Unmelted cosmic metal particles in the Indian Ocean

Abstract

Fe-Ni metal is a common constituent of most meteorites and is an indicator of the thermal history of the respective meteorites, it is a diagnostic tool to distinguish between groups/subgroups of meteorites. In spite of over a million micrometeorites collected from various domains, reports of pure metallic particles among micrometeorites have been extremely rare. We report here the finding of a variety of cosmic metal particles such as kamacite, plessite, taenite, and Fe-Ni beads from deep-sea sediments of the Indian Ocean, a majority of which have entered the Earth unaffected by frictional heating during atmospheric entry. Such particles are known as components of meteorites but have never been found as individual entities. Their compositions suggest precursors from a variety of meteorite groups, thus providing an insight into the metal fluxes on the Earth. Some particles have undergone heating and oxidation to different levels during entry developing features similar to I-type cosmic spherules, suggesting atmospheric processing of individual kamacites/taenite grains as another hitherto unknown source for the I-type spherules. The particles have undergone postdepositional aqueous alteration transforming finally into the serpentine mineral cronstedtite. Aqueous alteration products of kamacite reflect the local microenvironment, therefore they have the potential to provide information on the composition of water in the solar nebula, on the parent bodies or on surfaces of planetary bodies. Our observations suggest it would take sustained burial in water for tens of thousands of years under cold conditions for kamacites to alter to cronstedtite.

Reply to the comment by M. Genge and M. Van Ginneken on paper entitled “Unmelted cosmic metal particles from the Indian Ocean”

“…Genge and Van Ginneken also mention our sampling effort in Prasad et al (2013) where we reported ~5% of the total spherules to be either I-type or G-type and compare these with the abundances of the I-type and the G-type spherules which are close to 50% of the total collection thus implying that in the present area, a metal bolide had disrupted in the atmosphere to account for such large abundances of metals in the present study. We wish to clarify here that the samples used in Prasad et al (2013) and in Prasad et al (2017) are from the same geographic domains. The differences in the quantities of metallic particles between both the above methods of collection are because of the differences in the sampling methods employed in both these cases.…”

“…The MMs presented in our paper (fig. 6; Prasad et al 2017) have been plotted as they occurred. It is found that these MMs occur in different clusters defined by metallic minerals from different types of meteorites as presented by us in our fig.…”

“…1a). We examined the magnetic materials from four different samples and discovered MMs, Fe-Ni beads, etc., which were presented in Prasad et al (2017). Separation, mounting, and analyses of magnetic materials is a cumbersome process because all the particles are strongly magnetic and they tend to adhere to each other strongly and form "bunches" and it is akin to literally picking the proverbial needle in a haystack.…”

Comment on “Unmelted cosmic metal particles in the Indian Ocean” by Prasad et al.

“…The FeNi metal beads reported by Prasad et al (2017) show a much larger range of compositions than irregular grains and were suggested to form as (1) beads separated by immiscibility from chondritic objects during atmospheric entry, (2) unmelted spherical metal grains, or (3) the separated cores of metallic I-types. The compositions of the cores of Antarctic I-type spherules are shown in Fig.…”

“…The abundance of unmelted grains, therefore, would appear to be significantly larger than would be expected for low entry angle particles; however, a complete size distribution of I-type spherules as well as unmelted grains would be necessary to assess the degree of disparity. Prasad et al (2017) recognized that the large abundance of unmelted particles is not consistent with the entry heating of metal micrometeoroids. To reconcile the abundances, they suggested that metal grains could be liberated by fragmentation of larger dust particles at the top of the atmosphere causing deceleration and a decrease in heating.…”