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Primary
sources of kimberlite diamonds – the facts and hypotheses
Solov’eva 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 aren’t 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 T≈1250°C
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 [Solov’eva et al., 2008]. The
bottom of lithosphere plate was eroded, and higher lithospheric zone was
affected to metasomatic influence of asthenospheric fluids [Solov’eva 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 don’t 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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