Mitsunobu Tatsumoto

Isotopic composition of lead in oceanic basalt and its implication to mantle evolution

New data are given in this report for (1) Pb isotopic compositions and U, Th, and Pb concentrations of basalts from the island of Hawaii; (2) redetermined Pb isotopic compositions of some abyssal tholeiites; and (3) U, Th, and Pb concentrations of altered and fresh abyssal basalts, and basalt genesis and mantle evolution are discussed. The Th/U ratios of abyssal and Japanese tholeiites are distinctly lower than those of tholeiites and alkali basalts from other areas. It is thought that these low values reflect a part of the mantle depleted in large ionic lithophile elements. Thus a mantle evolution model is presented, in which Th/U ratios of the depleted zone in the mantle have decreased to ∼2, and U/Pb ratios have increased, showing an apparent ∼1.5-b.y. isochron trend in the 207Pb/204Pb vs. 206Pb/204Pb plot. The Pb isotopic compositions of basalts from the island of Hawaii are distinct for each of the five volcanoes, and within each volcano, Pbs of tholeiites and alkali basalts are similar. An interaction between partially melted material (hot plume?) of the asthenosphere and the lithosphere is suggested to explain the trend in the Pb isotopic compositions of Hawaiian basalts.

Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206

Measurements of the lead isotopic composition and the uranium, thorium, and lead concentrations in meteorites were made in order to obtain more precise radiometric ages of these members of the solar system. The newly determined value of the lead isotopic composition of Canyon Diablo troilite is as follows: 206Pb/204Pb = 9.307, 207Pb/2O4Pb = 10.294, and 208Pb/204Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the 206Pb/204Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage 207Pb/206Pb isochron based on the newly determined primordial lead value and the newly reported 235U and 838U decay constants, are 4.528 x 109 years for Sioux County and Nuevo Laredo and 4.555 x 109 years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 109 years and 4.620 x 109 years, respectively, and are therefore 40 to 70 x 106 years older than the 4.55 x 109 years age Patterson reported. The age difference of 27 x 106 years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial 87Sr/86Sr ratio of Angra dos Reis and that of seven basaltic achondrites observed by Papanastassiou and Wasserburg. The time difference is also comparable to that determined by 1291-129Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 109 years, and, here too, time differences in the formation of the parent bodies or later metamorphic events are indicated. Carbonaceous chondrites(C2 and C3) appear to contain younger lead components.

Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: implications for their origin from suboceanic-type mantle reservoirs

Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China. The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb/204Pb vs 206Pb/204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb/204Pb vs 206Pb/204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.

Mantle Plume Mixing Along the Reykjanes Ridge Axis: Lead Isotopic Evidence

Gradients of lead isotopic ratios from basalts erupted along the Reykjanes Ridge and Median Neovolcanic Zone ofIceland confirm mantle plume mixing with the depleted asthenosphere along the ridge axis. Geochemical studies of basalts erupted along the Reykjanes Ridge and its extension over the Median Neovolcanic Zone of Iceland have revealed striking gradients in minor and trace element concentrations (1). These studies and other geophysical and morphological evidence in the area have given strong support to the mantle plume hypothesis (2, 3) and led Schilling (1) to suggest a model consisting of two mantle sources, a mantle plume rising beneath Iceland and a depleted low velocity layer beneath the ridge. Mixing of these two sources at mantle depth was assumed to be the cause of the gradients, particularly in the La/Sm ratio. A subsequent 87Sr/ 86Sr test of the model (4) supported the concept presented by Schilling, but also suggested a less regular mixing along the transitional zone than was apparent from the gradient in the La/Sm ratio. Although challenged on various and debatable grounds (5-8), the binary mantle mixing model remains, we believe, the most important process in producing the gradients of large ionic lithophile (LIL) trace element ratios thus far observed along the Reykjanes Ridge-Iceland profile (1, 9). As further evidence in support of this model, we now present Pb iotopic compositions for 16 Reykjanes Ridge and 2 Iceland basalts, as well as Th, U, and Pb concentrations for 7 of these samples, whose locations are shown in Fig. 1. Results. Table I shows the Pb isotopic data obtained in this study and Table 2 the U, Th, and Pb concentrations (10, 11). All basalts analyzed are very fresh, tholeiitic in composition, and vary from slightly quartz normative to olivine normative (12). Figure 2 shows the 206Pb/204Pb and 208Pb/ 204Pb ratios of these basalts with respect to their distance from the southern tip of Iceland. Data for three samples from Sun and Jahn (13) and one from Welke et al (14), all from the Median Neovolcanic Zone of Iceland, are also plotted in Fig. 2. For comparison, 87Sr/86Sr and La/Sm ratios previously reported by Hart et al. (4) and Schilling (1), respectively, are also shown in Fig. 2. The 206Pb/204Pb profile shows high values over Iceland and low values south of 61°N to the Gibbs fracture zone. In be-

