When studying the bulk rock analysis of kimberlites a
significant correction for visible xenocrysts must be made
[1,2]. Such studies concluded that olivine in kimberlite has a
xenocrystic core and cognate margin. Previously, material
entrained by the melt and dissolved during transport has only
been qualitatively discussed [3]. We find bulk rock and
electron microprobe data strongly support dissolution of
orthopyroxene coupled with crystallization of cognate olivine
rims in the Majuagaa kimberlite sensu stricto dyke. This
reaction was suggested by the topography of phase diagrams
for systems containing Enstatite, Forsterite and CO2. The
‘parental’ melt for Majuagaa kimberlite was magnesiocarbonatite
which entrained 50wt% harzburgite, consistent with
melting experiments. The melt processed ~20wt% SiO2 but at
any given time had no more than ~5wt% SiO2. We explain
geochemical variations across the southern West Greenland
kimberlites sensu lato using similar processes. We may also
account for the intercraton variety in kimberlite composition
and mineralogy.
Mg# and Ni composition was obtained for transects across
olivine grains. Pure Rayleigh fractionation did not explain the
shape of the nickel transects. Diffusion between
homogeneous olivine grains and melt was also eliminated.
The data was replicated using a combination of growth and
minor later diffusion. Growth was modeled using AFC
equations [4]. Later diffusive equilibration between core and
margin was minor. Since kimberlite transport time was short
magmatic temperatures of ~900oC were supported.
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[1] Nielsen & Sand (2008), Canadian Mineralogist 46, 1043-
1061 [2] Ardnt, Guitreau, Boullier, Le Roex, Tommasi,
Cordier & Sobolev (2010), Journal of Petrology 51, 573-602
[3] Mitchell (2008), Journal of Volcanology and Geothermal
Research 174, 1-8 [4] De Paulo (1981), Earth and Planetary
Science Letters 53, 189-202