melt (MgCO3L) end member are calculated and the properties of (Fe,Mg)CO3 melt fit by a regular solution model with an interaction parameter of −7600 J/mol. The solution model reproduces the asymmetric melting loop and predicts the thermal minimum at 1240 °C near the siderite side at XMg = 0.2 (3.5 GPa). The solution model is applicable to pressures reaching to the bottom of the upper mantle and allows calculation of phase relations in the FeO–MgO–O2–C system. [ABSTRACT FROM AUTHOR]

The carbonatites of Brava Island, Cape Verde hot spot, allow to investigate whether they represent small mantle melt fractions or form through extreme fractionation and/or liquid immiscibility from CO2-bearing silicate magmas. The intrusive carbonatites on Brava Island are part of a strongly silica-undersaturated pyroxenite, ijolite, nephelinite, nepheline syenite, combeite–foiditite, carbonatite series. The major and trace element composition of this suite is reproduced by a model fractionating olivine, clinopyroxene, perovskite, biotite, apatite, titanite, sodalite and FeTi oxides, all present as phenocrysts in the rocks corresponding to their fractionation interval. Fractionation of ~90 wt% crystals reproduces the observed geochemical trend from the least evolved ultramafic dikes (bulk XMg = 0.64) to syenitic compositions. The modelled fractional crystallization leads to alkali enrichment, driving the melt into the carbonatite–silicate miscibility gap. An initial CO2 content of 4000 ppm is sufficient to saturate in CO2 at the point where the rock record suggests continuing unmixing carbonatites from nephelinites to nepheline syenites after 61 wt% fractionation. Such immiscibility is also manifested in carbonatite and silicate domains on a hand-specimen scale. Furthermore, almost identical primary clinopyroxene, biotite and carbonate compositions from carbonatites and nephelinites to nepheline syenites substantiate their conjugate character and our unmixing model. The modelled carbonatite compositions correspond to the natural ones except for their much higher alkali contents. The alkali-poor character of the carbonatites on Brava and elsewhere is likely a consequence of the release of alkali-rich CO2 + H2O fluids during final crystallization, which cause fenitization in adjacent rocks. We propose a general model for carbonatite generation during alkaline magmatism, where the fractionation of heavily Si-undersaturated, alkaline parent melts results in alkali and CO2 enrichment in the evolving melt, ultimately leading to immiscibility between carbonatites and evolved Si-undersaturated alkaline melts. Early saturation in feldspathoids or feldspars would limit alkali enrichment preventing the formation of carbonatites. The complete and continuous fractionation line from almost primitive melts to syenitic compositions on Brava underlines the possibly important role of intrusives for hot spot volcanism. [ABSTRACT FROM AUTHOR]

To evaluate compaction and interstitial melt expulsion during cumulate formation, a 20m cumulate section including the UG2 and UG3 chromitites from a 264m drill core through the Upper Critical Zone of the Bushveld Complex (South Africa) has been studied. The cumulates in the studied section are as follows: 3m plagioclase pyroxenite to pyroxenite, pegmatoid footwall pyroxenite at the lower contact to UG2, 0-7m UG2 chromitite, 6-8m pyroxenite, 0-24m UG3 chromitite, 2-0m plagioclase- rich pyroxenite changing locally to norite, the two 5cm leader stringers UG3a and UG3b, and 7m total of olivine pyroxenites grading into plagioclase pyroxenites. All pyroxenites are dominated by orthopyroxene (opx) and the cumulate sequence is topped by mottled anorthosite grading into norite. Stratigraphic concentrations of major and trace elements of 52 bulk-rock samples were determined. Bulk-rock Mg-numbers are 0-79-0-81 throughout the silicate cumulate units, and 0-40-0-46 in the chromitite layers. The stratigraphic distribution of six incompatible trace elements (K, Rb, Ba, Cs, Zr and Th) has been used to determine the amount of trapped liquid (FTL) or paleoporosity in the cumulate rocks. Final porosities (volume fractions), based on averages from the six trace elements, are 0-06-0-33 in the pyroxenites. In chromitite layers, trapped melt fractions of 0-12-0-36 are calculated from incompatible trace element concentrations, but bulk SiO2 concentrations and X-ray tomography yield 0-04-0-17 higher porosities. Hence, the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction, as poikilitic opx was crystallizing while the silicate melt still equilibrated. Using a previously derived experiment-based model for compaction time scales, gravitationally driven chemical compaction in the UG2-UG3- pyroxenite section is calculated to occur within 1-10 years. This time frame corresponds to the times necessary to cool a 20m layer by 10-50°C, the temperature interval argued to encompass the liquidus and almost complete solidification. Compaction within a decade can in fact easily develop the paleo-porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid. Contrary to previous assertions, melt expulsion from the cumulate pile does not hinder compaction; calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20m pile of cumulate. The pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites. Their entrapment below the chromitite layers suggests that these act as permeability barriers. This is in part due to their finer grain size compared with the pyroxenites, but is mainly due to the crystallization of large poikilitic opx during compaction. [ABSTRACT FROM AUTHOR]

Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na2Mg(CO3)2-K2Mg(CO3)2 join only the Na-end-member eitelite (R... space group), was reported in nature. The K-end-member (R...m) readily hydrates even at low temperatures, therefore, only baylissite, K2Mg(CO3)2⋅4H2O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2. Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V0 = 396.2(4), 381.2(5), and 347.1(3) ų and K0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)2, respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high-P phase is 1% denser than the low-P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25-600 °C and ambient pressure and are α0 = 14.31(5) x 10-5 K-1 and 16.73(11) x 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a- and c-axis: α0(a) = 2.84(6) x 10-5 and 4.78(5) x 10-5 K-1 and α0(c) = 10.47(11) x 10-5 and 8.72(5) x 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively. [ABSTRACT FROM AUTHOR]

Carbonatite and silicate rocks occurring within a single magmatic complex may originate through liquid immiscibility. We thus experimentally determined carbonatite/silicate melt partition coefficients (Dcarbonate melt/silicate melt, hereafter D) for 45 elements to understand their systematics as a function of melt composition and to provide a tool for identifying the possible conjugate nature of silicate and carbonatite magmas. Static and, when necessary, centrifuging piston cylinder experiments were performed at 1–3 GPa, 1150–1260°C such that two well-separated melts resulted. Bulk compositions had Na K, Na ∼ K, and Na K; for the latter we also varied bulk H2O (0–4 wt %) and SiO2 contents. Oxygen fugacities were between iron–wüstite and slightly below hematite–magnetite and were not found to exert significant control on partitioning. Under dry conditions alkali and alkaline earth elements partition into the carbonatite melt, as did Mo and P (DMo >8, DP= 1·6–3·3). High field strength elements (HFSE) prefer the silicate melt, most strongly Hf (DHf = 0·04). The REE have partition coefficients around unity with DLa/Lu = 1·6–2·3. Transition metals have D < 1 except for Cu and V (DCu ∼ 1·3, DV = 0·95–2). The small variability of the partition coefficients in all dry experiments can be explained by a comparable width of the miscibility gap, which appears to be flat-topped in our dry bulk compositions. For all carbonatite and silicate melts, Nb/Ta and Zr/Hf fractionate by factors of 1·3–3·0, in most cases much more strongly than in silicate–oxide systems. With the exception of the alkalis, partition coefficients for the H2O-bearing systems are similar to those for the anhydrous ones, but are shifted in favour of the carbonatite melt by up to an order of magnitude. An increase of bulk silica and thus SiO2 in the silicate melt (from 35 to 69 wt %) has a similar effect. Two types of trace element partitioning with changing melt composition can be observed. The magnitude of the partition coefficients increases for the alkalis and alkaline earths with the width of the miscibility gap, whereas partition coefficients for the REE shift by almost two orders of magnitude from partitioning into the silicate melt (DLa = 0·47) to strongly partitioning into the carbonatite melt (DLa = 38), whereas DLa/DLu varies by only a factor of three. The partitioning behavior can be rationalized as a function of ionic potential (Z/r). Alkali and alkaline earth elements follow a trend, the slope of which depends on the K/Na ratio and H2O content. Contrasting the sodic and potassic systems, alkalis have a positive correlation in D vs Z/r space in the potassic case and Cs to K partition into the silicate melt in the presence of H2O. For the divalent third row transition metals on the one hand and for the tri- and tetravalent REE and HFSE on the other, two trends of negative correlation of D vs Z/r can be defined. Nevertheless, the highest ionic strength network-modifying cations (V, Nb, Ta, Ti and Mo) do not follow any trend; understanding their behavior would require knowledge of their bonding environment in the carbonatite melt. Strong partitioning of REE into the carbonatite melt (DREE = 5·8–38·0) occurs only in H2O-rich compositions for which carbonatites unmix from evolved alkaline melts with the conjugate silicate melt being siliceous. We thus speculate that upon hydrous carbonatite crystallization, the consequent saturation in fluids may lead to hydrothermal systems concentrating REE in secondary deposits. [ABSTRACT FROM AUTHOR]

Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered (R...), but transforms with increasing temperature into a disordered higher symmetry structure (R...c). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300-1350 K and 2.6-4.2 GPa. The long-range order parameter s, quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20-30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and XFe. Complete disorder is attained in dolomite at ~1240 K, 100-220 K lower than previously thought, and in an ankeritic-dolomites.s. with an XFe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction. [ABSTRACT FROM AUTHOR]

The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ∼1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield <1100°C. Type-II spinel websterites are also primitive, but contain augitic clinopyroxene; their whole-rock compositions are characterized by high Ti, Ni, and Mg-number, intermediate Cr and trace element patterns with LREE depletion over the MREE and HREE. Type-III garnet pyroxenites, which include the famous diamond-pseudomorph-bearing garnet pyroxenites, are more evolved than Types-I and -II and have low and variable Mg-number correlating with an Fe-enrichment trend. High bulk-rock and garnet HREE to LREE ratios result from high-pressure fractionation of garnet and augitic cpx at calculated pressures of >45 to 20–30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na2O, CaO and Al2O3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (<10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800–980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantle. [ABSTRACT FROM AUTHOR]

Very low seismic velocity anomalies in the Earth's mantle may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper mantle is likely to be triggered by the presence of small amounts of carbonate. Such carbonates stem from subducted oceanic lithosphere in part buried to depths below the 660-kilometre discontinuity and remixed into the mantle. Here we demonstrate that carbonate-induced melting may occur in deeply subducted lithosphere at near-adiabatic temperatures in the Earth's transition zone and lower mantle. We show experimentally that these carbonatite melts are unstable when infiltrating ambient mantle and are reduced to immobile diamond when recycled at depths greater than ∼250 kilometres, where mantle redox conditions are determined by the presence of an (Fe,Ni) metal phase. This 'redox freezing' process leads to diamond-enriched mantle domains in which the Fe0, resulting from Fe2+ disproportionation in perovskites and garnet, is consumed but the Fe3+ preserved. When such carbon-enriched mantle heterogeneities become part of the upwelling mantle, diamond will inevitably react with the Fe3+ leading to true carbonatite redox melting at ∼660 and ∼250 kilometres depth to form deep-seated melts in the Earth's mantle. [ABSTRACT FROM AUTHOR]

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO and CaSiO perovskites were calculated for pressures and temperatures of the Earth's lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO perovskite completely in MgSiO perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO perovskite might completely dissolve in MgSiO perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO is increased by other major mantle constituents such as Fe and Al. [ABSTRACT FROM AUTHOR]

