Merwinite-structured phases as a potential host of alkalis in the upper mantle

Luca Bindi, Oleg G. Safonov, Dmitriy A. Zedgenizov

Результат исследования: Научные публикации в периодических изданияхстатья

5 Цитирования (Scopus)

Аннотация

Two previously unknown Na- and K-rich phases were synthesized near the solidus of the model CMAS lherzolite interacted with the CaCO3 + Na2CO3 + KCl melt at 7 GPa. They coexist with forsterite, garnet and chloride–carbonate melt. Stoichiometry and unit-cell parameters measured by means of powder diffraction indicate that one of the phases corresponds to (K,Na)2Ca4Mg2Si4O15 (with about 0.1 a.p.f.u. Al). Although single-crystal X-ray measurements of this phase did not allow the solution of the crystal structure, we suggest that the structure of this phase includes mixed SiO4 and Si2O7 units. Single-crystal diffraction experiments of the other alkali-rich phase with composition (Ca2.06Na0.86K0.08)Σ=3.00(Mg0.53Si0.45Al0.03)Σ=1.01Si2.00O8 showed that it exhibits the merwinite structure, space group P21/a, with lattice parameters a = 12.987(2), b = 5.101(1), c = 9.130(2) Å, β = 92.36(1)°, V = 604.3(2) Å3, and Z = 4. The structure was refined to R1 = 0.031 using 2619 independent reflections. In the structure, Na is hosted at the large Ca sites, whereas Si replaces Mg at the octahedral site and occurs in the usual tetrahedral coordination. Ordering-induced distortion provokes a change in coordination of the (Ca, Na) atoms with respect to pure merwinite. Merwinite phases with lower K + Na contents (0.08–0.18 a.p.f.u.) coexist with forsterite, clinopyroxene and immiscible carbonate–chloride and silicate melts at higher temperatures (up to 1510 °C) at 7 and 5.5 GPa. These phases (including alkali-rich ones at solidus) show a general formula [Ca3−2x(Na,K)2x][Mg1−xSix]Si2O8 (with x up to 0.45), where the Na + K content negatively correlates with Ca and positively correlates with Si. The present experimental and crystal-chemical data prove that merwinite-structured phases may be efficient hosts for alkalis in the upper mantle. They are mineralogical indicators of either the interaction of mantle peridotites with alkaline carbonatitic liquids or high-pressure crystallization of silica-undersaturated CaO and alkali-rich melts.

Язык оригиналаанглийский
Номер статьи14
ЖурналContributions to Mineralogy and Petrology
Том170
Номер выпуска2
DOI
СостояниеОпубликовано - 31 авг 2015

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