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Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones./
作者:	Eberhard, Lisa.
出版者:	Ann Arbor : ProQuest Dissertations & Theses, : 2021,
面頁冊數:	274 p.
附註:	Source: Dissertations Abstracts International, Volume: 82-09, Section: B.
Contained By:	Dissertations Abstracts International82-09B.
標題:	Minerals. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=28447938
ISBN:	9798209917007

Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones.
Eberhard, Lisa.

Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones. - Ann Arbor : ProQuest Dissertations & Theses, 2021 - 274 p.

Source: Dissertations Abstracts International, Volume: 82-09, Section: B.

Thesis (Ph.D.)--Universitaet Bayreuth (Germany), 2021.

This item must not be sold to any third party vendors.

The oceanic lithosphere sinks into the Earth's mantle at subduction zones, a process that provides an engine for material exchange between the surface and the Earth's deep interior. Serpentinisation fixes H2O in the oceanic lithosphere. This process also oxidises ferrous to ferric Fe, so that the subduction of serpentinites is an important process through which oxidised material is transported into the mantle. This transport has consequences not only for the redox state of the mantle but for the oxidation state of carbon- and sulfur-bearing volatile phases and their transport into the overlying mantle wedge. The main aim of this study is to provide the first experimental data to determine the relationship between the oxidation state of Fe in serpentinites and the oxygen fugacity f(O2). Using this relationship, the effect of ferric Fe on the phase relations within subducting slabs and the speciation of volatile components can be constrained.In the first part of this study multi-anvil experiments were performed between 2.5 and 5 GPa to examine the phase relations of antigorite- and lizardite-serpentinites. The f(O2) was buffered by various metal-oxide pairs. Mossbauer spectroscopy shows that Fe(3+) is charge balanced by a coupled substitution with Al in both serpentine minerals. Thermodynamic properties are derived to describe the substitution of both elements in both minerals. Lizardite displays a higher Fe(3+)/Fe(tot) ratio than antigorite under similar conditions, whereas the phase relations of antigorite and lizardite are found to be identical. Global Gibbs free energy minimisation calculations show that Al increases the stability of serpentine, whereas ferric and ferrous Fe decrease the stability. The effects are very small, however, and cannot explain differences among previous studies. Serpentine is found to dehydrate at lower temperatures with decreasing f(O2), due to a process termed redox dehydration.Most serpentinites have compositions that result in f(O2) in the range FMQ-0.5 to FMQ+2 at 500°C. As antigorite dehydrates at temperatures above 600°C, the f(O2), regardless of the initial bulk Fe3+/Fe(tot) ratio, will become buffered by the coexistence of magnetite and hematite. This oxidation state cannot be communicated to the mantle wedge through transfer of sulfate-rich fluids, since the f(O2) remains below the sulfide-sulfate equilibrium. The f(O2) during serpentinite subduction will also remain in the carbonate stability field. Previous observations of carbonate reduction to graphite associated with serpentinites and the disappearance of magnetite must result from the action of external reducing agents, such as H2.Calculations for the overlying mantle wedge, where antigorite forms from H2O released by the slab, show this to be one of the most reduced regions of the upper mantle. CO2 in fluids entering the wedge would consequently be reduced to CH4 and the mantle would be oxidised. This might explain the apparent raised oxidation state of island arc magmas. In the second part of the thesis phase relations in carbonated (CaCO3-bearing) antigorite-serpentinites, similar to ophicarbonates, were examined. Ca-Mg exchange results in the formation clinopyroxene, which replaces orthopyroxene and leads to a strong decrease in antigorite dehydration temperature. This will prevent the antigorite stability field from reaching conditions where dense hydrous magnesium silicates form. The presence of ophicarbonates will, therefore, favour the subduction of carbonate-rich but water-poor assemblages into the deep mantle.In the final part of this study a new technique was developed to measure permeabilities at high pressures and temperatures. The method was used to measure the permeability in serpentinites during dehydration. In multi-anvil experiments strongly foliated serpentine cylinders were embedded in MgO sleeves. Fluids, formed upon dehydration, migrate outward and react with MgO to produce brucite. The fluid flux is calculated from the amount of brucite formed. Using equations of state to determine the fluid overpressure, the permeability is subsequently calculated with Darcy's law. A slightly modified setup, using Al(OH)3 as the fluid source, was used to analyse the permeability prior to dehydration of antigorite. The results indicate a large increase in permeability of about 2 log units upon serpentine dehydration to near 1 · 10-18 m2 at 3 GPa, whereas serpentinites are found to be impermeable below the dehydration temperature. Although previous studies performed at near room pressure and temperature indicate that foliated serpentinites exhibit strong permeability anisotropy, the results reported here indicate that all anisotropy is lost once dehydration commences. An anomalously low fluid flux measured at 5 GPa provides the first experimental evidence for pore fluid underpressure upon antigorite dehydration at pressures above 3 GPa, that may prevent fluids from leaving the slab. Below this pressure, however, the large increase in permeability and the lack of permeability anisotropy as antigorite starts to dehydrate will favour pervasive rather than channelised fluid flow, which will promote the decarbonatisation of the slab by dissolution of carbonates in H2O.