Phase assemblages, melting relations and melt compositions of a dry carbonated pelite (DG2) and a carbonated pelite with 1·1 wt % H2O (AM) have been experimentally investigated at 5·5–23·5 GPa and 1070–1550°C. The subsolidus mineralogies to 16 GPa contain garnet, clinopyroxene, coesite or stishovite, kyanite or corundum, phengite or potassium feldspar (&leq;8 GPa with and without H2O, respectively), and then K-hollandite, a Ti phase and ferroan dolomite/Mg-calcite or aragonite + ferroan magnesite at higher pressures. The breakdown of clinopyroxene at >16 GPa causes Na-rich Ca-carbonate containing up to 11 wt % Na2O to replace aragonite and leads to the formation of an Na-rich CO2 fluid. Further pressure increase leads to typical Transition Zone minerals such as the CAS phase and one or two perovskites, which completely substitute garnet at the highest investigated pressure (23·5 GPa). Melting at 5·5–23·5 GPa yields alkali-rich magnesio-dolomitic (DG2) to ferro-dolomitic (AM) carbonate melts at temperatures 200–350°C below the mantle geotherm, lower than for any other studied natural composition. Melting reactions are controlled by carbonates and alkali-hosting phases: to 16 GPa clinopyroxene remains residual, Na is compatible and the magnesio- to ferro-dolomitic carbonate melts have extremely high K2O/Na2O ratios. K2O/Na2O weight ratios decrease from 26–41 at 8 GPa to 1·2 at 16 GPa when K-hollandite expands its stability field with increasing pressure. At >16 GPa, Na is repartitioned between several phases, and again becomes incompatible as at <3 GPa, leading to Na-rich carbonate melts with K2O/Na2O ratios 1. This leaves the pressure interval of c. 4–15 GPa for ultrapotassic metasomatism. Comparison of the solidus with typical subducting slab-surface temperatures yields two distinct depths of probable carbonated pelite melting: at 6–9 GPa where the solidus has a negative Clapeyron slope between the intersection of the silicate and carbonate melting reactions at ∼5 GPa, and the phengite or potassium feldspar stability limit at ∼9 GPa. The second opportunity is related to possible slab deflection along the 660 km discontinuity, leading to thermal relaxation and partial melting of the fertile carbonated pelites, thus recycling sedimentary CO2, alkalis and other lithophile and strongly incompatible elements back into the mantle. [ABSTRACT FROM AUTHOR]

Magmatic production on Earth is dominated by asthenospheric melts of basaltic composition that have mostly erupted at mid-ocean ridges. The timescale for segregation and transport of these melts, which are ultimately responsible for formation of the Earth’s crust, is critically dependent on the permeability of the partly molten asthenospheric mantle, yet this permeability is known mainly from semi-empirical and analogue models. Here we use a high-pressure, high-temperature centrifuge, at accelerations of 400g–700g, to measure the rate of basalt melt flow in olivine aggregates with porosities of 5–12 per cent. The resulting permeabilities are consistent with a microscopic model in which melt is completely connected, and are one to two orders of magnitude larger than predicted by current parameterizations. Extrapolation of the measurements to conditions characteristic of asthenosphere below mid-ocean ridges yields proportionally higher transport speeds. Application of these results in a model of porous-media channelling instabilities yields melt transport times of ∼1–2.5 kyr across the entire asthenosphere, which is sufficient to preserve the observed 230Th excess of mid-ocean-ridge basalts and the mantle signatures of even shorter-lived isotopes such as 226Ra (refs 5,11–14). [ABSTRACT FROM AUTHOR]

