TY  - JOUR
A1  - Cerantola, Valerio
A1  - Wilke, Max
A1  - Kantor, Innokenty
A1  - Ismailova, Leyla
A1  - Kupenko, Ilya
A1  - McCammon, Catherine
A1  - Pascarelli, Sakura
A1  - Dubrovinsky, Leonid S.
T1  - Experimental investigation of FeCO3 (siderite) stability in Earth's lower mantle using XANES spectroscopy
JF  - American mineralogist : an international journal of earth and planetary materials
N2  - We studied FeCO3 using Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy at pressures up to 54 GPa and temperatures above 2000 K. First-principles calculations of Fe at the K-edge in FeCO3 were performed to support the interpretation of the XANES spectra. The variation of iron absorption edge features with pressure and temperature in FeCO3 matches well with recently reported observations on FeCO3 at extreme conditions, and provides new insight into the stability of Fe-carbonates in Earth's mantle. Here we show that at conditions of the mid-lower mantle, ~50 GPa and ~2200 K, FeCO3 melts and partially decomposes to high-pressure Fe3O4. Carbon (diamond) and oxygen are also inferred products of the reaction. We constrained the thermodynamic phase boundary between crystalline FeCO3 and melt to be at 51(1) GPa and ~1850 K. We observe that at 54(1) GPa, temperature-induced spin crossover of Fe2+ takes place from low to high spin such that at 1735(100) K, all iron in FeCO3 is in the high-spin state. A comparison between experiment and theory provides a more detailed understanding of FeCO3 decomposition observed in X-ray absorption spectra and helps to explain spectral changes due to pressure-induced spin crossover in FeCO3 at ambient temperature.
KW  - Deep carbon cycle
KW  - siderite
KW  - decomposition
KW  - melting
KW  - spin transition
KW  - Earth in Five Reactions: A Deep Carbon Perspective
Y1  - 2019
U6  - https://doi.org/10.2138/am-2019-6428
SN  - 0003-004X
SN  - 1945-3027
VL  - 104
IS  - 8
SP  - 1083
EP  - 1091
PB  - Mineralogical Society of America
CY  - Chantilly
ER  - 
TY  - JOUR
A1  - Pohlenz, Julia
A1  - Rosa, A. D.
A1  - Mathon, O.
A1  - Pascarelli, S.
A1  - Belin, S.
A1  - Landrot, G.
A1  - Murzin, V.
A1  - Veligzhanin, A.
A1  - Shiryaev, A.
A1  - Irifune, T.
A1  - Wilke, Max
T1  - Structural controls of CO2 on Y, La and Sr incorporation in sodium-rich silicate - carbonate melts by in-situ high P-T EXAFS
JF  - Chemical geology : official journal of the European Association for Geochemistry
N2  - Carbonate-rich silicate and carbonate melts play a crucial role in deep Earth magmatic processes and their melt structure is a key parameter, as it controls physical and transport properties. Carbon-rich melts can be strongly enriched in trace elements, but the structural incorporation mechanisms of these elements are difficult to study because such melts generally cannot be quenched to glasses. In this contribution we investigate the influence of CO2 on the local environments of trace elements contained in silicate glasses with variable CO2 concentrations and in silicate and carbonate melts. The melts were studied in-situ at high pressure and temperature conditions using the Paris-Edinburgh press (2.2 to 2.6 GPa and 1200 to 1500 degrees C). The compositions studied include sodium-rich peralkaline silicate melts and glasses and carbonate melts similar to those occurring naturally at Oldoinyo Lengai volcano. The local environments of yttrium (Y), lanthanum (La) and strontium (Sr) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Main findings of the study suggest: (1) In peralkaline silicate glasses the local structure of Y is unaffected by the CO2 content. Contrary, a slight increase of oxygen bond lengths of Sr and La is inferred with increasing CO2 content in peralkaline glasses, while they remain constant in glasses of even higher peralkalinity independent of the CO2 content. (2) In silicate melts of different CO2 contents Y-O bond lengths are constant, while a slight increase within carbonate melt compositions is deduced. On the other hand, a steady bond lengths increase over the whole compositional range is inferred for La-O and Sr-O. This may well be explained by distinct preferences of these elements for specific local environments. Based on these new data, we suggest potential mechanisms for the structural incorporation of these elements, a key step towards understanding their partitioning behavior in natural magmatic systems.
KW  - Silicate- carbonate melts/glasses
KW  - Local structure Yttrium, Strontium, Lanthanum
KW  - EXAFS
KW  - Paris-Edinburgh press
Y1  - 2018
U6  - https://doi.org/10.1016/j.chemgeo.2017.12.023
SN  - 0009-2541
SN  - 1872-6836
VL  - 486
SP  - 1
EP  - 15
PB  - Elsevier
CY  - Amsterdam
ER  -