Kimberlite Groundmass Research Articles

POTASSIUM SULFIDES IN KIMBERLITE-HOSTED CHLORIDE-"NYEREREITE" AND CHLORIDE CLASTS OF UDACHNAYA-EAST PIPE, YAKUTIA, RUSSIA

Chloride-bearing clasts from the Udachnaya-East pipe kimberlites, Yakutia, Russia, contain abundant to accessory rasvumite, KFe2S3, and djerfisherite, K6Na0-1(Fe,Ni,Cu)(24)S26Cl, which have been previously described in the groundmass of the host kimberlite. Rasvumite occurs in chloride-nyerereite and carbonate clasts, where it forms prismatic crystals (up to 5 mm) or is associated with djerfisherite in a rim around pyrrhotite. Djerfisherite is the omnipresent sulfide in all types of chloride-containing clasts, in which it rims pyrrhotite and forms individual xenomorphic to subhedral grains and, more rarely, octahedral crystals at the contact with host kimberlite. In chloride clasts and the host kimberlites, the following sequence of crystallization was found: pyrrhotite - rasvumite - djerfisherite, reflecting an increase in the activity of Cl in the evolved melt. The composition of rasvumite from the Udachnaya-East pipe is close to ideal KFe2S3, with low abundances of Rb, Cs and Na (< 0.9, < 0.2 and <0.15 wt.%, respectively). Analyses of the rasvumite by the LA-ICP-MS technique indicates the presence of T1 (95-480), Ba (110-215), Pb (25-190), Te (up to 60), Se (20-135) and Co (25-100 ppm). These data also indicate compositional differences between unaltered rasvumite and its oxidized rim; the levels of Na, Mg, V, Mn, Ni, As, Sr, Sb and W increase by an order of magnitude. Unlike djerfisherite in the kimberlite groundmass, that in the chloride clasts is generally characterized by higher amounts of Fe and lower amounts of Cu, with low contents of Rb (< 2000 ppm), Cs (< 100 ppm), and Tl (< 330 ppm). Sulfides of potassium in chloride-bearing clasts are considered to be primary phases crystallized from an evolved kimberlitic melt.

Djerfisherite in the Udachnaya-East pipe kimberlites (Sakha-Yakutia, Russia): paragenesis, composition and origin

Djerfisherite, an unusual potassium- and chlorine-bearing sulphide K6Na(Fe,Ni,Cu)(24)S26Cl, is found in remarkably fresh rocks of the Udachnaya-East kimberlite pipe, including several varieties of kimberlite and a kimberlite-hosted phlogopite-spinel lherzolite xenolith. In both kimberlite breccia and monticellite kimberlite djerfisherite is a common groundmass mineral. Djerfisherite is also present as a daughter phase in olivine-hosted inclusions of trapped carbonate-chloride melt and sulphide melt. The mineral is present as irregular or rounded grains (up to 80-100 mu m) in association with magnetite and pyrrhotite in the kimberlite groundmass, and together with carbonates, Na-K-chlorides, silicates, magnetite, sulphates and Fe-Ni-sulphides in melt inclusions. Djerfisherite in the lherzolite xenolith is mainly interstitial (up to 100 mu m) and commonly rims primary mantle sulphides that show clear signs of replacement. Broad compositional variations in Fe, Ni and Cu are common in djerfisherite from different occurrences of the Udachnaya-East pipe. Textural relations, heating stage experiments with melt inclusions and compositional data, suggest a late magmatic origin of djerfisherite in the Udachnaya-East kimberlite groundmass, at shallow depths and at T <= 800 degrees C. In contrast, djerfisherite in the lherzolite xenolith appears to be a product of direct precipitation from evolved kimberlite magma infiltrating into lithospheric xenoliths or reactions of evolved kimberlite fluids/melts with primary minerals in xenoliths.

Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle

Kimberlite magmas, as the deepest probe into Earth's mantle (>150 km), can supply unique information about volatile components (hydrogen, carbon, chlorine, sulfur) in mantle-derived melts and fluids. All known kimberlite rocks are not suitable for studies of mantle volatiles because of their pervasive postmagmatic alteration; however, this study discusses an exceptionally fresh group I kimberlites (<0.5 wt% H2O) from the Udachnaya-East diamondiferous pipe in Siberia. Kimberlite groundmass, in addition to euhedral olivine and calcite, is extremely enriched (at least 8 wt%) in water-soluble alkali chlorides, alkali carbonates, and sulfates (ratio 5:3:1), and often shows immiscibility textures. A primary magmatic origin of alkali chlorides and alkali carbonates is confirmed by the study of strontium isotopes in the water- and dilute acid-leachates of the groundmass (Sr-87/Sr-86 = 0.7069 and 0.7050) that contrast with much more radiogenic isotope composition of the Cambrian platform sedimentary rocks and the Udachnaya-East mine-site brines. Melt inclusions in groundmass olivine, composed of halite, sylvite, alkali-Ca carbonates, phlogopite, olivine, and CO2 fluid, were used to determine the composition and evolution of the kimberlite melt prior to emplacement. Melt inclusions show immiscibility between chloride and carbonate liquids at <600oC in heating stage experiments. The chloride and carbonate enrichment in the kimberlite parental magma suggests the presence of a powerful agent for chemical modifications (metasomatism) in the mantle and crust.

