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Marc M. Hirschmann - One of the best experts on this subject based on the ideXlab platform.

  • Carbon-saturated monosulfide melting in the shallow mantle: solubility and effect on Solidus
    Contributions to Mineralogy and Petrology, 2015
    Co-Authors: Zhou Zhang, Nathan Lentsch, Marc M. Hirschmann
    Abstract:

    We present high-pressure experiments from 0.8 to 7.95 GPa to determine the effect of carbon on the Solidus of mantle monosulfide. The graphite-saturated Solidus of monosulfide (Fe0.69Ni0.23Cu0.01S1.00) is described by a Simon and Glatzel (Z Anorg Allg Chem 178:309–316, 1929) equation T (°C) = 969.0[P (GPa)/5.92 + 1]0.39 (1 ≤ P ≤ 8) and is ~80 ± 25 °C below the melting temperature found for carbon-free conditions. A series of comparison experiments using different capsule configurations and preparations document that the observed Solidus-lowering is owing to graphite saturation and not an artifact of different capsules or hydrogen contamination. Concentrations of carbon in quenched graphite-saturated monosulfide melt measured by electron microprobe are 0.1–0.3 wt% in monosulfide melt and below the detection limit (

  • A modified iterative sandwich method for determination of near-Solidus partial melt compositions. II. Application to determination of near-Solidus melt compositions of carbonated peridotite
    Contributions to Mineralogy and Petrology, 2007
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann
    Abstract:

    We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the Solidus of natural, fertile peridotite + CO2 at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the Solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-Solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO2, TiO2 and Al2O3 concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na2O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-Solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na2O concentrations in near Solidus carbonatitic partial melt likely account for the ∼150°C lower Solidus temperature of natural carbonated peridotite compared to the Solidus of synthetic peridotite in the system CMAS + CO2. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.

  • Effect of variable carbonate concentration on the Solidus of mantle peridotite
    American Mineralogist, 2007
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann
    Abstract:

    To explore the effect of variable CO 2 concentrations on the Solidus of natural carbonated peridotite, we determined near-Solidus phase relations of three different nominally anhydrous, carbonated lherzolite bulk compositions at 6.6 GPa. Starting mixes (PERC, PERC2, and PERC3) were prepared by adding variable proportions of a carbonate mixture that has the same Ca:Mg:Fe:Na:K ratio as the base silicate peridotite [MixKLB-1: Mg no. = 89.7; Ca no. = molar Ca/(Ca + Mg + Fe*) = 0.05]. For all three bulk compositions, the subSolidus assemblage includes olivine, orthopyroxene, clinopyroxene, garnet, and magnesite solid solutions. Above the Solidus, crystalline carbonate disappears and quenched Fe, Na-bearing dolomitic carbonatite melts were observed. For PERC3 (1.0 wt% bulk CO 2 ; Na 2 O/CO 2 weight ratio = 0.30), the observed Solidus is between 1190 and 1220 °C; for PERC (2.5 wt% bulk CO 2 ; Na 2 O/CO 2 = 0.12), it is between 1250 and 1275 °C; and for PERC2 (5.0 wt% bulk CO 2 ; Na 2 O/CO 2 = 0.06), it is between 1300 and 1330 °C. At 6.6 GPa, experimental solidi of natural magnesite peridotites are 100–200 °C lower than the CMAS-CO 2 Solidus, chiefly owing to the fluxing effect of alkalis, and Solidus temperatures increase with increasing bulk CO 2 (i.e., decreasing bulk Na 2 O/CO 2 ), owing to dilution of Na 2 O in near-Solidus melt. The effects of Mg no. and Ca no. on carbonated peridotite solidi appear to be less significant than that of Na 2 O/CO 2 . Trends of decreasing Solidus temperature with increasing Na 2 O/CO 2 and with decreasing CO 2 indicate that natural mantle peridotite with ~100–1000 ppm bulk CO 2 will have Solidus temperatures ~20° to ~100° lower than that determined experimentally. The Solidus of peridotite drops discontinuously by ~600 °C (at 6.6 GPa) at the CO 2 bulk concentration (~5 ppm) at which carbonate is stabilized, but then varies little with increasing bulk CO 2 . This result contrasts with the effect of H 2 O, which lowers the Solidus continuously with increasing concentration.

