Phase relations and formation of K-bearing Al-10 Å phase in the MORB+H2O system: Implications for H2O- and K-cycles in subduction zones

Abstract

The potassium (K) and water (H 2 O) cycles in subduction zones are predominately controlled by the stability of K- and H 2 O-bearing minerals, such as K-mica, lawsonite, and dense hydrous magnesium silicates (DHMS). K-micas (muscovite or phlogopite) are the principal H 2 O and K hosts in subduction zones and Earth’s upper mantle and play a significant role in the deep H 2 O and K cycles. The Mg-10 A phase, normally appearing in hydrated peridotite in high-pressure experiments, has been considered as an important water-carrier in subducted hydrated peridotite. In this study, we found a K-bearing Al-10 A phase in the MORB+H 2 O system (hydrated basalt) at high pressures according to X-ray diffraction and stoichiometry. We experimentally constrained its stability field at high pressure. By considering newly and previously documented compositions of the 10 A phase and micas, we confirmed a continuous solid solution or mixed layering between the 10 A phase and K-mica at the interlayer site, suggesting that the K cycle and the H 2 O cycle in subduction zones are coupled. From the discussion of the effect of f H 2 O on stability of the Al-10 A phase, we conclude that a cold subduction zone can host and carry more bulk H 2 O and K into Earth’s deep mantle than a hot one. This work expands the stability regions of the 10 A phase from the ultramafic system (Mg-10 A phase) to the mafic system (Al-10 A phase), and emphasizes the significance of the 10 A phase for the deep H 2 O and K cycle in subduction zone.