Redox systematics of a magma ocean with variable pressure-temperature gradients and composition

Abstract

The authors note the following: "The calculations that we presented for high-MgO concentration silicate melts are unduly sensitive to small variations in the K 2 O content of the liquid, stemming from the equation of state (EOS) [(1), hereafter referred to as Part IV EOS] that we utilized to calculate thermodynamic properties of the silicate liquid at high pressure. Although we attributed the strong decrease in ΔIW values for peridotite seen in Figs. 4 and 5 to the possibility of Fe 2 O 3 being stabilized in the presence of alkalis in the basalt composition (as well as to the effect of extremely high temperature conditions on the melt volume), the Part IV EOS does not include treatment of Fe 2 O 3 , so this argument may only be relevant to the natural system; in the Part IV EOS all oxidized iron is treated as FeO. Instead, the strong decrease can be attributed to K 2 O contents; above 20 GPa, the Part IV EOS calibration pinned the komatiite fit to a K 2 O content of 0.25 wt%; i.e., K 2 O is not an independent variable for high-MgO liquid above 20 GPa. A similar conclusion can be reached for the K 2 O of MORB liquid above 20 GPa, and to a lesser extent about the other alkali, Na 2 O. Given that there are more data constraints in the Part IV EOS to calibrate variation of Na 2 O content, we expect the sensitivity of derived thermodynamic properties on Na 2 O variation to be muted. However, due caution should be exercised and Na 2 O contents should not stray far from the komatiite and MORB calibrants used to derive the Part IV EOS model. We now conclude that the only consistent way to deal with high-magnesia liquids in the context of the Part IV EOS model is to use a K 2 O ∼0.25 wt%, instead of the 0.03 wt% K 2 O in natural peridotite. As a result, Figs. 4, 5, and 6 and their respective legends appeared incorrectly."Any use of the online fO 2 calculator above 20-25 GPa for peridotite, komatiite or similar high-MgO bulk composition liquids must fix K 2 O content of silicate melt to 0.25 wt% K 2 O, otherwise erroneous fO 2 values may result."The corrected figures and their corrected legends appear below. These errors do not affect the conclusions of the article.

A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres

“…At the base of a MO where metallic melts may pond before sinking into the core 15 , the oxygen fugacity is governed by the equilibrium between the metallic and silicate melts, and can be directly calculated given the compositions of these melts are known. Away from the base where metallicmeltis absent due to its rapid sinking velocity 16 , the MO redox state is controlled by the following redox buffering reaction 3,17 :…”

Effect of pressure on Fe3+/ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients

“…Reactions between silicate magmas and transiting core-forming metal establish geochemical partitioning between the nascent mantle and core (Righter, 2003;Rubie et al, 2003;Wade and Wood, 2005;Rubie et al, 2011; and large-scale planetary degassing from magma oceans and associated silicate-vapor equilibration creates massive primitive atmospheres (Matsui and Abe, 1986;Zahnle et al, 2007;Elkins-Tanton, 2008;Hamada et al, 2013). The parameterization of Fe 3+ /ΣFe as a function of T, P, and fO 2 can be applied to the problem of redox gradients in magma oceans (Hirschmann, 2012;Righter and Ghiorso, 2012a;2012b).…”

“…Meanwhile, silicate melts and overlying atmosphere react at low pressure and the oxygen fugacity may be different at high and low pressure if the magma ocean is well mixed and has a similar Fe 3+ /ΣFe over a range of depths (Hirschmann, 2012). Righter and Ghiorso (2012a) also noted that the pressure of metal-silicate equilibration can influence oxygen fugacities.…”

“…For example, Kress and Carmichael (1991) and O'Neill et al (2006) noted that basalts ascending from a shallow mantle source decrease in fO 2 compared to solid oxide buffers such as quartz-fayalite-magnetite (QFM). Magma oceans represent another environment in which changes in pressure can produce shifts in fO 2 relative to standard buffers (e.g., Hirschmann, 2012;Righter and Ghiorso, 2012a).…”

Magma Ocean Depth and Oxygen Fugacity in the Early Earth—Implications for Biochemistry

“…In particular, these elements are sensitive to oxygen fugacity (fO2), and there is a wide range of oxygen fugacities recorded in samples from the inner solar system (Earth, Moon, Mars, comets, and meteorites)-nearly 15 orders of magnitude from IW-6 to IW+9 (Righter et al 2006). It is not even clear if fO 2 evolved from reduced to oxidized (e.g., Rubie et al 2011), oxidized to reduced , or remained relatively constant (Righter and Ghiorso 2012) during Earth's accretion. In this contribution partitioning data are summarized, and a simple model is presented that includes mass balance and metal-silicate partitioning to illustrate how much S, C, N and H can be in the mantle just after core formation, and whether exogenous or indigenous additional sources of these elements are required.…”