Helium isotopes distribution in NW Iberian peninsula: evidences of a local neotectonic activity

Budapest, Hungary

In this work we report new data on He abundances and isotope ratios
(3He/4He) from gas associated to some thermal and CO2-rich mineral waters in
N-Portugal. Collected gas samples are mainly CO2-dominant except two sites
where gas is N2-rich. All the sampling sites are characterized by exceptionally
high helium contents with 3He/4He ratios, corrected for air contamination,
varying considerably from 0.09 to 2.68 Ra. In all sites, the 3He/4He ratios are
higher than that typical for stable continental areas thus indicating a variable but
not-negligible (up to 30%) contribution of mantle-derived primordial He. In all
the CO2-rich waters, CO2/3He ratios and 13CCO2 are comparable with mantle
values, thus suggesting a magmatic origin also for CO2. On the contrary, in the
N2-rich waters He is mainly radiogenic, and CO2 is organic in origin. Since no
recent volcanic activity is observed in NW Iberia, high 3He/4He values could be
due, at least, to three processes:
a) releasing of gas from the local upper mantle through deep extensional fault
systems; b) releasing of magmatic volatiles from crustal reservoir(s) formed
during past volcanic activity; c) degassing of a subsurface emplaced magma body.
Mantle He flux in N-Portugal has been estimated to be up to 3 orders of
magnitude higher than that typical for stable continental areas, thus suggesting,
in this area, the presence of a tensional tectonic regime. This implies that mantle
gases could migrate upward probably through inherited tectonic structures
reactivated by neotectonic activity. The third possible scenario seems to be less
plausible since seismic surveys carried out in NW Iberian did not find any
significant evidence of mantle intrusion in the crust. The observed spatial
variability in mantle-derived contribution could reflect the geometry of the
granitic plutons in this area, thus supporting the hypotheses of an upper mantle
degassing. Alternatively, it could be the result of a lateral migration of magmatic
volatiles stored in a crustal reservoir.