ISBN: 9798209917007Subjects--Topical Terms:

542841
Minerals.

Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones.
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100 1 $a Eberhard, Lisa. $3 3642057
245 1 0 $a Serpentinite Phase Relations: An Experimental Study on Redox Conditions and Fluid Migration in Subduction Zones.
260 1 $a Ann Arbor : $b ProQuest Dissertations & Theses, $c 2021
300 $a 274 p.
500 $a Source: Dissertations Abstracts International, Volume: 82-09, Section: B.
500 $a Advisor: Frost, Daniel;McCammon, Catherine.
502 $a Thesis (Ph.D.)--Universitaet Bayreuth (Germany), 2021.
506 $a This item must not be sold to any third party vendors.
506 $a This item must not be sold to any third party vendors.
520 $a The oceanic lithosphere sinks into the Earth's mantle at subduction zones, a process that provides an engine for material exchange between the surface and the Earth's deep interior. Serpentinisation fixes H2O in the oceanic lithosphere. This process also oxidises ferrous to ferric Fe, so that the subduction of serpentinites is an important process through which oxidised material is transported into the mantle. This transport has consequences not only for the redox state of the mantle but for the oxidation state of carbon- and sulfur-bearing volatile phases and their transport into the overlying mantle wedge. The main aim of this study is to provide the first experimental data to determine the relationship between the oxidation state of Fe in serpentinites and the oxygen fugacity f(O2). Using this relationship, the effect of ferric Fe on the phase relations within subducting slabs and the speciation of volatile components can be constrained.In the first part of this study multi-anvil experiments were performed between 2.5 and 5 GPa to examine the phase relations of antigorite- and lizardite-serpentinites. The f(O2) was buffered by various metal-oxide pairs. Mossbauer spectroscopy shows that Fe(3+) is charge balanced by a coupled substitution with Al in both serpentine minerals. Thermodynamic properties are derived to describe the substitution of both elements in both minerals. Lizardite displays a higher Fe(3+)/Fe(tot) ratio than antigorite under similar conditions, whereas the phase relations of antigorite and lizardite are found to be identical. Global Gibbs free energy minimisation calculations show that Al increases the stability of serpentine, whereas ferric and ferrous Fe decrease the stability. The effects are very small, however, and cannot explain differences among previous studies. Serpentine is found to dehydrate at lower temperatures with decreasing f(O2), due to a process termed redox dehydration.Most serpentinites have compositions that result in f(O2) in the range FMQ-0.5 to FMQ+2 at 500°C. As antigorite dehydrates at temperatures above 600°C, the f(O2), regardless of the initial bulk Fe3+/Fe(tot) ratio, will become buffered by the coexistence of magnetite and hematite. This oxidation state cannot be communicated to the mantle wedge through transfer of sulfate-rich fluids, since the f(O2) remains below the sulfide-sulfate equilibrium. The f(O2) during serpentinite subduction will also remain in the carbonate stability field. Previous observations of carbonate reduction to graphite associated with serpentinites and the disappearance of magnetite must result from the action of external reducing agents, such as H2.Calculations for the overlying mantle wedge, where antigorite forms from H2O released by the slab, show this to be one of the most reduced regions of the upper mantle. CO2 in fluids entering the wedge would consequently be reduced to CH4 and the mantle would be oxidised. This might explain the apparent raised oxidation state of island arc magmas. In the second part of the thesis phase relations in carbonated (CaCO3-bearing) antigorite-serpentinites, similar