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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Primary sources of kimberlite diamonds the facts and hypotheses

Soloveva L.V.*, Kalashnikova T.V.**

* - Institute of Earth crust, Siberian branch of RAS, Irkutsk, Russia

** - Vinigradov Institute of Geochemistry, Siberian branch of RAS, Irkutsk, Russia

Solv777@crust.irk.ru

 

The kimberlites are the main source of diamonds, but they arent environment for diamond growth. The contentious discussion of conditions and depth for diamonds origin was proceeding for 40 years at least. In 1985 well-known diamond researcher Prof. Henry Mayer wrote an article with intriguing title Genesis of diamond: mantle saga [1985].  In last 30 years large amount of researches about diamond as mineral, syngenetic diamond-inclusion and high-density fluids has been collected, extending our views about this mysterious mineral. At the same time data were obtained about sulfides and silicate-inclusions ages, physical-chemical character of fluids and redox-conditions of initial surroundings of diamond genesis. The geodynamic situation during diamondiferous kimberlite cycles in ancient craton was also investigated. It makes possible to judge about plume or plate-tectonic mechanisms exciting diamond-bearing kimberlite magmatism. Nevertheless, diamond and its initial origin is a mystery in spite of large amount of analyses and appearance of new methods.

In this report we attempted to summarize such question of principle for this problem as: chemical medium of diamond formation, age of this process, possible reasons  initiating diamonds development in bottom of craton mantle lithosphere, mantle transition zone and lower mantle. The following questions are expected to consider:

1. Age of diamondiferous kimberlite and diamonds;

2. Paternal matter for kimberlite diamonds;

3. Physical-chemical nature of diamond-forming melts and fluids;

4. Possible geodynamic mechanisms triggering kimberlite magmatism and influencing its diamondiferous.

Diamondiferous kimberlites generate within ancient cratons with thickened lithosphere (> 140 150 km). Different kimberlite cycles may be diamondiferous and may not contain diamonds. 7 epochs of kimberlite formation were assigned in Siberian craton based on new geological and isotopic data:  450-430, 420-400, 380-350, 250 230, 170 150, 110 100 and 60 50 Ma. Upper-Devonian cycle is the most diamond productive. Within Slave craton (Canada) 5 kimberlite cycles were identified: ~613, ~530, ~450, ~170 and 75-45 Ma [Helmstaedt, 2009]. Recently Middle-Paleoproterozoic kimberlites in Republic of South Africa may be considered as the most ancient kimberlites ~1950 Ma.

In modern scientific literature an idea is dominated that majority of diamonds is connected with metasomatic processes in late Archean and early Proterozoic. This viewpoint was proved corresponding Re-Os ages of sulfide inclusions in diamonds of peridotite (P) and eclogite (E) type [Griffin et al., 2002; Pearson et al., 1995]. However, the hypothesis of Archean - early Proterozoic diamonds meet with more and more difficulties. Spetsius et al. [2002] received Archean age for sulfides included in Paleozoic zircons from kimberlite pipe Mir. Harte [2012] attracts attention to marked contrast between phanerozoic ages of deep diamonds from Cretaceous kimberlite of South Africa and Archean Proterozoic Re-Os ages of diamond sulfides [Shirey & Richardson, 2011]. These facts may be evidence that ancient sulfides were included in young diamonds, which were resulted of diamond-forming fluids passing. The intriguing coincidence of sulfides model ages of P-type diamonds and ages of craton lithosphere and covering ancient crust [Griffin et al., 2002, Pearson et al, 1995] may be explain the influence of reduced fluids. The such fluids percolated through craton lithosphere, dissolving lithosphere sulfides and then precipitated them together with diamonds, preserving ancient Re-Os isotopic marks. 

Araujo et al. [2009] showed that diamonds from Diavik pipe (Slave craton, Canada) formed two genetic series: coating and fibrous (their crystallization preceded kimberlite intrusion within first millions hundreds of thousands years) and more earlier transparent octahedrons. The estimates of nitrogen aggregation time in earlier diamonds at T1250C showed 20-30 Ma [Araujo et al., 2009], that corresponded the beginning of kimberlite cycle. Sobolev et al [2009] also supposed young pre-kimberlite age of transparent colourless and poor-colored octahedral diamonds from Yakutia and Archangelsk regions.

The main mineral diamondiferous parageneses were discriminated based on numerous mineralogical researches. The parageneses are as follows: peridotitic (dunite-harzburgite and lherzolite- P), eclogitic (E) and very rare deep (transition zone mantle, lower mantle) [Harte, 2012]. The latter includes majorite, Ca-silicate perovskite, Mg-silicate perovskite, Fe-periclase, Mg-wustite, stishovite and products of their low-pressure alteration. The deep mineral assotiation is supposed to connect with primary peridotitic or eclogitic mediums. Its genesis is considered due to subduction of slabs and later rise to base of lithosphere plate by plumes [Harte, 2012]. The majority of diamonds formed at depth 150-250 km according to thermo-barometry estimates, that corresponded to the bottom of lithosphere plate or thermal boundary layer [Cartingny, 2005; McKenzie, 2005].

The crystallization conditions of most diamonds (except later cubic and coating forms) from reduced fluids correlated with high content of nitrogen showing incompatible properties in oxidized  and compatible properties in reduced fluids [Thomassot et al., 2007; Foley, 2011]. Thomassot et al. [2009] investigated isotopes of C, N in diamonds and S in included sulfides. They suggest that C-N source for eclogitic diamonds (E-type) was deep-seated reduced fluid. The sulfides were of ancient origin and preserved geochemical features of basic part of slab. Other hypotheses assume that transparent diamonds crystallized under the influence carbonatite melts, reduced fluid, CO2 + N2 fluids. The later cubic and coating pre-kimberlite diamonds are saturated high-density fluid inclusion. Their chemical composition represented the part of system: potassic- silicate melt carbonatite melt alkali brine, indicating the physical-chemical conditions of their growth.

By modern geodynamic approaches kimberlite magmatism is connected with initializing influence of thermo-chemical plumes. The example of this model is the rising Middle-Paleozoic Yakutian plume in Siberian craton [Ernst, Buchan, 1997] triggering diamondiferous kimberlite magmatism. The thermo-chemical plume underplating the base of lithosphere plate produced the melts parental to low-Cr megacrysts and percolation of solid rocks, modificating them to deformed peridotite [Soloveva et al., 2008]. The bottom of lithosphere plate was eroded, and higher lithospheric zone was affected to metasomatic influence of asthenospheric fluids [Soloveva et al., 2007]. The main stage of diamonds formation in lithospheric bottom on early stage of kimberlite-forming cycle thought to be connected with a such reduced fluids. On the other hand, the plumes bringing hot substance not rarely exposed to magmatic erosion that can delaminate the diamond-containing layer. Pokhilenko et al. [1999] suggested a decrease of Siberian craton lithosphere thickness in 70 km influenced by powerful trappean magmatism at the P-T boundary. Mather et al [2011] compared xenolith paleotherm with modern seismic geotherm beneath Gibeon pipe (70 Ma) and assumed lithosphere decrease in 50 km for this period. Nevertheless, processes of lithosphere thickening dont de eliminated on account for accretion of cold plume material or subducting oceanic crust. Perhaps, the latter process was realized in upper Proterozoic (2.1 Ga) at north-east border of South America (French Guyana). There proluvial diamonds were found, their protolite constituted oceanic sedimentary ooze [Smith et al., 2012].

 

The work was supported by grant RFFI 07-05-00589-a.

 

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