Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts
Jon Blundy, Elena Melekhova, Luca Ziberna, Madeleine C. S. Humphreys, Valerio Cerantola, Richard A. Brooker, Catherine McCammon, Michel Pichavant, Peter Ulmer
Abstract
Abstract The Fe–Mg exchange coefficient between olivine (ol) and melt (m), defined as $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:msup><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mi>T</mml:mi></mml:msup><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> = (Fe ol /Fe m )·(Mg m /Mg ol ), with all Fe T expressed as Fe 2+ , is one of the most widely used parameters in petrology. We explore the effect of redox conditions on $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:msup><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mi>T</mml:mi></mml:msup><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> using experimental, olivine-saturated basaltic glasses with variable H 2 O (≤ 7 wt%) over a wide range of f O 2 (iron-wüstite buffer to air), pressure (≤ 1.7 GPa), temperature (1025–1425 °C) and melt composition. The ratio of Fe 3+ to total Fe (Fe 3+ /∑Fe), as determined by Fe K-edge µXANES and/or Synchrotron Mössbauer Source (SMS) spectroscopy, lies in the range 0–0.84. Measured Fe 3+ /∑Fe is consistent (± 0.05) with published algorithms and appears insensitive to dissolved H 2 O. Combining our new data with published experimental data having measured glass Fe 3+ /∑Fe, we show that for Fo 65–98 olivine in equilibrium with basaltic and basaltic andesite melts, $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:msup><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mi>T</mml:mi></mml:msup><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> decreases linearly with Fe 3+ /∑Fe with a slope and intercept of 0.3135 ± 0.0011. After accounting for non-ideal mixing of forsterite and fayalite in olivine, using a symmetrical regular solution model, the slope and intercept become 0.3642 ± 0.0011. This is the value at Fo 50 olivine; at higher and lower Fo the value will be reduced by an amount related to olivine non-ideality. Our approach provides a straightforward means to determine Fe 3+ /∑Fe in olivine-bearing experimental melts, from which f O 2 can be calculated. In contrast to $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:msup><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mi>T</mml:mi></mml:msup><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> , the Mn–Mg exchange coefficient, $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:mtext>Mn</mml:mtext><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> , is relatively constant over a wide range of P–T– f O 2 conditions. We present an expression for $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:mtext>Mn</mml:mtext><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> that incorporates the effects of temperature and olivine composition using the lattice strain model. By applying our experimentally-calibrated expressions for $${\text{Kd}}_{{{\text{Fe}}^{T} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:msup><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mi>T</mml:mi></mml:msup><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> and $${\text{Kd}}_{{{\text{Mn}} {-} {\text{Mg}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mtext>Kd</mml:mtext><mml:mrow><mml:mtext>Mn</mml:mtext><mml:mo>-</mml:mo><mml:mtext>Mg</mml:mtext></mml:mrow></mml:msub></mml:math> to olivine-hosted melt inclusions analysed by electron microprobe it is possible to correct simultaneously for post-entrapment crystallisation (or dissolution) and calculate melt Fe 3+ /∑Fe to a precision of ≤ 0.04.