Dg. Pearson et al., RE-OS, SM-ND, AND RB-SR ISOTOPE EVIDENCE FOR THICK ARCHEAN LITHOSPHERIC MANTLE BENEATH THE SIBERIAN CRATON MODIFIED BY MULTISTAGE METASOMATISM, Geochimica et cosmochimica acta, 59(5), 1995, pp. 959-977
A suite of peridotite xenoliths from kimberlites intruding the Siberia
n craton indicate the pressence of lithospheric mantle over 150 km thi
ck at 350 Ma. We report Sr-Nd isotope data for minerals from the perid
otite xenoliths together with osmium isotopic compositions for whole-r
ocks and two olivine separates. Additionally, the osmium isotopic comp
osition of a carbonatite from Fort Portal, Uganda, has been measured i
n order to evaluate the effect of carbonatite metasomatism on mantle R
e-Os systematics. Osmium isotope compositions of peridotite xenoliths
from the Mir and Udachnaya kimberlites vary from those characteristic
of the oceanic mantle, to considerably less radiogenic values (Os-187/
Os-188, 0.16469 to 0.10812), comparable to those previously found in o
ther crationic peridotites. In contrast, two eclogite xenoliths from U
dachnaya have extremely radiogenic Os, Os-187/Os-188, up to 9.67. The
lowest peridotite osmium isotopic compositions require Re depletion in
the mid-Archaean (3.2 Ga) and this age is interpreted as the time of
differentiation of the Siberian cratonic lithospheric mantle. Archaean
depletion ages for spinel peridotites of relatively shallow origin an
d garnet peridotites and dunites containing diamond indicate that the
depleted lithosphere reached from the Moho to 150 Km depth at this tim
e and has been stable for 3 Ga. Re-Os and Sm-Nd model ages for two ecl
ogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an
ancient origin for the Siberian lithosphere. The oldest peridotite dep
letion ages and the eclogite model ages correspond to the oldest crust
al ages obtained from the Anabar Shield of the Siberian craton, and su
ggest that the initiation of major crust formation and stabilisation o
f a thick cratonic keel were coeval. In general, the Siberian low-temp
erature peridotites are not as enriched in incompatible elements as th
ose from the Kaapvaal craton yet their diopsides possess similar, low
Sm/Nd. The low incompatible element concentrations but LREE/MREE enric
hment seen in some Siberian lherzolites suggest they may be the produc
ts of disequilibrium melting. Neodymium and strontium isotopic composi
tions of minerals from the peridotites are extremely heterogeneous (ep
silon(Nd(350)), -55.1 to 491; Sr-87/Sr-86, 0.70253 to 0.72235). Subcal
cic garnets of diamond inclusion-like composition within megacrystalli
ne peridotites have epsilon(Nd(350)) values varying from -55.1 to -12.
1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an an
cient, enriched origin similar to that suggested for diamond inclusion
s (Richardson et al., 1984). Alternatively, consideration of the compl
ex garnet Sm-Nd isotope systematics and the presence of unsupported ra
diogenic Sr together with marked trace element zonation (Shimizu et al
., 1994) suggest that these subcalcic garnets crystallised recently (c
lose to the time of kimberlite eruption) from ancient, LREE-enriched,
high Rb/Sr precursors. We propose that the isotope systematics of subc
alcic garnet diamond inclusions can also be interpreted in terms of a
recent origin.