Phase stabilities and Fe/Sr/La partitioning between magnesite (MgCO3) and mantle silicates at lower mantle conditions.
Authors: Libon Lélia , Spiekermann Georg , Appel Karen , Biedermann Nicole , Albers Christian , Glazyrin Konstantin , Wilke Max 
Published: March 2020

&lt;p&gt;Carbonates appear to be one group of the main carbon-bearing minerals in the Earth&amp;#8217;s interior. Inclusions of carbonates in diamonds of lower mantle origin support the assumption that they are present even in the Earth&amp;#8217;s lower mantle. Although the carbonates&amp;#8217; phase diagrams have been intensively studied, their stability in presence of mantle silicates at deep mantle conditions (&gt;25 GPa) remains unclear. Furthermore, the carbonate inclusions show a high REE enrichment. This raises questions on the distribution of trace elements between carbonates and silicates and on the possible role of carbonates as trace element carrier in the Earth&amp;#8217;s mantle.&lt;/p&gt;&lt;p&gt;Numerous studies show that magnesite is likely to be the major solid carbonate carried by subduction into the Earth&amp;#8217;s lower mantle. We investigated the stability of MgCO&lt;sub&gt;3&lt;/sub&gt; in presence of mantle silicates and the Fe, Sr and La partitioning in high-pressure and high-temperature experiments. One set of experiments was conducted with multi-anvil presses at BGI, Bayreuth, at conditions ranging from 24 GPa to 30 GPa and 2000 K. The investigated reaction is between natural magnesite and (Mg,Fe)SiO&lt;sub&gt;3&lt;/sub&gt;-glasses doped with either Sr or La. Preliminary data from the multi-anvil press at 24 GPa and 2000K show the onset of carbonate melting which is consistent with the previous study of the melting curve in the enstatite-magnesite system [1]. Decomposition of MgCO&lt;sub&gt;3&lt;/sub&gt; is not observed, in contrast to experiments using magnesite and SiO&lt;sub&gt;2&lt;/sub&gt; as starting materials [2], suggesting that MgCO&lt;sub&gt;3&lt;/sub&gt; is stable at these conditions in the presence of silicates phases. The silicate glass react to bridgmanite (Mg,Fe)SiO&lt;sub&gt;3&lt;/sub&gt; as well as stishovite SiO&lt;sub&gt;2&lt;/sub&gt; and magnesiow&amp;#252;stite (Mg,Fe)O. The Fe-Mg partitioning coefficient between bridgmanite and magnesite calculated in this study is ~2 and in agreement with previous experiments at similar conditions [3].&lt;br&gt;Laser-heated diamond anvil cell (LH-DAC) experiments were performed at University of Potsdam [4] at conditions 30 to 40 GPa and 1800 to 2300 K. The run products were characterized in-situ at high-pressure by XRD and XRF mapping at the P02.2 beamline at PETRA III. Our data show a transformation of the starting silicate glass into bridgmanite. We also observed stishovite and magnesiow&amp;#252;stite in the center of the hotspot where the temperature had reached &gt;2000 K. In this case, the presence of magnesiow&amp;#252;stite might be the result of MgCO&lt;sub&gt;3 &lt;/sub&gt;decomposition at higher temperature. Additional TEM analyses on the post-mortem sample will allow us to further characterize the different phases present in the laser-heated hotspot.&lt;/p&gt;&lt;p&gt;[1] Thompson et al. (2014) Chemistry and mineralogy of the earth&amp;#8217;s mantle. Experimental determination of melting in the systems enstatite-magnesite and magnesite-calcite from 15 to 80 GPa. American Mineralogist 99(8-9), 1544-1554.&lt;br&gt;[2] Drewitt et al. (2019) The fate of carbonate in oceanic crust subducted into Earth&amp;#8217;s lower mantle. EPSL 511, 213-222&lt;br&gt;[3] Martinez, et al. (1998). Experimental investigation of silicate-carbonate system at high pressure and high temperature.&amp;#160;Journal of Geophysical Research: Solid Earth, 103(B3), 5143-5163.&lt;br&gt;[4] Spiekermann et al. (2020). A portable on-axis laser heating system for near-90&amp;#176; X-ray spectroscopy: Application to ferropericlase and iron silicide. Journal of Synchrotron Radiation. (accepted)&lt;/p&gt;