to ophicarbonates, were examined. Ca-Mg exchange results in the formation clinopyroxene, which replaces orthopyroxene and leads to a strong decrease in antigorite dehydration temperature. This will prevent the antigorite stability field from reaching conditions where dense hydrous magnesium silicates form. The presence of ophicarbonates will, therefore, favour the subduction of carbonate-rich but water-poor assemblages into the deep mantle.In the final part of this study a new technique was developed to measure permeabilities at high pressures and temperatures. The method was used to measure the permeability in serpentinites during dehydration. In multi-anvil experiments strongly foliated serpentine cylinders were embedded in MgO sleeves. Fluids, formed upon dehydration, migrate outward and react with MgO to produce brucite. The fluid flux is calculated from the amount of brucite formed. Using equations of state to determine the fluid overpressure, the permeability is subsequently calculated with Darcy's law. A slightly modified setup, using Al(OH)3 as the fluid source, was used to analyse the permeability prior to dehydration of antigorite. The results indicate a large increase in permeability of about 2 log units upon serpentine dehydration to near 1 · 10-18 m2 at 3 GPa, whereas serpentinites are found to be impermeable below the dehydration temperature. Although previous studies performed at near room pressure and temperature indicate that foliated serpentinites exhibit strong permeability anisotropy, the results reported here indicate that all anisotropy is lost once dehydration commences. An anomalously low fluid flux measured at 5 GPa provides the first experimental evidence for pore fluid underpressure upon antigorite dehydration at pressures above 3 GPa, that may prevent fluids from leaving the slab. Below this pressure, however, the large increase in permeability and the lack of permeability anisotropy as antigorite starts to dehydrate will favour pervasive rather than channelised fluid flow, which will promote the decarbonatisation of the slab by dissolution of carbonates in H2O.
520 $a Die ozeanische Lithosphare sinkt in den Erdmantel entlang von Subduktionszonen, ein Prozess der den Materialaustausch zwischen der Erdoberflache und dem Erdinneren antreibt. Serpentinisierung am Ozeanboden bindet H2O in der ozeanischen Lithosphare, fuhrt aber auch zur Oxidation von Fe2+ zu Fe3+. Die Subduktion von Serpentiniten ist deshalb ein wichtiger Prozess, der oxidiertes Material in den Mantel transportiert. Dieser Transport beeinflusst den Redoxzustand des Erdmantels sowie auch den Oxidationszustand von kohlenstoff- und schwefelhaltigen Fluiden und deren Transport in den daruberliegenden Mantelkeil. Das Hauptziel dieser Arbeit ist, die ersten experimentellen Daten zur Beziehung zwischen dem Oxidationszustand von Eisen in Serpentiniten und der Sauerstofffugazitat f(O2) zu liefern. Mit dieser Beziehung konnen Auswirkungen von Fe3+ auf die Phasenbeziehungen in subduzierten Platten sowie die Speziation von volatilen Komponenten berechnet werden.Im ersten Teil dieser Arbeit wurden Experimente mit einer Viel-Stempel-Presse im Bereich von 2.5 bis 5GPa durchgefuhrt, um die Phasenbeziehungen in Antigorit- und Lizardit-serpentiniten zu untersuchen. Die f(O2) wurde durch verschiedene Metall-Oxid-Paare gepuffert. Mittels Mossbauerspektroskopie kann gezeigt werden, dass in beiden Serpentinmineralen die Ladung von Fe3+ ausgeglichen wird durch eine gekoppelte Substitution mit Al. Thermodynamische Eigenschaften wurden hergeleitet, um die Substitution von beiden Komponenten in beiden Phasen zu beschreiben. Lizardit hat ein hoheres Fe3+/Fe(tot) Verhaltnis als Antigorit bei ahnlichen Bedingungen, wobei jedoch die Phasenbeziehungen gleich sind. Globale Minimierungen der freien Enthalpie zeigen, dass Al die Stabilitat