The chondritic impactor origin of the Ni-rich component in Australasian tektites and microtektites

Abstract

In the Earth’s crust, Ni is generally concentrated in mafic and ultramafic rocks and is coupled with Mg in Mg-olivine, Mg-pyroxene and spinel. Whether the Ni-rich, and in general, the mafic component of Australasian tektites and microtektites is terrestrial or meteoritic is still debated. To test the origin of the Ni-rich component, we studied the Ni versus Mg distribution in a large geochemical database of Australasian tektites (n = 208) and microtektites (n = 238) from the literature. Nickel contents of up to 428 µg/g in tektites and 678 µg/g in microtektites covary with Mg in tektites and in most (∼85%) of the microtektites defining a mixing trend between crustal and chondritic values, thereby documenting the chondritic origin of the Ni-rich component in Australasian tektites/microtektites. Mixing calculations indicate up to 4% and up to 6% by weight chondritic component in tektites and microtektites, respectively. A possible mafic component of terrestrial origin is observed in a minority of tektite and microtektite specimens. This finding is consistent with previous works suggesting a possible occurrence of a chondritic signature in high-Ni tektites, based on the study of highly siderophile elements and Os isotopes, and high-Ni microtektites, based on Ni, Co, and Cr ratios. The combined geochemical and isotopic analyses of high-Ni tektites and microtektites in collections worldwide may thus reveal the chondritic impactor type that generated one of the presumably largest impacts in the Cenozoic.