Ammonium is a significant reservoir of nitrogen in the Orgueil meteorite

1/ Introduction

The incorporation and evolution of nitrogen (N) in the early Solar System, as well as the origin of terrestrial nitrogen, are still very poorly understood. Across the Solar System, the isotopic composition of nitrogen is far from being uniform ­(from δ15N = –387 ± 8‰ in the Solar Wind to δ15N = +850 ± 150‰ in nitrogen species in cometary coma) and seems to reflect different processes leading to distinct isotopic reservoirs [e.g., 1]. As carbonaceous chondrites (CC) are the richest in volatiles [2] among primitive meteorites, the measurements of their nitrogen isotopic composition are a key to investigate the origins and evolutions of this element in the Solar System.

As for CCs [3], the nitrogen in comets is considered to be mainly present in the refractory organic material [4]. However, nitrogen in the form of ammonium salts have been identified on comet 67P/Churyumov-Gerasimenko [5] and may constitute the dominant reservoir of nitrogen in comets [5, 6]. Ammonium may also be present on several asteroids [5, 7] and might be present in Hayabusa2-returned samples among NH-rich compounds [8]. Ammonium could therefore constitute a dominant nitrogen reservoir of these objects, and may have been delivered in abundance to Earth through carbonaceous chondrites.

In 1864, 0.1wt% of ammonium was detected in the Orgueil meteorite, just after its fall [9], but has never been reported since then. Is the ammonium detected in 1864 still present in the meteorite nowadays? Is it a terrestrial contamination? If extraterrestrial, how much nitrogen is in the form of ammonium in Orgueil? How is nitrogen partitioned between ammonium, soluble and insoluble organic compounds? What are the genetic links between these nitrogen-containing phases? These are the questions we seek to address with the present work.

2/ Method

Because the Orgueil meteorite is known to be altered by the terrestrial environment [10] we have analysed two samples of Orgueil, that have been stored in different conditions. The so-called “Orgueil-Flask” (OF) is material recovered after the fall in 1864 and kept in a tightly sealed flask in a private collection. By contrast, the so-called “Orgueil-Museum” (OM) is a raw chip from the Orgueil sample kept in a sealed bell-jar, but possibly leaking, at the Museum Victor Brun de Montauban (France). OM grains/fragments are covered by relatively bright effloresced sulfates, which are minor in OF grains, indicating that the OF sample is probably less terrestrially altered.

To avoid any contamination by terrestrial ammonia, we have developed a contamination-free protocol to extract, quantify and measure the isotopic composition of nitrogen in Orgueil water-soluble ammonium (Figure 1). The meteorite is ground and solubilised in water to extract water-soluble ammonium. Grinding the meteorite fragment can heat the material, potentially leading to the evaporation of ammonium and/or to the decomposition of organic molecules forming ammonium or ammonia [11, 12, 13]. To overcome these issues and solubilise as much ammonium as possible, the meteorite fragment is ground under cryogenic conditions in a hermetic jar containing water ice. After grinding and water melting, ten series of leaching by ultra-sonification, followed by centrifugation, collection of the supernatant phase and re-solubilisation of the ammonium contained in the remaining powder are performed to extract all the water-soluble ammonium. The resulting solutions are analysed by Ion-exchange Chromatography (IC) and Isotopic Ratio Mass Spectrometer (IRMS) to quantify the ammonium content and measure its N-isotopy.

To investigate the partition of nitrogen in the meteorite, the nitrogen abundance and isotopic composition of the bulk samples and of the Insoluble Organic Material (IOM) extracted from these samples (adapted from the method described in [14]) are also measured by IRMS.

3/ Main results and conclusions

Duplicate analyses of the least altered sample (OF) reveal the presence of water-soluble ammonium at a concentration ranging from 0.053 to 0.083 weight percent by mass of meteorite with a mean value of 0.068 ± 0.015 wt% (Table 1). This concentration corresponds to a mean proportion of 28 ± 4% of the total nitrogen in Orgueil. This ammonium may come from ammonium salts and/or ammoniated phyllosilicates. The δ¹⁵N mean value of +132‰ confirm the extra-terrestrial origin of ammonium. The differences of isotopic composition observed between the N-bearing phases (Table 1) suggest a different origin of the nitrogen contained in organic matter and water-soluble ammonium.

Our results confirm that ammonium constitutes a substantial nitrogen reservoir in Orgueil. More broadly, studying the repartition of the nitrogen in the N-bearing phases, including ammonium, and their genetic links will help us to reveal the incorporation of nitrogen in the Solar System and ultimately on Earth.

4/ Acknowledgments

This work has been supported by grants from Labex OSUG@2020 (Investissements d’avenir – ANR10 LABX56) and from the Programme National de Planétologie (PNP) of CNRS-INSU co-funded by CNES.

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