Elsevier

Chemical Geology

Volume 244, Issues 1–2, 30 September 2007, Pages 186-201
Chemical Geology

A gas-chromatograph, continuous flow-isotope ratio mass-spectrometry method for δ13C and δD measurement of complex fluid inclusion volatiles: Examples from the Khibina alkaline igneous complex, northwest Russia and the south Wales coalfields

https://doi.org/10.1016/j.chemgeo.2007.06.014Get rights and content

Abstract

A new method is described for on-line extraction of complex mixtures of fluid inclusion volatiles for compound specific carbon- and hydrogen-isotope determination by gas-chromatography–continuous flow mass-spectrometry. Reproducibility for multiple aliquots of gas released from single samples and for duplicate samples is generally ≤ ± 0.7‰ for δ13C measurements of hydrocarbon and CO2 gases to concentrations as low as 20 nmol and ≤ ± 7‰ for δD measurements of hydrocarbon gases to concentrations as low as 100 nmol. This approach significantly improves upon the detection limits for trace gas analysis in fluid inclusions and enables rapid stable isotope measurements of multiple gas species in a single run.

Two examples are evaluated and discussed in their geological context. First, stable isotope results for CH4, CO2 and C2–C5 fluid inclusion gases hosted in the undersaturated peralkaline igneous complex of Khibina in northwest Russia indicate that CH4 was the juvenile magmatic gas phase (δ13C =  13 to − 8‰, δD =  120 to − 50‰). The δ13C values of the C2–C5 hydrocarbons are lower relative to CH4 (by up to 12‰), confirming their abiogenic origin. The δ13C values of CO2 are consistently lower than CH4 (by up to 9‰) and the δD values of C2H6 are 70–100‰ lower than the δDCH4 values. This pattern indicates that the CO2 and higher hydrocarbons were generated during sub-solidus, kinetic CH4-oxidation fractionation and polymerisation reactions.

Second, CH4, CO2 and C2H6 fluid inclusion gases were analysed from quartz veins associated with anthracite from the Nant Helen mine in the south Wales coalfields. The δ13C values of CH4 (− 27.9 ± 0.1‰), CO2 (− 3.1 ± 0.8‰) and C2H6 (− 20.7 ± 0.1‰) are typical of mature thermogenic coal-associated natural gases, as is the δDCH4 values (− 148 ± 1‰). Assuming equilibrium, the Δ13CCO2–CH4 value (+ 24.8‰) suggests that temperatures may have reached ∼ 300 °C in the anthracite zone.

Introduction

Fluid inclusions are ubiquitous in geological samples, can host significant concentrations of gases such as CO2, CH4, N2 and higher hydrocarbons and can provide valuable information about the physico-chemical conditions at the time of their trapping (see Roedder, 1984, Samson et al., 2003). A variety of techniques have been developed for the study of fluid inclusions, from non-destructive methods such as petrographic interpretation, microthermometry and laser-Raman microprobe analysis, which provide compositional information and the temperature and pressure at the time of entrapment (see Samson et al., 2003 and references therein), to destructive bulk volatile extraction techniques for compositional analysis of the entrapped fluids (see Salvi and Williams-Jones, 2003). The stable isotopic composition of fluid inclusion volatiles can also provide essential information about the origin and chemical characteristics of the fluids and related host rocks at the time of entrapment.

A variety of methods have been developed over the years to extract and prepare fluid inclusion volatiles for gas chromatographic and/or mass spectrometric analysis. In the past, two main off-line methods have been favoured. The first is in vacuo thermal decrepitation (Piperov and Penchev, 1973, Mironova et al., 1985, Barker and Smith, 1986, Jackson et al., 1988, Kesler et al., 1997). This approach can cause elevated concentrations of CO2, CH4, CO and H2 to be generated, because of thermal decomposition of minerals and interstitial carbonaceous material, both of which can affect the calculated fluid inclusion gas compositions and their isotopic values (Piperov and Penchev, 1973, Mironova et al., 1985, Kesler et al., 1997). The second method involves mechanical crushing in vacuo or under an inert gas at < 200 °C (Petersilie and Sørensen, 1970, Piperov and Penchev, 1973, Ikorskiy, 1986, Whelan, 1988, Simon, 2001, Potter and Siemann, 2004, Beeskow et al., 2006). The contributions of gases from the mineral host are minimised in this approach, but high blanks of common fluid inclusion components and high adsorption levels of polar species such as CO2 and H2O have been observed during crushing (Piperov and Penchev, 1973, Ikorskiy, 1986, Whelan, 1988).

A further disadvantage of off-line methods is that in most cases only a sub-sample of the extracted volatiles is transferred to the gas chromatograph and/or mass spectrometer for analysis, limiting detection levels (Salvi and Williams-Jones, 2003). Preparation of volatiles for isotopic measurement off-line can also be extremely time-consuming, requiring numerous steps to isolate and convert individual volatile species for isotopic analysis (Petersilie and Sørensen, 1970, Nivin et al., 1995, Kesler et al., 1997).

