A New Oxybarometer for Basalts Based on Olivine‐Melt Mn‐Fe2+ Exchange Coefficient and FeT/Mn Ratios in Olivine and Melt. Issue 2 (21st February 2022)

• Record Type: Journal Article
• Authors: Sun, Zhongxing; Xiong, Xiaolin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-21
• Subjects: Melt Fe3+/FeT ratios -- Oxygen fugacity -- Mn‐Fe exchange -- Olivine; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023312
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Melt Fe 3+ /Fe T ratios are widely used as a proxy for the oxygen fugacity ( f O2 ) in magma systems. A particular challenge is a difficulty in quantifying the Fe 2+ and Fe 3+ contents, which are not easily measurable by conventional techniques. Using the Mn–Fe 2+ exchange coefficient (KD) between olivine (ol) and melt (m), this study develops a novel approach to this problem using the relationship of Fe m 3 + ${\mathrm{Fe}}_{\mathrm{m}}^{3+}$ / Fe m T ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ = 1 − KD ol / m Mn − Fe 2 + ${\mathrm{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ * ( Fe ol T ${\mathrm{Fe}}_{\mathrm{ol}}^{\mathrm{T}}$ /Mnol )/( Fe m T ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ /Mnm ). The key for applying this equation is to combine well‐defined KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ with accurate Mn and Fe T analyses of olivine and coexisting melt. The Mn and Fe T analyses are readily measured by electron microprobe. To define KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$, we performed experiments at 1.0–1.75 GPa and 1250–1450°C under graphite‐buffered conditions, where Fe T can be used as Fe 2+ to calculate KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ . The results show that KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ = 0.730 ± 0.025 is nearly constant over a realistic range of experimental conditions (temperature, pressure, and melt composition) relevant to basaltic magmas, allowing calculation of melt Fe 3+ /Fe T ratios without the need for any further knowledge of these parameters. A comparison with published data, where melt Fe 3+ /Fe T ratios have been directly measured, demonstrates that our calculation method is as accurate as Mossbauer spectroscopy and XANES techniques, thus providing a very simple and accessible approach to melt Fe 3+ /Fe T ratio estimation. However, it should be noted that the accuracy of this method is critically reliant on: (a) accurate Mn and Fe analyses and (b) olivine‐melt chemical equilibrium. Plain Language Summary: The melt Fe 3+ /Fe T ratios are well constrained as a function of oxygen fugacity ( f O2 ) but difficult to measure. As nearly all Fe in olivine is Fe 2+ and all Mn in olivine and basaltic melt are Mn 2+, the Mn–Fe 2+ exchange coefficient (KD) between olivine (ol) and melt (m) can be developed into a simple back calculation method for melt Fe 3+ /Fe T using the formulation Fe m 3 + ${\mathrm{Fe}}_{\mathrm{m}}^{3+}$ / Fe m T ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ = 1 − KD ol / m Mn − Fe 2 + ${\mathrm{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ * ( Fe ol T ${\mathrm{Fe}}_{\mathrm{ol}}^{\mathrm{T}}$ /Mnol )/( Fe m T ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ /Mnm ). This equation only needs to know Mn and Fe T contents in olivine and a coexisting melt and KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ . The former is measurable accurately by careful electron microprobe analysis. The latter is obtained here by the graphite‐buffered experiments, where Fe T is used as Fe 2+ to calculate KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ . We find that KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ (0.730 ± 0.025) is close to constant over the temperature, pressure, and melt composition range relative to most basalts on Earth, which allows us to calculate the melt Fe 3+ /Fe T ratio with the above equation, providing a simple determination of the magma f O2 . This method has an accuracy comparable to more complicated and less accessible Mossbauer spectroscopy and XANES techniques. Key Points: A new oxybarometer is developed based on olivine‐melt Mn‐Fe 2+ exchange coefficient ( KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ ) KD ol / m Mn − Fe 2 + ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ is nearly constant with the variation of experimental conditions The new oxybarometer provides a simple and efficient method for melt Fe 3+ /FeT calculation