Mariana Convergent Margin and South Chamorro Seamount

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 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 19 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access materials from 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 the Mariana forearc and for billions of years on Earth.At least 19 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 farthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program 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 up to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the volcanic arc and backarc areas, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities.Expedition 366 focused on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes to compare with results from drilling at the two previously cored serpentinite mud volcanoes and with previously ...

Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366

“…1). They are formed by the eruption of mud flows consisting of unconsolidated serpentinite mud and containing variably serpentinized ultramafic clasts, as well as minor amounts of recycled Pacific plate and of Philippine Sea plate materials (Fryer, 2012;Fryer et al, 2018;Maekawa et al, 1993;Pabst et al, 2011). The serpentinite muds are derived from the forearc mantle where slab derived fluids interact with ultramafic lithologies and are buoyantly transported to the seafloor (Fryer, 2012 and reference therein).…”

“…It lies at 15°43′N latitude and 147°11′E longitude ( Fig. 1), at about 13 km above the subducting slab (Fryer et al, 2018;Hulme et al, 2010;Oakley, 2008). Previous studies estimated the temperature of the slab/ mantle interface below Yinazao at about 80°C (Hulme et al, 2010;Oakley, 2008).…”

“…Previous studies estimated the temperature of the slab/ mantle interface below Yinazao at about 80°C (Hulme et al, 2010;Oakley, 2008). Drilling took place at the flank (Site U1491) and the summit (Site U1492) of this mud volcano (Fryer et al, 2018). The recovered cores consisted mainly of an alternation of an uppermost altered units of red-brown pelagic mud, ranging from a few cm to up to 4 m thick, and units of green and bluegrey serpentinite pebbly mud (Fig.…”

“…2). The red-brown pelagic units are interpreted as paleo-seafloor horizons altered in contact with seawater between two mud eruptions (Fryer et al, 2018). Each unit contains between 5 and 10% ultramafic clasts, most of which are fully serpentinized.…”

“…The mud volcanoes are composed of serpentinite muds with embedded ultramafic clasts from the forearc mantle as well as mafic clasts from the subducting crust, originating from depths N15 km (Fryer et al, 2000;Maekawa et al, 1993). Here we take advantage of the recent International Ocean Discovery Program (IODP) Expedition 366 to study serpentinized ultramafic clasts contained in the serpentinite muds of three mud volcanoes: Yinazao (formerly known as Blue Moon), Fantangisña (Celestial) and Asùt Tesoru (Big Blue; Fryer et al, 2018). We show that clasts from the Mariana forearc mantle are variably serpentinized and preserve various stages of lizardite recrystallization into antigorite, brucite and magnetite.…”