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The density of melts within the Earth, or rather the density contrast between liquid and residual solid silicates, exerts a fundamental control on each stage of the genesis of magmas, from melt generation to final emplacement as intrusive or extrusive rock. Since magmas originate at depth in the Earth´s interior, the effect of pressure on density has to be taken in account to discuss magmatic processes. Combined with high pressure densities from static or dynamic density determinations, one bar densities provide a firm constraint for extrapolation of the densities of such melts to higher pressures. Unfortunately, calculations of melt densities of mantle melts involve extrapolation from the present experimental data base due to the very high liquidus temperatures and ready subliquidus crystallization of such melts.

Therefore, one bar density measurements at high temperatures (up to 1800 °C) were conducted in air on a number of mantle melts using the Ir-based double bob Archimedean method. The starting materials were three synthetic compositions in the simplified five component FeO-CaO-MgO-Al₂O₃-SiO₂ system: a FeO-rich peridotite (Fe-Peridotite in Fig. 3.7-6) which is considered to be representative of the composition of the silicate Earth (mantle + crust) and to be a model of the Martian mantle composition, a peridotite (PHN 1611) which is considered to be a common upper mantle composition, and a common Archean magma (Komatiite).

The densities of these melts, presented in the Fig. 3.7-6, ranges from 2.82 to 2.70 g/cm³ between their respective melting points and 1800 °C. The oxidation state of iron in these
 

Fig. 3.7-6: Variation of 1 bar density for some mantle melts as function of temperature.

samples was determined using thermogravimetric methods in the same temperature range as the density measurements. Sink/float experiments (i.e. the sinking, flotation or neutral buoyancy of forsterite and diamond crystals) have been conducted on similar compositions in the last years. Fitting the third order Birch-Murnaghan equation of state to both the high pressure densities and the new one bar densities yields new isothermal compression curves for these mantle melts. Their isothermal bulk moduli (K_T) and the pressure derivatives (K´_T) derived are reported in Table 3.7-1.