IS PHASE A RELEVANT TO THE EARTHS MANTLE?

Abstract

Dense hydrous magnesium silicates (DHMS), such as phase A [Mg7Si2O8(OH)6], are potential hosts for water in the Earth's mantle. Previous work has delineated the stability field of phase A (PhA) coexisting with clinoenstatite (CEn) and forsterite (Fo) in the system MgO-SiO2-H2O (MSH). In this study, the effects of the presence of magnesite (Mc), diopside (Di), and pyrope (Py) on the stability of coexisting PhA + CEn + Fo have been determined.In MSH, the breakdown reaction PhA + CEn = Fo + H2O was bracketed between 720 and 740°C at 7 GPa, between 800 and 820°C at 8 GPa, and between 860 and 880°C at 9 GPa. At 10 GPa, PhA + CEn transforms to a clinohumite (CH)-bearing assemblage between 920 and 940°C, which in turn dehydrates between 940 and 960°C to Fo + V. At higher pressures, PhA becomes unstable relative to phase E, another DHMS (Kanzaki, 1991).Addition of carbonate to MSH changes the breakdown reaction to PhA + CEn + Mc = Fo + V, where V is an H2O-CO2 fluid. At 8 GPa, this reaction occurs between 800 and 850°C, indistinguishable from the position of the CO2-free reaction in MSH. In the presence of diopside in the CaO-MgO-SiO2-H2O system, the reaction PhA + Enss = Fo + Diss + H2O occurs between 700 and 730°C at 8 GPa, where Enss and Diss are Ca-poor and Ca-rich pyroxene solid solutions, respectively. In the pyrope-containing assemblage in the MgO-Al2O3-SiO2-H2O system, the PhA-bearing assemblage reacts to form CH between 740 and 760°C at 8 GPa. The CH-bearing assemblage dehydrates to Fo + Py + V between 800 and 830°C. At 8 GPa in the system MgO-FeO-SiO2-H2O with molar Mg/(Mg + Fe) = 0.90, the reaction PhA + CEnss = CH + V occurs at <800°C, and the CEnss + CH = Fo + V reaction occurs between 850 and 900°C.In no case did the presence of additional components, such as CaO, FeO, Al2O3, or CO2, or the additional phases diopside, magnesite, or pyrope, stabilize the hydrous phases PhA or CH to significantly higher temperatures. Therefore, PhA will not be a stable phase in lherzolite assemblages under normal geothermal gradients in the mantle. In the interior of subducting slabs, temperatures may be sufficiently low that assemblages containing PhA may be stable to pressures greater than 10 GPa, allowing PhA to transport H2O to the deeper mantle. It is more likely that the higher-pressure phase E will be stabilized instead.