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  • GEOPHYSICS  (9)
  • 1995-1999
  • 1985-1989  (9)
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  • 1
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    Mantle-derived fluids in diamond micro-inclusions (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Microinclusions in diamonds from Zaire and Botswana differ in composition from the more common large inclusions of the peridotitic or eclogitic assemblages. These sub-micrometer inclusions resemble potassic magmas in their composition, but are enriched in H2O, CO2(3-), and K2O and depleted in MgO. This composition represents a volatile-rich fluid or melt from the upper mantle, which was trapped in the diamonds as they grew.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 335; 784-789
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  • 2
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    Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Solutions collected from 21 deg N, East Pacific Rise (Epr) and Guaymas Basin, Gulf of California, are analyzed for Nd isotopic composition and Sm and Nd concentrations. The results indicate extensive but not complete isotopic exchange with Sr in the depleted oceanic crust and that Sr concentrations in these solutions are buffered. In contrast, the Nd data exhibit a wide range in isotopic composition and concentration between vents. Many samples show substantial contributions from MORB, but all have isotopic compositions below MORB, in spite of enrichments in Nd up to 100 times seawater. It is shown that the fluids must exchange Nd with a sedimentary reservoir having an isotopic composition less than Pacific seawater. Low-temperature reactions with metalliferous sediments on the flanks of the EPR may provide such a source. Using a simple box method, estimates of the hydrothermal fluxes of Nd are compared to fluxes which are necessary to maintain a radiogenic isotopic composition of about -3 in the Pacific against the influx of Antarctic waters. It is concluded that erosion from island arcs is the main source of radiogenic Nd in the Pacific.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 72; 4 Ma
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  • 3
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    Sm-Nd in marine carbonates and phosphates - Implications for Nd isotopes in seawater and crustal ages (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The possibility of establishing a record of variations in the isotopic composition of Nd in seawater over geologic time is explored. To construct such a record, a phase must be identified which incorporated Nd with the same isotopic composition as seawater at the time of its formation, preserves that composition, and which is relatively common in sediments. To evaluate the suitability of carbonates and phosphates, the Rb, Sr, Sm, and Nd concentrations and the Nd and Sr isotopic composition of a variety of modern and ancient marine calcite, aragonite, and apatite samples have been measured and the results are presented and discussed.
    Keywords: GEOPHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 49; 503-518
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  • 4
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    Dating earthquakes with high-precision thorium-230 ages of very young corals (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Three corals from Vanuatu Islands, whose ages were known from counting annual growth bands, were analyzed by Th-230 age analysis to asses the accuracy of the Th-230 method. The comparison of Th-230 ages with the growth-band ages showed that the Th-230 ages were accurate within an error of + or - 3-5 y. Th-230 dates were then determined for two adjacent emerged heads from Santo Island; the dates were the same, indicating that the heads died at the same time, and consistent with the theory that they were killed by coseismic emergence around 1865 AD. The difference between this data and the data of the only major historically documented earthquake that caused an uplift (1973) suggests a seismic recurrence interval of 108 + or - 4 y for Santo. It is suggested that this approach may be extended back in time and to other localities.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 90; 4; 371-381
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  • 5
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    Rb-Sr and Sm-Nd isotope geochemistry and chronology of cherts from the Onverwacht Group (3.5 AE), South Africa (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: An Rb-Sr and Sm-Nd isotopic analysis of Archean chert samples from the Onverwacht Group, South Africa is presented, showing the same characteristic Rb and Sr concentrations as Phanerozoic cherts, with a very large range of Rb-87/Sr-86 ratios. A good correlation line in the Rb-87 to Sr-87 evolution diagram, corresponding to an age of about 2119 My and an initial Sr-87/Sr-86 ratio of about 0.72246, is derived which may be interpreted as reflecting the age of rehomogenization of the Sr in the protolith and the recrystallization of these cherts due to circulating hydrothermal fluids during regional metamorphism about 1.4 AE after deposition of the Onverwacht Group. The Sm-Nd systematics reflect an ancient source age of about 3.5 AE.