Sr, Nd, and Pb isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched components EMI and EMII in subcontinental lithosphere

The U-Th-Pb, Sm-Nd, and Rb-Sr isotopic systematics of mafic and ultramafic xenolithic rocks and associated megacrystic inclusions of aluminous augite and garnet, that occur in three alkalic volcanic suites: Kuandian in eastern Liaoning Province, Hanluoba in Hebei Province, and Minxi in western Fujian Province, China are described. In various isotopic data plots, the inclusion data invariably fall outside the isotopic ranges displayed by the host volcanic rocks, testifying to the true xenolithic nature of the inclusions. The major element partitioning data on Ca, Mg, Fe, and Al among the coexisting silicate minerals of the xenoliths establish their growth at ambient mantle temperatures of 1000–1100°C and possible depths of 70–80 km in the subcontinental lithosphere. Although the partitioning of these elements reflects equilibrium between coexisting minerals, equilibria of the Pb, Nd, and Sr isotopic systems among the minerals were not preserved. The disequilibria are most notable with respect to the206Pb/204Pb ratios of the minerals. On a Nd-Sr isotopic diagram, the inclusion data plot in a wider area than that for oceanic basalts from a distinctly more depleted component than MORB with higher143Nd/144Nd and a much broader range of87Sr/86Sr values, paralleling the theoretical trajectory of a sea-water altered lithosphere in Nd-Sr space. The garnets consistently show lowerμ andκ values than the pyroxenes and pyroxenites, whereas a phlogopite shows the highestμ andκ values among all the minerals and rocks studied. In a plot ofΔ207 andΔ208, the host basalts for all three areas show lowerΔ207 and higherΔ208 values than do the xenoliths, indicating derivation of basalts from Th-rich (relative to U) sources and xenoliths from U-rich sources. The xenolith data trends toward the enriched mantle components, EMI and EMII-like, characterized by high87Sr/86Sr andΔ207 values but with slightly higher143Nd/144Nd. The EMI trend is shown more distinctly by the host basalts. The EMII mantle domain may be present in the Chinese continental lithosphere just above the EMI domain of the basalt source at the lower part of the lithosphere. We argue that the ancient depleted continental lithosphere was metasomatized, imparting the EMI signature, in earlier times ( > 1000 m.y.), and U migrated upward, resulting in highTh/U ratios in the lower portion of the lithosphere. Observed highTh/U,Rb/Sr,87Sr/86Sr andΔ208, lowSm/Nd ratios, and a large negativeɛNd in phlogopite pyroxenite with a depleted mantle model age of 2.9 Ga, support our contention that metasomatized continental lower mantle lithosphere is the source for the EMI component. We also suggest that the EMII signature may have been introduced later (less than ∼ 500 Ma) by another metasomatic event during the subduction of an oceanic plate, which was partially responsible for some of the observed inter-mineral isotopic disequilibria.

