Roger H. Mitchell

Petrology of Lamproites

Petrology of Lamproites: Roger H. Mitchell, S.C. Bergman ... Geochemistry and petrology of the Early Miocene lamproites ... Petrology of lamproites (Book, 1991) [WorldCat.org] Petrology of Lamproites (030643556X) by Mitchell, Roger H ... CLASSIFICATION OF LAMPROPHYRES, LAMPROITES, KIMBERLITES ... Mineralogy of Lamproites | SpringerLink Lamproite Wikipedia Petrogenesis of Proterozoic Lamproites and Kimberlites ... Kimberlite Wikipedia Petrology of Lamproites from Smoky Butte, Montana ... Lamproite an overview | ScienceDirect Topics Petrology of Lamproites 1991, Roger H. Mitchell, S.C ...

Sr, Nd and Pb isotope and minor element geochemistry of lamproites and kimberlites

All samples studied here exhibit loweNd (lamproites; Western Australia = −10 to −19.2 and Smoky Butte = −21.6 to −25.9: Group II kimberlites: Finsch Mine = −5.8 to −7.4) and(87Sr/86Sr)i > Bulk Earth. They are all LREE enriched (up to 1000 × chondrite) and have high trace element contents (i.e. Rb, Rb, Ba, Th, K, Ta, Sr, P, Hf, Zr and Ti). Smoky Butte and Finsch rocks plot below the Pb-ore growth curve(206Pb204Pb) = 16.02−16.64 and 17.74−17.99, and207Pb204Pb= 15.19−15.28 and 15.48−15.57, respectively) consistent with two-stage evolutions whereas the Western Australia rocks exhibit high207Pb204Pb (15.71−15.75) with unradiogenic206Pb204Pb (17.23−17.57) indicative of a more complex three-stage evolution. These lamproites and Group II kimberlites were at least predominantly derived from source regions which had had low, but variableUP/b andRb/Sr for between 1 and 2.5 Ga. Such source regions are inferred to exist within the subcontinental mantle lithosphere. Different styles of trace element enrichment are recognised which with time develop different isotope-isotope trends. HighRb/Sr,Rb/Ba andK/Ti ratios observed in Group II kimberlites and metasomatised peridotites are probably related to the migration of H2O-rich fluids within the upper mantle, while the lamproites have features consistent with the introduction of small volume silicate melts to their source regions, i.e. highT/iK,Ta/Yb and lowRb/Ba. At Smoky ButteRb/Sr was also low, but the Western Australia rocks have highRb/Sr because they are relatively less enriched in Sr. Hence highRb/Sr ratios (and with time high87Sr/86Sr) are developed in mantle source with both high and lowRb/Ba. Inferred μ for the last stage of evolution of their source regions is < 7.5 for all the rocks studied and there is a striking positive correlation betweeneNd and206Rb/204Pb, which indicates that lowSm/Nd andU/Pb ratios coexist (over long periods of time) within the subcontinental mantle lithosphere.

Abundance and distribution of gold, palladium and iridium in some spinel and garnet lherzolites: implications for the nature and origin of precious metal-rich intergranular components in the upper mantle

Abstract The abundance and distribution of Au, Pd, Ir, Cu, Co and Cr has been determined in mantle-derived spinel lherzolite xenoliths in basanites from Mt Porndon (Victoria, Australia) and Kilbourne Hole (New Mexico, U.S.A.) and in garnet lherzolites from the Matsoku and Thaba Putsoa kimberlites (Lesotho). Minerals in the lherzolites concentrate Au, Pd and Ir in the following sequence of increasing platinum group element (PGE) content; garnet, olivine, orthopyroxene, clinopyroxene, spinel. and demonstrate that there exists a real crystallochemical control on the distribution of PGE. Whole rock PGE abundances calculated from the modal mineralogy are less than actually determined and indicate that the bulk of the PGE (60–80%) occur in a sulphide-rich intergranular component. A metasomatic origin for this component is considered to be unlikely and it is proposed that it represents an immiscible sulphide melt which has been retained in the mantle after extraction of a sulphur saturated basic partial melt. This component may in the case of garnet lherzolites have been modified by metasomatic events in the mantle leading to Au depletion and rare earth element addition. Spinel lherzolites are relatively homogeneous at a given locality but differ in their PGE content regionally. The weighted average abundances of PGE in a spinel lherzolite upper mantle are 0.6 ppb. Au, 4.0 ppb Pd. 3.6 ppb Ir. Garnet lherzolites are very heterogeneous and insufficient data is available to allow calculation of geochemically meaningful averages. Spinel lherzolite-basalt based pyrolite contains 0.9 ppb Au, 4.3 ppb Pd, 3.0 ppb Ir, and indicates that the mantle contains an apparent excess of Au over a calculated abundance based upon the siderophilic equilibrium distribution of Au between core and mantle. This excess is considered to be due to failure to consider the chalcophilic nature of Au in the mantle and not to the addition of a meteoritic component to a mantle equilibrated with the core.

