Oxygen and carbon isotope studies of carbonatites from the Laacher See District, West Germany and the Alnö District, Sweden

Abstract

Oxygen isotope analyses, given as per mil enrichment in O 18 relative to mean ocean water, were obtained for 8 calcites (6.3–8.2), 5 sanidines (7.0–7.4), 2 noseans (5.6–5.8), 4 augites (5.8–6.1), and 4 biotites (5.3–5.9) from volcanic ejecta in Pleistocene tuffs from the Laacher See district. Bock types studied include soevites, biotite-pyroxenites, foyaites, and alkali-syenites. Also, C 13 C 12 ratios relative to the Chicago PDB standard were determined for the calcites (−6.6 to −7.7) and for several CO 2 gas samples collected in the district (−4.9 to −5.1). There is a small, progressive decrease in O 18 and increase in C 13 as one moves along the postulated magmatic differentiation sequence in the Laacher See rooks, and oxygen isotope fractionations among coexisting minerals are approximately those expected for isotopic equilibrium at “magmatic temperatures”. Sanidines and augites in the carbonatites are very uniform in O 18 O 16 , with values almost identical to those of feldspars and pyroxenes from gabbroic and ultramafic rocks. The C 13 C 12 ratios of the calcites are also relatively uniform and are 5–15 per mil lower than in marine limestones. Carbon isotope ratios of the CO 2 gas samples are similar to values determined for CO 2 wells from Utah and New Mexico by Zartman et al. (1961). The uniformity of O 18 O 16 and C 13 C 12 in the Laacher See rocks contrasts sharply with previous isotopic studies of carbonatites (Alnö, Fen, Iron Hill, African occurrences etc.) by Baertschi (1957) and von Eckermann et al. (1952). Presumably, this is because the Laacher See rocks are geologically young samples brought up explosively from depth, thereby being fresh and free from weathering or hydrothermal alteration. However, we have analyzed 10 calcites from the Alnö area and have found them to be very uniform in O 18 O 16 with values of 6.9–8.4, almost identical with the calcites from Laacher See. Except for two fine-grained dike rocks, the C 13 C 12 ratios were also very uniform (−5.1 to −7.3). Weconclude: (1) the carbonatites of Laacher See and Alnö were not formed by assimilation of limestone, but likely were derived from gabbroic or ultramafic rocks at depth; (2) they give us the best available values for O 18 O 16 and C 13 C 12 in primary igneous carbonatite (+6.0 to +8.5, and −5.0 to −8.0, respectively); and (3) the CO 2 evolving in the Laacher See area, as well as CO 2 well samples from other localities, may well be primary also, with C 13 C 12 ratios determined by equilibration with carbonatite calcite at high temperatures.