Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle
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Abstract The subduction of hydrated slab mantle is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. The most critical unknowns are the initial hydration state and the dehydration behavior of the subducted oceanic mantle. Here we present a combined thermomechanical, thermodynamic, and geochemical model of the Kamchatka subduction zone that indicates significant dehydration of subducted slab mantle beneath Kamchatka. Evidence for the subduction of hydrated oceanic mantle comes from across‐arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. Our thermodynamic‐geochemical models successfully predict the complex geochemical patterns and the spatial distribution of arc volcanoes in Kamchatka assuming the subduction of hydrated oceanic mantle. Our results show that water content and dehydration behavior of the slab mantle beneath Kamchatka can be directly linked to compositional features in arc volcanic rocks. Depending on hydration depth of the slab mantle, our models yield water recycling rates between 1.1 × 10 3 and 7.4 × 10 3 Tg/Ma/km corresponding to values between 0.75 × 10 6 and 5.2 × 10 6 Tg/Ma for the entire Kamchatkan subduction zone. These values are up to one order of magnitude lower than previous estimates for Kamchatka, but clearly show that subducted hydrated slab mantle significantly contributes to the water budget in the Kamchatkan subduction zone.Keywords:
Slab
Volcanic arc
Adakite
Crustal recycling
Slab window
At a convergent margin large amounts of structurally bound water are carried into
the Earth’s interior and - as the subducting plate descends and the temperature
rises - are driven off to some extent into the mantle wedge, where they are thought
to trigger intermediate-depth earthquakes in the Wadati-Benioff zone and melting
under volcanic arcs. However, a largely uncertain fraction outlasts sub-arc fluid
release and hence enters the deeper mantle, which leads to a connection between
the oceans and the Earth’s deep water cycle. Thus, a detailed knowledge of the water
budget of a subduction zone is not only important to understand arc volcanism, but
as well to comprehend the chemical development of the Earth’s mantle. For this
purpose, profound information about the amount of water that is subducted along
with the oceanic plate is indispensable.
The present thesis uses geophysical methods to determine the degree of hydration
of the Cocos Plate offshore Nicaragua, which is subducted beneath the Caribbean
Plate.
In general it was assumed that structured water is transported into the slab
in sediments and the upper crust only, though in recent years growing evidence
suggested that lower crust and upper mantle might contain capacious amounts of
fluids as well, since the bending of the incoming oceanic plate leads to a reactivation
or creation of normal faults (bend-faults), which are visible in batrymetric data
and have been inferred to cut deep enough into the plate to provide a pathway
for seawater to penetrate into the lithosphere, changing ”dry” peridotites to ”wet”
serpentinites, which contain up to 13% of water. Such a mechanism could transport
much more fluids into the earth’s interior than any other considered possibility.
However, the cutting depth of these bend-faults and hence the depth that seawater
could penetrate into the mantle was not well-defined, for one reason since focal depth
of earthquakes associated with the bend-faults were poorly known. Yet previous
studies assumed cutting depths such that serpentinization is firstly restricted by its
thermal limit of 600± C.
This study uses openly accessible, global broadband data of earthquakes offshore
Central America as well as an unique dataset from a local long-period earthquake
monitoring network offshore Nicaragua, to determine typical focal depths off earthquakes
at the trench-outer rise and further relates these focal depths to the cutting
depths of bend-faults. In addition, a full 3d-tomographic inversion that consistently
integrates seismic airgun blasts and local as well as regional seismicity, could show
reduced seismic mantle velocities at the outer rise and nearby the deep sea trench
with an evolutionary trend towards it. Best explained is this by a fractured and
ii
partly serpentinized lithosphere. The use of regional sources (i.e. earthquakes in
distances of ¸200 km from the seismic network) in the tomographic inversion process
made it possible, for the first time, to reflect the entire brittle lithosphere. In
a second approach, relative arrival times of large earthquakes that occurred during
the deployment of the seismic network were investigated. Again, it could be shown
that seismic mantle velocities decrease in accordance with the onset of bend-faults
in the bathymetry.
But not only seismic velocities decrease nearby the trench, the average moment
magnitude of outer rise earthquakes does as well, though the number of events
increases significantly. We explain this a weakened lithosphere and hence a reduced
yield strain, which again suggests an occurrence of serpentinite.
However, tomographic images suggest that the area of reduced seismic velocities
and in turn possible serpentinization does not reach the cutting depth of bend-faults
nor the depth of the 600± C isotherm. Focal mechanisms of several earthquakes were
determined via moment tensor inversion and forward modelling respectively and it
could be shown that where seismic velocities are reduced only tensional ruptures
occur, which allow for water infiltration, meanwhile the area beneath is dominated by
compressional rupture behaviour, which presents a barrier for seawater. This result
does not only confirm and enlarge flexure models of subducting plates [Chapple
and Forsyth, 1979; Christensen and Ruff, 1988], but also establishes a coherent
connection between stress distribution in the incoming plate and penetration depth
of seawater and is the first study in this vein.
