Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions

Abstract

Abstract. Few data exist that provide insight into processes affecting the long-term carbon cycle at shallow forearc depths. To better understand the mobilization of C in sediments and crust of the subducting slab, we investigated carbonate materials that originate from the subduction channel at the Mariana forearc (< 20 km) and were recovered during International Ocean Discovery Program Expedition 366. Calcium carbonates occur as vein precipitates within metavolcanic and metasedimentary clasts. The clasts represent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and calcite and the lack of deformation within the veins suggest carbonate formation in a stress-free environment after peak metamorphism affected their hosts. Intergrowth with barite and marked negative Ce anomalies in carbonate attest the precipitation within a generally oxic environment, that is an environment not controlled by serpentinization. Strontium and O isotopic compositions in carbonate (87Sr∕86Sr = 0.7052 to 0.7054, δ18OVSMOW = 20 to 24 ‰) imply precipitation from slab-derived fluids at temperatures between ∼130 and 300 ∘C. These temperature estimates are consistent with the presence of blueschist facies phases such as lawsonite coexisting with the carbonates in some veins. Incorporated C is inorganic (δ13CVPDB = −1 ‰ to +4 ‰) and likely derived from the decarbonation of calcareous sediment and/or oceanic crust. These findings provide evidence for the mobilization of C in the downgoing slab at depths of < 20 km. Our study shows for the first time in detail that a portion of this C forms carbonate precipitates in the subduction channel of an active convergent margin. This process may be an important asset in understanding the deep carbon cycle since it highlights that some C is lost from the subducting lithosphere before reaching greater depths.

Mariana serpentinite mud volcanism exhumes subducted seamount materials: implications for the origin of life

“…These characteristics point to magma genesis from a locally enriched mantle source such as is abundant in Pacific Plate seamount provinces, but absent in the Mariana forearc [57,58]. The paragenesis of metamorphosed rocks of Pacific Plate origin collected from IODP Expedition 366 cores spans the gamut of metamorphic facies from zeolite [43], to greenschist [59], to blueschist [40,50,60] (figure 6).…”

Geochemistry and Microtextures of Vein Calcites Pervading the Izu‐Bonin Forearc and Rear Arc Crust: New Insights From IODP Expeditions 352 and 351

“…However, studies on calcite veins from the Izu-Bonin-Mariana trench in particular are rare. They are either confined to stable isotopic data (Alt et al, 1998) or focus on processes in the Mariana subduction channel at greater depth (Albers et al, 2019). Most of these geochemical studies on vein calcites from the Izu-Bonin-Mariana trench and other sections of oceanic crust including ophiolites have in common that they do not consider vein microtextures.…”

Shallow depth, substantial change: fluid-metasomatism causes major compositional modifications of subducted volcanics (Mariana forearc)

“…Petrographic descriptions and (mineral) geochemical compositions of the recycled OIBs from Fantangisña and Asùt Tesoru have been reported by Albers et al (2019), Fryer et al (2018, Deng et al (2021),…”

“…and Ca-Na amphibole, pumpellyite, and phengite at both seamounts, with prehnite, calcite, and zeolites (analcime, thomsonite, natrolite) exclusively reported from Fantangisña and lawsonite and Na pyroxene from Asùt Tesoru (Albers et al, 2019;Fryer et al, 2020;Ichiyama et al, 2021). Metamorphic vein precipitates that formed in apparent equilibrium with slab-derived fluids include pectolite and prehnite at Fantangisña and lawsonite and phengite at Asùt Tesoru; metamorphic calcite and aragonite occur at both settings (Albers et al, 2019). Na pyroxene mainly ranges in composition from aegirine to jadeite, but some analyses exhibit an increased augite component; amphibole is riebeckitic with ferric Fe/Al ratios of up to ~0.45 indicating a strong glaucophane component; phengite is Si-rich with up to 3.88 Si per formula unit (Albers et al, 2019;Fryer et al, 2020;Deng et al, 2021;Ichiyama et al, 2021).…”

“…and Ca-Na amphibole, pumpellyite, and phengite at both seamounts, with prehnite, calcite, and zeolites (analcime, thomsonite, natrolite) exclusively reported from Fantangisña and lawsonite and Na pyroxene from Asùt Tesoru (Albers et al, 2019;Fryer et al, 2020;Ichiyama et al, 2021). Metamorphic vein precipitates that formed in apparent equilibrium with slab-derived fluids include pectolite and prehnite at Fantangisña and lawsonite and phengite at Asùt Tesoru; metamorphic calcite and aragonite occur at both settings (Albers et al, 2019).…”