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 24-04-2015

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

carbonatites, Kimberlites and diamonds 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 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

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: 1 - 4 Ga! Dasgupta, 2013

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

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

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

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 (20-250 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)

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

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)

Cratonic diamonds Shirey, 2013

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 …. ?

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

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