Rare earth element geochemistry of the Fen alkaline complex, Norway

Determination of rare earth element (REE) abundances in rocks of the Fen complex has shown that within rocks of the first magmatic series REE abundances increase in the order urtite<ijolite<fine grained søvite <coarse grained søvite<silicocarbonatite. Low pressure differentiation of an ijolitic magma by crystallization of apatite, melanite, pyroxene and nepheline is reflected in the very variable La/Yb ratios (4–100) of the urtites and melteigites. Similar La/Yb ratios (40–60) in ijolite, tinguaite and søvite indicate that the søvites are not the result of extensive differentiation of a carbonated ijolitic magma. Within the second magmatic series REE increase in the order damtjernite<vibetoite-rauhaugite<rødberg. La/Yb ratios (30–551) are variable and reflect volatile transport of REE possibly as carbonate complexes. The effects of volatile transport, low and high pressure differentiation and liquid immiscibility on the Fen magmas are discussed and it is considered that parental magmas had relatively high La/Yb ratios (40–60). Utilizing petrological evidence from other alkaline complexes coupled with experimental studies it is considered that the parental magma was a carbonated nephelinite produced by limited (<10%) partial melting of the mantle. All the Fen rocks are placed in a petrogentic scheme in which a carbonated nephelinite magma undergoes liquid immiscibility, differentiation and volatile transport.

Carbonate-carbonate immiscibility, neighborite and potassium iron sulphide in Oldoinyo Lengai natrocarbonatite

Abstract Porphyritic natrocarbonatite lavas erupted from the Oldoinyo Lengai volcano (Tanzania) on 17 October 1995 and 15-19 December 1995 differ from previously studied lavas in that they preserve textures indicative of groundmass carbonate-carbonate immiscibility. The immiscible fractions are considered to involve: a Na-K-Ca-CO2-Cl-rich, F-bearing fluid crystallizing gregoryite, sodian sylvite, potassium neighborite as well as a complex Ba-rich carbonate; and a Na-rich, Cl-poor carbonate liquid approximating to a nyerereite-gregoryite cotectic composition. Compositional data are given for potassium neighborite, this mineral being the first recognized occurrence of a fluorine-based perovskite group mineral in a magmatic environment. New compositional data are also given for a previously recognized potassium iron sulphide which indicate that this phase is probably a solid solution between the ternary sulphides, KFe3S4, K2Fe3S4, and KFe2S3. Textural and paragenetic data are interpreted to suggest that these recent lavas are more evolved than previously investigated Oldoinyo Lengai lavas and that natrocarbonatite is a highly evolved rather than a primitive magma.

Composition of olivine, silica activity and oxygen fugacity in kimberlite

Abstract Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo94 Fo91) and euhedral groundmass olivines (Fo91 Fo88-5). Rounded phenocrysts less magnesian than Fo88 are considered to be mantle derived xenocrysts; such crystals comprise up to 40% of the phenocrysts material in kimberlite. Calculated silica activity ranges from 10−1,5 at 1200 °C, 50 kbs, to between 10−1,6 and 10−2.4 at 600 °C, 0.5–1.0 kb. Silica buffers involving olivines, pyroxenes and garnet are considered. Oxygen fugacities on the order of 10−20 bars indicate that kimberlite magmas are highly reduced at the time of groundmass formation.

