Kanenori Suwa

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 δ13C in the sodium carbonate lava is thought to be due to the partitioning of heavy carbon into a CO2 gas phase during vulcanicity. It appears to suggest that no genetical relationship exists between sodium carbonatite lava and evaporite trona.

Ammonium contents of biotites from Precambrian rocks in Finland: The significance of NH4+ as a possible chemical fossil

Abstract The ammonium contents of biotites in Precambrian rocks from Finland have been determined, to examine the possibility that ammonium in biotites represents a chemical fossil. If biologic activity existed in Precambrian sedimentary environments, biotites formed from the sediments should contain distinctly larger quantities of NH 4 + than biotites from igneous rocks. Biotites from Svecokarelidic metasediments, which were originally products of sedimentation about 2400-1900 Ma ago, have high NH 4 + contents (hundreds of ppm), while biotites from Svecokarelidic plutonic rocks of 1800–1900 Ma old and Postsvecokarelian rapakivi granites of 1650–1700 Ma old have low NH 4 + contents (tens of ppm). The results may indicate that biologic activity was plentiful during sedimentation of the Svecokarelian strata. This agrees with evidence for ancient life found in dolomites and greywacke-slates of the Svecokarelides. Biotites from Presvecokarelidic schists of more than 2600–2800 Ma old have only tens of ppm of NH 4 + . This value is at the same level as those of biotites from the rapakivi granites and the Svecokarelidic plutonic rocks. Considering the rarity of fossils in Presvecokarelidic rocks, the low NH 4 + content of biotite from a Presvecokarelidic pelitic schist may indicate less biologic activity during the sedimentation of the original sediments. These data emphasize that ammonium in biotites of Precambrian metasediments may be a useful chemical fossil for identifying the geologic sites of ancient life.

Zn Mn Ilmenite in the Kuiqi Granite from Fuzhou, Fujian Province, East China

SummaryZn-Mn ilmenite occurs as a principal constituent of the miarolitic cavities in the Kuiqi granite from Fuzhou. In the cavities potassium feldspar, albite, quartz, fluorite, aegirine and arfvedsonite also occur with accessory zircon, magnetite, hematite and sphalerite. Zn-Mn ilmenite forms a trigonal platy euhedral crystal up to 30 mm in length. Its chemical composition ranges from 37.3 to 63.8 mol.% FeTiO3 (ilmenite molecule), 61.1 to 22.4 mol.% MnTiO3 (pyrophanite molecule) and 0.6 to 15.3 mol.% ZnTiO3 (Znmetatitanate molecule). ZnO content ranges from 0.34 wt.% to 7.63 wt.%. Zonal structure is noticeable in the Zn-Mn ilmenite. FeTiO3 and ZnTiO3 molecules increase towards the crystal rim, while MnTiO3 molecule decreases towards the rim. Unit cell parameters of the rim and the core area = 5.092(6)Å,c = 14.08(2)Å,V = 316.1Å3 anda = 5.106(7)Å,c = 14.02(3)Å,V = 316.6Å3, respectively. Coexisting minerals, except for arfvedsonite and sphalerite, are very low in ZnO content. It is suggested that complete isomorphous replacement between FeTiO3 MnTiO3 and ZnTiO3 may be possible. Oxygen fugacity conditions for crystallization of Zn-Mn ilmenite are considered to be in the vicinity of the magnetite-hematite and quartz-fayalite-magnetite buffers.ZusammenfassungZn-Mn Ilmenit ist ein grundsätzlicher Bestandteil in miarolitischen Hohlräumen in den Kuiqi Graniten aus Fuzhou. Daneben treten Kalifeldspäte, Albit, Quarz, Fluorit, Ägirin und Arfvedsonit sowie akzessorisch Zirkon, Magnetit, Hämatit und Sphalerite in den Hohlräumen auf.Zn-Mn Ilmenit bildet idiomorphe, tabular trigonale Kristalle mit bis zu 30 mm Länge aus. Die chemische Zusammensetzung variiert zwischen 37,3 und 63,8 Mol.% FeTiO3 (Ilmenit) 61,1 und 22,4 Mol.% MnTiO3 (Pyrophanit) sowie zwischen 0,6 und 15,3 Mol.% ZnTiO3 (Zn-Metatitanat-Molekül). Die ZnO-Gehalte schwanken von 0,34 Gew.% bis 7,63 Gew.%. In dem Zn-Mn Ilmenit wurden Zonierungen beobachtet. FeTiO3 und ZnTiO3 Moleküle nehmen zum Rand des Kristalls hin zu, MnTiO3 Moleküle hingegen ab.Die Parameter der Elementarzelle sind am Randa = 5,092(6)Å,c = 14.08(2)Å,V = 316.1Å3 und im Kerna = 5,106(7)Å,c = 14.02(3)Å,V = 316,6Å3. Mit Ausnahme von Arfvedsonit und Sphalerit sind die ZnO Gehalte in koexistierenden Mineralen sehr niedrig.Es wird daher angenommen, daß zwischen FeTiO3, MnTiO3 und ZnTiO3 ein vollständiger isomorpher Ersatz möglich ist. Die Sauerstoff-Fugazitäten die während der Kristallisation von Zn-Mn Ilmenit herrschten, bewegten sich zwischen MagnetitHämatit und Quarz-Fayalit-Magnetit.

