Carbonatite Dykes Research Articles

Hydrogen, carbon and oxygen isotope studies of megacryst and matrix minerals from lesothan and South African kimberlites

Hydrogen carbon and oxygen isotope analyses were determined, where applicable, on phlogopite megacrysts (two samples), serpentine megacrysts (two), unweathered matrix (nine), carbonated granulite xenoliths (two), a carbonatite dyke and weathered yellow kimberlite from diatremes in Lesotho (Kao, Matsoku) and South Africa (Monastery, New Elands, Premier, Wesselton) and dykes and sills in Lesotho (Kaunyane) and South Africa (Benfontein, Du Plessis). The D H ratios of phlogopite megacrysts are in the range previously reported for possible upper mantle derived phlogopites: −40 to −70 per mil, SMOW. D H ratios of the serpentinized megacrysts and the hydrous phases of the matrix, dominantly serpentine and/or phlogopite, are depleted in D by 20 to 55 per mil relative to the phlogopite megacrysts from, in some cases, the same sample. Serpentine megacrysts and serpentine-rich matrix samples are depleted in 18O by up to 6 per mil relative to both phlogopite-rich and olivine-rich matrix, and “normal” ultramafic values. The H- and O-isotope data indicate that meteoric-hydrothermal fluids interacted with the kimberlite during the major serpentinization processes. The origin of fluids associated with matrix phlogopite cannot be defined specifically: magmatic, metamorphic or exchanged meteoric waters with very low water/rock ratios (< 0.2 at.% oxygen) or some combination are possible. The isotopic “thermometers” indicate very approximately that the matrix phlogopite crystallized at temperatures on the order of 200° to 500°C and serpentinization process occurred at 50° to 200°C. Carbonates from the matrix, carbonatite dykes and xenoliths have δ 18O = +7 to +21 per mil, SMOW and δ 13C = −3 to −7.5 per mil, PDB. The C-isotope ratios are similar to those for most diamonds and primary igneous carbonatites and are similarly variable at a single locality. They are compatible with a genetic relationship among these carbon sources. The 18O enrichment of most kimberlite carbonates is a result of the relatively low temperature of exchange with other matrix minerals and/or with meteoric-hydrothermal fluids. Loss of CO 2 + H 2 O fluid during degassing may contribute to the H- and C-isotope variations in a kimberlite. The water content of kimberlite magmas were probably substantially less than the 5 to 10 wt.% H 2O observed in many diatreme-facies kimberlites. The preservation of diamonds in a kimberlite may be critically dependent on both the initial water content of the mell and the stage and temperature at which large-scale influx of ground waters into the kimberlite occured. Weathered yellow kimberlite is isotopically distinctive from our matrix samples.

Isotope geochemistry and petrology of African carbonatites

The carbonatite bodies examined are the Palabora, Spitskop and Premier Mine carbonatites of Precambrian age, the Mbeya carbonatite of Mesozoic age, Tertiary Homa Mountain carbonatite and the Recent eruptions of the Oldoinyo Lengai carbonatite. The field of oxygen and carbon isotopic ratios of the Palabora carbonatite, especially of the older carbonatite, coincides with that of primary igneous carbonatites. Spitskop carbonatite shows a slight deviation in oxygen and carbon isotopic ratios from primary igneous carbonatites. The Spitskop carbonatite forms a pipe-like body whose composition was modified after emplacement in several stages by circulating fluids which showed progressive enrichment in magnesium and iron. In the Premier Mine, the main carbonatite dyke mass intruded the centre of the black kimberlite. The oxygen isotopic compositions show a significant enrichment in their 18O compared to the “sub-volcanic” type carbonatites. Carbon and especially oxygen isotopic compositions show a considerable range in the Mbeya carbonatite. The fractionation factors of oxygen and carbon isotopes between the coexisting calcites and dolomites in the primary Mbeya carbonatites suggest that the carbonates crystallized at temperatures from 800°C to 380°C and > 700°C to 380°C, respectively. Carbon and oxygen isotopic compositions of the Homa Mountain carbonatite show a considerable spread and this is probably due to interaction with atmospheric oxygen and meteoric water during eruption. Sodium carbonate lava from Oldoinyo Lengai is deliquescent and readily alters in air. The lower value of δ 13C in the sodium carbonate lava is thought to be due to the partitioning of heavy carbon into a CO 2 gas phase during vulcanicity. It appears to suggest that no genetical relationship exists between sodium carbonatite lava and evaporite trona.

Plutonic nodules in lamprophyric carbonatite dykes near Frederikshåb, South-West Greenland

A swarm of thin NW-SE lamprophyric carbonatite dykes of Mesozoic age occurs south of Frederikshåb associated with a contemporaneous, parallel swarm of thick dolerites. Apart from local country rock material, inclusions in the lamprophyric carbonatites are mainly of the following types: 1) Single crystals of olivine which were probably mainly derived from the upper mantle. 2) Relatively unmodified garnet- and pyroxene-granulite nodules brought up from a lower crustal level. 3) Nodules, and single crystals, consisting mainly of hornblende and salite which are considered to have formed by metasomatic reaction between the carbonatite magma and mainly acid to intermediate lower crustal rocks, possibly at relatively low levels in the dykes. Hornblende shows various stages of growth from initially small, iron-rich crystals to larger, iron-poor crystals which have commonly replaced pyroxenes. The pyroxenes show a similar but less pronounced development. 4) Alkaline nodules which are again thought to have developed by metasomatic reaction between the magma and country rock inclusions, but possibly at higher levels in the dykes. 5) Phlogopite megacrysts which may be partly xenocrystal but which are thought to have mainly crystallised from the contaminated magma. Complete chemical analyses of lamprophyric carbonatites and partial analyses of individual minerals are presented.