ECLOGITIC INCLUSIONS IN DIAMONDS - EVIDENCE OF COMPLEX MANTLE PROCESSES OVER TIME

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].