Subduction Zone Processes
EAS 302 Lecture 29
Fuego Volcano, Guatemala
1
Importance of Subduction Zone
Processes

Mantle-to-crust mass transfer

Crust-to-Mantle transfer
 Principal present way in which new continental crust is created
 oceanic crust returned to mantle
 Along with it, products of reaction between seawater and oceanic crust;
sediment
 Carbonates in the oceanic crust may either be




subducted (producing loss of CO2 from the surface of the Earth)
Broken down by metamorphic reactions, with CO2 released by volcanism to the
atmosphere
Both have important implications for long term climate regulation.
Volcanism and Seismicity
 Subduction zones are very seismically active areas.
 Subduction-related volcanoes erupt violently
 Significant hazard to humans and their structures

El Salvador, Kobi Japan, Mt. St. Helens
 SO2 and particulates can have important climatic effects


Global cooling recorded following Tambora (1815), Krakatau (1883), El
Chichon (1983) and Mt. Pinatubo (1991)
Reduced stratospheric ozone.
Society and Subduction Zones:
Volcanic Eruptions
 300,000
volcanorelated deaths in last
1000 years
 Tambora, 1815, 92,000
 Krakatau, 1883, 36,000
 Mont Pelée, 1902,
29,000
 Nevado del Ruiz, 1985,
26,000
 Unzen, 1792, 15,000
 Laki, Iceland, 1783,
9,000
Pyroclastic Flow
Augustine Volcano, Alaska
2
Climatic Impact of Large Volcanic
Eruptions
Atmospheric SO2 following June 19,1991 eruption of Mt. Pinatubo
Climatic Impact of Large Volcanic
Eruptions
Summer Temperature Anomalies follow eruption of Mt. Pinatubo
3
Society and Subduction Zones:
Earthquakes
Seattle, 2001
Society and Subduction Zones:
Earthquakes
Kobi, Japan 1995
4
How do we study subduction zones?

Seismology
 Benioff Zone
earthquakes define
path of subducting
lithosphere

Chemistry of the
magmas
100120 km
 Reveal processes
beneath subduction
zone volcanoes.
Chemical Characteristics of
Subduction Zone Magmas

Volatile (mainly water) rich
 Directly and indirectly responsible for explosive
nature

Rich in SiO2
 “andesite” the most common magma type
 andesite is in most instances a product of
fractional crystallization of basalt
 thicker crust, water-rich nature, cooler
environment leads to more extensive fractional
crystallization
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Volcanic Eruptions
in Subduction
Zones

High SiO2 leads
to high viscosity
 Makes it difficult
for gas to escape

Combined with
water-rich
magmas,
produces
explosive
eruptions, such
as Mt. Pinatubo.
Incompatible Element Characteristics of
Subduction Zone Magmas
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What is the role of subducting
sediment?

In cases where the sedimentary pile is
particularly thick (West Indies, Indonesia), much
of the sediment is scraped off, creating
accretionary prisms.
 Seismic studies and drilling, however, indicate that
lowermost layers (100 m in W. Indies) is subducted.


In cases where sediment is thin (Marianas, S.
America), much or all of the sediment
“disappears down the tube”; there are no
accretionary prisms.
Isotopic evidence:


“Sediment-like” Pb isotope ratios
Presence of 10Be
Accretionary Prism in Cross Section
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Pb isotope ratios of subduction zone
magmas
15.9
15.8
15.7
207
204
Sediments
Banda
Sunda
Marianas
S. Sandwich
Pb 15.6
Pb
15.5
Taiwan
MORB
Aleutians
Indian MORB
Philippines
15.4
15.3
17
17.5
18
L. Antilles
18.5
Pb/204Pb
19
19.5
20
206

High 207Pb/204Pb & steep arrays on Pb isotope plots
suggesting mixing between sediment and mantle
10Be : The Smoking Gun
 10Be

created in the
atmosphere by cosmic ray
spallation.
Half-life of 1.6 million years
 Exists only on surface of the
Earth.


Removed from atmosphere
by rain and adsorbed by
clays.
If it were found in
subduction-related lavas,
but not in other lavas, this
would prove they contain
material that was recently at
the surface of the Earth.
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Island Arcs
A
Cosmogenic 10Be
in subduction
zone magmas



Subduction zone
lavas (at least
some) contain
10Be.
Other lavas
contain none.
Therefore,
subduction zone
lavas contain
material recently
at the surface,
namely sediments.
A
A
Japan
Peru
Central America
24
Aleutians
0
5
10 Be
10
15
(106 atoms/g)
All Others
Other Non-arc Volcanos
Flood basalts
MORB
n = 26
Average = 0.3 x 106 g–1
0
5
10 Be
10
15
(106 atoms/g)
How do subduction zone magmas
acquire their unique characteristics?

Like MORB, subduction zone magmas are a
product of melting of the mantle
 and not the subducting oceanic crust, except in rare
circumstances.
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How do subduction zone magmas
acquire their unique characteristics?


Like MORB, subduction zone magmas are a
product of melting of the mantle.
Derived mainly from the “mantle wedge”:
region of mantle above the subducting
oceanic lithosphere.
How do subduction zone magmas
acquire their unique characteristics?



Like MORB, subduction zone magmas are a
product of melting of the mantle.
Derived mainly from the “mantle wedge”:
region of mantle above the subducting
oceanic lithosphere.
Dehydration of oceanic crust (and perhaps
melting of sediments) generates fluids that
rise into the overlying mantle wedge.
 In part, this water came from ocean ridge
hydrothermal activity.
 Hydrated minerals not stable at high temperature.
10
How do subduction zone magmas
acquire their unique characteristics?




Like MORB, subduction zone magmas are a
product of melting of the mantle
Derived mainly from the “mantle wedge”:
region of mantle above the subducting
oceanic lithosphere
Dehydration of oceanic crust (and perhaps
melting of sediments) generates fluids that
rise into the overlying mantle wedge.
Presence of water reduces the solidus
temperature of peridotite. Thus water
released by crust induces melting in
overlying mantle.
How do subduction zone magmas
acquire their unique characteristics?





Like MORB, subduction zone magmas are a product
of melting of the mantle.
Derived mainly from the “mantle wedge”: region of
mantle above the subducting oceanic lithosphere.
Dehydration of oceanic crust (and perhaps melting
of sediments) generates fluids that rise into the
overlying mantle wedge.
Presence of water reduces the solidus temperature
of peridotite. Thus water released by crust induces
melting in overlying mantle.
Fluids also transfer incompatible elements from the
subducting oceanic crust to the magma generation
zone.
 Most soluble elements (alkalis, alkaline earths, Pb) transferred
readily.
 Insoluble elements, Ta, Nb, not transferred efficiently.
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Explosive, waterrich eruptions
Some fluid
expelled here
Fluid rises into
overlying hot wedge,
induce melting
P & T reach point where hydrous
minerals break down, release water;
fluids dissolve soluble elements
Sediment, and more rarely oceanic crust,
May eventually partially melt
Summary: Significance of
Subduction-related Processes


Societal impact of volcanism and
seismicity
Destruction, or recycling, of continental
crust


Transfer of crustal material to the mantle


perhaps can explain some of the chemical variation
observed in the mantle
Significance for elemental budgets,
particularly for carbon


continental crust is not entirely permanent
does loss of carbon in subduction zones play a role in
the carbon cycle and regulating atmospheric CO2?
Creation of new continental crust
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