Download Melting of the mantle

Melting of the mantle
‰ Increasing temperature: Intraplate igneous
activity (OIB, continental basalts etc.)
‰ Decreasing
D
i pressure: Divergent
D
plate
l
boundaries (MORBs, rifts, back-arc basins
etc.))
‰ Adding volatiles: Convergent plate
boundaries (arc lavas, continental margins,
etc.)
Plate Tectonic - Igneous
g
Genesis
1. Mid-Ocean Ridges
g
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental
Margins
g
5. Back-Arc Basins
6. Ocean Island Basalts
7. Miscellaneous IntraC ti t l Activity
Continental
A ti it
kimberlites, carbonatites,
anorthosites...
Ocean-ocean → Island Arc (IA)
Ocean-continent → Continental Arc or
A i Continental
Active
C i
l Margin
M i (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.
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
• C
Complex
l tectonic
i situation
i i andd broad
b d spectrum off
volcanic products
• High
Hi h proportion
ti off basaltic
b lti andesite
d it andd andesite
d it
– Most andesites occur in subduction zone settings
Table 16-1. Relative Proportions
p
of Analyzed
y
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
L
Lesser
A
Antilles
till
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
Talasea,
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!
Classification of
Igneous Rocks
Figure 2.3. A classification and nomenclature
of volcanic rocks
rocks. After IUGS
IUGS.
Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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
tl (violet),
( i l t) which
hi h
undergoes partial melting as
amphibole (A) and
phlogopite (B) dehydrate.
From Tatsumi (1989), J.
Geophys. Res., 94, 4697
4697-4707
4707
and Tatsumi and Eggins
(1995). Subduction Zone
Magmatism. Blackwell.
Oxford.
Island Arc Petrogenesis
‰Altered oceanic crust begins to dehydrate at depths ~ 50 km or less,
as chlorite, phengite, and other hydrous phyllosilicates decompose.
‰Further dehydration takes place at greater depths as other hydrous
phases become unstable, including amphibole at about 3 GPa.
‰The slab crust is successively converted to blueschist, amphibolite,
and finally anhydrous eclogite as it reaches about 80-100 km depth.
‰In most (mature) arcs, the temperature in the subducted crust is
below the wet solidus for basalt,, so the released water cannot cause
melting, and most of the water is believed to rise into the overlying
mantle wedge, where it reacts with the lherzolite to form a pargasitic
amphibole and probably phlogopite (yellowish area)
‰Slightly hydrous mantle immediately above the slab is carried
downward by induced convective flow where it heats up, dehydrates,
andd melts
l at A (120
(
k )
km)
‰Fractional crystallization happens in the arc crust
Continental Arc
Magmatism
Figure 17.2. Schematic diagram to illustrate how a
shallow dip of the subducting slab can pinch out the
asthenosphere from the overlying mantle wedge.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Continental Arc Magmatism
Potential differences with respect to Island Arcs:
• Thick sialic crust contrasts greatly with mantlederived partial melts Æ more pronounced effects of
contamination
• Low density of crust may retard ascentÆ stagnation
of magmas and more potential for differentiation
• Low meltingg point
p
of crust allows for ppartial meltingg
and crust-derived melts
Continental Arc Petrogenesis
Figure 17.23. Schematic cross section of an active continental margin subduction zone, showing the dehydration of the subducting slab,
hydration and melting of a heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle), crustal underplating of
mantle-derived melts where MASH processes may occur, as well as crystallization of the underplates. Remelting of the underplate to
produce tonalitic magmas and a possible zone of crustal anatexis is also shown. As magmas pass through the continental crust they may
differentiate further and/or assimilate continental crust. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.
Chapter 17: Continental Arc Magmatism
Figure 17-24. Pressure-temperature phase diagram showing the solidus curves for H2O-saturated and dry granite. An H2O-saturated
granitoid just above the solidus at A will quickly intersect the solidus as it rises and will therefore solidify. A hotter, H2O-undersaturated
granitoid at B will rise further before solidifying. Note: the pressure axis is inverted to strengthen the analogy with the Earth, so a
negative dP/dT Clapeyron slope will appear positive. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Overall Conclusions and Andesite Petrogenetic
g
Model: I
‰Strong link between convergent plate boundaries and CalcCalc
alkaline volcanism (and associated intrusives).
‰Andesite is the dominant rock type found in most island
arcs; more common in older, more mature arcs, such as
Japan; while basalts and basaltic-andesite are more common
on younger,
younger less mature arcs
arcs, such as the Marianas
Marianas.
‰Volcanism is commonly aligned along a front that is
between 100 to 200 km above the top
p of the subducting
g slab.
Overall Conclusions and Andesite Petrogenetic
g
Model: II
‰Most island arc lavas are extremely phenocryst rich; their
bulk composition then do not likely represent true liquid
compositions.
‰Smooth
‰S
th Harker
H k di
diagrams ffor allll major
j elements.
l
t IImplies
li
common liquid-line of descent driven by fractional
crystallization
y
of SiO2-poor
p
and FeO-rich such as Timagnetite and amphibole.
‰Sr-enrichment from seawater alteration of hydrated
basaltic oceanic crust and Pb-enrichment from terrigenous
sediments (1-3%).
Overall Conclusions and Andesite Petrogenetic Model: III
‰Volatiles
‰V
l til from
f
descending
d
di slab
l b are lib
liberated
t d iinto
t overlying
l i
mantle wedge. Initiates partial melting yielding water-bearing
basalts. Basalts are enriched in the “subduction” component
p
(LIL and LREE, Sr, Pb enriched). The basalts transit mantle
wedge and in older arcs likely pond at MOHO, where they
may melt the lower crust and differentiate by fractional
crystallization. More evolved magmas (lower density) rise
into mid-crust and periodically erupt.
‰Some evidence for repeated basalt injections into midcrustal storage
g zones in the form of zoned crystals,
y
variable
Fe-Ti oxide-derived temperatures, and textures of “mafic”
inclusions.