Yoshihiro Asahara

JNdi-1 : a neodymium isotopic reference in consistency with LaJolla neodymium

Abstract A neodymium oxide with relative 143Nd/144Nd ratio 1.000503±1(1 σm) to LaJolla Nd was prepared as a new isotopic reference. The neodymium reagent was selected from two points of view as follows. The first is low abundance of neighboring elements Ce and Sm, which affects isobaric interference. The second is high 143Nd/144Nd ratio, which is closer to those of chondritic and mantle-derived materials. The 143Nd/144Nd ratio of the reagent was measured alternately with LaJolla Nd to get a coherency with LaJolla Nd using 12 mass spectrometers in 11 laboratories in Japan. Aliquots of this neodymium oxide reagent named JNdi-1 are available upon request from the Geological Survey of Japan and may be useful for precise interlaboratory calibration of Nd isotopes.

Asian continental nature of87Sr/86Sr ratios in north central Pacific sediments

Strontium isotopic compositions were determined for silicate detritus in 152 deep-sea and continental shelf surface sediment samples from the Pacific Ocean to trace their provenance. The major characteristics of the spatial variation of the Sr isotopes and the interpretation of this variation may be summarized as follows: (1) Sediments in the Pacific have a lower average87Sr/86Sr ratio (0.7112) with a narrower range (0.703–0.724) than in the Atlantic (0.7166; 0.704–0.743). Many western Pacific volcanoes together with young continental crust supply material with low87Sr/86Sr ratios to the Pacific, in contrast to the large old cratonic crust that supplies material with high Sr ratio to the Atlantic. (2) The87Sr/86Sr ratio is high in the north central Pacific (0.711–0.719). The Izu-Ogasawara and Mariana arcs supply low87Sr/86Sr material to the western Pacific, but farther to east the abundant supply of loess carried by middle-latitude westerlies from the Asian continent (87Sr/86Sr=ca.0.720) dominates the north central Pacific.

Provenance of the north Pacific sediments and process of source material transport as derived from Rb–Sr isotopic systematics

Abstract Rb–Sr isotopic systematics of 111 samples of sediments in nine cores from the north Pacific of Quaternary and Pliocene ages have been investigated. They provide information on the provenance, the process of particle transport and temporal variation in the flux of source material. The Rb–Sr isotopic systematics of the core sediments show well-correlated pseudo isochrons. The pseudo isochrons reflect the mixing of two types of material, i.e., the Asian continental material with high 87 Rb / 86 Sr ratios (4.5–6.5) and high 87 Sr / 86 Sr ratios (0.723–0.727) and the volcanic material with low 87 Rb / 86 Sr ratios and low 87 Sr / 86 Sr ratios, from island–arc volcanics such as the Izu–Ogasawara–Mariana and the Japanese Islands and oceanic islands such as the Hawaiian Islands (0.0–2.0; 0.703–0.708). The clearness of the pseudo isochrons implies that the Rb–Sr isotopic composition of the weathering products derived from the Asian continental crust is extremely homogenized. The fine fraction (a few μm) with a high 87 Rb / 86 Sr ratio (5.0–6.5) is widely transported into the north Pacific by the middle-latitude westerlies and contributes largely to pelagic sediments. The coarser fraction with a lower 87 Rb / 86 Sr ratio (

Systematic variations in C, O, and Sr isotopes and elemental concentrations in Neoproterozoic carbonates in Namibia: implications for a glacial to interglacial transition

Abstract We analyzed C, O, and Sr isotopic compositions of Neoproterozoic cap carbonates overlying a glacial deposit in Namibia to trace environmental changes following the glaciation. The three isotopic records showed coherent and phased changes that corresponded to the stratigraphic variation. A lower rhythmite unit had three intervals, which were distinguished by isotopic variations and mineral type. In the basal interval, δ 13 C increased exponentially from a negative value (−4.3‰, Pee Dee Belemnite [PDB]). δ 13 C values were relatively constant in the middle interval, and increased again to positive values in the top interval, indicating a phased removal of 13 C -depleted carbon from the surface ocean. The gradual increase to larger values (>6‰) in the upper stromatolite unit was interpreted to reflect a change in seawater composition resulting from biological activity after environmental recovery from the glacial episode. δ 18 O values and Sr isotopic ratios decreased in the basal interval and approached stable values in the middle interval. They increased in the upper interval and fluctuated in the stromatolite unit. On the basis of geochemical screening criteria for detecting diagenetic alteration of Sr isotopes, the middle interval of the rhythmite unit, with a minimum 87 Sr / 86 Sr ratio of 0.70685, was interpreted to have retained the primary value, or to have the least-altered Sr isotopic composition. The basal interval, which had ratios (∼0.718) at its base as high as those of old, sialic rocks, was interpreted as having altered Sr isotopic ratios. The high Sr isotopic ratios at the base of this interval gradually decreased to near the minimum value at the top of the interval, and other isotopic compositions and trace element concentrations showed systematic variations. Such systematic variations, except for the variation in Na concentration, can be explained by a progressive diagenetic fluid–rock interaction. Alternatively, they might reflect a compositional transition from a local, transport-limited water following the glaciation to global, well-mixed seawater.

