Vladimir N. Sobolev

Diamondiferous eclogites from Yakutia, Siberia: evidence for a diversity of protoliths

Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Neal 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are inconsistent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application to Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However, reconstructed REE compositions using various garnet: clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (either directly or through partial melting) as “normal” or Type-I ocean floor basalt. Others, however, require material of eclogitic or pyroxenitic affinities to undergo partial melting; this facilitates the depletion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of “anomalous” or Type II ocean floor basalt. This fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification.

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

The first ion-probe trace element analyses of clinopyroxene-garnet pairs both included within diamonds and from the eclogite host xenoliths are reported; these diamondiferous eclogites are from the Udachnaya and Mir kimberlite pipes, Yakutia, Russia. The major and trace element analyses 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, evaluate 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 diversity of relationships with their eclogite hosts. No single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples. Garnet and clinopyroxene inclusions in the diamonds from two samples (U-65/3 and U-66/3) have lower Mg#s, lower 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 enrichment in diamond-inclusion MREE enrichment relative to the eclogite host and may indicate a metasomatic event prior to, or during, diamond formation. Bulanova [2] found striking differences between inclusions taken from within different portions of the very same diamond. Clinopyroxene inclusions taken from the central (early) portions of Yakutian diamonds were lower in Mg# and Mg contents (by up to 25%) than those later inclusions at the rims of diamonds. These trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and/or P-T conditions in the mantle. Garnet trace element compositions are similar in relative proportions, but variable in abundances, between diamond inclusions and host eclogites. This is probably due to the rapid diffusion of trace elements in garnet under mantle 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 clinopyroxene does show enrichment in the LREE compared to that in the inclusion and may be attributed to mantle metasomatism, not related to kimberlite transport. In another eclogite, M-46, the host clinopyroxene is depleted in the LREE and Fe, and enriched in the HREE and Mg, relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation. SmNd ratios in clinopyroxenes appear to be little affected by these processes for most samples, allowing SmNd isotopic studies to yield important information about ancient protoliths. Eclogitic mineral inclusions in Yakutian diamonds appear consanguineous with the diamonds, a contention supported by the observations of Bulanova [2]. Therefore, ReOs whole-rock and SmNd clinopyroxene age determinations of the Udachnaya eclogites also yield the time of diamond formation, approximately 2.9 Ga [32,33].

Precise Mossbauer milliprobe determination of ferric iron in rock-forming minerals and limitations of electron microprobe analysis

Abstract For estimations of P - T conditions of igneous and metamorphic rocks, Fe3+ in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe2+ - Mg2+ exchange can be significantly affected by either neglecting Fe3+ or using incorrect values. Ratios of Fe3+/ΣFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe3+ contents in garnet and clinopyroxene on temperature estimations were evaluated. The following Fe3+/ΣFe (in at%) values were obtained (EMPA/Mössbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22.0. The effects of Fe3+ in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na2O contents from 5.67 to 5.74 wt% (< 2.0% relative error) would lead to increasing Fe3+/ΣFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 °C. Various Fe3+/ΣFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe3+/ΣFe whereas for garnet, T increases with increasing Fe3+/ΣFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe3+ corrected vs. uncorrected microprobe analyses. Little correlation exists between EMPA-calculated and Mössbauer-measured Fe3+/ΣFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe3+ may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe3+ calculated from EMPA of silicate minerals leads to serious questions with regard to KD values obtained from natural assemblages.

Nd and Sr isotopes from diamondiferous eclogites, Udachnaya Kimberlite Pipe, Yakutia, Siberia: Evidence of differentiation in the early Earth?

