The influence of water on melting of mantle peridotite

This experimental study examines the effects of variable concentrations of dissolved H₂O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H₂O dissolved in the liquid. The effects of increasing the concentration of dissolved H₂O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO₂, FeO, MgO, and CaO. The concentration of Al₂O₃ is unaffected. The lower SiO₂ contents of the hydrous melts result from an increase in the activity coefficient for SiO₂ with increasing dissolved H₂O. The lower concentrations of FeO and MgO result from the lower temperatures at which H₂O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO₂/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO₂ rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H₂O-rich component introduced into the mantle wedge from the subducted slab. Received: 7 April 1997 / Accepted: 9 January 1998     

Phase relations of phlogopite with magnesite from 4 to 8 GPa

To evaluate the stability of phlogopite in the presence of carbonate in the Earth’s mantle, we conducted a series of experiments in the KMAS–H₂O–CO₂ system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl₉₀-mag₁₀; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1,150 to 1,550°C. We bracketed the solidus between 1,200 and 1,250°C at pressures of 4, 5 and 6 GPa and between 1,150 and 1,200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus, magnesite is the first phase to react out, and enstatite and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from ~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO₂-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a cold (40 mWm⁻²) subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and can coexist with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite and carbonate at a depth of 180–200 km.     

The origin and evolution of the parental magmas of frontal volcanoes in Kamchatka: Evidence from magmatic inclusions in olivine from Zhupanovsky volcano

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO₂, ∼2.5 wt % H₂O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx: (Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO₂, Na₂O, and K₂O and depleted in MgO, CaO, and Al₂O₃. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H₂O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (∼45 wt % SiO₂) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H₂O-rich fluid to higher temperature H₂O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka was estimated at 4°C/km.     

Experimental study of eclogite melting with participation of the H<Subscript>2</Subscript>O-CO<Subscript>2</Subscript>-KCl fluid at 5 GPa

In order to model the processes of formation of the highly alkaline (potassic) melts during the partial melting of the eclogite nodules in kimberlites, experiments on the melting of the model and natural eclogites in presence of the H₂O-CO₂ and H₂O-CO₂-KCl fluids at 5 GPa and 1200 and 1300°C are performed. A comparative analysis of the phase relations in the systems with H₂O-CO₂ and H₂O-CO₂-KCl demonstrate that KCl in the fluid equilibrated with eclogites intensifies their melting. It is related to both high Cl concentration in the forming silicate melt (2.0–5.5 wt %) and its enrichment in K₂O owing to the K-Na exchange reactions with the immiscible chloride melt. Because of these reactions, the K₂O/Cl ratio in the melts increases with the KCl content in the system and reaches 2.5–3.5 in the silicate melts coexisting with the immiscible chloride liquid. However, the ratio KCl/(H₂O + CO₂ + KCl) in the fluid does not influence on the ratio K₂O/Cl in the melts. Thus, the solubility KCl in the melts, apparently, does not depend on presence of the H₂O-CO₂ fluid, at least, within the concentration range used in the experiments (up to 20 wt %). The experiments show that the deliberated chloride liquid is necessary to form the potassium-rich chlorine-bearing silicate melts during the eclogite melting. It corresponds to the KCl content in the system above 5 wt %.     

