Ferric Iron in the Upper Mantle and In Transition Zone Assemblages: Implications for Relative Oxygen Fugacities in the Mantle

2013 ◽  
pp. 73-88 ◽  
Author(s):  
H. St.C. O'Neill ◽  
D. C. Rubie ◽  
D. Canil ◽  
C. A. Geiger ◽  
C. R. Ross ◽  
...  
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Ferric Iron

scholarly journals Gravity constraints on structure of the East-West Antarctic lithospheric transition zone

2021 ◽  
Author(s):  
Sam Treweek
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Geophysical Methods ◽  
East Antarctica ◽  
Gravity Models ◽  
West Antarctica ◽  
Asthenospheric Mantle ◽  
Surface Uplift ◽  
Mantle Viscosity ◽  
Taylor Valley

<p><b>The differing structural evolution of cratonic East Antarctica and younger West Antarctica has resulted in contrasting lithospheric and asthenospheric mantle viscosities between the two regions. Combined with poor constraints on the upper mantle viscosity structure of the continent, estimates of surface uplift in Antarctica predicted from models of glacial isostatic adjustment (GIA) and observed by Global Satellite Navigation System (GNSS) contain large misfits. This thesis presents a gravity study ofthe lithospheric transition zone beneath the Taylor Valley, Antarctica, conducted to constrain the variation in lithological parameters such as viscosity and density of the upper mantle across this region.</b></p> <p>During this study 119 new gravity observations were collected in the ice-free regions of the Taylor Valley and amalgamated with 154 existing land-based gravity observations, analysed alongside aerogravity measurements of southern Victoria Land. Gravity data are used to construct 2D gravity models of the subsurface beneath this region. An eastward gradient in Bouguer anomalies of ~- 1.6 mGal/km is observed within the Taylor Valley. Models reveal thickening of the Moho from 23±5 km beneath the Ross Sea to 35±5 km in the Polar Plateau (dipping at 24.5±7.2°), and lithospheric mantle 100 km thicker in East Antarctica (~200±30 km) than West Antarctica (~90±30 km). </p> <p>Models of predicted surface uplift history are used to estimate an asthenospheric mantle viscosity of 2.1x1020 Pa.s at full surface recovery beneath the Ross Embayment, differing by ~14% from the viscosity at 50% recovery. The temperature contrast between lithospheric and asthenospheric mantle is estimated as ~400°C, equivalent to a viscosity that decreases by a factor of about 30 over the mantle boundary.</p> <p>Results demonstrate that the history of surface uplift in the study area may be complicated, resulting in observations of uplift, or subsidence, at GNSS stations. Future work should incorporate additional geophysical methods, such as seismicity and electrical resistivity, improving constraints on gravity models. A better understanding of the surface uplift (or subsidence) history in the Transantarctic Mountains is critical, with implications in reducing uncertainty in GIA models.</p>

scholarly journals An Overview of the Experimental Studies on the Electrical Conductivity of Major Minerals in the Upper Mantle and Transition Zone

Materials ◽  
2020 ◽  
Vol 13 (2) ◽  
pp. 408 ◽  
Author(s):  
Lidong Dai ◽  
Haiying Hu ◽  
Jianjun Jiang ◽  
Wenqing Sun ◽  
Heping Li ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Transition Zone ◽  
Oxygen Fugacity ◽  
Upper Mantle ◽  
Activation Enthalpy ◽  
Small Polaron ◽  
Experimental Studies ◽  
Transport Processes ◽  
Structural Water ◽  
Nominally Anhydrous Minerals

In this paper, we present the recent progress in the experimental studies of the electrical conductivity of dominant nominally anhydrous minerals in the upper mantle and mantle transition zone of Earth, namely, olivine, pyroxene, garnet, wadsleyite and ringwoodite. The main influence factors, such as temperature, pressure, water content, oxygen fugacity, and anisotropy are discussed in detail. The dominant conduction mechanisms of Fe-bearing silicate minerals involve the iron-related small polaron with a relatively large activation enthalpy and the hydrogen-related defect with lower activation enthalpy. Specifically, we mainly focus on the variation of oxygen fugacity on the electrical conductivity of anhydrous and hydrous mantle minerals, which exhibit clearly different charge transport processes. In representative temperature and pressure environments, the hydrogen of nominally anhydrous minerals can tremendously enhance the electrical conductivity of the upper mantle and transition zone, and the influence of trace structural water (or hydrogen) is substantial. In combination with the geophysical data of magnetotelluric surveys, the laboratory-based electrical conductivity measurements can provide significant constraints to the water distribution in Earth’s interior.