Rare earth element geochemistry of kimberlite

Abstract Abundances of rare earth elements (REE), Sc, Co, Hf, Ta and Th have been determined by neutron activation analysis in kimberlites from the Wesselton pipe, in micaceous kimberlites from the Swartruggens fissure and kimberlite from the Monastery Mine, Ison Creek and Somerset Island pipes. Kimberlites are characterized by a high REE content and have chondrite normalized REE distribution patterns which show extreme fractionation of the REE group, e.g. kimberlite La Yb = 100 ; micaceous kimberlite La Yb = 140 . Distribution patterns are linear, with weak negative Eu anomalies evident in the micaceous types. It is considered that the major sites of REE are apatite, perovskite and carbonate and that Eu anomalies are due to the presence of perovskite or mica. REE distributions are analysed in terms of partial melting and eclogite crystallization models by means of REE crystal-liquid distribution coefficients. Partial melting models indicate that the La Yb ratios of kimberlites and micaceous kimberlites can be produced independently by differing amounts of partial melting ( La Yb ratio (ca. 5) which was produced by extensive (15–20%) partial melting of garnet lherzolite mantle. REE distributions do not provide evidence in favour of any one petrogenetic model, especially with regard to La and Yb abundances; this is considered to be a reflection of errors in the magnitude of the distribution coefficients. Evidence bearing on the possibility of eclogite crystallization from kimberlitic magmas is reviewed (i.e. eclogite distribution, age, mineralogy of kimberlite and of inclusions in diamonds) and is interpreted to indicate that eclogite fractionation is the least likely means of generating kimberlite liquids. The relation of kimberlite magmatism to continental basaltic magmatism is considered in terms of a partial melting model in which the extent of partial melting of the mantle is dependent upon heat flow variations with time. The small volumes of liquid required in partial melting hypotheses are thought to be concentrated into kimberlitic magmas by shearing processes.

Oxidation states of the upper mantle recorded by megacryst ilmenite in kimberlite and type A and B spinel lherzolites

The intrinsic oxygen fugacities of homogeneous, inclusion-free, megacryst ilmenites from the Frank Smith, Excelsior, Sekameng and Mukorob kimberlite pipes in southern Africa, and the alnöitic breccia in the Solomon Islands have been determined. Similar measurements have been made of the type A and B spinel peridotites from San Carlos in Arizona. The type A peridotites are characterised by oxygen fugacities close to the iron-wüstite buffer, similar to those of equivalent peridotite specimens from other continental and island arc environments. In strong contrast, the type B peridotites and all of the ilmenite megacrysts range between the oxygen fugacities defined by the nickelnickel oxide and fayalite-magnetite-quartz buffers. A close relationship between type B peridotites, oxidized metasomatizing fluids in the upper mantle and oxidized, silicaundersaturated magma types is suggested. It is unlikely that a solid elemental carbon phase can be an equilibrium crystallization product of kimberlite magmas if the ilmenite megacrysts represent the redox state of kimberlite melts. The ultimate source of the oxidizing fluids and the development of such a wide dispersion (>4 orders of magnitude) in oxygen fugacities of the upper mantle is not clear, but may involve recycled lithosphere, fluids from the lower mantle or result from the relatively rapid diffusion of H2, compared with other potential volatile species, in the mantle.

Geochemistry of magnesian ilmenites from kimberlites in South Africa and Lesotho

Abstract Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, Sekameng, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Mg, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilite series. Lamellar ilmenites from Frank Smith and Monastery differ in composition but both show a limited range in composition which lies within the compositional range shown by discrete ilmenites from these pipes. The ilmenites are enriched in Nb, Zr, Cr, Co, and Ni and depleted in Mn, Cu, and Zn relative to Mg-poor ilmenites from basic intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted to imply that the geochemistry cannot be interpreted in terms of a differentiation hypothesis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodules, which are considered to be phenocrysts in a proto-kimberlite magma. Lamellar ilmenite-pyroxene intergrowths are unlikely to have had a eutectic origin, and show no simple relationship to the discrete ilmenites.

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda, Africa

Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.