1. Kimberlites, Carbonatites and diamonds
Redox conditions of the mantle
Anouk Borst, PhD Student
Geological Survey of Denmark and Greenland
PhD Course Earth and Planetary Materials and Dynamics UiO/CEED

2. Outline Carbonatite, kimberlite and diamond formation
The carbon cycle of the mantle
What can we learn about the oxidation state of the mantle?
And why do we care?
Oxygen state of the mantle influences:
Melt production in the mantle
Water and carbon storage capacity of the mantle
Rheology of the mantle

3. carbonatites, Kimberlites and diamonds
Deep-seated Ca-Mg-volatile-rich (C-O-H) rich melts
% partial melt of carbonated/metasomatised peridotite
Relatively oxidixed  can’t host diamonds
Kimberlites:
Ultramafic, alkaline (potassic) and volatile-rich (CO2) melts
> 1% partial melt of carbonated/metasomatised peridotite
Slightly more reduced  able to transport diamonds
Continuum?
Kimberlites carry mantle xenoliths /xenocrysts (Cr-garnet, Cr-spinel, Cr-cpx, Mg-ilmenite) and diamonds
These provide a unique window into the cratonic lithosphere providing us with invaluable information about the underlying mantle, its mineralogy and physical properties
Diamondiferous kimberlites are spatially restricted to Archaean cratons with cold, thick mantle keels (Clifford’s rule)
Shirey et al., 2013

4. Mantle Carbon cycle
Total carbon budget in Earth’s interior greater than the exterior!
Origin of carbon in the mantle
Primordial carbon from accretion
Recycled carbon: exchange of carbon between mantle and atmosphere
Significant C-influx into the mantle through subduction of carbonated oceanic lithosphere
2/3 from hydrothermally altered oceanic basalts
1/3 from top carbonates
Significant C-outflux through volcanism
Efficiency of carbonate-subduction and melt production greatly influenced by oxidation state
In turn influencing the residence times of C in mantle: Ga!
Dasgupta, 2013

5. Speciation of Carbon
Low solubility of C in mantle silicates (<ppm-levels)
C mainly occurs in accessory phases
Solids (immobile): Carbonates (calcite, dolomite, magnesite etc)
Graphite (<150 km), Diamond (>150 km), Fe-Ni-carbides (FexCx) and Fe-Ni metals
Volatiles (mobile): CO2 vs CH4 melt/fluids/vapors
Speciation of carbon depends on fO2 , controlled by Fe-C mineral equilibria
Dasgupta and Hirschman, 2010

6. Redox conditions of the mantle
Cratonic mantle (~250 km) – fO2 calculations from peridotite xenoliths/diamond inclusions
Range from 3 – 1 relative to Iron-Wustite buffer
Close to Ni-precipitation curve, below which Fe-Ni-metals are stable
Marks lower boundary for fO2 as large amounts of FeO have to be reduced to lower the oxygen fugacity
Below 250 km: experimental results
Increasing majorite component in garnet with depth
Increasing Fe3+/∑Fe ratios in majoritic garnet
Below 660 km:
Al-perovskite high Fe3+ /∑Fe ratios
Missing Fe3+ provided by disproportionation of iron:
FeO = Fe2O3 + Fe
Mantle is very reducing and Fe-metal saturated
0.5% Fe-metal at base of transition zone and 1% in lower mantle
Frost et al., 2004, 2008; Rohrbach et al., 2007; Rohrbach and Schmidt,2011
Shirey, 2013

7. Diamond stability Requirements: High P, High T > 120 km at 900 °C
Reduced conditions - upper fO2 limit: EMOD
Elevated C concentrations – otherwise dissolved in Fe-metals or carbide
Frost et al., 2008
Dasgupta and Hirschman., 2011

8. Diamond Stability In the lithosphere In the astenosphere
Only cratonic lithosphere is thick (>150 km) and cold enough to retain diamonds
Occasionally diamonds can be formed in UHP metamorphic terranes
In the astenosphere
In principle, diamond is stable anywhere below EMOD and G/D transition!
But C-contents generally too low ( ppm), such that they are dissolved in Fe-carbides or in Fe-metals
Need input of Carbon!
Lithospheric geotherms
Shirey and Shigney, 2013 (adapted from Tappert and Tappert, 2011)

