Chapter 16. Island Arc Magmatism


Arcuate volcanic island chains along
subduction zones
Distinctly different from mainly basaltic
provinces thus far
 Composition more diverse and silicic
 Basalt generally subordinate
 More explosive
 Strato-volcanoes most common volcanic
landform
Igneous activity related to convergent plate
situations- subduction of one plate beneath
another
 The initial petrologic model:
 Subducted oceanic crust is partially melted
 Partial melts- more silicic than source
 Melts rise through the overriding plate 
volcanoes just behind leading plate edge
 Unlimited supply of oceanic crust to melt
This simple elegant model fails to explain many
aspects of subduction magmatism

The Subduction Factory
From Tatsumi, Y. (2005)
The subduction factory:
How it operates in the
evolving Earth. GSA
Today, 15, 4-10.
Ocean-ocean  Island Arc (IA)
Ocean-continent  Continental Arc or
Active Continental Margin (ACM)
Figure 16.1. Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding
plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.
Subduction Products



Characteristic igneous associations
Distinctive patterns of metamorphism
Orogeny and mountain belts
Complexly
Interrelated
Structure of an Island Arc
Figure 16.2. Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites
and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)
Volcanic Rocks of Island Arcs


Complex tectonic situation and broad spectrum of
volcanic products
High proportion of basaltic andesite and andesite
 Most andesites occur in subduction zone settings
Table 16-1. Relative Proportions of Analyzed
Island Arc Volcanic Rock Types
Locality
B
B-A
A
D
R
2
Mt. Misery, Antilles (lavas)
17
22
49
12
0
2
Ave. Antilles
17
( 42 )
39
2
1
Lesser Antilles
71
22
5
( 3 )
1
Nicaragua/NW Costa Rica
64
33
3
1
0
1
W Panama/SE Costa Rica
34
49
16
0
0
1
Aleutians E of Adak
55
36
9
0
0
1
Aleutians, Adak & W
18
27
41
14
0
2
Little Sitkin Island, Aleutians
0
78
4
18
0
2
Ave. Japan (lava, ash falls)
14
( 85 )
2
0
1
Isu-Bonin/Mariana
47
36
15
1
<1
1
Kuriles
34
38
25
3
<1
2
Talasea, Papua
9
23
55
9
4
1
Scotia
65
33
3
0
0
1
from Kelemen (2003a and personal comunication).
2
after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite
A = andesite, D = dacite,
R = rhyolite
Basalts are still very
common and important!
Major Elements and Magma Series
Tholeiitic (MORB, OIT)
Alkaline (OIA)
Calc-Alkaline (~ restricted to SZ)
Characteristic
Plate Margin
Series
Convergent Divergent
Alkaline
yes
Tholeiitic
yes
yes
Calc-alkaline
yes
Within Plate
Oceanic Continental
yes
yes
yes
yes
Major Elements and
Magma Series
a. Alkali vs. silica
b. AFM
c. FeO*/MgO vs. silica
diagrams for 1946 analyses from
~ 30 island and continental arcs
with emphasis on the more
primitive volcanics
Figure 16.3. Data compiled by Terry
Plank (Plank and Langmuir, 1988)
Earth Planet. Sci. Lett., 90, 349-370.
Figure 16.4 The three andesite series
of Gill (1981). A fourth very high K
shoshonite series is rare. Contours
represent the concentration of 2500
analyses of andesites stored in the
large data file RKOC76 (Carnegie
Institute of Washington).
Figure 16.6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K,
diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably
dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by
vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.
Figure 16.6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows
represent differentiation trends within a series.
Figure 16.6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. The gray arrow
near the bottom is the progressive fractional melting trend under hydrous conditions of Grove et al. (2003).
6 sub-series if combine tholeiite and C-A (some are rare)
May choose 3 most common:
•
Low-K tholeiitic
• Med-K C-A
• Hi-K mixed
Figure 16.5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calcalkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The
points represent the analyses in the appendix of Gill (1981).
Tholeiitic vs. Calc-alkaline differentiation
Figure 16.7. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Calc-alkaline differentiation