Nd and Sr isotopic data from diamond-bearing eclogites found in the Udachnaya kimberlite, Yakutia, Siberia, are interpreted as indicating an early (>1 4 Ga) differentiation event, whereby the mantle split into complementary depleted and enriched reservoirs. Reconstructed whole-rock 87Sr/S6Sr ratios (present-day) range from 0.70151 to 0.70315 and are consistent with a mantle origin for these rocks. The Nd isotopic evolution lines of four samples (U-5, U-37, U-41 and U-79) converge at 2.2-2.7 Ga. Sample U-5 is unique in exhibiting the most enriched signature of any of the samples yet analyzed (present-day end of --20), and this sample points unequivocally to an old, enriched component. A complementary depleted mantle component is suggested by two of the eclogite samples, U-86 and U-25, which yield ENd values (at 2.2 Ga) of + 13 and + 7, respectively. The two mantle reservoirs possibly formed prior to 4 Ga and evolved separately until 2.2-2.7 Ga. At that time, the reservoirs were melted forming eclogites both as residues (from the enriched reservoir) and as partial melts of peridotite (from the depleted reservoir), resulting in demonstrably different histories for eclogites from the same locality.

Diamondiferous Eclogites from the Udachnaya Kimberlite Pipe, Yakutia

Xenoliths from the upper mantle have undergone a wide variety of processes at varied temperatures and pressures, as recorded by mineral compositions and textures. Eclogite xenoliths in kimberlites are a unique source from which to obtain information about such processes. Furthermore, eclogites that contain accessory diamond yield important compositional constraints on the deeper upper mantle. Whether eclogites have mantle or crustal origins is still a subject of controversy. Mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite in Yakutia, Siberia are presented and combined with previous studies of these eclogites by our group (Jerde et al., 1993, 1994; Snyder et al., 1993a; Sobolev et al., 1994). Five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low-jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets. Eclogites ...

Extreme Chemical Diversity in the Mantle during Eclogitic Diamond Formation: Evidence from 35 Garnet and 5 Pyroxene Inclusions in a Single Diamond

We report major- and trace-element variations in 35 garnet inclusions extracted from a single eclogitic diamond from the Mir kimberlite pipe, Yakutia. The range in CaO and Mg# in these garnets is large-from 3.84 to 9.66 wt% and 45 to 56, respectively. These ranges cover nearly half of the total range in eclogitic garnet compositions from diamonds worldwide. An extremely wide range in trace elements such as Y, Zr, and Sr also covers nearly the total range recorded for garnets included in diamonds, as well as that known for rock-forming minerals of diamondiferous eclogite xenoliths-15.1 to 48.9 ppm Y; 2.2 to 40.8 ppm Zr; and 0.5 to 9.1 ppm Sr. The widest ranges in REE between different grains (by an order of magnitude) are noted for LREE and MREE. Such compositional ranges are the first recorded for garnets available as inclusions in a single eclogitic diamond and cover nearly the complete range of compositions known for all Yakutian eclogites. Five clino-pyroxene inclusions exhibit more moderate ranges in ...

Significance of eclogitic and related parageneses of natural diamonds

Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents ...

Quantifying the Effects of Metasomatism in Mantle Xenoliths: Constraints from Secondary Chemistry and Mineralogy in Udachnaya Eclogites, Yakutia

In mantle xenoliths, metasomatism is recorded by compositional variations within and between minerals, and by the introduction of secondary minerals. However, metasomatism has not been quantitatively evaluated as a process with respect to the fluid composition involved. Diamondiferous eclogites from the Udachnaya kimberlite provide a unique suite of samples that allow a semiquantitative estimation of metasomatic fluid composition. The basis of our analysis involves comparison of reconstructed whole-rock compositions with measured whole-rock analyses. Primary minerals in these samples are relatively homogeneous, and permit the use of modal analyses and mineral chemistry for reconstruction of “pristine” whole-rock compositions. The metasomatic overprint, which is similiar in all samples studied, has produced depletions in SiO2, Na2O, and FeO and enrichments in TiO2, K2O, MgO, and LREE. Secondary minerals from the samples are interpreted as the direct result of metasomatism (i.e., typical metasomatic mineral...