Komatiites and the Early Evolution of the Earth · • Komatiite mantle melting process: a new-old...
Transcript of Komatiites and the Early Evolution of the Earth · • Komatiite mantle melting process: a new-old...
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Komatiites and the Early Evolution of the Earth
Stephen Parman, Jay Barr, Tim Grove and
Jesse Dann, Marten de Wit (UCT), Alan Wilson (WITS)
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Outline
• Earth Evolution: theories and constraints • Komatiites: evidence for a subduction
origin • Komatiite mantle melting process: a new-
old model
• Implications for the Early Earth
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Heat Sources in the Early Earth
• Radiogenic Heat
• Core Segregation
• Giant Impact/Accretionary Heat
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Heat Sources in the Early Earth
Turcotte and Schubert, Geodynamics (2002)
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Geodynamic Models - basic
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Geodynamic Models
episodic cooling due to phase
transitions
Davies, EPSL (1995)
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Observational Constraints
• Metamorphic Gradients - inconclusive • Tectonic Style (rigid vs gooey) - inconclusive
• Archean magmatic products - promising – Direct evidence of temperatures in the interior of
the early Earth – Fundamental constraint on all thermal and
chemical evolution models
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Eastern Kaapvaal
Craton
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Not too different in the Archean • Magmatic products in greenstone belts (to
3.5 Ga) and gneisses (to 4.0 Ga) are primarily basalts and granites. No suggestion of higher temperatures.
• 4.4 Ga zircons (Wilde et al., Nature, 2001) contain granitic inclusions and provide isotopic evidence for liquid surface water.
• Primary difference - presence of komatiites in many igneous sequences
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1.7 m thick unit in Komati
Fm.
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~10% of stratigraphic thickness is
komatiite
after J. Dann
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Really, really, really high MgO
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High MgO means high T
1 atm.
3.0 GPa
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Plume model
Herzberg, JGR (1992)
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Inklings of a Problem
• Much of upper mantle would be molten
• Many komatiites interlayered with subduction related magmas
• Mounting evidence for high H2O contents
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Not compatible with geophysical constraints
too hot!
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Comes close to lower mantle
temperatures for episodic models
But melts in the lower mantle would
sink!
Davies, EPSL (1995)
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Plume-Arc Interaction
Sproule et al., 2002
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Evidence for H2O in komatiite magmas
• Pyroxene compositions in Barberton and in Commondale komatiite
• Amphibole in komatiite and melt inclusions
• High Pressure Phase Equilibria
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Experiments on
komatiites -
TZM-bomb experimental
set-up
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ZHM/MHC-bomb
sample
External T.C.
• up to 3 kbar & 1350oC • gas pressure media • fO2 buffered by Ni-NiO
pressure in
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ZHM/MHC-bomb and
furnace
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1-atm (dry) and 2-kbar (H2O sat.) Experiments
Augite
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Augite Wo contents in Barberton
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Augite Al contents
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Preserved Igneous Pyroxenes Orthoenstatite in Commondale
1 cm
From the center of a several meter-thick flow
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Equilibrium Pyroxene Growth
H2O Saturated (when first opx appeared) = ~4.5 wt% H2O
Commondale Komatiites
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Chemical Trends Caused By Undercooling
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Undercooled, Hydrous Komatiites Commondale komatiite
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Amphibole in Proterozoic komatiites
Hanski, 1992
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High-P melting experi-ments
deeper = wet
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Other Potential Evidence for High H2O
• Melt Inclusions
• Spinifex textures
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Melt Inclusions
Shimizu et al., 2001
• 1.1-2.6 wt.% H2O • devolatilization? • re-equilibration?
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MI H2O contents
in modern magmas
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Spinifex texture
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Subduction-like Geochemical Features
• Subduction-like characteristics – high SiO2, MgO, CaO/Al2O3, low TiO2 – HFSE depletions – variable LREE
• Boninite-like characteristics – high MgO, – Ti/Zr and Nb/Th systematics – Positive Zr and Hf anomalies
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High SiO2
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Low TiO2
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LILE variability, HFSE depletions
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North Caribou, Superior Province
Hollings and Kerrich, 1999
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U-shaped pattern
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LILE depleted boninites
Gd
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Mixing of fluid and dep. mantle
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Bimodal trend
Nelson et al., 1984 Cameron et al., 1983 Sobolev&Danu., 1994
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Bimodal trend
Nelson et al., 1984 Cameron et al., 1983 Sobolev&Danu., 1994
Bonin Cyprus New Zealand
N. Tonga trench
Cape Vogel New Caledonia New Zealand Cyprus Victoria
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Weaknesses
• Differences in Ti, Fe, Nb/Th, Nd isotopes • Distinguish chemical signature of slab fluid
from crustal contamination – Zr anomalies, Nd isotopes?
