Geochemistry of Darwin glass and target rocks from Darwin crater, Tasmania, Australia

Abstract

Darwin glass formed about 800,000 years ago in western Tasmania, Australia. Target rocks at Darwin crater are quartzites and slates (Siluro-Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO2 (85%), Al2O3 (7.3%), TiO2 (0.05%), FeO (2.2%), MgO (0.9%), and K2O (1.8%). Group 2 has lower average SiO2 (81.1%) and higher average Al2O3 (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.910; Eu/Eu* = 0.55-0.65) and overlapping trace element abundances. 87Sr/86Sr ratios for the glasses (0.80778-0.81605) fall in the range (0.76481-1.1212) defined by the rock samples. ε-Nd results range from -13.57 to -15.86. Nd model ages range from 1.2-1.9 Ga (CHUR) and the glasses (1.2-1.5 Ga) fall within the range defined by the target samples. The 87Sr/86Sr versus 87Rb/86Sr regression age (411 ± 42 Ma) and initial ratio (0.725 ±  0.016), and the initial 43Nd/144Nd ratio (0.51153 ±  0.00011) and regression age (451 ±  140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elements except FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile.