Hydrous magnesium-rich magma genesis at the top of the lower mantle

Several igneous activities occur on the surface of the Earth, including island arcs, mid-ocean ridges and hot spots. Based on geophysical observations, melting phenomena in the interior also occur at the asthenosphere’s top and the upper mantle’s bottom. Additionally, a seismological low-velocity anomaly was observed at the top of the lower mantle that may result from mantle melting due to dehydration decomposition of ringwoodite to bridgmanite and ferropericlase with a downward flow. However, the corresponding high-pressure experimental data are too poor to understand the melting phenomena under the lower mantle condition. Herein, we conducted hydrous peridotite melting experiments at pressures from 23.5 to 26 GPa and at temperatures from 1300 to 1600 °C for demonstrating the melt composition and the gravitational stability of magma at the top of the lower mantle. The melt had a SiO2-poor and MgO-rich composition, which is completely different than that of dry peridotite melting experiments. Compared with the seismological lower mantle, the experimental melt is gravitationally lighter; thus, a similar melt could be observed as seismological low-velocity zone at the lower mantle’s top. The generated magma plays as a filter of down-welling mantle and can contribute to a formation of a silicate perovskitic lower mantle.

Hydrous ringwoodite with about 1.5 wt.% H 2 O 1 and phase egg [2][3][4] has been discovered in the diamond inclusions of mantle xenoliths. Ringwoodite is a high-pressure polymorph of olivine and a significant mineral in the mantle transition zone. Phase egg is one of several hydrous aluminosilicate minerals which is stable under the conditions found in the mantle transition zone 2,3 . Identifying these hydrous minerals of the mantle transition zone would be natural evidence supporting the hypothesis that the mantle transition zone is a water reservoir.
As wadsleyite and ringwoodite are major constituent minerals of the mantle transition zone, their water capacities have been experimentally investigated. The water solubilities of wadsleyite and ringwoodite have been estimated to be about 1-3 wt.% 5,6 . As a basis of comparison, the water solubilities of major constituent minerals of the mantle, olivine and bridgmanite, are about 0.1 wt.% 6 and 0.2 wt.% 7 , respectively.
The water-rich mantle transition zone plays an important role as a trigger of mantle melting. Owing to the large difference in water solubilities between olivine and wadsleyite, dehydration melting can occur at the 410 km discontinuity at the beginning of the transition zone 8 . Geophysical observations support the existence of melt at the base of the upper mantle based on low seismic wave velocity anomalies 9,10 and high electrical conductivity 10 . In addition, the gravitational stability of hydrous magma was experimentally confirmed by Sakamaki et al. 11 . However, water-induced mantle melting can occur not only above the mantle transition zone but also below the transition zone. Significantly, a low-velocity anomaly has been detected at the top of the lower mantle [12][13][14] .
Melting experiments on mantle materials have been widely performed [15][16][17] . The composition of partial melt becomes magnesium (Mg)-rich and silicon (Si)-poor with increasing pressure 15 . In the case of melting under the lower mantle condition, the liquidus phase is bridgmanite, and the composition of the coexisting melt is richer in iron (Fe) and Calcium (Ca) than that in bridgmanite 16,18-21 . Kawamoto conducted experiments with a water-saturated KLB-1 peridotite melt at 14-24 GPa and 900 °C-1400 °C 22 . A clear difference was observed in crystal-melt partitioning between the dry and hydrous systems. The melt exhibited a Ca-rich and Si-, Mg-and aluminium (Al)-poor composition as compared with the anhydrous system. A problem with this experiment was the valence of iron in the starting materials. Although samples with divalent iron (Fe 2+ ) have generally been Comparing the chemical composition of crystal and the partial melt. When comparing the composition of bridgmanite with that of melts, no difference was observed in MgO content ( Fig. 3; Supplementary  Table S1). This trend is similar to the results of previous experiments performed with anhydrous systems ( Table 2). The important point is that less difference in iron content between melt and bridgmanite (Fe melt / Fe Brg = 0.9). Compared to anhydrous system (Fe melt /Fe Brg = 2-3) 18,19 , preferred concentration of Fe into melt is not observed. In the case of melting experiment of Fe 2+ -bearing sample 22 , the Fe content ratio of bridgmanite to melt is also about 0.9 (see Table 2). The Fe-Mg exchange partition K D Fe Mg ( / ) between bridgmanite and melt of anhydrous peridotite is about 0.4 18,19 , while that of this study (hydrous system) is 1.1.
Density of the melt. To discuss the gravitational stability of the melt in the lower mantle, the melt density was calculated using Eq.  www.nature.com/scientificreports www.nature.com/scientificreports/ www.nature.com/scientificreports www.nature.com/scientificreports/ In Eq. (1), P, K, V, α and T represent pressure, bulk modulus, volume, the thermal expansion coefficient and temperature, respectively. Because water concentrates in the melt of run products, considering the effect of H 2 O on the density of silicate melts is necessary. The partial molar volume of H 2 O in magma at high pressure and temperature was calculated using the Eq. (2): is the zero-pressure partial molar volume as described by Bouhifd et al. 28 and K T is the isothermal bulk modulus as described by Sakamaki 29 . Owing to the feature of EDS, measurement of light elements, such as H, is impossible; moreover, if H is concentrated in the sample, the average of the total value (wt.%) is estimated to be low. It is not conclusive what leads to the low total value because other influences such as vacancy must also be considered, but coexisting solid phases are anhydrous minerals; hence, we estimated that the remaining weight ratio of the melt was concentrated water (see Supplementary Fig. S4 for detail). We calculated the chemical compositions of hydrous magma based on the average composition of obtained results and the water content of 29.9 ± 4.1 wt.%. The compression curve of the obtained melt is calculated by the extrapolation of the Eqs (1) and (2). Figure 4a shows two calculated patterns of hydrous melt (29.9 ± 4.1 wt.% H 2 O) and dry melt (0 wt.% H 2 O). Comparing with the preliminary reference earth model (PREM) 30 , both melt densities of this study become lighter than the lower mantle. This means that the melt in this study would be gravitationally stable around the top of the lower mantle regardless of the melt's water content.

