TY  - THES
A1  - Libon, Lélia
T1  - Stability of magnesite in the Earth lower mantle: insight from high-pressure and high-temperature experiments
N2  - Carbonates carried in subducting slabs may play a major role in sourcing and storing carbon in the deep Earth’s interior. Current estimates indicate that between 40 to 66 million tons of carbon per year enter subduction zones, but it is uncertain how much of it reaches the lower mantle. It appears that most of this carbon might be extracted from subducting slabs at the mantle wedge and only a limited amount continues deeper and eventually reaches the deep mantle. However, estimations on deeply subducted carbon broadly range from 0.0001 to 52 million tons of carbon per year. This disparity is primarily due to the limited understanding of the survival of carbonate minerals during their transport to deep mantle conditions. Indeed, carbon has very low solubility in mantle silicates, therefore it is expected to be stored primarily in accessory phases such as carbonates. Among those carbonates, magnesite (MgCO3), as a single phase, is the most stable under all mantle conditions. However, experimental investigation on the stability of magnesite in contact with SiO2 at lower mantle conditions suggests that magnesite is stable only along a cold subducted slab geotherm. Furthermore, our understanding of magnesite’s stability when interacting with more complex mantle silicate phases remains incomplete. In the first part of this dissertation, laser-heated diamond anvil cells and multi-anvil apparatus experiments were performed to investigate the stability of magnesite in contact with iron-bearing mantle silicates. Sub-solidus reactions, melting, decarbonation and diamond formation were examined from shallow to mid-lower mantle conditions (25 to 68 GPa; 1300 to 2000 K). Multi-anvil experiments at 25 GPa show the formation of carbonate-rich melt, bridgmanite, and stishovite with melting occurring at a temperature corresponding to all geotherms except the coldest one. In situ X-ray diffraction, in laser-heating diamond anvil cells experiments, shows crystallization of bridgmanite and stishovite but no melt phase was detected in situ at high temperatures. To detect decarbonation phases such as diamond, Raman spectroscopy was used. Crystallization of diamonds is observed as a sub-solidus process even at temperatures relevant and lower than the coldest slab geotherm (1350 K at 33 GPa). Data obtained from this work suggest that magnesite is unstable in contact with the surrounding peridotite mantle in the upper-most lower mantle. The presence of magnesite instead induces melting under oxidized conditions and/or foster diamond formation under more reduced conditions, at depths ∼700 km. Consequently, carbonates will be removed from the carbonate-rich slabs at shallow lower mantle conditions, where subducted slabs can stagnate. Therefore, the transport of carbonate to deeper depths will be restricted, supporting the presence of a barrier for carbon subduction at the top of the lower mantle. Moreover, the reduction of magnesite, forming diamonds provides additional evidence that super-deep diamond crystallization is related to the reduction of carbonates or carbonated-rich melt.

The second part of this dissertation presents the development of a portable laser-heating system optimized for X-ray emission spectroscopy (XES) or nuclear inelastic scattering (NIS) spectroscopy with signal collection at near 90◦. The laser-heated diamond anvil cell is the only static pressure device that can replicate the pressure and temperatures of the Earth’s lower mantle and core. The high temperatures are reached by using high-powered lasers focused on the sample contained between the diamond anvils. Moreover, diamonds’ transparency to X-rays enables in situ X-ray spectroscopy measurements that can probe the sample under high-temperature and high-pressure conditions. Therefore, the development of portable laser-heating systems has linked high-pressure and temperature research with high-resolution X-ray spectroscopy techniques to synchrotron beamlines that do not have a dedicated, permanent, laser-heating system. A general description of the system is provided, as well as details on the use of a parabolic mirror as a reflective imaging objective for on-axis laser heating and radiospectrometric temperature measurements with zero attenuation of incoming X-rays. The parabolic mirror improves the accuracy of temperature measurements free from chromatic aberrations in a wide spectral range and its perforation permits in situ X-rays measurement at synchrotron facilities. The parabolic mirror is a well-suited alternative to refractive objectives in laser heating systems, which will facilitate future applications in the use of CO2 lasers.
