Selective Disparity of Ordinary Chondritic Precursors in Micrometeorite Flux

All known extraterrestrial dust (micrometeoroids) entering the Earth's atmosphere is anticipated to have a significant contribution from ordinary chondritic precursors, as seen in meteorites, but this is an apparent contradiction that needs to be addressed. Ordinary chondrites represent a minor contribution to the overall meteor influx compared to carbonaceous chondrites, which are largely dominated by CI and/or CM chondrites. However, the near-Earth asteroid population presents a scenario with sufficient scope for generation of dust-sized debris from ordinary chondritic sources. The bulk chemical composition of 3255 micrometeorites (MMs) collected from Antarctica and deep-sea sediments has shown Mg/Si largely dominated by carbonaceous chondrites, and less than 10% having ordinary chondritic precursors. The chemical ablation model is combined with different initial chondritic compositions (CI, CV, L, LL, H), and the results clearly indicate that high-density (≥2.8 g cm⁻³) precursors, such as CV and ordinary chondrites in the size range 100–700 μm and zenith angle 0°–70°, ablate at much faster rates and lose their identity even before reaching the Earth's surface and hence are under-represented in our collections. Moreover, their ability to survive as MMs remains grim for high-velocity micrometeoroids (>16 km s⁻¹). The elemental ratio for CV and ordinary chondrites are also similar to each other irrespective of the difference in the initial chemical composition. In conclusion, MMs belonging to ordinary chondritic precursors' concentrations may not be insignificant in thermosphere, as they are found on Earth's surface

<SUP>26</SUP>Al in chondrules from unequilibrated L chondrites: onset and duration of chondrule formation in the early solar system

Al-Mg isotope records in sixteen ferromagnesian chondrules from five unequilibrated ordinary chondrites (UOCs), four L chondrites with petrologic grades varying from 3.0 to 3.2 and one L/LL chondrite with petrologic grade 3.1, were studied using an ion microprobe to look for presence of the now-extinct nuclide 26Al at the time of their formation. UOCs of low petrologic grades were selected to avoid possible thermal perturbation of Al-Mg isotope records in the analyzed chondrules. The presence of resolved 26Mg excess (due to decay of 26Al) in Al-rich phases, such as glassy mesostasis and plagioclase, was found in all the chondrules, barring a sole exception. The inferred initial 26Al/27Al ratios in the chondrules range from (0.6-1.9) &#215; 10- 5, and most of them have values lying between (0.6-1.1) &#215; 10- 5. Our data supports a late formation of chondrules relative to CAIs, suggested in previous studies, and indicates a time lag of at least 1.5 Ma between CAI formation and onset of an intense episode of chondrule formation in the early solar system that lasted less than a million years. The paucity of chondrules (10%) with initial 26Al/27Al 10- 5 rules out active chondrule forming events at earlier epochs. The presence of rare chondrules with initial 26Al/27Al 0.5 &#215; 10- 5 in UOCs of low petrologic grades (3.0 to 3.2) analyzed by us and also reported previously in UOCs of petrologic grade ranging up to 3.4, most probably represent parent body processes leading to disturbance of Al-Mg isotope systematics in these cases. An extended duration of chondrule formation lasting more than a couple of million years, suggested in some earlier studies, needs reassessment. A long duration of chondrule formation is also difficult to reconcile with the problem of storage, isolation, mixing and assimilation of chondrules into their meteorite parent bodies

Presence of <SUP>60</SUP>Fe in eucrite Piplia Kalan: a new perspective to the initial <SUP>60</SUP>Fe/<SUP>56</SUP>Fe in the early solar system

Fe-Ni isotope measurements of ferrous pyroxenes of the Piplia Kalan eucrite using Secondary Ion Mass Spectrometer revealed the presence of 60Ni excess corresponding to the initial 60Fe/56Fe of (5.2 ± 2.4) x 10-9. Combining this ratio with the inferred initial 26Al/27Al ratio of (7.5 ± 0.9) X 10~7 in plagioclase of the Piplia Kalan that has been already reported in the literature suggests initial 60Fe/56Fe of (5.2 ± 2.4) x 10-8 in the early solar system, which is significantly lower than the value inferred from Fe-Ni isotope measurements of chondrules in silicates and sulphides from unequili-brated ordinary chondrites. We attribute the difference in the initial 60Fe/56Fe to be solely due to the closure temperature of Fe-Ni isotope systematics, which is less compared to Al-Mg isotope systematics. With the initial 60Fe/56Fe values found in the Piplia Kalan, it is possible that the Fe-Ni isotope systematics was disturbed for another ~ 5 Ma (million years) after closure of the Al-Mg isotope systematics. The closure temperature of ~ 450-650°C for Fe-Ni isotope system seems feasible and we anticipate a cooling rate of ~20-60°C/Ma in the crust region of the parent body of Piplia Kalan, thereby matching the initial 60Fe/56Fe to ~ 5 X 10-7. This is consistent with the initial value found in the silicate phases in chondrules of least metamorphosed meteorites. However, the presence of 60Ni excess in Piplia Kalan does not confirm 60Fe to be a major heat source for the early thermal evolution of meteorite parent bodies. Also, it seems that a massive star probably has contributed the two short-lived nuclides, 26Al and 60Fe, to the early solar system

<SUP>26</SUP>Al records in chondrules from unequilibrated ordinary chondrites: II. Duration of chondrule formation and parent body thermal metamorphism

Fossil records of the now-extinct short-lived nuclide 26Al were analyzed in twelve chondrules from four unequilibrated ordinary chondrites (UOCs), Semarkona (3.0), Bishunpur and Y-791324 (both 3.1) and ALHA76004 (3.3), belonging to the LL group to infer their time of formation. The present data along with those reported by us previously firmly establish a short duration of less than a million years for the major episode of UOC chondrule formation. Nebular processes responsible for chondrule formation started ~ 1 Ma after formation of CAIs and effectively stopped within another one and a half million years in the inner region of the asteroidal belt. Modeling of thermal evolution of chondritic parent bodies suggest that the longer duration of UOC chondrule formation inferred in some of the earlier studies could be attributed to thermal metamorphism experienced by some chondrules in UOC parent bodies. The duration of formation of UOC chondrules is significantly shorter than that for carbonaceous chondrite chondrules

Micrometeorite collections: a review and their current status

Micrometeorites are estimated to represent the main part of the present flux of extraterrestrial matter found on the Earth’s surface and provide valuable samples to probe the interplanetary medium. Here, we describe large and representative collections of micrometeorites currently available to the scientific community. These include Antarctic collections from surface ice and snow, as well as glacial sediments from the eroded top of nunataks—summits outcropping from the icesheet—and moraines. Collections extracted from deep-sea sediments (DSS) produced a large number of micrometeorites, in particular, iron-rich cosmic spherules that are rarer in other collections. Collections from the old and stable surface of the Atacama Desert show that finding large numbers of micrometeorites is not restricted to polar regions or DSS. The advent of rooftop collections marks an important step into involving citizen science in the study of micrometeorites, as well as providing potential sampling locations over all latitudes to explore the modern flux. We explore their strengths of the collections to address specific scientific questions and their potential weaknesses. The future of micrometeorite research will involve the finding of large fossil micrometeorite collections and benefit from recent advances in sampling cosmic dust directly from the air. This article is part of the theme issue ‘Dust in the Solar System and beyond’