what on earth happened 2.7 billion years ago? · tectonic settings in time. 3.0 2.5 2.0 1.5 1.0 0.5...
TRANSCRIPT
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WHAT ON EARTH
HAPPENED 2.7 BILLION
YEARS AGO?
Kent C. Condie
Dept. Earth and Environmental Science
New Mexico Tech
Socorro, NM 87801 USA
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EXOGENIC
ENDOGENIC
107-108 y
< 103 y
PLANETARY CATASTROPHIES
Atmosphere
Ocean
Biosphere
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00.51.01.52.02.53.0AGE (Ga)
Accretionary Orogens
Large Igneous Provinces (LIPS)
Arc Systems
2.7 Ga
TECTONIC SETTINGS IN TIME
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00.51.01.52.02.53.0AGE (Ga)
2.7 Ga
1.9 Ga
JUVENILE CONTINENTAL CRUST AGE
DISTRIBUTION
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TTGs
2.5 Ga Southern
India
2.7 Ga Zimbabwe
4.0 Ga
Yellowknife
Canada
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DEFORMED GRANITOID ZIRCON AGES
WITH END +1
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DEFORMED GRANITOID ZIRCON AGES
WITH END +1
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CATASTROPHIC
MANTLE PLUME EVENT
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c = crustal preservation rate
e = crustal extraction rate
r = crustal recycling rate
s = sediment subduction
b = subduction erosion
d = lithosphere delamination
r = s + b + d
c = e - r
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01.02.03.04.0
Time (Ga)
2
3
4
Production
Recycling
Preservation
2.7 Ga Event
1.88-1.62 Ga Events
c = e - r
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Valley et al. 2005 CMP
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00.51.01.52.02.53.0
AGE (Ga)
2.7 Ga
Superior
2.7 Ga
OROGENIC GOLD DEPOSITS
Yilgarn
2.65 Ga
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00.51.01.52.02.53.0
AGE (Ga)
2.7 Ga
Carajas
2.6 Ga
IRON OXIDE COPPER-GOLD
(IOCG)
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CHEMICAL INDEX OF ALTERATION (CIA)
50 60 70 80 90 100
Residual ClaysAmazon Muds
Average Shale
Glacial Clay
Average Upper
Continental Crust
CIA = [Al2O3/(Al2O3+CaO+Na2O+K2O)] x 100
Molecular values
includes silicate CaO only
Nesbitt and Young (1982)
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Age (Ga)
CIA
Mean
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Modern Stromatolites from Sharks
Bay, Western Australia
2.7-Ga
Stromatolites
From Eastern
Canada
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Stromatolite occurrencesPer unit volume of
Juvenile crust
1.9 Ga
2.7 Ga
Age (Ga)
Data from Hofmann 1998
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-60
-50
-40
-30
-20
-10
0
1.2 1.6 2 2.4 2.8 3.2 3.6 4
13C
org
Age (Ga)
Data from Pavlov et al 2001 Geol 29: 1003-1006
13C = {[(13C/12C)s/(13C/12C)st] - 1} x 103
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CH4 + O2 CH2O + H2O
VOLCANIC
CH4, CO2
CO2 + H2O CH2O + O2
CH2O CO2 + CH4
Methanogens
Methanotrophs
Oxidative Photosynthesis
FRACTIONATION OF CARBON ISOTOPES IN ORGANICS
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0.1
1
0 0.5 1 1.5 2 2.5 3 3.5 4
0.5
2
Ce/
Ce*
Age (Ga)
BIF
Fe-Mn
ppts
After Kato et al. 2006
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0.5
0.6
0.7
0.8
0.91
2
3
4
5
0 0.5 1 1.5 2 2.5 3 3.5 4
Eu
/Eu
*
Age (Ga)
Eu Anomalies in BIF
ModernHydrothermal
deposits
After Kato et al. 2006
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-60
-50
-40
-30
-20
-10
0
1.2 1.6 2 2.4 2.8 3.2 3.6 4
13C
org
Age (Ga)
2.7-Ga GLOBAL MANTLE PLUME EVENT
onsetend
CH4 + CO2
Normal
Range
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GLOBAL WARMING (CH4, CO2)
Increased intensity of chemical weathering
Negative 13C excursion in organic carbon
Enhanced CIA values in shales
Biodiversity/Biomass increase
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Age (Ga)
Perc
ent
kom
ati
ite
KOMATIITES
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�3.1
3.0
2.9
2.8
2.7
2.6
2.5
2.4
PILBARA KAAPVAAL
Ventersdorp
2725-2710 MaFortescue
2764-2756
MaAG
E (
Ga)
FLOOD BASALTS
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0
