Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust

Abstract

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, Ne-20, and N-2 concentrations and delta(15)N ratios. All gases are N-2-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 He-3/He-4 values ranging from 0.6 to 4.5 Ra. All measured nitrogen isotope ratios are heavier than air with delta(15)N values ranging from 0.5 to 5.0parts per thousand. The relative N-2 excess with respect to air-saturated water computed on the basis of N-2 and Ne-20 contents indicates the contribution of a nonatmospheric N-2 source. All the samples show a good correlation between delta(15)N and the relative excess of N-2 with delta(15)Nsimilar to+5.3parts per thousand for the maximum N-2 excess of 100%. Due to a presumed lack of seafloor sediment involved in the subduction process, such a delta(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.