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Origin of the mantle
xenoliths with green garnets in kimberlites
(Newlands dyke, South
Africa and Nyurbinskaya pipe, Yakutia)
Sablukov S.M.
* , Sablukova L.I. * , Stegnitsky Yu.B. ** ,
Karpenko M.A. ***
*“RUSGEO”
Limited, Moscow, Russia;
**
NIGP ALROSA Co. Ltd., Mirny, Russia;
***
Nyurbinskaya Mine, ALROSA Co. Ltd., Mirny, Russia.
sablukoff@rambler.ru
Green garnets
occur in concentrates of diamondiferous kimberlite bodies in Yakutia (Udachnaya,
Zimnaya, Mir, etc.), South Africa (Newlands, Bellsbank), Venezuela (Guaniamo
sills), and Canada (Mud Lake field) (Clarke
and Carswell, 1977). Mantle xenoliths of
rocks containing such garnets are very rare (Kharkiv, 1978).
We found
peridotite xenoliths with green garnet in situ in kimberlites of
the Newlands dike. Xenoliths are irregular in form, 4.5*1.9 cm,
1.5*0.8 cm
(sample NL-11),
and 1.0*0.5 cm in size (sample NL-21), and have similar modal
compositions: gar(70)+ol(28)+sp(2), gar(9)+ol(90)+sp(1) and
gar(50)+ol(30)+sp(20). Rock texture is medium-crystalline, while
structure is massive. We also identified a garnet macrocryst of 0.5*0.4
cm in size with a pale green kelyphytic rim (sample NL-31) and 1.7*1.5
cm in size intergrowth: green garnet (14) + altered olivine (75) +
chrome spinel small inclusions (1) (sample NL-41).
Garnet
composition in the studied samples is quite constant and is
characterized by the high Cr2O3 content
(10.94-11.99%) and CaO content (19.52-24.94%) at the reduced contents of
TiO2 (0.24-0.52%). The chrome spinel is high Cr2O3
(55%) content and the low TiO2 (0.5-0.6%) content. Olivine
is high-Mg (Fo95), but elevated CaO content (0.09%).
Isotopic composition of
oxygen in garnet (d18O = 4.05-4.25 pm) and olivine (d 18O
= 4.91 pm) differs drastically from the mantle values (d 18О
= 5,5+/-0,4 о/оо
after Mattey
et al., 1994).
Rb-Sr
and Sm-Nd isotopic composition
(sample
NL-11)
show the relatively "young"
model age of the sample relative to the depleted mantle (1.78 billion
years), the age of formation of this rocks is also relatively "young" –
probable mezoproterozoic.
In
kimberlites and placers of the Nyurbinskaya pipe (Nakyn field, Yakutia)
there are 4 green garnet grains of 0.5-2.0 mm in size, including one
intergrowth gar+sp. Most garnets are characterized by the higher CaO
(18.06-22.87%) and TiO2 (1.46, 1.65, 1.75%) contents not
noted before for similar garnets.
Studied
green garnets have the similar "sine wave" type of REE distribution for
low-Ti garnets and a "raised" type of REE distribution with enrichment
in medium and light REE for high-Ti garnet.
All
green garnets are characterized by an increased content of light REE and
Sc. High-Ti garnets are characterized by an increased content of light
and middle REE, as well as titanium, and a particularly sharply
increased content of Zr (up to 1042 ppm!).
Paragenesis ol+sp is formed
at 805o and 23.4 kbar, and paragenesis ol+gar is formed at
1080o and 23.8 kbar.
The rocks are characterized by
nonequilibrium paragenesis ol+sp+gar and formation at moderate depths
(80-90 km) under conditions of high heat flow (52-55
mW/m2).
Judging from
variations modal composition of studied xenoliths, absence of
clinopyroxene, variations in chemical compositions and trace element
compositions,
relatively "young" model age
and non-mantle isotopy of oxygen in garnets, these rocks are not
“wehrlites” and likely represent metasomatic rocks such as
uvarovite-chromite veins or schlierens at the moderate depths of upper
mantle – it is
similar to uvarovite-chromite veins of the
metasomatic or a
hydrothermal origin in the crustal serpentinites.
References
Clarke D.B. and Carswell
D.A. (1977) Green garnets from the
Newlands kimberlite, Cape Province, South Africa.
Earth and Planetary Science Letters
34 . p. 30- 38.
Kharkiv A.D. (1978) A find
of the ore wehrlite xenoliths in “Zimnaya” kimberlite pipe
(Verhne-Munsky diamondiferous region, Yakutia) Dokl. Akad. Nauk SSSR.
252(3), 707-711 (in Russian).
Mattey, D., Lowry, D.,
Macpherson, C. (1994). Oxygen isotope composition of mantle peridotite.
Earth Planet. Sci. Lett. 128, P.
231-241. |