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Chris Ballhaus - One of the best experts on this subject based on the ideXlab platform.
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Experimental Evidence for a Reduced Metal-saturated Upper Mantle
Journal of Petrology, 2011Co-Authors: Arno Rohrbach, Chris Ballhaus, Peter Ulmer, U. Golla-schindler, Dirk SchönbohmAbstract:The Uppermost Mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite^wu« stite and fayalite^ferrosilite^magnetite equilibria.Whether this range in oxygen fugacity is a shallow Mantle signature or representative of the entire Upper Mantle still is unclear and a matter of debate because Mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper Upper Mantle, we performed experiments from 1 to 14 GPa and 1220 to 16508C on a model peridotite composition, encompassing the convecting asthenospheric Mantle down to the Transition Zone at 410 km depth.The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron^wu« stite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the Upper Mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 1GPa, corresponding to 250 30 km depth in the Upper Mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile Upper Mantle (i.e. 2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized Upper Mantle near the fayalite^ferrosilite^magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth’s present-day Mantle is likely to be saturated with metallic iron.
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metal saturation in the Upper Mantle
Nature, 2007Co-Authors: Arno Rohrbach, Chris Ballhaus, Peter Ulmer, U Gollaschindler, Vadim S Kamenetsky, D V KuzminAbstract:High-pressure experiments are used to show that large parts of the Earth's asthenosphere are metal-saturated, demonstrating that oxidation is only a shallow phenomenon restricted to an Upper veneer only about 250 km depth. The oxygen fugacity of the Earth’s Mantle is one of the fundamental variables in Mantle petrology. Through ferric–ferrous iron and carbon–hydrogen–oxygen equilibria, influences the pressure–temperature positions of Mantle solidi and compositions of small-degree Mantle melts1,2,3. Among other parameters, affects the water storage capacity and rheology of the Mantle4,5. The Uppermost Mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts6,7, but it is not known whether the shallow Mantle is representative of the entire Upper Mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the Mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the Upper Mantle can no longer be regarded as being representative for the Earth’s Upper Mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an Upper veneer only about 250 km in thickness.
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Is the Upper Mantle metal-saturated?
Earth and Planetary Science Letters, 1995Co-Authors: Chris BallhausAbstract:The equilibria controlling silica activity and oxygen fugacity (fO2) in the Upper Mantle are related and all sensitive to pressure. With increasing pressure, both silica activity andfO2 will fall. Provided there is no systematic change in Mantle bulk chemistry, the rate of reduction per unit GPa pressure increase may be of the order of −0.6 log bars relative to the fayalite-magnetite-quartz buffer. At that rate, one may predict (1) that C-H-O volatiles in the convecting Upper Mantle are dominated by methane and hydrogen, and (2) that Fe metal saturation may occur at pressures around 9 GPa.fO2 may be a key variable to explain the fractionation of platinum-group elements between a Mantle residue and a partial melt.
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oxygen fugacity controls in the earth s Upper Mantle
Nature, 1990Co-Authors: Chris Ballhaus, R F Berry, David H GreenAbstract:DESPITE significant progress in recent years1–3, there is still considerable debate surrounding the oxygen fugacity (fo2) of the Earth's Upper Mantle and the chemical reactions that buffer it. Electrochemical measurements1,3 give heterogeneous oxidation states, with one group near the fayalite–magnetite–quartz (FMQ), and the other near the iron-wustite buffer (IW). Thermobarometric calculations based on Fe3+/Fe2+ ratios in basaltic glasses4, Mantle-derived spinels5,6, ilmenites7 and garnets8 support uniform oxidation states bounded by wustite-magnetite (WM) and FMQ. Here we compare the behaviour of four 'oxygen barometers'5,6,9,10 based on the compositions of coexisting minerals, including a new empirical olivine–orthopyroxene–spinel barometer10 that has been calibrated experimentally in spinel Iherzolite at pressures, temperatures and compositions appropriate to the Upper Mantle. Application of this barometer to Mantle-derived rocks and basaltic melts of different tectonic settings suggests that the Upper Mantle is only weakly buffered by Fe3+/Fe2+ equilibria. Large-scale heterogeneity in fo2 structure is governed by recycling processes and injection of oxidized crustal material into a moderately reduced, poorly buffered asthenosphere.
