Simplicity in melt densification in multicomponent magmatic reservoirs in Earth’s interior revealed by multinuclear magnetic resonance

Abstract

Pressure-induced changes in properties of multicomponent silicate melts in magma oceans controlled chemical differentiation of the silicate earth and the composition of partial melts that might have formed hidden reservoirs. Although melt properties show complex pressure dependences, the melt structures at high pressure and the atomistic origins of these changes are largely unknown because of their complex pressure-composition dependence, intrinsic to multicomponent magmatic melts. Chemical constraints such as the nonbridging oxygen (NBO) content at 1 atm, rather than the structural parameters for melt polymerization, are commonly used to account for pressure-induced changes in the melt properties. Here, we show that the pressure-induced NBO fraction in diverse silicate melts show a simple and general trend where all the reported experimental NBO fractions at high pressure converge into a single decaying function. The pressure-induced changes in the NBO fraction account for and predict the silica content, nonlinear variations in entropy, and the transport properties of silicate melts in Earth's mantle. The melt properties at high pressure are largely different from what can be predicted for silicate melts with a fixed NBO fraction at 1 atm. The current results with simplicity in melt polymerization at high pressure provide a molecular link to the chemical differentiation, possibly missing Si content in primary mantle through formation of hidden Si-rich mantle reservoirs. E arly in Earth history, during the magma-ocean phase, the chemical differentiation of the silicate earth, formation of core, and evolution of atmosphere were controlled by the properties of silicate melts at high pressure (1-6). Pioneering experimental studies show that these thermodynamic and transport properties relevant to the chemical evolution of the Earth vary nonlinearly with changes in pressure (7-9). For instance, the solubility of Ar into melts increases with increasing pressure and then decreases drastically with a further increase in pressure, with data suggesting a strong composition dependence (7,10,11). Similarly, complex behaviors were reported for the diffusivity and viscosity of silicate melts at high pressure (8). Although the trend in the silica activity in the melts at high pressure is not known, phase relations of mantle melts and minerals imply varying activity coefficients of the oxide in silicate melts with changes in pressure (12)(13)(14). Changes of up to two orders of magnitude in the element partitioning coefficient between melts and crystal/coexisting phases have been reported stemming mostly from the effect of the melt composition, constraining the fate of radioactive nuclides in the Earth's interior (15-18).The key to understanding these nonlinear changes in melt properties with pressure is the melt structure at high-pressure in a short-(e.g., coordination number) to medium-range scale (19)(20)(21). While recent progress in mineral physics provides the link between the macroscopic properties and the structures of...

A NMR and molecular dynamics study of CO2-bearing basaltic melts and glasses

“…This increase is also suggested by the results obtained from FPMD simulations with an average Al coordination increasing from 4.6 to 4.9 at 5.0 and 8.0 GPa, respectively. This clear increase of Al coordination both in glass and high temperature melt with increasing pressure is currently well-accepted (Allwardt et al, 2005(Allwardt et al, , 2007Lee, 2010Lee, , 2011Lee et al, 2012). As suspected, 1) the average Al coordination number is comparable in ex-situ glass and in-situ melt and 2) the impact of CO 2 on the average Al coordination number is weak.…”

Structure and disorder in basaltic glasses and melts: Insights from high-resolution solid-state NMR study of glasses in diopside–Ca-tschermakite join and diopside–anorthite eutectic composition

“…NBO fraction in CaO-Na 2 O-Al 2 O 3 -SiO 2 glasses is also reported to decrease with pressure (Lee, 2011).…”

“…The structures (coordination number and network connectivity) and the extent of chemical and topological disorder in multi-component basaltic glasses and melts provide insights into the macroscopic properties (e.g., energy, entropy, viscosity, diffusivity, crystal-melt partitioning coefficients, and activity coefficients of silica) and the dynamics of magmas in the Earth's crust and interior (e.g., Navrotsky et al, 1983;Neuville et al, 2004b;Lee, 2005Lee, , 2011Mysen and Richet, 2005;Neuville, 2006;Potuzak and Dingwell, 2006;Giordano et al, 2008;Kelsey et al, 2008a;Lee and Stebbins, 2009;Karki and Stixrude, 2010 and references therein). They also allow us to understand geochemical processes involving magmas (i.e., melting, migration, and emplacement) and have implications for the chemical differentiation of the early Earth (e.g., Ohtani et al, 1986;Stebbins, 1995;Tinker et al, 2003;Mysen and Richet, 2005;Stixrude and Karki, 2005;Ohtani, 2009;Asimow and Ahrens, 2010;Lee et al, 2010a;Lee, 2011 and references therein).…”

High-pressure radiative conductivity of dense silicate glasses with potential implications for dark magmas

“…Alternatively, silicate glasses have been studied as analogues for quenched silicate melts to simulate high-pressure melt behaviour [16][17][18][19][20][21] . Recent experimental research on the possible spin-pairing electronic transition of iron in silicate glasses at midlower mantle pressures 22 has renewed interest in understanding the thermal transport mechanisms in dense silicate melts at the bottom of the mantle, as changes in the optical properties of ironbearing phases associated with spin transitions may significantly influence the radiative component of thermal conductivity.…”