Pb, Sr, Nd, and Hf isotopic constraints on the origin of Hawaiian basalts and evidence for a unique mantle source

Abstract Pb, Sr, Nd, and Hf isotopic relationships among basalts from the Hawaiian Islands suggest that these basalts were derived from three sources; the oceanic lithosphere (Kea end member), the depleted asthenosphere (posterosional end member) and a deep-mantle plume (Koolau end member). Hawaiian tholeiites are derived within the lithosphere and the isotopic trends collectively defined by the tholeiite data are interpreted as a plume-lithosphere mixing trend. The isotopic characteristics of late-stage basalts are derived from the tholeiite source (lithosphere + plume) with additional input from the lithosphere, asthenosphere, or both. These basalts probably originate from near the asthenosphere-lithosphere boundary. Posterosional basalts are derived from the depleted asthenosphere, but their isotopic characteristics have been slightly modified by either the plume or the source of previously erupted volcanics. The isotopic data require little or no mixing of asthenospheric material into the plume during tholeiite production and thus are consistent with the concept of a rapidly ascending, fluid-rich plume. In addition to providing a source of heat, the plume may supply volatiles to both the sources of tholeiites and posterosional basalts. The isotopic characteristics of the Koolau (plume) component are unique among OIB sources. If undifferentiated or “primitive” mantle material still exists, then the radiogenic-isotope data for Koolau in combination with rare gas data for Hawaiian basalts in general suggest that the Hawaiian plume may be derived from such material. In any case, the Hawaiian Islands data, when compared to those of other OIB, serve to illustrate the isotopically diverse nature of mantle sources.

Genetic Relations of Oceanic Basalts as Indicated by Lead Isotopes

The isotopic compositions of lead and the concentrations lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206: lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206: lead-204 ratios between 19.3 and 20.4 It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from and original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.

Potassium, Rubidium, Strontium, Thorium, Uranium, and the Ratio of Strontium-87 to Strontium-86 in Oceanic Tholeiitic Basalt

The average concentrations of potassium, rubidium, strontium, thorium, and uranium in oceanic tholeiitic basalt are (in parts per million) K, 1400; Rb, 1.2; Sr, 120; Th, 0.2; and U, 0.1. The ratio Sr87 to Sr86 is about 0.702, that of K to U is 1.4 x 104, and of Th to U is 1.8. These amounts of K, Th, U, and radiogenic Sr87 are less than in other common igneous rocks. The ratios of Th to U and Sr87 to Sr86 suggest that the source region of the oceanic tholeiites was differentiated from the original mantle material some time in the geologic past.

EVOLUTION OF THE ISOTOPIC COMPOSITION OF URANIUM AND THORIUM IN SOIL PROFILES.

Mass-spectrometric and alpha-spectrometric analyses for the isotopic composition of U 238 , U 235 , U 234 , Th 232 , and Th 230 have been made on several soils and soil profiles in glacial-derived parent material, volcanic rock, and shale. Three deep profiles from Minnesota represent typical well-developed soils on till and loess of Wisconsin age continuously sampled to depths of 6, 7, and 10 feet, respectively. Results on individual soil horizons, in these three profiles, show similar trends in the variation of U 234 /U 238 and Th 230 /U 238 ratios, the predominant features being excess Th 230 and deficient U 234 compared to U 238 . The other soils and soil profiles have different variations which can be correlated by use of a model to describe uranium migration. The data collected provided information for interpreting the causes of uranium migration in soils and for constructing a tentative model to explain the isotopic evolution of uranium and thorium in soil profiles. The proposed model indicates that (1) uranium was leached at depth in the profile, (2) preferential leaching of U 234 was continuous in the soil, and (3) upward capillary migration of a fraction of the uranium with above-normal U 234 /U 238 ratio tended to make uranium of high U 234 ratio available for isotopic exchange in upper soil horizons and for assimilation in organic complexes in surface soils which are rich in organic matter. Thus some of the organic-rich surface soils, which have had considerable time to develop, contain uranium with excess U 234 compared to U 238 . Interpretations from this model suggest that remixing of radioisotopes by geochemical processes contributes significantly, along with the usual physical processes of radioactive growth and decay, toward producing an isotopic composition near radioactive equilibrium.

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/204Pb vs 204Pb/204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.