Transcript General

Dry Mantle Melting and
the Origin of Basaltic
Magma
GEOS 508 Lec 14-15-16
Mantle melting
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Needs special condition to melt, usually
solid;
Melt fractions usually low, under 2% at any
given time;
Regardless of melting conditions, yields one
or another variety of basaltic (low silica)
melts - andesites, dacites, do not come out
of the mantle;
2 principal types of basalt in
the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Common petrographic differences between tholeiitic and alkaline basalts
Tholeiitic Basalt
Groundmass
Usually fine-grained, intergranular
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
Interstitial glass rare, and quartz absent
Olivine rare, unzoned, and may be partially resorbed
Phenocrysts
Alkaline Basalt
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).
Each is chemically distinct
Evolve via FX as separate series
along different paths
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Tholeiites are generated at mid-ocean ridges
 Also generated at oceanic islands,
subduction zones
Alkaline basalts generated at ocean islands
 Also at subduction zones
Sources of mantle material
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Ophiolites
 Slabs
of oceanic crust and upper mantle
 Thrust at subduction zones onto edge of continent
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Dredge samples from oceanic fracture zones
Nodules and xenoliths in some basalts
Kimberlite xenoliths
 Diamond-bearing
pipes blasted up from the
mantle carrying numerous xenoliths from depth
Lherzolite is probably fertile unaltered mantle
Dunite and harzburgite are refractory residuum after basalt has been
extracted by partial melting
Tholeiitic basalt
15
10
Brown and Mussett, A. E. (1993),
The Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall/Kluwer.
5
Lherzolite
Harzburgite
Dunite
0
0.0
0.2
Residuum
0.4
Wt.% TiO2
0.6
0.8
Lherzolite: A type of peridotite
with Olivine > Opx + Cpx
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Phase diagram for aluminous
4-phase lherzolite:
Al-phase =
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Plagioclase
 shallow
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Spinel
 30-80
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km
Garnet
 80-400

(< 30 km)
km
Si  VI coord.

> 400 km
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.
How does the mantle melt??
1) Increase the temperature
Melting by raising the temperature.
2) Lower the pressure
 Adiabatic
rise of mantle with no conductive heat loss
 Decompression melting could melt at least 30%
Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the
solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.
HW 7
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Use pMELTS to determine if the sub SWUS
upwelling mantle would melt along an adiabat that
contains the PT point of 14500C and 15 kbar. A
typical composition of the SWUS mantle is given
via a San Carlos peridotite; calculate the fraction
of melt and phases in equilibrium with the liquid?
What is the composition and melt fraction at 10
kbar (at the Moho), assume 1390C?
3) Add volatiles (especially H2O)
Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
15%
Fraction melted is
limited by
availability of water
From Burnham and Davis (1974). A J Sci., 274, 902940.
20%
50% 100%
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Heating of amphibole-bearing peridotite
1) Ocean geotherm
2) Shield geotherm
Phase diagram (partly schematic) for a
hydrous mantle system, including the H2Osaturated lherzolite solidus of Kushiro et
al. (1968), the dehydration breakdown
curves for amphibole (Millhollen et al.,
1974) and phlogopite (Modreski and
Boettcher, 1973), plus the ocean and shield
geotherms of Clark and Ringwood (1964)
and Ringwood (1966). After Wyllie (1979). In
H. S. Yoder (ed.), The Evolution of the
Igneous Rocks. Fiftieth Anniversary
Perspectives. Princeton University Press,
Princeton, N. J, pp. 483-520.
Melts can be created under
realistic circumstances
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Plates separate and mantle rises at midocean ridges
 Adibatic rise  decompression melting
Hot spots  localized plumes of melt
Fluid fluxing may give LVL
 Also important in subduction zones and
other settings
Generation of tholeiitic and
alkaline basalts from a
chemically uniform mantle
Variables (other than X)
 Temperature
 Pressure
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.
Pressure effects:
Figure 10-8 Change in the eutectic
(first melt) composition with
increasing pressure from 1 to 3 GPa
projected onto the base of the basalt
tetrahedron. After Kushiro (1968), J.
Geophys. Res., 73, 619-634.
Liquids and residuum of melted pyrolite
Initial Conclusions:
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Tholeiites favored by shallower melting
 25% melting at <30 km  tholeiite
 25% melting at 60 km  olivine basalt
Tholeiites favored by greater % partial melting
 20 % melting at 60 km  alkaline basalt

