Chapter 1.INTRODUCTION TO OXYGEN FUGACITY AND ITS PETROLOGIC IMPORTANCE

https://openresearch-repository.anu.edu.au/bitstream/1885/202254/6/The%20Oxidation%20state%20of%20iron%20AAM.pdf

The oxidation state of iron in Mid-Ocean Ridge Basaltic (MORB) glasses: Implications for their petrogenesis and oxygen fugacities

The concentration of carbon in primary mid-ocean ridge basalts (MORBs), and the associated fluxes of CO2 outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2 concentrations for ridges that have Ba and/or Rb data. When combined with quality-controlled volatile-element data from the literature (n = 2,446), these data constrain a range of primary CO2 abundances that vary from 104 ppm to 1.90 wt%. Segment-scale data reveal a range in MORBmagma flux varying by a factor of 440 (from 6.8 × 10 to 3.0 × 10 m/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km/year. When combined with CO2/Ba and CO2/Rb-derived primary magma CO2 abundances, the calculated segment-scale CO2 fluxes vary by more than 3 orders of magnitude (3.3 × 10 to 4.0 × 10 mol/year) and sum to an integrated global MORB CO2 flux of 1:32þ0:77 0:85 × 10 12 mol/year. Variations in ridge CO2 fluxes have a muted effect on global climate; however, because the vast majority of CO2 degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long-term variations in climate via degassing directly into the atmosphere in shallow-water areas or where the ridge system is exposed above sea level. Plain Language Summary Estimated CO2 contents of primary mid-ocean ridge basalts (MORB), calculated on a segment-by-segment basis, vary from 104 ppm to 1.9 wt%. CO2-enriched MORB are present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO2 fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary CO2 content. This study provides the most detailed and accurate estimate to date of the integrated total flux of CO2 from mid-ocean ridges of 1:32 þ0:77 0:85 × 10 12 mol/year.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GC007630

Carbon Fluxes and Primary Magma CO2 Contents Along the Global Mid-Ocean Ridge System

https://hal-insu.archives-ouvertes.fr/insu-02291811/document

Mantle plumes are oxidised

Abstract Oxygen fugacity (fo2) exerts first-order control on the geochemical evolution of planetary interiors, and the Fe3+/ΣFe ratios of silicate glasses provide a useful proxy for fO2. Fe K-edge micro-X-ray absorption near-edge structure (XANES) spectroscopy allows researchers to micro-analytically determine the Fe3+/ΣFe ratios of silicate glasses with high precision. In this study we characterize hydrous and anhydrous basalt glass standards with Mössbauer and XANES spectroscopy and show that synchrotron radiation causes progressive changes to the XANES spectra of hydrous glasses as a function of radiation dose (here defined as total photons delivered per square micrometer), water concentration, and initial Fe3+/ΣFe ratio. We report experiments from eight different radiation dose conditions and show that Fe in hydrous silicate glasses can undergo rapid oxidation upon exposure to radiation. The rate and degree of oxidation correlates with radiation dose and the product of water concentration and ferrous/ferric iron oxide ratio on a molar basis (Φ = XHO0.5·XFeO/XFeO1.5). For example, a basalt glass with 4.9 wt% dissolved H2O and Fe3+/ΣFe = 0.19 from its Mössbauer spectrum may appear to have Fe3+/ΣFe ≥ 0.35 when analyzed over several minutes at a nominal flux density of ~2 × 109 photons/s/μm2. This radiation-induced increase in Fe3+/ΣFe ratio would lead to overestimation of fO2 by about two orders of magnitude, with dramatic consequences for the interpretation of geological processes. The sample area exposed to radiation shows measureable hydrogen loss, consistent with radiation-induced breaking of O–H bonds, associated H migration and loss, and oxidation of Fe2+. This mechanism is consistent with the observation that anhydrous glasses show no damage under any beam conditions. Cryogenic cooling does not mitigate, but rather accelerates, iron oxidation. The effects of beam damage appear to persist indefinitely. We detect beam damage at the lowest photon flux densities tested (3 × 106 photons/s/ μm2); however, at flux densities ≤6 × 107 photons/s/µm2, the hydrous glass calibration curve defined by the centroid (derived from XANES spectra) and Fe3+/SFe ratios (derived from Mössbauer spectra) is indistinguishable from the anhydrous calibration curve within the accuracy achievable with Mössbauer spectroscopy. Thus, published Fe3+/ΣFe ratios from hydrous glasses measured at low photon flux densities are likely to be accurate within measurement uncertainty with respect to what would have been measured by Mössbauer spectroscopy. These new results demonstrate that to obtain accurate Fe3+/ΣFe ratios from hydrous, mafic, silicate glasses, it is first necessary to carefully monitor changes in the XANES spectra as a function of incident dose (e.g., fixed-energy scan). Defocusing and attenuating the beam may prevent significant oxidation of Fe in mafic water-bearing glasses.

A Mössbauer-based XANES calibration for hydrous basalt glasses reveals radiation-induced oxidation of Fe

https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=1740&context=gsofacpubs

Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB

Corrigendum to “Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB” [Volume 479, 20 February 2018, pages 166-175]

Hongluo L. Zhang

Papers

Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB

Corrigendum to “Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB” [Volume 479, 20 February 2018, pages 166-175]