Oliver A. Schaeffer

39Ar-40Ar laser probe dating of terrestrial rocks

We have shown that for metamorphosed granites, as well as for metamorphosed sediments, it is possible to obtain a reliable K-Ar age from a sample as small as 0.2 μg. Samples with ages as young as 40 m.y. are easily studied. There is no reason to believe that the method could not be used for samples considerably younger. The main application of the laser will probably be for cases where the same mineral species has been formed during two distinct metamorphic events in the same rock. In this case, it is difficult, if not impossible, to make a mineral separate suitable for a 39 Ar- 40 Ar study. We have shown that in the case of an old granite the biotite in one sample distinctly shows thermal events at 1600 m.y. whileits primary cooling age is 2100 m.y. The thermal release study yields a good plateau age, roughly an average age of the two thermal events. An other interest of this method is demonstrated by argon dating of two different kinds of glaucophane, only one of theseshowing argon in excess.

Total nitrogen content of deep sea basalts

Abstract Various models have been suggested concerning the origin and evolution of the earths atmosphere. An estimate of the nitrogen content of the mantle could further constrain atmospheric models. Total nitrogen content was determined by thermal neutron activation analysis via 14N(n,p)14C. The 14C was converted to carbon dioxide and counted in miniature low level proportional counters. The total nitrogen content of U.S.G.S. standards BCR-1 and G-2 as determined by different laboratories is variable, probably due to atmospheric adsorption by the finely ground samples. Total nitrogen content was determined in deep sea basalt glasses from three regions: East Pacific Rise ( 15 ± 4, 18 ± 4 , and 7 ± ppm 2 N ), Mid-Atlantic Rift (FAMOUS Region: 22 ± 5, 18 ± 3 , and 10 ± 2 ppm N ) and the Juan de Fuca Ridge ( 17 ± 4 ppm N ). Matrix material from the same samples as the glasses was available from the East Pacific Rise ( 37 ± 6, 26 ± 4 , and 34 ± 6 ppm N ) and the Mid-Atlantic Rift ( 39 ± 4 ppm N ) which are about 50 to 100% greater than the associated glasses. The increased matrix abundance may be due to incorporation of chemically bound nitrogen from sea water rather than dissolved molecular nitrogen. The nitrogen content of the FAMOUS samples are inconsistent with the model of Langmuir et al. (1977) for petrogenesis based on trace element data. Factors which can affect the observed nitrogen content in the basalts and the interpretation in terms of the mantle nitrogen abundance are discussed (e.g. partial melting and degassing of the basalts). A lower limit of about 2 ppm N in the mantle can be estimated.

Solubility of noble gases in serpentine: Implications for meteoritic noble gas abundances

Experiments to study the incorporation of noble gases during synthesis of serpentine and during equilibration of serpentine with the atmosphere are reported. Hydrothermal synthesis and exchange experiments were conducted at 340°C and 1 kbar with partial pressures of noble gases from 2 × 10−8 to 10−1 atm. Exchange experiments were also conducted with the terrestrial atmosphere at lower temperatures. The values of the measured distribution coefficients (cm3 STP/g atm) for noble gases are not sufficiently high to account for the trapped noble gases in carbonaceous chondrites by exchange in the solar nebula if meteoritic minerals have comparable distribution coefficients. Of additional significance is the fact that serpentine gains and loses noble gases to approach equilibrium solubility values with the terrestrial atmosphere. Consequently, exposure to the terrestrial atmosphere may have influenced the noble gas abundances in phyllosilicate minerals of carbonaceous chondrites. The dispersion of K-Ar ages of carbonaceous chondrites could be the result of phyllosilicate minerals approaching equilibrium solubility of atmospheric40Ar.

AN EVALUATION OF THE URANIUM--HELIUM METHOD OF DATING OF FOSSIL BONES.

Abstract From our study of 6 bones of Oligocene age from different sites of one locality (the Fayum beds of Egypt) and 3 bones of Pleistocene or Pliocene age we conclude that the general applicability of uranium-helium method of dating is not possible for fossil bones or other apatite deposits. The major problem appears to be continuous helium loss rather than recent uranium gain.

Trapped solar wind He, Ne, and Ar and energetic He in Surveyor 3

Abstract The Apollo 12 mission brought back sections of the Surveyor 3 vehicle suitable for mass spectrometric studies of implanted solar wind and solar cosmic rays. Using this method, we have determined an average solar wind 4 He flux of 6.1 × 10 6 ions/cm 2 sec for the 31 months of exposure. We have also measured 4 He/ 3 He= 2700 ± 50; 4 He/ 20 Ne= 410 ± 30; 20 Ne/ 22 Ne= 13.5 ± 0.2; 20 Ne/ 36 Ar= 24.5 ± 2.5; and 36 Ar/ 38 Ar= 5.41 ± 0.20. These measurements provide solar wind values averaged over considerably longer periods of time than the Apollo Solar Wind Composition experiments and suggest that the short term SWC measurements during a period of high solar activity may not be a reliable measure of average solar wind composition.

A new approach to decipher the origin of the carbonaceous chondrite fission krypton and xenon

It is suggested that the carbonaceous chondrite fission krypton and xenon, as measured in the primitive meteorites, may have been produced by nuclear fission induced by CNO flare particles in the few-MeV/nucleon energy range on very heavy target elements such as Au, Hg, Tl, Pb, and Bi. It is speculatively proposed that the locale of this process has been the T-Tauri phase of our sun.