Stability of phlogopite in ultrapotassic kimberlite-like systems at 5.5–7.5 GPa

Hydrous K-rich kimberlite-like systems are studied experimentally at 5.5–7.5 GPa and 1200–1450?°C in terms of phase relations and conditions for formation and stability of phlogopite. The starting samples are phlogopite–carbonatite–phlogopite sandwiches and harzburgite–carbonatite mixtures consisting of Ol?+?Grt?+?Cpx?+?L (±Opx), according to the previous experimental results obtained at the same P–T parameters but in water-free systems. Carbonatite is represented by a K- and Ca-rich composition that may form at the top of a slab. In the presence of carbonatitic melt, phlogopite can partly melt in a peritectic reaction at 5.5 GPa and 1200–1350?°C, as well as at 6.3–7.0 GPa and 1200?°C: 2Phl?+?CaCO₃ (L)?Cpx?+?Ol?+?Grt?+?K₂CO₃ (L)?+?2H₂O (L). Synthesis of phlogopite at 5.5 GPa and 1200–1350?°C, with an initial mixture of H₂O-bearing harzburgite and carbonatite, demonstrates experimentally that equilibrium in this reaction can be shifted from right to left. Therefore, phlogopite can equilibrate with ultrapotassic carbonate–silicate melts in a?≥?150?°C region between 1200 and 1350?°C at 5.5 GPa. On the other hand, it can exist but cannot nucleate spontaneously and crystallize in the presence of such melts in quite a large pressure range in experiments at 6.3–7.0 GPa and 1200?°C. Thus, phlogopite can result from metasomatism of peridotite at the base of continental lithospheric mantle (CLM) by ultrapotassic carbonatite agents at depths shallower than 180–195 km, which creates a mechanism of water retaining in CLM. Kimberlite formation can begin at 5.5 GPa and 1350?°C in a phlogopite-bearing peridotite source generating a hydrous carbonate–silicate melt with 10–15 wt% SiO₂, Ca# from 45 to 60, and high K enrichment. Upon further heating to 1450?°C due to the effect of a mantle plume at the CLM base, phlogopite disappears and a kimberlite-like melt forms with SiO₂ to 20 wt% and Ca#?=?35–40.     

Phase relations and melting of carbonated peridotite between 10 and 20 GPa: a proxy for alkali- and CO<Subscript>2</Subscript>-rich silicate melts in the deep mantle

We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite + 5.0 wt % CO₂ and PERC: fertile peridotite + 2.5 wt % CO₂) at 10–20 GPa and 1,500–2,100 °C and constrain isopleths of the CO₂ contents in the silicate melts in the deep mantle. At 10–20 GPa, near-solidus (ACP: 1,400–1,630 °C) carbonatitic melts with < 10 wt % SiO₂ and > 40 wt % CO₂ gradually change to carbonated silicate melts with > 25 wt % SiO₂ and < 25 wt % CO₂ between 1,480 and 1,670 °C in the presence of residual majorite garnet, olivine/wadsleyite, and clinoenstatite/clinopyroxene. With increasing degrees of melting, the melt composition changes to an alkali- and CO₂-rich silicate melt (Mg# = 83.7–91.6; ~ 26–36 wt % MgO; ~ 24–43 wt % SiO₂; ~ 4–13 wt % CaO; ~ 0.6–3.1 wt % Na₂O; and ~ 0.5–3.2 wt % K₂O; ~ 6.4–38.4 wt % CO₂). The temperature of the first appearance of CO₂-rich silicate melt at 10–20 GPa is ~ 440–470 °C lower than the solidus of volatile-free peridotite. Garnet + wadsleyite + clinoenstatite + carbonatitic melt controls initial carbonated silicate melting at a pressure < 15 GPa, whereas garnet + wadsleyite/ringwoodite + carbonatitic melt dominates at pressure > 15 GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10–20 GPa. The liquidus is likely to be at ~ 2,050 °C or higher at pressures of the present study, which gives a melting interval of more than 670 °C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20 GPa at near mantle adiabat or even at plume temperature. These alkali- and CO₂-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the zone of metal saturation, CO₂ and H₂O flux melting can take place and kimberlite parental magmas can be generated. These mechanisms might be important for mantle dynamics and are potentially effective metasomatic processes in the deep mantle.     

The beginnings of hydrous mantle wedge melting

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na₂O, 0.03 wt% K₂O) in the presence of excess H₂O (14.5 wt% H₂O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H₂O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from the H₂O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H₂O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H₂O-saturated melting curve. Chlorite can therefore provide sufficient H₂O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H₂O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H₂O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies free H₂O to trigger melting at the H₂O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer of chloritized peridotite up to 1.5 km thick could contain all the H₂O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones.     

Solubility of carbon dioxide in melts of andesite,tholeiite, and olivine nephelinite composition to 30 kbar pressure

