Deep carbon:
carbonates etc
University College London: Adrian P Jones and coworkers
Judith Milledge (Emeritus),
UCL APJ Postgrads: Emma Tomlinson, Su Trickett, Dan
Howell, Sami Mikhail, (Emma Bowden, Rachel Hazael,
Gianluigi Rosatelli, Matt Genge, Dave Dobson)
OPEN UNIVERSITY: Alex Verchovsky, Monica Grady..
DEEP CARBON CYCLE
Model of CO2 Pacific
atmospheric distribution
Rosetta CO2
Mt Erebus effect
(Eruption?)
Volcanic eruptions
outgas CO2
What is the bulk C of the Earth?
What is the volcanic flux of CO2?
-Steady state vs catastrophic
-Eurocarb….
4.6 Ga Protoearth
4.56 Moon
Diamond + O2 = CO2
core
Geodynamic interior
Diamond as a major C reservoir
C delivery
meteorites
Deep carbon APJ perspective
• Carbon-rich volcanism
– Degassing
• Mantle Carbonate
– Where, why
• Planetary perspective
– Venus, Earth, the beginning.
• Mantle Diamond
– Reservoir and age
• Impact behaviour of carbonate, diamond..
Carbonatite volcanism
•
•
•
•
Oldoinyo Lengai
CO2 degassing
Mantle carbon
1966, 2007
events, plus
continuously
active
• Regional carbon
footprint
Natrocarbonatite to carbonatite in ~1 year
at <43oC.: density
2006 Zaitsev and Keller,
Lithos 91, 191-207
2007 UCL unpublished data*
d13C
d18O
2006
-6.8
+6.5
2007*
-6.7
+24.8
Nyerereite
Gregoryite
Calcite
density
2.42
2.27
2.71
Crystal
orthorhombic
hexagonal
trigonal
chemistry
Na2Ca(CO3)2
(Na2,K2,Ca)CO3
CaCO3
Observational tools
•
•
•
•
•
•
Geology, rocks and minerals
Meteorites, rocks and minerals
Mineralogy, petrology, geochemistry
Carbon inorganic vs organic
Field observations
Laboratory Experiments
Carbonate melt mantle: Canary
• Direct observation of
carbonate in mantle
xenoliths is becoming
more widely recognised
100000
10000
1000
100
10
1
0.1
La
13
9
e1
40
Pr
14
1
N
d1
46
Sm
14
7
Eu
15
3
G
d1
57
Tb
15
9
D
y1
63
H
o1
65
Er
16
6
Tm
16
9
Yb
17
2
Lu
17
5
0.01
C
• Cryptic metasomatism of
mantle xenoliths from
transient carbonatites
often leaves a distinctive
geochemical “smell”
Carbonate melt mantle: Tanzania
(Rudnick ~1999,2000)
~100 km
Transition zone ULM
• Martinez et al 1998 (JGR 103)
– suggested carbonate minerals and melts in the
transition zone
• Superdeep diamond inclusions provide precious
samples (akin to meteorites but smaller, rarer
and arguably much more valuable)
– Carbonate is very rare (maybe 2 grains worldwide)
– New data on metal carbide inclusions maybe from
~20 GPa
– Some doubt about pressure = depth
Jagersfontein c-type (chondritic
relics) diamonds?
32 out of 148 diamonds contained dark inclusions;
13 have native siderophile metal/carbide
?
C-type
Jagersftn
d13C
-20+/- 4
Silicate
inclusions
Maj, pvsk,
fe-per,
ilm
Fe-per (1)
Mg-pvsk (3)
Ca-pvsk (3)
Ilm (1)
Metallic
inclusions
Fe-Ni-CoC
Fe-Ni-Cr-C
(9)
Sulphides
troilite
FeS
Troilite (1)
FeCr (9)
NiS
(millerite)
(1)
Ni (1)
FeNiS
Pentlandite
(3)
Co (minor)
~3000 km (no samples)
Isshiki et al Nature 2003
High PT exeriments
Ono et al Am Mineral 2005 calcite post aragonite
Also Seto et al 2007 subduction reactions
To ~2000 km 3000K (Phys Chem Mineral)
Modelling (Oganov): stable
structure of CaCO3 at 150 GPa
Is carbon oxidised or reduced
in the lower mantle? (Oganov)
Modelling: Oganov et al
Modelling cont:
So, carbonate reservoirs are
plentiful
• But reservoirs are hypothetical, where are the deepest,
and oldest samples?
• Carbonate as a carbon reservoir is almost certainly
dynamic, and involved in the convective cycle; hinges on
oxygen and T
• How much is really subducted (<2%?)
• Carbonate may be so dynamic that the carbon cycle in
the upper mantle is isolated from the lower mantle.
Carbon bottleneck
• No consensus on bulk Earth carbon
– Cosmochemical the best?
• Need more information from meteorites
– And it is rapidly evolving
– Did the moon-forming event change
everything?
• Carbon isotopes
– We can construct a mass balance model for
the whole Earth, but major assumptions
Deep carbon: Europe
• European Eurocores proposal (link from
previous ESF Eurocarb) 2009?
– NASA Orbiting carbon observatory; volcanic
16 km footprint (eg Etna)
• ?UK consortium of volcano CO2 monitors
– NERC, ESF
Meteoritic carbon
• Evolving view of
carbon chemistry
from meteorites (see
figure)
• New hypothesis for
bulk Earth (Grady,
Open University) –
APJ separate short
presentation?
El Goresy et al 2005
1: Carbon: planetary perspective
• M.M. Grady, Verchovsky, A. B., & Wright, I. P. Magmatic
carbon in Martian meteorites: attempts to constrain the
carbon cycle on Mars. Int J Astrobiol 3, 117-124 (2004)
– Abstract…”[Mars meteorites] show that the magmatic component
has a very variable abundance of 1-100 ppm, with d13C ~-20+/4%0. This value is close to magmatic carbon determined for
Moon and for Vesta (parent body of HED basaltic meteorites),
but very different from that of the Earth.”
– Conclusions…(4) ..perhaps the d13C of -5%0 on Earth does not
represent the bulk planet.