Early crystallization of Fe-Ti oxide
Probably related to the high water content of calcalkaline magmas in arcs, dissolves  high fO2
High PH2O also depresses plagioclase liquidus  more
An-rich
As hydrous magma rises, DP  plagioclase liquidus
moves to higher T  crystallization of considerable Anrich-SiO2-poor plagioclase
The crystallization of anorthitic plagioclase and lowsilica, high-Fe hornblende may be an alternative
mechanism for the observed calc-alkaline differentiation
trend
Other Trends
Spatial


“K-h”: low-K tholeiite near trench  C-A 
alkaline as depth to seismic zone increases
Some along-arc as well
 Antilles  more alkaline N  S
 Aleutians is segmented with C-A prevalent
in segments and tholeiite prevalent at ends
Temporal

Early tholeiitic  later C-A and often latest
alkaline is common

REEs





Trace Elements
Slope within series is
similar, but height varies
with FX due to removal of
Ol, Plag, and Pyx
(+) slope of low-K  DM
 Some even more depleted
than MORB!
Others have more normal
slopes
 heterogeneous mantle
sources
HREE flat, so no deep garnet
Figure 16.10. REE diagrams for some representative Low-K (tholeiitic),
Medium-K (calc-alkaline), and High-K basaltic andesites and andesites.
An N-MORB is included for reference (from Sun and McDonough, 1989).
After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag.
MORB-normalized Spider diagrams
 Intraplate OIB has typical hump
Figure 14.3. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall. Data from Sun and
McDonough (1989) In A. D. Saunders and M. J. Norry (eds.),
Magmatism in the Ocean Basins. Geol. Soc. London Spec.
Publ., 42. pp. 313-345.
MORB-normalized Spider diagrams
 IA: decoupled HFS - LIL (LIL are hydrophilic)
What is it about subduction zone setting that
causes fluid-assisted enrichment?
Figure 14.3. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall. Data from Sun and
McDonough (1989) In A. D. Saunders and M. J. Norry (eds.),
Magmatism in the Ocean Basins. Geol. Soc. London Spec.
Publ., 42. pp. 313-345.
Figure 16-11a. MORB-normalized spider diagrams for
selected island arc basalts. Using the normalization and
ordering scheme of Pearce (1983) with LIL on the left and
HFS on the right and compatibility increasing outward from
Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in
yellow.
Isotopes
New Britain, Marianas, Aleutians, and South Sandwich volcanics
plot within a surprisingly limited range of DM
Figure 16.12. Nd-Sr
isotopic variation in some
island arc volcanics.
MORB and mantle array
from Figures 13-11 and
10-15. After Wilson
(1989), Arculus and
Powell (1986), Gill
(1981), and McCulloch et
al. (1994). Atlantic
sediment data from
White et al. (1985).
Pb is quite scarce in the mantle
•
Low-Pb mantle-derived melts susceptible to Pb contamination
•
U, Pb, and Th are concentrated in continental crust (high radiogenic
daughter Pb isotopes)
• 204Pb
non-radiogenic: 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb increase
as U and Th decay
•
Oceanic crust also has elevated U and Th content (compared to the
mantle)
•
Sediments derived from oceanic and continental crust
•
Pb is a sensitive measure of crustal (including sediment) components in
mantle isotopic systems
•
93.7% of natural U is 238U, so 206Pb/204Pb will be most sensitive to a
crustal-enriched component
9-20
9-21
9-22
 234U  206Pb
235U  207Pb
232Th  208Pb
238U
H
I
M
U
Figure 16.13. Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the
Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of
207Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).
10Be
created by cosmic rays + oxygen and nitrogen in upper atmos.
•  Earth by precipitation
• Readily  clay-rich oceanic sediments
• Half-life of only 1.5 Ma
•

