Transcript Introduction to Environmental Geochemistry

Petrology Lecture 6
Generation of Basaltic Magma
GLY 4310 - Spring, 2015
1
Magma Series Definition
• A group of rocks that share some chemical,
and sometimes mineralogical, characteristics
• They share patterns on chemical variation
diagrams which suggests a genetic
relationship – beyond the scope of this course
2
Major Magma Series
• J.P. Iddings, 1892
J.P. Iddings,
1857-1920
(left)
 Alkaline
 Sub-alkaline
• C.E. Tilley (1950) split
sub-alkaline into:
 Tholeittic
 Calc-alkaline
C.E. Tilley,
1894-1973
(right)
3
Relationship of Magma Series
to Plate Tectonics
4
Geotherms
• Estimated ranges of
oceanic (hatched) and
continental (solid)
steady-state geotherms
to a depth of 100 km
using upper and lower
limits based on heat
flows measured near
the surface
5
Sources of Upper Mantle Rocks
• Ophiolites
• Dredge Samples from oceanic fracture zones
• Nodules in basalts
 Autoliths
 Restites
• Xenoliths in kimberlites
• Stony meteorites – analogous to upper mantle
of a broken planet
6
Basaltic Partial Melt
• TiO2 vs. Al2O3
• Extraction of a
basaltic partial melt
from lherzolite can
result in solid
refractory
harzburgite or
dunite
•
Brown and Mussett, A. E.
(1993), The Inaccessible
Earth: An Integrated View
of Its Structure and
Composition. Chapman &
Hall/Kluwer.
7
Lherzolite: A type of peridotite
with Olivine > Opx + Cpx
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Figure 2.2 C After IUGS
Clinopyroxene
8
Aluminous Lherzolite Phase Diagram
• Al – phase
 Plagioclase
o

Spinel
o

50-80 km
Garnet
o

shallow (< 50 km)
80-400 km
Si  VI coord.
o
> 400 km
Figure 10.2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus
reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
9
Mantle
Melting
•Increase in temperature
•Problem: No realistic mechanism
•Perhaps with local hot spots, with very limited area
10
Decompression
Melting
• Decompression melting under adiabatic
conditions
• When adiabat crosses solidus, melting begins
• Dashed lines represent approximate % melting
• At least 30% melting is realistic
11
Addition of Volatiles
Figure 10.5. Dry
peridotite solidus
compared to
several
experiments on
H2O-saturated
peridotites
12
Effect of Pressure on Initial Melting
• Change in eutectic
position with
increasing pressure
• First melting occurs
at the eutectic
• After Kushiro, 1968
13
Pyrolite
Melting
A.E. Ringwood,
1930-1993
• Nature of the liquids and refractory residua
associated with partial melting of pyrolite
• After Green and Ringwood (1967)
14
Fractional
Crystallization
of Basaltic
Magmas
• Minerals
fractionating are
listed near arrows
• After Wyllie,
1971
Peter Wyllie,
1930-
15
Partial Melting and Fractional
Crystallization
16
Trends in Partial
Melts
• Spinel lherzolites, from 6 to 35% partial melt
• Low melt leads to alkaline basalt, higher % melts to
more tholeittic compositions
• After Hirose and Kushiro, 1991
17
Basalt Petrogenesis
18
Magma Types
• Primary Magma - one that forms by melting at
depth, without any later modification
• Derivative - A primary magma that has been
modified by some magma differentiation
process on the way to the surface
• Parental - Most primitive magma type within a
given magma series, it may or may not be
primary
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Multiple saturation

Low P


Ol then Plag then Cpx as
cool
~70oC T range
Figure 10.13 Anhydrous P-T phase relationships for
a mid-ocean ridge basalt suspected of being a
primary magma. After Fujii and Kushiro (1977).
Carnegie Inst. Wash. Yearb., 76, 461-465.
Multiple saturation

Low P
 Ol then Plag then Cpx as
cool
 70oC T range

High P

Cpx then Plag then Ol
Figure 10.13 Anhydrous P-T phase relationships for
a mid-ocean ridge basalt suspected of being a
primary magma. After Fujii and Kushiro (1977).
Carnegie Inst. Wash. Yearb., 76, 461-465.
Multiple saturation

Low P
 Ol then Plag then Cpx as
cool
 70oC T range

High P


Cpx then Plag then Ol
25 km get all at once


= Multiple saturation
Suggests that 25 km is the
depth of last equilibrium
with the mantle
OIB and MORB
• Upper, REE
diagram
• Lower, Spider
diagram
• Data from Sun and
McDonough, 1989
23
LREE depleted
or unfractionated
REE Variation
LREE enriched
LREE depleted
or unfractionated
LREE enriched
• Chondritenormalized REE
diagrams for spinel
and garnet
lherzolites
• After Basaltic
Vocanism Study
Project, 1981
24
143Nd/144Nd
vs.
87Sr/86Sr
• Upper, oceanic basalts
• Lower, ultramafic
xenoliths from
subcontinental mantle
• MAR = Mid-Atlantic
Ridge
• EPR = East Pacific Rise
• IR = Indian Ocean Ridge
25
Mantle
Convection
Models
• After Basaltic
Vocanism
Study Project,
1981
26
Partial Melting Experiments
• “Opx out” and
“Cpx out” =
degree of melting
at which these
phases are
completely
consumed
• Left, depleted lherzolites; Right, fertile lherzolites
• Dashed lines = % partial melt produced
• Shaded area = condition required for the generation of
alkaline basaltic magmas
27
Changing ΔV
• ΔV approaches zero
on going from solid
to liquid as pressure
increases
• Causes both solidus
and liquidus to
change slope as
depth increases
28