Abstract
AbstractCosmic spherules are micrometeorites that melt at high altitude as they enter Earth's atmosphere, and their oxygen isotope compositions are partially or completely inherited from the upper atmosphere, depending on the amount of heating experienced and the nature of their precursor materials. In this study, the three oxygen isotope compositions of 137 cosmic spherules are determined using 277 in situ analyses by ion probe. Our results indicate a possible correlation between increasing average δ18O compositions of silicate‐dominated (S‐type) spherules along the series scoriaceous < porphyritic < barred < cryptocrystalline < glass < CAT (calcium‐aluminum‐titanium) spherules (~12‰, 20‰, 22‰, 25‰, 26‰, and 50‰). This is consistent with the evolution of oxygen isotopes by mass fractionation owing to increased average entry heating and thus suggests mass fractionation dominates changes in isotopic composition, with atmospheric exchange being less significant. The Δ17O values of spherules, therefore, are mostly preserved and suggest that ~80% of particles are samples of C‐type asteroids. The genetic relationships between different S‐types can also be determined with scoriaceous, barred, and cryptocrystalline spherules mostly having low Δ17O values (≤0‰) mainly derived from carbonaceous chondrite (CC)‐like sources, while porphyritic spherules mostly have positive Δ17O (>0‰) and are largely derived from ordinary chondrite (OC)‐like sources related to S (IV)‐type asteroids. Glass and CAT spherules have variable Δ17O values indicating they formed by intense entry heating of both CC and OC‐like materials. I‐type cosmic spherules have a narrow range of δ17O (~20–25‰) and δ18O (~38–48‰) values, with Δ17O (~0‰) suggesting their oxygen is obtained entirely from the Earth's atmosphere, albeit with significant mass fractionation owing to evaporative heating. Finally, G‐type cosmic spherules have unexpected isotopic compositions and demonstrate little mass fractionation from a CC‐like source. The results of this study provide a vital assessment of the wider population of extraterrestrial dust arriving on Earth.
Highlights
Micrometeorites are the most abundant extraterrestrial materials reaching the Earth’s surface on an annual basis (Love and Brownlee, 1993)
The oxygen isotope composition of the upper atmosphere (i.e. > 60 km) has not been determined yet and any isotopic information above this altitude need to be deciphered from micrometeorites as most of these particles have undergone heating leading to isotopic exchange between
We have classified 137 cosmic spherules and partially melted scoriaceous micrometeorites based on classification scheme by Genge et al (2008) in order to understand the nature of oxygen isotope composition
Summary
Micrometeorites are the most abundant extraterrestrial materials reaching the Earth’s surface on an annual basis (Love and Brownlee, 1993). Micrometeorites provide information on the nature of their Extraterrestrial parent bodies, the processes operating during atmospheric entry and the composition of the Earth’s atmosphere. Extraterrestrial material that enters the upper atmosphere at >100 km altitude experiences gas drag heating thereby changing their original chemical and isotopic properties (Kurat et al 1994; Beckerling and Bischoff 1995; Brownlee et al 1997; Greshake et al 1998; Engrand et al., 2005; Genge et al, 2008; Cordier et al, 2011; Rudraswami et al, 2012, 2015, 2016). The oxygen isotope compositions of particles that survive atmospheric entry are related to those of both their precursors and the upper atmosphere. The oxygen isotope composition of the upper atmosphere (i.e. > 60 km) has not been determined yet and any isotopic information above this altitude need to be deciphered from micrometeorites as most of these particles have undergone heating leading to isotopic exchange between
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