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Carbonatites are rare igneous rocks that host the largest resources of REE and Nb, yet, their genesis and evolution are far from clear. The leading models of carbonatite formation are the direct melting of carbonate-bearing peridotites, silicate-carbonatite liquid immiscibility, and fractionation of carbonated silicate melts. The validity of these models has never been robustly addressed through combining the available experimental results with the natural rock record. We thus re-evaluate the presently 633 alleged carbonatite occurrences including carbonatite type, bulk composition, mineralogy, and field exposure, followed by a review of experimental data pertinent to carbonatite genesis and evolution. Based on the available data, 454 carbonatite occurrences are magmatic, of which 87 without and 338 with spatially associated alkaline magma, 9 with kimberlites, and 20 with ultramafic cumulates only. Eighty-four percent of the magmatic occurrences contain calcite carbonatite (of which 1/3 also contain dolomite carbonatite), only 9% have dolomite but not calcite carbonatite, the incidence of dolomite carbonatite being similar for occurrences with or without associated silicate magmas. Available experimental data show that crystallization of calcite, dolomite, ankerite, and siderite at crustal conditions requires moderately alkaline and/or hydrous carbonate melts with ≥20 to 25 wt % (Na,K)2CO3 + H2O. It follows that carbonatite rocks, poor in these elements, are at best magmatic cumulates (if not carbo- or hydrothermal) that lost these ephemeral components. Carbonatitic melts could form in the lithospheric mantle, but these are always dolomitic and cannot deviate from close-to-minimum compositions when rising, their strong adiabatic cooling keeping them on the solidus until they decompose to olivine, clinopyroxene (cpx), and CO2 when reaching <2.1 GPa, i.e. the carbonated peridotite solidus ledge, which renders their extraction from the mantle highly unlikely. Furthermore, dolomitic carbonate melts crystallize periclase + calcite at crustal conditions. Only when containing ≥15 wt % (Na,K)2O + H2O they may crystallize dolomite and form dolomitic carbonatites. This value is far above the 2 to 5 wt % (Na,K)2O of mantle-derived carbonatitic melts. Liquid immiscibility from CO2-bearing close-to-natural melilititic, nephelinitic, and phonolitic melts requires 10 to 15 wt % Na2O + K2O in the silicate melt, increasing with SiO2. Extensive differentiation of primitive alkaline melilititic or basanitic parents is hence required to achieve immiscibility. The experimental data show that evolved nephelinites and phonolites unmix calcic carbonatitic melts, while melilitites and undifferentiated nephelinites with >4 wt % MgO may also unmix dolomitic carbonatitic melts. The latter may hence arise from liquid immiscibility or develop through fractionation from calcic carbonatitic melts. Finally, carbonatites may also derive through fractionation of CO2-rich ultramafic melts, but a continuous increase in dissolved CO2 from a carbonated silicate melt to a carbonatitic melt requires ≥3 GPa. We conclude that the combination of the natural rock record with melting and crystallization phase relations excludes a direct mantle origin for almost all carbonatites found in the crust. Instead, their vast majority forms through immiscibility from an alkali-rich differentiated silicate melt that stems from a mantle-derived alkaline parent, consistent with the common spatial association with alkaline complexes and similar isotopic compositions of carbonatite and alkaline silicate rocks. Direct fractionation from silicate melts may occur for kimberlitic or ultramafic lamprophyric melts, but only at ≥3 GPa, i.e. within the lithospheric mantle. To make progress in this field, we suggest a more rigorous distinction of magmatic and carbo- or hydrothermal carbonatite rocks in each occurrence, and to focus on mineral compositions in the carbonatite and associated silicate magmas, as bulk rocks are at best cumulative in nature. Additional experimental work to understand the role of alkalis and H2O in the formation and evolution of carbonatites, in particular crystallization and fluid saturation at crustal conditions, will be essential to provide a more complete understanding of carbonatite petrogenesis. [ABSTRACT FROM AUTHOR]

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]

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8-9 km depth (north) to its bottom at 25-35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite-trondhjemite-granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones. Keywords Kohistan batholith * Al-in-hornblende barometry * Magmatic epidote * Granitoid suites * TTG * Island arc * Tectonics, Introduction The origin of the granitic (s.l.) felsic upper part of the continental crust is now discussed for decades. In general, granitoids encompass a highly diverse suite of rocks dominated [...]