  • The effect of bulk composition on the Solidus of carbonated eclogite from partial melting experiments at 3 GPa
    Contributions to Mineralogy and Petrology, 2005
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann, Nikki Dellas
    Abstract:

    To explore the effect of bulk composition on the Solidus of carbonated eclogite, we determined near-Solidus phase relations at 3 GPa for four different nominally anhydrous, carbonated eclogites. Starting materials (SLEC1, SLEC2, SLEC3, and SLEC4) were prepared by adding variable proportions and compositions of carbonate to a natural eclogite xenolith (66039B) from Salt Lake crater, Hawaii. Near-Solidus partial melts for all bulk compositions are Fe–Na calcio-dolomitic and coexist with garnet + clinopyroxene + ilmenite ± calcio-dolomitic solid solution. The Solidus for SLEC1 (Ca#=100 × molar Ca/(Ca + Mg + Fe_T)=32, 1.63 wt% Na_2O, and 5 wt% CO_2) is bracketed between 1,050°C and 1,075°C (Dasgupta et al. in Earth Planet Sci Lett 227:73–85, 2004), whereas initial melting for SLEC3 (Ca# 41, 1.4 wt% Na_2O, and 4.4 wt% CO_2) is between 1,175°C and 1,200°C. The Solidus for SLEC2 (Ca# 33, 1.75 wt% Na_2O, and 15 wt% CO_2) is estimated to be near 1,100°C and the Solidus for SLEC3 (Ca# 37, 1.47 wt% Na_2O, and 2.2 wt% CO_2) is between 1,100°C and 1,125°C. Solidus temperatures increase with increasing Ca# of the bulk, owing to the strong influence of the calcite–magnesite binary Solidus-minimum on the Solidus of carbonate bearing eclogite. Bulk compositions that produce near-Solidus crystalline carbonate closer in composition to the minimum along the CaCO_3-MgCO_3 join have lower Solidus temperatures. Variations in total CO_2 have significant effect on the Solidus if CO_2 is added as CaCO_3, but not if CO_2 is added as a complex mixture that maintains the cationic ratios of the bulk-rock. Thus, as partial melting experiments necessarily have more CO_2 than that likely to be found in natural carbonated eclogites, care must be taken to assure that the compositional shifts associated with excess CO_2 do not unduly influence melting behavior. Near-Solidus dolomite and calcite solid solutions have higher Ca/(Ca + Mg) than bulk eclogite compositions, owing to Ca–Mg exchange equilibrium between carbonates and silicates. Carbonates in natural mantle eclogite, which have low bulk CO_2 concentration, will have Ca/Mg buffered by reactions with silicates. Consequently, experiments with high bulk CO_2 may not mimic natural carbonated eclogite phase equilibria unless care is taken to ensure that CO_2 enrichment does not result in inappropriate equilibrium carbonate compositions. Compositions of eclogite-derived carbonate melt span the range of natural carbonatites from oceanic and continental settings. Ca#s of carbonatitic partial melts of eclogite vary significantly and overlap those of partial melts of carbonated lherzolite, however, for a constant Ca-content, Mg# of carbonatites derived from eclogitic sources are likely to be lower than the Mg# of those generated from peridotite.

  • deep global cycling of carbon constrained by the Solidus of anhydrous carbonated eclogite under upper mantle conditions
    Earth and Planetary Science Letters, 2004
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann, Anthony C Withers
    Abstract:

