Shun'ichi Nakai

Provenance of dust in the Pacific Ocean

Eolian dust preserved in deep-sea sediment cores provides a valuable indicator of past atmospheric circulation and continental paleoclimate. In order to identify the provenance of eolian dust, Nd and Sr isotopic compositions and Rb, Sr and rare earth element (REE) concentrations have been determined for the silicate fractions of deep-sea sediments from the north and central Pacific Ocean. Different regions of the Pacific Ocean are characterized by distinct air-borne inputs, producing a large range in eNd (--10 tO + 1), STSr//86Sr (0.705-0.721), La/Yb (5-15), EUN/EU ~ (0.6-1.0) and Sr/Nd (4-33). The average Nd isotopic composition of Pacific deep-sea sediments (ENO = --6), is more radiogenic than the average from the Atlantic (eNa = --8). In contrast, the average 1475m//14aNd ratio for Pacific sediments (0.114) is identical to that of Atlantic sediments and to that of global average riverine suspended material. The values of ENa and 147Sm/144Nd are positively correlated for the Pacific samples but negatively correlated for Atlantic samples, reflecting a fundamental difference between the dominant components in the end members with radiogenic Nd (island-arc components in the Pacific and LREE-enriched intraplate ocean island components in the Atlantic). Samples from the north central Pacific have distinctive unradiogenic eNd values of --10, 87Sr/86Sr > 0.715, high La/Yb (> 12), and low EuN/Eu ~ (0.6) and Sr/Nd (3-6). These data are virtually identical to the values for loess from Asia and endorse the use of these sediments as indicators of Asian paleoclimate and paleowind directions. Island-arc contributions appear to dominate in the northwest Pacific, resulting in higher end (--1 to + 1) and lower 87Sr/86Sr (= 0.705) and La/Yb (~-5). Sediments from the eastern Pacific tend to have intermediate Sr and Nd isotopic compositions but regionally variable Sr/Nd and REE patterns; they appear to be derived from the west margin of the North and South American continents, rather than from Asia. Our results confirm that dust provenance can be constrained by isotopic and geochemical analyses, which will facilitate reconstructions of past atmospheric circulation and continental paleoclimate.

Lithium isotopic systematics of the mantle-derived ultramafic xenoliths: implications for EM1 origin

Isotopic signatures of mantle-derived xenoliths have provided much information on the evolution of their mantle source regions. A recently developed multiple-collector inductively coupled plasma mass spectrometry method allows precise and accurate lithium isotopic determinations of Li-poor samples such as peridotites. We present Li–Sr–Nd isotopic systematics of clinopyroxenes (CPXs) in mantle-derived ultramafic xenoliths. The results show that Ichino-megata (Northeastern Japan) and Bullenmerri (Southeastern Australia) samples have positive δ7Li values (δ7Li∼+4 to +7‰, δ7Li=[[7Li/6Li]sample/[7Li/6Li]L-SVEC standard−1]×1000) common to values previously reported for terrestrial volcanic rocks. By contrast, unusually low δ7Li values (δ7Li∼−17‰) are observed in many samples from the Far East region of Russia (Sveyagin, Ennokentiev, and Fevralsky) and southwestern Japan (Kurose and Takashima). The δ7Li values of Sikhote-Alin (Sveyagin and Ennokentiev) samples vary widely from −17.1‰ to −3.1‰, while the δ7Li values are positively correlated with 143Nd/144Nd, and negatively correlated with 87Sr/86Sr. On the other hand, the δ7Li values of the Bullenmerri samples are essentially constant (δ7Li=+5.0 to +6.0‰), while the 87Sr/86Sr (0.7027∼0.7098) and 143Nd/144Nd ratios (0.51224∼0.51297) vary widely. These features can be explained by the results of a binary mixing between a depleted component (low-87Sr/86Sr, and high-143Nd/144Nd) and an enriched component (high-87Sr/86Sr, and low-143Nd/144Nd). The enriched component (metasomatic agent) in the mantle beneath the Sikhote-Alin area has extraordinarily low δ7Li value ( altered MORB) from the subducted slab during dehydration at low temperature. Consequently, it is deduced that Li of metasomatic agent with an extremely low δ7Li value is derived from subducted highly altered basalt. The enrichment of isotopically light Li (low δ7Li) may be a general property of EM1 mantle reservoir. The Li isotopic data suggest further that the EM1 and HIMU sources originate from different parts of a recycling oceanic crust. This is essentially the same as the models proposed previously based on the radiogenic isotopic data, but with the Li isotopic data requiring uppermost, highly altered basaltic crust as well as pelagic sediment in the EM1 source, but not so in the HIMU end-member. Because of the apparent sensitivity of Li isotopic composition to the alteration profile of subducted MORB, it may provide complementary information to Sr, Nd, and Pb isotopic compositions regarding the mantle source.

