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Lecture 3: Subduction

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Title: Lecture 3: Subduction


1
Lecture 3 Subduction
  • Questions
  • What is subduction and why does it cause
    volcanism?
  • What controls the differences in lava composition
    at arc volcanoes?
  • What controls the geomorphology and eruption
    style of volcanoes?

1
2
Hydrothermal vents
  • One of the coolest things about mid-ocean ridges
    is that the heat from the magma drives vigorous
    hydrothermal systems. This has many important
    consequences
  • It supports ecosystems of microorganisms, plants,
    and animals that function in the total absence of
    photosynthesis. Some believe this is the
    environment where life originated and/or survived
    big impacts
  • It has large effects on the chemical and isotopic
    composition of seawater
  • It drives alteration of the oceanic crust and
    lithosphere, and hence the storage of water in
    materials that will be subducted. This water may
    drive arc volcanism at subduction zones or be
    recycled to the mantle.

2
3
Hydrothermal venting at mid-ocean ridges
  • As new ocean crust cools, it fractures and
    becomes permeable to hydrologic flows. Seawater
    invades the crust, is heated, reacts with the
    rock, and is expelled from warm or hot vents.
  • The chemistry of the hydrothermal fluid is
    controlled by exchange with the basalt, by phase
    separation into vapor and brine, by microbial
    activity, and by mixing on discharge with cold
    seawater

3
4
Hydrothermal venting at mid-ocean ridges
  • These diagrams show chemistry of hydrothermal
    vent fluids with increasing time after a dike
    injection event that provided a new heat source
    and a new cycle of high-temperature venting. The
    component mixed with seawater evolved from a
    low-Cl, low-Fe vapor to a high-Cl, high-Fe brine
    as the temperature of venting decreased

4
5
Hydrothermal venting at mid-ocean ridges
  • Looking at extinct oceanic crust preserved in
    ophiolite complexes, evidence of the hydrothermal
    alteration processes that occurred near the ridge
    is preserved in the crustal section. Besides the
    presence of sulfide deposits and hydrated
    minerals, one signature of water-rock interaction
    is the shift in the oxygen isotope ratio 18O/16O
    in the minerals.
  • Seawater-rock interaction at low temperature
    leaves the rock enriched in 18O, whereas at high
    temperature it leaves the rock depleted in 18O.
    In the Oman ophiolite we can therefore see
    evidence of low-temperature circulation in the
    extrusive lavas and dikes, and high-temperature
    circulation in the gabbro section.

The fact that the average value in this whole
section is close to the primary mantle value led
Gregory and Taylor to propose that alteration at
mid-ocean ridges determines the value of seawater
18O/16O.
5
6
Subduction and associated magmatism
6
  • Observation Most of the worlds volcanoes (and
    nearly all the hazardous ones) sit above
    subduction zones!
  • The basic problem slabs are cold...why does
    subduction cause volcanism?

7
Subduction and associated magmatism
  • Key observations
  • Volcanic front is typically 100 km above the
    Wadati-Benioff zone, independent of subduction
    angle or velocity.
  • Subduction zone magmas are wet, often several
    percent water (that is why arc-type volcanoes
    erupt explosively, more on this later).
  • Geochemical tracers show signature of subducted
    material (sediment, altered basalt) coming up in
    the arc lavas.

7
8
Evolution of slab and mantle wedge
  • Thermal environment is set by the temperature
    structure of the downgoing lithosphere, by
    induced convection in the mantle wedge, and by
    heat transport and generation within the system.
  • Models show isotherms dragged far down the slab
    and predict moderately low temperature to great
    depth along the slab-wedge interface.

8
9
Evolution of slab and mantle wedge
  • What materials are available to contribute to
    magmatic activity at subduction zones?
  • Downgoing slab is made of sediments, altered
    oceanic crust, and depleted (serpentinized?)
    lithospheric mantle peridotite.
  • The mantle wedge is made of peridotite, possibly
    modified first by melt extraction in the back-arc
    basin, then by addition of mobile components from
    the slab.
  • The overriding plate is made of old oceanic or
    continental crust and earlier products of ongoing
    subduction processes.
  • We can understand the possible environments for
    melt generation by mapping the phase
    relationships of these materials onto thermal
    models of slab-wedge thermal convection
  • If we know what is there, and in the laboratory
    we measure where these materials melt or
    dehydrate, perhaps we can develop a sensible
    model for the unseen processes the explains the
    observed products.

9
10
Evolution of slab and mantle wedge
10
  • First consider the metamorphism of
    hydrothermally altered basaltic rocks in the
    crust of the downgoing slab.
  • This is our fist look at the metamorphic facies
    diagram.

It shows the generalized phase assemblages that
occur on metamorphism of hydrated basaltic rocks
at various Pressure and Temperature
conditions. The metamorphic conditions seen by
rocks of any composition are named by these
facies, but here it is convenient that the facies
are named for hydrated basaltic rocks, since we
are looking at subduction of altered oceanic
crust.
11
Evolution of slab and mantle wedge
11
  • Hydrothermal alteration of the crust creates
    hydrous minerals and stores water in the rock.

Plotting the P-T path of slabs shows that
subducting basalt passes through blueschist into
eclogite facies. Eclogite is an anhydrous rock,
so by 40 km we have almost total dehydration of
basalt. We follow this water to understand the
melting process. Note the red curve is the
water-saturated solidus, not relevant if water is
dehydrated at lower T.
12
Evolution of slab and mantle wedge
  • Water promotes melting of rocks at high
    pressure, where the solubility of water in the
    melts becomes large (think freezing point
    depression).
  • This diagram shows the solidus curves for basalt
    and peridotite when dry (lines at high
    temperature) and with excess water (concave
    upwards curves at lower T).
  • It also shows the stability limit of amphibole,
    the primary mineral able to store water and
    prevent it from enhancing melting.
  • If all the water can be tied up by amphibole
    formation (2), the effective solidus is formed
    by the amphibole stability limit and the
    water-saturated solidus (hatched).

AmP
AmB
AP
WP
Note reversed sign on Pressure axis!
(Petrologists plot pressure increasing upwards,
geophysicists plot depth increasing downwards)
12
13
Evolution of slab and mantle wedge
  • The next step is to take these phase boundaries
    and draw them on the P-T field generated by a
    thermal convection model of a subduction zone, to
    show the likely regions of dehydration and melt
    generation.
  • In this model, the slab dehydrates (at AmB), but
    never melts.
  • Peridotite melting is restricted to this area
    here, at higher P and T than the intersection of
    amphibole stability and the wet solidus of
    peridotite.

13
14
Evolution of slab and mantle wedge
14
  • A more general, complete and intuitive
    qualitative version is shown in this diagram by
    Prof. Wyllie
  • Things to note in this mess
  • The key observation that the volcanic front sits
    100 km above the Benioff zone could be explained
    by the shape of the Amphibole stability curve.
  • Shows possibility of slab melting if any water
    is retained to gt100 km. Probable only in
    Archaean or for very young slabs.
  • Mantle melts likely to pond and evolve at base
    of low-density crust.
  • Sorry about the reversed sense of
    subductionpeople draw it both ways.

15
Evolution of slab and mantle wedge
15
  • The slab can lose its water in several ways
    dehydration and/or melting of sediments, basalt,
    and/or serpentinite.
  • In most modern subduction zones, the basaltic
    component of the slab probably does not melt.
    This may have been different in the past when
    slabs were typically hotter.
  • Instead there are three principal additions of
    fluid to the mantle wedge first, shallow
    sediment melting later, basalt dehydration
    finally, serpentinite dehydration.

Old model (Davies and Stevenson) transports water
as amphibole in peridotite parallel to the slab
until a higher pressure (AmP line), when it
returns to vapor that rises vertically across the
amphibole stability line and becomes bound again.
Eventually it could zig-zag across until it
crosses the wet peridotite solidus (WP or
location N in above figures). The problem with
this model it takes too long.
16
Evolution of slab and mantle wedge
  • Timing of transport from slab to arc
  • 10Be, a cosmogenic nuclide with a half-life of
    1.6 Ma, is subducted with sedimentary fluids. It
    turns up, still alive, in arc volcanics, which
    implies that the amphibolite zig-zag method of
    transport from the slab to beneath the volcanic
    front is, at least, not the only thing going on.

U-Th disequilibria in some arcs there are two
slab components a sediment melt, a delay of gt350
ka to return to U-Th secular equilibrium, then a
hydrous fluid from basalt dehydration, followed
within 30 ka by eruption For a subduction rate
of 10 cm a-1 and a slab dip of 60, this implies
dehydration of the basalt 30 km deeper than
melting of the sediment, and rapid transport to
melting site, melt extraction, and eruption.
16
17
More recent model of slab/wedge evolution
  • Hebert et al. (2008) variable viscosity model
    with tracing of water in vapor, melt, hydrous
    minerals, and nominally anhydrous minerals

18
More recent model of slab/wedge evolution
19
Compositions of typical Arc volcanic rocks
  • Arc volcanics are usually calc-alkaline suites
    with the differentiation sequence
    basaltandesitedaciterhyolite. Variants depend
    on K2O content.
  • Low-K continental arc assemblages follow a
    sequence gabbrogabbrodioritetonalitelow-K
    granodiorite.
  • High-K island arcs generate trachybasalt-trachyand
    esitephonolite.
  • Broadly speaking there is a time progression in
    the evolution of an island arc from low-K
    (tholeiitic), to medium-K (calc-alkaline), to
    high-K (shoshonitic).
  • This is best understood as an increase in the
    extent of contamination and assimilation of new
    primary magmas by older rocks of the same arc.

Low-K, normal, High-K
19
20
Mature arcs 2 MVB Cascades
21
Compositions of typical Arc volcanic rocks
  • Low-K (or calcic or tholeiitic) series is
    dominated by basalt and basaltic andesite. Low-K
    dacites and rhyolites are rare. This is common in
    juvenile island arcs (isolated islands built
    directly on oceanic crust.
  • Calc-alkaline suite is dominated by andesite,
    although the whole sequence from basalt to
    rhyolite is usually found. This suite dominates
    the building of mature island arcs (Japan,
    Indonesia), where islands have joined together
    and built a continuous arc of proto-continent
    above sea-level.
  • Calc-alkaline andesites also dominate continental
    margin arcs like the Andes and Cascades.
  • When the arc is mature, or subduction stops,
    high-K series become common. These are dominated
    again by more mafic rocks, basalt to basaltic
    andesite.

21
22
gt23700 rocks!
23
Compositions of typical Arc volcanic rocks
  • The frequency of eruption of rocks at various
    stages of differentiation, as well as the
    occurrence of the different series, is best
    thought of in terms of density filtering.
  • It is difficult for magma to rise though less
    dense country rock. Broadly speaking the density
    of rock decreases with increasing SiO2 content or
    decreasing FeOMgO content.
  • When the overlying plate consists only of
    basaltic oceanic crust, arc-related basalts can
    rise directly and erupt with little
    differentiation and with little opportunity for
    assimilation.
  • Thus, early in the life of an arc, the low-K
    series occurs and is dominated by mafic members.
  • As the crust evolves and thickens, it becomes
    harder for basalt to rise through it. Instead
    primary basalts are likely to pond at the base of
    the crust and undergo differentiation and
    assimilation.
  • The result is calc-alkaline andesite, dacite, and
    eventually rhyolite.
  • High-K and shoshonite series presumably pond for
    long times and undergo extensive assimilation,
    but eruption of mafic members of the series is
    promoted by extensional tectonics that set in as
    subduction slows.

23
24
Eruption Styles, Volcano Morphology, Lava
Composition
Arc Volcanoes Steep cones, explosive eruptions
Help!
24
25
Eruption Styles, Volcano Morphology, Lava
Composition
Hot-spot Volcanoes Low, shield-profile cones,
effusive eruptions
Cool!
25
26
Eruption Styles, Volcano Morphology, Lava
Composition
  • The way that a magma erupts is determined by its
    composition, by the flux of magma and by local
    tectonics
  • The most important variables are viscosity and
    volatile content.
  • More viscous magmas build steeper-sided cones.
  • Viscosity is determined by composition,
    temperature, crystal content, and volatile
    concentration. Generally, more SiO2-rich lavas
    erupt at lower temperature, carry more
    phenocrysts, and are more viscous at equal
    temperature, so all these effects work together
    to make viscosity increase with SiO2 content.
  • Basalts make shield volcanoes. Andesites and
    basaltic andesites make stratovolcanoes.
    Rhyolites make domes.
  • More volatile-rich magmas yield more explosive
    eruptions, especially if they are viscous enough
    to retard bubble escape.
  • Basalts are fairly dry and tend to experience
    effusive eruptions a wet basalt can fragment and
    make a cinder cone.
  • Rhyolites, especially, are very volatile-rich and
    viscous, so bubbles can accumulate to the point
    of fragmentation, leading to explosive ash
    eruptions.

26
27
Volcano morphology basaltic
  • Shield volcano, formed by repeated effusion of
    low-viscosity magma from a central vent and flank
    rift systems

Cinder cone, a small-scale feature (100 m high)
formed by fragmentation of wet basaltic magma.
27
28
Volcano morphology andesite-rhyolite
  • Stratovolcano cone, formed by alternating
    effusion of lava flows and ejection of
    pyroclastic debris

A caldera, formed by collapse of the roof of a
shallow magma chamber after sudden eruption and
emptying of the chamber. Calderas range from 1
km to 50 km in diameter on Earth. Contrast a
caldera, formed by collapse, with a crater,
formed right at a surface vent by direct
expulsion of material at the surface.
28
29
Eruption Styles, Volcano Morphology, Lava
Composition
  • Explosive eruptions are of two general types
  • driven by magmatic fragmentation from exsolution
    of dissolved volatiles, or
  • phreatomagmatic explosions resulting from the
    boiling of external water brought into contact
    with hot magma.
  • If an explosive eruption generates a large column
    of ash that reaches the stratosphere, it is a
    plinian eruption.
  • Explosive eruptions, instead of making lava
    flows, make pyroclastic flows and pyroclastic
    deposits.
  • These include nuées ardentes, or glowing clouds,
    which are very fast moving incandescent masses of
    turbulent air-ash mixtures (example, Mt. Pelée on
    Martinique, 1902), and
  • lahars or volcanic mudflows, essentially
    water-induced landslides of unconsolidated
    volcanic debris on steep slopes.

29
30
Volcanic DepositsLahar, or volcanic mudflow
30
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