Time Differences in the Formation of Meteorites as Determined from the Ratio of Lead-207 to Lead-206

Abstract

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.

Lead isotopes in mantle derived xenoliths from Japan and South Africa.

“…2 In order to interpret the lead isotope data on the basis of the isochrons obtained above, we now use the two stage model which was adopted in a previous work by MORIOKA and KIGOSHI (1975). In this model, each inclusion has been held in a closed system from the time of crystal lization, and also a parent material of these inclusions had been held in a closed system from 4.57 billion years ago (TATSUMOTO et al, 1973) to the time of crystallization. Such a model is similar to that of ULRYCH (1967) who believed that the upper mantle existed in a closed system for the greater part of the earth's history.…”

“…The 1311U_204 Pb ratio of the primary material of inclusions must have been 15 -16 4.57 bil lion years ago in order to grow up to the cal culated lead isotopic compositions from primor dial lead (TATSUMOTO et al, 1973). (MORIOKA and KIGOsHi, 1975); d = first stage growth curve.…”

Early archean inheritance in zircon from Mesozoic dalongshan granitoids in the Yangtze Foldbelt of Southeast China.

“…Within the framework of a restricted U-Pb three-stage model (Ulrych, 1969), it is calculated that the investigated zircons have identical initial Pb isotope ratios 206Pb / 204Pb =10.63 and 207Pb / 204Pb =12 .01 at time t, = 3330 Ma, and that original system had a 238U/204Pb ratio (,ul value) of 3.73 for time interval from t0 to t,, given that we take the meteoritic parameters de termined by Tatsumoto et al (1973) for starting conditions (i.e.,ao=9.307,8o=10.294 and to=4.57Ga). This calculation has to require that the early Archean basement originated from a primitive mantle with a u value of about 3-4.…”

SHRIMP measurements of U and Pb isotopes in the Koongarra secondary ore deposit, Northern Australia.

“…Thus, the radiogenic component is absent within the instrumental measurement error. Values of 9.307 for 206 Pb/ 204 Pb and 10.294 for 207 Pb/ 204 Pb were reported as the primordial lead ratios by Tatsumoto et al (1973). The current average terrestrial lead (mentioned above) essentially contains the radiogenic lead formed since the formation of the Earth, as well as the primordial lead, and is here referred to as "common lead".…”