10
20
30
40
50
60
70
0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4
Archean cratonsProterozoic cratons
TRD
(Ga)
TRD
Re Model ages Mantle Xenoliths
187 Re 187 Os
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4
6
8
10
12
14
5 10 15 20 25
PRIMARY MAGMA COMPOSITIONS
FeO
(T
) (%
)
MgO (%)
Mantle Plumes
Primary Magmas Calculated forArchean Cratonic Mantle
MORB and Arcs
After Herzberg (2004)
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"Log-jam Tectonics"Stacked slices of hydrated oceanic
crust, plateaus, arcs
partial meltingof hydrated roots
DOWNWELLING
TTG
"Plume-head Underplating"
Older crustFlood Basalts
partial meltingin plume head
Superior and Zimbabwe Provinces Kaapvaal and Pilbara Provinces
2.7 Ga Tectonic Scenarios
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CATASTROPHIC MANTLE
PLUME EVENT
• Juvenile continental crust
• Supercontinent
• Mineral Deposits
• Thick lithosphere (plume restite)
• Global warming
• Increase in biologic activity
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LARGE IGNEOUS PROVINCES
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YOUNG LIPS
Flood basalts in Iceland with
Columnar Joints
Karoo Basalts at
Victoria Falls
Zimbabwe
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GIANT DYKE SWARMS Mackenzie Swarm 1267 Ma
Northern Canada
Amerilyk Dykes 2.9 Ga SW
Greenland
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Age (Ma)
LIP EVENTS
JUVENILE CRUST
EVENTS
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BANDED IRON
FORMATION
Transvaal Group
South Africa
2.5 Ga
Lake Superior Region
USA-Canada
2.0 Ga
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Superior
Yilgarn
Banded Fe
Formation
India, S Africa,
Brazil, Ukraine
LIP
Events
Age (Ma)
Hamersley
N China
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CONTROLS ON BIF DEPOSITION
BIF
Submarine Volcanism
Marginal Basins
Upwelling Photosynthesis
Fe+2
OxygenSea Level R
ise
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BIF
Submarine Volcanism
Marginal Basins
Upwelling Photosynthesis
Fe+2
Oxygen
Sea Level R
ise
POTENTIAL TRIGGERS FOR 2.5-Ga BIF
EVENT: Fe+2, CO2, Sea level rise
1. Supercontinent breakup
2. Global mantle plume event
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1. Juvenile Crust Events
2. LIP Events
TWO TYPES OF MANTLE
PLUME EVENTS ?
OR The onset of the first episode of global plate tectonics
(Juvenile Crust Event)
And a global mantle plume event
(LIP Events)
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1. JUVENILE CRUST EVENTS (Catastrophic Mantle Event)
2. LIP EVENTS
(Large Plate Shielding)
Growth Continents
Global warming
Biologic expansion
Thick lithosphere
Mineral deposits
BIF
Giant dyke swarms
Flood basalts
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1. SLAB AVALANCHE (Condie, 1998)
2. CORE RESONANCE (Greff-Lefftz & Legros, 1999)
3. ASTEROID IMPACT (Abbott & Isley, 2002)
POSSIBLE TRIGGERS FOR
CATASTROPHIC MANTLE PLUME EVENTS
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AN UPDATE OF THE CONTINUING SAGA
DURING THE LATE ARCHEAN
• High resolution U/Pb zircon ages suggest the existenceof a global tectonic/thermal event at 2.7 (perhapsmultiple events over < 50 My).
• This resulted in rapid growth of continental crust andmantle lithosphere. This may have been caused bycatastrophic collapse of slabs through the 660-kmseismic discontinuity initiating a global mantle plumeevent. Alternatively, this may reflect the onset ofglobal plate tectonics.
• Plume-derived CH4 (± CO2) may be responsible forglobal warming and increased 13 C fractionation inorganics at 2.7 Ga.
• A “shielding” type mantle plume event at 2.5 Ga maybe responsible for deposition of enormous amounts ofBIF at this time.
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Proterozoic Prokaryotic microfossils. Methanotrophs?
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-4
-2
0
2
4
6
8
10
0 1000 2000 3000 4000
Epsilo
n N
d (
T)
Age (Ma)
DM
CHUR
ENd Evolution of Depleted Mantle
After Bennett 2003
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2.7-Ga Event