Achondrite investigations and their bearing on the origin of tektites

Abstract

A common origin for chondrites achondrites, and tektites seems reasonable. A lunar impactite origin for tektites seems doubtful because their content of cosmic-ray-induced 26Al and 10Be indicates much longer times of travel as small bodies in space than is possible in passing from the moon to the earth. A planet with a chondritic centre, an achondritic subcrust, and a tektitic crust is suggested as a possible place of origin. At the time of its disruption, the outer portions of the planet must have been hot enough so that the tektitic crust was molten and the achondritic sublayer was semi-molten. Achondrite compositions are examined from this viewpoint. With increasing SiO 2 the composition of the achondrite series varies as follows: FeO, CaO and Al 2O 3 undergo fractionations of 20 : 1, 6 : 1 and 7:1, respectively, while MgO is enriched by a factor of 8. This variation could not have been produced by crystal fractionation in a basic magma, but might have been produced by partition of chemical components between the achondrite and tektite layers due to a condition of liquid immiscibility in such a basic magma. This variation could also have been produced by crystal fractionation in an extremely acidic magma but the acidic magma itself would have had to have been produced by a segregation due to liquid immiscibility. Both methods of accounting for the observed compositional variations among achondrites would result in the formation of a siliceous phase similar in composition to tektites.