    Keywords: GEOPHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 51; 973-984
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  • 6
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    Rb-Sr and Sm-Nd systematics of cherts and other siliceous deposits (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: An analysis of marine and lacustrine cherts demonstrates that Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts, and that the Rb-Sr system may be used for dating. All samples have high Rb/Sr ratios, and though the site of these elements in cherts was not established, they appear to be correlated with Al content. Sm/Nd ratios were typical of continental sources, and the results show the Rb-Sr system in cherts to be isolated from the associated carbonate, with little Sr exchange. Data suggest that in many cases cherts are formed by dissolved SiO2 nucleating on clay-like material which preserves the isotropic characteristics of the detrital source, while in some cases it is possible that cherts were formed from silica-saturated waters from a nonmarine source.
    Keywords: GEOPHYSICS
    Type: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 51; 959-972
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  • 7
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    Trends of Sr and Nd isotopes through time near the Japan Sea in northeastern Japan (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Nd and Sr isotopic analyses were performed on selected volcanic rocks of 27-6 Ma age in the Green Tuff region of northeastern Japan, in adjacent areas, and in two ultramafic inclusions from Ichinomegata. The isotopic data plot generally along the region of the mantle array, and the volcanic rocks vary in epsilon(Nd) from -0.9 to 5.7, and in epsilon(Sr) from 14.1 to -16.3. It is found that the volcanic rocks are part of an evolutionary pattern over a time period of 27 Ma, and that the pattern goes from an initially enriched source to that of a very depleted source. A greater-than-15-Ma preopening stage, characterized by an enriched isotopic signature, and a less-than-15-Ma postopening stage, characterized by a depleted signature, are identified. It is suggested that the degree of continental crust involvement is high during the preopening stages of a back-arc basin when it is associated with a rifted continental mass.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 78; 2-3; 157-167
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  • 8
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    Precise timing of the last interglacial period from mass spectrometric determination of thorium-230 in corals (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The development of mass spectrometric techniques for determination of Th-230 abundance has made it possible to reduce analytical errors in (U-238)-(U-234)-(Th-230) dating of corals even with very small samples. Samples of 6 x 10 to the 8th atoms of Th-230 can be measured to an accuracy of + or - 3 percent (2sigma), and 3 x 10 to the 10th atoms of Th-230 can be measured to an accuracy of + or - 0.2 percent. The time range over which useful age data on corals can be obtained now ranges from about 50 to about 500,000 years. For young corals, this approach may be preferable to C-14 dating. The precision with which the age of a coral can now be determined should make it possible to critically test the Milankovitch hypothesis concerning Pleistocene climate fluctuations. Analyses of a number of corals that grew during the last interglacial period yield ages of 122,000 to 130,000 years. The ages coincide with, or slightly post-date, the summer solar insolation high at 65 deg N latitude which occurred 128,000 years ago. This supports the idea that changes in Pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth's orbit and rotation axis.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 236; 1547-155
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  • 9
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    U-238-U-234-Th-230-Th-232 systematics and the precise measurement of time over the past 500,000 years (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: A method is presented for the high-precision measurement of the Th-230 abundance in corals by isotope-dilution mass spectrometry using techniques developed by Chen and Wasserburg (1980, 1981) and Chen et al. (1986). It is shown that 6 x 10 to the 8th atoms of Th-230 can be measured to + or - 30 percent (2 sigma) and 2 x 10 to the 10th atoms of Th-230 to + or - 2 percent. The time over which useful age data on corals can be obtained ranges from a few years to about 500 ky, with the uncertainty in age ranging from 5 y for a 180-y-old coral, to 44 y for a 8294-y-old coral, to 1.1 ky for a 123.1-ky-old coral. Ages were determined with high analytical precision for several corals that grew during high sea-level stands about 120 ky ago, supporting the view that the dominant cause of Pleistocene climate change was Milankovitch forcing.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 81; 2-3; 175-192
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