The Andesite line Our
and purpose
Benioff zones Use the exotic chemistry of arc lavas to constrain the geology and dynamics of subduction...
Benioff, 1954
Marshall, 1911
...and use the geology and dynamics of subduction to understand
the exotic chemistry of arc lavas.
Conceptual models old and new
andesitic
crust
basaltic crust
hydrous,
andesitic
melt
han
gi
foo ngwa
t wa l
l
peridotite
dehydration
metamorphism
peridotite
back-arc
rifting
and volcanism
mixing
Trench
Basalt
depletedresidues
peridotitic
lithosphere
segregation and
differentiation
'wet' garnet
pyroxenite
melting
decompression
melti ng (±added
water)
dehydration;
metamorphism
eclogite melting
?
pyrolite
Slab melting drives arc magmatism
Ringwood, 1974;
Eiler, 2003, after various sources
Lithospheric
mantle
fluxed
melting
f
noasalt
o
i
t
b
ra
hyducted
e
dubd
s
gite
o
l
c
de
cte
u
ubd
s
y
pl
dee
Slab dehydration drives arc magmatism
Basaltic portions of subducted slabs should not melt
except under unusual circumstances
5
'typical' slab
geotherms
wet basalt
solidus
4
P
(GPa)
3
Hbl
out
2
Ec
1
'Bs'
'Gs'
dry basalt
solidus
'Gr'
'Am'
200
Peacock, 1996
400
600
T (˚C)
800
1000
1200
Basalts and basaltic andesites are the most common
products of arc volcanism
Dacite and Rhyolite
P
Basalt
augite
Siliceous Andesite
olivine
p
Basaltic Andesite
3400 lavas from recently active arc volcanoes
Aleutians, Antilles, Bismark, Central America,
Hokkaido, Japan, Java, Kermadec, Izu-BoninMarianas, Mexico, Philippines, Ryuku, Solomons,
South Sandwich, Sumatra, Tonga, Vanuatu
o
Ol
projected from plagioclase
p
x
i
g
Qtz
• Many (most?) of the more siliceous lavas are differentiation+mixing
products of basaltic magmatic systems
• Most primitive arc basalts are derived from, or last equilibrated with,
Lherzolites (fertile perditotites).
Gill, 1981; Grove, 1987
Water in terrestrial basalts
P (kb)
0.5 1.0
2.0
0.1
3.0
MORB
lavas
10
0
inclusions
4
0
Back-arc
lavas
10
0
inclusions
4
0
Arc
lavas
4
0
inclusions
4
0
2
4
wt. % H2O
6
Stability limits of hydrous minerals and the zone of melting
beneath convergent margins
0
amphibole
2
1100˚C
Melting
1400˚C
4
zoisite
P
(GPa) 6
e
rp
e
s
8
lawsonite
10
phengite
Schmidt and Poli, 1998
'A'
in
nt
e chloritoid
Peculiar reasons why slabs melt
176 ˚W
156 ˚W
Bering sea
S
Shiveluch
Kluchevskoy
67
K
63
SiO2
59
55
Pacific
51
2
4
FeO*/MgO
K
S
Yogodzinski et al., 2001