Keywords

1 Introduction

The most important mechanism of large-scale exchange of matter between the crust and mantle is the subduction of the oceanic crust, leading to the eclogitization of basalts and the formation of reservoir in the mantle with protoliths of the subducted crust (Taylor and Neal 1989). The volatile enriched fluid has an effective effect on the phase composition, melting point of the mantle, the composition of the formed fluid-containing silicate and salt (carbonate, chloride, sulfide) melts. High extracting and transport properties of fluids at high pressures lead to mobilization and transport of main and impurity elements. The processes of mantle metasomatism are related to their interaction with the mantle substrate.

The experimental model of the mantle reservoir with protoliths of the subducted oceanic crust is the system of peridotite-basalt-fluid. This type of system has been experimentally studied under “dry” conditions (Yaxley 2000; Tumiati et al. 2013; Mallik and Dasgupta 2012). Fluid-containing systems have been studied to a lesser extent. The system of peridotite-basalt in the presence of water and water-carbonate fluids was studied experimentally in the interval of 1250–1400 °C, 1.5–2.5 GPa (Gorbachev 1990). Instability of olivine in the presence of water-containing fluid was revealed. It is shown that the formation of magnesia magmas of the picritobasalt type does not require overheating of the mantle, their formation occurs at temperatures close to the mantle adiabatic point (Gorbachev 2000).

An important feature of fluid-containing silicate systems is the existence of critical relations between silicate melt and fluid at high pressures and temperatures, caused by their high mutual solubility. At critical PCTC, there is a complete mixing between them with the formation of supercritical fluid melt, and in the 2nd end critical point (2PCTC) with the equality of P-T silicate solidus and PCTC equilibrium melt fluid—a complete mixing between the liquidus phases, melt and fluid, which makes it difficult to determine P-T silicate solidus (Keppler and Audetat 2005; Litasov and Ohtani 2007).

The existence in the upper mantle of critical relations and composition of supercritical fluids-melt is proved by fluid-melt inclusions in diamonds, in minerals of mantle xenoliths of peridotite and eclogites from kimberlites and alkaline basalts, as well as in tectonically embedded carbonate-containing peridotite and eclogite massifs. Three main final types of such fluids are distinguished: carbonate enriched with Ca, Mg, Fe; salt enriched with Na, K, H2O and silicate enriched with Si, Al (Klein-Bendavid et al. 2007; Navon et al. 1988; Weiss et al. 2009; Bogatikov et al. 2010). There is a complete mixture between alkaline and carbonate, carbonate and silicate end compositions of mantle fluids, and at pressure and temperature of the second critical point—between silicates and fluid (Wyllie and Ryabchikov 2000; Kessel et al. 2005).

Critical ratios were experimentally studied in water-containing silicate systems of monomineral (SiO2, albite, nepheline, jadeite) and granite compositions. In these systems the critical pressure and temperature of PCTC and 2PCTC lie in the range of 0.7–2.3 GPa, 550–1050 °C, increasing in the sequence of quartz–nepheline–albite–jadeite–granite. Critical pressures decrease when components (e.g. fluorine) are added to the fluid, increasing the mutual solubility of the melt and fluid, and increase when a component is added to the fluid, reducing the mutual solubility of the melt and fluid (e.g. CO2) (Bureau and Keppler 1999). It has been established that at H2O + CO2 composition of the fluid in the system of peridotite-basalt critical ratios of silicate melt—fluid do not occur up to pressures of 4 GPa, but are observed at the water composition of the fluid. It was shown that 4 GPa and 1400 °C in the system of peridotite-basalt-H2O characterize parameters of the 2 end critical points (Gorbachev 2000).

In the main and ultrabasic systems, much attention was paid to the PCTC of the second end critical point of 2PCTC. In the Fo–En–H2O system, 2PCTC pressure is estimated at 12–13 GPa (Stalder et al. 2001), eclogite-H2O at 5–6 GPa (Kessel et al. 2005), basalt-peridotite-H2O at 3.8–4.0 GPa (Gorbachev 2000), and peridotite-H2O at 3.8 GPa (Mibe et al. 2007). The Fo + En + H2O + CO2 2PCTC system is estimated at 12–15 GPa (Willy and Rhyabchikov 2000). Critical ratios also exist between silicate and carbonate melts (Kisseva et al. 2012; Dasgupta et al. 2005; Gorbachev et al. 2015a, b).