Subsolidus and melting experiments were performed at 2·0–3·7 GPa and 750–1300°C on a carbonated pelite in the model system K2O–CaO–MgO–Al2O3–SiO2–H2O–CO2 to define stabilities of potassic micas and fluid-present melting reactions. The biotite to phengite reaction occurs at pressures between 2· 4 and 2·6 GPa for temperatures of 750–850°C, and the amphibole to clinopyroxene reaction from 2·0 GPa, 875°C to 2·5 GPa, 740°C. Dolomite is the carbonate phase stable at subsolidus conditions. The biotite to phengite reaction preserves K2O, but is not H2O conservative, as a fluid is produced from the decomposition of zoisite. Phengite + quartz control fluid-saturated melting at a pressure (P) >2·6 GPa, whereas biotite + quartz dominate at P <2· 4 GPa. Incongruent melting occurs through the reactions phengite or biotite + zoisite + quartz/coesite + fluid = silicate melt + clinopyroxene + kyanite. Overstepping of the solidus, located at 850–950°C, results in 7–24 wt % metaluminous K-rich granitic melts. The experiments define the melting surface of the model system, projected from kyanite + quartz/coesite + fluid onto the K2O–CaO–MgO plane. The solidus melts in the studied system occur at a peritectic point consuming mica + zoisite and forming clinopyroxene. With increasing temperature (T), carbonated pelites then evolve along a peritectic curve along which further clinopyroxene is produced until zoisite is exhausted. This is then followed by a peritectic curve consuming clinopyroxene and producing garnet. A comparison of CO2-bearing with CO2-free experiments from the literature suggests that the main effect of adding calcite to a continental sediment is not the minor shift of typically 20–30°C of reactions involving fluid, but the change in bulk Ca/(Mg + Fe) ratio stabilizing calcic phases at the expense of ferromagnesian phases. The experiments suggest that in most subduction zones, CO2, H2O and K2O will be carried to depths in excess of 120–150 km through carbonates and K-micas, as partial melting occurs only at temperatures at the uppermost end of thermal models of subduction zones. Nevertheless, the release of fluid through P-induced decomposition of amphibole and zoisite provides some H2O for arc magma formation. Melting at higher temperatures (e.g. resulting from slower burial rates or from incorporation of subducted crust into the mantle) will produce potassic granitic melts and provide a substantial volatile and K source for the formation of arc magmas. [ABSTRACT FROM PUBLISHER]

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (∼1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5–1·0 GPa, 1200–1350°C, Pt–graphite capsules) investigated the melting behaviour of a model amphibole–olivine–clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1190°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma–rock interaction (assimilative recycling) in the arc crust. [ABSTRACT FROM AUTHOR]

Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism. The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown. Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120–180 km and temperatures of 700–1,200 °C. The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements. The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements). At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures. This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa. [ABSTRACT FROM AUTHOR]

CaO-rich, Al2O3-poor ultracalcic primitive melts occur at mid-ocean-ridges, back-arc basins, ocean islands and volcanic arcs. They are subdivided into a “nepheline-normative” alkaline-rich, silica-poor group uniquely found in arcs and in “hypersthene-normative” fairly refractory melts which occur in all of the above environments. The high CaO contents (to 19.0 wt%) and CaO/Al2O3 ratios (to 1.8) exclude an origin from fertile lherzolites at volatile-absent conditions. Experimental investigation of the liquidus of a hypersthene-normative and a nepheline-normative ultracalcic melt results in quite distinct pressure-temperature conditions of multiple saturation: whereas the hypersthene-normative liquid saturates in olivine + clinopyroxene at 1.2 GPa and 1,410°C, this occurs at 0.2 GPa and 1,220°C for the nepheline-normative ultracalcic liquid. Our results in combination with melting experiments from the literature suggest that hypersthene-normative melts result from melting of a refractory olivine + clinopyroxene ± orthopyroxene source at elevated mantle temperatures. Contrasting, nepheline-normative ultracalcic melts form from wehrlitic cumulates in the arc crust; to account for the high alkaline and low silica contents, and the relatively low temperatures, source wehrlites must have contained amphibole. [ABSTRACT FROM AUTHOR]

