ALBERT

Notes: Abstract Textural interpretation of ore-mineral assemblages, such as bornite-chalcopyrite (bn-cpy) intergrowths, should be based on definite experimentation. Appropriate exsolution and coarsening experiments were performed using sealed, evacuated, silicaglass tubes; synthetic bn-cpy solid solutions were annealed between 100° and 350°C for times of 20 min to 10 weeks. Early-formed textures develop through nucleation and growth and depend on the initial degree of supersaturation and the metal diffusivities. Final textures, due to additional growth and coarsening, are very sensitive to temperature and may serve as geothermometers. Mutual boundary textures which form above 250°C can originate by: (1) simultaneous precipitation; (2) exsolution during slow cooling from above the solvus; and (3) metamorphism to temperatures above about 250°C. Widmanstätten textures are not compatible with slow cooling, but indicate: (1) low-temperature replacement of bn; or (2) exsolution of cpy lamellae from anomalous bn heated to around 200°–250°C during mild metamorphism.

Notes: Abstract Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo85–93), garnet (0.21–9.07% Cr2O3), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56–0.88% Cr2O3), and Cr-poor orthopyroxene (〈0.03–0.34% Cr2O3) in a matrix of olivine (Fo88–92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79–0.86) and CaO (4.54–7.10%) with Cr2O3 content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr2O3 and increase in TiO2 (0.38–0.56%) with decreasing mg (0.87–0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06–0.38% Cr2O3) and less Cr-rich than peridotite clinopyroxenes (1.39–1.46% Cr2O3). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09–1.16% CaO and 1.16–1.18% Al2O3) and low-Ca, low-Al (0.35–0.46% CaO and 0.67–0.74% Al2O3). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86–0.98% CaO and 0.95–1.01% Al2O3). The high-Ca, high-Al group (Group I) has lower mg (0.88–0.90) than low-Ca, low-Al group (Group II) with mg=0.92–0.93; low mg orthopyroxenes (Group Ia) have lower Cr2O3 and higher TiO2 than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr2O3 than peridotite orthopyroxenes (0.46–0.57% Cr2O3). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165°–1,390° C and 1,295°–1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165°–1,255° C and 51–53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970°–1,020°C and 46–56 kb (± 5kb). Garnet peridotites equilibrated at 1,240°–1,360° C and 47–49 kb. Spinel peridotites have discordant temperatures of 720°–835° C (using spinel-olivine Fe/Mg) and 865°–1,125° C (Al in orthopyroxene). Megacrysts probably precipitated from a fractionating liquid at 〉150 km depth. They are not disaggregated peridotite because: (1) of large crystal size (up to 1.5 cm), (2) compositions are distinctly different from peridotite phases, and (3) they display fractionation trends. The high mg, low T orthopyroxenes and the clustering of olivine rims near Fo89–90 reflect liquid changes to higher MgO contents due to (1) assimilation of wall-rock and/or (2) an increase in Fe3+/Fe2+ and subsequently MgO/FeO as a result of an increase in f o.

Notes: Abstract Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the mineral compositions and textures. Exsolution of garnet from clinopyroxene is one such texture, occurring in a minority of mantle eclogites. New analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at Bellsbank (South Africa) and Obnazhënnaya (Yakutia, Russia) are presented here, and these are combined with data from the literature. Exsolution of garnet from clinopyroxene is generally lamellar, although lens-shaped garnets are also present. Major- and trace-element characteristics show a wide range of compositions and include eclogite Groups A, B, and C. Rare-earth element (REE) concentrations of garnet and pyroxene were determined by SIMS, and the REE patterns are subtly different from those in “ordinary” eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differences in the slopes of chondrite-normalized REE patterns. It is possible that these “signatures” are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstructed whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is indicative of a Ca-Eskola component. Reconstructed “pyroxenes” from kyanite- and corundum-rich samples contain unrealistic amounts of aluminium for mantle pyroxenes. A protolith (or parental pyroxene) “threshold” of ∼24% Al2O3 may exist, above which (as in a plagioclase cumulate) the final assemblage is kyanite- and/or corundum-bearing.

Notes: Abstract Caporuscio and Smyth have presented crystal chemical study on eleven mantle-derived eclogites from the Bellsbank (8) and Roberts Victor (3) kimberlites, South Africa. They combine these results with experimental partition coefficients and geochemical modelling to argue for a high pressure igneous cumulate origin from a MORB-like melt for these eclogites. In particular, they highlight the kyanite-bearing eclogites (grospydites), especially the presence of a “middle rare-earth-element (MREE) enrichment”, which may also be considered in terms of LREE and HREE depletions. Caporuscio and Smyth, as well as Smyth et al., cite this phenomenon as evidence for an origin by igneous accumulation of hyperaluminous clinopyroxene at high pressure. However, this type of REE pattern has also been interpreted as depicting a positive Eu anomaly inherited from a subducted, plagioclase-rich oceanic crustal protolith (Shervais et al., Taylor and Neal, Neal et al.). This manuscript presents an alternative interpretation of the data presented by Caporuscio and Smyth. The results presented here demonstrate that high-pressure igneous accumulation of hyperaluminous pyroxene from a MORB-like liquid, followed by exsolution of major amounts of garnet and kyanite, is unlikely to account for all chemical signatures exhibited by grospydites. Our approach is to undertake quantitative geochemical modelling of these processes using the actual samples and literature values quoted by Caporuscio and Smyth.

Notes: Abstract 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.

Notes: Abstract Olivine samples (Fa 11) have been oxidized in air (f O2 = 0.2 atm) at temperatures ranging from 350–700 °C and examined by Mössbauer spectroscopy, transmission electron microscopy, X-ray powder diffraction and thermomagnetic analysis. Oxidation of olivine was found to result in ferriolivine, magnesioferrite (major oxide phase) and magnetite (minor oxide phase) formation. Ferriolivine forms planar (001) precipitates, 0.6 nm in thickness, in the olivine host; the composition is likely to be Mg0.5 v 0.5(Fe3+)1.0SiO4. Magnesioferrite MgFe2O4 exsolves as fine-grained precipitates (5–6 nm in size) filling interstices between the ferriolivine planar precipitates. Oxidation kinetic data at 700 °C show two stages of oxidation corresponding to formation of ferriolivine in the first stage and magnesioferrite in the second stage. The linear rate law with a rate constant k Fol = 1.23 · 10-3 s-1 was found for the first stage whereas a parabolic rate-law with a constant of k oxi = 3.28 · 10-3 s-1 was determined for the second stage of oxidation. It was found that ferriolivine is not an intermediate metastable phase in the oxidation process, terminated by magnesioferrite formation. The ferriolivine and magnesioferrite are considered to have formed by independent reactions which do not necessarily proceed simultaneously.