N2  - Karbonate, die von subduzierenden Platten mitgeführt werden, könnten eine wichtige Rolle bei der Transport und Speicherung von Kohlenstoff im tiefen Erdinneren spielen. Aktuellen Schätzungen zufolge gelangen pro Jahr zwischen 40 und 66 Millionen Tonnen Kohlenstoff über Subduktionszonen ins Erdinnere, aber es ist unbekannt, wie viel davon den unteren Erdmantel erreicht. Es gibt Hinweise darauf, dass der größte Teil dieses Kohlenstoffs aus den subduzierenden Platten am Mantelkeil extrahiert wird und nur eine begrenzte Menge den tiefen Erdmantel erreicht. Die Schätzungen über tief subduzierten Kohlenstoff reichen von 0,0001 bis 52 Millionen Tonnen Kohlenstoff pro Jahr. Diese Diskrepanz ist in erster Linie auf das begrenzte Wissen über die Stabilität von Karbonatmineralen während ihres Transports in den tiefen Erdmantel zurückzuführen. In der Tat hat Kohlenstoff eine sehr geringe Löslichkeit in Mantelsilikaten, daher wird erwartet, dass er hauptsächlich in akzessorischen Phasen wie Karbonaten gespeichert wird. Unter diesen Karbonaten nur Magnesit (MgCO3) ist unter allen Mantelbedingungen stabil. Experimentelle Untersuchungen über die Stabilität von Magnesit im Kontakt mit SiO2 bei niedrigeren Mantelbedingungen legen jedoch nahe, dass Magnesit nur entlang einer kalten subduzierten Plattengeotherme stabil ist. Im ersten Teil dieser Dissertation wurden Hochdruck- und Hochtemperaturexperimente unter Verwendung der laserbeheizten Diamantstempelzellen und Vielstempel-Pressen durchgeführt, um die Stabilität von Magnesit im Kontakt mit eisenhaltigen Mantelsilikaten bei Bedingungen im unteren Mantel zu untersuchen. Die aus dieser Arbeit gewonnenen Daten legen nahe, dass Magnesit im Kontakt mit dem umgebenden Peridotitmantel im oberen Teil des unteren Mantel instabil ist. Das Vorhandensein von Magnesit induziert stattdessen das Schmelzen unter oxidierten Bedingungen und/oder fördert die Bildung von Diamanten unter reduzierten Bedingungen in einer Tiefe von ~700 km. Infolgedessen werden die Karbonate aus den Platten entfernt und nicht in größere Tiefen transportiert, was für das Vorhandensein einer Barriere für die Kohlenstoffsubduktion an der Spitze des unteren Mantels spricht. Darüber hinaus liefert die Reduktion von Magnesit, aus der Diamanten entstehen, einen zusätzlichen Beweis dafür, dass die Kristallisation von Diamanten in großer Tiefe mit der Reduktion von Karbonaten oder karbonatreicher Schmelze zusammenhängt.
Im zweiten Teil dieser Dissertation wird die Entwicklung eines portabel Laserheizsystems vorgestellt, das für die Röntgenemissionsspektroskopie (XES) und die Spektroskopie mit nuklearer inelastischer Streuung (NIS) optimiert ist. Das Signal kann hierbei aus einer Diamantstempelzellein einem Winkel von nahezu 90◦ gesammelt werden. Die laserbeheizte Diamantstempelzelle ist das einzige statische Druckgerät, das den Druck und die Temperaturen des unteren Erdmantels und des Erdkerns erzeugenkann. Die hohen Temperaturen werden durch den Einsatz von Hochleistungslasern erreicht, die auf die Probe gerichtet sind, welche sich zwischen den Diamantstempeln befindet. Darüber hinaus ermöglicht die Transparenz von Diamanten im Wellenlängenbereich von Röntgenstrahlung in-situ röntgenspektroskopische Messungen, mit denen die Probe unter Hochtemperatur- und Hochdruckbedingungen untersucht werden kann. Um hierbei ausreichende Intensitäten im ausgehenden Signal zu erreichen, wurde das portabel Laserheizsystem für den Einsatz an Synchrotronanlagen konzipiert, die über keine eigenen Laserheizanlagen für Diamantstempelzellen verfügen. Beschrieben wird der allgemeine Aufbau des Systems. Außerdem werden Einzelheiten der Verwendung eines Parabolspiegels als reflektivem abbildenden Objektivs für die Laserheizung dargelegt. Der Parabolspiegel ist eine gut geeignete Alternative zu refraktiven Objektiven in Laserheizsystemen und wird künftige Anwendungen beim Einsatz von CO2-Lasern erleichtern.
KW  - deep carbon
KW  - magnesite
KW  - laser heating
KW  - diamond anvil cells
KW  - lower mantle
KW  - carbonate stability
KW  - Karbonat-Stabilität
KW  - tiefer Kohlenstoff
KW  - Diamantstempelzellen
KW  - Laserheizsystem
KW  - unterer Mantel
KW  - Magnesit
Y1  - 2023
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-604616
ER  - 
TY  - JOUR
A1  - Kaa, Johannes M.
A1  - Sternemann, Christian
A1  - Appel, Karen
A1  - Cerantola, Valerio
A1  - Preston, Thomas R.
