Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

“…This may be reconciled with experimental studies showing immobility of Mo in low fO 2 -bearing slab fluids and hydrous slab melts (Bali et al, 2012;Skora et al, 2017;Chowdhury et al, 2022). These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo 4þ is immobile in fluids in the presence of rutile (Bali et al, 2012) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo 4þ relative to Mo 6þ into the residual phases such as sulfides or rutile (Skora et al, 2017;Chowdhury et al, 2022). These findings are also consistent with subduction of organic matter (OM)-rich black shales in the Lesser Antilles Arc, where lavas south of Martinique exhibit high δ 98/95 Mo along with lower Mo/Ce (Freymuth et al, 2016;Gaschnig et al, 2017), suggesting the minute contribution of unfractionated slab-derived Mo to the mantle sources relative to melts originating from more oxidising sediments.…”

“…The trend towards higher observed (co-) variations of δ 98/95 Mo with increasing degrees of mantle source enrichment within the S-MAR suite therefore implies recycling of a sedimentary δ 98/95 Mo or even a total Mo budget unaffected by dehydration and melting during subduction. This may be reconciled with experimental studies showing immobility of Mo in low fO 2 -bearing slab fluids and hydrous slab melts (Bali et al, 2012;Skora et al, 2017;Chowdhury et al, 2022). These experiments were conducted at subduction zone P-T and at reducing conditions, and showed that Mo 4þ is immobile in fluids in the presence of rutile (Bali et al, 2012) and mobilisation of Mo is inefficient in melts due to increased partitioning of Mo 4þ relative to Mo 6þ into the residual phases such as sulfides or rutile (Skora et al, 2017;Chowdhury et al, 2022).…”

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

“…L. Li et al, 2020;Tomkins & Evans, 2015). Our study emphasizes the fact that slab fluids have the potential to carry reduced species (such as carbon-, sulfur-, and iron-bearing species) upon slab dehydration and/or slab melting (Chowdhury et al, 2022;Evans & Frost, 2020;Piccoli et al, 2019), and this scenario could possibly explain the low, MORB-like oxidation state of the Troodos protoarc magmas (Figure 13).…”

“…For instance, it has been proposed that slab‐derived sulfur would be one of the most important elements that could modulate the redox state of the mantle wedge, because of its valence state (from −2 to +6). Slab‐derived fluids would thus carry either reduced (H 2 S, HS − ) or oxidized (SO 4 2‐ ) sulfur species that could reduce or oxidize the asthenospheric mantle, respectively (e.g., Chowdhury et al., 2022; Evans et al., 2012; J. L. Li et al., 2020; Tomkins & Evans, 2015). Our study emphasizes the fact that slab fluids have the potential to carry reduced species (such as carbon‐, sulfur‐, and iron‐bearing species) upon slab dehydration and/or slab melting (Chowdhury et al., 2022; Evans & Frost, 2020; Piccoli et al., 2019), and this scenario could possibly explain the low, MORB‐like oxidation state of the Troodos proto‐arc magmas (Figure 13).…”

“…For instance, it has been proposed that slab-derived sulfur would be one of the most important elements that could modulate the redox state of the mantle wedge, because of its valence state (from −2 to +6). Slab-derived fluids would thus carry either reduced (H 2 S, HS − ) or oxidized (SO 4 2-) sulfur species that could reduce or oxidize the asthenospheric mantle, respectively (e.g., Chowdhury et al, 2022;Evans et al, 2012;J. L. Li et al, 2020;Tomkins & Evans, 2015).…”