Salts produced by hydrothermal alteration of soluble organic matter in CI chondrite parent bodies

Introduction:

The presence of a range of secondary alteration minerals including phyllosilicates, extensive magnetite, and carbonates in CI chondrites have been inferred to represent aqueous alteration of their parent body asteroids¹. Many simultaneous organic synthesis processes have been identified as potentially occurring within parent bodies². Analysing the soluble organic matter (SOM) produced under analogous hydrothermal conditions can elucidate the chemical pathways involved in this alteration within CI chondrite precursor asteroids³, and even asteroids Ryugu and Bennu ^4,5.
Various asteroidal analogue experiments have previously been performed, but these have been non-specific in terms of conditions and minerals utilised^3,6. While other minerals are often added, the influence of carbonates on SOM produced from parent body alteration processes remains to be investigated. This project will therefore examine the effects of carbonate on organic synthesis in an analogue system for hydrothermal alteration in a CI chondrite parent body. 

Samples and methods:

Hexamethylenetetramine (HMT) solutions, and combinations of HMT and carbonate, were heated at 150ᵒC and pH 10 for a 30-day period. The samples were dried down and resuspended in 100μl of Milli-Q water.
Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to investigate the solutions’ elemental composition to determine salt abundances. Dilution of 10μl of the resuspended samples with 3% nitric acid was performed to make 2ml. 
From January 2025 onwards, gas chromatography-mass spectrometry (GC-MS) and ultra-high-throughput liquid chromatography-mass spectrometry (UPLC-MS) will be conducted, as well as more ICP-OES and Fourier-transform infrared (FTIR) spectroscopy. 

Preliminary results:

The six samples yielded a variety of elemental compositions (Fig. 1). Comparison of the concentrations of Al, Ca, Fe, Mg and Na shows that the different samples show significant distinctions in the concentrations of these elements present. The sample with both carbonate and saponite heated for 7 days and the sample with saponite heated for 30 days contain high concentrations of Ca, Mg and Na.

Discussion:

The elevated concentrations of certain elements in these samples suggests the presence of multiple types of salt - potentially even organic salts such as magnesium formate and iron acetate. Despite the insensitivity to salts that has characterised similar experimental procedures⁶, the larger amount of salt associated with certain saponite-containing samples may be significant. These salts have been produced from minerals like peridot and troilite under similar conditions, and interpreted to facilitate specific reaction pathways⁷. In our future investigation, it might be expected that different minerals have affected the organic composition of these solvents.

References: 
[1] Brearley A. J. (2006) Meteorites and the early solar system II, 943, 587-624. 
[2]Kebukawa Y. et al. (2017) Science advances, 3(3). 
[3] Vinogradoff V. et al. (2017) Icarus, 305, 358-370. 
[4] Chan Q. et al. (2023) In Hayabusa2 International Symposium.
[5] Zega T. J. et al. (2024) LPI Contributions, 3036, 6450.
[6] Vinogradoff V. et al. (2020) ACS Earth Space Chem., 4, 1398-1407.
[7] Serra C. et al. (2024) Icarus, 423, 116-273.

Fig. 1: A bar chart showing the concentrations of different elements within the analogue samples containing different mixtures of minerals, as measured through ICP-OES.