Presentation on theme: "LOWER MANTLE MATERIAL IN THE SOURCE OF KIMBERLITES Igor Ryabchikov (1) and Felix Kaminsky (2) (1) Russian Academy of Sciences, Moscow (2) KM Diamond Exploration."— Presentation transcript:
LOWER MANTLE MATERIAL IN THE SOURCE OF KIMBERLITES Igor Ryabchikov (1) and Felix Kaminsky (2) (1) Russian Academy of Sciences, Moscow (2) KM Diamond Exploration Ltd, West Vancouver, British Columbia, Canada
Calculation of ferropericlase composition depends on the apparent Mg/Fe exchange constant K*=(Fe tot /Mg) MPv /(Fe/Mg) FP Sinmyo & Hirose, 2013
Calculated Mg#’s of ferropericlase for pyrolite bulk composition along lower mantle mean adiabate. F(met) – fraction of Fe-Ni alloy
With the exclusion of the data for Brasilian diamonds, which contain an unusually high proportion of Fe-rich ferropericlase, 92 % of the calculated Mg-numbers for ferropericlase inclusions in diamond fall within the range of calculated values. It is consistent with the assumption that the composition of lower mantle is close to pyrolite. Similar conclusion was reached on the basis of seismic wave velocities. (Zhang, Stixrude, Brotholbt, 2013)
92 % of the calculated Mg-numbers for ferropericlase inclusions in diamond fall within the range of calculated values. It suggests that the composion of lower mantle is close to pyrolite. The same conclusion was reached on the basis of seismic waves velocities (Zhang, Stixrude, Brodholtb, 2013)
2 (Mg,Fe,Ni)O = 2 Fe,Ni + O 2 2/3 (Mg,Fe)O + 1/3C (diam) = 2/3 Fe 3 C + O 2 (Mg,Fe)CO 3 = (Mg,Fe)O + C(diam) + O 2 Fields of stability of C-bearing minerals in equilibrium with phase assemblage of pyrolite bulk composition were calculated at P-T of lower mantle. Thermodynamic data and EOS from Holland & Powell, 2011 and Holland, Hudson, Powell, Harte, Formation of diamonds require more oxidizing environment than should be typical for the prevailing part of this geosphere ( f O2 below IW buffer).
2 (Mg,Fe,Ni)O = 2 Fe,Ni + O 2 2/3 (Mg,Fe)O + 1/3C (diam) = 2/3 Fe 3 C + O 2 (Mg,Fe)CO 3 = (Mg,Fe)O + C(diam) + O 2 It suggests that metallic phase is absent from the environment of sublithospheric diamond formation.
Ni is a monitor of the presence of metallic alloys in mineral assemblage: in the absence of alloy Ni in FP is close to 1%, at F(met)=0.01 this value is significantly lower.
In the presence of metallic alloy Ni content in FP should be significantly lower by comparison with alloy-free phase assemblage. In fact for FP with Mg#’s above 0.8 Ni content is close to 1%, which is significantly higher than in lherzolite with 1% of metallic alloy. This implies that disproportionation of FeO is not a factor governing the composition of FP’s captured by sublithospheric diamonds.
2 (Mg,Fe,Ni)O = 2 Fe,Ni + O 2 2/3 (Mg,Fe)O + 1/3C (diam) = 2/3 Fe 3 C + O 2 (Mg,Fe)CO 3 = (Mg,Fe)O + C(diam) + O 2 The formation of diamonds in lower mantle requires relatively oxidizing environment.
This is confirmed by recently published experimental data and measurements of Fe 3+ in FP’s log units above IW buffer. Otsuka et al, 2013 (EPSL)
2 (Mg,Fe,Ni)O = 2 Fe,Ni + O 2 2/3 (Mg,Fe)O + 1/3C (diam) = 2/3 Fe 3 C + O 2 (Mg,Fe)CO 3 = (Mg,Fe)O + C(diam) + O 2 The estimates of f O2 based on Fe 3+ content in FP are close to stability field of carbonate phases.
There may be several factors controlling this redox differentiation including subduction of oxidized crustal material, loss of metallic phase produced by FeO disproportionation etc. Perhaps the leading role belongs to effect of increasing temperature on redox reactions.
Using available experimental data we calculated equilibrium constant of the reaction 3FeO(FP) = Fe 2 O 3 (MPv) + Fe 0 (met) and used very simplified model for FeO and Fe 2 O 3 activities for pyrolite bulk composition.
Below 1750 o C phase assemblage of pyrolitic composition includes metallic phase. At this stage f O2 is controlled by FP+Met. Above 1750 o C metallic phase disappears and f O2 is controlled by FP+MPv, leading to significant increase of normalized f O2 with further heating.
Such temperature dependence of f O2 is similar to phase relations in Fe-O system even at low pressures. With increasing temperature the extent of FeO disproportionation drastically decreases, and f O2 normalized to IW buffer rises with further heating. 1/2Fe3O4=3/2Fe+O2 3Fe3O4=6FeO+O2 2FeO=2Fe+O2
If temperature increases by several hundreds of degrees the carbonate stability field may be reached
It is possible that in hot mantle plumes relatively oxidizing conditions exist which lead to the formation of diamond, whereas in the prevailing part of lower mantle with temperature close to AMA carbon is mainly present in the form of carbides.
An important role for the formation of diamonds may be played by melts formed during the ascent of mantle plumes. Under relatively oxidizing conditions near-solidus melts must be represented by carbonate liquids, or considering similar contents of carbon and phosphorus in the primitive mantle these melts should have carbonate+phosphate composition (Ryabchikov & Hamilton, 1993).
Products of solidification of such melts were recently described by Kaminsky et al (2013) Inclusion in diamond from the Juina area, Mato Grosso, Brazil. The inclusion is composed of dolomite, magnesite, phosphate Na 4 Mg 3 (PO 4 ) 2 (P 2 O 7 ) (phosph), and pores (fluid). Note the euhedral faces of the inclusion (‘negative’ inclusion) and zonal structure of magnesite near contact with diamond.
Migration of such melts in lower mantle into the colder and less oxidized domains may result in the formation of diamond due to the reduction of carbonates.
The appearance of relatively oxidized material in sublithospheric zones is reflected in the presence of magmas with elevated redox potential in LIP’s, which are linked to the most powerful mantle plumes. They include meimechites, alkaline picrites and intrusive rocks of the Guli massif which belong to the Siberian trap province.
The formation of diamond in lower mantle requires more oxidizing conditions by comparison with the prevailing part of this geosphere, because in the presence of metallic alloy carbon would be present as iron carbide. An important reason of elevated oxygen fugacities in some domains of lower mantle may be the effect of temperature on redox equilibria: with increasing temperature the extent of disproportionation of FeO may decrease and relative oxygen fugacity may grow faster than for reference IW buffer. The link between sublithospheric diamond formation and high temperatures of certain mantle domains confirms that the formation of diamonds in deep geospheres and genesis of kimberlite magmas are related to mantle plumes. CONCLUSIONS