incompatibles (alkalis)  initial melts
 30
% melting at 60 km  tholeiite
Crystal Fractionation of magmas
as they rise
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Tholeiite  alkaline
by FX at med to high P
Not at low P
 Thermal divide
Al in pyroxenes at Hi P
 Low-P FX  hi-Al
shallow magmas
(“hi-Al” basalt)
Primary magmas
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Formed at depth and not subsequently modified by
FX or Assimilation
Criteria
 Highest Mg# (100Mg/(Mg+Fe)) really  parental
magma
 Experimental results of lherzolite melts
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Mg# = 66-75
Cr > 1000 ppm
Ni > 400-500 ppm
Multiply saturated
Multiple saturation
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Low P
 Ol then Plag then
Cpx as cool
 ~70oC T range
Figure 10-12 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
Anhydrous P-T phase relationships for a midocean 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
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High P
 Cpx

then Plag then Ol
25 km get all at once
=
Multiple saturation
 Suggests that 25 km is
the depth of last eqm
with the mantle
Summary
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A chemically homogeneous mantle can
yield a variety of basalt types
Alkaline basalts are favored over tholeiites
by deeper melting and by low % PM
Fractionation at moderate to high depths can
also create alkaline basalts from tholeiites
At low P there is a thermal divide that
separates the two series
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QuickTime™ and a
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QuickTime™ and a
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QuickTime™ and a
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QuickTime™ and a
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Review of REE
sample/chondrite
10.00
8.00
6.00
4.00
2.00
0.00
La Ce
Nd
Sm Eu
Tb
Er
atomic number
increasing incompatibility
Yb Lu
Review of REE
Rare Earth concentrations
(normalized to chondrite) for
melts produced at various
values of F via melting of a
hypothetical garnet lherzolite
using the batch melting model
(equation 9-5). From Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall.
increasing incompatibility
REE data for oceanic basalts
increasing incompatibility
REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean
ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Data from Sun and McDonough (1989).
Spider diagram for oceanic basalts
increasing incompatibility
Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean
ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Data from Sun and McDonough (1989).
LREE depleted
or unfractionated
LREE enriched
REE data
for UM
xenoliths
LREE depleted
or unfractionated
Chondrite-normalized REE diagrams for spinel (a)
and garnet (b) lherzolites. After Basaltic
Volcanism Study Project (1981). Lunar and
Planetary Institute.
LREE enriched
Review of Sr isotopes
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l = 1.42 x 10-11 a
Rb (parent) conc. in enriched reservoir (incompatible)
Enriched reservoir
develops more
87Sr over time
Depleted reservoir
(less Rb)
develops less
87Sr over time
87Rb  87Sr
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
Review of Nd isotopes
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l = 6.54 x 10-13 a
Nd (daughter)  enriched reservoir > Sm
Enriched res.
develops less
143Nd over time
Depleted res.
(higher Sm/Nd)
develops higher
143Nd/144Nd
over time
147Sm  143Nd
REE diagram
Nd
Sm
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.
mantle model II
Upper depleted mantle = MORB source
 Lower undepleted & enriched OIB source

