Precambrian ammonium biogeochemistry: a study of the Moine metasediments, Scotland
Résumé
The content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of Precambrian age. Whole-rock samples and mica separates from the Moine Succession (Proterozoic) of Scotland were analysed by a combination of capacitance manometry and static vacuum mass-spectrometry. Prior to the analyses, the presence of the ammonium ion was confirmed by obtaining infrared (IR) absorption spectra from the micas, principally biotites. Whole-rock ammonium contents are high for amphibolite facies metasediments (140–422 ppm). The ammonium ion was not detected in the IR absorption spectra of biotites from one schist; however, this sample contains neither garnet nor white mica. For most of the samples, the δ15N values are very positive, between +12.4 and +16.6‰, with one sample having a lower value of +8.4‰. The major variations between samples can be interpreted in terms of original sedimentary features. For the ammonium-bearing samples, muscovites have concentrations between 365 and 633 ppm, biotites between 845 and 1739 ppm. For individual samples, [NH4+]Mu/[NH4+]Bi varies between 0.31 and 0.43, a much narrower range than has been reported previously. There are no significant or consistent differences in δ15N between co-existing biotite and white mica, nor between the δ15N values of the mineral separates and their respective whole-rock values. The latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar, the only other potential host for ammonium in the rocks. Assuming that the ammonium has been derived from organic matter originally present at the time of deposition, and by comparison with previous studies, it can be inferred that the original δ15N value of the Moine sediments was somewhere between +5 and +10‰, similar to recent sediments. This suggests that the nitrogen cycle in the Proterozoic shelf-sea, from which the Moine sediments were deposited, was broadly similar to the present day. Overall the study underlines how under-developed the N-system is compared to other stable isotope systems, notably C and S, and we use the opportunity to highlight some areas that could be developed in the future. However, one aspect is clear: future ammonium studies of metamorphic rocks will need to be performed at high resolution (visible scale of heterogeneity), with strong mineralogical, petrological and field controls.