    Abstract We present partial melting experiments that constrain the near Solidus phase relations of carbonated eclogite from 2 to 8.5 GPa. The starting material was prepared by adding 5 wt.% CO2 in the form of a mixture of Fe–Mg–Ca–Na–K carbonates to an eclogite from Salt Lake crater, Oahu, Hawaii and is a reasonable approximation of carbonated oceanic crust from which siliceous hydrous fluids have been extracted during subduction. Melt-present versus melt-absent conditions are distinguished based on textural criteria. Garnet and clinopyroxene appear in all the experiments. Between 2 and 3 GPa, the subSolidus assemblage also includes ilmenite±calcio-dolomitess±CO2, whereas above the Solidus (1050–1075 °C at 3 GPa) calcio-dolomitic liquid appears. From 3 to 4.5 GPa, dolomitess is stable at the Solidus and the near-Solidus melt becomes increasingly dolomitic. The appearance of dolomite above 3 GPa is accompanied by a negative Clapeyron slope of the Solidus, with a minimum located between 995 and 1025 °C at ca. 4 GPa. Above 4 GPa, the Solidus rises with increasing pressure to 1245±35 °C at 8.5 GPa and magnesite becomes the subSolidus carbonate. Dolomitic melt coexists with magnesite+garnet+cpx+rutile along the Solidus from 5 to 8.5 GPa. Comparison of our results to other recent experimental studies [T. Hammouda, High-pressure melting of carbonated eclogite and experimental constraints on carbon recycling and storage in the mantle, Earth Planet. Sci. Lett. 214 (2003) 357–368; G.M. Yaxley, G.P. Brey, Phase relations of carbonate-bearing eclogite assemblages from 2.5 to 5.5 GPa: implications for petrogenesis of carbonatites, Contrib. Mineral. Petrol. 146 (2004) 606–619] shows that carbonate minerals are preserved in anhydrous or slightly hydrous carbonated eclogite to temperatures >1100 and >1200 °C at 5 and 9 GPa, respectively. Thus, deep subduction of carbonate is expected along any plausible subduction geotherm. If extrapolated to higher pressures, the carbonated eclogite Solidus is likely to intersect the oceanic geotherm at a depth close to 400 km. Carbonated eclogite bodies entering the convecting upper mantle will thus release carbonate melt near the top of the mantle transition zone and may account for anomalously slow seismic velocities at depths of 280–400 km. Upon release, this small volume, highly reactive melt could be an effective agent of deep mantle metasomatism. Comparison of the carbonated eclogite Solidus with that of peridotite-CO2 shows a shallower Solidus–geotherm intersection for the latter. This implies that carbonated peridotite is a more likely proximal source of magmatic carbon in oceanic provinces. However, carbonated eclogite is a potential source of continental carbonatites, as its Solidus crosses the continental shield geotherm at ca. 4 GPa. Transfer of eclogite-derived carbonate melt to peridotite may account for the geochemical characteristics of some oceanic island basalts (OIBs) and their association with high CaO and CO2.

Eiji Ohtani - One of the best experts on this subject based on the ideXlab platform.

  • the Solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32 gpa and carbonatite liquid in the deep mantle
    Earth and Planetary Science Letters, 2010
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–stishovite at 10 GPa via garnet–corundum–stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The Solidus temperature was defined by the appearance of partial melt. The Solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of Solidus curve is less steep at 10–32 GPa than at lower pressures. The Solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.

  • Solidus and phase relations of carbonated peridotite in the system cao al2o3 mgo sio2 na2o co2 to the lower mantle depths
    Physics of the Earth and Planetary Interiors, 2009
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    Abstract Melting phase relations have been determined in a model carbonated peridotite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressures from forsterite/wadsleyite–clinoenstatite/akimotoite–garnet–clinopyroxene/Ca-perovskite at 10–20 GPa to Mg-perovskite–periclase–Ca-perovskite at 27–32 GPa. Magnesite is the only carbonate stable in peridotite through the studied pressure range. The Solidus temperature was defined by the appearance of quenched carbonatite melt, which occurs at slightly lower temperature than that of disappearance of magnesite. Accordingly, Solidus of carbonated peridotite in the present study is bracketed at 1380–1460 °C at 10.5 GPa, 1550–1650 °C at 16.5 GPa, 1620–1720 °C at 20 GPa, 1710–1850 °C at 27 GPa, and 1750–1890 °C at 32 GPa. The slope of Solidus curve is more gradual at 10–32 GPa than at lower pressures. The Solidus temperature was found to be in agreement with previous works on carbonated peridotite at pressures below 10 GPa with comparable alkali and CO 2 contents in the starting material. Partial melts formed by melting of carbonated peridotite at 10.5–32.0 GPa have magnesiocarbonatitic compositions with moderate variations in Ca/Mg ratio and have high Na 2 O-contents. It has been demonstrated that alkali-rich magnesiocarbonatite melt can be generated by partial melting of carbonated peridotite at pressure up to at least 32 GPa, i.e. to the lower mantle depths. The generation of calciocarbonatite by melting of carbonated peridotite is unlikely in the deep mantle. Determined Solidus temperatures allow stability of magnesite along the normal mantle geotherm, however minor heating or addition of potassium to the system can cause melting of carbonates.

