The water–basalt system at 4 to 6 GPa: Phase relations and second critical endpoint in a K-free eclogite at 700 to 1400 °C

Kessel, R.; Ulmer, Peter; Pettke, Thomas; Schmidt, M. W.; Thompson, Alan Bruce

doi:10.1016/j.epsl.2005.06.018

Cited by 292 publications

(180 citation statements)

References 45 publications

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“…Kessel et al (40) reported that the second critical endpoint in a K-free basalt-H 2 O system occurs at around 5.5 GPa, which is 2.1 GPa higher than the result determined in the present study. Kessel et al (40) determined the solubility of silicate components in H 2 O with changing Tunder constant P. The solubility of silicate components changes continuously with T when the experimental P is above the second critical endpoint, whereas the solubility changes discontinuously with T when the experimental P is below the second critical endpoint.…”

Section: Discussioncontrasting

confidence: 57%

See 1 more Smart Citation

Slab melting versus slab dehydration in subduction-zone magmatism

Mibe

Kawamoto

Matsukage

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

156

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The second critical endpoint in the basalt-H 2 O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770°C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.water | island arc | silicate melt | synchrotron X-ray | high-pressure research W ater plays an important role in subduction-zone magmatism because it can reduce the melting temperature of rocks in subduction zones and hence can generate magmas (1-7). There is a long-standing debate about whether the fluids released from a subducting slab are aqueous fluid, hydrous silicate melt, supercritical fluid (SCF), or a combination of these (2,4,(8)(9)(10)(11)(12). Whether the subducting slab melts or dehydrates depends on the thermal structure and the phase relation of slab materials under hydrous conditions. Therefore, this long-standing question cannot be answered until the detailed stability fields of these fluids are clarified.Under high-pressure (P) and high-temperature (T) conditions, it has been shown that the solubility of both water in silicate melt (13-16) and silicate in aqueous fluid (13, 17-25) increases with increasing P. As a result, silicate melt and aqueous fluid in the interior of the Earth are expected to become SCF, and the hydrous solidus of the system can no longer be defined beyond a certain critical condition (26-36). This condition is called the second (or upper) critical endpoint (26) and is the point of intersection between the critical curve and hydrous solidus. Basalt is one of the dominant constituents of a subducting slab and is considered the main carrier of water that triggers the melting of rocks in subduction zones. Therefore, the second critical endpoint in the basalt-H 2 O system has to be determined in order to understand fully the subduction-zone magmatism.In some silicic silicate-H 2 O systems, the location of the second critical endpoint has been reported [e.g., 1.0 GPa, 1,080°C in the SiO 2 -H 2 O system (13); 1.5 GPa, 670°C in the system NaAlSi 3 O 8 -H 2 O (21); 1.5 GPa, 800°C in the system KAlSi 3 O 8 -H 2 O (36)]. In mafic systems, however, the determination of the second critical endpoints is not easy because of the difficulty in identifying phases (aqueous fluid versus hydrous silicate melt) in the recovered samples quenched from high-P and high-T conditio...

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Section: Discussioncontrasting

confidence: 57%

“…2 B versus C) is valid under the experimental P by determining the solubility only. Using their indirect method (40), which involved some uncertainty in determining the solubility of silicates, it is considered that Kessel et al (40) were unable to determine precisely the P of the second critical endpoint.…”

Section: Discussionmentioning

confidence: 99%

Slab melting versus slab dehydration in subduction-zone magmatism

Mibe

Kawamoto

Matsukage

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

156

View full text Add to dashboard Cite

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“…At P Ͼ P2 cP, as melting degree increased, the fluid/melt composition should continuously vary between end-members, from aqueous through carbonatitic to carbonate/silicate liquids (6). Recent experimental studies (6,80) show that such change of the fluid/melt composition took place in systems with compositions corresponding to mantle rocks. This process might have initiated spontaneous crystallization of diamond.…”

Section: Discussionmentioning

confidence: 99%

The role of mantle ultrapotassic fluids in diamond formation

Palyanov

Shatsky

Sobolev

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Analysis of data on micro-and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400 -1,800°C, with different compositions of melts and fluids in the KCl/K 2CO3/H2O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes. high-pressure experimentA n important stage in solving problems of diamond genesis is constructing a clear physical model of diamond formation (1). One of the key parameters to be modeled is the composition of the diamond crystallization medium, with a fluid being largely considered as a crucial agent in diamond formation. It is generally agreed that the mantle media of diamond crystallization were volatile-saturated melts or fluids (1-7). Carbonbearing fluids and carbonates could be sources of carbon, and the medium could have been saturated with it by redox reactions. Recent studies have been aimed at searching and examining fluid and fluid-containing inclusions in genetically different diamonds. Also, experiments have been performed to model the processes of diamond crystallization in the media compositionally corresponding to the inclusions. It is the combination of these two approaches that permits us to gain deeper insight into diamond genesis and to better understand the specific character of processes of deep-seated mineral formation.Studies of mineral inclusions in diamonds have determined the main types of diamondiferous parageneses: ultramaffic, eclogitic (4,8,9), and intermediate websterite (10) and calc-silicate types (11). Some mineral inclusions, e.g., intergrowths of phlogopite with pyroxene (8, 12, 13) and phlogopite with calcite (14), are unambiguously evidence that K and H 2 O were present in the diamond-forming medium. It is pertinent to note that, as early as 30 years ago, neutron activation analysis of diamonds without visible inclusions, taken from three South African deposits, revealed a mixture of mostly K and of other components, which was interpreted as evidence of the presence of entrapped melt (15). Principally, more recent information has accrued from micro-and nano-inclusions in mantle-derived and metamorphic diamonds. Analysis of the composition of the inclusions in fibrous or cloudy mantle diamonds shows that, at the time of entrapment, the inclusion matter was a specific fluid whose composition belongs to one of the three main types: (i) ...

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“…Here, we obtain deeper insights into the crystallisation mechanism by the analysis of solution-solid equilibrium in a model MgO-Al 2 O 3 -SiO 2 -H 2 O (MASH) system and the application of principles of mass conservation. If aqueous fluids released by the slab equilibrate with solid phases, their molal composition would be close to 0.11, 0.18, and 3.7, in terms of dissolved MgO, Al 2 O 3 , and SiO 2 components, in accordance with the experimental composition of the fluid phase in equilibrium with a K-free eclogite at 4 GPa and 800 °C (Kessel et al, 2005; Fig. 3).…”

Section: Resultsmentioning

confidence: 60%

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

Campione¹,

Tumiati²,

Malaspina³

2017

Geochem. Persp. Let.

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Multiphase inclusions represent microenvironments where the interaction between fluid and host mineral is preserved during the rock geological path. Under its peculiar chemical-physical constraints, the entrapped solute-rich fluid might follow a crystallisation mechanism which is not predictable through simple equilibrium arguments. In this letter, by the modelling of solid-solution equilibrium and the application of principles of mass conservation, we demonstrate that cavities in mantle garnet filled with slab-derived fluids can re-equilibrate to a pyrope + spinel + chlorite assemblage at the same high P-T of their formation. The basis of this occurrence is a dissolution-reprecipitation mechanism, triggered by a dilute, non-equilibrated slab fluid.

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The water–basalt system at 4 to 6 GPa: Phase relations and second critical endpoint in a K-free eclogite at 700 to 1400 °C

Cited by 292 publications

References 45 publications

Slab melting versus slab dehydration in subduction-zone magmatism

Slab melting versus slab dehydration in subduction-zone magmatism

The role of mantle ultrapotassic fluids in diamond formation

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

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