A1  - Albers, Christian
A1  - Elbers, Mirko
A1  - Libon, Lelia
A1  - Makita, Mikako
A1  - Pelka, Alexander
A1  - Petitgirard, Sylvain
A1  - Plückthun, Christian
A1  - Roddatis, Vladimir
A1  - Sahle, Christoph J.
A1  - Spiekermann, Georg
A1  - Schmidt, Christian
A1  - Schreiber, Anja
A1  - Sakrowski, Robin
A1  - Tolan, Metin
A1  - Wilke, Max
A1  - Zastrau, Ulf
A1  - Konopkova, Zuzana
T1  - Structural and electron spin state changes in an x-ray heated iron carbonate system at the Earth's lower mantle pressures
JF  - Physical review research
N2  - The determination of the spin state of iron-bearing compounds at high pressure and temperature is crucial for our understanding of chemical and physical properties of the deep Earth. Studies on the relationship between the coordination of iron and its electronic spin structure in iron-bearing oxides, silicates, carbonates, iron alloys, and other minerals found in the Earth's mantle and core are scarce because of the technical challenges to simultaneously probe the sample at high pressures and temperatures. We used the unique properties of a pulsed and highly brilliant x-ray free electron laser (XFEL) beam at the High Energy Density (HED) instrument of the European XFEL to x-ray heat and probe samples contained in a diamond anvil cell. We heated and probed with the same x-ray pulse train and simultaneously measured x-ray emission and x-ray diffraction of an FeCO3 sample at a pressure of 51 GPa with up to melting temperatures. We collected spin state sensitive Fe K beta(1,3) fluorescence spectra and detected the sample's structural changes via diffraction, observing the inverse volume collapse across the spin transition. During x-ray heating, the carbonate transforms into orthorhombic Fe4C3O12 and iron oxides. Incipient melting was also observed. This approach to collect information about the electronic state and structural changes from samples contained in a diamond anvil cell at melting temperatures and above will considerably improve our understanding of the structure and dynamics of planetary and exoplanetary interiors.
Y1  - 2022
U6  - https://doi.org/10.1103/PhysRevResearch.4.033042
SN  - 2643-1564
VL  - 4
IS  - 3
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Krstulović, Marija
A1  - Rosa, Angelika D.
A1  - Ferreira Sanchez, Dario
A1  - Libon, Lélia
A1  - Albers, Christian
A1  - Merkulova, Margarita
A1  - Grolimund, Daniel
A1  - Irifune, Tetsuo
A1  - Wilke, Max
T1  - Effect of temperature on the densification of silicate melts to lower earth's mantle conditions
JF  - Physics of the earth and planetary interiors
N2  - Physical properties of silicate melts play a key role for global planetary dynamics, controlling for example volcanic eruption styles, mantle convection and elemental cycling in the deep Earth. They are significantly modified by structural changes at the atomic scale due to external parameters such as pressure and temperature or due to chemistry. Structural rearrangements such as 4- to 6-fold coordination change of Si with increasing depth may profoundly influence melt properties, but have so far mostly been studied at ambient temperature due to experimental difficulties. In order to investigate the structural properties of silicate melts and their densification mechanisms at conditions relevant to the deep Earth's interior, we studied haplo basaltic glasses and melts (albite-diopside composition) at high pressure and temperature conditions in resistively and laser-heated diamond anvil cells using X-ray absorption near edge structure spectroscopy. Samples were doped with 10 wt% of Ge, which is accessible with this experimental technique and which commonly serves as a structural analogue for the network forming cation Si. We acquired spectra on the Ge K edge up to 48 GPa and 5000 K and derived the average Ge-O coordination number NGe-O, and bond distance RGe-O as functions of pressure. Our results demonstrate a continuous transformation from tetrahedral to octahedral coordination between ca. 5 and 30 GPa at ambient temperature. Above 1600 K the data reveal a reduction of the pressure needed to complete conversion to octahedral coordination by ca. 30 %. The results allow us to determine the influence of temperature on the Si coordination number changes in natural melts in the Earth's interior. We propose that the complete transition to octahedral coordination in basaltic melts is reached at about 40 GPa, corresponding to a depth of ca. 1200 km in the uppermost lower mantle. At the core-mantle boundary (2900 km, 130 GPa, 3000 K) the existence of non-buoyant melts has been proposed to explain observed low seismic wave velocity features. Our results highlight that the melt composition can affect the melt density at such extreme conditions and may strongly influence the structural response.
KW  - Silicate melts
KW  - Densification
KW  - High pressure and high temperature;
KW  - XANES
KW  - Coordination number
KW  - Ultra-low velocity zones
Y1  - 2022
U6  - https://doi.org/10.1016/j.pepi.2021.106823
SN  - 0031-9201
SN  - 1872-7395
VL  - 323
PB  - Elsevier
CY  - Amsterdam
ER  -