Fryer, and 31 co-authors, reported in 2019 that the subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, the authors presented:
(1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope;
(2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust;
(3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone;
(4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents;
(5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor
and/or seamounts; and
(6) the implications for effects of these processes with regard to evolution of life.
The authors concluded that their observation of an intact section of Cretaceous reef exposed in the Mariana Trench inner slope shows, for the first time, that large expanses of impinging seamounts can remain undeformed at first contact. It also confirms uplift of the surrounding forearc region. They observed that forearc relief is enhanced where thicker subducting crust or large seamounts impinge, thus adding to the vertical tectonic deformation of the forearc region.
ODP and IODP drill cores from five serpentinite mud volcanoes contain a range of subducted plate and seamount lithologies. The mudflow matrix composition and the broad variation in degree of metamorphism of the rocks recovered suggest that the relief of the subduction channel beneath the Mariana forearc is likely much higher than generally thought.
High relief in the subduction channel would affect the temperature and pressure conditions, as well as mineralogical and pore fluid compositions of muds that erupt at the active Mariana forearc serpentinite mud volcanoes. A given eruption may involve materials plucked from conduit walls at a variety of depths. This can explain the wide range of degree of serpentinization of ultramafic clasts contained in the mudflows and the wide range of metamorphic facies and degree of deformation observed in metabasites derived from the down-going Pacific Plate and subducted seamounts.
If subducting seamounts pass through the shallow to intermediate depths of the subduction channel relatively intact, microbial communities present in the sub-seafloor on their flanks or within the abyssal Pacific Plate’s lithospheric slab could also be exhumed from varying depths. Serpentinization of forearc mantle fault gouge, in contact with rising fragments of the subducted plate, likely selects for those microbes that prefer the resulting high pH (up to 12.5)
and reducing conditions of the serpentinizing pore fluids [13].
Researchers have suggested a variety of serpentinization environments that may be supportive of the earliest formation of life [82–85,89–99]. These mainly stress hydrothermal vent environments. The focus on mid-ocean ridges and off-ridge hydrothermal regions results from discovery of vast accumulations of vent fauna in the oceans today. The focus on subaerial hydrothermal pools near volcanoes stresses the hydration/dehydration potential for
concentrating prebiotic molecules. However, hydrothermal activity is sporadic and ephemeral on geologic time scales along mid-ocean ridges, and is spatially miniscule by comparison with the 70% global expanse of the ocean floor.
The environment of shallow (approx. 0–20 km) subduction at convergent plate margins is cooler, more contiguously widespread and supports optimum geochemical conditions over geologically greater timescales. So, perhaps we must ask what other geologic settings would have been conducive to enabling the transition from the milieu of probiotic molecules to the development of living cells. If we consider the possibility of the presence of an ocean on the early (4.5 Ga) Earth and the advent of cyclic crustal generation and consumption via some early form of plate tectonic activity, then the temporal continuity and spatial breadth of convergent plate margins is likely far greater than that of hydrothermal vent locations. As we ponder the most favorable environments for
the development of chemosynthetic life on the basis of serpentinization, either on Earth or other planets, it would be well to consider processes other than those just of hydrothermal systems.
Even discarding speculations about ancient environments, the causes of and conditions during current, active episodes of escape/venting of material from the subduction channel in any phase of plate subduction remain uncertain. If seismic events trigger episodes of eruption at modern serpentinite mud volcanoes, is it possible to take the pulse of subduction? Cased boreholes left at the summits of four of the drilled ODP and IODP Sites at summits of four serpentinite mud volcanoes provide a regional laboratory for the study of geophysical, geochemical and geobiological processes associated with the subduction channel beneath the Mariana forearc.
The planned and funded deployment of instrumentation in these seafloor observatory holes will permit further analysis of interrelationships between seismic activity, pore fluid flux, eruption mechanics and effects on microbial communities of the serpentinite mud volcanoes. The results will impact current paradigms of lithospheric deformation, mass cycling and physical conditions at the plate contact zone and help define potential constraints on the mechanism by which subsurface microbes could be exhumed from the subduction channel and ‘recycled’ without being subjected to sterilizing conditions.
Data accessibility.
The analytical methods text and an additional table with data supporting the major and trace element data in this article have been uploaded as part of the electronic supplementary material.
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