• Correct for effects of alteration – high SiO2
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Ion Probe analyses of
unaltered augite
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Measured and Estimated Augite REE
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Assessment of TE mobility
gain
loss
Barberton
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Subduction summary
• Fits geochemistry • Fits evidence for high H2O • Fits tectonic models of greenstone belt
formation • Requires a single, currently observed process
= subduction
• What are implications for Earth Evolution?
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Predicted melting conditions
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Subduction initiation
Based on boninite model of Stern and Bloomer, 1992
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Mature subduction zone
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Experimental Estimates of Hydrous Melting Conditions
1-6 wt.% H2O
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Geodynamic Models - basic
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Geodynamic Models
episodic cooling due to phase
transitions
Davies, EPSL (1995)
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Sunrise at Spinifex Stream, Barberton Mountainland
Subduction zone model explains most observations with a single, currently active process
Suggests Archean sub-arc mantle ~100oC hotter than present sub-arc mantle
Little evidence requires plume model
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Western Barberton
Greenstone belt
after Byerly
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Evidence that does not constrain H2O contents
• Presence of vesicles (tuffs)
• Depletion of source (epsilon Nd(t)>0)
• Eruption on surface
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High CaO/Al2O3
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U-shaped pattern
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majorite TE partitioning
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majorite fractionation
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Influence of H2O on the Development
of Spinifex Textures in Komatiites
Experimental and Field
Constraints,
Barberton Mountainland,
South Africa
S. Parman, J. Dann, M. de Wit, J. Barr, A. Wilson
Conditions for formation of olivine
spinifex in komatiites
• Experimental Evidence for spinifex formation
• Experimental Containers influence nucleation and growth
• H2O changes nucleation and growth conditions
• Field evidence of initial conditions
• Melt arrives undercooled (supersaturated)
• Cooling rate is slow, but variable chill geometry influencescooling rate
• H2O in melt is important
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Olivine-spinifex
Chill
Radiate
-texture
Bladed
Hopper – like surface texture of olivine blade in coarse, bladed
spinifex zone. Individual crystals ~30–40 cm length, grow 1 to 4
meters from heat loss boundary
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Petrologic Evidence suggest 4 – 6 wt. % H2O
pre-emplacement for Barberton komatiites(Parman et al., 1997)
H2O can promote development of spinifextextures
1) Changes in melt physical properties
Lowers nucleation rate
Increases growth rate
2 Undercooling by decompression ofhydrous magma
200 microns
Compositions of igneous
pyroxene in spinifex record
evidence of 4 – 6 wt.% pre-
eruptive H2O
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Augite Wo contents
Experimental Evidence on
Spinifex formation
Container size controls on spinifex development
• 1 Larger container = lower nucleation rate
• 2 Leads to more dominant control of growth rate oncrystal morphology
• H2O lowers melt viscosity, lowers
• Lowers nucleation rate
• Speeds up growth rate
• Leads to growth of a few BIG crystals
• Ultramafic Pegmatites
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Donaldson (1976) Olivine morphology vs coolingrate & undercooling
Micro-spinifex
develops at rapid
cooling rates
No spinifextextures were
produced a at slow
cooling rates
Experimental Approach
Munro township komatiite SSK 17 % MgO
!Tinitial 1370 oC @ 0.1 MPa “DRY”
1300 oC @ 200 MPa H2O-saturated “WET”
Variable cooling rate 3 to 100 oC/hr
Variable container
composition - volume (ml) - texture
MgO 0.4 granular-hopper
Al2O3 0.7 – 60 hopper-chain
SanCarlos 0.3 chain-euhedral
Pt-Fe loops 0.03 chain-euhedral
AuPd (200 MPa) 0.27 hopper-chain
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Large container dry ~ 5 times larger
Small container “wet” ~ 7 times larger
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70
MgOAl
2O
3
FePtAuPd WET
Container volume (ml)
0
5
10
15
20
1 10 100
MgOAl
2O
3
FePt
AuPd WET
Cooling Rate (oC/hr)
Possible to get a complete
transition in crystal
morphology at constant
cooling rate
From many equant small
crystals - shown here in a
cooling rate experiment
performed in an MgO
capsule
To spinifex – just by
lowering the the surface
area to volume ratio
- reducing heterogeneous
nucleation sites
- grow fewer longer crystals250 microns
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Both equant and
spinifex crystal
forms
Large container – 60 ml
crystals extend to edges
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Large container dry ~ 5 times larger
Small container “wet” ~ 7 times larger
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 60 70
MgOAl
2O
3
FePtAuPd WET
Container volume (ml)
0
5
10
15
20
1 10 100
MgOAl
2O
3
FePt
AuPd WET
Cooling Rate (oC/hr)
Adding H2O increases crystal size and decreases
nucleation density dramatically. 3 oC/hr expt. 200 MPa
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10oC/hr, 6 wt.% H2O
1 mm
10oC/hr, 6 wt.% H2O
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N = Nucleation density
!T = undercooling
U = Growth rate
Influence of H2O is to put
peak in growth rate at
LOWER undercooling
At the same time H2O
addition reduces
nucleation rate.
