U-238 U-234-TH-230 CHRONOMETRY OF FE-MN CRUSTS - GROWTH-PROCESSES AND RECOVERY OF THORIUM ISOTOPIC-RATIOS OF SEAWATER

被引:39
作者
CHABAUX, F
COHEN, AS
ONIONS, RK
HEIN, JR
机构
[1] UNIV CAMBRIDGE, DEPT EARTH SCI, CAMBRIDGE CB2 3EQ, ENGLAND
[2] US GEOL SURVEY, MENLO PK, CA 94025 USA
关键词
D O I
10.1016/0016-7037(94)00379-Z
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Comparison of (U-234)excess/(U-238) and (Th-230/Th-232) activity ratios in oceanic Fe-Mn deposits provides a method for assessing the closed-system behaviour of U-238-U-234-Th-230, as well as variations in the initial uranium and thorium isotopic ratios of the percipitated metal oxides. This approach is illustrated using a Fe-Mn crust from Lotab seamount (Marshall Islands, west equatorial Pacific). Here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust, and from two depth profiles of the outermost rim of the same botryoid. The decrease of (U-234)excess/(U-238) and (Th-230/Th-232) activity ratio with depth in the two profiles gives mean growth rates, for the last 150 ka, of 7.8 +/- 2 mm/Ma and 6.6 +/- 1 mm/Ma, respectively. All data points (surface and core samples) but one, define a linear correlation in the Ln (Th-230/Th-232) - Ln [(U-234)excess/(U-238)] diagram. This correlation indicates that for all points the U-Th system remained closed after the Fe-Mn layer percipitated, and that the different samples possessed the same initial Uranium and thorium isotope ratios. Furthermore, these results show that the preserved surface of this Fe-Mn crust may not be the present-day growth surface, and that the thorium and uranium isotopic ratios of seawater in west equatorial Pacific have not changed during the past 150 ka. The initial thorium activity ratio is estimated from the correlation obtained between Ln (Th-230/Th-232) and Ln [(U-234)excess/(U-238)].
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收藏
页码:633 / 638
页数:6
相关论文
共 32 条
[1]   DISTRIBUTION OF THORIUM ISOTOPES BETWEEN DISSOLVED AND PARTICULATE FORMS IN THE DEEP-SEA [J].
BACON, MP ;
ANDERSON, RF .
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, 1982, 87 (NC3) :2045-2056
[3]   RESIDENCE TIME OF THORIUM IN SURFACE SEA-WATER AND ITS IMPLICATIONS REGARDING RATE OF REACTIVE POLLUTANTS [J].
BROECKER, WS ;
KAUFMAN, A ;
TRIER, RM .
EARTH AND PLANETARY SCIENCE LETTERS, 1973, 20 (01) :35-44
[4]   THE ROLE OF OCEAN-ATMOSPHERE REORGANIZATIONS IN GLACIAL CYCLES [J].
BROECKER, WS ;
DENTON, GH .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1989, 53 (10) :2465-2501
[5]   DOES THE OCEAN-ATMOSPHERE SYSTEM HAVE MORE THAN ONE STABLE MODE OF OPERATION [J].
BROECKER, WS ;
PETEET, DM ;
RIND, D .
NATURE, 1985, 315 (6014) :21-26
[6]  
CHABAUX F, 1993, EOS, V73, P339
[7]  
CHABAUX F, 1994, US G SURVEY CIRCULAR, V1107, P50
[8]   ISOTOPIC DETERMINATION OF URANIUM IN PICOMOLE AND SUBPICOMOLE QUANTITIES [J].
CHEN, JH ;
WASSERBURG, GJ .
ANALYTICAL CHEMISTRY, 1981, 53 (13) :2060-2067
[9]   HIGH-PRECISION URANIUM, THORIUM AND RADIUM ISOTOPE RATIO MEASUREMENTS BY HIGH DYNAMIC-RANGE THERMAL IONIZATION MASS-SPECTROMETRY [J].
COHEN, AS ;
BELSHAW, NS ;
ONIONS, RK .
INTERNATIONAL JOURNAL OF MASS SPECTROMETRY, 1992, 116 (01) :71-81
[10]   U-238 U-234-TH-230-TH-232 SYSTEMATICS AND THE PRECISE MEASUREMENT OF TIME OVER THE PAST 500000 YEARS [J].
EDWARDS, RL ;
CHEN, JH ;
WASSERBURG, GJ .
EARTH AND PLANETARY SCIENCE LETTERS, 1987, 81 (2-3) :175-192