Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust
Por:
Inguaggiato S., Taran Y., Grassa F., Capasso G., Favara R., Varley N., Faber E.
Publicada:
1 ene 2004
Resumen:
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, 20Ne, and N2 concentrations and ?15N ratios. All gases are N2-dominant (>84%) with the exception of one sample (Rio Purificacion), which has a significant CH4 content (about 50%). All collected gas samples are relatively high in He, up to 1500 ppm vol and with 3He/4He values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with ?15N values ranging from 0.5 to 5.0%. The relative N2 excess with respect to air-saturated water computed on the basis of N2 and 20Ne contents indicates the contribution of a nonatmospheric N2 source. All the samples show a good correlation between ?15N and the relative excess of N2 with ?15N ? +5.3% for the maximum N2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a ?15N 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. Copyright 2004 by the American Geophysical Union.
Filiaciones:
Istituto Nazionale di Geofisica e Vulcanologia-Seizione di Palermo, Via Ugo La Malfa 153, Palermo 90146, Italy
Instituto de Geofisica, UNAM, Coyoacan, D.F. 04510, Mexico
Facultad de Ciencias, Universidad de Colima, Colima, Mexico
Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, Germany
All Open Access; Bronze
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