In the majority of experimental works the transition of the system to the supercritical state and critical equilibria of melt and fluid were determined in situ at extreme (supra-liquidus) temperatures. The complete melt and fluid miscibility has been recorded either visually or by the homogenization of the 2-phase melt fluid association on diamond anvil equipment (Bureau and Keppler 1999) or by X-ray radiography (X-ray radiography) (Mibe et al. 2007). In some studies, the quenching method was used, where the transition from subcritical to supercritical state in the eclogite-H2O system was fixed by the composition of quenching water-bearing glass and fluid in diamond traps (Kessel et al. 2005). However, these methods do not allow us to study the peculiarities of the phase composition in the interaction of supercritical fluid melts with restite. Moreover, most of the experiments were carried out at extreme temperatures. Taking into account the large temperature interval between solidus and liquidus in the main and ultrabasic systems, as well as the formation of mantle magmas in the partial melting of the mantle substance, it is of great interest to study the critical relations between partial (subliquidus) melts and fluid in the presence of refractory restite. In the works (Gorbachev 2000; Gorbachev et al. 2015a, b), the texture and phase composition of experimental samples can be used as a critical ratio test in such experiments. The composition of supercritical fluid melts and the nature of their interaction with the silicate substrate depend on the ratio of silicate and fluid components. It is believed that at intermediate compositions in supercritical fluid melts there may exist clusters, typical for both melt and fluid (Wyllie and Rhyabchikov 2000). The dual nature of supercritical fluid melts may appear in the structure and phase composition of the material formed during quenching after the experiment.

To obtain new data on the influence of fluid composition on melting, phase composition and critical ratios in the fluid-containing upper mantle, the system of peridotite-basalt-(Na, K)2CO3 under “dry” conditions and in the presence of H2O, H2O + CO2 and H2O + HCl fluids is studied at P = 4 GPa, T = 1400 °C as an experimental model of a mantle reservoir with protoliths of subducted oceanic crust.

2 Experiment

The experiments were carried out in IEM RAS on the anvil with hole apparatus (NL-40) using a multi-ampoule technique with a Pt-peridotite ampoule (Gorbachev 1990). A specially prepared peridotite ampoule was filled with a mixture of powders of tholeiite basalt and sodium and potassium carbonates. Start composition (wt%): peridotite—55 wt%, basalt—25 wt%, sodium and potassium carbonate ~10 wt%. Chemical compositions of peridotite and basalt are given in Table 12.1. The source of fluids (20 wt% versus silicate) was distilled H2O, oxalic acid dihydrate H2C2O4 × 2H2O, 1H solution HCl. Chromite was added as accessory minerals, and Ni–Cu–Pt containing pyrrhotite was used in experiments under “dry” conditions. The charged peridotite ampoule was placed in a “working” Pt ampoule (d = 5 mm), which was hermetically welded. This ampoule was placed in a larger Pt ampoule (d = 10 mm) containing a fO2 buffer association close to the quartz-fayalite-magnetite buffer. Temperature was measured by Pt30Rh/Pt6Rh thermocouple, pressure at high temperatures was calibrated according to the quartz-coesite equilibrium curve. The accuracy of the temperature and pressure tests is estimated at ±5 °C and ±1 kbar (Litvin 1991). The duration of the experiment was 18–24 h. The products of the experiments—polished preparations of quenching samples—were studied on the electronic scanning microscope CamScan MV2300 with YAG detector of secondary and reflected electrons and energy dispersive X-ray microanalyzer with semiconductor Si(Li) detector Link INCA Energy.

Table 12.1 Chemical compositions (in wt%) of the initial peridotite and basalt in experiments
Full size table

