Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.

Exhumation of subducted mafic rocks in a dynamically evolving thermal structure: constraints from phase equilibria modelling

<p>The dominant mechanisms that control the exhumation of subducted rocks and how these mechanisms evolve through time in a subduction zone remain unclear. Dynamic models of subduction zones suggest that their thermal structures evolve from subduction initiation to maturity. The series of metamorphic reactions that occur within the slab, resultant density, and buoyancy with respect to the mantle wedge will co-evolve with the thermal structure. We combine dynamic models of subduction zone thermal structure with phase equilibria modeling to place constraints on the dominant controls on the depth limits of exhumation. This is done across the temporal evolution of a subduction zone for various endmember lithologic associations observed in exhumed high-pressure terranes: sedimentary and serpentinite mélanges, and oceanic tectonic slices.</p><p>Initial modeling suggests that both serpentinite and sedimentary mélanges remain positively buoyant with respect to the mantle wedge throughout all stages of subduction (up to 65 Myr), and for the spectrum of naturally constrained ratios of mafic blocks to serpentinite/sedimentary matrix. In these settings, exhumation depth limits and the “point of no return” (c. 2.3 GPa) are not directly limited by buoyancy, but potentially rheological changes in the slab at the blueschist-eclogite transition stemming from: the switch from amphibole-dominated to pyroxene-dominated rheology and/or dehydration embrittlement. These mechanisms may increase the possibility of brittle failure and hence promote detachment of the slab top into the subduction channel. For the range of temperatures recorded by exhumed serpentinite mélanges, the locus of dehydration for altered MORB at the slab top coincides with the point of no return (2.3 GPa) between 35 and 40 Myr, suggesting a strong temporal dependence on deep exhumation in the subduction channel. </p><p>Tectonic slices composed of 50% mafic rocks and 50% serpentinized slab mantle show a temporal dependence on the depth limits of positive buoyancy. For the range of temperatures recorded by exhumed tectonic slices, the upper pressure limit of positive buoyancy is ~2 GPa, and is only crossed between ~30 and 40 Myr after subduction initiation. Some exhumed tectonic slices record much higher pressures (2.5 GPa); thus, other mechanisms or lithologic combinations may also play a significant role in determining the exhumation limits of tectonic slices. </p><p>Future work includes constraining how the loci of dehydration vary through time for different degrees of oceanic crust alteration, how exhumation limits and mechanisms may change with different subducting plate ages, and calculating how initial exhumation velocities may vary through time. Further comparison with the rock record will constrain the parameters that control the timing and limits of exhumation in subduction zones.</p>

Tracing the Evolution of Slab Fluids during Progressive Subduction: Insights from Serpentinite Mud Volcanoes in the Mariana Forearc

<p>Geological processes in subduction zones strongly influence seismicity, igneous activity, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and overlying mantle wedge. Direct study of active slab evolution is inhibited by the great depths at which these processes occur and there is a dearth of physical samples to assess the state of water-rock-sediment reactions, thermal and pressure conditions, and physical properties of materials within the subduction channel.</p><p>The drilling of serpentinite mud volcanoes in the Mariana forearc provides a telescope into these deep processes and allows us to sample fluids and xenoliths from the subducting slab and forearc mantle.  Fluid-laden serpentinite is transported along active extensional faults in the upper plate and seeps out at mud volcano edifices. There is widespread evidence for episodic voluminous serpentine eruptions, likely related to seismic events. Mud volcanoes are found across the forearc and sample the slab interface from 13 to 19 km depth. Samples obtained over three Scientific ocean drilling legs (ODP Legs 125 and 195; IODP Leg 366) and additional ROV expeditions elucidate the evolution of fluid production, reaction and exchange, during the progressive subduction of the down-going plate.</p><p>Fluid analyses show clear trends in pore water chemical and isotopic composition with progressive subduction. These parameters can be used to assess the thermal state of the subduction channel at different depths, identify the reactions controlling fluid releases, and to estimate fluid fluxes. Pore waters from the shallowest depths-to-slab (13-16 km) are Ca and Sr-enriched compared to seawater, but otherwise solute poor, low alkalinity fluids of pH ~11. In contrast, more deeply derived fluids (>18 km) have higher pH (12.5), reduced concentrations of Ca and Sr and elevated DIC, Na and Cl, as well as B and K compared to seawater – these changes are associated with the breakdown of slab sheet silicate phases. These waters also have higher δD and δ<sup>11</sup>B values than shallower waters (δD values up to +16 ‰; δ<sup>11</sup>B ~ 14-15 ‰ cf. δD < 0‰; δ<sup>11</sup>B ~ 12-13 ‰). PHREEQC modelling indicates pore water chemical evolution reflects mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate; however, a component from sediment sources is a likely contributor, especially for those mud volcanoes near the trench.</p><p>Our new data indicate that the lawsonite-epidote mineral transformation boundary (~250 °C, >18 km depth) is an important source of devolatilization waters and may also drive slab carbonate breakdown, despite its apparent thermodynamic stability at such temperatures and pressures. At shallower depths, the main reactions controlling fluid liberation are sediment compaction (<13 km) followed by clay diagenesis and desorbed water release (>13 km depth). This study thus provides direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate.</p>

Brittle-ductile deformation in high-pressure continental units and deep episodic tremor and slip

<p>The geological record of deep seismic activity in subduction zones is generally limited due to common rock overprinting during exhumation and only a few regions allow studying well-preserved exhumed deep structures. The Northern Apennines (Italy) are one such area, granting access to continental units (Tuscan Metamorphic Units) that were subducted to high-pressure conditions, were affected by brittle-ductile deformation while accommodating deep tremor and slip and then exhumed back to surface, with only minor retrogression.</p><p>Our approach is based on detailed fieldwork, microstructural and petrological investigations. Field observations reveal a metamorphosed broken formation composed of boudinaged metaconglomerate levels enveloped by metapelite displaying a pervasive mylonitic foliation. Shear veins occur in both lithologies, but are more common and laterally continuous in the metapelite. They are mostly parallel to the foliation and composed of iso-oriented stretched quartz and Mg-carpholite (XMg>0.5) fibres, which are single-grains up to several centimetres long. These fibres define a stretching direction coherent with that observed in the metaconglomerate and metapelite, which is marked by K-white mica and quartz. Thermodynamic modeling constrains the formation of the high-pressure veins and the mylonitic foliation to ~ 1 GPa and 350°C, corresponding to c. 30-40 km depth in the subduction channel.</p><p>Shear veins developed in subducted (meta)sediments are a key indicator of episodic tremor and slip (e.g. <sup>1</sup>). We propose that these structures reflect the repeated alternation of localised brittle failure, with shear veins development, and more diffuse viscous deformation. These cycles were probably related to the fluctuation of pore pressure that repeatedly reached lithostatic values. Concluding, these structures can be considered the geological record of episodic tremors and slip occurring at >30 km of depth in the Apenninic subduction channel.</p><p>1. Fagereng, Å., Remitti, F. & Sibson, R. H. Incrementally developed slickenfibers — Geological record of repeating low stress-drop seismic events? Tectonophysics <strong>510</strong>, 381–386 (2011).</p><p>This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 839779.</p>