Experimental Spinel Standards for Ferric Iron (Fe3+) Determination During Peridotite Partial Melting

Abstract:

The presence of ferric iron (Fe³⁺) in the mantle plays a significant role in the oxygen fugacity (fO₂) of the Earth’s interior. This has a wide range of implications for Earth related processes ranging from the composition of the atmosphere to magmatic phase relations during melting and crystallization processes [1]. A major source of Earth’s mantle magmas is spinel peridotite. Despite its low abundance, spinel (Fe³⁺/ƩFe = 15-34%, [2]) is the main contributor of Fe³⁺to the melt upon partial melting.

Analyses of Fe³⁺ on small areas of spinel and melt are required to study the Fe³⁺ behavior during partial melting of spinel peridotite. Fe K-edge X-ray Absorption Near Edge Structure (XANES) combines both high precision and small beam size, but requires standards with a wide range of Fe³⁺ content to obtain good calibration. Glasses with varying Fe³⁺ content are easily synthesized [3, 4]. Spinel, however, presents a challenge for experimental standards due to the low diffusion of Cr and Al preventing compositional homogeneity. Natural spinel standards are often used, but only cover a narrow Fe³⁺ range. Thus, there is a need for better experimental spinel standards over a wider range of fO₂.

Our study involves making experimental mantle spinels with variable Fe³⁺ content. We used a sol-gel auto-combustion method to synthesize our starting material [5]. FMQ-2, FMQ+0, and air fO₂ conditions were established using a gas mixing furnace. Piston cylinder experiments were performed at 1.5GPa, and 1310 -1370°C to obtain solid material for XANES. To maintain distinct oxidizing conditions, three capsules were used: graphite for reduced, Re for intermediate and AuPd for oxidized conditions. The spinels were analyzed by Mössbauer spectroscopy. Fe³⁺/ƩFe ranged from 0.3 to 0.6. These values are consistent with the Fe edge position obtained using XANES analyses, between 7130 and 7132 eV, respectively. Our spinels are thus suitable standards for Fe³⁺ measurements in peridotite systems and can be used to determine Fe³⁺/ΣFe ratio of spinel during magma genesis from a peridotite source.

[1] Canil et al. (1994), EPSL 123, 205-220. [2] Frost & McCammon (2008), Annu Rev Earth Planet Sci 36, 389–420. [3] Cottrell & Kelley (2011), EPSL 305, 270-282. [4] Falloon et al. (2008) J. Petrol. 49, 591-613. [5] Vader et al. (2013), J mater Sci 25, 765-771.