Density of magmas at depth

Abstract

International audienceKnowing the density of silicate liquids at high pressure is essential to answer questions relevant to the presence of magmas at depth, whether that be in the present Earth or in its earliest times, during differentiation of the planet. Melts have unique physical and chemical properties, which vary as a function pressure, and chemical composition. The focus here will be on in situ measurements of the density of magmas, with a presentation of the available methods and of the main results obtained so far, including why some magmas may be trapped at depth. Understanding the macroscopical physical properties of magmas requires an accurate microscopic structural description. Structural descriptions of compressed magmas are becoming more widely available, from experiments and from theoretical calculations. These structural inputs are used to understand the compression mechanisms at stake in the densification of magmas, e.g. the collapse of voids, coordination increase for the major cations, and bond compressibility. These densification processes profoundly affect not only the physical properties of the melt, but also its chemical properties, i.e. the way elements partition between the magma and a metallic melt or between the magma and crystals

Formation of bridgmanite-enriched layer at the top lower-mantle during magma ocean solidification

“…The negative pressure dependence is due to either the Si-O bond weakening by the pressure-induced bending of the Si-O-Si angle 21,22 or possibly the increasing concentration of five-fold Si-O coordination species 23,24 . The complex pressure dependence correlates nicely with the mechanisms of silicate melt densification described previously 22,25 (details in Supplementary Note 1 and Supplementary Fig. 4).…”

Viscosity of mafic magmas at high pressures

“…There is no obvious reason why viscosity should go up again at higher P unless the interparticle interaction changes. This might occur in relation with Si coordination change above 15 GPa, while changes of CN of Ca, Mg, and Fe are finished around 10–15 GPa and that of Al is mostly completed (see Sanloup [], for a compilation). However, molten basalts laser heated using diamond‐anvil cells systematically quench as fully crystalline assemblages from P above 10 GPa and up to 60 GPa, while they quench as glasses below 10 GPa [ Sanloup et al , ].…”

Pressure-induced structural change in MgSiO 3 glass at pressures near the Earth’s core–mantle boundary

“…Considering the similarity in the pressure-induced structure changes in silicate glass and melt at very high pressure conditions corresponding to the Earth's lower mantle (13), we anticipate that a similar ultrahigh-pressure structural change may also occur in MgSiO 3 melt under similar pressure conditions. We note that the pressure condition of the structural change in MgSiO 3 glass at 88 GPa at room temperature is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO 3 (28,31,32) (Fig.…”

“…Only a few studies have successfully measured the structure of silicate melt above 10 GPa (12). Given these difficulties, silicate glasses have been studied as an alternate approach to understand structural changes of silicate melts at high pressures, because of similarities in the pressure-induced structural changes in silicate melts and glasses (13).…”