Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones.
more »
« less
Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks
Metamorphic devolatilization of subducted slabs generates aqueous fluids that ascend into the mantle wedge, driving the partial melting that produces arc magmas. These magmas have oxygen fugacities some 10–1,000 times higher than magmas generated at mid-ocean ridges. Whether this oxidized magmatic character is imparted by slab fluids or is acquired during ascent and interaction with the surrounding mantle or crust is debated. Here we study the petrology of metasedimentary rocks from two Tertiary Aegean subduction complexes in combination with reactive transport modelling to investigate the oxidative potential of the sedimentary rocks that cover slabs. We find that the metasedimentary rocks preserve evidence for fluid-mediated redox reactions and could be highly oxidized. Furthermore, the modelling demonstrates that layers of these oxidized rocks less than about 200 m thick have the capacity to oxidize the ascending slab dehydration flux via redox reactions that remove H2, CH4and/or H2S from the fluids. These fluids can then oxidize the overlying mantle wedge at rates comparable to arc magma generation rates, primarily via reactions involving sulfur species. Oxidized metasedimentary rocks need not generate large amounts of fluid themselves but could instead oxidize slab dehydration fluids ascending through them. Proposed Phanerozoic increases in arc magma oxygen fugacity may reflect the recycling of oxidative weathering products following Neoproterozoic–Palaeozoic marine and atmospheric oxygenation.
more » « less- Award ID(s):
- 1855208
- NSF-PAR ID:
- 10474558
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Geoscience
- Volume:
- 15
- Issue:
- 4
- ISSN:
- 1752-0894
- Format(s):
- Medium: X Size: p. 320-326
- Size(s):
- p. 320-326
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Crystallization of the 2.06 Ga Bushveld magma formed a 9 km (maximum) sequence of ultramafic and mafic rocks that generated a large volume of country fluid as it thermally metamorphosed a 3+ km section of previously unaltered underlying sedimentary rocks of the Transvaal sequence – a geometry similar to that seen as subducting lithospheric slabs are heated by overlying mantle rocks. The presence of a diatreme (breccia pipe) and other large, pipe-like features in the Bushveld Complex located proximal to diapiric upwelling of the basement rocks suggest that overpressured fluids generated during dehydration of the footwall sediments are focused by the diapiric structures such that the country fluids rapidly penetrate the Bushveld rock. A re-examination of existing stable and radiogenic isotopic evidence is consistent with contamination of Main Zone magmas by 1–2% country fluid. Numeric modelling of the footwall dehydration similarly shows that most of the country fluids will be confined to pipe-like channels as it percolates into the Bushveld sill. Modelling also suggests that the maximum extent of the metamorphic aureole was reached at about the same time that the Main Zone began to crystallize. It is proposed that rapid inflation of the Bushveld sill induced the sudden and catastrophic expulsion of overpressured country fluids to both generate the diatreme and contaminate the Main Zone magma, resulting in the Main Zone enrichment in crustal stable and radiogenic isotopic signatures (Sr, Nd, O and others). By analogy, it is also suggested that hydration melting in the mantle wedge is episodically driven by similar sudden influxes of slab fluids that are able to retain their geochemical and isotopic character by rapid channelled influx. This can be aided by flow focusing at diapirs structures at the upper slab-mantle contact.more » « less
-
Abstract Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO2) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ34S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H2O‐NaCl solutions to calibrate a high‐pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at logmore » « less
f O2from FMQ‐2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite atf O2conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur‐rich and high δ34S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke34S‐rich subducted lithologies. -
Abstract Whether and how subduction increases the oxidation state of Earth's mantle are two of the most important unresolved questions in solid Earth geochemistry. Using data from the southern Cascade arc (California, USA), we show quantitatively for the first time that increases in arc magma oxidation state are fundamentally linked to mass transfer of isotopically heavy sulfate from the subducted plate into the mantle wedge. We investigate multiple hypotheses related to plate dehydration and melting and the rise and reaction of slab melts with mantle peridotite in the wedge, focusing on electron balance between redox-sensitive iron and sulfur during these processes. These results show that unless slab-derived silicic melts contain much higher dissolved sulfur than is indicated by currently available experimental data, arc magma generation by mantle wedge melting must involve multiple stages of mantle metasomatism by slab-derived oxidized and sulfur-bearing hydrous components.more » « less
-
more » « less
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, CO2below 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.
