Mamoru Adachi

Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites

Electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite-sillimanite transition (620 ± 15°C) and sillimanite-orthoclase (680 ± 15°C) zones of the Cretaceous Ryoke metamorphic belt, southwest Japan. Monazites from pelitic gneisses are of metamorphic origin, euhedral to subhedral and chronologically homogeneous, giving chemical ThU-total Pb isochron (CHIME) ages of 98.8 ± 3.3–98.0 ± 3.2 Ma. Two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca. 1700 Ma. Most detrital monazite grains are heterogeneous in the ThO2 and UO2 concentrations and have multiple or single rims as young as ca. 100 Ma. Several detrital monazite grains are well rounded in form, exhibit homogeneous Th and U distributions and show a Pb diffusion profile in the margin. The width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss: 18–22 μm for 620°C and 48–58 μm for 680°C. Assuming the isothermal diffusion of Pb from homogeneous monazite spheres during a 5 Ma duration of peak metamorphism, we obtain diffusion coefficients of 1.9(±0.3) × 10−21 and 1.5(±0.3) × 10−20 cm2/s at 620°C and 680°C, respectively. These data derive an activation energy of 2.44(+2.85/−1.26) × 105 J/mol and a frequency factor of 3.4 × 10−7 (8.5 × 10−12–2.2 × 107 cm2/s, taking account of uncertainties of ±15°C in the temperatures and of ±20% in the diffusion coefficients. The diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for Pb in monazite that has been poorly understood so far.

Middle precambrian provenance of Jurassic sandstone in the Mino Terrane, central Japan: Th-U-total Pb evidence from an electron microprobe monazite study

Precise electron microprobe analyses have been made on detrital monazites in Jurassic sandstones of the Mino Terrane, central Japan, and their Th-U-Pb relations are examined in terms of PbO and ThO2∗ (ThO2∗ = ThO2 plus the equivalent of UO2). The PbO/ThO2∗ ratios of most detrital monazites (403 of 581 grains) cluster in the range from 0.0763 to 0.0618. Other grains show the ratio of about 0.0541 (46 grains), 0.0364 (29 grains), 0.0170 (24 grains) and 0.0116 (79 grains). The PbO/ThO2∗ ratio of 0.0763-0.0618 is nearly identical with that of monazites in a middle Precambrian (1740 Ma) gneiss clast from the Kamiaso Conglomerate of the Mino Terrane; this strongly suggests that the detrital monazites were derived from gneisses and/or granitoids of middle Precambrian age. The ratio of 0.0116 is nearly the same as that of monazites in a Permian (274 Ma) paragneiss of the Hida Terrane north to the Mino Terrane. The PbO/ThO2∗ monazite data, coupled with palaeocurrent analyses and petrological characteristics of sandstones, show that middle Precambrian high-grade metamorphic rocks and granitoids were widespread both in the northern and southern source regions of the E-W trending Jurassic basin of the Mino Terrane. The PbO/ThO2∗ ratio of detrial monazites is a useful new measure for the provenance analysis of clastic rocks.

Middle Precambrian detrital monazite and zircon from the hida gneiss on Oki-Dogo Island, Japan: their origin and implications for the correlation of basement gneiss of Southwest Japan and Korea

Abstract Monazite and zircon from seven paragneiss samples of the upper amphibolite facies grade from Oki-Dogo Island in the Hida terrane, western Japan, were analyzed for Th, U and Pb on an electron microprobe. Most monazite grains are chronologically unzoned and give unequivocal CHIME (chemical Th-U-total Pb isochron) ages of 250 Ma, but some grains show a distinct core-overgrowth relationship. The cores retain evidence of pre-250 Ma ages and are interpreted to be of detrital origin, suggesting that monazite can be more immune to diagenetic and low-grade metamorphic decompositions than had been thought previously. Zircon grains, rounded to nearly euhedral, give CHIME ages ranging from 3000 to 350 Ma. Of particular interest are several rounded 3000 Ma recycled zircons which perhaps originated from an Archean terrain in the Sino-Korean craton. The youngest age of detrital zircons constrains that the gneiss protolith deposited after the 350 Ma event. The CHIME monazite and zircon ages for the paragneisses from Oki-Dogo Island, and from the main part of the Hida terrane, reveal that the gneisses formed from Permo-Carboniferous clastic sediments during a single metamorphism at ca. 250 Ma. The results are in marked contrast to the current view that (1) the Hida gneiss is a polymetamorphosed Precambrian complex; (2) the central part (‘Hida gneiss’ region) and the eastern marginal part (Unazuki schist’ region) of the Hida terrane are correlative with the Precambrian Kyonggi massif and the Paleozoic Okchon zone in the Korean Peninsula, respectively. Instead, we consider that the Okchon zone alone can be correlatable with the Paleozoic Hida terrane.

