Be,La,U-rich spherules as microtektites of terrestrial laterites: What goes up must come down

TL;DR

This study identifies Be, La, U-rich spherules as terrestrial microtektites from the Australasian strewn field, challenging previous extraterrestrial interpretations and linking them to a 788,000-year-old impact event in Indochina.

Contribution

It demonstrates that certain exotic-looking spherules are actually terrestrial microtektites, providing new insights into impact ejecta and their geochemical signatures.

Findings

Spherules are terrestrial microtektites from Australasian strewn field.

They contain Be, La, U, Ba, and other elements, with terrestrial Fe isotopic signatures.

Estimated 3% of Australasian microtektites are similar terrestrial tektites.

Abstract

Recently Loeb et al. (2024, "Recovery and Classification of Spherules from the Pacific Ocean Site of the CNEOS 2014 January 8 (IM1) Bolide", Res. Notes. Amer. Astron. Soc. 8, 39) reported the magnetic collection of millimeter-sized spherules from the seafloor near Papua New Guinea. About 22% had Mg/Si < 1/3 and were identified as a new "differentiated" variety of cosmic spherule ("D-type"). In a subset of 26 of these "D-type" spherules, 12 "BeLaU" spherules were found to be dominated by Fe and Al, marked by low Si and even lower Mg content, depletions of volatile species like Pb and Cs, and remarkable enrichments of Be, La, U, Ba, and other elements. Loeb et al. claimed these have exotic compositions different from other Solar System materials. We show that in fact samples with these compositions are not just found on Earth, they are from Earth; specifically, we identify them as microtektites of terrestrial lateritic sandstone. Based on the location of the sample site, we associate them with the Australasian tektite strewn field, generated 788 kyr ago by an impactor that melted and ejected ~10^8 tons of sandstone, including a lateritic layer, from Indochina. A tektite origin for the spherules is corroborated by their terrestrial Fe isotopic compositions and the compound, non-spherical nature of many of them, which preclude formation as ablation spherules from a bolide. Due to the restriction of laterites to the tropics, iron-rich tektites may be uncommon, but we predict they should comprise ~3% of the Australasian microtektites.