Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe

Abstract

In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO 2-rich (>99 vol.%) and have δ 13C values of −1.95 and −4.29‰, respectively. With distance from the center of CO 2 degassing (Bublak) the δ 13C values decrease, most likely due to physico-chemical fractionation of CO 2 between gaseous and aqueous phases rather than to admixture of organic/biogenic CO 2. The δ 15N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by 3He/ 4He ratios decreasing from 5.9 R a in the center (Bublak) to 0.8 R a in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest 3He/ 4He ratio (5.9 R a) is accompanied by the highest 40Ar/ 36Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ 15N value of ≈−4.5 ± 1.0‰ represents the first free gas δ 15N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the 3He/ 4He ratios, the δ 15N values are equal for ESCM and MORB, respectively.