Rb-Sr dating of sphalerites from Mississippi Valley-type (MVT) ore deposits

Abstract Mississippi Valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, barite, dolomite, calcite, and quartz. Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America. Sphalerites from Immel mine, Mascot-Jefferson City district, east Tennessee, define a Rb-Sr age of 347 ± 20 Ma consistent with a Rb-Sr age of 377 ± 29 Ma for sphalerites from Coy mine in the same district, but inconsistent with models that ascribe their genesis to the effects of the late Paleozoic Alleghenian orogeny. Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniels Harbour mine, New foundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. Sphalerites from the Pine Point district, Northwest Territories, Canada, define a Rb-Sr age of 361 ± 13 Ma, indicating that the mineralization took place shortly after the deposition of the middle Devonian host carbonate rocks. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations (less than 0.1 ppm). The Rb-Sr data do not form isochrons and the sphalerites have higher 87 Sr 86 Sr ratios than expected, given their Rb Sr ratios and reasonable constraints on their ages. The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr.

Contribution of slab melting and slab dehydration to magmatism in the NE Japan arc for the last 25 Myr: Constraints from geochemistry

[1]xa0Hf isotope compositions, coupled with Pb-Sr-Nd isotopes and trace element compositions, for the Miocene to the Quaternary volcanic rocks from the NE Japan arc have documented the geochemical evolution in the magma source compositions in association with back-arc opening (23–14 Ma). Clear temporal variation in Hf isotope ratios is demonstrated, from the lowest ɛHf (+9) in the early Miocene to the highest ɛHf (+14) in the late Miocene and the Quaternary, whereas there are little changes in Sr and Nd isotope composition. Hf isotope ratios are clearly correlated with some trace elemental ratios. The observed temporal variation is attributed to a change in metasomatic agents derived from the subducted slab, which could have been caused by cooling of the mantle wedge during the syn and post stages of the back-arc opening. In the early stage of the back-arc opening, when injection of hot asthenospheric mantle induced high-temperature conditions in the mantle wedge, slab melting contributed to low ɛHf coupled with low ɛNd, relatively low La/Nb, and high Th/Yb in volcanic rocks. On the other hand, the later stage volcanism, characterized by high ɛHf, decoupling of ɛHf and ɛNd, high and variable La/Nb and Ba/La, is best explained by addition of fluids from subducted sediments and oceanic crust to the mantle wedge.

Mantle-derived noble gases in natural gases from Songliao Basin, China

Abstract Abundances and isotopic compositions of noble gases have been measured in six natural gas samples (CO 2 and CH 4 -rich) from the Songliao Basin, Jilin Province, in northeastern China. The samples contain noble gases of mantle origin. The 3 He 4 He ratio reaches 5.0 times the atmospheric ratio. In a three-isotope plot of neon, the 20 Ne 22 Ne (up to 10.9) and 21 Ne 22 Ne (up to 0.051) ratios make a positive correlation array together with natural gases from other continental areas. Compared with a correlation band for MORB, the natural gases have a lower slope with more nucleogenic 21 Ne. The natural gas samples contain radiogenic argon with 40 Ar 36 Ar ratios up to 7700. A positive correlation between 40 Ar 36 Ar and 20 Ne 22 Ne ratios indicates occurrence of mantle-derived Ar. Slight excess of 38 Ar can be attributed to a nuclear reaction like 35 C1 (α, p ) 38 Ar. Apparent excesses of 129 2Xe, 132–136 Xe are recognized in four samples. The excess of 129 Xe (up to 3%) can be attributed to a decay of extinct 129 I. Excess 132−136 Xe is not large enough to determine if the origin of the excess is 238 U or 244 Pu. Anomaly in 129 Xe 130 Xe ratio is correlated with that of 136 Xe 130 Xe . The isotopic features of the natural gases with radiogenic 4 He and nucleogenic 21 Ne can be produced within the crust. Alternatively, they may reflect the geochemical features of the subcontinental mantle which has been enriched in U, Th. We can not distinguish the two possibilities. However, a natural gas from another basin in eastern China with a different reservoir age contains mantle derived neon which falls on the neon correlation line formed by the samples from the Songliao Basin. This consistency suggests that the isotopic features of the natural gases arent necessarily ascribable to surface contamination of radiogenic and nucleogenic isotopes.