  • Solidus of carbonated peridotite from 10 to 20 GPa and origin of magnesiocarbonatite melt in the Earth's deep mantle
    Chemical Geology, 2009
    Co-Authors: Sujoy Ghosh, Konstantin D Litasov, Eiji Ohtani, Hidenori Terasaki
    Abstract:

    article i nfo Article history: Accepted 23 December 2008 We have experimentally determined the Solidus of an alkali-bearing carbonated peridotite (with 5 wt.% CO2) between 10 and 20 GPa. Based on K-deficit in all low-temperature runs we assumed that some melt could be present in the low temperature runs and the true Solidus of an alkali-bearing carbonated peridotite is placed below 1200 °C. However, based on the disappearance of magnesite and the appearance of the visible quenched melt coexisting with silicate phases, the 'apparent' Solidus, which may be applicable for peridotite with low alkali contents, was identified. The 'apparent' Solidus temperature increases from ~1380 °C at 10 GPa to ~1525 °C at 15 GPa and the 'apparent' Solidus curve becomes almost flat from 15 GPa to 20 GPa, where it is located near 1550 °C. At 10 GPa, the 'apparent' Solidus of carbonated peridotite is ~550 °C lower than the Solidus of CO2-free natural anhydrous peridotite. The Solidus of the present study was also ~120 °C lower than the Solidus determined by Dasgupta and Hirschmann (Dasgupta, R., Hirschmann, M.M., 2006. Melting in the Earth's deep upper mantle caused by carbon dioxide. Nature, 440, 659-662.) for natural carbonated peridotite. The drop in the Solidus temperature is mainly due to the effect of alkalis (Na2O, K2O). The melt near the 'apparent' Solidus has high CO2 (N40 wt.%) and contains b6.0 wt.% SiO2, b0.30 wt.% Al2O3 and b0.25 wt.% TiO2. The composition of near-Solidus partial melt is close to that observed at 6-10 GPa in the CMS-CO2 and CMAS-CO2 systems, and natural carbonated peridotite, with some variations in Ca/Mg-ratio. High alkali contents in measured and calculated partial melts are consistent with the compositions of deep- seated fluids observed as inclusions in diamonds and may be consistent with the compositions of parental melt, reconstructed for natural magnesiocarbonatite. We have demonstrated that magnesiocarbonatite-like melt can be generated by partial melting of carbonated peridotite at pressure up to at least 20 GPa. The generation of calciocarbonatite and ferrocarbonatite is unlikely to be possible during melting of carbonated peridotite in the deep mantle.

  • Hydrous Solidus of CMAS‐pyrolite and melting of mantle plumes at the bottom of the upper mantle
    Geophysical Research Letters, 2003
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    [1] We showed in previous experiments that the melting temperature of hydrous pyrolite, at the transition boundary between wadsleyite and olivine, is abruptly reduced by the presence of 2 wt.% H2O. In this paper we determine the apparent Solidus for CaO-MgO-Al2O3-SiO2-pyrolite with lower and geologically more reasonable H2O contents (0.5 wt.%). Phase relations and melt compositions have been determined at pressures of 13.5–17.0 GPa and temperatures of 1600 to 2100°C. There was no abrupt decrease of Solidus temperature along the phase boundary between olivine and wadsleyite in pyrolite with 0.5 wt.% H2O. However significant gradual decrease of the Solidus temperature at pressures below 15–16 GPa still supports previous models for a hydrous origin of some ancient komatiites by dehydration melting of rising wet plumes at pressures of 4–10 GPa.

Konstantin D Litasov - One of the best experts on this subject based on the ideXlab platform.

  • the Solidus of carbonated eclogite in the system cao al2o3 mgo sio2 na2o co2 to 32 gpa and carbonatite liquid in the deep mantle
    Earth and Planetary Science Letters, 2010
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    Abstract Melting phase relations have been determined in a model carbonated eclogite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressure from garnet–omphacite–kyanite–stishovite at 10 GPa via garnet–corundum–stishovite at 16–20 GPa to Mg–perovskite–Ca–perovskite–CF phase–stishovite at 27–32 GPa. Magnesite is the only carbonate stable in this system through the studied pressure range. The Solidus temperature was defined by the appearance of partial melt. The Solidus of carbonated eclogite is bracketed at 1380–1460 °C at 10.5 GPa, 1460–1560 °C at 16.5 GPa, 1530–1630 °C at 20 GPa, and 1600–1790 °C at 27 and 32 GPa. The slope of Solidus curve is less steep at 10–32 GPa than at lower pressures. The Solidus curve of Fe-free carbonated eclogite roughly coincides with an average mantle geotherm. Partial melts formed by melting of carbonated eclogite at 10.5–32.0 GPa have magnesiocarbonatite compositions with Ca/Mg ratios higher than in similar melts in peridotite assemblages, and contain high Na 2 O-contents. It has been demonstrated that carbonatite-like melt can be generated by partial melting of carbonated eclogites at pressure up to at least 32 GPa, i.e. to lower mantle depths.