Gero Kurat - One of the best experts on this subject based on the ideXlab platform.
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Fluid inclusions in Upper Mantle xenoliths from Northern Patagonia, Argentina: Evidence for an Upper Mantle diapir
Mineralogy and Petrology, 1997Co-Authors: M. E. Varela, Ernesto Bjerg, R. Clocchiatti, C. H. Labudia, Gero KuratAbstract:Three generations of fluid inclusions can be recognized in Upper Mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (“early”, “primary”) one consists of dense CO2 inclusions which were trapped in the Mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The “late” (“pseudosecondary” and “secondary”) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO2 (L1) and vapor CO2 (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the Upper Mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO2 inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising Upper Mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the Upper Mantle underneath Northern Patagonia, a fairly unusual feature for a continental Upper Mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the Upper Mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future.
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Fluid inclusions in Upper Mantle xenoliths from Northern Patagonia, Argentina: Evidence for an Upper Mantle diapir
Mineralogy and Petrology, 1997Co-Authors: M. E. Varela, Ernesto Bjerg, R. Clocchiatti, C. H. Labudia, Gero KuratAbstract:Three generations of fluid inclusions can be recognized in Upper Mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (“early”, “primary”) one consists of dense CO_2 inclusions which were trapped in the Mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The “late” (“pseudosecondary” and “secondary”) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO_2 (L1) and vapor CO_2 (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the Upper Mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO_2 inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising Upper Mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the Upper Mantle underneath Northern Patagonia, a fairly unusual feature for a continental Upper Mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the Upper Mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future. Erdmantel - Xenolithe in Alkali-Basalten des Somoncure Massivs, Nord-Patagonien, Argentinien, führen drei Generationen von Fluid-Einschlüssen. Die erste (“frühe”, “primäre”) Generation besteht aus dichten CO_2-Einschlüssen, welche offenbar in der Mantel-Kruste Grenzzone (22–36 km Minimum-Tiefe) eingeschlossen wurden. CO_2-Einschlüsse sind kogenetisch mit Silikat-Schmelzeinschlüssen. Dies erlaubt die Abschätzung der Einschließ-Temperatur mit minimal 1200°C, was auf ein Hochtemperatur-Ereignis in einer deutlich kühleren Umgebung hinweist. Die “späten” (“pseudosekundäre” und „sekundäre”) CO_2- Fluid-Einschlüsse bilden zwei Generationen von denen die eine in die flüssige (L1), die andere in die Dampfphase (L2) homogenisieren. Die minimale Einschließ-Tiefe für die L1 Generation ist etwa 16 km. Dies bedeutet - auch bei Berücksichtigung der mit diesem Wert verbundenen Ungenauigkeit - daß diese Erdmantel-Gesteine einige Zeit in der mittleren Erdkruste verbrachten und ein metasomatisches Ereignis erlebten, bevor sie von den Basalten zur Erdoberfläche gebracht wurden. Die Tatsache, daß dieses Ereignis die frühen Einschlüsse nicht zerstörte, kann nur bedeuten, daß es von kurzer Dauer war. Die L2-Generation besteht aus Niedrigdruck CO_2-Einschlüssen mit einer Minimum-Einschließtiefe von nur 2 km. Dies könnte in einer seichten Magmakammer des Wirt Basaltes geschehen sein. Die Abfolge von Fluid-Einschlüssen deutet auf einen relativ schnell aufsteigenden oberen Erdmantel unterhalb von Patagonien hin. Die Petrologie und Mineralchemie der peridotitischen Xenolithe unterstützen das. Die Vorherrschaft von Harzburgiten im Erdmantel unterhalb von Nord-Patagonien deutet auf umfangreiche Bildung partieller Schmelzen und deren Abfuhr hin — eine für einen kontinentalen Mantel ungewöhnliche Situation. Sowohl die Verarmungsereignisse, als auch die metasomatischen Veränderungen (einschließlich jene, welche Spuren in Form von Fluid Einschlüssen hinterließen) machen das Vorhandensein eines schnell aufsteigenden Daipirs im oberen Erdmantel dieser Gegend wahrscheinlich. Der Aufstieg kann vom Stabilitätsbereich der Granat-Peridotite bis in die mittlere Kruste verfolgt werden und muß daher relativ schnell erfolgt sein. Unterschiede in Gesteins-, Mineral und Fluid-Eigenschaften zwischen verschiedenen Lokalitäten legen einen diffusen und differenziellen Diapirismus nahe. Zukünftige Studien sollten es ermöglichen, das Gesamtausmaß und die unterschiedlichen Aufstiegshöhen des Diapirs zu kartieren und Hinweise auf seine Entstehung und zukünftige Entwicklung zu erhalten.