After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
Mantle convection model I
After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute.
Nd and Sr isotopes of Ocean Basalts
“Mantle Array”
Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic basalts. From Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).
Zindler-Hart
0.5135
DMMb
DMMa
0.5130
PREMA
143Nd/144Nd
HIMU
BSE
0.5125
EMI
0.5120
0.701
0.702
0.703
0.704
0.705
87Sr/86Sr
0.706
0.707
0.708
0.709
Zindler-Hart
DMMb
0.5135
DMMa
MORB
0.5130
PREMA
143Nd/144Nd
HIMU
BSE
0.5125
EMI
EMII
0.5120
15
16
17
18
19
206Pb/204Pb
20
21
22
Zindler-Hart
16.0
15.9
15.8
EMII
HIMU
15.7
BSE
15.6
207Pb/204Pb
15.5
PREMA
EMI
15.4
DMMa
15.3
15.2
DMMb
15.1
15.0
15
16
17
18
19
206Pb/204Pb
20
21
22
OAHU
Pali
Koolau
C al de ra
1.
Al iman u
Mak alapa
2.
Kah li
Pali
S alt Lak e
Nu u an u
Kaau
Significance of the
Koolau component
From Lassiter &Hauri, 1996
Koolau component-recycled crust?
Lassiter and Hauri, 1998
Eiler et al., 1996
2mm (a,b,c), 1mm (d)
Plg-lherzolite-a,b,c, Sp lherzolite-d
a
c
Take a look at hand specimen too!
b
d
Sr-Nd isotopes
@Pali, Salt Lake Crater
& Koolau
Nd and Sr isotopes of Kimberlite Xenoliths
Initial 143Nd/144Nd vs. 87Sr/86Sr for mantle xenoliths. From Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer. Data from Zindler et al. (1982) and Menzies (1983).
Dm , bse, em1, em2, himu
Chemical dynamics- a word of
caution
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LAB should be defined based on rheology not
chemistry; T=1250 C is where olivine starts
behaving ductily;
Asthenosphere becomes lithosphere and viceversa
- thus chemistry is not a good indicator;
Small enriched domains in a chemically
heterogenous mantle can supply most melts if F is
small.
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decompressor
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Experiments on melting enriched
vs. depleted mantle samples:
1. Depleted Mantle
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Tholeiite easily created
by 10-30% PM
More silica saturated
at lower P
Grades toward alkalic
at higher P
Results of partial melting experiments on depleted lherzolites.
Dashed lines are contours representing percent partial melt
produced. Strongly curved lines are contours of the normative
olivine content of the melt. “Opx out” and “Cpx out” represent
the degree of melting at which these phases are completely
consumed in the melt. After Jaques and Green (1980). Contrib.
Mineral. Petrol., 73, 287-310.
Experiments on melting enriched
vs. depleted mantle samples:
2. Enriched Mantle


Tholeiites extend to
higher P than for DM
Alkaline basalt field
at higher P yet
 And lower % PM
Results of partial melting experiments on fertile lherzolites.
Dashed lines are contours representing percent partial melt
produced. Strongly curved lines are contours of the normative
olivine content of the melt. “Opx out” and “Cpx out” represent
the degree of melting at which these phases are completely
consumed in the melt. The shaded area represents the
conditions required for the generation of alkaline basaltic
magmas. After Jaques and Green (1980). Contrib. Mineral.
Petrol., 73, 287-310.
Need to parametrize melting
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Will do this for dry melting only;
Aim to explain major elements;
Assume adiabatic melting;
Need a melting function;
Need a start depth and an end depth;
Assume that SiO2 does not change much;
Use fractionall melting in increments of 1 kbar
Parametrization

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Melting is linear as a function of depth;
Source is only peridotite;
Shape of melting domain is triangular; no
extra wings to scavenge traces;
Based on McKenzie and Bickle (1988);
Langmuir et al. (1992) and Wang et al.
(2002).
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QuickTime™ and a
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Assumptions
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Ti is used as a perfectly incompatible
element;
Fe and Na will constrain the depth where
melting starts and the length of melting
column respectively;
Thickness of melt column is also calculated
(e.g. for MORB it should be 6 km);
K influence the calculation - I forget why.
Comparing against data
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Plot the major elements of your set against MgO
(Harker type diagrams);
Find the FeO, Na2O, TiO2 and K2O corresponding
to the most primitive composition;
Those are the values to compare against the
forward model;
Works for any adiabatic melting assuming that
only peridotite is the source. You can mess with
fertility (% cpx source), amount of MgO, Na2O,
K2O, FeO in source.
Na2O=2.8
FeO=9
6.00
Series1
5.00
Na2O (wt%)
4.00
3.00
2.00
1.00
.00
6.00
7.00
8.00
9.00
FeO (wt%)
10.00
11.00
12.00
Best match
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Start at 23 kbar
Stop at 15 kbar
8 kbar column of melt, stops exactly at crust
-mantle boundary (about 50 km under the
Puna);
Predicts 2.5 km of basalt accumulated in the
crust; average melting 7%;
Is this any good?
Hits solidus at around 1450 C
Other constraints



Use ol-glass thermometer for magma temp;
Get xenoliths to find out how
depleted/fertile a peridotite from under the
Puna is;
Crustal and lithospheric thickness
constraints from seismo people;
HW8
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
Use Blondes et al major element data for
the Papoose flows only to determine the
FeO and Na2O corresponding to the most
primitive MgO;
Use LPK model to determine the melt
starting pressure, ending pressure, melt
thickness and average F