  • Solidus and phase relations of carbonated peridotite in the system cao al2o3 mgo sio2 na2o co2 to the lower mantle depths
    Physics of the Earth and Planetary Interiors, 2009
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    Abstract Melting phase relations have been determined in a model carbonated peridotite (5 wt.% CO 2 ) at 10.5–32.0 GPa and 1300–1850 °C. The assemblage of silicate minerals coexisting with partial melts changes with pressures from forsterite/wadsleyite–clinoenstatite/akimotoite–garnet–clinopyroxene/Ca-perovskite at 10–20 GPa to Mg-perovskite–periclase–Ca-perovskite at 27–32 GPa. Magnesite is the only carbonate stable in peridotite through the studied pressure range. The Solidus temperature was defined by the appearance of quenched carbonatite melt, which occurs at slightly lower temperature than that of disappearance of magnesite. Accordingly, Solidus of carbonated peridotite in the present study is bracketed at 1380–1460 °C at 10.5 GPa, 1550–1650 °C at 16.5 GPa, 1620–1720 °C at 20 GPa, 1710–1850 °C at 27 GPa, and 1750–1890 °C at 32 GPa. The slope of Solidus curve is more gradual at 10–32 GPa than at lower pressures. The Solidus temperature was found to be in agreement with previous works on carbonated peridotite at pressures below 10 GPa with comparable alkali and CO 2 contents in the starting material. Partial melts formed by melting of carbonated peridotite at 10.5–32.0 GPa have magnesiocarbonatitic compositions with moderate variations in Ca/Mg ratio and have high Na 2 O-contents. It has been demonstrated that alkali-rich magnesiocarbonatite melt can be generated by partial melting of carbonated peridotite at pressure up to at least 32 GPa, i.e. to the lower mantle depths. The generation of calciocarbonatite by melting of carbonated peridotite is unlikely in the deep mantle. Determined Solidus temperatures allow stability of magnesite along the normal mantle geotherm, however minor heating or addition of potassium to the system can cause melting of carbonates.

  • Solidus of carbonated peridotite from 10 to 20 GPa and origin of magnesiocarbonatite melt in the Earth's deep mantle
    Chemical Geology, 2009
    Co-Authors: Sujoy Ghosh, Konstantin D Litasov, Eiji Ohtani, Hidenori Terasaki
    Abstract:

    article i nfo Article history: Accepted 23 December 2008 We have experimentally determined the Solidus of an alkali-bearing carbonated peridotite (with 5 wt.% CO2) between 10 and 20 GPa. Based on K-deficit in all low-temperature runs we assumed that some melt could be present in the low temperature runs and the true Solidus of an alkali-bearing carbonated peridotite is placed below 1200 °C. However, based on the disappearance of magnesite and the appearance of the visible quenched melt coexisting with silicate phases, the 'apparent' Solidus, which may be applicable for peridotite with low alkali contents, was identified. The 'apparent' Solidus temperature increases from ~1380 °C at 10 GPa to ~1525 °C at 15 GPa and the 'apparent' Solidus curve becomes almost flat from 15 GPa to 20 GPa, where it is located near 1550 °C. At 10 GPa, the 'apparent' Solidus of carbonated peridotite is ~550 °C lower than the Solidus of CO2-free natural anhydrous peridotite. The Solidus of the present study was also ~120 °C lower than the Solidus determined by Dasgupta and Hirschmann (Dasgupta, R., Hirschmann, M.M., 2006. Melting in the Earth's deep upper mantle caused by carbon dioxide. Nature, 440, 659-662.) for natural carbonated peridotite. The drop in the Solidus temperature is mainly due to the effect of alkalis (Na2O, K2O). The melt near the 'apparent' Solidus has high CO2 (N40 wt.%) and contains b6.0 wt.% SiO2, b0.30 wt.% Al2O3 and b0.25 wt.% TiO2. The composition of near-Solidus partial melt is close to that observed at 6-10 GPa in the CMS-CO2 and CMAS-CO2 systems, and natural carbonated peridotite, with some variations in Ca/Mg-ratio. High alkali contents in measured and calculated partial melts are consistent with the compositions of deep- seated fluids observed as inclusions in diamonds and may be consistent with the compositions of parental melt, reconstructed for natural magnesiocarbonatite. We have demonstrated that magnesiocarbonatite-like melt can be generated by partial melting of carbonated peridotite at pressure up to at least 20 GPa. The generation of calciocarbonatite and ferrocarbonatite is unlikely to be possible during melting of carbonated peridotite in the deep mantle.