Petrology of peridotite nodules from Ndonyuo Olnchoro, samburu district, central Kenya

Sodic alkaline volcanic rocks were extruded repeatedly in enormous volumes from Miocene time onward in Kenya, eastern Uganda and northern Tanzania. The volcanic cone of Ndonyuo Olnchoro in central Kenya consists of Recent olivine melanephelinite agglutinates and lava that contain many ejected ultramafic nodules. The nodules are divisible into two groups. The first group contains rocks consisting of magnesian olivine, orthopyroxene, and small amounts of chromian spinel and clinopyroxene. Harzburgite and lherzolite comprise this group and account for more than 95% of the peridotite nodules by volume. The second group of ultramafic nodules consists of websterite consisting of orthopyroxene, clinopyroxene and small amounts of chromian spinel. This group comprises less than 5% of the peridotite nodules by volume. These two groups of nodules are of different origins. The pressure and temperature of formation of the first group are estimated as 1200–1350°C36–38 kbar. Their MgOσFeO ratios fall within the limit of variation of nodules from kimberlite and are higher than those for oceanic lherzolites. Mineralogical and chemical characters of the phases in the first group indicate more affinities with those in mantle-derived rocks than with peridotites derived by accumulation from a basaltic melt. The nodules were originally garnet peridotites and were transported upward into a lower pressure regime presumably by mantle convection. The pressure and temperature of formation of the second group of nodules are estimated as 1090°C16 kbar. They were produced by crystallization from an alkaline magma in relatively deep levels of the continental crust. Both groups of nodules were transported to the surface when olivine melanephelinite was erupted.

Plagioclase twin laws and fabrics in three specimens of anorthosite

Abstract Twinning patterns and petrofabrics of plagioclases are examined in three specimens of anorthosite from the Bushveld Complex, the Quebec Massif, and the Fiskenaesset Complex. Their plagioclases have petrographical characteristics exhibiting their different petrogeneses. In an anorthosite from the Bushveld Complex, plagioclase grains are twinned after the albite-Carlsbad, pericline, albite and Carlsbad laws. Frequency percentage of the albite-Carlsbad and Carlsbad laws reaches 43% Plagioclase grains in the adcumulate layers are developed with their composition plane (010) subparallel to the cumulate plane, whereas those in the heteradcumulate layers are developed with their composition plane (010) subperpendicular to the cumulate plane. In an equigranular anorthosite from the Quebec Massif, plagioclase grains are polysynthetically twinned after the albite and pericline laws with rare examples of the albite-Carlsbad and Carlsbad laws. Frequency percentage of the latter two laws is only 1% together. Some regularities are recognized in the petrofabrics of c-axis and (010) plane. In a calcic anorthosite from the Fiskenaesset Complex, plagioclase grains are polysynthetically twinned, exclusively according to the pericline law or a combination of pericline and albite laws. The pericline law is predominant and reaches 64% and this twinning pattern cleaarly differs from that of the former two anorthosites.

Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya

K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

47 – 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 δ13C in the sodium carbonate lava is thought to be due to the partitioning of heavy carbon into a CO2 gas phase during vulcanicity. It appears to suggest that no genetical relationship exists between sodium carbonatite lava and evaporite trona.

21 – PETROLOGY OF PERIDOTITE NODULES FROM NDONYUO OLNCHORO, SAMBURU DISTRICT, CENTRAL KENYA

Sodic alkaline volcanic rocks were extruded repeatedly in enormous volumes from Miocene time onward in Kenya, eastern Uganda and northern Tanzania. The volcanic cone of Ndonyuo Olnchoro in central Kenya consists of Recent olivine melanephelinite agglutinates and lava that contain many ejected ultramafic nodules. The nodules are divisible into two groups. The first group contains rocks consisting of magnesian olivine, orthopyroxene, and small amounts of chromian spinel and clinopyroxene. Harzburgite and lherzolite comprise this group and account for more than 95% of the peridotite nodules by volume. The second group of ultramafic nodules consists of websterite consisting of orthopyroxene, clinopyroxene and small amounts of chromian spinel. This group comprises less than 5% of the peridotite nodules by volume. These two groups of nodules are of different origins. The pressure and temperature of formation of the first group are estimated as 1200–1350°C/36–38 kbar. Their MgO/Z FeO ratios fall within the limit of variation of nodules from kimberlite and are higher than those for oceanic lherzolites. Mineralogical and chemical characters of the phases in the first group indicate more affinities with those in mantle-derived rocks than with peridotites derived by accumulation from a basaltic melt. The nodules were originally garnet peridotites and were transported upward into a lower pressure regime presumably by mantle convection. The pressure and temperature of formation of the second group of nodules are estimated as 1090°C/16 kbar. They were produced by crystallization from an alkaline magma in relatively deep levels of the continental crust. Both groups of nodules were transported to the surface when olivine melanephelinite was erupted.