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