Download 1 Island Arc Magmatism

Island Arc Magmatism
Wilson p. 153-225
•
In this lecture:
–
Where & what are island arcs?
•
Why so important?
–
Diverse magma sources
–
Partial melting
–
Magma segregation, ascent, storage
–
Magma series
–
Trace elements in arc magmas
–
Isotopic compositions
•
Subducted components
•
Differentiation mechanisms
–
Petrologic model
–
Implications for mantle evolution
Island arcs
Eleven major active island arcs
–
Destructive margins
–
Subduction of oceanic plate
beneath adjacent ocean plate
Importance
–
Hazards
Explosive volcanism; Earthquakes
Millions inhabit arcs (Indonesia,
Japan, Phillipines, etc.)
–
Chemical exchange & recycling
Crust ÍÎ Mantle
Genesis & evolution of arc magmas
–
Critical to understand origin of
hazards & element recycling
1
Sources of island arc magma
JOIDES Resolution
Mantle wedge above subducted slab
1. 40-70 km thick oceanic lithosphere
depleted mantle = refractory lherzolite+harzburgite
2. Asthenospheric mantle
fertile lherzolite; thickness a function of slab dip
Ocean crust
1. Metamorphosed basalt, gabbro
note: facies vary w/increasing P-T of slab
2. Ocean sediment (clay, CaCO3, clastics)
May become involved:
Deep: upper part of subducted slab
Shallow: base of island arc volcanic sequence
Sea water
1. H2O component fundamental to arc
magmatism
2. Incorporated during hydrothermal alteration
and ocean-floor (low P) metamorphism of
ocean crust
Thermal structure & partial melting
JOIDES Resolution
Thermal structure critical
magma generation
seismicity
Numerical Models
convection in asthensophere
dehydration of subducting crust
frictional heating upper slab surface
2
Thermal structure & partial melting
JOIDES Resolution
Temperature distribution
a major control on location of partial
melting within the mantle wedge
or the subducted slab
Enigma
cold slab “refrigerates” the mantle
the slab is too cold to melt under most
circumstances
yet, melting and volcanism occur
WPS:
wet peridotite
solidus
Thermal structure & partial melting
JOIDES Resolution
Partial Melting of potential sources
Subducted ocean crust
1. Meta-basalt
amphibolite or eclogite
2. Meta-sediments
2. Fluid
Prograde metamorphism
Progressive dehydration at higher P-T
H2O-saturated vs. anhydrous melting of basalt
Arc
Basalt
Liquidus
Temperature of wet melting does not match
arc basalt liquidus range
Wet eclogite melting >150 km depth?
does not explain arc position (see fig. 6.8)
Subducted sediment melting?
the jury is……..out.
3
Thermal structure & partial melting
JOIDES Resolution
Partial Melting of potential sources
The mantle wedge
Lherzolite fluxed with:
1. H2O-rich fluid
2. H2O-rich partial melt from slab
Lowers dry solidus below mantle
wedge geotherm
Partial melting experiments
indicate that basalt can be generated
from lherzolite in presence of small %
of H2O
Basalt is parent magma to spectrum
of andesite-rhyolite in island arcs
Segregation, ascent, storage of magma
Partial melt segregates from asthenosphere
polybaric melting & segregation
percolative flow vs. fracture transport?
Ponding of magma in high-level reservoirs
fractional crystallization
ground-surface deformation
caldera formation
S-wave attenuation
Low-P fractional crystallization
Harker diagrams
cumulate-textured plutonic xenoliths:
oliv + cpx + opx + plag + amph + mag
plagioclase suggests P<10 kbar (<30 km)
amphibole implies hydrous melts at depth
lack of amphibole in erupted lavas due to resorption
4
Magma series and differentiation
JOIDES Resolution
K2O vs. SiO2
low-K series (island arc tholeiite series)
calc-alkaline series
high-K series
shoshonitic series (alkaline series)
FeO*/MgO vs. SiO2 or AFM
tholeiitic series
calc-alkaline series
Island arcs may contain both CA and
TH volcanoes! (see Aleutian examples)
Aleutian Island arc
Seguam Island: Tholeiitic shield volcano
Kanaga Island: calc-alkaline stratovolcano
5
Pyroclastic flows
Geologic
features at
Seguam
Older, Pleistocene lavas
5 km
1993 Basalt
1977 Basalt
Eastern collapse calderas
93.1 + 9.5 ka
Seguam Island, central Aleutians
Pleistocene lavas and tephras
Holocene basalt
6
Kanaga
Volcano
Kanaga Island
Kanaton Ridge
199.1 + 2.5 ka
198.1 + 2.1 ka
Tkb
QTb
Kana
to n R
i dg e
Photos courtesy of
Dörte Mann
7
JOIDES Resolution
Major and trace element composition of magmas
Major elements
Island arc basalts similar to other oceanic basalts except lower in Ti
Fractional crystallization produces more SiO2 rich magmas
Trace elements (relative to N-MORB)
Enriched in low ionic potential elements; LILE + Th
fluid mobilized elements added to mantle wedge?
Low in high ionic potential elements; Ta, Nb, Zr, Hf, REE
larger degree of melting ? residual phase(s) [rutile, zircon, sphene] during melting ?
JOIDES Resolution
Trace element composition of magmas
Island arc basalts distinguished from MORB
LILE and REE variations, e.g., Ba/La vs. La/Sm
MORB-normalized differences
Negative Nb anomaly
Nb
Slab
component
8
JOIDES Resolution
Radiogenic isotopes
Sr and Nd isotopes
mixing between mantle & crustal components (compare to MORB and OIB)
mass balance of Sr and Nd: source contamination vs. crustal assimilation
terriginous sediment in source of Lesser Antilles & Sunda arcs
Ocean
basalt
array
lower
older
crust
upper
younger
crust
JOIDES Resolution
Radiogenic isotopes
Pb isotopes
Pb contents (>20 ppm) and isotopic ratios of sediments very high
Pb content (< 1 ppm) and isotopic ratios of mantle are low
Thus Pb is a sensitive tracer of sediment involvement in magma source
Lesser Antilles arc lavas
Pb ratios both higher and
lower than Atlantic sediments
source contamination and
crustal assimilation?
9
JOIDES Resolution
Beryllium isotope data
The isotope 10Be
produced by cosmic ray induced rxns in atmosphere
transported to surface pelagic sediments via rain & snow
half-life is 1.5 x 106 yr
tracer for young marine sediment in arc magma source
10Be
contents of basalts
below detection in MORB, OIB
high in basalt from some arcs
thus in some arcs:
uppermost, young sediments are
not accreted, they subduct and
either melt or release 10Be rich
fluid to the mantle in < 10 myr
General model of island arc magmatism
JOIDES Resolution
Major element, trace element and
isotopic ratios indicate that at
most a very small weight %of arc
magma comprises subducted
crustal elements.
Thus most of the subducted crust
bypasses the arc and descends
into the deeper mantle.
Perhaps in cases like the Farallon
plate crust travels all the way to
the core-mantle boundary
10
Modes of mantle convection
The “hybrid” model
One layer or two?
1. Neither the 410 or 660
discontinuity seem to act
as a barrier to flow
One layer: the transition zone phase
transitions do not prevent mass flux across
the 410/660 discontinuity
2. Still need a chemically
distinct source
Two layers: There still needs to be
chemically distinct regions
3. New boundary: around
1600 km depth with
small density contrast
Kellogg 1999
Crust generated at ridges is stirred into the deep mantle below island arcs
11