Archean mantle heterogeneity and the origin of diamondiferous
eclogites, Siberia: Evidence from stable isotopes and hydroxyl in garnet
GREGORY A. SNYDER, LAWRENCE A. TAYLOR, ERIC A. JERDE
Planetary Geosciences Institute
Department of Geological Sciences, University of Tennessee, Knoxville,
Tennessee 37996, U.S.A
ROBERT N. CLAYTON, TOSHIKO K. MAYEDA
Enrico Fermi Institute
University of Chicago, Chicago, Illinois 60637, U.S.A.
PETER DEINES
Department of Geosciences
Pennsylvania State University, University Park, Pennsylvania 16802,
U.S.A.
GEORGE R. ROSSMAN
Division ofGeological and Planetary Sciences
California Institute of Technology, Pasadena, California 91125, U.S.A.
NIKOLAI V. SOBOLEV
Institute of Mineralogy
and Petrography
Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
ABSTRACT
Data are presented for theO isotopic composition of clinopyroxene and
garnet, the C isotopic composition of diamond, and the OH content of
gamet from eclogite xenoliths brought to the surface by the Udachnaya
kimberlite pipe, Yakutia, Siberia. Radiogenic isotopic data suggest
that the eclogites could have been derived from
an ultradepleted mantle at approximately 2.9 Ga (Pearson et al., 1995;
Snyder et al., in preparation). O isotopic compositions generally show
equilibration between the eclogitic minerals (elta Cpx-Grt = 0.11-0.41
per mil) and have delta 18OSMOW
for both garnet and clinopyroxene that lie near the range of accepted
mantle values of 5.7 ± 0.7 per mil. However, several
eclogites indicate significant deviations from this range, at higher
values of
6.8-7.0 per mil. Also, two eclogites lie at the lower end of the mantle
range, at values of 4.8 and 5.0 per mil (all in clinopyroxene). C
isotopic compositions of diamonds all have delta 13CPDB
in the range of -1 to -7 per mil and are centered at approximately -5
per mil, also within the range of accepted mantle values. The OH
contents of the garnet are generally between 0 and 22 ppm (as H2O),
although two samples exceed 70 ppm. This range of OH is similar to
eclogitic garnet from the Kaapvaal craton of southern Africa.
The mantle C isotopic values of associated diamonds, the majority of O
isotopic data, and the low OH
content of the minerals, although suggesting a general lack of crustal
participation in the formation of the Udachnaya eclogites, do not rule
out the participation of some ancient crustal material.
That these eclogites include both 18O-enriched
and 18O-depleted types
suggests that the protoliths may have been
overprinted by both low- and high-temperature hydrothermal
events (cf. Jacob et al., 1994). A
positive correlation between delta 18O and 87Sr/86Sr
allows the interpretation
of these eclogites as representing a cross section of an Archean
ophiolite. However, the lack of a single coherent grouping on a plot
of delta 18O vs
87Sr/86Sr
suggests that, if the
Udachnaya eclogites were
derived from oceanic crust, they cannot be cogenetic and must represent
at least two separate ophiolite sequences. Conversely, if the eclogites
are found to be cogenetic, then a totally different process affected
their formation and a probable metasomatic mechanism was operative.
Because of the total lack of correlation of delta 18O with other geochemical
parameters, we find no compelling evidence that all eclogites are
derived ultimately from oceanic crust.