Composition of Cosmic Spherules from Ferromanganese Crusts of the Magellan Seamounts

Abstract

—2720 cosmic spherules extracted from ferromanganese crusts sampled at two guyots of the Magellan Seamounts were studied using a scanning electron microscope. In comparison with collections of modern cosmic spherules, our samples are significantly richer in I-type spherules (consisting of Fe oxides, often with a Fe–Ni metal core). The compositions of 406 metal cores were analyzed. Six spherules with cores significantly enriched in Co (>5 wt %) were found; these were the first spherules of this composition ever found worldwide. Such a high Co content in the cores cannot be explained by the evolution of a micrometeorite of chondrite composition at its melting. Some groups of the spherules have either elevated or lower Co concentrations than those expected according to the evolutionary trend of spherules at oxidation during the atmosphere entry. These groups apparently reflect the composition of the initial micrometeorites. It is proposed to use the composition of the cores of cosmic spherules to reveal the time variation in the composition of cosmic dust entering the Earth. Several spherules were found with small cores compared to the oxide shell; these cores are characterized by the presence of a significant amount of platinum group elements (PGE) (up to 2.4 wt % PGE). The averaged composition of PGE in the high-Ni core, which is the richest in PGE, shows a pattern close to that of chondrite. The collection contains 23 spherules that host PGE nuggets. In three spherules, micrometer-sized nuggets with various PGE were found (one nugget is Os + Ir + Ru, and two others contain all PGE). In 20 spherules, nanometer-sized nuggets of rhodium platinum were found. The largest nugget (3 μm) has PGE ratios close to chondritic, with the exception of a significant depletion in Pd.