  • Hydrous Solidus of CMAS‐pyrolite and melting of mantle plumes at the bottom of the upper mantle
    Geophysical Research Letters, 2003
    Co-Authors: Konstantin D Litasov, Eiji Ohtani
    Abstract:

    [1] We showed in previous experiments that the melting temperature of hydrous pyrolite, at the transition boundary between wadsleyite and olivine, is abruptly reduced by the presence of 2 wt.% H2O. In this paper we determine the apparent Solidus for CaO-MgO-Al2O3-SiO2-pyrolite with lower and geologically more reasonable H2O contents (0.5 wt.%). Phase relations and melt compositions have been determined at pressures of 13.5–17.0 GPa and temperatures of 1600 to 2100°C. There was no abrupt decrease of Solidus temperature along the phase boundary between olivine and wadsleyite in pyrolite with 0.5 wt.% H2O. However significant gradual decrease of the Solidus temperature at pressures below 15–16 GPa still supports previous models for a hydrous origin of some ancient komatiites by dehydration melting of rising wet plumes at pressures of 4–10 GPa.

Rajdeep Dasgupta - One of the best experts on this subject based on the ideXlab platform.

  • A modified iterative sandwich method for determination of near-Solidus partial melt compositions. II. Application to determination of near-Solidus melt compositions of carbonated peridotite
    Contributions to Mineralogy and Petrology, 2007
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann
    Abstract:

    We performed modified iterative sandwich experiments (MISE) to determine the composition of carbonatitic melt generated near the Solidus of natural, fertile peridotite + CO2 at 1,200–1,245°C and 6.6 GPa. Six iterations were performed with natural peridotite (MixKLB-1: Mg# = 89.7) and ∼10 wt% added carbonate to achieve the equilibrium carbonatite composition. Compositions of melts and coexisting minerals converged to a constant composition after the fourth iteration, with the silicate mineral compositions matching those expected at the Solidus of carbonated peridotite at 6.6 GPa and 1,230°C, as determined from a sub-Solidus experiment with MixKLB-1 peridotite. Partial melts expected from a carbonated lherzolite at a melt fraction of 0.01–0.05% at 6.6 GPa have the composition of sodic iron-bearing dolomitic carbonatite, with molar Ca/(Ca + Mg) of 0.413 ± 0.001, Ca# [100 × molar Ca/(Ca + Mg + Fe*)] of 37.1 ± 0.1, and Mg# of 83.7 ± 0.6. SiO2, TiO2 and Al2O3 concentrations are 4.1 ± 0.1, 1.0 ± 0.1, and 0.30 ± 0.02 wt%, whereas the Na2O concentration is 4.0 ± 0.2 wt%. Comparison of our results with other iterative sandwich experiments at lower pressures indicate that near-Solidus carbonatite derived from mantle lherzolite become less calcic with increasing pressure. Thus carbonatitic melt percolating through the deep mantle must dissolve cpx from surrounding peridotite and precipitate opx. Significant FeO* and Na2O concentrations in near Solidus carbonatitic partial melt likely account for the ∼150°C lower Solidus temperature of natural carbonated peridotite compared to the Solidus of synthetic peridotite in the system CMAS + CO2. The experiments demonstrate that the MISE method can determine the composition of partial melts at very low melt fraction after a small number of iterations.

  • Effect of variable carbonate concentration on the Solidus of mantle peridotite
    American Mineralogist, 2007
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann
    Abstract:

