Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

Henrik Svensen, Gray Edward Bebout, Andreas Kronz, Long Li, Sverre Planke, Luc Chevallier, Bjørn Jamtveit

Research output: Contribution to journalArticle

34 Citations (Scopus)

Abstract

We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium NH4 +-bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ15Nair between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ15N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ15N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in 15N, with δ15N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N2). We suggest that the relatively high δ15N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity.

Original languageEnglish
Pages (from-to)4929-4947
Number of pages19
JournalGeochimica et Cosmochimica Acta
Volume72
Issue number20
DOIs
Publication statusPublished - Oct 15 2008
Externally publishedYes

Fingerprint

Geochemistry
Vents
hydrothermal vent
fluid flow
Flow of fluids
Nitrogen
geochemistry
tracer
Bearings (structural)
nitrogen
basin
fluid
Fluids
Rocks
Shale
feldspar
Ammonium Compounds
shale
ammonium
Minerals

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa. / Svensen, Henrik; Edward Bebout, Gray; Kronz, Andreas; Li, Long; Planke, Sverre; Chevallier, Luc; Jamtveit, Bjørn.

In: Geochimica et Cosmochimica Acta, Vol. 72, No. 20, 15.10.2008, p. 4929-4947.

Research output: Contribution to journalArticle

Svensen, Henrik ; Edward Bebout, Gray ; Kronz, Andreas ; Li, Long ; Planke, Sverre ; Chevallier, Luc ; Jamtveit, Bjørn. / Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa. In: Geochimica et Cosmochimica Acta. 2008 ; Vol. 72, No. 20. pp. 4929-4947.
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