Geochemical evidence for the origin of australites

Abstract

Nineteen new analyses of australites are given. The average composition of 43 australites (including 24 analyses from Taylor, 1962a) is as follows: P, 200 ppm; Si, 34.25% (73.31 % SiO 2); Al, 6.18% (11.68% Al 2O 3); Ga, 8.3 ppm; Cr, 75 ppm; Mg, 1.22% (2.02% MgO); Li, 42 ppm; Ti, 4130 ppm; Ni, 30 ppm; Co, 15 ppm; Cu, 7.3 ppm; Fe, 3.54% (4.03% FeO; 0.60% Fe 2O 3); V, 76 ppm; Zr, 410 ppm; Mn, 760 ppm; Sc, 12 ppm; Na, 0.93% (1.25% Na 2O); Ca, 2.50% (3.50% CaO); Sr, 200ppm; K, 1.88% (2.26% K 2O); Ba, 620 ppm; Kb, 84 ppm; Cs, 2.7 ppm. A discussion is given of possible regional variation in composition and comparison is made with Darwin Glass and impact glasses. In contrast to these, no significant meteoritic contribution to australite composition is observed. Elements whose abundance is particularly significant for tektite origin are discussed. These are: Cs; Mg, Li and K; Ti and Zr; U and Th; Ni, Co and Cr; Ca, Sr and Ba; Ga, Cu and Pb; and the rare earths. The abundances of these elements, both relative and absolute, are similar to those in well-mixed terrestrial surface material. They are not consistent with igneous rock abundances, nor have they been altered significantly by selective distillation of volatile elements. The place of origin of tektites is discussed with special reference to a terrestrial or lunar origin. The major and trace-element abundances, with the lead and strontium isotope ratios, support an origin for the parent material from the earth, or an entirely similar planet. Our current ideas about the composition and history of the lunar surface are given and a section is included on the probable composition of lunar acid differentiates. Our present knowledge precludes the lunar surface from being a source area for australites.