Sediment Underplating Beneath Central America: Insights From N-He, δ15N Systematics of Volatile Discharges in Nicaragua
Abstract
We investigated N-isotope systematics and geochemistry of gases emitted from fumarolic and geothermal sources along the Nicaraguan volcanic front. The goals of this study are: 1) to quantify the contributions from different reservoirs to the volatile output along the volcanic front; 2) to map along-arc variations in sediment contribution to magmas. We report relative abundances of N2, Ar, and He, as well as N isotopes, for geothermal fluid samples from Nicaraguan volcanoes. Samples from Masaya, Tipitapa, Xiloa, Momotombo geothermal wells, Cerro Negro, and San Cristobal (from SE to NW) have N2/He ratios in the range 1,600 - 25,000 - typical of arcs. The southernmost sample, Volcan Mombacho, has N2/He =144, consistent with upper mantle derivation (N2/He ~150). N isotopes identify sources for the geothermal gases. A positive δ15N value indicates N contribution from subducted hemipelagic sediment (δ15N = +7‰ ). The upper mantle, in contrast, is characterized by a δ15N = -5‰ . Samples from Masaya, Xiloa, a Momotombo well, and Cerro Negro have δ15N = +1.2, +4.9, +2.1, and +4.9‰ , a strong sediment signature. The sample from Mombacho has a δ15N = +0.5‰ , indicative of mantle nitrogen. These Nicaraguan data contrast with reported N2-He-δ15N systematics of central Costa Rica, where gas discharges sample primarily mantle wedge volatiles (Zimmer et al., AGU 2001, Fischer et al., Science 2002). Most Nicaraguan samples have N2/He and δ15N values that indicate subduction of hemipelagic sediments, as is typical of other arcs. In Costa Rica and southern Nicaragua (Mombacho), underplating prevents hemipelagic sediments from reaching the zone of melting and results in low N2/He and negative δ15N values. Southern Nicaragua represents the transition from sediment underplating (in the south) to sediment subduction (in the north). The N-systematics are therefore consistent with other geochemical indicators (Ba/La, 10Be, e.g.) showing significant sedimentary contribution to Nicaraguan volcanic output.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.V21B1209E
- Keywords:
-
- 8499 General or miscellaneous