Download Igneous Geochemistry OUTLINE

11/14/16
Igneous
Geochemistry
B
Al2O3
BA
A
D RD
R
16
14
10
6
8
MgO
Fe2O3
2
4
0
8
4
4
Na2O
CaO
0
3
2
4
2
K2O
0
45
55
65
75
Wt. % SiO2
OUTLINE
Reading this week:
White Ch 7
Note: Thu will be in-class exercise (hands-on)
Today
1.  Finish making the crust
2.  Major elements
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11/14/16
Continental crust age distribution
Low density continental crust does not subduct, it just folds.
⇒ Continents up to 4 Ga, only continental mass recycled is small
amounts of sediment on oceanic plates (small flux)
⇒ Land
keeps
being
added
Where to add to a continent?
• At convergent plate margins (volcanic arcs) – water added to the
mantle from the subducted lithosphere causes melting - flux
melting - calc-alkaline basalt (so still not silicic)
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Adding mass to a
continent
Step 1: accrete terranes to
the continental margin; i.e.
blocks of unrelated origin
got assembled together
Model would be initially to
have island arcs collide
Make the granitoids
•  Within the continental arcs
•  Great example: coastal batholiths
•  What we think happens:
–  Existing low(er) SiO2 rocks get reheated by
repeated intrusion and remelt/mix (just the
low-temperature melting components)
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Compositions by Goldschmidt’s classes
Split “primitive mantle” to crust, mantle; elements divided:
Lithophiles mostly in crust; ionic bonds; large ions. O, Mg, Fe, Si
in mantle too
Chalcophiles
split between
mantle, crust,
core; covalent
Siderophiles
mostly in the
core (metal)
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Element
O
Si
Al
Fe
Ca
Mg
Na
Wt % Oxide Atom %
60.8
59.3
21.2
15.3
6.4
7.5
2.2
6.9
2.6
4.5
2.4
2.8
1.9
Major elements: usually greater than 1%
SiO2 Al2O3 FeO* MgO CaO Na2O K2O H2O
Minor elements: usually 0.1 - 1%
TiO2 MnO P2O5 CO2
Trace elements: usually < 0.1%
everything else
Ta ble 8-3. Che mica l a na lyse s of some
re pre se nta tive igne ous rocks
Peridotite
Basalt Andesite Rhyolite Phonolite
SiO2
42.26
49.20
57.94
72.82
56.19
TiO2
0.63
1.84
0.87
0.28
0.62
Al2O3
4.23
15.74
17.02
13.27
19.04
Fe2O3
3.61
3.79
3.27
1.48
2.79
FeO
6.58
7.13
4.04
1.11
2.03
MnO
0.41
0.20
0.14
0.06
0.17
MgO
31.24
6.73
3.33
0.39
1.07
CaO
5.05
9.47
6.79
1.14
2.72
Na2O
0.49
2.91
3.48
3.55
7.79
K2O
0.34
1.10
1.62
4.30
5.24
H2O+
3.91
0.95
0.83
1.10
1.57
Total
98.75
99.06
99.3
99.50
99.23
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Major elements reflect melts vs crystals
Major elements: make up essential parts of crystal lattice
⇒ Distribution between melt and crystals follows
stoichiometry of minerals (mineral compositional formula)
⇒ Mostly: melt and crystals are not identical in composition:
separating melt from crystals allows for compositional
change
⇒ This works for both partial melting, fractional crystallization
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Balance liquid and crystals
to bulk composition (B)
= mass balance
Key:
B=bulk, L=liquid, P-S=crystals
In cooling melt, crystals
CONTROL liquid path
(arrow points away from
crystal composition)
Example for olivine
crystallization: melt
follows olivine control
line.
Melt evolution = liquid line
of descent
B
Harker diagram
–  Tight (smooth) trends
–  Model with 3 assumptions:
1 Rocks are related
2 Trends = liquid line of
descent (mineral control)
3 The basalt is the parent
magma from which the
others are derived
Al2O3
BA
A
D RD
R
16
14
10
6
8
MgO
Fe2O3
2
4
0
8
4
4
Na2O
CaO
0
3
2
4
2
B=basalt, BA=basaltic-andesite, A=andesite,
D=dacite, RD=rhyo-dacite, R=rhyolite
K2O
0
45
55
65
75
Wt. % SiO2
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11/14/16
http://www.geol.lsu.edu/henry/Geology3041/lectures/12LayeredMafic/Fig12-15.jpg
End-members
80%
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Other ways:
Assimilation
Chemical classification of volcanic rocks
13
fractional
crystallization
partial
melting
9
Phonolite
Tephriphonolite
11
Wt.% Na2O+K2O
⇒ partial
melting,
fractional
crystallization
are simplest
ways to move
around in this
plot
Phonotephrite
(Foid)ite
Trachyte
Trachy- Trachydacite
andesite
Rhyolite
Basaltic
trachyTephrite
Basanite Trachy- andesite
7
basalt
5
3
Basalt
Dacite
Basaltic
Andesite
Andesite
Picrobasalt
1
37
41
ULTRABASIC
45
45
49
BASIC
53
57
61
52 INTERMEDIATE
65
63
69
73
77
ACIDIC
wt% SiO2
Winter Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica.
After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.
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12
%Na2O + K2O
10
Alkaline
8
6
4
2
Subalkaline
40
35
45
50
55
60
65
%SiO2
Differences in ocean basins
Tholeiitic Basalt and Alkaline Basalt
Tholeiitic Basalt
Groundmass
Usually fairly coarse, intergranular to ophitic
No olivine
Olivine common
Clinopyroxene = augite (plus possibly pigeonite)
Titaniferous augite (reddish)
Orthopyroxene (hypersthene) common, may rim ol.
Orthopyroxene absent
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may occur
Interstitial glass and/or quartz common
Olivine rare, unzoned, and may be partially resorbed
Phenocrysts
Alkaline Basalt
Usually fine-grained, intergranular
Interstitial glass rare, and quartz absent
Olivine common and zoned
or show reaction rims of orthopyroxene
Orthopyroxene uncommon
Orthopyroxene absent
Early plagioclase common
Plagioclase less common, and later in sequence
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).
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Tholeiites and alkali basalts: both made in Hawai‘i
Lots of melt in the middle = tholeiite, periphery = alkalic
(Ribe & Christiansen 1999)
=> Major
elements
depend on P,T
thus location in
plume
Bianco et al., 2008
Tholeiites and alkali basalts: both made in Hawai‘i
Decompression in
hotspots limited by plate
thickness; due to high T,
melt relatively deep up to
plate’s bottom
(Ribe & Christiansen 1999)
At ridge, plate = crust @ axis, so
decompression up to ~8 km:
melt from less deep, but all the
way to ~8 (instead of ~80) km:
makes high degree melt
tholeiites
Dickin’s book
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