MR13C-2713
Evolution of the Oxidation State of the Earth’s Mantle: Challenges of High Pressure Quenching

Monday, 14 December 2015
Poster Hall (Moscone South)
Lisa Renee Danielson, Jacobs Technology, NASA Johnson Space Center, Houston, TX, United States, Kevin Righter, NASA Johnson Space Center, Houston, TX, United States, Lindsay P Keller, NASA Johnson Space Ctr, Mail Code XI2, Houston, TX, United States and Zia Rahman, Jacobs/JETS Contract, NASA Johnson Space Center, Mail Code XI2, Houston, TX, United States
Abstract:
The oxidation state of the Earth’s mantle during formation remains an unresolved question, whether it was constant throughout planetary accretion [1], transitioned from reduced to oxidized [2,3,4], or from oxidized to reduced [1,5]. We investigate the stability of Fe3+ at depth, in order to constrain processes (water, late accretion, dissociation of FeO) which may reduce or oxidize the Earth’s mantle.

Experiments of more mafic compositions and at higher pressures commonly form a polyphase quench intergrowth composed primarily of pyroxenes, with interstitial glass which hosts nearly all of the more volatile minor elements.

In our previous experiments on shergottite compositions, variable fO2, T, and P <4 GPa, Fe3+/ΣFe decreased slightly with increasing P, similar to terrestrial basalt [6,7,8]. For oxidizing experiments < 7GPa, Fe3+/ΣFe decreased as well [9], but it’s unclear from previous modelling whether the deeper mantle could retain significant Fe3+ [1,10]. Our current experiments expand our pressure range deeper into the Earth’s mantle and focus on compositions and conditions relevant to the early Earth. Experiments with Knippa basalt as the starting composition were conducted at 1-8 GPa and 1800 °C, using a molybdenum capsule to set the fO2 near IW, by buffering with Mo-MoO3. TEM and EELS analyses revealed the run products from 7-8 GPa quenched to polycrystalline phases, with the major phase pyroxene containing approximately equal Fe3+/2+.

A number of different approaches have been employed to produce glassy samples that can be measured by EELS and XANES. A more intermediate andesite was used in one experiment, and decompression during quenching was attempted after [11], but both resulted in a finer grained polyphase texture. Experiments are currently underway to test how different capsule materials may affect quench texture. A preliminary experiment using liquid nitrogen to greatly enhance the rate of cooling of the assembly has also been attempted and this technique will be refined in further experiments.

[1]Righter and Ghiorso, 2012; [2]Rubie et al., 2011; [3]Wood et al., 2006; [4]Wänke and Dreibus, 1988; [5]Siebert et al., 2013; [6] Righter et al., 2013; [7] O’Neill, et al., 2006; [8] Kress and Carmichael, 1991; [9] Zhang et al., 2013; [10] Hirschmann, 2012 [11] Elkins et al., 2000.

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