C.J. Allegre | USGS Science

USGS1999

Osmium isotope variations in the oceans recorded by Fe-Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe-Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re/Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya-Tibet region is not a major source for Os. Recently formed Fe-Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (???4 km) possess a remarkably unradiogenic Os isotope composition (187Os/186Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Earth and Planetary Science Letters↗

USGS1973

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: 206 Pb/ 204 Pb = 9.307, 207 Pb/ 2O4 Pb = 10.294, and 208 Pb/ 204 Pb = 29.476. The leads of Angra dos Reis, Sioux County, and Nuevo Laredo achondrites are very radiogenic, the 206 Pb/ 204 Pb values are about 200, and the uranium-thorium-lead systems are nearly concordant. The ages of the meteorites as calculated from a single-stage 207 Pb/ 206 Pb isochron based on the newly determined primordial lead value and the newly reported 235 U and 838 U decay constants, are 4.528 x 10 9 years for Sioux County and Nuevo Laredo and 4.555 x 10 9 years for Angra dos Reis. When calculated with the uranium decay constants used by Patterson, these ages are 4.593 x 10 9 years and 4.620 x 10 9 years, respectively, and are therefore 40 to 70 x 10 6 years older than the 4.55 x 10 9 years age Patterson reported. The age difference of 27 x 10 6 years between Angra dos Reis and the other two meteorites is compatible with the difference between the initial 87 Sr/ 86 Sr 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 129 1- 129 Xe chronology. The ages of ordinary chondrites (H5 and L6) range from 4.52 to 4.57 x 10 9 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.

Science↗