We have developed a novel analytical technique for diamond-trap experiments to directly analyze high-pressure, high-temperature fluid and melt compositions in equilibrium with mantle material. Experiments were conducted at a pressure of 6 GPa and temperatures between 900-1200 °C in a multi-anvil apparatus with a synthetic K-free eclogite doped with 860 ppm Cs, ∼20 wt% H2O, and a layer of diamond aggregates serving as a fluid/melt trap. Experiments at identical conditions were analyzed with two different methods. In the new, "freezing," approach, the capsule was frozen prior to opening and kept frozen during laser-ablation ICP-MS analysis, thus ablating the quenched fluid (precipitates together with water that unmixed upon quenching) in a solid state. Cesium, fractionating completely into the fluid or melt phase, was used as an internal standard for calculating the fluid compositions. Calculated uncertainties on H20 content in the fluid composition are 0.7-2.5%. In the conventional "evaporation" approach, water from the unmixed fluid was first evaporated from the capsule, then the remaining fluid precipitates were analyzed by LA-ICP-MS. The compositions of the residual eclogitic minerals were measured by electron microprobe, and the fluid composition was then determined by mass-balance. Uncertainties in mineral compositions lead to poor precision in fluid composition in this latter approach. Results of the two methods of fluid analysis were found to be in good agreement. Because the "freezing" approach analyzes the entire fluid directly and does not rely on mass balance for calculating fluid compositions, our new method provides a superior means for determining the composition of fluids. Secondly, it avoids loss of cations that remain soluble in water (e.g., Cs, K) after quenching the experiment. [ABSTRACT FROM AUTHOR]

The subducted lithosphere is composed of a complex pattern of chemical systems that undergo continuous and discontinuous phase transformation, through pressure and temperature variations. Volatile recycling plays a major geodynamic role in triggering mass transfer, melting, and volcanism. Although buoyancy forces are controlled by modal amounts of the most abundant phases, usually volatile-free, petrogenesis and chemical differentiation are controlled by the occurrence of minor phases, most of them volatile-bearing. Devolatilization of the subducted lithosphere is a continuous process distributed over more than 300 km of the slab-mantle interface. Melting of the subducted crust, if any, along sufficiently hot P-T paths, is governed by fluid-absent reactions, even though the difference between fluid and melt vanishes at pressures above the second critical end point. The density distribution at a depth of 660 km suggests episodic penetration in space and time of subducted slabs into the lower mantle and sinking down to the D″ region at the core-mantle boundary. [ABSTRACT FROM AUTHOR]

Presents a study which synthesized potassic white micas in a solid solution of muscovite and celadonite. Crystal chemistry; Crystallography; Volume of mixing aluminoceladonite; Discussion.

Presents information on an experiment conducted on tonalite and granodiorite to establish the magmatic stability field of epidote. Water saturated conditions and fo2 as stability field in tonalite; Findings base on epidote stability in the presence of water; Information on experimental work relevant to magmatic epidote stability; Materials used in the experiment; Procedure used for the analysis of findings.

Studies the upper pressure stability qualities of lawsonite. Decomposition of lawsonite to grossular; Occurrence of lawsonite in basaltic and andesitic compositions; Formation of higher density hydrous phases.

Investigates the structure and composition of a high-pressure hydrous phase of AlSiO3OH. Preparation of starting materials; Structure differentiation and refinement; Reference to Schreinemaker analysis and stability of the compound; Conclusions drawn regarding this compound.

Examines the pressure stability of kyanite with the use of a multianvil apparatus. Description of the behavior; How thermodynamic calculation can be performed; What the experimental results indicate.