The HVEE Tandetron AMS system at Nagoya University

Abstract In 1996/97, we installed a second-generation AMS system (model 4130-AMS) manufactured by HVEE, B.V., The Netherlands. Although we encountered a lot of problems during the installation and tuning up of the machine, our new system has now reached good stability and reproducibility in performing 14 C / 12 C and 13 C / 12 C measurements. According to the reproducibility tests conducted in January 1999, where six graphite targets prepared from HOxII standard material were measured, the standard deviation for 14 C / 12 C reproducibility is around 1.6 ‰ (comparable with counting statistics) and that for 13 C / 12 C is 0.28 ‰ . The background level of the AMS system was measured using a pure graphite (commercial graphite rod). It lies between 50 and 55 ka BP. We are still on the way to reduce the background. However, we are now prepared to start routine measurements of unknown-age samples.

RbSr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from central Japan

Abstract Whole-rock samples of metamorphic and granitic cobbles and boulders from the Kamiaso conglomerate in central Japan yield well-defined Rb Sr isochron ages of 1985 ± 25 my and 1820 ± 40 my. These ages are the oldest yet obtained for rocks in the Japanese Islands, and provide key evidence for the middle Precambrian metamorphic and igneous events in the provenance of these rocks. The age of 1985 my defined by six samples of quartzo-feldspathic gneiss may be the time of emplacement of the original granitic rocks. The 1820 my age indicates the time of extensive regional metamorphism and igneous activity. Precambrian episodes in the provenance of the Kamiaso conglomerate are summarized as follows: (1) 2000 my — formation of granitic rocks, (2) 1800–1600 my — high grade metamorphism accompanied by igneous activity, (3) 1200–1000 my — some significant thermal event. Judging from rock types and geochronological data, it can be said that metamorphic rocks in the Kamiaso conglomerate are remarkably similar to those of the Matenrei and Nangnim systems in North Korea. The Precambrian complex from which the metamorphic and granitic rocks were derived, was exposed to the north not far from the present site of the Kamiaso conglomerate in the late Paleozoic time, and it was probably a part of the large Precambrian continent in East Asia.

Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes

Simultaneous monitoring of He, H2, Ar, N2, CH4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H2, as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H2. We suggest that the selective and preseimic increases of H2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress-corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H2 production by mechanochemical reaction of fractured rock surface with water in pre-existing fault planes as groundwater conduits.

Late Paleozoic geosynclinal basalt and tectonism in the Japanese Islands

Abstract In the Japanese Late Paleozoic group basaltic volcanics are commonly found whose chemical natures are very similar to the abyssal basalt of the present mid-oceanic ridge, characterized, above all, by low K 2 O, low Fe 3+ /Fe 2+ and high K/Rb. Two types of the basaltic rocks are distinguished, especially in their distribution pattern of rare-earth elements, the one displaying the logarithmically rectilinear relationship of chondrite-normalized pattern and the other with the inflection point. Recently, Precambrian metamorphic cobbles dated at 1,700-1,500 m.y. have been found in Permian conglomerates of central Japan. Paleocurrent data indicate lateral filling of clastic materials derived from the north, where a vast expanse of Precambrian complex is supposed to have occurred. Stratigraphy and structure of the Paleozoic terrain of central Japan suggest that the Japanese Paleozoic geosyncline was grown on the Precambrian continental crust. From geochemical properties and the distribution of the geosynclinal basalt in the Japanese Islands, it is concluded that the basaltic rocks were derived from the upper mantle, the original magma having been created by partial melting of the mantle and moved upward through a rifting zone in the Precambrian crust. The early stage of tectonism of the geosynclinal belt was thought to be under a tensional condition rather than a compressional one.

