THE ORIGIN OF METHANE AND EXCESS NITROGEN IN CENTRAL AMERICAN GEOTHERMAL SETTINGS: EVIDENCE FROM STABLE ISOTOPES, NOBLE GASES, AND IODINE-129

Excess molecular nitrogen in island arc fumaroles, hot springs, and geothermal wells has been frequently cited as evidence for the recycling of subducted marine sediments. The occurrence of methane and other hydrocarbons, on the other hand, is almost certainly the result of metamorphic reactions of organic matter in the shallow crust. We present stable isotope data for carbon and nitrogen as well as noble gas data in order to distinguish between crustal and magmatic nitrogen sources in production wells from 4 geothermal fields in Central America. Methane and nitrogen are closely correlated suggesting a common source. The Momotombo geothermal field of Nicaragua presents exceptionally high excess nitrogen and methane. While this is not uncommon for geothermal methane fields found in sedimentary basins along plate boundaries, the Momotombo geothermal field is unique in that it presents both high N₂/³He ratios while preserving essentially magmatic ³He/⁴He ratios. Analysis of iodine-129 indicates both a subducted organic component (25-30 Ma), as well as a much older crustal component (40-65 Ma) throughout much of Central America. The high excess-nitrogen component of Nicaragua is related to the older iodine end-member. This is likely the result of the westward migration of the arc system in the past 15 Ma, and the incorporation of organic matter deposited in the western margins of the Nicaraguan Depression. Although the older crustal source is also present in the Miravalles Geothermal field in Costa Rica, and in the Berlín and Ahuachapán geothermal fields of El Salvador, the lack of methane and excess nitrogen indicates that the incorporation of crustal organic matter is not significant to the northwest and southeast of the Nicaraguan portion of the arc.