Adakite
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Volcanic arc
Slab window
Slab
Crustal recycling
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Slab
Slab window
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Adakite
Peridotite
Eclogitization
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Processes in subduction zones such as slab and mantle‐wedge metamorphism, intraslab earthquakes, and arc volcanism vary systematically with the age‐dependent thermal state of the subducting slab. In contrast, the configuration of subduction zones is rather uniform in that the arc is typically situated where the slab is ∼100 km deep. Toward reconciling the diversity and uniformity, we developed numerical thermal models with a nonlinear mantle rheology for seventeen subduction zones, spanning a large range of slab age, descent rate, and geometry. Where there are adequate observations, such as in Cascadia, northeast Japan, and Kamchatka, we find that surface heat flows can be explained if the interface between the slab and the mantle wedge is decoupled to a depth of 70–80 km. Models with this common decoupling depth predict that the region of high mantle temperatures and optimal fluid supply from the dehydrating slab, both required for melt generation for arc volcanism, occurs where the slab is ∼100 km deep. These models also reproduce the variations of the metamorphic, seismic, and volcanic processes with the thermal state of the slab. The shallow decoupling results in a stagnant fore arc whose thermal regime is controlled mainly by the subducting slab. The deeper coupling leads to a sudden onset of mantle wedge flow that brings heat from greater depths and the back arc, and its thermal effect overshadows that of the slab in the arc region. Our results serve to recast the research of subduction zone geodynamics into a quest for understanding what controls the common depth of decoupling.
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The oceanic crust is created at mid-ocean ridges and returned to the Earth's deep mantle at subduction zones. A clear picture on the dynamics and distribution of the subducted oceanic crust in the deep mantle is crucial for understanding the nature of compositional heterogeneities and the chemical evolution of the deep Earth. This chapter reviews observational constraints, mineral physics experiments, and geodynamic studies on the distribution, physical properties, and dynamics of the subducted oceanic crust in the deep mantle. It has been found that the subducted oceanic crust may exhibit a variety of behaviors when interacting with the surrounding mantle, depending on the volume, density, and viscosity of the subducted crustal materials and the surrounding mantle dynamics. The subducted oceanic crust may sink directly to the lowermost mantle, or pond at the base of the mantle transition zone with some part episodically sinking to the bottom of the mantle. The subducted oceanic crust in the lowermost mantle may either be directly carried up to shallower depths by entrainment, or accumulate on the core–mantle boundary with some part entrained into, and carried upward by, mantle plumes. The entrained oceanic crust may be directly carried to the base of the lithosphere, pond at a variety of mantle depths, or sink downward due to its negative buoyancy, and it is eventually mixed into the background mantle. The cycling of subducted oceanic crust throughout the deep mantle provides an understanding on the geochemical complexities in volcanic rocks and the seismic anomalies in the deep mantle.
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Adakites are unusual felsic igneous rocks commonly associated with asthenospheric slab window opening or fast subduction of a young (<25 Ma) oceanic plate that may allow slab melting at shallow depths. Their genesis has been extensively debated, as they are also observed in other geodynamic settings where thermal models do not predict slab melting in the fore-arc region. Here, we present a new approach that provides new constraints on adakite petrogenesis in hot subduction zones (e.g. the Philippines) and above an asthenospheric window (e.g. Baja California, Mexico). We use amphibole compositions to estimate magma storage depths and the composition of the host melts to test the hypothesis that adakites are pristine slab melts. We find that adakites from the Philippines and Baja California fore-arcs formed in two distinct petrogenetic scenarios: in the Philippines, water-rich mantle melts stalled and crystallized within lower and upper crustal magma storage regions to produce silica-rich melts with an adakitic signature; in Baja California, slab melts that percolated through the mantle wedge mixed or mingled with water-rich mantle melts within a lower crustal (∼30 km depth) magma storage region before stalling in the upper arc crust (∼7–15 km depth). Alternatively, the Baja California adakites may represent mixing products between high-pressure differentiated mantle melts and mantle melts in a lower crustal magma reservoir, periodically refluxed by mantle melts. Thus, slab melting is not necessarily required to produce an adakitic geochemical fingerprint in hot subduction zones. The hot downgoing plate may cross the 'adakitic window' and melt in specific geodynamic settings such as the opening of a slab tear, as beneath Baja California.
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Abstract A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism.
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