Carbon dioxide solubilities in H₂O-free hydrous silicate melts of natural andesite (CA), tholeiite (K 1921), and olivine nephelinite (OM1) compositions have been determined employing carbon-14 beta-track mapping techniques. The CO₂ solubility increases with increasing pressure, temperature, and degree of silica-undersaturation of the silicate melt. At 1650° C, CO₂ solubility in CA increases from 1.48±0.05 wt % at 15 kbar to 1.95±0.03 wt % at 30 kbar. The respective solubilities in OM1 are 3.41±0.08 wt % and 7.11±0.10 wt %. The CO₂ solubility in K1921 is intermediate between those of CA and OM1 compositions. At lower temperatures, the CO₂ contents of these silicate melts are lower, and the pressure dependence of the solubility is less pronounced. The presence of H₂O also affects the CO₂ solubility (20–30% more CO₂ dissolves in hydrous than in H₂O-free silicate melts); the solubility curves pass through an isothermal, isobaric maximum at an intermediate CO₂/(CO₂+H₂O) composition of the volatile phase. Under conditions within the upper mantle where carbonate minerals are not stable and CO₂ and H₂O are present a vapor phase must exist. Because the solubility of CO₂ in silicate melts is lower than that of H₂O, volatiles must fractionate between the melt and vapor during partial melting of peridotite. Initial low-temperature melts will be more H₂O-rich than later high-temperature melts, provided vapor is present during the melting. Published phase equilibrium data indicate that the compositional sequence of melts from peridotite +H₂O+CO₂ parent will be andesite-tholeiite-nephelinite with increasing temperature at a pressure of about 20 kbar. Examples of this sequence may be found in the Lesser Antilles and in the Indonesian Island Arcs.     

Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province,Brazil: genesis and constraints on mantle metasomatism

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO₂; 5.2–6.7 wt.% ZrO₂) occurs in peridotite xenoliths rimming chromite (～50 wt.% Cr₂O₃) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.     

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO₂ and especially K₂O but low CaO and Al₂O₃ relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO₂ contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO₂ in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al₂O₃ and do not resemble lamproites. These experiments show that low CaO and Al₂O₃ in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P₂O₅ or TiO₂ depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.     

Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

Crystallization (dacite) and interaction (dacite–peridotite)experiments have been performed on the 1991 Pinatubo dacite(Luzon Island, Philippines) to constrain its petrogenesis. Inthe dacite–H₂O system at 960 MPa, magnetite and eitherclinopyroxene (low H₂O) or amphibole (high H₂O) are the liquidusphases. No garnet is observed at this pressure. Dacite–peridotite interaction at 920 MPa produces massive orthopyroxenecrystallization, in addition to amphibole ± phlogopite.Amphibole crystallizing in dacite at 960 MPa has the same compositionas the aluminium-rich hornblende preserved in the cores of amphibolephenocrysts in the 1991 dacite, suggesting a high-pressure stageof dacite crystallization with high melt H₂O contents (>10wt %) at relatively low temperature (<950°C). The compositionsof plagioclase, amphibole and melt inclusion suggest that thePinatubo dacite was water-rich, oxidized and not much hotterthan 900°C, when emplaced into the shallow magma reservoirin which most phenocrysts precipitated before the onset of the1991 eruption. The LREE-enriched REE pattern of the whole-rockdacite demands garnet somewhere during its petrogenesis, whichin turn suggests high-pressure derivation. Partial melting ofsubducted oceanic crust yields melts unlike the Pinatubo dacite.Interaction of these slab melts with sub-arc peridotite is unableto produce a Pinatubo type of dacite, nor is a direct mantleorigin conceivable on the basis of our peridotite–daciteinteraction experimental results. Dehydration melting of underplatedbasalts requires unrealistically high temperatures and doesnot yield dacite with the low FeO/MgO, and high H₂O, Ni andCr contents typical of the Pinatubo dacite. The most plausibleorigin of the Pinatubo dacite is via high-pressure fractionationof a hydrous, oxidized, primitive basalt that crystallized amphiboleand garnet upon cooling. Dacite melts produced in this way weredirectly expelled from the uppermost mantle or lower crust toshallow-level reservoirs from which they erupted occasionally.Magmas such as the Pinatubo dacite may provide evidence forthe existence of particularly H₂O-rich conditions in the sub-arcmantle wedge rather than the melting of the young, hot subductingoceanic plate. KEY WORDS: Pinatubo dacite; slab melt; experimental petrology; arc magmas     

The system granite-peridotite-H2O at 30 kbar,with applications to hybridization in subduction zone magmatism