Intermediate water formation in the Bering Sea during glacial periods: Evidence from neodymium isotope ratios

Changes in the flux and location of overturning circulation may have large effects on marine ecosystems and CO 2 exchange between the ocean and atmosphere. However, unlike the Atlantic, little is known about ocean circulation and ventilation under glacial boundary conditions in the North Pacific, especially in regard to intermediate water circulation. Here we present new records of neodymium (Nd) isotopes (e Nd ) in Fe-Mn oxyhydroxides of Bering Sea sediments (884 m water depth). We found a systematic variation between radiogenic values (up to +0.8 e Nd ) during cold periods and relatively less radiogenic values ( Nd ) during warm periods. There are no water masses with such radiogenic Nd isotope signatures in the intermediate or deep North Pacific. Potential sources of radiogenic values in the subarctic North Pacific are limited to surface waters adjacent to the Aleutian Arc and Kamchatka Peninsula. Therefore, the radiogenic e Nd values of Fe-Mn oxyhydroxides observed at the intermediate depth during glacial periods are best explained by subduction of the surface water to the intermediate depth (at least ∼800 m) due to brine rejection. Our data strongly indicate that the northwestern Bering Sea (off northeastern Kamchatka) was a possible source region of glacial intermediate water in the Bering Sea and the subarctic North Pacific.

Juvenile granite in the Sanandaj–Sirjan Zone, NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low 87Sr/86Sr(i) (i represents initial) and high 143Nd/144Nd(i) ratios, resulting in positive ϵt Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ϵt Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.

87Sr/86Sr variation in north pacific sediments: a record of the Milankovitch cycle in the past 3 million years

Abstract Temporal variations of Sr isotopic compositions in the detrital component of north Pacific sediments have been precisely examined. The Sr isotopic composition is controlled by the flux of eolian material from the Asian continent with a high 87 Sr/ 86 Sr ratio (0.724–0.726) relative to volcanic material with a low ratio (0.703–0.705) from the Izu-Ogasawara-Mariana arc and oceanic islands such as the Hawaiian Islands. Assuming that the volcanic flux is constant, the temporal variation in Sr isotopic ratio reflects the amount of eolian input from the continent. The major characteristics of the temporal variations and the interpretation of them are summarized as follows. (1) The 87 Sr/ 86 Sr records show cyclic fluctuations of 400 ky and 100 ky periodicities which are associated with the eccentricity of the earths orbit (Milankovitch cycle). (2) Between 3 and 0.8 Ma, the 87 Sr/ 86 Sr ratio in the north central Pacific sediment increases gradually. This reflects an increased eolian input from the arid region in east Asia. The increase must be related to aridification of the Asian continent. (3) The decreased 87 Sr/ 86 Sr ratio during the past 0.8 m.y. implies a decreased eolian input. The age of 0.8 Ma may relate to the climatic event known as the Middle Pleistocene shift. All of these phenomena reflect a fluctuation of the eolian flux corresponding to the paleoclimatic cycle of aridity in the Asian continent. The Sr isotopic composition of pelagic sediment in the north Pacific is sensitive to changes in the eolian input reflecting to the aridity of the Asian continent.

Geochemistry and geodynamics of the Mawat mafic complex in the Zagros Suture zone, northeast Iraq

The Iraqi Zagros Orogenic Belt includes two separate ophiolite belts, which extend along a northwest-southeast trend near the Iranian border. The outer belt shows ophiolite sequences and originated in the oceanic ridge or supra-subduction zone. The inner belt includes the Mawat complex, which is parallel to the outer belt and is separated by the Biston Avoraman block. The Mawat complex with zoning structures includes sedimentary rocks with mafic interbedded lava and tuff, and thick mafic and ultramafic rocks. This complex does not show a typical ophiolite sequences such as those in Penjween and Bulfat. The Mawat complex shows evidence of dynamic deformation during the Late Cretaceous. Geochemical data suggest that basic rocks have high MgO and are significantly depleted in LREE relative to HREE. In addition they show positive ɛNd values (+5 to+8) and low 87Sr/86Sr ratios. The occurrence of some OIB type rocks, high Mg basaltic rocks and some intermediate compositions between these two indicate the evolution of the Mawat complex from primary and depleted source mantle. The absence of a typical ophiolite sequence and the presence of good compatibility of the source magma with magma extracted from the mantle plume suggests that a mantle plume from the D″ layer is more consistent as the source of this complex than the oceanic ridge or supra-subduction zone settings. Based on our proposed model the Mawat basin represents an extensional basin formed during the Late Paleozoic to younger along the Arabian passive margin oriented parallel to the Neo-Tethys oceanic ridge or spreading center. The Mawat extensional basin formed without creation of new oceanic basement. During the extension, huge volumes of mafic lava were intruded into this basin. This basin was squeezed between the Arabian Plate and Biston Avoraman block during the Late Cretaceous.

Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Quaternary high-Nb basalts: existence of young oceanic crust under the Sanandaj–Sirjan Zone, NW Iran

Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO2, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO2 (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their 87Sr/86Sr values range from 0.7049 to 0.7053 and their ϵ0Nd values lie between –0.2 and 1.1. The small negative values of ϵ0Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.