Elemental, isotopic, and structural changes in Tagish Lake insoluble organic matter produced by parent body processes

Abstract

AbstractHere, we present the results of a multitechnique study of the bulk properties of insoluble organic material (IOM) from the Tagish Lake meteorite, including four lithologies that have undergone different degrees of aqueous alteration. The IOM C contents of all four lithologies are very uniform and comprise about half the bulk C and N contents of the lithologies. However, the bulk IOM elemental and isotopic compositions vary significantly. In particular, there is a correlated decrease in bulk IOM H/C ratios and δD values with increasing degree of alteration—the IOM in the least altered lithology is intermediate between CM and CR IOM, while that in the more altered lithologies resembles the very aromatic IOM in mildly metamorphosed CV and CO chondrites, and heated CMs. Nuclear magnetic resonance (NMR) spectroscopy, C X‐ray absorption near‐edge (XANES), and Fourier transform infrared (FTIR) spectroscopy confirm and quantitate this transformation from CR‐like, relatively aliphatic IOM functional group chemistry to a highly aromatic one. The transformation is almost certainly thermally driven, and probably occurred under hydrothermal conditions. The lack of a paramagnetic shift in 13C NMR spectra and 1s‐σ* exciton in the C‐XANES spectra, both typically seen in metamorphosed chondrites, shows that the temperatures were lower and/or the timescales were shorter than experienced by even the least metamorphosed type 3 chondrites. Two endmember models were considered to quantitatively account for the changes in IOM functional group chemistry, but the one in which the transformations involved quantitative conversion of aliphatic material to aromatic material was the more successful. It seems likely that similar processes were involved in producing the diversity of IOM compositions and functional group chemistries among CR, CM, and CI chondrites. If correct, CRs experienced the lowest temperatures, while CM and CI chondrites experienced similar more elevated temperatures. This ordering is inconsistent with alteration temperatures based on mineralogy and O isotopes.