3 Results

3.1 Peridotite-Basalt-Na2CO3–K2CO3 System

Microphotographs of quenching samples are given in Fig. 12.1, chemical compositions of coexisting phases are given in Table 12.2. In “dry” conditions the quenched samples inherit the structure of peridotite ampoule. The inner “basalt” part, the “reaction” zone at the contact basalt-peridotite and the outer “peridotite” part of the ampoule are separated. The texture is massive. Although there was no disintegration of the initial peridotite ampoule during the experiment, the mineral composition of the zones is similar as a result of the reactions occurring during the experiment. Their main resemblance is the absence of olivine. The samples consist of isolated secretions of pyroxenes, phlogopite, chromium-spinel, cemented with intergranular alkaline melt (Table 12.2, Fig. 12.1b, c). Pyroxene grains of 20–50 μ in size have a zonal structure. The reaction relations of clinopyroxene ← orthopyroxene in the peridotite part of the sample are observed. The central part of the crystal is characterized by a higher content of MgO (up to 33 wt%), low content of CaO (~3 wt%) is represented by orthopyroxene. The edges of the grain are lighter colored due to a decrease in the concentration of MgO (up to 20–22 wt%) and an increase in the content of CaO (up to 12–14 wt%), belong to the pigeonites (Fig. 12.1c, Table 12.2). In the reaction zone of the ampoule isolated orthopyroxene crystals coexist with clinopyroxenes of avgite composition. Silicate melt has a phonolite composition. SiO2 content ~55 wt%, K2O + Na2O ~ 11–13 wt%, K2O/Na2O ~ 0.15. Sulfides are concentrated in the reaction zone. The oval, molten form of sulfides indicates partial melting of the original pyrrhotite. Sulfide globula matrix is represented by Fe–Ni sulfide of pentlandite and Ni-pyrrhotite composition with inclusions of PtS and Pt–Fe phase (Table 12.2, Fig. 12.1b).

Fig. 12.1
figure 1

Peridotite-basalt-Na2CO3–K2CO3 system. BSE images of experimental run products. a General view of the sample consist of peridotite ampoule and basalt; b phase composition of reaction zone consist of: orthopyroxene (Opx), clinopyroxene (Cpx), phlogopite (Phl), chromite (Chr), with inclusions of sulphide globules (LSulph), cemented by alkaline silicate glass (LSil); c peridotite ampoule consist of orthopyroxene (Opx), clinopyroxene (Cpx), phlogopite (Phl) and chromite (Chr), cemented by alkaline silicate glass (LSil)

Full size image
Table 12.2 Chemical compositions (in wt%) of coexisting phases in peridotite–basalt–(Na, K)2CO3 system
Full size table

3.2 Peridotite-Basalt-Na2CO3–K2CO3–(H2O + CO2) System

Microphotographs of the samples are shown in Fig. 12.2, chemical compositions of the coexisting phases-in Table 12.3. The hardened sample has a solid texture. The initial structure of the peridotite ampoule is not preserved. The texture and phase composition of peridotite and its basaltic part are similar. The experimental sample consists of clinopyroxene, K-clinopyroxene, phlogopite, single grains of orthopyroxene. Notice the absence of olivine among restite minerals. Klinopyroxene is moderately aluminous (up to 9 wt% Al2O3), concentration of micro impurities TiO2, Cr2O3 does not exceed 0.5 wt%, Na2O content is about 0.8 wt%. Potassium clinopyroxene is more aluminous (up to 12 wt% Al2O3), with a reduced concentration of SiO2 (46–47 wt%), concentration of micro impurities does not exceed 0.5 wt% for TiO2 and 0.7 wt% for Cr2O3, enriched with Na2O (up to 2.5 wt%), K2O (up to 0.7 wt%), ratio of K2O/Na2O ~ 0.3. Phlogopite is found in the form of thin, filiform secretions. Silicate melt of moderate alkalinity refers to trachiandezybasalts. The alkali content in the melt (K2O + Na2O) is about 6 wt%, Na2O/K2O ~ 2.3.

Fig. 12.2
figure 2

Peridotite-basalt-Na2CO3–K2CO3–(H2O + CO2) system. BSE images of experimental run products. a General view of the sample. During melting, the composition averaged. Contact between the peridotite and basalt parts is almost not observed; b basalt, c peridotite part of the ampoule. Their texture and phase composition (Ca-clinopyroxene (Cpx), K-clinopyroxene (K-Cpx), phlogopite (Phl) and intergranular silicate glass (LSil)) are similar

Full size image
Table 12.3 Chemical compositions (in wt%) of coexisting phases in peridotite–basalt–(Na, K)2CO3–(H2O + CO2) system
Full size table

3.3 Peridotite-Basalt-Na2CO3–K2CO3–H2O System

Microphotographs of the samples are shown in Fig. 12.3, chemical compositions of coexisting phases-in Table 12.4.

