Low Melting Temperature of Anhydrous Mantle Materials at the Core‐Mantle Boundary

Abstract

One of the central challenges in accurately estimating the mantle melting temperature is the sensitivity of the probe for detecting a small amount of melt at the solidus. To address this, we used a multichannel collimator to enhance the diffuse X-ray scattering from a small amount of melt and probed an eutectic pyrolitic composition to increase the amount of melt at the solidus. Our in situ detection of diffuse scattering from the pyrolitic melt determined an anhydrous melting temperature of 3,302 ± 100 K at 119 ± 6 GPa and 3,430 ± 130 K at the core-mantle boundary (CMB) conditions, as the upper bound temperature. Our CMB temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling and higher concentrations of S, C, O, and/or H in the region, and nonlinear, advocating the basal magma ocean hypothesis. Plain Language Summary The heat stored in the Earth's deep interior has been the primary fuel for a range of global processes from mantle convection to surface tectonics, but quantitative estimation of the heat remains uncertain. The melting temperature of mantle materials is one of the key parameters to understanding the thermal evolution and present-day state of the Earth's interior, but it has been poorly constrained, with recent measurement discrepancies as large as 600 K. Here, we report melting temperatures of mantle compositions measured over a wide range of pressures expected for the lower mantle. We investigated a eutectic mantle composition using multichannel collimator filtered X-ray diffraction in combination with the laser-heated diamond-anvil cell. Fitting our melting data over the range of 46-145 GPa led to a solidus temperature of 3,430 ± 130 K at the core-mantle boundary. This temperature is approximately 700 K lower than the previous estimates, implying much faster secular cooling at the lower mantle than previously believed. Furthermore, our solidus curve constrained for a wide pressure range is strongly nonlinear and thus supports the basal magma ocean hypothesis. Recently, Fiquet et al. (2010) reported direct detection of diffuse X-ray scattering from partial melt in peridotite at the pressures expected for the CMB in a laser-heated diamond-anvil cell (LHDAC) and estimated a solidus temperature of 4,180 ± 150 K. Subsequently, Andrault et al. (2011) inferred a similar solidus temperature for a chondritic composition at the CMB based on the combination of plateau in the temperature-laser power and rapid recrystallization signatures in diffraction images. Most recently, Nomura et al. (2014) reported a significantly lower solidus temperature of 3,570 ± 200 K at the CMB through X-ray computed tomography as diagnostics for the degree of melting using the temperature quenched samples. Another important ©2020. American Geophysical Union. All Rights Reserved.

Post‐Perovskite Phase Transition in the Pyrolitic Lowermost Mantle: Implications for Ubiquitous Occurrence of Post‐Perovskite Above CMB

“…In addition, runs #4 and #5 were conducted at 156–166 GPa and 3570–3860 K under subsolidus conditions (Figure 3a), which is supported by observations that the number of peaks and their relative intensities in XRD patterns did not change upon quenching temperature. The present above‐solidus and subsolidus experiments support the relatively low solidus temperature of pyrolite at the CMB pressure (Kim et al., 2020; Nomura et al., 2014).…”

“…Even in this case, post-perovskite is stable (bridgmanite does not appear) at the bottom of the mantle as far as the CMB temperature is less than 3550 K (Figure 3b). Since the lowermost mantle is not globally molten, such relatively low CMB temperature is supported by recent determinations of the solidus temperature of pyrolite at 135 GPa; 3570 ± 200 K by Nomura et al (2014) and 3430 ± 130 K by Kim et al (2020).…”

“…In order to avoid kinetic hindering of phase transformation especially in such a multi‐component system, heating was made on an amorphous starting material at a single P ‐ T condition in each run. In run #1, the sample was compressed and then heated to 3910 K at 122 GPa, higher than the solidus temperature of pyrolite (Kim et al., 2020; Nomura et al., 2014) (Figure 3a). The XRD spectrum collected in situ during heating shows that bridgmanite and minor post‐perovskite, in addition to ferropericlase and davemaoite (CaSiO 3 perovskite), grew from the amorphous sample (Figure 2a).…”

Liquidus Phase Relations and Solid‐Liquid Partitioning in the Fe‐Si‐C System Under Core Pressures

“…(2014) and Kim et al. (2020). T CMB > 4620 K is much higher than any of these estimates and therefore T ICB > 6280 K is unlikely, otherwise the lowermost mantle should be molten not only locally at ultralow velocity zones but globally.…”

Structure, Materials and Processes in the Earth’s Core and Mantle

“…A recent investigation of the melting curve of pure iron, using diamond anvil cell resistance heating at pressures up to 290 GPa (Sinmyo et al 2019), resulted in a curve which is about 600 K below that of Anzellini et al (2013), determined by laserheating. A lower melting curve for iron at core conditions, combined with recent result indicating considerably lower solidus temperatures for peridotite at the lowermost mantle conditions (Nomura et al 2014;Kim et al 2020) compared to previous determinations, might possibly lead to a reduction of the average CMB temperature estimates from values of 4200-4300 K (e.g., Nimmo 2015;Vocadlo 2015) to values in the 3600-3800 K range.…”