D V Kuzmin - One of the best experts on this subject based on the ideXlab platform.
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metal saturation in the Upper Mantle
Nature, 2007Co-Authors: Arno Rohrbach, Chris Ballhaus, Peter Ulmer, U Gollaschindler, Vadim S Kamenetsky, D V KuzminAbstract:High-pressure experiments are used to show that large parts of the Earth's asthenosphere are metal-saturated, demonstrating that oxidation is only a shallow phenomenon restricted to an Upper veneer only about 250 km depth. The oxygen fugacity of the Earth’s Mantle is one of the fundamental variables in Mantle petrology. Through ferric–ferrous iron and carbon–hydrogen–oxygen equilibria, influences the pressure–temperature positions of Mantle solidi and compositions of small-degree Mantle melts1,2,3. Among other parameters, affects the water storage capacity and rheology of the Mantle4,5. The Uppermost Mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts6,7, but it is not known whether the shallow Mantle is representative of the entire Upper Mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the Mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the Upper Mantle can no longer be regarded as being representative for the Earth’s Upper Mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an Upper veneer only about 250 km in thickness.
Rocco Malservisi - One of the best experts on this subject based on the ideXlab platform.
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lateral variation in Upper Mantle viscosity role of water
Earth and Planetary Science Letters, 2004Co-Authors: Jacqueline Eaby Dixon, Timothy H Dixon, D R Bell, Rocco MalservisiAbstract:Differences in the viscosity of the earth’s Upper Mantle beneath the western US (f10 18 –10 19 Pa s) and global average values based on glacial isostatic adjustment and other data (f10 20 –10 21 Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and Mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the Upper Mantle, using a power law creep rheological model. These data and calculations suggest that the low viscosities of the western US Upper Mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US Upper Mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the Upper Mantle. D 2004 Elsevier B.V. All rights reserved.
Arno Rohrbach - One of the best experts on this subject based on the ideXlab platform.
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Experimental Evidence for a Reduced Metal-saturated Upper Mantle
Journal of Petrology, 2011Co-Authors: Arno Rohrbach, Chris Ballhaus, Peter Ulmer, U. Golla-schindler, Dirk SchönbohmAbstract:The Uppermost Mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite^wu« stite and fayalite^ferrosilite^magnetite equilibria.Whether this range in oxygen fugacity is a shallow Mantle signature or representative of the entire Upper Mantle still is unclear and a matter of debate because Mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper Upper Mantle, we performed experiments from 1 to 14 GPa and 1220 to 16508C on a model peridotite composition, encompassing the convecting asthenospheric Mantle down to the Transition Zone at 410 km depth.The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron^wu« stite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the Upper Mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 1GPa, corresponding to 250 30 km depth in the Upper Mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile Upper Mantle (i.e. 2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized Upper Mantle near the fayalite^ferrosilite^magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth’s present-day Mantle is likely to be saturated with metallic iron.
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metal saturation in the Upper Mantle
Nature, 2007Co-Authors: Arno Rohrbach, Chris Ballhaus, Peter Ulmer, U Gollaschindler, Vadim S Kamenetsky, D V KuzminAbstract:High-pressure experiments are used to show that large parts of the Earth's asthenosphere are metal-saturated, demonstrating that oxidation is only a shallow phenomenon restricted to an Upper veneer only about 250 km depth. The oxygen fugacity of the Earth’s Mantle is one of the fundamental variables in Mantle petrology. Through ferric–ferrous iron and carbon–hydrogen–oxygen equilibria, influences the pressure–temperature positions of Mantle solidi and compositions of small-degree Mantle melts1,2,3. Among other parameters, affects the water storage capacity and rheology of the Mantle4,5. The Uppermost Mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts6,7, but it is not known whether the shallow Mantle is representative of the entire Upper Mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the Mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the Upper Mantle can no longer be regarded as being representative for the Earth’s Upper Mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an Upper veneer only about 250 km in thickness.
