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..

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?)

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

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2006 Zaitsev and Keller, Lithos 91, 191-2072007 UCL unpublished data*

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

• 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*

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

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)

~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)

Modelling (Oganov): stable structure of CaCO3 at 150 GPa

Is carbon oxidised or reducedin 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 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.