Polymerization during melting of ortho- and meta-silicates: Effects on Q species stability, heats of fusion, and redox state of mid-ocean range basalts (MORBs)

2020 ◽  
Vol 105 (5) ◽  
pp. 716-726 ◽  
Author(s):  
H. Wayne Nesbitt ◽  
G. Michael Bancroft ◽  
Grant S. Henderson
Keyword(s):  
Upper Mantle ◽  
Ferric Iron ◽  
Spectroscopic Studies ◽  
29Si Nmr ◽  
Anionic Complex ◽  
Free Oxygen ◽  
Raman Spectroscopic ◽  
Melting Temperatures ◽  
The Right ◽  
Midocean Ridge

Abstract 29Si NMR and Raman spectroscopic studies demonstrate that fusion of crystalline orthosilicates and metasilicates produces melts more polymerized than their precursor crystals. Forsterite, for example, consists of 100% Q0 species, whereas its melt consists of ~50 mol% of Q1 species (Q = a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms in the tetrahedron). Polymerization during melting can be rationalized from an energetics perspective. Si-NBO-M moieties of Q species are more susceptible to librational, rotational, and vibrational modes than are Si-BO-Si moieties (NBO = non-bridging oxygen; BO = bridging oxygen; M = counter cation). Thermal agitation activates these additional modes, thus increasing the CP and free energy of melts. The reaction of Qn to Qn+1 species during melting eliminates Si-NBO-M moieties and produces Si-O-Si moieties that are less susceptible to the additional modes, thereby minimizing the CP of melts. By decreasing the abundances of Q0, Q1, and Q2 species in favor of Q3 and Q4 species, melts become more stable. In the absence of polymerization, melting temperatures of minerals would be appreciably greater than observed. Polymerization involves formation of Si-O bonds, which are strongly endothermic (Si-O bond dissociation is ~798 kJ/mol). The large heats of fusion (ΔHf) of orthosilicates result primarily from polymerization reactions during melting (ΔHf of forsterite, fayalite, and tephroite are ~142, ~92, and ~90 kJ/mol). The fusion of metasilicates and sorosilicates (e.g., pyroxenes and melilites) involves endothermic polymerization and exothermic depolymerization reactions, although the former dominates. These reactions tend to negate each other during melting, yielding less positive ΔHf values than observed for orthosilicate fusion (e.g., ΔHf of enstatite, diopside, pseudowollastonite, and åkermanite are ~73, ~69, ~57, and ~62 kJ/mol). Where polymerization and depolymerization reactions are absent ΔHf is low and is due mostly to disordering during melting (e.g., ΔHf of cristobalite iŝ8.9 kJ/mol). Experimental evidence indicates that ferric iron is present as a negatively charged oxy-anionic complex in melts (e.g., [FeO2]1–) so that oxidation of Fe2+ should proceed according to: 4Femelt2+ + 1O2 + 6Omelt2−→4[FeO2]melt1−. Free oxygen (O2–), a by-product of polymerization reactions, drives the reaction to the right. Midocean ridge basalts (MORBs) consequently should be more oxidized than their source (e.g., lherzolites) or their residues (e.g., harzburgites). Extraction of melt from the upper mantle and deposition in the crust should produce a crust more oxidized than its upper mantle source. Production of O2– during melting and its presence in alkali-rich magmas also explains the alkali-ferric iron effect.

Phase relations of MgFe2O4at conditions of the deep upper mantle and transition zone

2017 ◽  
Vol 102 (3) ◽  
pp. 632-642 ◽  
Author(s):  
Laura Uenver-Thiele ◽  
Alan B. Woodland ◽  
Tiziana Boffa Ballaran ◽  
Nobuyoshi Miyajima ◽  
Dan J. Frost
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Phase Relations
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