2013

Melting of the model peridotite under influence of H2O-NaCl fluid at 6 GPa.

Butvina V.G., Safonov O.G.

IEM RAS, Chernogolovka, Moscow district, butvina@iem.ac.ru

Support: MD – 222.2012.5, RFBR 13-05-00353, 12-05-31017_mol_a

At the conditions of diamond and kimberlite melts stability, KCl and NaCl are important constituents of complex carbonate-silicate, carbonatite and carbonate-chloride fluid-melts, whose inclusions are known in diamonds world-wide (e.g. [1]). In depths 45 - 75 km, alkali chlorides participate in aqueous-carbonic fluids, that is supported by findings of Cl-bearing apatites, amphiboles and micas, as well as brine inclusions in minerals of spinel peridotites nodules [2-5]. Despite these data, experimental data on interaction of the chloride-bearing fluids with peridotites are extremely scarce. In the most cases, conclusions on influence of chlorides on phase relations in upper-mantle assemblages are based on extrapolation of results of experiments at pressures below 2 GPa and in the systems unrelated to peridotite (e.g. [6]). Experiments by Chu et al. [7] on melting of Mg₂SiO₄+MgSiO₃ at presence of the H₂O+KCl fluid at 5 GPa show, that solidus temperature of this assemblage increases with increasing of KCl content in the fluid, that is related to reduction of the H2O activity with addition of salts [8]. These experiments were conducted in model system, which did not contain such important components as Al₂O₃, CaO, Na₂O. Presence of such components can have a significant influence on interaction of peridotites with brine fluids. For example, Al has strong affinity to alkalis destabilizing Al-rich silicate minerals. Previously, we have shown experimentally [9] that the H₂O-KCl fluids destabilize the assemblage of orthopyroxene with Al-bearing phases (garnet, spinel, amphibole), but do not affect clinopyroxene and olivine. As a result of these reaction, the clinopyroxene-olivine-phlogopite assemblage forms, whose solidus temperature decreases. Thus, the effect of chemical interaction of chlorides and silicate minerals predominates over the effect of the reduction of the H₂O activity. The effect of NaCl, apparently, is related to stability of pargasite amphibole. However, at pressures above 3 GPa, where amphibole is unstable, the NaCl effect on phase relationships in H₂O-bearing peridotite is less obvious.

In order to study the effect of NaCl on the Al₂O₃, CaO, Na₂O-rich H₂O-bearing peridotite transformation at pressures above 3 GPa, we have conducted the experiments on interaction of model peridotite Fo₅₇En₁₇Prp₁₄Di₁₂ with the H₂O-NaCl fluid at 6 GPa and 1050-1450^ОС. Starting materials were mixtures of oxides, Mg(OH)₂ and jadeite (mg): SiO₂ (37.04); Al₂O₃ (3.66); CaO (3.62); Mg(OH)₂ (48.30); MgO (4.14), NaAlSi₂O₆ (3.24). NaCl added at 8 wt. %, that corresponds to mole fraction of X_NaCl = NaCl/(NaCl+H₂O) in the fluid 0.05. The present experiments were performed using Pt capsules of 0.02 mm of the wall thickness.

In absence of NaCl in the fluid, the assemblage Fo+Opx+Cpx+Grt was observed in the solidus of the model peridotite, while melting began at about 1200-1300⁰С. Clinopyroxene shows the increase of jadeite content with increasing temperature. Addition of NaCl reduces melting temperature down to 1050-1100⁰С. Amount of orthopyroxene and garnet in solidus decreases, the Al content in orthopyroxene decreases, and the jadeite content of clinopyroxene increases in presence of NaCl. These relationships can be described by the following reactions:

(1)  1/4 Mg-Ts + 7/4 En + 1/2 NaCl + 1/4 H₂O = Fo + 1/2 Jd + 1/2 HCl,

(2)  1/4Prp + 5/4 En + 1/2 NaCl + 1/4 H₂O = Fo + 1/2 Jd + 1/2 HCl

(3)  1/4 Grs + 11/4 En + 1/2 NaCl + 1/4 H₂O = Fo + 3/4 Di +1/2 Jd + 1/2 HCl

(where Mg-Ts – Mg- Tschemack molecule, MgAl₂SiO₆ in orthopyroxene, En – enstatite, Fo – forsterite, Prp – pyrope, Grs – grossular, Di – diopside, Jd – jadeite). These reactions show the destabilization of Grt-Opx association in the presence of the H₂O-NaCl fluid.

            Thus, addition NaCl in the H₂O-peridotite system does not influence on phase assemblages. However, changing of mineral compositions, mostly increase of the jadeite content in clinopyroxene, results in the decrease of melting temperature. Present experiments further support our conclusion [9] that the effect of interaction of alkali chlorides with silicates in complex peridotite assemblages overpowers the effect of the reduced H₂O activity in the brine fluid [7].

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