8th International Symposium on Applied Isotope Geochemistry

F irst Aggiungi un titolo nNitrogen isotope datameasurements on fumarolic gaseshydrothermal fluids from Azores (S.ão Miguel, Terceira and Graciosa) Islands, Portugal from Sao Miguel, Terceira and Graciosa islands (Azores Islands, Portugal)

G. Capasso1, F. Grassa1, P.M. Carreira2, M.R. Carvalho3, J.M. Marques4, and J.C. Nunesz5

1 Istituto Nazionale di Geofísica e Vulcanologia Sezione di Palermoia, Via U. La Malfa, 153, 90146 Palermo, Italy.

2 Instituto Tecnológico e Nuclear, Estrada Nacional nº 10, 2686-953 Sacavém, Portugal,

3 Fac.uldade de Ciências de Lisboa, Departamento de Geologia, CeGUL, Ed. C6, 1Campo Grande, 1749-016 Lisboa, Portugal

4 Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

5 Department of Geosciences, University of the Azores, Portugal.

Univ Azores. Ponta Delgada, Azores, Portugal

We present the first Nnitrogen isotope data, collected from fumarolic gas and hydrothermal fluids of Graciosa, Terceira and S.ãoao Miguel Islands (Azores, Portugal), Here are presented for the first timetogether with . Hhelium isotope composition. 15N values are slightly enriched in light isotopes (from -0.7‰ to -2.2‰) with respect to air, while 3He/4He ratiosn of these gases are also repoted rangeing helium and, for the first time, also nitrogen isotope data collected from fumarolic gas and hydrothermal fluids of Graciosa, Terceira and Sao Miguel Islands (Azores, Portugal). Helium isotope ratios ranges from 5 to 6 Ra in Sao Miguel island and from 8 to 9.1 Ra in Graciosa and Terceira islands. respectively. The latter se values are similar to those found in olivine phenocrysts of basalts reported (by Moreira et al., (1999; ) and by Madureira et al., (2005) in olivine phenocrysts of basalts. Such isotope signatures seems to point to the presence of two different deeply-derived end-members:

a 3He-rich primitive end-member evident in Terceira and Graciosa Islands islands samples and ;

a radiogenic 4He3He-rich poor end-member characterizing samples from S.ao Miguel island.

According to Madureira et al., (2005), the He primitive component which is mainly more evident in the central partsportions of the Azores archipelago has to be related to a the lower mantle contribution ofin lower mantle to the prevailing MORB component. On the contrary, the question regarding the origin of this the radiogenic crustal component is still open and debated. The most ??? between the contribution of recycled terrigenous sediments (Turner et al., 1997) or and oceanic mantle lithosphere (Schaefer et al., 2002).

Since d15N 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 (d15NN2), in some hydrothermal fluids with the aim to provide additional information of on the mantle composition beneath the Azores archipelago.

The obtained d15N values are slightly enriched in light isotopes (from -0.7‰ to -2.2‰) with respect to air. Based on the N2/36Ar ratio we computed for each samples we have can calculated firstly the atmospheric nitrogen component. Therefore, assuming that nitrogen result from a simply binary mixing between an atmospheric and a deep component, the d15N values of the non-atmospheric nitrogen (d15Ndeep) for all the samples were was computed the d15N values of the non-atmospheric nitrogen (d15Ndeep). for each samples.

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 (d15N = from 1‰ to 4‰) and upper mantle (d15NMORB = from -3‰ to÷????? -7‰). On the other hand, the d15Ndeep feeding the fumaroles at S. ãoao Miguel is close to -5.5‰. Such a 15N-depleted values seem to indicate that a probabley nitrogen originates from a deep source originated 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. rather than from the recycling of terrigenous sediments transported on top of the descending plate that should have lead to 15N-enrichments.

References

Madureira P., Moreire M., Mata, J. and Allegre, C.J., 2005. Primitive neon isotopes in Terceira Island (Azores archipelago). Earth Planetary Science Letters., 233, 429-440.

Moreira M., Doucelance, R, Kurz, M.D., Duprè, B. and Allegre C.J., 1999. Helium and lead isotope geochemistry of the Azores Archipelago. Earth Planetary Science Letters 169, 189-205.

Schaefer, B.F., Turner, S., Parkinson, I., Rogers, N. and C. Hawkesworth. 2002. Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume. Nature, 420, 304-307.

Turner, S., Hawkesworth C., Rogers, N. and King P., 1997. U-Th isotope disequilibria and ocean island basalt generation in the Azores. Chemical Geology. 139, 145-164. since the subduction of sediments transported on top of the descending plate should have lead to 15N-enrichments.

La Malbaie, Quebec, Canada August 30th to September 4th, 2009