Open AccessNitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust
Author(s) -
Inguaggiato S.,
Taran Y.,
Grassa F.,
Capasso G.,
Favara R.,
Varley N.,
Faber E.
Publication year - 2004
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2004gc000767
Subject(s) - geology , continental crust , subduction , forearc , crust , nitrogen , seafloor spreading , isotopes of nitrogen , metamorphism , geochemistry , volcanism , oceanic crust , tectonics , craton , seismology , paleontology , physics , quantum mechanics
The Jalisco Block (JB) is a geologically and tectonically complex part of northwestern Mexico characterized by active subduction‐type volcanism, rifting, and old stable structures. Thermal springs and groups of springs are widely distributed over JB. Bubbling gas from seven thermal springs located within different tectonic environments of the JB was analyzed for He, 20 Ne, and N 2 concentrations and δ 15 N ratios. All gases are N 2 ‐dominant (>84%) with the exception of one sample (Rió Purificación), which has a significant CH 4 content (about 50%). All collected gas samples are relatively high in He, up to 1500 ppm vol and with 3 He/ 4 He values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with δ 15 N values ranging from 0.5 to 5.0‰. The relative N 2 excess with respect to air‐saturated water computed on the basis of N 2 and 20 Ne contents indicates the contribution of a nonatmospheric N 2 source. All the samples show a good correlation between δ 15 N and the relative excess of N 2 with δ 15 N ∼ +5.3‰ for the maximum N 2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a δ 15 N positive value seems to reflect the addition to the fluids of a heavy nitrogen originating from metamorphism processes of rocks occurring within the overlying continental crust.