Carbonatite compositions resulting from melting of magnesian calcite + olivine + clinopyroxene were experimentally determined in the system CaO–MgO–SiO2–CO2–H2O as a function of temperature and bulk H2O contents at 1.0 and 1.5 GPa. The melting reaction and melt compositions were found to be highly sensitive to H-loss or -gain during experiments. We hence designed a new hydrogen-trap technique, which provided sufficient control to obtain consistent results. The nominally dry solidus temperatures at 1.0 and 1.5 GPa are 1225–1250 °C and 1275–1300 °C, respectively. At 1.0 GPa, the solidus temperature decreases with H2O increasing to 3.5 wt% (1025–1050 °C), then remains approximately constant at higher H2O concentrations. Our nominally dry solidus temperatures are up to 140 °C higher than in previous studies that did not take measures to limit hydrogen infiltration and hence suffered from H2O formation in the capsule. The near-solidus anhydrous melts have 7–8 wt% SiO2 and molar Ca/(Ca + Mg) of 0.78–0.82 (XCa). Melting temperatures decrease by as much as 200 °C with increasing X H 2 O in the coexisting COH-fluid. Concomitantly, near-solidus melt compositions change with increasing bulk H2O from siliceous Ca-rich carbonate melts to Mg-rich silico-carbonatites with up to 27.8 wt% SiO2 and 0.55 XCa. The continuous compositional array of Ca–Mg–Si carbonatites demonstrates the efficient suppression of liquid immiscibility in the alkali-free system. Diopside crystallization was found to be sensitive to temperature and bulk water contents, limiting metasomatic transformation of carbonated upper mantle to wehrlite at 1.0–1.5 GPa to < 1175 °C and < 7 wt% bulk H2O. [ABSTRACT FROM AUTHOR]

The Letšeng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86−88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24-26 wt% SiO2, 20-21 wt% MgO, 16-19 wt% CaO and 1.9-2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09-0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5-5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10-12 wt% CO2, H2O is estimated to 4-5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i) = 0.703602-0.703656, 143Nd/144Nd(i) = 0.512660 and 176Hf/177Hf(i) = 0.282677-0.282679 indicate that the Letšeng kimberlite originates from the convective upper mantle. U-Pb dating of groundmass perovskite reveals an emplacement age of 85.5 ± 0.3 (2σ) Ma, which is significantly younger than previously proposed for the Letšeng kimberlite. [ABSTRACT FROM AUTHOR]

The article discusses the speech of Max W. Schmidt accepting the Dana Medal of the Mineralogical Society of America (MSA) for 2013. He thanked all the people who had contributed to his successes including his collaborators Stefano Poli and Daniel Vielzeuf with whom he co-authored research on geology and mineralogical sciences. He explained how he perceived the award as a reward for his hard work. He also expressed his gratitude to the MSA for selecting him for the award.

The reaction 2 talc + 2 forsterite = 5 enstatite + 2 H2O has been investigated between 1 and 3.5 GPa. Here, we report the results from 2 sets of experiments: (1) "conventional" static quenching experiments performed several years ago in non-end-loaded and end-loaded piston-cylinder apparatus, and (2) a new set of experiments conducted in a novel type of apparatus, a "rocking" piston cylinder. This new technique was employed for the reinvestigation to overcome segregation problems encountered during static experiments. The run-products of the latter experiments are homogenous throughout the entire capsule. The reaction was bracketed to 680-690 °C at 1 GPa, to 660-680 °C at 1.7 GPa, to 650-660 °C at 2.5 GPa, to 600-640 °C at 3 GPa, and to 550-600 °C at 3.5 GPa. The location of the reaction in P-T-space is consistent with low-pressure experimental data of Chernosky et al. (1985), and is in reasonable agreement with the "conventional" high-pressure experiments of this study. The new experimental results were used to refine the existing thermodynamic properties of talc within the framework of the Holland and Powell (1998) database. The standard state enthalpy and entropy amount to Hf298o = 5912.358 k J/mol and S298o = 239.1 J/(K⋅mol). The refined standard entropy is about 9% lower than the entropy obtained calorimetrically. When the clinoenstatite-orthoenstatite transition is taken into account, S298o of talc improves by ∼6 J/K. We assume that a further increase of the standard state entropy of talc can be achieved by varying the activity of H2O in the fluid as a function of pressure and temperature. [ABSTRACT FROM AUTHOR]