Redox conditions of the mantle
KIMBERLITES, CARBONATITES AND
DIAMONDS
Anouk Borst, PhD Student Geological Survey of Denmark and Greenland
PhD Course Earth and Planetary Materials and Dynamics UiO/CEED 24-04-2015
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
OUTLINE
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)
CARBONATITES, KIMBERLITES AND DIAMONDS
Kimberlites: • Ultramafic, alkaline (potassic) and
volatile-rich (CO2) melts• > 1% partial melt of
carbonated/metasomatised peridotite
• Slightly more reduced able to transport diamonds
Carbonatites:• Deep-seated Ca-Mg-volatile-rich
(C-O-H) rich melts• 0.01 - 0.5 % partial melt of
carbonated/metasomatised peridotite
• Relatively oxidixed can’t host diamonds
Shirey et al., 2013
Continuum?
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
Effi ciency of carbonate-subduction and melt production greatly influenced by oxidation state In turn influencing the residence times of C in mantle: 1 - 4 Ga!
MANTLE CARBON CYCLE
Dasgupta, 2013
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
SPECIATION OF CARBON
Dasgupta and Hirschman, 2010
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
REDOX CONDITIONS OF THE MANTLE
Frost et al., 2004, 2008; Rohrbach et al., 2007; Rohrbach and Schmidt,2011
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
DIAMOND STABILITY
Frost et al., 2008 Dasgupta and Hirschman., 2011
In the lithosphere 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 (20-250 ppm), such that they are dissolved in Fe-carbides or in Fe-metals
Need input of Carbon!
DIAMOND STABILITY
Shirey and Shigney, 2013 (adapted from Tappert and Tappert, 2011)
Lithospheric geotherms
CARBONATE INFLUX THROUGH SUBDUCTION
Rohrbach and Schmidt, 2011
1) Redox freezing of oxidized C-O-H fluids/carbonate bearing peridotite in reducing ambient mantle - Diamond formation after Fe-metals/carbide saturation with C
2) Redox melting – if caught in upwelling mantle, above 660 km diamonds are re-oxidized to CO2 resulting in carbonatite melt formation
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
ROLE OF CARBON IN MELTING
Shirey, 2013 (adapted from Dasgupta, 2013)
CRATONIC DIAMONDS
Shirey, 2013
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 …. ?
SUMMARY
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
THANKS!
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