A contribution of slab-melts to the formation of high-Mg andesite magmas; Hf isotopic evidence from SW Japan

[1]xa0Hf isotope ratios were measured for high-Mg andesites (HMA) and basalts from the Setouchi volcanic belt, Southwest Japan, in order to examine whether fluid or melt from subducted slabs act as metasomatic agents for HMA magma genesis. Since Hf is one of the incompatible elements that may not be readily transported by aqueous fluids, Hf isotopes should provide strong constraints on the origin of such slab-derived components. Setouchi HMAs and basalts are distinct in their lower 176Hf/177Hf (eHf +13), suggesting significant contribution of subducted sediments to Setouchi magma genesis. It should be further stressed that Hf isotopes are well correlated with Sr-Nd isotopes and La/Nb ratios among the Setouchi magmas. Geochemical modeling suggests that such characteristics can be best explained by addition of slab-derived components with sediment melts rather than aqueous fluids to the HMA source.

Helium-3 emission in and around Teide Volcano, Tenerife, Canary Islands, Spain

Air corrected ³He/4He ratios of fluid samples collected in and around Teide volcano, Tenerife, Canary Islands, ranged from 5.97 to 8.06 Ra. Helium-3 emission spatial distribution has a quite different pattern from other stratovolcanoes which show commonly a decreased trend for the ³He/4He ratio with respect to distance from the summit crater. This finding might be related to the volcano-tectonic setting differences between subduction-type and oceanic island-type statrovolcanoes. Radiogenic helium production is a significant process for the dilution of uprising derived-mantle fluids in and around subduction-type stratovolcanoes while it can be almost negligible for polygenetic volcanoes at oceanic islands.

Geochemistry of Ce and Nd isotopes and REE abundances in the Amîtsoq gneisses, West Greenland

Abstract Ce and Nd isotope ratios and rare earth element (REE) abundances were determined on the Archean Amitsoq gneisses, West Greenland; samples used were four grey gneisses, two white gneisses and one banded gneiss from the Isukasia area and one banded grey gneiss from the Godthaab area. All of the Amitsoq gneisses studied here have negative e Nd and positive e Ce values for the present Nd and Ce isotopic compositions. Calculated initial values of e Nd and e Ce give the following evolution histories of the REE patterns. The grey gneiss in the Godthaab area is considered to have been derived from a chondritic source with (La/Ce) N and (Nd/Sm) N ratios of 1.1 ± 0.4 and 0.99 ± 0.02 in the chondrite-normalized values. On the other hand, it is concluded that the grey gneisses and the banded gneiss in the Isukasia area were formed from a light-REE-depleted source with (La/Ce) N = 0.6 ± 0.1 and (Nd/Sm) N = 0.97 ± 0.03. Furthermore, it is suggested that the white gneisses in the Isukasia area may have been produced from a source with a convex REE pattern. Our study on Ce and Nd isotopes and REE abundances provides further evidence that fractionated REE patterns were established already in the very early stages of the Archean.

A strontium and neodymium isotopic study of Apollo 17 high-Ti mare basalts - Resolution of ages, evolution of magmas, and origins of source heterogeneities

