Micrometer‐ and nanometer‐sized platinum group nuggets in micrometeorites from deep‐sea sediments of the Indian Ocean

Abstract

Abstract-We examined 378 micrometeorites collected from deep-sea sediments of the Indian Ocean of which 175, 180, and 23 are I-type, S-type, and G-type, respectively. Of the 175 I-type spherules, 13 contained platinum group element nuggets (PGNs). The nuggets occur in two distinct sizes and have distinctly different elemental compositions: micrometer (lm)-sized nuggets that are >3 lm contain dominantly Ir, Os, and Ru (iridium-platinum group element or IPGE) and sub-lm (or nanometer)-sized (<1 lm) nuggets, which contain dominantly Pt, Rh, and Pd (palladium-PGE or PPGE). The lm-sized nuggets are found only one per spherule in the cross section observed and are usually found at the edge of the spherule. By contrast, there are hundreds of nanometer-sized nuggets distributed dominantly in the magnetite phases of the spherules, and rarely in the wu¨stite phases. Both the nugget types are found as separate entities in the same spherule and apparently, nugget formation is a common phenomenon among I-type micrometeorites. However, the lm-sized nuggets are seen in fewer specimens (2.5% of the observed I-type spherules). In all, we analyzed four nuggets of lm size and 213 nanometer-sized nuggets from 13 I-type spherules for platinum group elements. Chemically, the lm-sized PGNs contain chondritic ratios of Os ⁄ Ir, but are depleted in the more volatile PGE (Pt, Rh, and Pd) relative to chondritic ratios. On the other hand, the nanometer-sized nuggets contain dominantly Pt and Rh. Importantly, the refractory PGEs are conspicuous by their absence in these nanometer nuggets. Palladium, the most volatile PGE is highly depleted (<1.1%) with respect to chondritic ratios in the lm-sized PGNs, and is observed in only 17 of 213 nanometer nuggets with concentrations that are just above the detection limit ( ‡0.2%). Distinct fractionation of the PGE into IPGE (Ir, Os, Ru) and PPGE seems to take place during the short span of atmospheric entry. These observations suggest several implications: (1) The observation of fractionated PGE in an Fe-Ni system gives rise to the possibility that Earth's core could contain fractionated PGE. (2) The present data support the processes suggested for the fractionated PGE patterns observed in the ejecta of ancient meteorite impacts. (3) Meteoric metals released in the troposphere could contain fractionated PGNs in large numbers.

Micrometeorite flux on Earth during the last ~50,000 years

“…Five spherules show the presence of nanometer‐sized Pt‐group nuggets similar to the ones reported by Rudraswami et al . [] recently (Figure b). In addition to Fe and Ni, these nuggets contain only Pt, except for the large (>200 nm) ones with Pt contents > ~5%, for which Rh is also detected in proportions consistent with the chondritic Pt:Rh ratio of 7 [ Tagle and Berlin , ].…”

Ordinary chondritic micrometeorites from the Indian Ocean

“…; Rudraswami et al. ). The spherules that are ablation products of meteorites, although rare, can be distinguished generally by their major element ratios; however, if they undergo severe heating and ablation during atmospheric entry, then they develop glassy textures and would be indistinguishable from cosmic spherules from other sources (Genge and Grady ).…”

Oxygen isotopic and chemical composition of chromites in micrometeorites: Evidence of ordinary chondrite precursors

“…Micrometeorites from Indian Ocean DSS (74–76°E, 10–13°S) were collected using a grab sampler measuring 50 cm long × 50 cm wide × 15 cm deep with the capacity to lift 45 kg of sediments from the seafloor depth of ~5000 m (Rudraswami et al. , ; Prasad et al. ).…”