Takeshi Kuritani

Precise isotope analysis of nanogram-level Pb for natural rock samples without use of double spikes

A simple technique has been developed for the precise analysis of lead isotope in natural rock samples by thermal ionization mass spectrometry (TIMS). Two-stage column chromatography, using 100 and 10 μl columns, was used to minimize the amounts of impurities in separated lead samples. This dramatically improved the reproducibility of mass fractionation during mass spectrometry using an ion emitter made of a mixture of silicic acid and phosphoric acid. This improvement made it possible to precisely determine the Pb isotopic compositions of very small sample sizes, employing “zero-time correction” for mass discrimination, without requiring a double-spike technique. Using the present method, analytical reproducibility of 208Pb/204Pb of 0.02% and 0.06% (2σ) was attained for 100 and 1 ng of Pb, respectively, separated from natural rock samples. Furthermore, we obtained a reproducibility of 0.06% (2σ) for 208Pb/204Pb for 10 ng of Pb separated from GSJ JP-1 (peridotite), in which the Pb concentration was 0.09 ppm. The measured isotope compositions of USGS standard rocks AGV-1 and BCR-1 were comparable with the published values using the double-spike technique. These observations suggest that our simple technique is reliable in terms of both accuracy and precision for the determination of the Pb isotopic compositions of natural rock samples irrespective of rock chemistry and sample sizes from 1 to 100 ng of Pb.

Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan

[1] 238 U- 230 Th- 226 Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1-4) starting >7000 BP. 238 U- 230 Th- 226 Ra disequilibria observed in lavas with large 238 U and 226 Ra excesses imply metasomatism of depleted mantle by fluid-related processes. This metasomatism is also suggested by trace element and Sr-Nd-Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial ( 230 Th/ 232 Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab-derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226 Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid-release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3). Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several-kiloyear timescale.

Highly precise and accurate isotopic analysis of small amounts of Pb using 205Pb–204Pb and 207Pb-204Pb, two double spikes

A new, simple, and practical method has been developed for the accurate and precise isotopic analysis of extremely small amounts of Pb (<∼3 ng) by thermal ionization mass spectrometry. Two different types of double spikes, one consisting of 205Pb and 204Pb and another enriched in 207Pb and 204Pb, are used to reduce “204Pb error” and to correct mass fractionation during mass spectrometry. Using this technique, replicate analyses of 1.5 ng of Pb from NBS981 were performed, and an external precision of 0.02% (2σ) was attained for the 208Pb/204Pb ratio. Compared with results for 1.5 ng of Pb by the normal double spike method using a single 207Pb–204Pb double spike, the external precision for the isotopic ratios involving 204Pb is reduced by about 60–70%, simply by addition of the 205Pb–204Pb double spike. The two double spikes method was also applied to isotopic analyses of natural rock samples, and we obtained an external precision of 0.06% (2σ) for 208Pb/204Pb in ∼1.5 ng of Pb separated from fresh peridotite, in which the Pb concentration is as low as 3.2 ppb. This new technique is superior to any previous method for precise and accurate isotopic analyses of extremely small amounts of Pb, and will be a highly powerful tool in the future of earth science and environmental science.

Phenocryst crystallization during ascent of alkali basalt magma at Rishiri Volcano, northern Japan

Abstract Phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers. A fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions, on the basis of their large sizes. However, this assumption is not always true as demonstrated here for a porphyritic alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan. All phenocryst phases in the Kutsugata lava, plagioclase, olivine, and augite, have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals. Rarely, a core region with distinct composition is present in all phenocryst phases. Phenocrysts, excluding this core, are occasionally in direct contact with each other, forming crystal aggregates. The equilibrium liquidus temperature of plagioclase, the dominant phase (∼35 vol%) in the Kutsugata lava, can never exceed the estimated magmatic temperature, unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent. This suggests that most phenocrysts in the Kutsugata lava were formed by decompression of the magma during ascent in a conduit, rather than by cooling during residence in a magma reservoir. In the magma chamber before eruption, probably located at depth of more than 7 km, only cores of the phenocrysts were present and the magma was nearly aphyric ( 4 wt.%) in the magma chamber, and liquidus temperatures of phenocryst phases were significantly suppressed. Large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution. Low ascent rate of the Kutsugata magma, which is suggested by pahoehoe lava morphology and no association of pyroclastics, gave sufficient time for crystallization. Furthermore, the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size, which characterizes the porphyritic nature of the Kutsugata lava. Alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems.

