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Volume 81, Issue 3
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with special reference to the primitive tholeiiteToshiaki Takimoto1986 Volume 81 Issue 3 Pages 93-104
Published: March 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSVolcanic rocks of middle-upper Miocene age are widely distributed in the Tomari district, eastern part of the Shimokita Peninsula, Northeast Japan. High magnesian tholeiitic basalts in the Tomari volcanics are found as pillow lava and dikes These basalts are characterized by high Ni (47-276ppm), Cr (67-1044ppm) contents and low FeO*/MgO ratios (0.78-1.87). The bronzite bearing augite olivine basalt has very high MgO, Ni and Cr contents (11.70%, 276ppm and 1044ppm respectively), and its magnesian olivine phenocrysts have a narrow compositional range(Fo85-91) and high NiO content(0.21-0.32%). The basalt also contains chromite as inclusions in the olivine phenocrysts and rarely as isolated grains in the groundmass. Fe-Mg, Ni-Mg partitionings of the basalt suggest that the phenocrysts of magnesian olivine (Fo89.91 respectively) are in equiliblium with bulk rock composition.
These petrological features strongly indicate that the bronzite bearing augite olivine basalt has primitive nature and may represent the primary tholeiitic magma generated in the upper mantle.View full abstractDownload PDF (787K) -
Osamu Ujike, Tatsunori Soya, Koji Ono1986 Volume 81 Issue 3 Pages 105-115
Published: March 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSX-ray fluorescence analysis was carried out for major elements on 13 volcanic rocks and for Rb, Sr, Y and Zr on 24 volcanic rocks from the Kikai Caldera. The 30 available major-element analyses indicate that the rocks constructing the land portions of Kikai Caldera are tholeiitic and calc-alkalic and their SiO2 contents show two separate ranges, 50-60% and 69-73%. No clear correlation is observed between the stage of caldera evolution to which the samples belong and the chemical composition of the samples. Most of all the samples seem to have been more or less affected by mixing of magmas with mafic and felsic compositions.
All the analyses are in limited ranges of ratios between highly incompatible elements ; K/Rb=270-340 and Zr/Rb=2.2-2.9. This implies that all the Kikai magmas were ultimately derived from a single source. The rhyolites are likely to be derivatives by partial melting of materials formed from a batch (or batches) of basalt magma emplaced within the crust ahead of others. The Rb/Y-Zr/Y systematics strongly suggests that the basalt source region was enriched in Rb by mantle metasomatism.View full abstractDownload PDF (840K) -
Congruent dissolution of plagioclaseSHIGERU OKUMURA1986 Volume 81 Issue 3 Pages 116-128
Published: March 05, 1986
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSWeathering process of plagioclase in the middle depth of weathering crust which develops at the top of Nabari gabbroic body, Southwest Japan was examined mineralogically and geochemically.
Unweathered core stones are coated, first by shell and then by saprolite in the upper depth (Zone A) and in the middle depth (Zone B), whereas they are directly coated by saprolite in the lower depth (Zone C). In the Zone B, the shell shows a sequence of different types of weathering products of plagioclase, which occur in layering from outer part to inner fresh part within the shell as follows:
-compact kaolinite (Type 1);
-sparse halloysite (Type 2);
-void space with minor amount of gel (Type 3);
-unweathered plagioclase.
As for the formation of void space (Type 3), it is inferred that plagioclase dissolves congruently by chemical weathering at the middle depth of the weathering crust. As plagioclase dissolves, released constituent cations, including aluminum, are almost removed out from the shell, and no secondary minerals neoform in the immediate vicinity of dissolving plagioclase.
The congruent dissolution indicates that the dissolution process is controlled kinetically by mineral-water interfacial reaction in non-equilibirium and that the released constituents are all removed out by transport of pore solution and/or by ion diffusion in aqueous media in open system. The lack of secondary minerals shows the need to introduce kinetics of non-equilibrium process also in precipitation.View full abstractDownload PDF (1723K)
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