Diversification 2
Today
Updates:
•Lab progress?
Topics:
•Evolving melt compositions:
Magma mixing
Assimilation
Magmatic Differentiation
Stages in ascent







Eruption
(Fragmentation)
Vesiculation
Renewed ascent
Storage
 mixing
 assimilation
 crystallization
Buoyant ascent
Partial melting
Processes during storage in
magma chambers
Magma Mixing
+
=
Magma Mixing
15
End-members
10
5
80%
0
45
50
55
60
SiO
65
70
75
22
Mixing vs
fractionation
10
Al2O3
MgO
17
5
0
12
15
FeO* 10
10
5
CaO
5
0
0
4
6
3
Na2O
4
2
2
0
45
1
0
50
55
60
SiO2
65
70
75 45
50
55
60
SiO2
65
70
75
K2O
Mixing vs mingling
Comingled basalt-Rhyolite
Mt. McLoughlin, Oregon
Figure 11-8 From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall
Basalt pillows
accumulating at the bottom
of a in granitic magma
chamber, Vinalhaven
Island, Maine
A Magma Mixing/Mingling
B
Assimilation
Assimilation-diffusion
http://iramis.cea.fr/Images/astImg/308_2.jpg
Assimilation-xenoliths
Zone melting
Melt
Crystallize
Detecting and assessing assimilation
Figure 9-13.
After Wilson
(1989). Igneous
Petrogenesis.
Unwin
Hyman/Kluwer.
Combinations
Compositional effects of magma
chamber processes
Renewed ascent







Eruption
(Fragmentation)
Vesiculation
Renewed ascent
Storage
 mixing
 assimilation
 crystallization
Buoyant ascent
Partial melting
Common evidence for mixing:
Eruptions (renewed ascent) driven
by new magma entering magma
chamber
At or near surface







Eruption
(Fragmentation)
Vesiculation
Renewed ascent
Storage
 mixing
 assimilation
 crystallization
Buoyant ascent
Partial melting
Vesicles show gasses unmix:
volatile phases escape from melt
Does not affect major or (most)
trace elements
Possible Origin
Eruptions are triggered by
mixing events.
Mixing in magma chambers
generates andesite and dacite
mantle
Partial melting of “crust” ->
rhyolitic magma
Storage -> crystal
fractionation: basaltic andesite
Partial melting in mantle yields
basaltic magma