Thermal and compositional evolution of a cooling magma chamber by boundary layer Fractionation: model and its application for primary magma estimation

A boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented. The model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body. The magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer. The consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space. The model is used to evaluate a primary magma estimation from volcanic rock series. It is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation. This can cause significant errors in estimating the conditions at which the primary magma was generated.

Boundary layer fractionation constrained by differential information from the Kutsugata lava flow, Rishiri volcano, Japan

Detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber. Kutsugata lava, a suitable target of such study, is a Quaternary alkali basalt (51.3–53.2 wt% in SiO2) from Rishiri Volcano, northern Japan. Despite the narrow range in the whole rock compositional variation, chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition. In the North lava (51.3–51.9 wt% in SiO2) the less differentiated portion of the Kutsugata lava, most crystals which include low-Ni olivine and plagioclase were derived from the mushy boundary layer. The main part of the magma body was principally aphyric ( 30 vol% by such mechanisms as compaction and compositional convection. This fractionated melt was mixed with the overlying main magma, causing differentiation of the Kutsugata magma. The average temperature of the extracted melt is 1010°C, significantly lower than 1100°C estimated for the main magma. A quantitative model of magmatic differentiation, which includes thermal and compositional evolution of a mushy boundary layer, can successfully reproduce the observed compositional trends of the North lava.

Mantle hydration and the role of water in the generation of large igneous provinces

The genesis of large igneous provinces (LIP) is controlled by multiple factors including anomalous mantle temperatures, the presence of fusible fertile components and volatiles in the mantle source, and the extent of decompression. The lack of a comprehensive examination of all these factors in one specific LIP makes the mantle plume model debatable. Here, we report estimates of the water content in picrites from the Emeishan LIP in southwestern China. Although these picrites display an island arc-like H2O content (up to 3.4 by weight percent), the trace element characteristics do not support a subduction zone setting but point to a hydrous reservoir in the deep mantle. Combining with previous studies, we propose that hydrous and hot plumes occasionally appeared in the Phanerozoic era to produce continental LIPs (e.g., Tarim, Siberian Trap, Karoo). The wide sampling of hydrous reservoirs in the deep mantle by mantle plumes thus indicates that the Earth’s interior is largely hydrated.The genesis of large igneous provinces (LIPs) remains controversial. Here, the authors examine the water contents of picrites from the Emeishan LIP and find that despite high water contents, the elevated temperature and trace elements suggest a mantle plume from a hydrous deep reservoir rather than subduction zone related.

Origin of ultra rear‐arc magmatism at Rishiri Volcano, Kuril Arc

The Rishiri Volcano is located at the very rear of the Kuril Arc at its junction with the NE Japan Arc, and its 300 km depth to the slab surface is one of the deepest among the active arc volcanoes in the world. In this study, the origin of this ultra rear-arc magmatism was investigated by analyzing the basaltic lavas from the volcano. The lavas consist of low-K and high-K groups, with the low-K lavas predating the high-K lavas. Since it is unlikely that the high-K magmas are derivatives of the low-K magmas, the two magmas are thought to be derived from different source mantle materials. Analyses using multicomponent thermodynamics suggest that these magmas were both generated through the ∼2% melting of a source mantle with 0.04–0.11 wt.% H2O at 1280–1330°C and ∼2.3 GPa. The temperatures at the surface of the subducting Pacific slab, from which the slab fluids were released, were estimated to be 860–960°C for the low-K magmas and 930–1040°C for the high-K magmas. These temperatures of the slab surface are remarkably higher than those predicted by thermal models. The estimated high temperatures of the slab surface and the latest detailed seismic tomography results suggest that the low-K and high-K magmatism resulted from the progressive production of fluids at the slab surface due to heating by the injection of hot mantle materials into a relatively large-scale fracture in the distorted portion of the subducting Pacific plate. This article is protected by copyright. All rights reserved.

High-Nb hawaiite–mugearite and high-Mg calc-alkaline lavas from northeastern Iran: Oligo-Miocene melts from modified mantle wedge

ABSTRACT Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit), constitute vast outcrops (up to 250 km2) of high-Mg basaltic andesites to dacites that are associated with high-Nb hawaiites and mugearites. Whole-rock 40Ar/39Ar ages show a restricted range of 24.1 ± 0.4–22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of 87Sr/86Sr and 143Nd/144Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in the high-Mg basaltic andesites–dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27–100) and the isotopic composition of the subalkaline high-Mg basaltic andesites–dacites indicate derivation from a mantle modified by slab and sediment partial melts. Evidence such as reverse zoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate that magma mixing with mafic melts and concurrent fractional crystallization lead to the compositional evolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodic alkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and a wide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr–Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from a mantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggested to have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatized by slab-derived melts.