SECRETARY'S REPORT 157 



kamacite and retained in taenite. Measurements of tritium in the 

 Sputnik IV fragment and studies by Dr. Tilles of tritium retention in 

 a proton-irradiated target have provided additional data on the reten- 

 tion and loss of tritium in iron and steel. 



Dr. Tilles has nearly completed assemblmg the parts of the high- 

 sensitivity mass spectrometer ^ for studies of noble gases in meteorites. 

 Anticipated research studies with the spectrometer will include meas- 

 urements of noble gas abundance and isotopic composition in separated 

 phases of meteorites. 



Problems in the mineralogy and petrology of meteorites, with spe- 

 cial reference to their temperature-pressure history and age, are being 

 considered. In the course of these studies," Mrs. Ursula B. Marvin 

 discovered zircon, heretofore milaiown in meteorites, in the Vaca 

 Muerta mesosiderite. The zircon, which is radioactive, is of special 

 significance in age determmations of any meteorite where it occurs. 

 As part of a long-term project in collaboration with Dr. Fireman, 

 Mrs. Marvin has separated mineral concentrates of high purity from 

 Indarch, a stony meteorite abnormally rich in xenon and contammg 

 the rare minerals CaS and MgS. She will study the mineralogy and 

 petrology of this meteorite in detail. The radioisotope group will 

 make age determmations on the separated fractions and a bulk sample. 



Initiating a program of study of the chemical compositions of micro- 

 structures in chondrites, Dr. Jolm A. Wood used the electron micro- 

 probe in the University of Chicago Division of Geological Sciences 

 as an analytical tool.^^ At present, the focus of the study is the grains 

 and particles of nickel-iron metal present in chondrites. The composi- 

 tions and compositional gradients in these are determined by the 

 thermal history of the chondrite containing them. This study should 

 hence yield information about the nature and thermal history of the 

 planet from which the chondrites were derived. 



Dr. Wood has also made a detailed theoretical study of the prop- 

 erties of the most common class of meteorite, the chondrites, in an 

 attempt to understand the processes that operated to produce them.^* 

 He also studied the thermal history of nickel-iron phases and their 

 compositional gradients in iron meteorites. Tliis involved the use 

 of a digital computer to solve the diffusion equation of nickel in nickel- 

 iron alloys for various postulated cooling rates and thermal histories." 

 He found a thermal history that yielded the same nickel diffusion 

 profiles observed in iron meteorites. Preliminary results indicate 

 that the medium octahedrite iron meteorites originated in a small 

 planet, about 200 km in radius; that this object originally accreted 

 at a rate of --^0.5 cm per year; and that it originally contained a 

 short-lived radionuclide ('^100 ppm of Al ^e or the equivalent), which 



See footnotes on p. 164. 



