Please use this identifier to cite or link to this item:
http://hdl.handle.net/2122/5664
Authors: | Capasso, G.* Grassa, F.* Carreira, P.M.* Carvalho, M.R.* Marques, J.M.* Nunes, J.C.* |
Title: | Nitrogen isotope measurements on hydrothermal fluids from Azores (S. Miguel, Terceira and Graciosa) Islands, Portugal | Issue Date: | 30-Aug-2009 | Keywords: | Nitrogen isotopes Hydrothermal fluids Azores Islands Volcanic gases |
Subject Classification: | 04. Solid Earth::04.07. Tectonophysics::04.07.06. Subduction related processes 04. Solid Earth::04.08. Volcanology::04.08.01. Gases 04. Solid Earth::04.08. Volcanology::04.08.07. Instruments and techniques 05. General::05.02. Data dissemination::05.02.01. Geochemical data |
Abstract: | We present the first nitrogen isotope data from hydrothermal fluids of Graciosa, Terceira and S. Miguel Islands (Azores, Portugal), together with helium isotope composition. 15N values are slightly enriched in light isotopes (from -0.7‰ to -2.2‰) with respect to air, while 3He/4He ratios range from 5 to 6 Ra in Sao Miguel island and from 8 to 9.1 Ra in Graciosa and Terceira islands. The latter values are similar to those found in olivine phenocrysts of basalts (Moreira et al., 1999; Madureira et al., 2005). Such isotope signatures seem to point to the presence of two different deeply-derived end-members: a 3He-rich primitive end-member evident in Terceira and Graciosa islands samples and a 3He-poor end-member characterizing samples from S. Miguel island. According to Madureira et al. (2005), the He primitive component which is more evident in the central parts of the Azores archipelago has to be related to a lower mantle contribution into the prevailing MORB component. On the contrary, the origin of the radiogenic crustal component is still open and debated between the contribution of recycled terrigenous sediments (Turner et al., 1997) and oceanic mantle lithosphere (Schaefer et al., 2002). Since 15N values from fluids vents and inclusions in basalt glasses were discovered to be good geochemical tracers of magma genetic processes, we determined molecular and isotope nitrogen composition (15NN2) in some hydrothermal fluids with the aim to provide additional information on the mantle composition beneath the Azores archipelago. Based on the N2/36Ar ratio we computed for each samples the 15N values of the non-atmospheric nitrogen (15Ndeep). Volatiles emitted from Graciosa and Terceira islands seem to have a common deep nitrogen isotope signature of about -1.5‰ likely resulting from a mixing between lower mantle (15N from 1‰ to 4‰) and upper mantle (15NMORB from -3‰ to -7‰). On the other hand, the 15Ndeep feeding the fumaroles at S. Miguel is close to -5.5‰. Such a 15N-depleted values seem to indicate a probable nitrogen origin from a deep source derived from the recycling of ancient oceanic plate into the upper mantle (MORB). rather than from the recycling of terrigenous sediments transported on top of the descending plate that should have lead to 15N-enrichments. |
Appears in Collections: | Conference materials |
Files in This Item:
File | Description | Size | Format | |
---|---|---|---|---|
AIG-8_Abstract_Capasso et al. Azores.doc | Abstract | 54.5 kB | Microsoft Word | View/Open |
Page view(s)
154
checked on Apr 24, 2024
Download(s) 50
89
checked on Apr 24, 2024