deep carbon: carbonates etc university college london: adrian p jones and coworkers judith milledge...
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Deep carbon: carbonates etc
University College London: Adrian P Jones and coworkersJudith 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..
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DEEP CARBON CYCLE
What is the bulk C of the Earth?What is the volcanic flux of CO2?-Steady state vs catastrophic -Eurocarb….
Geodynamic interior
Volcanic eruptions outgas CO2
4.6 Ga Protoearth4.56 Moon
C delivery meteorites
Diamond as a major C reservoir
Diamond + O2 = CO2core
Model of CO2 Pacific atmospheric distribution
Rosetta CO2Mt Erebus effect(Eruption?)
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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..
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Carbonatite volcanism
• Oldoinyo Lengai• CO2 degassing• Mantle carbon• 1966, 2007
events, plus continuously active
• Regional carbon footprint
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Natrocarbonatite to carbonatite in ~1 year at <43oC.: density
Nyerereite Gregoryite Calcite
density 2.42 2.27 2.71
Crystal orthorhombic hexagonal trigonal
chemistry Na2Ca(CO3)2 (Na2,K2,Ca)CO3 CaCO3
2006 2007*
13C -6.8 -6.7
18O +6.5 +24.8
2006 Zaitsev and Keller, Lithos 91, 191-2072007 UCL unpublished data*
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Observational tools
• Geology, rocks and minerals
• Meteorites, rocks and minerals
• Mineralogy, petrology, geochemistry
• Carbon inorganic vs organic
• Field observations
• Laboratory Experiments
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Carbonate melt mantle: Canary
• Direct observation of carbonate in mantle xenoliths is becoming more widely recognised
• Cryptic metasomatism of mantle xenoliths from transient carbonatites often leaves a distinctive geochemical “smell”
0.01
0.1
1
10
100
1000
10000
100000
Carbonatite*
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Carbonate melt mantle: Tanzania(Rudnick ~1999,2000)
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~100 km
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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
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Jagersfontein c-type (chondritic relics) diamonds?
32 out of 148 diamonds contained dark inclusions; 13 have native siderophile metal/carbide
?
C-type
Jagersftn
13C -20+/- 4
Silicate inclusions
Maj, pvsk, fe-per,
ilm
Fe-per (1)
Mg-pvsk (3)
Ca-pvsk (3)
Ilm (1)
Metallic inclusions
Fe-Ni-Co-C
Fe-Ni-Cr-C (9)
FeCr (9) Ni (1) Co (minor)
Sulphides troilite FeS
Troilite (1)
NiS (millerite) (1)
FeNiS
Pentlandite (3)
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~3000 km (no samples)
Isshiki et al Nature 2003High PT exeriments
Ono et al Am Mineral 2005 calcite post aragonite
Also Seto et al 2007 subduction reactionsTo ~2000 km 3000K (Phys Chem Mineral)
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Modelling (Oganov): stable structure of CaCO3 at 150 GPa
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Is carbon oxidised or reducedin the lower mantle? (Oganov)
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Modelling: Oganov et al
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Modelling cont:
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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.
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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
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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
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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
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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 13C ~-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 13C of -5%0 on Earth does not represent the bulk planet.