Variations in Fe3+/∑Fe of Mariana Arc Basalts and Mantle Wedge fO2

Abstract

When collecting Fe-µ-XANES spectra on olivine-hosted melt inclusions, it is important to avoid hitting the olivine crystal with the beam during analysis. Olivine contains several weight percent of Fe 2+ and even a very small amount of olivine interference will "contaminate" the pre-edge structure of Fe-µ-XANES spectra collected for melt inclusions and bias the result towards more reduced values. The region of XANES spectra at higher energies than the Fe-Kα absorption edge contains information related to Fe-coordination and can be used to distinguish glass structure (random and on average, uncoordinated) from olivine signal (strong coordination, Fig. A1). All melt inclusion and seafloor glass spectra were visually inspected and compared to spectra taken on San Carlos olivine and standard glasses from Cottrell et al. (2009) in order to screen for crystal contamination. Any spectra demonstrating signs of spectral features similar to those observed in San Carlos olivine were not considered in this study and additional spectra were collected to accommodate for this elimination. Model liquid lines of descentTo constrain the effects of fractional crystallization on magmatic Fe 3+ /∑Fe ratios, model liquid lines of descent that match the observed major element variations were generated using PetroLog3 (Danyushevsky & Plechov, 2011). The mineral-melt models that most closely replicate the natural data were chosen for each location, resulting in some variation in the models used from volcano to volcano. Individual model parameters

Diffuse Extension of the Southern Mariana Margin

“…The IMI-30 was deployed in two main survey areas-along the Malaguana Gadao Ridge (MGR), the magmatically inflated southern Mariana backarc seafloor spreading center (Becker et al, 2010), and the SEMFR area ( Figure 2) at the eastern end of the rifted margin. Stern, P. Fryer, K. Kelley, and Y. Ohara. Results of the geochemical sampling in the SEMFR area and surrounding margin are reported in , , Ribeiro et al (2015Ribeiro et al ( , 2017, and Brounce et al (2014). Geophysical results from the MGR and its westward continuation as the diffuse spreading zone (DSZ) extending to the Southwest Mariana Rift (SWMR, Figure 2) are reported in a manuscript in preparation: "Diffuse spreading, a new mode of crustal accretion in the southern Mariana Trough backarc basin" by J. D. Sleeper, F. Martinez, R. J.…”

Stable vanadium isotopes as a redox proxy in magmatic systems?

“…Both methods indicate that Mariana arc basalts are generally ~1-2 log units more oxidised compared to Icelandic basalts from Hekla (e.g., de Moor et al, 2005;Moune et al, 2007;Brounce et al, 2014;Shorttle et al, 2015). We chose forty whole rock samples from three well-studied lava suites: 1) primitive arc lavas of the Mariana Central Island Province (CIP; Elliott et al, 1997), 2) co-genetic lavas from Anatahan volcano in the Mariana arc (Wade et al, 2005) and 3) co-genetic lavas from Hekla volcano, Iceland (Savage et al, 2011).…”

“…In order to explain how oxidised arc basalts and more reduced MORBs can be derived from sources with similar fO 2, processes subsequent to magma generation such as degassing and crystal fractionation have been invoked (e.g., Mallmann and O'Neill, 2009;Lee et al, 2010). Evidence gleaned from both experimental and natural suites is conflicting, with cases of Fe 3+ /∑Fe increasing, decreasing and remaining invariant during magmatic degassing and crystallisation (e.g., Brounce et al, 2014;Waters and Lange, 2016). Thus lingering uncertainty as to whether the elevated Fe 3+ /∑Fe in arc lavas is derived from their sources or during differentiation fuels continued debate.…”

Redox variations in Mauna Kea lavas, the oxygen fugacity of the Hawaiian plume, and the role of volcanic gases in Earth’s oxygenation

“…Estimates of fO 2 of Archean magmas are not this low, arguing for alternative explanations for the oxygenation of the atmosphere. /ΣS), than midocean ridge basalt (MORB) magmas (1)(2)(3)(4)(5)(6)(7). Based on the known relationships between Fe 3+ /ΣFe, S 6+ /ΣS, oxygen fugacity (fO 2 ), and melt composition (8,9) and on recent observations that Fe 3+ /ΣFe of subduction-related magmas are only weakly dependent on factors such as crystal−liquid fractionation and degassing (7,10,11), the higher Fe 3+ /ΣFe ratios of arc and back-arc basalts relative to MORBs have been attributed by most authors to elevated fO 2 in their mantle sources resulting from addition of oxidized hydrous fluids or silicate melts from subducted slabs (7,11,12).…”

“…/ΣS), than midocean ridge basalt (MORB) magmas (1)(2)(3)(4)(5)(6)(7). Based on the known relationships between Fe 3+ /ΣFe, S 6+ /ΣS, oxygen fugacity (fO 2 ), and melt composition (8,9) and on recent observations that Fe 3+ /ΣFe of subduction-related magmas are only weakly dependent on factors such as crystal−liquid fractionation and degassing (7,10,11), the higher Fe 3+ /ΣFe ratios of arc and back-arc basalts relative to MORBs have been attributed by most authors to elevated fO 2 in their mantle sources resulting from addition of oxidized hydrous fluids or silicate melts from subducted slabs (7,11,12). Ocean island basalts (OIBs) are more variable in fO 2 , with Fe 3+ /ΣFe and S 6+ /ΣS ratios that range from lower than to higher than MORBs (4,(13)(14)(15)(16)(17), but it is unclear whether these differences relate to variable amounts of ancient, more oxidized subducted material in the mantle sources of OIB (e.g., ref.…”