Long enough to be subducted

After about 10 Ma 10Be is no longer detectable

Not a part of main mantle systems
10Be/9Be
averages about 5000 x 10-11 in the uppermost
oceanic sediments
• In mantle-derived MORB and OIB magmas, & continental
crust, 10Be is below detection limits (<1 x 106 atom/g) and
10Be/9Be is <5 x 10-14
Boron is a stable element
• Very brief residence time deep in subduction zones
• B in recent sediments is high (50-150 ppm), but has a greater
affinity for altered oceanic crust (10-300 ppm)
• In MORB and OIB it rarely exceeds 2-3 ppm
10Be/Be
total
vs. B/Betotal diagram (Betotal  9Be because 10Be so rare)
Figure 16.14. 10Be/Be(total)
vs. B/Be for six arcs. After
Morris (1989) Carnegie Inst.
of Washington Yearb., 88,
111-123.
Petrogenesis of Island Arc Magmas
Why is subduction zone magmatism a paradox?
Main variables that can affect the isotherms in subduction
zone systems:
1) Rate of subduction
2) Age of subduction zone
3) Age of subducting slab
4) Extent to which subducting slab induces flow in the
mantle wedge
5) Effects of frictional shear heating along W-B zone
Other factors, such as:
• Dip of slab
• Endothermic metamorphic reactions
• Metamorphic fluid flow
are now thought to play only a minor role


Typical thermal model for a subduction zone
Isotherms will be higher (i.e. the system will be hotter) if
a) Convergence rate is slower
b) Subducted slab is young and near the ridge (warmer)
c) Arc is young (< 50-100 Ma according to Peacock, 1991)
yellow curves
= mantle flow
Figure 16.15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).
The principal source components  IA magmas
1. Crustal portion of the subducted slab
1a Altered oceanic crust (hydrated by circulating seawater,
and metamorphosed in large part to greenschist facies)
1b Subducted oceanic and forearc sediments
1c Seawater trapped in pore spaces
Figure 16.15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).
The principal source components  IA magmas
2. Mantle wedge between slab and arc crust
3. Arc crust
4. Lithospheric mantle of subducting plate
5. Asthenosphere beneath slab
Figure 16.15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).



Left with the subducted crust and mantle wedge
Trace element and isotopic data  both contribute
to arc magmatism.
How, and to what extent?
 Dry peridotite solidus too high
 LIL/HFS ratios of arc magmas  water plays a
significant role in arc magmatism
Sequence of pressures and temperatures a rock subjected to
during burial, subduction, metamorphism, uplift, etc. is called
a pressure-temperature-time (P-T-t) path
P-T-t paths for subducted crust
Based on subduction rate of 3 cm/yr
(length of each curve = ~15 Ma)
Subducted Crust
Yellow paths =
various arc ages
Red paths =
different ages of
subducted slab
Figure 16.16. Subducted crust
pressure-temperature-time (P-Tt) paths for various situations of
arc age (yellow curves) and age
of subducted lithosphere (red
curves, for a mature ca. 50 Ma
old arc) assuming a subduction
rate of 3 cm/yr (Peacock, 1991,
Phil. Trans. Roy. Soc. London,
335, 341-353).
Add solidi for dry and water-saturated melting of basalt
and dehydration curves of likely hydrous phases
Subducted Crust
Figure 16.16. Subducted crust
pressure-temperature-time (P-Tt) paths for various situations of
arc age (yellow curves) and age
of subducted lithosphere (red
curves, for a mature ca. 50 Ma
old arc) assuming a subduction
rate of 3 cm/yr (Peacock, 1991).
Included are some pertinent
reaction curves, including the
wet and dry basalt solidi (Figure
7-20), the dehydration of
hornblende (Lambert and
Wyllie, 1968, 1970, 1972),
chlorite + quartz (Delaney and
Helgeson, 1978). Winter (2001).
An Introduction to Igneous and
Metamorphic Petrology.
Prentice Hall.
Mature arcs (lithosphere > 25 Ma): Dehydration D releases water in
No slab melting!
Slab melting M in
arcs subducting
young lithosphere.
Dehydration of chlorite
or amphibole releases
water above the wet
solidus  (Mg-rich)
andesites directly.
Subducted Crust
Newer models allow for temperature and stress dependence of mantle
wedge viscosity. Indicates much higher temperatures in the
shallowest part of the subducted slab.
Figure 16.17. P-T-t paths at a depth of 7 km into the
slab (subscript = 1) and at the slab/mantle-wedge
interface (subscript = 2) predicted by several
published dynamic models of fairly rapid subduction
(9-10 cm/yr). ME= Molnar and England’s (1992)
analytical solution with no wedge convection. PW =
Peacock and Wang (1999) isoviscous numeric model.
vK = van Keken et al. (2002a) isoviscous remodel of
PW with improved resolution. vKT = van Keken et al.
(2002a) model with non-Newtonian temperature- and
stress-dependent wedge viscosity. After van Keken et
al. (2002a) © AGU with permission.
Subducted Crust
Slab melting in
mature arcs no
longer precluded
by models.
Debate renewed.