Result is fewer, bigger,
faster growing crystals..
Fenn, 1976
Here are predicted cooling rate
vs distance and growth rate vs
cooling rate for conductive
cooling.
0
50
100
150
2000 20 40 60 80 100
Cooling rate (oC/hour)
0
50
100
150
2000 50 100 150 200 250
olivine blade length (mm)
Cooling rate and crystal
size are related, slow
cooling bigger crystals.
Predicted from wet cooling
expts.
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Field evidence in Barberton komatiites
• Equant olivine present in chill margins
• Close spatial association of spinifex
with granular-textured olivine
• Largest bladed spinifex forms at slowest
cooling rates
Equant olivine in chill margins =
Emplacement at undercooled
conditions
Tem
p o
C
1450
1350
Pressure MPa
0 400200
Liq
Ol + Liq
Ol + Liq + V
!T
Pre-emplacement
conditions
H2O
dissolved
in melt
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NO olivine in chill margins = Emplacement atsuperheated conditions
Tem
p o
C
1750
1350
Pressure MPa
0 400200
Liq
Ol + Liq
Ol + Liq + V
!T
Pre-emplacement
conditions
DRY
melt
50 microns
Chill
MarginEquant
Olivine
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More equant olivine ~ 1 cm
from chill – consistent with
emplacement at undercooled
conditions
Inflation features - Spinifex veins intruding granular,
equant olivine cumulate
Grove et al., 1996
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Spinifex
Equant
olivine in
cumulate
Thin pillows –
no spinifex
Thicker
– only
radiate -
texture
Grove et al., 1996
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Chill
Thin radiate zone
Bladed
Cumulate
zone
Examples of spinifex in 4 to
10 m thick inflated flows
Note variable
thickness of
zones &
discontinuities
Grove et al., 1996
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Inflation features imply conductive cooling
conditions and overpressures that allow for slow
cooling and development of spinifex textures
Spinifex blade maximum length vs distance from
upper chill margin in Barberton komatiites0
100
200
300
400
500
600
0 100 200 300 400
4-4spinifex
mm - max
0
100
200
300
400
5000 100 200 300 400
mm - max
7 m cooling
unit
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Spinifex blade length and
prediction from wet expts
0
100
200
300
400
5000 100 200 300 400
mm - max
0
100
200
300
400
500
600
0 100 200 300 400
4-4spinifex
mm - max 7 m cooling
unit
Require a conductive layer
on the top. Flow inflation
under pillow pile
Conclusions
H2O controls on development of spinifex textures
1) Undercooling by decompression/devolatilization of
hydrous magma
2) H2O changes melt physical properties
•Lowers nucleation rate – destroys nucleii
•Decreases melt viscosity – increases growth
rate at undercooled conditions
3) Injection of melt into cumulates and Flow inflation
processes modify thermal conditionsleading to
spinifex growth at slow cooling rates
Spinifex flows = Ultramafic pegmatites
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Crystal size
vs. cooling
rate
calculated
with dry
small
volume data
Grove et al., 1996
High-P
melting
experi-
ments
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Experimental Estimates of Hydrous
Melting Conditions
1-6 wt.% H2O
majorite fractionation
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Mixing of fluid and dep. mantle