Fig. 12.3
figure 3

Peridotite-basalt-Na2CO3–K2CO3–H2O system. BSE images of experimental run products. a Peridotite-basalt contact; b basalt part, represented by a fine mixture of quenching silicate phases; c disintegrated peridotite part; d–f different areas of peridotite ampoules, characterizing the features of the phase composition, due to the reaction ratios in the peridotite restead with Opx ← Cpx ← K-Amp substitutions, newly formed Phl, Cb and quenched glass globules (LSil)

Full size image
Table 12.4 Chemical compositions (in wt%) of coexisting phases in peridotite–basalt–(Na, K)2CO3–H2O system
Full size table

Disintegration of the peridotite ampoule was observed in the experimental products. As can be seen in Fig. 12.3a, the cross section of the sample only shows the components of the initial sample: disintegrated peridotite ampoule (Fig. 12.3c), reaction zone on the border of peridotite-basalt and “basalt” (Fig. 12.3b, c) the part of the initial sample formed during the quenching of supercritical fluid-melt consisting of a mixture of small crystals (up to 5 μ) of silicates, carbonates, sulfide and aluminosilicate microglobules. In contrast to subcritical conditions, massive silicate glass is not formed in the peridotite of the ampoule. The experimental sample consists of isolated relics or intergrowths of peridotite restite (olivine + orthopyroxene + clinopyroxene). Reaction relations between them and newly formed minerals with the replacement of type: orthopyroxene ← clinopyroxene ← K-amphibole (Fig. 12.3d, e), as well as the formation of newly formed and quenched phases—phlogopite, carbonate, Al–Si globules of phonolite glass (Fig. 12.3f). Figure 12.3(d–f) shows the types of reaction relations in peridotite restite, and Table 12.4 shows their chemical composition.

3.4 Peridotite-Basalt-Na2CO3–K2CO3–(H2O + HCl) System

Microphotographs of quenching samples are shown in Fig. 12.4, chemical compositions of coexisting phases-in Table 12.5. The texture and phase composition of the quenching sample resembles that of the H2O + CO2 fluid experiment (Fig. 12.3). They differ in the presence of halite, riodacite composition of intergranular and quenched (in the form of Al–Si globules) melts. The matrix of experimental samples consists of relics and aggregates of restite minerals clinopyroxene, olivine, orthopyroxene, phlogopite.

Fig. 12.4
figure 4

Peridotite-basalt-Na2CO3–K2CO3–(H2O + HCl) system. BSE image of experimental run products. The formation of NaCl crystals in a silicate matrix consisting of olivine (Ol), clinopyroxene (Cpx, K-Cpx), phlogopite (Phl), silicate melt (LSil)

Full size image
Table 12.5 Chemical compositions (in wt%) of coexisting phases in peridotite–basalt–(Na, K)2CO3–(H2O + HCl) system
Full size table

4 Discussion

4.1 Peridotite-Basalt-Na2CO3–K2CO3 System

Partial melting of the original sample is observed. Although after the experiment the structure of peridotite ampoule is preserved, but as a result of reactions occurring during the experiment, the mineral composition of peridotite and basalt is similar. Their main feature is the absence of olivine, formation of alkaline melt of phonolite composition, concentration of sulfide melt in the reaction zone. Reaction ratios in pyroxenes of the type of pigeonite ← orthopyroxene is observed in the peridotite part of the ampoule enriched with MgO.

4.2 Peridotite-Basalt-Na2CO3–K2CO3–(H2O + CO2) System

In the case of partial melting of the initial sample, the structure of the initial peridotite ampoule is not preserved. The phase relations were determined by reactions between the fluid, alkaline fluid-containing melt and peridotite. As a result of reactions, olivine is dissolved, clinopyroxene, K-amphibole, and phlogopite are formed that coexist with the trachiandezybasalt melt.

4.3 Peridotite-Basalt-Na2CO3–K2CO3–H2O System

At 4 GPa, 1400 °C, critical relations between the carbonated silicate melt and the fluid were achieved. This is evidenced by the peculiarities of texture and phase composition of the samples: disintegration of peridotite of the ampoule, absence of intergranular silicate glass in peridotite. Peridotite ampoule, which was filled with a mixture of basalt and carbonates during the assembly of the initial sample, after the experiment consisted of a compressed mixture of micron-sized particles of silicate and carbonate composition, formed during the quenching of fluid-melt. Reaction relations in restite with substitutions of olivine ← orthopyroxene ← clinopyroxene ← K-amphibole are determined by the interaction with supercritical fluid-melt. Wide development of reaction relations among minerals of peridotite restite testifies to high chemical activity of supercritical fluid-melt. The presence of quenching phases—phlogopite, Al–Si globules, carbonate—characterize the composition of supercritical fluid-melt. At melting of the supercritical mantle reservoir with protoliths of the subducted oceanic crust at P-T, exceeding the critical values, there should also be observed decompaction of the substrate as a result of disintegration of peridotite.