Abstract A combined Sr and Nd isotopic study of 15 Apollo 17 high-Ti mare basalts was undertaken to investigate geochronological and compositional differences between previously identified magma types (A, B1, B2, and C). Whole-rock and mineral separates for one of the least-evolved Type B1 basalts, 70139, yield Sm-Nd and Rb-Sr isochron ages of 3.71 ± 0.12 Ga and 3.65 ± 0.13 Ga, respectively. A more-evolved, Type A basalt, 71539, exhibits a slightly older Sm-Nd isochron age of 3.75 ± 0.07 Ga and a Rb-Sr isochron age of 3.67 ±0.10 Ga. Although these two ages are non-resolvable by themselves, compilation of all available geochronological data allows resolution of Type A and B1/B2 ages at high levels of confidence (>99%). The most reliably dated samples, classified according to their geochemical type, yield weighted average ages of 3.75 ± 0.02 Ga for Type A (N = 4) and 3.69 ± 0.02 Ga for Type B1/B2 (N = 3) basalts. Insufficient geochronological data are available to place the rare, Type C basalts within this stratigraphy. We propose that age differences correlate with geochemical magma type, and that early magmatism was dominated by eruption of Type A basalts while later activity was dominated by effusion of Type B1 and B2 basalts. Whole-rock isotopic data yield distinct differences in initial Sr and Nd isotopic compositions between Types A, B1, B2, and C basalts. Types A, B1, and C exhibit restricted intra-group compositional variations and lie along well-defined whole-rock isochrons. These data are consistent with petrogenetic models involving closed-system fractionation of observed microphenocrysts from chemically and isotopically distinct parental magmas. In contrast, a wide range of Type B2 initial isotopic compositions indicates mixing of several distinct components during magma evolution. The Sm-Nd whole-rock isochron age for Type A, Bl, and C basalts of 3.79 ± 0.15 Ga is within error of Apollo 17 eruptive activity. However, the very well-defined Sr whole-rock isochron age of 4.02 ± 0.05 Ga is 270 to 330 Ma older than eruptive ages. Isotopic and petrological arguments indicate that extensive Rb/Sr fractionation did not occur at the time of melt generation. Therefore, the 4.0 Ga Sr whole-rock isochron age records a significant event at which time geochemical heterogeneities were established within the originally homogeneous basalt source regions. Types A and C sources were enriched in Rb/Sr, with little or no concurrent modification of 87 Sr 86 Sr , Sm/Nd, or 143 Nd 144 Nd . Infiltration of similar-aged KREEP magmas into mantle cumulate sources cannot explain both Sr and Nd isotopic data. Instead, we suggest a metasomatic origin in which Rb, transported as a chloride complex in halogen-rich fluids, was preferentially mobilized relative to Sr and the REEs.

MORB-TYPE NEON IN AN ENRICHED MANTLE BENEATH ETNA, SICILY

Abstract Noble gas elemental and isotopic compositions were determined for five CO 2 –CH 4 samples collected around Etna, Sicily, to investigate the geochemical features of the mantle beneath the volcano. The samples contain mantle-derived noble gases. The measured helium isotopic ratios ( 3 He/ 4 He) vary between 5.9 and 6.4 times atmospheric ratio ( R a =1.4×10 −6 ), which are comparable to the ratios of olivines (6.1–8.2 R a ) in the lavas of the same volcano [1]. Neon in the samples is enriched in both 20 Ne and 21 Ne ( 20 Ne/ 22 Ne 9.95–10.7, 21 Ne/ 22 Ne 0.030–0.037), indicating derivation from the mantle. The δ ( 20 Ne/ 22 Ne)/ δ ( 21 Ne/ 22 Ne) values are identical with that of mid-ocean ridge basalts (MORB), indicating a similarity in the time-integrated (U/Ne) ratio between the Etnean magma source and the depleted upper mantle (MORB source). Argon in the samples has 40 Ar/ 36 Ar ratios up to of 1800, which are higher than in atmosphere. These ratios are positively correlated with the 20 Ne/ 22 Ne ratios, indicating a mantle origin of the radiogenic argon. Compared with olivines from the Etnean lavas [1], the argon in our natural gas samples is less contaminated by atmospheric argon. Two samples from a CO 2 well show small but resolvable excess of 129 Xe and 134,136 Xe. The volcanic rocks of Etna, ranging from tholeiites to alkaline, are enriched in incompatible elements. Nd, Sr and Pb isotopes of the volcanic rocks indicate that the magma source is isotopically heterogeneous and contain a component with high Sr, Pb and low Nd isotopic ratios derived from a mantle region which has been enriched in incompatible elements for a few million years [2,3,4] relative to the depleted upper mantle. The helium isotopic ratios of the gas samples are lower than those of MORB and are consistent with geochemical signatures of the solid elements in the Etnean volcanic rocks. However, the observed MORB-type neon is on a MORB correlation array without accumulation of nucleogenic 21 Ne. This apparent decoupling may be explained by a recent mixing of the depleted upper mantle (MORB source) with fluid enriched in both incompatible elements and radiogenic 4 He. When the fluid was formed with a small degree of partial melting, fractionation of radiogenic 4 He from nucleogenic 21 Ne could have occurred because of smaller partition coefficient of He than Ne.