Ken-ichiro Aoki

The major element composition of the upper mantle estimated from the composition of lherzolites

The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.

Pyroxenes from lherzolite inclusions of Itinome-gata, Japan

Abstract The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermaks molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermaks component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermaks components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions.

Titanochondrodite and titanoclinohumite derived from the upper mantle in the Buell Park Kimberlite, Arizona, USA

Kimberlite from Buell Park, Arizona, which was intruded into Permian sediments about 30 m.y. ago, is characterized by the hydrous silicates titanochondrodite and titanoclinohumite. Titanochondrodite is the first finding in kimberlites. Optical properties, chemical compositions and cell dimensions of these two minerals are determined.

Petrology and geochemistry of the Nyamuragira volcano, Zaire

Abstract Nyamuragira volcano in the Western Rift of the East Africa Rift Valley is characterized by violent volcanic eruptions, the most recent of which occurred in 1967, 1971, 1976–1977, 1980 and 1981–1982. Its extrusives consist of a highly potassic suite of olivine basanite-tephritic phonolites among which phonolitic tephrite is the commonest. The petrographic features of the whole suite such as phenocryst and groundmass mineral assemblages and modal variations are rather simple. Major elements vary smoothly from olivine basanite to phonolitic interstitial glass without any abrupt change with increasing K 2 O. In addition, the typical incompatible trace elements, Ba, Ce, F, Nb, Rb and Sr increase continuously with the increase of K 2 O. These suggest that chemical variations were controlled mainly by the effective fractionation of olivine and clinopyroxene associated with plagioclase and titanomagnetite. Furthermore, in the K 2 O-incompatible trace-element diagrams, trend lines extrapolated to zero, converge on the coordinate axes of the vertical and horizontal lines. This suggests that these incompatible elements were preferentially moved toward the primitive basaltic magma during the partial melting of metasomatized upper-mantle peridotites. Furthermore, these elements are concentrated almost completely in residual liquids during fractional crystallization.

Origin of phlogopite and potassic richterite bearing peridotite xenoliths from South Africa

Peridotite xenoliths containing primary phlogopite with or without potassic richterites as major constituent (up to 12 vol. %) are rarely found in kimberlite from the Bultfontein Floors. Chemically, these rocks are similar in compositions with those of the granular type garnet peridotite xenoliths from South Africa and Lesotho, except for an abnormally high content of K2O in the former.Phlogopite and potassic richterite are thought to have the following genesis: garnet peridotites at a depth from 170 to 100 km suffered local introduction of a potash-rich fluid, and garnet and enstatite reacted with this fluid to form phlogopite and diopside. Potassic richterite may have been produced by the reaction between diopside and fluid at the same time as crystallization of phlogopite at depths shallower than 120 km.

Petrology of kaersutite-bearing ultramafic and mafic inclusions in Iki Island, Japan

Kaersutite-bearing peridotite, clinopyroxenite, gabbro and hornblendite inclusions up to 5 cm in size and megacrysts of kaersutite, andesine and titanomagnetite occur in alkali basalt scoria at Takenotsuji, Iki Island, Japan. New analyses are presented for seven kaersutite-bearing basaltic rocks, three inclusions, three clinopyroxenes and six kaersutites.From the petrography and chemistry of the inclusions and recent experimental work at high pressures and temperatures on natural rock systems, it is suggested that kaersutite-bearing inclusions have been produced from alkali basalt magmas under hydrous conditions at a depth of about 25 to 30 km in the lowest part of the crust.

Trace element variations in the volcanic rocks from the Nasu zone, northeast Japan

Trace element concentrations were analyzed in 46 volcanic rocks from twelve recent volcanoes of the outer volcanic zone (Nasu zone) of Northeast Japan. The Ba, Hf, Th, U and REE contents in the rocks are low as expected from their low K 2 O contents. The tholeiitic rocks and calc-alkaline rocks can be distinguished by their geochemical character as well as by the character of their groundmass pyroxenes. The magmas of the tholeiitic series and calc-alkaline series appear to have been generated independently in the upper mantle so far as their trace element variations are concerned, though a few calc-alkaline rocks are derivatives of a tholeiitic magma. The two magmas have been derived from two different source materials. However, the trace element variations in the two volcanic rock series can be interpreted as the product of different degrees of partial melting of the same peridotitic upper mantle.

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Abstract Fluorine contents in about 160 representative Quaternary volcanic rocks and 15 hornblende and biotite phenocrysts in a calc-alkali series in Japan have been determined by a selective ion-electrode method. Tholeiites have the lowest contents and the narrowest range (58–145 ppm), while alkali basalts have the highest contentws and the widest range (301–666 ppm), high-alumina basalts have intermediate values (188–292 ppm). F contents in basalts clearly increase from east to west across the Japanese Islands, as do alkalies, P2O5 REE, U, Th and H2O. The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO2 or advancing fractionation, F contents show either progressive enrichment; or (2) with increasing fractionation, F contents show rather constant values. The former is produced by fractionation of anhydrous phases from basalt to mafic andesite magmas; the tholeiite series of Nasu volcanic zone (outer zone), northeastern, Japan is a typical example. The latter group is derived through separation of amphibole-bearing phases from basaltic magmas at various depths from upper mantle (about 30 km) to upper crust; the alkali series in southwestern Japan and the calc-alkali series of Chokai volcanic zone (inner zone), northeastern Japan, are examples.

Fluorine contents of some hydrous minerals derived from upper mantle and lower crust

Abstract Fluorine contents have been determined in about forty samples of amphibole, mica and apatite in alkali basalt and kimberlite and their incorporated xenoliths. They show a wide variation ranging from 15,000 to 100 ppm, corresponding to about 40 to 0.2 per cent substitution of F for OH in hydroxyl site of hydrous minerals. Fluorine abundances in these minerals reflect those of their host magmas or rocks; Itinome-gata xenoliths are the lowest and South African kimberlites and their xenoliths are the highest. F/OH and also. D/H (Kuroda et al. 1975) ratios in coexisting phlogopite-potassic richterite from peridotite and mica nodules are thoughts to have formed under no simple equlibrium conditions.

Phlogopites and potassic richterites from mica nodules in South African kimberlites

Mica nodules from the Dutoitspan and Wesselton mines, Kimberley can be divided into two groups: diopside-phlogopite nodules and potassic richterite-phlogopite nodules. The latter is characterized by the presence of abundant potassic richterite (less than 50% by volume) and large size of the crystals (up to 1 cm in length). It seems this type nodule is the first finding.Phlogopites and potassic richterites show a rather wide variations of major elements they have low Al2O3 and high Fe2O3, with Fe+3 in addition to Al+3 in the tetrahedral sites. It is believed that the mica nodules would be produced from kimberlitic and related magmas under moderate temperature and pressure conditions.