MG ISOTOPIC COMPOSITIONS OF STONY SPHERULES FROM DEEP-SEA SEDIMENTS

The isotopic compositions of Mg in 5 individual stony spherules from deep-sea sediments have been measured. The spherules are composed of olivine, glass, and magnetite, and considered to be of extraterrestrial origin. No significant mass fractionation of up to 2 per mil per amu was recognized. One sample showed a small but significant non-linear Mg-26 isotopic effect (delta-Mg-26 = + 1.2 per mil) similar to that reported in interplanetary dust particles (IDPs). Based on Mg isotopic compositions of the spherules, we suggest that either vaporization mass loss of parent meteoroids of stony spherules was not so large, usually less than approximately 20%, or ablative mass loss occurred effectively during entry into the Earth's atmosphere, and that physicochemical fractionations took place in parent meteoroids could be related to those of primitive IDPs. The processes responsible for the non-linear effect of Mg-26, however, remain undefined.