An experimental determination of the liquidus in the system CaCO3-MgCO3 and a thermodynamic analysis of the melting of carbonated mantle melting
Authors: Poli Stefano , Zhao Sutao , Schmidt Max W. 
Published: March 2022

&lt;p&gt;The system CaCO&lt;sub&gt;3&lt;/sub&gt;-MgCO&lt;sub&gt;3&lt;/sub&gt; has been used since the '60s for reconstructing the petrogenesis of carbonated lithologies, notably of carbonatite magmas possibly generated in the Earth's mantle.&amp;#160;Yet, experimental results at high temperatures and pressures remain contradictory, and a thermodynamic model for the carbonate liquid in this binary is still lacking.&lt;/p&gt;&lt;p&gt;We experimentally investigated the melting of aragonite and magnesite to pressures of 12 GPa, and of calcite-magnesite mixtures at 3 and 4.5 GPa, and at variable Mg/(Mg+Ca) (X&lt;sub&gt;Mg&lt;/sub&gt;). Results show that the melting of aragonite, and of magnesite have similar slopes, magnesite melting &amp;#8776; 30 &amp;#176;C higher than aragonite. The minimum on the liquidus surface is at X&lt;sub&gt;Mg&lt;/sub&gt; &amp;#8776; 0.35-0.40, 1200 &amp;#176;C at 3 GPa, and 1275 &amp;#176;C at 4.5 GPa, which, when combined with data from Byrnes and Wyllie (1981) and M&amp;#252;ller et al. (2017), imply that minimum liquid composition remains approximately constant with pressure increase. We present the first thermodynamic model for CaCO&lt;sub&gt;3&lt;/sub&gt;-MgCO&lt;sub&gt;3&lt;/sub&gt; liquids, retrieved from the experimental data available. Although carbonate liquids should be relatively simple molten salts, they display large non-ideality and a three-component (including a dolomite component), pressure dependent, asymmetric solution model is required to model the liquidus surface. Attempts to use an end-member two-component model fail, invariably generating a very wide magnesite-liquid loop, contrary to the experimental evidence.&lt;/p&gt;&lt;p&gt;The liquid model is used to evaluate results of experimentally determined phase relationships for carbonated peridotites modelled in CaO-MgO-SiO&lt;sub&gt;2&lt;/sub&gt;-CO&lt;sub&gt;2&lt;/sub&gt; (CMS-CO&lt;sub&gt;2&lt;/sub&gt;), and CaO-MgO-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt;-CO&lt;sub&gt;2&lt;/sub&gt; (CMAS- CO&lt;sub&gt;2&lt;/sub&gt;). Computations highlight that the liquid composition in the CMS-CO&lt;sub&gt;2&lt;/sub&gt; and CMAS-CO&lt;sub&gt;2&lt;/sub&gt; and in more complex systems do not represent &quot;minimum melts&quot; but are significantly more magnesian at high pressure, and that the pressure-temperature position of the solidus, as well as its &lt;em&gt;dP/dT&lt;/em&gt; slope, depend on the bulk composition selected, unless truly invariant assemblages occur. Calculated phase relationships are somewhat dependent on the model selected for clinopyroxene, and to a lesser extent of garnet.&lt;/p&gt;&lt;p&gt;Byrnes A.P. and Wyllie P.J. (1981) Subsolidus and melting relations for the join CaCO&lt;sub&gt;3&lt;/sub&gt;-MgCO&lt;sub&gt;3&lt;/sub&gt; at 10 kbar. Geochim. Cosmochim. Acta 45, 321-328&lt;/p&gt;&lt;p&gt;M&amp;#252;ller I.A., M&amp;#252;ller M. K., Rhede D., Wilke F.D.H. and Wirth R. (2017) Melting relations in the system CaCO&lt;sub&gt;3&lt;/sub&gt;-MgCO&lt;sub&gt;3&lt;/sub&gt; at 6 GPa. Am. Mineral. 102, 2440-2449.&lt;/p&gt;