Ocean Island arcs

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Ocean Island arcs

• Ocean-ocean collision zones

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(No Transcript)
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Volcanic Rocks of Island Arcs

• Complex tectonic situation and broad spectrum
• High proportion of basaltic andesite and andesite
• Most andesites occur in subduction zone settings

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Figure 16-6. b. AFM diagram distinguishing
tholeiitic and calc-alkaline series. Arrows
represent differentiation trends within a series.
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Tholeiitic vs. Calc-alkaline differentiation

• C-A shows continually increasing SiO2 and lacks
  dramatic Fe enrichment
• Tholeiitic magmas shallow partial melting of
  mantle
• Calc-alkaline restricted to subduction zones.
  Why?

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Major Elements and Magma Series

• Tholeiitic (MORB, OIB)
• Alkaline (OIB)
• Calc-Alkaline ( restricted to subduction zone)

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Island Arc Petrogenesis
Figure 16-11b. 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.
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Island Arc Petrogenesis

• Altered ocean crust dehydrates at 50 km
• Chlorite dehydrates first
• Amphibole dehydrates at 110 km (point A)
• Slab metamorphosed until 80-100 km
• Water rises into mantle wedge
• Hydrous mantle heats, melts, rises

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How to get calc-alkaline trend

• Crystallize hornblende, anorthite (Ca-rich plag),
  olivine
• How to get hornblende to crystallize?
• Pond magmas in the crust
• Get farther down Bowens reaction series

olivine
Calcic plagioclase
(Spinel)
Mg pyroxene
Calcic-alkalic plagioclase
Continuous Series
Mg-Ca pyroxene
alkali-calcic plagioclase
Discontinuous Series
amphibole
alkalic plagioclase
biotite
Temperature
potash feldspar
muscovite
quartz
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Magma trends in island arcs

• 1st magma tholeiites
• More primitive
• Melts of mantle like OIBs
• 2nd calc-alkaline
• Crust gets thicker
• Magmas stall out more

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Chapter 16. Island Arc Magmatism

• Activity along arcuate volcanic island chains
  along subduction zones
• Distinctly different from the mainly basaltic
  provinces thus far
• Composition more diverse and silicic
• Andesite most common rock
• Basalt generally occurs in subordinate quantities
• Also more explosive than the quiescent basalts
• Strato-volcanoes are the most common volcanic
  landform

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• Igneous activity is related to convergent plate
  situations that result in the subduction of one
  plate beneath another
• The initial petrologic model
• Oceanic crust is partially melted
• Melts rise through the overriding plate to form
  volcanoes just behind the leading plate edge
• Unlimited supply of oceanic crust to melt

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• 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.
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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)
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Structure of an Island Arc

• Subduction rate 0.9-10.8 cm/yr
• Subduction angle 30-90 (45 average)
• Younger the subduction slab, the shallower dip
• Earthquakes as deep at 700 km
• Volcanoes 110 km above slab

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Major Elements and Magma Series

• a. Alkali vs. silica (alkaline rocks minor)
• b. AFM (both types)
• 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.
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Sub-series of Calc-Alkaline

• K2O is an important discriminator ? 3 sub-series
• Low K usually basalt
• Medium and high-K are andesites

Figure 16-4. The three andesite series of Gill
(1981) Orogenic Andesites and Plate Tectonics.
Springer-Verlag. Contours represent the
concentration of 2500 analyses of andesites
stored in the large data file RKOC76 (Carnegie
Institute of Washington).
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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.
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Figure 16-6. c. FeO/MgO vs. SiO2 diagram
distinguishing tholeiitic and calc-alkaline
series.
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Figure 16-6. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
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• 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. calc-alkaline 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).
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• Magmas are differentiated (not primary)
• Only most Mg-rich are nearly primitive mamgas
• Spread in data is due to fractionation of Ol,
  opx, cpx, plag
• Similar to MORB magma source it seems

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Magma source

• Need little Fe-enrichment, Na and K enrichment
• If melting a depleted magma source, need water to
  facilitate melting
• More water can have amphiboles, biotite
• More water changes phase diagram
• Larger olivine field (less Mg, Fe, Ni, Cr in
  melt)
• Smaller plag field (more Na and K in melt)

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Calc-alkaline differentiation

• Early crystallization of an Fe-Ti oxide phase
• Probably related to the high water content of
  calc-alkaline magmas in arcs, dissolves
• The crystallization of anorthitic plagioclase and
  low-silica, high-Fe hornblende is an alternative
  mechanism for the observed calc-alkaline
  differentiation trend

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Petrogenesis of Island Arc Magmas

• Why is subduction zone magmatism a paradox?

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• Rocks heat as subducted
• On normal geotherm, basalt melts at 40 km
• In subduction zones, not until 200 km
• All volcanoes sit 110 km above slab

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Of the many variables that can affect the
isotherms in subduction zone systems, the main
ones are 1) the rate of subduction 2) the age
of the subduction zone 3) the age of the
subducting slab
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• Typical thermal model for a subduction zone
• Isotherms will be higher (i.e. the system will be
  hotter) if
• a) the convergence rate is slower
• b) the subducted slab is young and near the ridge
  (warmer)
• c) the arc is young (lt50-100 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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
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The principal source components ? island arc
magmas
1. The 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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
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The principal source components ? Island arc
magmas
2. The mantle wedge between the slab and the arc
crust 3. The arc crust
Figure 16-15. Cross section of a subduction zone
showing isotherms (red-after Furukawa, 1993, J.
Geophys. Res., 98, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
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• Left with the subducted crust and mantle wedge
• The trace element and isotopic data suggest that
  both contribute to arc magmatism. How, and to
  what extent?
• Dry peridotite solidus too high for melting of
  anhydrous mantle to occur anywhere in the thermal
  regime shown
• water plays a significant role in arc magmatism

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• A multi-stage, multi-source process
• Dehydration of the slab provides the LIL
  enrichments
• These components, plus other dissolved silicate
  materials, are transferred to the wedge in a
  fluid phase (or melt?)
• The mantle wedge provides the depleted and
  compatible element characteristics

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• Trace element data underscore the importance of
  slab-derived water and a MORB-like mantle wedge
  source
• The flat HREE pattern argues against a
  garnet-bearing (eclogite) source
• Thus modern opinion has swung toward the
  non-melted slab for most cases

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• 10Be created by cosmic rays oxygen and nitrogen
  in upper atmos.
• ? Earth by precipitation readily ? clay-rich
  oceanic seds
• Half-life of only 1.5 Ma (long enough to be
  subducted, but quickly lost to mantle systems).
  After about 10 Ma 10Be is no longer detectable
• 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
  (lt1 x 106 atom/g) and 10Be/9Be is lt5 x 10-14

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• B 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

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• 10Be/Betotal vs. B/Betotal diagram (Betotal ?
  9Be since 10Be is 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.