von Serpentin erhoht, Fe3+ sowie auch Fe2+ jedoch die Stabilitat verringern. Die Auswirkungen sind allerdings klein und konnen die Unterschiede zwischen fruheren Arbeiten nicht erklaren. Mit abnehmender f(O2) dehydriert Serpentin bei tieferer Temperatur, ein Phanomen, das als Redoxdehydratation bezeichnet werden kann.Die Zusammensetzung der meisten Serpentinite fuhrt zu einer f(O2) im Bereich von FMQ-0,5 und FMQ+2 bei 500°C. Wenn Antigorit bei Temperaturen uber 600°C dehydriert, wird die f(O2) unabhangig vom ursprunglichen Bulk-Fe3+/Fe(tot) Verhaltnis durch die Koexistenz mit Magnetit und Hamatit gepuffert. Dieser Oxidationszustand kann durch sulfatreiche Flussigkeiten nicht an den Mantelkeil ubertragen werden, da die f(O2) unterhalb des Sulfid-Sulfat-Gleichgewichtes bleibt. Die f(O2) von Serpentiniten in Subduktionszonen bleibt auch im Karbonatstabilitatsbereich. Fruhere Beobachtungen wie die Reduktion von Karbonat zu Graphit in Gesteinen assoziiert mit Serpentiniten, sowie auch das Verschwinden von Magnetit konnen deshalb nur durch ein externes Reduktionsmittel wie H2 erklart werden.Fur den daruberliegenden Mantelkeil, in welchem sich Antigorit durch von der sub- duzierten Platte freigesetztes H2O bildet, zeigte sich, dass dies eine der reduziertesten Regionen im oberen Erdmantel ist. In Flussigkeiten gelostes CO2 wird reduziert zu CH4, wobei der Erdmantel oxidiert wird. Dies konnte den hohen Oxidationszustand in Inselbogenmagmen erklaren.Im zweiten Teil dieser Arbeit wurden die Phasenbeziehungen von karbonatisierten (CaCO3-haltigen) Antigoritserpentiniten, ahnlich zu Ophikarbonaten, untersucht. Der Ca-Mg-Austausch bildet Klinopyroxen, welcher Orthopyroxen ersetzt und die Dehydratationstemperatur von Antigorit stark erniedrigt. Dies wiederum verhindert, dass das Antigoritstabilitatsfeld Bedingungen erreicht unter welchen sich dichte wasserhaltige Magnesiumsilikate bilden. Das Vorhandensein von Ophikarbonaten begunstigt demnach die Subduktion von karbonatreichen aber wasserarmen Gesteinen in den tiefen Mantel.Im letzten Teil dieser Arbeit wurde eine neue Methode zur Bestimmung von Permeabilitaten bei hohem Druck und Temperatur entwickelt. Die Methode wurde angewendet, um die Permeabilitat in Serpentiniten wahrend der Dehydratation zu bestimmen. In Experimenten in einer Viel-Stempel-Presse wurden Zylinder aus blattrigen Serpentiniten in eine Hulle aus MgO eingebettet. Wahrend der Dehydratation gebildete Fluide migrieren nach ausen und reagieren mit MgO zu Brucit. Der Fluidfluss wurde aus der Menge des gebildeten Brucits berechnet. Mittels Zustandsgleichungen wurde der Uberdruck berechnet und daraus mit Hilfe des Gesetzes von Darcy die Permeabilitat bestimmt. Eine leicht abgeanderte Konfiguration mit Al(OH)3 als Wasserquelle wurde benutzt, um die Permeabilitat vor der Antigoritdehydratation zu bestimmen. Diese Resultate zeigen, dass die Permeabilitat wahrend der Dehydratation um 2 Grosenordnungen zunimmt auf nahezu · 10-18 m2 bei 3 GPa. Unterhalb der Dehydratationstemperatur sind Serpentinite jedoch undurchlassig. Obwohl fruhere Studien zeigten, dass bei nahezu Raumdruck und -temperatur Antigorit eine starke Permeabilitatsanisotropie aufweist, zeigen diese Resultate auch, dass samtliche Anisotropie beim Einsetzen der Dehydratation verloren geht. Ein anomal tiefer Fluidfluss bei 5 GPa liefert den ersten experimentellen Nachweis eines Unterdrucks des Porenfluides bei der Dehydratation von Antigorit oberhalb 3 GPa, welcher das Fluid in der subduzierten Platte zuruckhalten kann. Unterhalb dieses Druckes jedoch begunstigt die starke Zunahme der Permeabilitat sowie der Verlust der Anisotropie einen pervasiven Fluidfluss gegenuber einem kanalisierten Fluidflusses, was wiederum die Dekarbonatisierung durch Auflosung von Karbonaten in H2O begunstigt.
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