Experiments with mixtures of granite, peridotite and H₂O at 30 kbar were designed as a first step to test the hypothesis that the calc-alkaline igneous rocks of subduction zones are formed by differentiation of magmas derived by partial melting of hybrid rocks generated in the mantle wedge, by reaction between hydrous siliceous magma rising from subducted oceanic crust, and the overlying mantle peridotite. Experiments were conducted in gold capsules in half-inch diameter piston-cylinder apparatus. Results are presented in a 900° C isotherm, and in a projection of vapor-present phase fields onto T-granite-peridotite. Isobaric solution of peridotite in hydrous, H₂O-undersaturated granite liquid at 900° C causes only small changes in liquid composition, followed by precipitation of orthopyroxene until about half of the liquid has solidified; then orthopyroxene is joined by jadeitic clinopyroxene, garnet, and phlogopite. Phlogopite-garnet-websterite continues to be precipitated, with evolution of aqueous vapor, until all of the liquid is used up. The product of hybridization is a pyroxenite without olivine. The products of partial melting of this material would differ from products derived from peridotite because there is no olivine control, and the clinopyroxenes contain up to 7% Na₂O, compared with less than 1% Na₂O in peridotite clinopyroxenes. The reaction products are directly analogous to those in the model system KAlSiO₄-Mg₂SiO₄-SiO₂-H₂O, where, with decreasing SiO₂ in the hydrous siliceous liquid, the field for phlogopite expands, and phlogopite instead of orthopyroxene becomes the primary mineral. If this occurs with less siliceous magmas from the subducted oceanic crust, there is a prospect for separation of discrete bodies of phlogopite-rock as well as phlogopite-garnet-websterite. We need to know the products of hybridization, and the products of partial melting of the hybrid rocks through a range of conditions.     

Beginning of melting and composition of first melts in the system Qz-Ab-Or-H2O-CO2

Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO₂)-Ab(NaAlSi₃O₈)-Or(KAlSi₃O₈) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH₂O) between 0.0 and 1.0 were obtained by using H₂O-CO₂ mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH₂O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH₂O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH₂O=0.1 is 120°C. The influence of pure CO₂ on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH₂O=0.5 between 2 and 10 kbar and approximatively 830°C at aH₂O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.     

Dehydration melting of tonalites. Part I. Beginning of melting

 The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An₄₅) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions of biotite varying from iron-free phlogopite to iron-rich Ann₇₀, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase (An₄₅) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and 730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar). The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration melting of biotite (Ann₅₀) + plagioclase (An₄₅) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene + clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f _{O 2} in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium, and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann₅₀) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann₇₀-bearing starting composition begins at 730°C, and in the Ann₂₅-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of biotite (Ann₂₅ and Ann₅₀) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5 kbar). Received: 27 December 1995 / Accepted: 7 May 1996     

Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂–O₂–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.     

Experimental study of the phase and melting relations of homogeneous basalt + peridotite mixtures and implications for the petrogenesis of flood basalts

Flood basalt provinces may constitute some of the most catastrophic volcanic events in the Earth's history. A popular model to explain them involves adiabatic ascent of plumes of anomalously hot peridotite from a thermal boundary layer deep in the mantle, across the peridotite solidus. However, peridotitic plumes probably require unreasonably high potential temperatures to generate sufficient volumes of magma and high enough melting rates to produce flood volcanism. This lead to the suggestion that low melting eclogitic or pyroxenitic heterogeneities may be present in the source regions of the flood basalts. In order to constrain petrogenetic models for flood basalts generated in this way, an experimental investigation of the melting relations of homogeneous peridotite + oceanic basalt mixtures has been performed. Experiments were conducted at 3.5 GPa on a fertile peridotite (MPY90)–oceanic basalt (GA1) compositional join. The hybrid basalt + peridotite compositions crystallised garnet lherzolite at subsolidus temperatures plus quenched ne-normative picritic liquids at temperatures just above the solidus, over the compositional range MPY90 to GA1₅₀MPY90₅₀. The solidus temperature decreased slightly from ∼1500 °C for MPY90 to ∼1450 °C for GA1₅₀MPY90₅₀. Compositions similar to GA1₃₀MPY90₇₀ have 100% melting compressed into a melting interval which is approximately 50–60% smaller than that for pure MPY90, due to a liquidus minimum. During adiabatic ascent of hybrid source material containing a few tens of percent basalt in peridotite, the lower solidus and compressed solidus–liquidus temperature interval may conspire to substantially enhance melt productivity. Mixtures of recycled oceanic crust and peridotite in mantle plumes may therefore provide a viable source for some flood volcanics. Evidence for this would include higher than normal Fe/Mg values in natural primary liquids, consistent with equilibration with more Fe-rich olivine than normal pyrolitic olivine (i.e. <Fo_89–92). Modelling of fractionation trends in West Greenland picrites is presented to demonstrate that melts parental to the Greenland picrites were in equilibrium at mantle P–T conditions with olivine with Fo_84–86, suggesting an Fe-enriched source compared with normal peridotite, and consistent with the presence of a basaltic component in the source. Received: 29 October 1999 / Accepted: 3 February 2000     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Mantle melting in equilibrium with an Iron–Wüstite–Graphite buffered COH-fluid