Discussions
The mantle transition zone can act as a water reservoir/source, and a hydrous condition can be created at the top of the lower mantle. The solidus of peridotite rocks under anhydrous conditions is much higher than that under the mantle geotherm 16, 18,19,21 . However, the presence of water lowers the melting point of the rock, and as a result, the rock begins to produce a partial melt. This MgO-rich and SiO 2 -poor melt becomes lighter owing to the higher concentrations of water and lower concentrations of iron. Thus, the generated melt at the top of the lower mantle can be separated from the surrounding mantle (Fig. 4b) and be observed as a low velocity anomaly [12][13][14] . This melt is caused by a downward flow such as a slab 13,14 and may react with the mantle transition zone. Even if this melt is absorbed by the mantle transition zone, the melt can be continuously generated at the top of lower mantle as long as the down-welling mantle continues.
The residual mantle rocks separate from the melt due to the large density difference. The downward mantle rock is poor in melt component, that is, the lower mantle composition below the melt can be MgO-poor and SiO 2 -rich, which is consistent with silicate perovskitic composition. The silicate perovskitic lower mantle has been proposed based on the comparison of experimentally determined sound velocity of lower mantle minerals with seismological observation 31 . The melt at the top of lower mantle plays as a filter and causes a chemical contrast between upper and lower mantles. Our results can provide a possible mechanism for formation of the silicate perovskitic lower mantle. High pressure and temperature experiment. High pressure and temperature experiments were conducted using a Kawai-type multi-anvil press (Hymag 1,000-tonne press) at Bayerisches Geoinstitut (BGI) in Germany. We used a 32-mm second-stage anvil (ha7%Co/hawedia) and a 7/3 cell assembly. The sample was enclosed in a gold capsule. Supplementary Fig. S4 shows a detailed design of the cell assembly.
Pressure and temperature conditions were 23.5 and 26 GPa and 1300, 1400, 1500 and 1600 (or 1590) °C, respectively (Table 1 and Supplementary Fig. S1). In the experiment where the duration was doubled (60 min became 120 min) at 26 GPa and 1400 °C, crystal growth was observed; however, no significant difference was observed in chemical composition. After maintaining the temperature for 15 to 120 min, the samples were recovered via quenching.  www.nature.com/scientificreports www.nature.com/scientificreports/ Analysis of the recovered samples. Recovered samples were polished using sand papers and abrasives ranging from #80 to #3000, and then finished with 3-μm and 1-μm diamond paste. Field emission scanning electron microscope (FE-SEM; JSM-7001F/JEOL) and EDS (SN: 60154/Oxford Instruments) instruments were used to analyse the samples. The acceleration voltage was 15.0 kV, and the emission current was 77-80 μA.
We estimated the Fe 3+ /Fe 2+ ratio of minerals in the recovered sample based on stoichiometry of analyzed chemical composition using EDS. Based on the estimation, bridgmanite is Fe 3+ -rich and ferropericlase is Fe 2+ -rich. However, we could not determine the Fe 3+ /Fe 2+ ratio of the melt. The EDS measurements of melt composition have been performed under the same analysis conditions. The melt composition shown in the Supplementary Table S1 takes the average of ten times measurements.
Calculations of melt density. The reference temperature T 0 was 2500 K, and the temperature of the required density T was set to 1873 K. Consequently, calculations yielded that K T 0, 0 , V T 0, 0 and α T 0, 0 are 64.4 GPa, 17.0 cm 3 /mol and 15.4 × 10 −5 /K, respectively. In the case of dry peridotitc melt, these parameters were calculated to be 54 GPa, 18.0 cm 3 /mol and 11.5 × 10 −5 /K, respectively 27 .
In Eq. (1), it is assumed that this is in an anhydrous system, and the Fe valence is perfectly Fe 2+ (=FeO). However, we used Fe 2 O 3 in this study.

Data Availability
The data used to obtain the conclusions in this study are provided in the main article and the Supplementary Information.  Table 3. Chemical composition of the starting material based on McDonough et al. 34 . The ratio of ferrous iron to ferric one (Fe 2+ /Fe 3+ ) is 0 in the starting material.