Tetracarbonate melts and the fate of primordial carbon in the deep Earth

Much of Earth’s carbon is thought to have been stripped away from the silicate mantle by dense metallic-iron to form the core1. However, recent studies2,3 suggest that a considerable part of it could have remained stranded in the deep mantle due to a change in its affinity to dissolve into iron metal-alloys at the extreme pressures and temperatures of the deep Earth. The underlying physical phenomena that would render carbon less siderophile at extreme conditions remain elusive. Here we describe the compaction mechanisms and structural evolution of a simple carbonate glass to deep mantle pressures by monitoring the evolution of the electronic state and atomic structure of the glass upon compression. Our new experiments demonstrate a pressure-induced change in hybridization of carbon from sp2 to sp3 starting at 40 GPa, due to the conversion of [3]CO32- groups into [4]CO44- units, which is completed at ~112 GPa. The pressure-induced increase of carbon coordination number from three to four increases possibilities for carbon-oxygen interactions with lower mantle silicates and increased compatibility4,5. Tetracarbonate melts provide a mechanism for changing the presumed siderophile nature of deep carbon and instead imply storage of carbon in the deep mantle as a possible source for carbon-rich emissions registered at the surface in intra-plate and near-ridge hot spots6,7

Phase stabilities and Fe/Sr/La partitioning between magnesite (MgCO3) and mantle silicates at lower mantle conditions.

<p>Carbonates appear to be one group of the main carbon-bearing minerals in the Earth’s interior. Inclusions of carbonates in diamonds of lower mantle origin support the assumption that they are present even in the Earth’s lower mantle. Although the carbonates’ phase diagrams have been intensively studied, their stability in presence of mantle silicates at deep mantle conditions (>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’s mantle.</p><p>Numerous studies show that magnesite is likely to be the major solid carbonate carried by subduction into the Earth’s lower mantle. We investigated the stability of MgCO<sub>3</sub> 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<sub>3</sub>-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<sub>3</sub> is not observed, in contrast to experiments using magnesite and SiO<sub>2</sub> as starting materials [2], suggesting that MgCO<sub>3</sub> is stable at these conditions in the presence of silicates phases. The silicate glass react to bridgmanite (Mg,Fe)SiO<sub>3</sub> as well as stishovite SiO<sub>2</sub> and magnesiowüstite (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].<br>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 magnesiowüstite in the center of the hotspot where the temperature had reached >2000 K. In this case, the presence of magnesiowüstite might be the result of MgCO<sub>3 </sub>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.</p><p>[1] Thompson et al. (2014) Chemistry and mineralogy of the earth’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.<br>[2] Drewitt et al. (2019) The fate of carbonate in oceanic crust subducted into Earth’s lower mantle. EPSL 511, 213-222<br>[3] Martinez, et al. (1998). Experimental investigation of silicate-carbonate system at high pressure and high temperature. Journal of Geophysical Research: Solid Earth, 103(B3), 5143-5163.<br>[4] Spiekermann et al. (2020). A portable on-axis laser heating system for near-90° X-ray spectroscopy: Application to ferropericlase and iron silicide. Journal of Synchrotron Radiation. (accepted)</p>

Experimental Modeling of Silicate and Carbonate Sulfidation under Lithospheric Mantle P,T-Parameters

: Interactions of mantle silicates with subducted carbonates, sulfides, and sulfur-rich fluids are experimentally simulated in the olivine-ankerite-sulfur and olivine-ankerite-pyrite systems using a multi-anvil high-pressure split-sphere apparatus at 6.3 GPa and range of 1050–1550 °C. Recrystallization of Fe,Ni-bearing olivine and ankerite in a sulfur melt was found to be accompanied by sulfidation of olivine and carbonate, involving partial extraction of metals, carbon, and oxygen into the melt, followed by the formation of pyrite (±pyrrhotite), diopside, and Fe-free carbonates. The main features of metasomatic alteration of Fe,Ni-olivine by a reduced sulfur fluid include: (i) a zonal structure of crystals (Fe-rich core, Mg-rich rim); (ii) inclusions of pyrite and pyrrhotite in olivine; (iii) certain Raman spectral characteristics of olivine. At T > 1350 °C, two immiscible melts, a predominantly sulfur melt with dissolved components (or a Fe–Ni–S–O melt) and a predominantly carbonate one, are generated. The redox interaction of these melts leads to the formation of metastable graphite (1350–1550 °C) and diamond growth (1550 °C). The studied olivine-ankerite-sulfur and olivine-ankerite-pyrite interactions may be considered as the basis for simulation of metasomatic processes accompanied by the formation of mantle sulfides during subduction of crustal material to the silicate mantle.

Subduction-transition zone interaction: A review

As subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20–50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of –1 to –2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ∼2 orders of magnitude higher than background mantle (effective yield stresses of 100–300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.