Andrawes and Gibson (1979) developed an on-line crushing technique for compositional analysis of fluid inclusion volatiles by gas chromatography. Samples were crushed in a high helium flow and sent directly to a gas chromatographic column for subsequent separation and analysis of the individual fluid components. This approach was developed further by Bray et al. (1991) and Salvi and Williams-Jones (1997a). Their procedure allows for transfer of all extracted volatiles to the gas chromatograph column, thereby improving the detection levels for trace volatiles hosted in fluid inclusions. Crushing in a helium flow at low temperatures (∼ 120 °C) also decreases/eliminates problems such as adsorption and chemical reactions, which can arise upon crushing (Bray et al., 1991, Salvi and Williams-Jones, 1997a, Salvi and Williams-Jones, 2003). However, this method requires high flow rates (> 20 ml/min) because of large dead volumes within the system; high flow rates, in turn, necessitate the use of fused silica megabore (≥ 0.53 mm ID) and stainless steel-packed (≥ 3.18 mm OD) gas chromatographic columns. In contrast, low flow rates (1–2 ml/min) are required for isotopic determination by mass spectrometry and therefore gas chromatographic fused silica columns with ≤ 0.32 mm internal diameters are generally used. The use of megabore and packed gas chromatographic columns requires the majority of the sample to be vented for mass spectrometric analysis, with only ∼ 1% reaching the mass spectrometer. As a result, off-line extraction and preparation of volatiles for isotopic analysis has remained the preferred approach of most researchers (Kesler et al., 1997, Beeskow et al., 2006).

Here, for the first time, an on-line gas-chromatography continuous flow-isotope ratio mass-spectrometry (GC/C-irMS) method is described and evaluated for extraction, separation and δ13C and δD measurement of complex mixtures of fluid inclusion volatiles. This method incorporates the advantages of the on-line crushing method for compositional analysis (Bray et al., 1991, Salvi and Williams-Jones, 1997a). As well, a new high-flow/low-flow interface is described for moving gases extracted during crushing to the gas-chromatograph mass-spectrometer for analysis by isotope-ratio mass-spectrometry. This provides a means for rapid and efficient transfer of entire aliquots of extracted volatiles, and thus, enables the isotopic determination of, not only major, but also minor trace gases present in fluid inclusions. This approach expands greatly upon the initial ideas of Potter and Siemann (2004) for analysis of CH4 contained in fluid inclusions from evaporites. Potter and Siemann (2004) described a means of releasing fluid inclusion gases by in vacuo crushing, off-line, before manual transfer to the gas-chromatograph mass-spectrometer system. Two examples are also presented here, to demonstrate the potential of this new method for δ13C and δD measurement of fluid inclusion volatiles.

Section snippets

Analytical device

The crushers are largely based on the design of Bray et al. (1991) with minor modifications. Three units were assembled. A single unit comprises a sample chamber, a piston chamber and a heating block controlled by two cartridge heaters (Fig. 1). The piston and sample chambers are connected by a rotating lock and sealed by silicon O-rings (Fig. 1). Both chambers have 1/8ʺ stainless steel connections that attach to 1/16ʺ stainless steel tubing. The units are placed in a steel frame with a

Results for standard and blanks tests

Several gases of known isotopic composition were analysed to test the methods described in Section 2. Natural gas standards RM 8559 (NGS 1) and RM 8560 (NGS 2) were obtained from the National Institute for Standards and Technology. The carbon-isotope compositions of the CH4, C2H6, C3H8 and CO2 in these gas mixtures have been calibrated by NIST versus NBS 19 (assigned value of + 1.95‰). Ranges of hydrogen-isotope compositions for CH4 and C2H6 have also been reported, evaluated versus VSMOW and

Case study 1: Khibina alkaline igneous complex, northwest Russia

The Devonian Khibina alkaline igneous complex is located in the Kola Alkaline Province in northwest Russia (Fig. 6). These peralkaline igneous rocks host unusually high concentrations of reduced gases entrapped predominantly as secondary fluid inclusions (Potter et al., 1998, Beeskow et al., 2006). The gases are similar to those found in other peralkaline igneous complexes worldwide (Petersilie and Sørensen, 1970, Salvi and Williams-Jones, 1997b, Konnerup-Madsen et al., 1985, Markl et al., 2001

Conclusions

The analytical method described here (crushing on-line in a constant helium flow) makes possible analysis of complex mixtures of fluid inclusion gases. Non-condensable and condensable fluid inclusion gases released upon crushing are captured using a 13 nm molecular sieve immersed in liquid nitrogen and then transferred to a gas chromatograph–continuous flow-isotope ratio mass-spectrometer system for compound-specific carbon- and hydrogen-isotope analysis. The CH4, CO2 and higher hydrocarbons

Acknowledgements

We thank Colin Bray, Ed Spooner, Paul Middlestead, Andreas Hilkert, Stef Salvi, Kim Law and Li Huang for their invaluable advice during development of this method and the technical staff at the University Machine and Physics Machine Shops (UWO) for supplying various parts to specification. We also gratefully acknowledge Bettina Beeskow, Andrew Rankin and Peter Treloar for their helpful discussions and provision of samples from Kingston University collections (collected by BB, JP and AR), and

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