Covariation of Slab Tracers, Volatiles, and Oxidation During Subduction Initiation

Abstract

Subduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Secular Variations in Provenance of Sedimentary Components in the Western North Pacific Ocean Constrained by Sr Isotopic Features of Deep‐Sea Sediments

“…In the early stage, the IBM arc was aligned east-west near the equator; however, it gradually rotated to a north-south alignment and migrated northward (Hall, 2002;Taylor & Goodliffe, 2004). Recent studies revealed that the volcanism in the early stage was characterized by forearc basalt at 52.5 Ma, and boninite for the infant arc edifice building at 51.3-46 Ma (Arculus et al, 2015;Brandl et al, 2017;Brounce et al, 2021;Ishizuka, Tani, et al, 2011, Ishizuka et al, 2018Robertson et al, 2018), followed by the mature arc edifice building (40-28 Ma;Brandl et al, 2017;Johnson et al, 2021). In particular, the volcanism in the latest Eocene to early Oligocene (∼37-28 Ma) was dominated by subaerial and subaqueous explosive volcanism producing the vitric ash, accompanied by the growth of the initial Izu-Bonin-Mariana arc, including the section called the Kyushu-Palau Ridge (KPR;Shipboard Scientific Party, 2002;Johnson et al, 2021).…”

The Redox State of Incipient Oceanic Subduction Zones: An Example From the Troodos Ophiolite (Cyprus)

“…The f O 2 value and the timescale of the IBM forearc is after Brounce et al. (2015, 2021) and Ishizuka et al. (2011), respectively.…”

“…Therefore, such secondary processes might not account for the high fO 2 values of the IBM proto-arc boninites. Indeed, Brounce et al (2021) estimated that the fO 2 of the IBM boninitic magmas may have decreased by ∼0.5 orders of magnitude if sulfur was lost to the gas phase, and so the calculated fO 2 values would only represent minimum estimates. Therefore, S degassing as well as other shallow-level processes would not account for the high fO 2 values of the IBM boninites (Brounce et al, 2021).…”

Spinel harzburgite–derived silicate melts forming sulfide-bearing orthopyroxenite in the lithosphere. Part 2: Sulfide compositions and their chalcophile and highly siderophile trace element signatures

“…Indeed, despite being primarily classified as highly siderophile, PGE also have a strong affinity for S 2− dissolved in silicate melts (Laurenz et al, 2013) or in sulfide liquids (Mungall and Brenan, 2014). Since Hamlyn and Keays (1986), technical progress has allowed the analyses of chalcophile and highly siderophile trace elements with greater accuracy and precision in boninites, further supporting that these magmas are undersaturated in sulfide at relatively low-P conditions (e.g., on the ocean floor; Woodland et al, 2002;Dale et al, 2012;Valetich et al, 2019;Brounce et al, 2021). However, neither PPGE enrichments nor sulfide undersaturation are specific to boninites under these low-P conditions.…”

“…Furthermore, studies of sub-arc mantle peridotites have shown that both HCB and LCB endmember can be characterized by high volatile contents and an elevated oxidation state at a primitive stage (Bénard et al, 2016;Bénard et al, 2017b;Bénard et al, 2018b;Bénard et al, 2018c;Bénard et al, 2022). However, large variations in volatile contents and oxidation state are sometimes observed in boninites sampled on the ocean floor, even though they are not as primitive as those in mantle peridotites (Brounce et al, 2015;Brounce et al, 2021).…”