From basin to basement: the movement of surface fluids into the crust

doi:10.1016/S0375-6742(00)00114-X

Journal of Geochemical Exploration

Volumes 69–70, June 2000, Pages 527-531

https://doi.org/10.1016/S0375-6742(00)00114-X Get rights and content

Abstract

Quartz veins from Precambrian, high grade, basement rocks in Southern Norway contain both aqueous and biogenically sourced hydrocarbon fluid inclusions. Aqueous fluids show a wide range in salinities (0–40 wt% NaCl eq.) and densities (0.80–1.16 g/cm³) but the ultimate source of this salinity is unclear. The oxygen isotopic composition of most of the veins is dominated by the host-rock compositions. Surface derived fluids migrated downwards as a result of extension of dry crystalline basement rocks with pore fluid pressures less than hydrostatic. Quartz precipitation occurred in the veins initially at near hydrostatic pressures but later quartz generations were precipitated at pressures closer to lithostatic.

Introduction

The presence of saline brines in crystalline basement rocks is a well-documented phenomenon from many stable cratons (e.g. Frape and Fritz, 1987, Möller et al., 1997). Such brines have often equilibrated with their host rocks over very long residence times so that chemical and isotopic tracers no longer record their origins.

Quartz veins in high grade rocks in Southern Norway contain aqueous and hydrocarbon inclusions that have been trapped at pressures of 1–2 kbars and temperatures of 230–300°C (Munz et al., 1995). The biogenic nature of the hydrocarbon fluids in these veins strongly suggest that in this case sedimentary fluids have infiltrated crystalline basement rocks. These fluids were heated as they moved down into the basement rocks, resulting in the in situ cracking of the hydrocarbons (Munz et al., 1995). In this paper we present fluid inclusion and oxygen isotopic data to elucidate the origins and mechanisms of formation of these quartz veins.

Section snippets

Background geology and field relations

The Modum Complex of Southern Norway comprises a series of Precambrian high grade metasediments and metagabbros that form part of the Fennoscandian shield. A series of albite rimmed quartz veins±actinolite that cross-cut all metamorphic fabrics and foliations are found throughout these amphibolite facies rocks. The veins are of variable thicknesses and lateral extent and have no preferred orientation, reflecting the heterogeneities of the host-rocks. The veins commonly contain large (<10 cm)

Petrography

Quartz in these veins is strongly growth zoned with alternating bands of fluid inclusion-rich and inclusion-free quartz. Five main types of fluid inclusions have been identified. Type 1, 2 and 3 inclusions are 1 and 2 phase hydrocarbon±CO₂ inclusions, which have been described by Munz et al., 1998, Munz et al., 1995 and are not discussed further. Aqueous inclusions are abundant in all samples. The most common type in samples from Modum are 2 phase liquid and vapour inclusions (Type 4) that vary

Analytical techniques

A Linkam MDS 600 heating freezing stage was used for microthermometric analyses. Calibration was carried out in the temperature range −56.6 to 300°C using a mixture of CO₂ standards and the melting points of standard solids using the techniques described in Shepherd et al. (1985). Precision on freezing point temperatures is 0.2°C and on homogenisation/halite dissolution temperatures is 1°C. Salinity data are reported in wt% NaCl equiv. and are calculated using the equations of Bodnar (1994) for

Microthermometric results

The bulk of first ice melting temperatures in Type 4 inclusions occur in the range −19.2 to −25.0°C and suggest that the fluids in these veins are dominated by NaCl±KCl salts. Last ice melting in Type 4 inclusions occurs over a wide range in temperatures from −0.1 to −25.1°C. Samples from Døviken, Høgås and Rasmusbråten show a similar range in values, however, mean ice melting temperatures from Embretsfoss are lower at −22.4°C.

With the exception of Type 4 inclusions from Embretsfoss,

Oxygen isotopes

Results from the oxygen isotopic study are presented in Table 1. Quartz veins from Døviken, Høgås and Rasmusbråten have a similar range of values (7.9–11.1‰), although veins from Embretsfoss have values, which are significantly lower (5.0–8.5‰). Albite selvages from Embretsfoss and Høgås have lower δ¹⁸O values (Table 1) and actinolite values are the highest recorded from these veins. Two quartz separates from the meta-gabbro have a δ¹⁸O values of 7.0 and 7.1‰.

Density of aqueous fluids

Fluid densities have been calculated using the equations of Zhang and Frantz (1987). There is a wide range in fluid densities over all the dataset, ranging from 0.82 to 1.16 g/cm³. Variations in density can result from variations in salinity, pressure or temperature of trapping. Fig. 1is a salinity-Th plot contoured with densities for NaCl solutions and demonstrates both the enormous spread in fluid densities and the different patterns of variation in different samples. For example the sample

Discussion and conclusions

The distribution of data from Embretsfoss (Fig. 1) is consistent with inclusion trapping from near hydrostatic to near lithostatic pressures. If the temperatures estimation of Munz et al., (1995) for the formation of these veins is correct and applicable to all localities, then the data suggest that in most of the other veins trapping took place at higher than hydrostatic pressures.

The origin of the wide range in salinity observed in some of these samples is unclear. It may reflect the tapping

Acknowledgements

This work was funded by the Natural Environment Research Council through research grant GR3/11087.

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