9. Carbonate influx through subduction
Oxidized C-O-H fluids/melts released from subducting slab
‘Redox freezing’ in reducing ambient mantle
If C contents higher than buffering capacity of Fe-Ni metals  diamonds form
If carbonated peridotites or eclogites are caught in upwelling mantle, carbonatitic melts are produced by ‘Redox melting’
As long as the mantle is saturated with Fe-metals (660 – 250 km) carbonatitic melts continuously reduced to diamond or carbides (immobile!)
Redox freezing of oxidized C-O-H fluids/carbonate bearing peridotite in reducing ambient mantle - Diamond formation after Fe-metals/carbide saturation with C
Redox melting – if caught in upwelling mantle, above 660 km diamonds are re-oxidized to CO2 resulting in carbonatite melt formation
Rohrbach and Schmidt, 2011

10. Role of carbon in melting
Presence of carbonate (CO2 or CO3) in peridotite drastically lowers the solidus
Below 300 km: solidus = parallel to adiabat
Small degree melts can form at great depths
Continuously reduced to diamonds as long as it encounters Fe-metals
Adiabatic upwelling mantle crosses the solidus of CO2-bearing peridotite at ~300 km
Producing carbonatitic melts below base of the cratonic lithosphere
Only underneath cratons carbonatitic melts are separated from upwelling mantle which is slowed down below SCLM
These evolve to kimberlitic melts with increasing melt fractions (>1% partial melt) and continued reduction by reduced ambient mantle
Shirey, 2013 (adapted from Dasgupta, 2013)

11. Cratonic diamonds
Shirey, 2013

12. Summary
Diamonds can form anywhere in mantle below G/D transition, if C contents high enough
Subduction transports C deep into the mantle (in oxidized form)
Produces C-rich (diamond/Fe-carbides) peridotite + eclogites by redox freezing of released C-O-H-bearing fluids/melts
C-rich metasomatised domains caught in upwelling mantle produce carbonatitic small-degree melts by redox melting and decompression melting
Archean cratonic roots (depleted in Fe, rather reduced) provide ideal window for diamond formation between 150 and 250 km by fluxing with carbonatitic melts/C-O-H rich fluids during many cycles of subduction
Cratonic diamonds can be stored for long periods of time (>3 Ga) until they are picked up by much younger kimberlite melts
Kimberlites formed through continued redox and decompressional melting of carbonated peridotite/eclogite at the base of the cratonic lithospheric
Along margins of LLVSP’s …. ?

13. Thanks!
Dalton, J.A. and Presnall, D.C., 1998, The Continuum of Primary Carbonatitic– Kimberlitic Melt Compositions in Equilibrium with Lherzolite: Data from the System CaO–MgO–Al2O3–SiO2–CO2 at 6 Gpa, Jounal of Petrology 39
Dasgupta, R. and Hirschman, M.M., 2010, The deep carbon cycle and melting in Earth’s interior, Earth and Planetary Science Letters
Dasgupta, R., 2013, Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time, Reviews in Mineralogy and Geochemistry
Frost, D.J., et al., 2004, Experimental evidence for the existence of iron-rich metal in the Earth’s lower mantle, Nature 428
Frost, D.J. and McCammon, C.A., 2008, The redox state of the Earth’s mantle, Annual Reviews of Earth and Planetary Sciences
Shirey, S.B., et al., 2013 Diamonds and the geology of mantle carbon, Reviews in Mineralogy and Geochemistry
Shirey, S.B., and Shigley, J.E., 2013, Recent advances in understanding the geology of diamonds, Gems and Gemology
Stachel, T., Brey, G.P., Harris, J.W., 2005 Inclusions in sublithospheric diamonds: Glimpses of Deep Earth, Elements
Stagno, V., et al., 2013, The oxidation state of the mantle and the extraction of carbon from Earth’s interior, Nature
Rohrbach, A. et al., 2007, Metal saturation in the upper mantle, Nature
Rohrbach, A. and Schmmidt, M.W., 2011, Redox freezing and melting in the Earth’s deep mantle resulting from carbon-iron redox coupling,, Nature
Woodland, A.B. and Koch, M., 2003, Variation in oxygen fugacity with depth in the upper mantle beneath the Kaapvaal Craton, Southern Africa, Earth and Planetary Science Letters

14. Redox melting reactions
Melting by oxidation (either O2 (a) and Fe3+ (b) as oxidants) of diamond
Melting by oxidation of metal-carbide
Redox freezing
Dasgupta, 2013