LIL/HFS trace element data underscore the
importance of slab-derived water and a
MORB-like mantle wedge source
Flat HREE pattern argues against a garnetbearing (eclogite) source
Modern opinion has swung toward the nonmelted slab for most cases
Mantle Wedge P-T-t Paths


Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first?
Figure 16.19. Calculated P-T-t paths for
peridotite in the mantle wedge as it follows paths
similar to the flow lines in Fig 16.15. Included
are dehydration curves for serpentine, talc,
pargasite, and phlogopite + diopside +
orthopyroxene. Also the P-T-t path range for the
subducted crust in a mature arc, and the wet and
dry solidi for peridotite. Subducted crust
dehydrates, and water is transferred to the wedge
(labeled arrows). Areas in which the dehydration
curves are crossed by the P-T-t paths below the
wet solidus for peridotite are stippled and labeled
D for dehydration. Areas in which the
dehydration curves are crossed above the wet
solidus are hatched and labeled M for melting.
Note that although the slab crust usually
dehydrates, the wedge peridotite melts as
pargasite dehydrates (Millhollen et al., 1974)
above the wet solidus. An alternative model
involves dehydration of serpentine  chlorite
nearer the wedge tip (lower-case d) with H2O
rising into hotter portions of the wedge (gray
arrow) until H2O-exess solidus is crossed (lowercase m). A second melting may also occur as
phlogopite dehydrates in the presence of two
pyroxenes (Sudo, 1988). After Peacock (1991),
Tatsumi and Eggins (1995). Winter (2001). An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Crust and
Mantle
Wedge
Island Arc Petrogenesis
Figure 16.18. A proposed
model for subduction zone
magmatism with particular
reference to island arcs.
Dehydration of slab crust
causes hydration of the
mantle (violet), which
undergoes partial melting as
amphibole (A) and
phlogopite (B) dehydrate.
From Tatsumi (1989), J.
Geophys. Res., 94, 4697-4707
and Tatsumi and Eggins
(1995). Subduction Zone
Magmatism. Blackwell.
Oxford.
A multi-stage, multi-source process


Mantle wedge  HFS and other depleted and
compatible element characteristics
Slab dehydration (and perhaps melting)  LIL, 10Be,
B, etc. enrichments + enriched Nd, Sr, and Pb
isotopic signatures
 These components, plus other dissolved silicate
materials, are transferred to the wedge in a fluid
phase (or melt in some cases?)


Phlogopite is stable beyond amphibole breakdown
Wedge P-T-t paths reach phlogopite dehydration at ~ 200 km depth
Fractional crystallization takes place at a number of levels
From Peacock
(2003) Geophysical
Monograph 138 Am.
Geophys. Union