The first stepwise crushing data on C, N and Ar isotopic and elemental ratios in Guli carbonatites fluid inclusions

Buikin, A. I.; Verchovsky, A. B.; Grinenko, V. A. and Kogarko, L. N. (2011). The first stepwise crushing data on C, N and Ar isotopic and elemental ratios in Guli carbonatites fluid inclusions. In: Goldschmidt Conference 2011, 14-19 Aug 2011, Prague, Czech Republic.

URL: http://www.goldschmidt2011.org/abstracts/originalP...

Abstract

To get insight into the fluid regime evolution during formation of Guli massif (Maymecha-Kotuy magmatic complex, Syberia) carbonatites we have studied C, N and Ar isotopic and elemental ratios in foure carbonatite samples by stepwise crushing method. Mineral separates representing different formation stages of the massif have been selected for the investigation: two early calcites (Cal) and a late stage dolomite (Dol) and siderite (Sid).

The early calcites are carachterized by significantly lower CO₂content and lower δ¹³C values than the late Dol and Sid (-14.1, -13.6‰ and -9.0, -10.5‰ in average respectively). Fractionation during melt degassing (when system is closing) could lead to a higher CO₂content with higher d 13 C values in the late minerals. But the results on C, N and Ar elemental compositions in the fluid inclusions have shown that C/N and C/Ar ratios also dramatically increase from the early to the late samples (C/N: from 7 in Cal to 210 in Sid and 2100 in Dol; C/Ar ; from 870 in Cal to 12300 in Sid and 159000 in Dol), which could not be caused by a simple magmatic fractionation. An additional source of CO₂could appear at the late stages of the fluid-magmatic evolution of the massif. The data on C, N and Ar concentration variations in crushing steps support this assumption: well-defined correlations between concentrations of these elements in fluid inclusions are observed in the early Cal (i.e. all gases in the inclusions have the same elemental composition and concequently the same source). For the late Sid and Dol the situation is different: when N and Ar concentrations decrease with crushing steps, the C concentration is increasing, suggesting different sources for (N+Ar) and for most of CO₂. Moreover, ⁴⁰Ar/³⁶Ar ratios in early and late samples are quite different: 3680 in Cal and 657 and 549 in Sid and Dol, respectively. This suggests the air-like argon component to be domineted in fluids during formation of the late minerals.

Thus, relationships between C, N and Ar concentrations as well as differences in C and Ar isotopic compositions in fluid inclusions of the early and late carbonitites suggest that at the late stages of Guli massif carbonatites formation an additional CO₂source with havier carbon and atmosphere-like Ar have contributed to the system.

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