ABSTRACTS
Bulletin of the Geological Survey of Japan
Vol.55 No.3/4 2004

Oxygen isotopic constraints on the geneses of the Cretaceous granitoids in the Kitakami and Abukuma terrains, Northeast Japan

Shunso Ishihara and Yukihiro Matsuhisa

@Oxygen isotopic ratios (18O/16O) were measured on 48 whole rock samples for the Early Cretaceous granitoids of the Kitakami Mountains and the Abukuma Highland. Together with 10 published data, origins of these granitoids are considered.
@In the Kitakami Mountains, the granitoids generally belong to magnetite series and their initial Sr isotopic ratios are low (0.70363 - 0.70463). The Zone I granitoids are lowest in 18O value, indicating they were originated in 18O-depleted mafic rocks, such as underplated tholeiite. The 18O values of the Zone II plutons where slab-melting has been proposed for the high-87Sr granitoids and those of the Zone Va Tono-Kesengawa plutons are not particularly high but close to the average plots for the Kitakami granitoids. The 18O values are relatively high in the K-alkalic Hinomiko and calc-alkaline Hitokabe plutons. Oxidized and K-rich metasomatized upper mantle may be needed to produce such a potassic magma at the Hinomiko body, but mafic igneous source rocks with intercalated terrestrial sediments may be enough to generate the calc-alkaline magma of the Hitokabe pluton.
@The Abukuma granitoids generally belong to ilmenite series and their initial Sr isotopic ratios are around 0.70518. Their 18O values are higher than those of the Kitakami granitoids. The Abukuma granitoids of the western zone where metamorphic grade of the intruded rocks are higher than in the eastern zone, have slightly higher 18O values than in the eastern zone. The two-mica granites of the western zone have the highest 18O values and are least in magnetic susceptibility. These granites have the largest amount of sedimentary components of terrestrial origin in their protolith, while the other quartz diorite - granodiorite has had predominantly mafic igneous rocks in their source region.

The early Cenomanian (Cretaceous) ammonite fauna from the Soeushinai area of Hokkaido, North Japan
(Studies of the Cretaceous ammonites from Hokkaido and Sakhalin-XCVII)

Tatsuro Matsumoto, Tamio Nishida and Seiichi Toshimitsu

@The ammonoid fauna from the main part of the Lower Cenomanian sedimentary series in the Soeushinai area of northwestern Hokkaido is prolific. It forms the assemblage of species here called the Stoliczkaia ( Lamnayella) japonica Assemblage Zone. It is situated above the basal Cenomanian Graysonites wooldridgei Zone and below the well traced Mantelliceras saxbii Zone. It, thus, represents the main part of the Lower Cenomanian Substage in the studied area. The correlation of this zone with otherwise defined zones, home and abroad, is discussed. Systematic descriptions are given for the zonal indices and several selected species.

DMAP.m: A Mathematica® program for three-dimensional mapping of tortuosity and porosity of porous media

Yoshito Nakashima and Tetsu Yamaguchi

@We developed a Mathematica® program for three-dimensional mapping of the porosity and normalized apparent diffusion coefficient (tortuosity) of isotropic heterogeneous porous media. The program, DMAP.m, is a package-type program for Mathematica® version 4 or later. DMAP.m accepts three-dimensional (3D) digital image data for the porous media as an input. Such data may be obtained by, for example, X-ray computed tomography (CT) as a set of text files of two-dimensional contiguous CT slices (square matrices). DMAP.m reads the text files and divides the image set into sub-cubes, then executes a non-sorbing random walk (lattice walk) through the discrete pore space in each sub-cube. A specified number of voxels are chosen randomly as the start position of the random walk. If the chosen voxel falls within a pore, random walk simulation is carried out until the walker exits the sub-cube. If the chosen voxel falls within a solid, the random walk is not performed. The porosity of each sub-cube is calculated as the probability of a successful hit on a pore voxel in this random choice of the start position (Monte Carlo integral). The time required for the walkers to escape from each sub-cube is recorded in the random walk simulation, representing an gout-diffusionh or gout-leachingh numerical simulation. The tortuosity (apparent diffusion coefficient in the free space divided by that in porous media) is calculated by fitting the time-dependent cumulative number of walkers that have escaped from the sub-cube to a theoretical curve. DMAP.m was applied successfully here to the 3D X-ray CT image of a monosized sand pack. DMAP.m is available for free download on the authorfs website (URL = http://staff.aist.go.jp/nakashima.yoshito/progeng.htm) to facilitate study on porous media by X-ray CT or nuclear magnetic resonance imaging.