4.4 Peridotite-Basalt-Na2CO3–K2CO3–(H2O + HCl) System

The quenching samples do not inherit the structure of the peridotite ampoule. At partial melting as a result of reactions between peridotite restite, fluid and fluid-containing melt there was an averaging of phase composition of peridotite and basalt components of the initial sample, formation without olivine association coexisting with riodacite melt. The texture and phase composition of the quenched samples are similar to the samples from the experiments with H2O + CO2 fluid. The formation of halite is observed.

4.5 K-Containing Phases

In experiments with H2O + CO2, H2O + HCl fluids, potassium phases are represented by K-containing clinopyroxenes with a similar chemical composition. In experiments with aqueous potassium-rich fluids, the K-amphibole phase is represented. Compared to K-clinopyroxene, K-amphibole is depleted in Na, Mg, Si and contains higher concentrations of K, Fe, Al, Ti (Table 12.6).

Table 12.6 Chemical compositions (in wt%) of K-containing phases in systems with different fluid (H2O + CO2, H2O + HCl, H2O)
Full size table

5 Conclusions

The composition of the fluid has a significant influence on the phase composition and critical ratios in mantle reservoirs with protoliths of the oceanic crust. In “dry” conditions and at acid composition of fluid alkaline melts of Na series are formed. The effect of acid-base interaction is clearly manifested, acidification of the fluid shifts the composition of the melt from trachiandezybasalt at H2O + CO2 composition of the fluid to riodacites with H2O + HCl fluid. Alkaline melts coexist with non-olivine metasomatically altered restite of pyroxene-phlogopite composition.

Instability of olivine at partial melting of peridotite-basalt mixture in the presence of acid fluid is established. The interaction of fluid and fluid-containing melts with restite in experiments with acid fluid (H2O + CO2, H2O + HCl) was accompanied by pyroxenization and phlogopitization of peridotite:

$$\begin{aligned} & {\text{Mg}}_{2} {\text{SiO}}_{4 } (Ol) \, + {\text{MgSiO}}_{3 } (Opx) \, + \, (3{\text{CaCO}}_{3} + \, 4{\text{SiO}}_{2} )\left( {L_{Sil } + Fl} \right) \\ & = 3{\text{CaMgSi}}_{2} {\text{O}}_{6 } (Cpx) \, + \, 3{\text{CO}}_{2 } (Fl); \\ \end{aligned}$$
$$\begin{aligned} & 2{\text{Mg}}_{ 2} {\text{SiO}}_{4} \left( {Ol} \right) + 2{\text{MgSiO}}_{3} \left( {Opx} \right) + ({\text{K}}_{ 2} {\text{O}} + {\text{Al}}_{ 2} {\text{O}}_{ 3} + 2{\text{SiO}}_{ 2} + 2{\text{H}}_{ 2} {\text{O)}}(L_{Sil} + Fl) \\ & = 3{\text{CaMgSi}}_{ 2} {\text{O}}_{ 6} \left( {Cpx} \right) + 3{\text{CO}}_{ 2} (Fl). \\ \end{aligned}$$

The acid fluid produces K-containing clinopyroxene, the aqueous fluid produces K-containing amphibole, and the H2O + HCl fluid produces magmatic halite.

Critical relations between fluid and silicate melt are observed in the water-bearing system. Interaction of supercritical fluid melts with peridotite restitis leads to the formation of clinopyroxene, K-amphibole, phlogopite, carbonate, quenching silicate globules. Newly formed clinopyroxene and K-amphibole are in reactionary relations with olivine, orthopyroxene and clinopyroxene of peridotite restitis. Modal metasomatism of the upper mantle under the influence of water-containing supercritical fluid-melt leads to secondary enrichment—”refertilization” of depleted restite of the harzburgite composition, replacement of peridotite olivine-orthopyroxene association of newly formed phlogopite-clinopyroxene association. The texture and phase composition of the samples under supercritical conditions allow us to draw a conclusion about the zonal structure of reservoirs with protoliths of the subducted oceanic crust: the outer zone—metasomatically altered under the influence of supercritical fluid-melt disintegrated restite of dunite or harzburgite composition, underwent “refertilization” as a result of the formation of new minerals in it, the inner zone—isolated lenses of supercritical fluid-melt. Disintegration of fluid-containing mantle peridotite substrate at supercritical pressures can lead to the formation of tectonically weakened zones, fluid paths and upper mantle plumes.