The effect of potassium on aluminous phase stability in the lower mantle

The aluminous calcium-ferrite type phase (CF) and new aluminous phase (NAL) are thought to hold the excess alumina produced by the decomposition of garnet in MORB compositions in the lower mantle. The respective stabilities of CF and NAL in the nepheline-spinel binary (NaAlSiO4-MgAl2O4) are well established. However with the addition of further components the phase relations at lower mantle conditions remain unclear. Here we investigate a range of compositions around the nepheline apex of the nepheline-kalsilite-spinel compositional join (NaAlSiO4-KAlSiO4-MgAl2O4) at 28-78 GPa and 2000 K. Our experiments indicate that even small amounts of a kalsilite (KAlSiO4) component dramatically impact phase relations. We find NAL to be stable up to at least 71 GPa in potassium-bearing compositions. This demonstrates the stabilizing effect of potassium on NAL, because NAL is not observed at pressures above 48 GPa on the nepheline-spinel binary. We also observe a broadening of the CF stability field to incorporate larger amounts of potassium with increasing pressure. For pressures below 50 GPa only minor amounts (< 0.011 ( 1 ) K K + N a + M g ) of potassium are soluble in CF, whereas at 68 GPa, we find a solubility in CF of at least 0.088 ( 3 ) K K + N a + M g . This indicates that CF and NAL are suitable hosts of the alkali content of MORB compositions at lower mantle conditions. For sedimentary compositions at lower mantle pressures, we expect K-Hollandite to be stable in addition to CF and NAL for pressures of 28-48 GPa, based on our simplified compositions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00410-024-02129-w.

Experimental evidence supporting a global melt layer at the base of the Earth's upper mantle

The low-velocity layer (LVL) atop the 410-km discontinuity has been widely attributed to dehydration melting. In this study, we experimentally reproduced the wadsleyite-to-olivine phase transformation in the upwelling mantle across the 410-km discontinuity and investigated in situ the sound wave velocity during partial melting of hydrous peridotite. Our seismic velocity model indicates that the globally observed negative Vs anomaly (−4%) can be explained by a 0.7% melt fraction in peridotite at the base of the upper mantle. The produced melt is richer in FeO (~33 wt.%) and H₂O (~16.5 wt.%) and its density is determined to be 3.56–3.74 g cm⁻³. The water content of this gravitationally stable melt in the LVL corresponds to a total water content in the mantle transition zone of 0.22 ± 0.02 wt.%. Such values agree with estimations based on magneto-telluric observations.

A 56–60 km thick low velocity layer exists at the base of the Earth’s upper mantle. Here, the authors experimentally reproduced the wadsleyite-to-olivine transition in the upwelling mantle and show that the low velocity anomaly can be explained by melting of hydrous peridotite.

Hydrous olivine unable to account for conductivity anomaly at the top of the asthenosphere

The oceanic asthenosphere is observed to have high electrical conductivity, which is highly anisotropic in some locations. In the directions parallel and normal to the plate motion, the conductivity is of the order of 10(-1) and 10(-2) S m(-1), respectively, which cannot be explained by the conductivity of anhydrous olivine. But because hydrogen can be incorporated in olivine at mantle pressures, this observation has been attributed to olivine hydration, which might cause anisotropically high conductivity by proton migration. To examine this hypothesis, here we report the effect of water on electrical conductivity and its anisotropy for hydrogen-doped and undoped olivine at 500-1,500 K and 3 GPa. The hydrous olivine has much higher conductivity and lower activation energy than anhydrous olivine in the investigated temperature range. Nevertheless, extrapolation of the experimental results suggests that conductivity of hydrous olivine at the top of the asthenosphere should be nearly isotropic and only of the order of 10(-2) S m(-1). Our data indicate that the hydration of olivine cannot account for the geophysical observations, which instead may be explained by the presence of partial melt elongated in the direction of plate motion.

Grain boundaries as reservoirs of incompatible elements in the Earth's mantle

The concentrations and locations of elements that strongly partition into the fluid phase in rocks provide essential constraints on geochemical and geodynamical processes in Earth's interior. A fundamental question remains, however, as to where these incompatible elements reside before formation of the fluid phase. Here we show that partitioning of calcium between the grain interiors and grain boundaries of olivine in natural and synthetic olivine-rich aggregates follows a thermodynamic model for equilibrium grain-boundary segregation. The model predicts that grain boundaries can be the primary storage sites for elements with large ionic radius--that is, incompatible elements in the Earth's mantle. This observation provides a mechanism for the selective extraction of these elements and gives a framework for interpreting geochemical signatures in mantle rocks.