PRECAMBRIAN AMMONIUM BIOGEOCHEMISTRY - A STUDY OF THE MOINE METASEDIMENTS, SCOTLAND

The content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of Precambri an age. Whole-rock samples and mica separates from the Moine Successio n (Proterozoic) of Scotland were analysed by a combination of capacita nce manometry and static vacuum mass-spectrometry. Prior to the analys es, the presence of the ammonium ion was confirmed by obtaining infrar ed (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 nei ther garnet nor white mica. For most of the samples, the delta(15)N va lues are very positive, between +12.4 and +16.6 parts per thousand, wi th one sample having a lower value of +8.4 parts per thousand. The maj or variations between samples can be interpreted in terms of original sedimentary features. For the ammonium-bearing samples, muscovites hav e concentrations between 365 and 633 ppm, biotites between 845 and 173 9 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 delta(15)N betwe en co-existing biotite and white mica, nor between the delta(15)N valu es of the mineral separates and their respective whole-rock values. Th e latter suggests minimal selective partitioning of isotopes between m icas and plagioclase feldspar, the only other potential host for ammon ium in the rocks. Assuming that the ammonium has been derived from org anic matter originally present at the time of deposition, and by compa rison with previous studies, it can be inferred that the original delt a(15)N value of the Moine sediments was somewhere between +5 and + 10 parts per thousand, similar to recent sediments. This suggests that th e nitrogen cycle in the Proterozoic shelf-sea, from which the Moine se diments were deposited, was broadly similar to the present day. Overal l the study underlines how under-developed the N-system is compared to other stable isotope systems, notably C and S, and we use the opportu nity to highlight some areas that could be developed in the future. Ho wever, one aspect is clear: future ammonium studies of metamorphic roc ks will need to be performed at high resolution (visible scale of hete rogeneity), with strong mineralogical, petrological and field controls . (C) 1998 Elsevier Science B.V.