Very low seismic velocity anomalies in the Earth's mantle (1,2) may reflect small amounts of melt present in the peridotite matrix, and the onset of melting in the Earth's upper [...]

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 [...]

The ~ 570 Ma old Khantaishir ophiolite is built by up to 4 km harzburgitic mantle with abundant pyroxenites and dunites followed by ~ 2 km of hornblende-gabbros and gabbronorites and by a ~ 2.5 km thick volcanic unit composed of a dyke + sill complex capped by pillow lavas and some volcanoclastics. The volcanics are mainly basaltic andesites and andesites (or boninites) with an average of 58.2 ± 1.0 wt% SiO.sub.2, X.sub.Mg = 0.61 ± 0.03 (X.sub.Mg = molar MgO/(MgO + FeO.sup.tot), TiO.sub.2 = 0.4 ± 0.1 wt% and CaO = 7.5 ± 0.6 wt% (errors as 2[sigma]). Normalized trace element patterns show positive anomalies for Pb and Sr, a negative Nb-anomaly, large ion lithophile elements (LILE) concentrations between N- and E-MORB and distinctly depleted HREE. These characteristics indicate that the Khantaishir volcanics were derived from a refractory mantle source modified by a moderate slab-component, similar to boninites erupted along the Izu-Bonin-Mariana subduction system and to the Troodos and Betts Cove ophiolites. Most strikingly and despite almost complete outcrops over 260 km.sup.2, there is no remnant of any pre-existing MORB crust, suggesting that the magmatic suite of this ophiolite formed on completely denudated mantle, most likely upon subduction initiation. The architecture of this 4-5 km thick early arc crust resembles oceanic crust formed at mid ocean ridges, but lacks a sheeted dyke complex; volcanic edifices are not observed. Nevertheless, low melting pressures combined with moderate H.sub.2O-contents resulted in high-Si primitive melts, in abundant hornblende-gabbros and in a fast enrichment in bulk SiO.sub.2. Fractional crystallization modeling starting from the observed primitive melts (56.6 wt% SiO.sub.2) suggests that 25 wt% pyroxene + plagioclase fractionation is sufficient to form the average Khantaishir volcanic crust. Most of the fractionation happened in the mantle, the observed pyroxenite lenses and layers in and at the top of the harzburgites account for the required cumulate volumes. Finally, the multiply documented occurrence of highly depleted boninites during subduction initiation suggests a causal relationship of subduction initiation and highly depleted mantle. Possibly, a discontinuity between dense fertile and buoyant depleted mantle contributes to the sinking of the future dense subducting plate, while the buoyant depleted mantle of the future overriding plate forms the infant mantle wedge., Author(s): Omar Gianola [sup.1] , Max W. Schmidt [sup.1] , Oliver Jagoutz [sup.2] , Oyungerel Sambuu [sup.3] Author Affiliations: (1) Institute of Geochemistry and Petrology, ETH Zurich, 0000 0001 2156 [...]

The reaction 2 talc + 2 forsterite = 5 enstatite + 2 [H.sub.2]O 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 [degrees]C at 1 GPa, to 660-680 [degrees]C at 1.7 GPa, to 650-660 [degrees]C at 2.5 GPa, to 600-640 [degrees]C at 3 GPa, and to 550-600 [degrees]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 [H.sup.o.sub.f298] = 5912.358 kJ/mol and [S.sup.o.sub.298] = 239.1 J/(K x mol). The refined standard entropy is about 9% lower than the entropy obtained calorimetrically. When the clinoenstatite-orthoenstatite transition is taken into account, [S.sup.o.sub.298] 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 [H.sub.2]O in the fluid as a function of pressure and temperature. Keywords: MgO-Si[O.sub.2]-[H.sub.2]O system, experimental petrology, high pressures, uncertainties, thermodynamics, database, entropy, enthalpy

Author(s): Ronit Kessel (corresponding author) [1, 2]; Max W. Schmidt [2]; Peter Ulmer [2]; Thomas Pettke [3] Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into [...]

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[degrees]C in a multi-anvil apparatus with a synthetic K-free eclogite doped with 860 ppm Cs, ~20 wt% [H.sub.2]O, 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 [H.sub.2]O 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.