Serpentinization of phlogopite phenocrysts from a micaceous kimberlite

Phenocrysts of phlogopite from a micaceous kimberlite contain finely interlayered serpentine. These phenocrysts occur in the kimberlite groundmass and are altered along the mica layers and are slightly deformed. Lizardite is the predominant serpentine mineral, but chrysotile and mixed structures also occur. The lizardite occurs as lamellae within phlogopite, oriented such that (001) layers of the two minerals are parallel and the [100] direction of lizardite is parallel to the [100] or 〈110〉 directions of phlogopite. The serpentinized regions of phlogopite are localized and extensive along the (001) layers. Chrysotile fibers and chrysotile-like curled serpentine occur within regions of disrupted material, and polygonal structures occur in folded lizardite lamellae. Textural relations suggest three events: 1) replacement of phlogopite by lizardite, 2) deformation of the phenocrysts, and 3) partial transformation of the lizardite to chrysotile-like structures. Deformation features include openings along (001), folds, and regions of disrupted or broken material. The folded and broken material consists of lamellar lizardite and phlogopite, indicating that lamellar replacement preceded deformation. Intergrowths of lizardite and curled serpentine are associated with cleavage openings and voids in disrupted material, suggesting that a partial transformation of lizardite to chrysotile occurred within openings created by deformation. Clay minerals also occur within phlogopite as either a minor product of serpentinization or of late-stage alteration.

Petrological Significance of Mineral Chemistry in the Agathla Peak and the Thumb Minettes, Navajo Volcanic Field

The Agathla Peak and The Thumb minettes from the Navajo province, southwestern USA, are characterized by macrocrysts of phlogopite and diopside plus rare olivine () set in a crystalline to glassy groundmass containing diopside, phlogopite, sanidine, apatite, Ti-magnetite and analcime. The diopside has low Al, Ti, and Na like late-stage diopside in kimberlite groundmass. Ti-rich phlogopite is zoned with increasing Fe, Ti and Ba from a Cr, Ni-bearing core. High Sr in the minettes is expressed in groundmass sanidine (up to 1.2 wt % SrO), apatite (0.5-0.9) and diopside (0.03-0.11). Xenocrystal apatite is chemically distinct from phenocrystal apatite in The Thumb minette in having low SrO (< 0.04 wt %) and a distinct REE distribution which matches apatites in garaet-granulite xenoliths. All chemical properties of the minerals are consistent with derivation of the minette magmas from a region of the upper mantle containing olivine, clinopyroxene, phlogopite and ilmenite and enriched in K, Ti, Ba, Sr, P, REE, H, and halogens, as proposed by earlier workers.

Further data on the occurrence of pectolite in kimberlite

AbstractPectolite has been identified, using petrographic, chemical, and X-ray methods, in kimberlites from the De Beers and Dutoitspan Mines, Kimberley, South Africa and the Letseng-La-Terai Mine, Lesotho. It occurs as radiating aggregates of fibrous or acicular crystals not only in the groundmass of certain kimberlites but also in altered xenoliths included within kimberlite. The occurrence of Na-rich minerals in kimberlite is unusual. It is suggested that it can occur as an apparently primary mineral resulting from the incorporation and assimilation of foreign Na-bearing material prior to final consolidation, and/or as a secondary mineral after metasomatic introduction of Na-bearing fluids.

RbSr age determinations on South African kimberlite pipes

A study has been made to establish to what extent reliable age-measurements can be obtained by the RbSr isochron method on phlogopite micas from kimberlite and its associated inclusions. Essentially linear isochrons were usually obtained for the micas when grouped according to their source. The data for the Wesselton and Roberts Victor pipes are examined in detail to assess the validity of the method. The available evidence suggests that mica which occurs in kimberlite groundmass ahd associated peridotite and eclogite inclusions yields valid ages for pipe emplacement by the isochron method. Phlogopite nodule samples appear to yield an age that is anomalously high due to radiogenic Sr retention, and an association with carbonatite is implied. Results for the following pipes are reported: Wesselton, DuToitspan, De Beers, Bultfontein and Monastery (mean age 86±3 m.y.); Roberts Victor (127±3 m.y., possibly less reliable); Swartruggens (147±4 m.y.); Gross Brukkaros (∼84 m.y., not considered reliable).