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Abstract

Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program (IODP) Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 18 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access this zone.Active conduits, resulting from fractures in the forearc, are prompted by along-and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at this site and for billions of years on Earth.At least 10 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the furthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program (ODP) Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of about 18 to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the forearc and volcanic arc, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities.Expedition 366 centered on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes defined by the two previously cored serpentinite mud volcanoes and the trench. Three serpentinite mud volcanoes (Yinazao, Fa...

Origin of Short-Chain Organic Acids in Serpentinite Mud Volcanoes of the Mariana Convergent Margin

“…By contrast, moderate-pH mud fluids show a gradual decrease but sustain significantly higher Mg 2+ concentrations throughout the cored intervals. These Mg 2+ concentrations cannot be explained with measured drilling fluid contamination (Fryer et al, 2017; Lever et al, in prep.) and indicate that moderate-pH pore fluids are a mixture of seawater and/or shallower sedimentary pore fluids and deeply-sourced serpentinitic fluids.…”

“…In a few high-pH samples, concentrations of up to ~5 mM are reached, which correspond to ~10% of seawater values (~54 mM). Drilling fluid (seawater) contamination is a likely source of these elevated Mg 2+ values (Figure 2B) according to drilling fluid intrusion estimates based on perfluorocarbon tracer compounds (Fryer et al, 2017; Lever et al, in prep.). Mg 2+ concentrations in high-pH fluids increase steeply near the seafloor, in the same intervals where pH values decrease, indicating significant fluid exchange, e.g., by diffusive mixing with seawater, in the top meters of sediment.…”

Episodicity of structural flow in an active subduction system, new insights from mud volcano's carbonate veins – Scientific Ocean drilling expedition IODP 366

“…Serpentine mudflows were at least recorded from Neogene until present day, as attested by, among other indications, Miocene or Pliocene nanofossils sampled in the first hundreds meters cored on Fantangisña seamount flank (U1498A, Fryer et al, 2017),…”

“…Site U1497 drilling reached depths at Fantangisña Seamount summit (Figure 1B) between 2018.30 and 2019.24 mbsl, recovering 42.38 m of material. Site U1498 is located on the flank of the edifice, drilling between 3284.70 and 3496.21 mbsl and recovering 103.41 m of core (Fryer et al, 2017). This work focuses on carbonate veins sampled at the contact between a metabasite and a chert limestone (Figure 2), in cores U1498A 3R 2W, U1498B-8R and U1498B-21R through U1498B-23R.…”

“…This hard rock sequence is embedded in pebbly serpentinite mud. Following a macroscopic structural analysis of the vein network on-board (Fryer et al, 2017;, the various carbonate vein families were sampled so as to describe the mineral growth mechanisms and diagenesis processes. Polished thin sections and slabs were respectively imaged with a petrographic microscope and microbeam X-Ray Fluorescence (XRF) (Bruker M4 TORNADO TM ) imaging.…”