Sources and Depositional Environments of Some Permian and Triassic Cherts: Significance of Rb‐Sr and Sm‐Nd Isotopic and REE Abundance Data

Rb‐Sr and Sm‐Nd isotopic data, rare earth element (REE ) abundances, and major‐element compositions are reported for the Triassic cherts of the Mino Terrane in the Inner Zone of southwest Japan and for the Permian and Triassic cherts of the Sambosan Terrane in the Outer Zone of southwest Japan. Rb‐Sr isotopic data of the Mino and Sambosan cherts define separate isochron lines, and the Rb‐Sr ages of ca. 210 and 240 Ma reflect the chert deposition and the end of the subsequent chemical diagenesis during which amorphous silica was transformed into quartz. It is concluded that the Rb‐Sr isotopic system of the cherts becomes essentially homogenized among biogenic silica, detrital components and hydrogeneous components during deposition, and the subsequent chemical diagenesis before lithification due to high mobility of Rb and Sr. The Sr initial ratio (0.71363) of the Triassic Mino cherts is clearly higher than those (0.7079 and 0.7068) of the Triassic and Permian Sambosan cherts, which are close to the estimated oceanic Sr ratios of 0.7066–0.7081 in the Permian and Triassic. On the other hand, Sm‐Nd isotopic data do not define isochron lines. This might be due to incomplete homogenization of the system among biogenic silica, detrital components and hydrogeneous components during deposition, and chemical diagenesis because of low mobility of REE. Initial Sr and Nd isotopic ratios of the Mino cherts can be interpreted by mixing typical continental crustal rocks, represented by aeolian loess compositions, with a small amount of Precambrian rocks (<10%), while those of the Sambosan cherts can be explained by mixing continental crustal rocks with oceanic volcanic rocks such as MORB (mid‐ocean ridge basalt). In the mixing model between Chinese loess and MORB, a proportion of loess is ca. 30%–60% for the Middle Permian Sambosan cherts and ca. 60%–90% for the Triassic Sambosan cherts. Al2O3/(Al2O3 + Fe 2O3) ratios and Ce anomalies in the REE patterns also show a higher contribution of continental components in the Triassic Sambosan and Mino cherts than the Permian Sambosan cherts, suggesting deposition in a marginal sea or on the continental shelf and slope for the Triassic Sambosan and Mino cherts. Our study shows that the geochemical and isotopic features of the Mino and Sambosan cherts essentially retain the nature of their formation and subsequent chemical diagenesis before lithification. Alteration or metamorphic effects during subduction and accretion processes cannot be found in the chemical and isotopic features of these cherts. Our result requires the reexamination of the previous paleogeographical reconstructions of the Japanese Islands as well as the relationship between the Mino and Sambosan Terranes in Permian and Triassic time.

Permian hydrothermal deposits in the Mino Terrane, central Japan: Implications for hydrothermal plumes in an ancient ocean basin

Abstract Geochemical features of metalliferous deposits, cherts and basalts from the Early Permian Hashikadani Formation in the Mino Terrane, central Japan were studied. The metalliferous deposits occur as thin Fe-rich layers about 5 cm thick and Mn-rich crusts up to 30 cm thick in cherts overlying basalt. The Fe-rich layers contain large amounts of magnetite and are intercalated in the massive chert unit just above the basal basalt. The Mn-rich crusts occur in the bedded chert unit, which always overlies massive cherts, and their dominant Mn-mineral is braunite. These metalliferous deposits show highly fractionated MnO/Fe 2 O 3 ∗ (total iron as Fe 2 O 3 ) values and are depleted in such elements as Ni, Co, Cu and Zn; these features indicate their hydrothermal origin. The values of Fe 2 O 3 /FeO and MnO/Fe 2 O 3 ∗ in samples abruptly increase above the lithologic boundary between the massive chert and the bedded chert units. This stratigraphic change of lithofacies and chemical characteristics can be attributed to the precipitation of Fe-oxides prior to Mn-oxides from a hydrothermal solution that was gradually diluted with seawater and became oxic. The formation process of the Permian metalliferous deposits from central Japan can be inferred by analogy with modern hydrothermal deposits recently discovered along active spreading centres.

Provenance of Creataceous sandstones in the southeastern Yongdong Basin, Korea: CHIME geochronology of detrital monazites

CHIME (chemical Th-U-total Pb isochron method) ages were determined on detrital monazites in Cretaceous sandstones, southeastern part of the Yongdong Basin and in granitic gneiss (?foliated granite) of the Yongnam Massif, northern part of the Kimchon area. Most detrital monazites in the sandstone samples are chronologically unzoned, and yield CHIME ages of about 180 and 250 Ma. Some detrital monazites show age signatures of middle Precambrian (ca. 1200 and 1750 Ma) and possible Silurian (420 Ma). Monazites in the granitic gneiss yield an unequivocal age of 251.2±3.0 Ma. The age spectrum of detrital monazites coupled with paleoflows and petrographic data suggests that clastic materials in the southeastern part of the Yongdong Basin were largely derived from the granitic and metamorphic rocks of the Yongnam Massif in the Sangju—Kimchon—Keochang area.