Partial melting experiments on a San Carlos peridotite were done in a Walker type multi-anvil press at pressures from 5 to 12.5 GPa. Experiments were done in the presence of a COH-fluid and at oxygen fugacity controlled by the Fe–FeO buffer. Olivine, clinopyroxene, garnet and orthopyroxene are stable in all but the highest temperature 10 GPa experiments where olivine and garnet coexist, and the highest temperature 5 GPa experiments where olivine is the single crystalline phase. The solidus at 5 GPa was found to be at approximately 1,200°C and the liquidus was estimated to be at 1,325°C, which is ∼500°C lower than has been reported for dry melting of peridotite. The aluminum concentration of the melts decreases with increasing melt fraction and decreases also with increasing pressure. At 5 GPa the melts have a CaO/Al₂O₃-ratio of 0.85–1.0, which is similar to that of undepleted komatiites; major element concentrations are also identical to those of undepleted komatiites such as the Munro komatiites. At 10 and 12.5 GPa the partial melts have CaO/Al₂O₃-ratios above 1.5 and major element composition almost identical to aluminum depleted komatiites such as the Barberton komatiites. We therefore conclude that in the presence of a reducing COH-fluid both aluminum-depleted and -undepleted komatiites could have formed at temperatures much lower than generally accepted.     

Fluids in the peridotite–water system up to 6 GPa and 800°C: new experimental constrains on dehydration reactions

The phase assemblages and compositions in a K-free lherzolite + H₂O system were determined between 4 and 6 GPa and 700–800°C, and the dehydration reactions occurring at subarc depth in subduction zones were constrained. Experiments were performed on a rocking multi-anvil apparatus using a diamond-trap setting. The composition of the fluid phase was measured using the recently developed cryogenic LA–ICP–MS technique. Results show that, at 4 GPa, the aqueous fluid coexisting with residual lherzolite (~85 wt% H₂O) doubles its solute load when chlorite transforms to the 10-Å phase between 700 and 750°C. The 10-Å phase breaks down at 4 and 5 GPa between 750 and 800°C and at 6 GPa between 700 and 750°C, leaving a dry lherzolite coexisting with a fluid phase containing 58–67 wt% H₂O, again doubling the total dissolved solute load. The fluid fraction in the system increases from 0.2 when a hydrous mineral is present to 0.4 when coexisting with a dry lherzolite. Our data do not reveal the presence of a hydrous peridotite solidus below 800°C. The directly measured fluid compositions demonstrate a fundamental change in the (MgO + FeO) to SiO₂ mass ratio of fluid solutes occurring at a depth of ca. 120–150 km (in the temperature window of 700–800°C), from (MgO–FeO)-dominated at 4 GPa [with (MgO + FeO)/SiO₂ ratio of 1.41–1.56] to SiO₂-dominated at 5–6 GPa (ratios of 0.61–0.82). The mobility of Al₂O₃ increases by more than one order of magnitude across this P–T interval and demonstrates that Al₂O₃ is compatible in an aqueous fluid coexisting with the anhydrous ol-opx-cpx ± grt assemblage. This shift in the fluid composition correlates with changes in the phase assemblage of the residual silicates. The hitherto unknown fundamental change in (MgO + FeO)/SiO₂ ratio and prominent increase in Al₂O₃ of the aqueous fluid with progressive subduction will likely inspire novel concepts on mantle wedge metasomatism by slab fluids.     

Anatectic melt volumes in the thermal aureole of the Etive Complex,Scotland: the roles of fluid‐present and fluid‐absent melting

Pelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ～770 °C and yield <11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H₂O, modelled melt modes increase dramatically within ～13 °C of the solidus, in some cases to >60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H₂O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the diorite magma chamber associated with a second phase of intrusion, or by sub‐solidus thermal contraction in the diorite pluton during the early stages of inner‐aureole cooling.