Water and Iron effect on the P-T-x coordinates of the 410-km discontinuity in the Earth upper mantle

Abstract

We performed multi-anvil experiments in the system MgO-SiO 2 ± H 2 O at 13.0-13.7 GPa and 1,025-1,300°C and in the system MgO-FeO-SiO 2 ± H 2 O, under reducing conditions, at 11.0-12.7 GPa and 1200°C, to depict the effect of H 2 O on the P-T-x coordinates of the 410-km discontinuity, i.e. the olivine-wadsleyite phase boundary. The charges were investigated with where wad-ring and ol-ring coexist, we observe, however, an unexpected broadening of the loops with a shift to higher iron contents. In total the stability field of hydrous wad expands in both directions, to lower and higher pressures. Fe 3+ concentrations as determined by EELS are very low and are expected to play no role in the broadening of the loops.

Partition coefficients of trace elements between carbonates and melt and suprasolidus phase relation of Ca-Mg-carbonates at 6 GPa

“…In run D melt-pockets occur in triple-junctions between homogenous Ca-magnesite crystals suggesting equilibrium (Hunter and McKenzie 1989). The 'hourglass' texture in run A, B and C, with solids in a semicircle on both ends of the capsule and the liquid phase in the middle, has previously been reported for fluid saturated systems (Deon et al 2011). This hourglass texture might be due to a small thermal gradient with the hottest part in the centre of the capsule.…”

Ferric iron and water incorporation in wadsleyite under hydrous and oxidizing conditions: A XANES, Mossbauer, and SIMS study

“…Koch-Muller et al (2009) have indeed observed that under oxidizing and wet conditions ringwoodite breaks down into a spinelloid phase (which could be isostructural to wadsleyite) and a pyroxene. In agreement with all observations we can conclude that under oxidizing and wet conditions wadsleyite is stabilized relative to ringwoodite and olivine, due to its higher water incorporation capacity (as shown by many authors including Kawamoto et al 1996;Deon et al 2011;Inoue et al 2010) and ferric iron content (Deon et al 2011). In particular, Deon et al (2011) have reported that 2.5 times more water is incorporated into wadsleyite than into ringwoodite, with a D Fe3 + ringwoodite/wadsleyite (defined as [Fe 3+ ] ringwoodite /[Fe 3+ ] wadsleyite ) of 3.…”

“…under oxidizing conditions (coexistence with rhenium metal). Deon et al (2011) observed a maximum iron content of 27 Fe no. under wet and reducing conditions (coexistence with Fe-FeO).…”

Low‐Velocity Layer Atop the Mantle Transition Zone in the Lower Yangtze Craton from P Waveform Triplication

“…In this paper, the depth of the 410 beneath the lower Yangtze craton is found to be a gradient layer with a thickness of 20 km. Compared with the first-order discontinuity of the 410 in IASP91, the gradient layer indicates the existence of water or other volatiles (Smyth & Frost, 2002;Deon et al, 2011). The LVL, as a result of partial melting, can be stable atop the 410 km discontinuity if the water content is less than 6.7%±0.6% (Sakamaki et al, 2006).…”

“…Because of the positive Clapeyron slope, the 410 could be elevated in a cold anomaly (subducted slab) (Collier et al, 2001), while it will be depressed at a warm anomaly (mantle plume or mid-oceanic ridge) (Deusss, 2007). The additions of water and/or iron to olivine elevate the 410 to a shallower depth and broaden the two-phase loop (Smyth & Frost, 2002;Deon et al, 2011). Based on the seismic tomography results, cold subducted slabs in the mantle are generally not a simple subduction structure (Li & van der Hilst, 2010;Zhao et al, 2011); thus, the existence and topography of the 410 should be quite complicated.…”