La. Taylor et al., ECLOGITIC INCLUSIONS IN DIAMONDS - EVIDENCE OF COMPLEX MANTLE PROCESSES OVER TIME, Earth and planetary science letters, 142(3-4), 1996, pp. 535-551
The first ion-probe trace element analyses of clinopyroxene-garnet pai
rs both included within diamonds and from the eclogite host xenoliths
are reported; these diamondiferous eclogites are from the Udachnaya an
d Mir kimberlite pipes, Yakutia, Russia. The major and trace element a
nalyses of these diamond-inclusion and host-rock pairs are compared in
order to determine the relative ages of the diamonds, confirm or deny
genetic relationships between the diamonds and the eclogites, evaluat
e models of eclogite petrogenesis, and model igneous processes in the
mantle before, during, and after diamond formation. The most striking
aspect of the chemical compositions of the diamond inclusions is the d
iversity of relationships with their eclogite hosts. No single distinc
t pattern of variation from diamond inclusion minerals to host mineral
s is found for all four samples. Garnet and clinopyroxene inclusions i
n the diamonds from two samples (U-65/3 and U-66/3) have lower Mg#s, l
ower Mg, and higher Fe contents, and lower LREE than those in the host
eclogite. We interpret such variations as due to metasomatism of the
host eclogite after diamond formation. One sample, U-41/3 shows enrich
ment in diamond-inclusion MREE enrichment relative to the eclogite hos
t and may indicate a metasomatic event prior to, or during, diamond fo
rmation. Bulanova [2] found striking differences between inclusions ta
ken from within different portions of the very same diamond. Clinopyro
xene inclusions taken from the central (early) portions of Yakutian di
amonds were lower in Mg# and Mg contents (by up to 25%) than those lat
er inclusions at the rims of diamonds. These trends are parallel to th
ose between diamond inclusions and host eclogites determined for four
of the five samples from the present study and may merely represent ch
anging magmatic and/or P-T conditions in the mantle. Garnet trace elem
ent compositions are similar in relative proportions, but variable in
abundances, between diamond inclusions and host eclogites. This is pro
bably due to the rapid diffusion of trace elements in garnet under man
tle temperatures and consequent alteration of the garnet, and not due
to juvenile diamonds 'locking in' source heterogeneities (c.f., [3]).
Trace element compositions of clinopyroxenes included in diamonds are
generally similar to those in the host eclogite. However, one host cli
nopyroxene does show enrichment in the LREE compared to that in the in
clusion and may be attributed to mantle metasomatism, not related to k
imberlite transport. In another eclogite, M-46, the host clinopyroxene
is depleted in the LREE and Fe, and enriched in the HREE and Mg, rela
tive to the inclusion and is consistent with partial melting of the ec
logite subsequent to diamond formation. Sm/Nd ratios in clinopyroxenes
appear to be little affected by these processes for most samples, all
owing Sm-Nd isotopic studies to yield important information about anci
ent protoliths. Eclogitic mineral inclusions in Yakutian diamonds appe
ar consanguineous with the diamonds, a contention supported by the obs
ervations of Bulanova [2]. Therefore, Re-Os whole-rock and Sm/Nd clino
pyroxene age determinations of the Udachnaya eclogites also yield the
time of diamond formation, approximately 2.9 Ga [32,33].