Fate of MgSiO ₃ melts at core–mantle boundary conditions

Abstract

One key for understanding the stratification in the deep mantle lies in the determination of the density and structure of matter at high pressures, as well as the density contrast between solid and liquid silicate phases. Indeed, the density contrast is the main control on the entrainment or settlement of matter and is of fundamental importance for understanding the past and present dynamic behavior of the deepest part of the Earth's mantle. Here, we adapted the X-ray absorption method to the small dimensions of the diamond anvil cell, enabling density measurements of amorphous materials to unprecedented conditions of pressure. Our density data for MgSiO 3 glass up to 127 GPa are considerably higher than those previously derived from Brillouin spectroscopy but validate recent ab initio molecular dynamics simulations. A fourth-order Birch-Murnaghan equation of state reproduces our experimental data over the entire pressure regime of the mantle. At the core-mantle boundary (CMB) pressure, the density of MgSiO 3 glass is 5.48 ± 0.18 g/cm 3 , which is only 1.6% lower than that of MgSiO 3 bridgmanite at 5.57 g/cm 3 , i.e., they are the same within the uncertainty. Taking into account the partitioning of iron into the melt, we conclude that melts are denser than the surrounding solid phases in the lowermost mantle and that melts will be trapped above the CMB.silicate glass density | X-ray absorption | basal magma ocean V ariations in seismic velocities observed in the deep mantle could be attributed to melting phenomena (1), accumulation of dense matter (2), core-mantle interactions (3), or even a deep hidden geochemical reservoir that dates from the early differentiation of the Earth (4). Indeed, melting phenomena play a critical role in the formation and evolution of terrestrial planets. In the early Earth's history, large-scale melting events and magma oceans facilitated the segregation of the iron-rich core and the partitioning of elements between the different layers of the Earth (5). Catastrophic events like the moon-forming giant impact may have melted the entire Earth and modified the thermal and chemical state of the planet's interior (6). It has long been thought that the mantle crystallized from the bottom to the top, with crystals being denser than melts (7). However, the formation of a dense basal magma ocean (BMO) at the bottom of the mantle concomitant to the final accretion of the Earth was proposed as a consequence of partial melting (2). The formation of a BMO requires the accumulation of dense iron-rich silicate melts at the core-mantle boundary (CMB). In this scenario, the mantle starts to crystallize at an intermediate depth, and a raft of crystals continues to grow toward both the bottom and top of the mantle. In the modern mantle, seismic observations highlight ultralow velocity zones (ULVZs) near the CMB (8) that may be caused by melting of deep mantle material (9), potentially feeding sources of hot spots. Alternatively, the ULVZs may be dense remnants from an initial BMO (10, 11). Both scenar...

Fate of Hydrous Fe‐Rich Silicate Melt in Earth's Deep Mantle

“…A few extant studies using static compression are limited to upper mantle conditions (e.g., Matsukage et al, 2005). While experiments using diamond anvil cells can reach up to Earth's core mantle boundary (CMB) pressure, only MgSiO 3 glass (an analog of MgSiO 3 melt) has been measured at ambient temperature (Petitgirard et al, 2015). Dynamic (shock-wave) experiments can directly determine the equation of state of silicate melts at simultaneous high pressure-high temperature conditions, but these measurements have so far focused on water-free systems, such as molten MgSiO 3 (Akins, 2004;Mosenfelder et al, 2009), Mg 2 SiO 4 (Mosenfelder et al, 2007), and Fe 2 SiO 4 (Thomas et al, 2012).…”

Density of jadeite melts under high pressure and high temperature conditions

“…This well-established technique allows us to measure density of liquid under desired pressure and temperature conditions with much improved precision and accuracy. The technique is also applied to diamond anvil cell for density measurement of silicate glasses at much higher pressures (Sato and Funamori, 2008;Petitgirard et al, 2015). Jadeite (NaAlSi 2 O 6 ) is a basic sodium aluminosilicate and one of the important mantle minerals.…”

Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

“…Silicate glasses have been used as possible analogs for silicate melts to understand the nature of dense magmas in the lower mantle (e.g., Bass 2010, 2011;Petitgirard et al 2015). Although the structural changes that silicate glasses undergo cannot be assumed to be a completely valid model for silicate melts, previous experimental works have shown that the Si-O coordination numbers of SiO 2 glass and a molten basalt appear to change in generally similar ways (Meade et al 1992;Lin et al 2007; Funamori 2008, 2010;Benmore et al 2010;Zeidler et al 2014;Sanloup et al 2013).…”