    To explore the effect of variable CO 2 concentrations on the Solidus of natural carbonated peridotite, we determined near-Solidus phase relations of three different nominally anhydrous, carbonated lherzolite bulk compositions at 6.6 GPa. Starting mixes (PERC, PERC2, and PERC3) were prepared by adding variable proportions of a carbonate mixture that has the same Ca:Mg:Fe:Na:K ratio as the base silicate peridotite [MixKLB-1: Mg no. = 89.7; Ca no. = molar Ca/(Ca + Mg + Fe*) = 0.05]. For all three bulk compositions, the subSolidus assemblage includes olivine, orthopyroxene, clinopyroxene, garnet, and magnesite solid solutions. Above the Solidus, crystalline carbonate disappears and quenched Fe, Na-bearing dolomitic carbonatite melts were observed. For PERC3 (1.0 wt% bulk CO 2 ; Na 2 O/CO 2 weight ratio = 0.30), the observed Solidus is between 1190 and 1220 °C; for PERC (2.5 wt% bulk CO 2 ; Na 2 O/CO 2 = 0.12), it is between 1250 and 1275 °C; and for PERC2 (5.0 wt% bulk CO 2 ; Na 2 O/CO 2 = 0.06), it is between 1300 and 1330 °C. At 6.6 GPa, experimental solidi of natural magnesite peridotites are 100–200 °C lower than the CMAS-CO 2 Solidus, chiefly owing to the fluxing effect of alkalis, and Solidus temperatures increase with increasing bulk CO 2 (i.e., decreasing bulk Na 2 O/CO 2 ), owing to dilution of Na 2 O in near-Solidus melt. The effects of Mg no. and Ca no. on carbonated peridotite solidi appear to be less significant than that of Na 2 O/CO 2 . Trends of decreasing Solidus temperature with increasing Na 2 O/CO 2 and with decreasing CO 2 indicate that natural mantle peridotite with ~100–1000 ppm bulk CO 2 will have Solidus temperatures ~20° to ~100° lower than that determined experimentally. The Solidus of peridotite drops discontinuously by ~600 °C (at 6.6 GPa) at the CO 2 bulk concentration (~5 ppm) at which carbonate is stabilized, but then varies little with increasing bulk CO 2 . This result contrasts with the effect of H 2 O, which lowers the Solidus continuously with increasing concentration.

  • The effect of bulk composition on the Solidus of carbonated eclogite from partial melting experiments at 3 GPa
    Contributions to Mineralogy and Petrology, 2005
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann, Nikki Dellas
    Abstract:

    To explore the effect of bulk composition on the Solidus of carbonated eclogite, we determined near-Solidus phase relations at 3 GPa for four different nominally anhydrous, carbonated eclogites. Starting materials (SLEC1, SLEC2, SLEC3, and SLEC4) were prepared by adding variable proportions and compositions of carbonate to a natural eclogite xenolith (66039B) from Salt Lake crater, Hawaii. Near-Solidus partial melts for all bulk compositions are Fe–Na calcio-dolomitic and coexist with garnet + clinopyroxene + ilmenite ± calcio-dolomitic solid solution. The Solidus for SLEC1 (Ca#=100 × molar Ca/(Ca + Mg + Fe_T)=32, 1.63 wt% Na_2O, and 5 wt% CO_2) is bracketed between 1,050°C and 1,075°C (Dasgupta et al. in Earth Planet Sci Lett 227:73–85, 2004), whereas initial melting for SLEC3 (Ca# 41, 1.4 wt% Na_2O, and 4.4 wt% CO_2) is between 1,175°C and 1,200°C. The Solidus for SLEC2 (Ca# 33, 1.75 wt% Na_2O, and 15 wt% CO_2) is estimated to be near 1,100°C and the Solidus for SLEC3 (Ca# 37, 1.47 wt% Na_2O, and 2.2 wt% CO_2) is between 1,100°C and 1,125°C. Solidus temperatures increase with increasing Ca# of the bulk, owing to the strong influence of the calcite–magnesite binary Solidus-minimum on the Solidus of carbonate bearing eclogite. Bulk compositions that produce near-Solidus crystalline carbonate closer in composition to the minimum along the CaCO_3-MgCO_3 join have lower Solidus temperatures. Variations in total CO_2 have significant effect on the Solidus if CO_2 is added as CaCO_3, but not if CO_2 is added as a complex mixture that maintains the cationic ratios of the bulk-rock. Thus, as partial melting experiments necessarily have more CO_2 than that likely to be found in natural carbonated eclogites, care must be taken to assure that the compositional shifts associated with excess CO_2 do not unduly influence melting behavior. Near-Solidus dolomite and calcite solid solutions have higher Ca/(Ca + Mg) than bulk eclogite compositions, owing to Ca–Mg exchange equilibrium between carbonates and silicates. Carbonates in natural mantle eclogite, which have low bulk CO_2 concentration, will have Ca/Mg buffered by reactions with silicates. Consequently, experiments with high bulk CO_2 may not mimic natural carbonated eclogite phase equilibria unless care is taken to ensure that CO_2 enrichment does not result in inappropriate equilibrium carbonate compositions. Compositions of eclogite-derived carbonate melt span the range of natural carbonatites from oceanic and continental settings. Ca#s of carbonatitic partial melts of eclogite vary significantly and overlap those of partial melts of carbonated lherzolite, however, for a constant Ca-content, Mg# of carbonatites derived from eclogitic sources are likely to be lower than the Mg# of those generated from peridotite.

