Metal rich cosmic spherules from Calama (Atacama Desert) and Walnumfjellet (Antarctica): a textural, chemical and isotopic comparison

Introduction. Each year, approximately 40,000 ± 20,000 metric tons of extraterrestrial material reaches Earth, primarily in the form of micrometeorites [1]. Micrometeorites form by collisions between asteroids or as the result of evaporation of comets, and derive from a variety of parent bodies, including carbonaceous and ordinary chondrites [2]. Micrometeorites sample parent bodies distinct from those of meteorites, offering a unique perspective on the origin and evolution of the Solar System. They also provide valuable insights into potential dust-related hazards for space exploration. Melted micrometeorites, also termed cosmic spherules, are categorized into S-, I-, and G-types [3]. G-types, and to a lesser extent I-types, are rare in modern collections (1-2% are G-types) and their formation remains poorly understood, with no precise classification or parent body assignment [4]. This study focuses on I- and G-type particles recovered from both Antarctica and the Atacama Desert, to allow a direct comparison of relative particle abundance, state of preservation, and types of alteration in contrasting sedimentary environments. By comparing distinct I- and G-type collections, their classification can be improved, and their Solar System origins refined.

Methods. To assess the modern micrometeorite flux and distinct state of preservation, sediments are collected from two locations: near Calama in the Atacama Desert (Chile), and Walnumfjellet in the Sør Rondane Mountains (Antarctica). In the Atacama Desert, sediment is gathered from a 1 m² surface at 2 cm depth; Antarctic material is collected from sediment traps near Walnumfjellet mountain#5 in 2018. Around 500 g of sediment from each location is sieved into six size fractions between 2 mm and 32 µm. Magnetic separation is used to extract the magnetic micrometeorite-rich fraction from the bulk sediment. Candidate spherules are picked under a binocular microscope. Micro X-ray fluorescence (µXRF) analysis performed at AMGC (Vrije Universiteit Brussel) maps the major element composition of the selected spherules to obtain a preliminary identification. Internal textures and geochemical composition are analyzed using scanning electron microscope – energy dispersive spectrometry (SEM-EDS) at VUB. To determine parent body origin and atmospheric entry effects, in situ triple-oxygen isotopic compositions are measured using secondary ion mass spectrometry (SIMS) at CRPG in Nancy.

Results. Following SEM-EDS analysis, 5 I- and 11 G-type well-preserved cosmic spherules were identified in the Atacama sample (Fig. 1), with diameters ranging from 55 to 139 µm. Typical internal textures of I-type micrometeorites are observed, with or without metal beads. G-types consist of magnetite dendrites in a glassy mesostasis. Notably, the G-types exhibit different textures with variable thickness (0.2 µm to few micrometers wide) of magnetite dendrites and abundances of glassy mesostasis. This observed textural diversity highlights the need for a refined classification for G-types. Triple-oxygen data for four I-type and five G-type spherules indicate most G-types may derive from a similar source that aligns with CR or CV carbonaceous chondrites, while a single I-type can be linked to an ordinary chondrite source (Group 3) (Fig. 2). The remaining cosmic spherules plot near the terrestrial fractionation line (TFL, ~ δ¹⁷O = 0.52 x δ¹⁸O [5]), suggesting terrestrial atmospheric or diagenetic overprint.

The Antarctic sediment sample yields a larger number of micrometeorites than the Atacama sample, with 995 unclassified spherules that will be analyzed using µXRF and SEM in the upcoming months.  For now, the texture and composition of 13 well-preserved I-type spherules is observed using SEM-EDS (Fig. 1). SIMS data of triple-oxygen isotopes for 11 I-type cosmic spherules indicates strong oxidization during atmospheric entry, with ten spherules that may be linked to carbonaceous chondrites precursor. A single Antarctic I-type plots at a much lower δ¹⁸O value, possibly suggesting a link to an ordinary chondritic precursor. Despite the low number of particles analyzed and the measurement uncertainty, a difference in precursor material may explain the differences observed in isotopic compositions for the Atacama and Antarctic I-type spherules.

Conclusions. This study highlights the importance of comparing micrometeorite sampling sites and subpopulations directly, to assess variations in abundance and alteration of the micrometeorites. The Atacama location is promising for recovering rare G-types, which is crucial in refining the existing cosmic spherule classification schemes. Ongoing analysis of the Antarctic spherules will expand the dataset.

References. [1] Love and Brownlee (1993) Science 262:550-553. [2] Burns et al. (1979) Icarus 40:1-48. [3] Folco & Cordier (2015) EMU Notes in Mineralogy 15(9)253-297. [4] van Ginneken et al. (2024) Phil. Trans. R. Soc. A 382:20230195. [5] Clayton (1993) Annu. Rev. Earth. Planet. Sci. 21:115-149. [6] Thiemens and Brenninkmeijer (1995) Geophys. Res. Lett. 22(3)255-257. [7] Suavet et al. (2010) Earth Planet. Sci. Lett. 293:313-320.

Figure 1. SEM images of the internal textures of selected I- and G-type cosmic spherules from Calama (Atacama Desert) (A, E-I) and Walnumfjellet (Antarctica) (B-D). (A) I-type with metal bead (At-Cal-M1-26), (B) I-type with metal bead (WN18-M2-G5), (C) I-type (WN18-M2-F2), (D) I-type (WN18-M2-F2), (E) I-type (At-Cal-M2-41), (F) G-type (At-Cal-M3-63), (G) G-type (At-Cal-M2-44), (H) G-type (At-Cal-M2-06), (I) G-type (At-Cal-M2-35). Scalebar of 10 µm for image B, E-I, and 20 µm for A, C, D. 

Figure 2. (A) Triple-oxygen data of 9 micrometeorites from Calama (Atacama) and (B) 11 micrometeorites from Walnumfjellet (Antarctica) measured using SIMS. The results are plotted as Δ¹⁷O (‰) versus δ¹⁸O (‰). G- and I-type micrometeorites are plotted in relation to previously identified micrometeorite groups and meteorite parent body sources. The solid line represents the TFL (~ δ¹⁷O = 0.52 x δ¹⁸O; [5]), with associated analytical uncertainty as determined during SIMS sessions. The grey star represents the present average atmospheric isotopic composition of oxygen around the stratosphere-mesosphere transition (δ¹⁸O ~ 23.5‰ and δ¹⁷O ~ 11.8‰ [6]). The average uncertainty is 2SD. The plot is made after [7].