  • deep global cycling of carbon constrained by the Solidus of anhydrous carbonated eclogite under upper mantle conditions
    Earth and Planetary Science Letters, 2004
    Co-Authors: Rajdeep Dasgupta, Marc M. Hirschmann, Anthony C Withers
    Abstract:

    Abstract We present partial melting experiments that constrain the near Solidus phase relations of carbonated eclogite from 2 to 8.5 GPa. The starting material was prepared by adding 5 wt.% CO2 in the form of a mixture of Fe–Mg–Ca–Na–K carbonates to an eclogite from Salt Lake crater, Oahu, Hawaii and is a reasonable approximation of carbonated oceanic crust from which siliceous hydrous fluids have been extracted during subduction. Melt-present versus melt-absent conditions are distinguished based on textural criteria. Garnet and clinopyroxene appear in all the experiments. Between 2 and 3 GPa, the subSolidus assemblage also includes ilmenite±calcio-dolomitess±CO2, whereas above the Solidus (1050–1075 °C at 3 GPa) calcio-dolomitic liquid appears. From 3 to 4.5 GPa, dolomitess is stable at the Solidus and the near-Solidus melt becomes increasingly dolomitic. The appearance of dolomite above 3 GPa is accompanied by a negative Clapeyron slope of the Solidus, with a minimum located between 995 and 1025 °C at ca. 4 GPa. Above 4 GPa, the Solidus rises with increasing pressure to 1245±35 °C at 8.5 GPa and magnesite becomes the subSolidus carbonate. Dolomitic melt coexists with magnesite+garnet+cpx+rutile along the Solidus from 5 to 8.5 GPa. Comparison of our results to other recent experimental studies [T. Hammouda, High-pressure melting of carbonated eclogite and experimental constraints on carbon recycling and storage in the mantle, Earth Planet. Sci. Lett. 214 (2003) 357–368; G.M. Yaxley, G.P. Brey, Phase relations of carbonate-bearing eclogite assemblages from 2.5 to 5.5 GPa: implications for petrogenesis of carbonatites, Contrib. Mineral. Petrol. 146 (2004) 606–619] shows that carbonate minerals are preserved in anhydrous or slightly hydrous carbonated eclogite to temperatures >1100 and >1200 °C at 5 and 9 GPa, respectively. Thus, deep subduction of carbonate is expected along any plausible subduction geotherm. If extrapolated to higher pressures, the carbonated eclogite Solidus is likely to intersect the oceanic geotherm at a depth close to 400 km. Carbonated eclogite bodies entering the convecting upper mantle will thus release carbonate melt near the top of the mantle transition zone and may account for anomalously slow seismic velocities at depths of 280–400 km. Upon release, this small volume, highly reactive melt could be an effective agent of deep mantle metasomatism. Comparison of the carbonated eclogite Solidus with that of peridotite-CO2 shows a shallower Solidus–geotherm intersection for the latter. This implies that carbonated peridotite is a more likely proximal source of magmatic carbon in oceanic provinces. However, carbonated eclogite is a potential source of continental carbonatites, as its Solidus crosses the continental shield geotherm at ca. 4 GPa. Transfer of eclogite-derived carbonate melt to peridotite may account for the geochemical characteristics of some oceanic island basalts (OIBs) and their association with high CaO and CO2.

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  • carbonatitic melts along the Solidus of model lherzolite in the system cao mgo al2o3 sio2 co2 from 3 to 7 gpa
    Contributions to Mineralogy and Petrology, 1998
    Co-Authors: John A Dalton, Dean C Presnall
    Abstract:

    We have experimentally determined the Solidus position of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 (CMAS.CO2) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subSolidus reactions at the Solidus involving carbonate generates two invariant points, I1A and I2A, which mark the transition from CO2-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO2, we find I1A at 2.6 GPa/1230 °C and I2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the Solidus has also been determined. In the pressure range investigated, Solidus melts are carbonatitic with SiO2 contents of <6 wt%, CO2 contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO2 system show that in a carbonate-bearing mantle, Solidus and near-Solidus melts will be CO2-rich and silica poor. The widespread evidence for the presence of CO2 in both the oceanic and continental upper mantle implies that such low degree SiO2-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.