Publication Date:
2021-06-25
Description:
We present the first helium isotope data for thermal waters and gas emissions on the islands of Terceira,
Graciosa, Faial, Pico and Flores, as well as new data for Sao Miguel. The results allow us to track current mantle
degassing associated with the Azores hot spot, to delineate its spatial distribution and to discuss its possible
origin. As a general rule, we find that free gases tend to display somewhat higher 3He/4He ratio than
groundwaters.We argue that this difference is likely due to radiogenic helium inputs to aquifers duringwater–
rock interactions and, therefore, that gas phases are the fluid carriers with the most representative of mantle
source signature. The measured 3He/4He ratios (normalized to the air ratio, Ra) range from lower-than-MORB
values (5.23–6.07 Ra) on central Sao Miguel, to MORB values on Faial (8.53 Ra) and Flores (8.04 Ra) – located on
either side of the Mid-Atlantic Ridge – and to plume-type values on Graciosa (11.2 Ra) and Terceira (13.5 Ra)
where free gases also display ten times higher-than-MORB CO2/3He ratios (1.8–2.6×1010). Such a wide He
isotopic range and its spatial distribution corroborate with available data for volcanic rocks, indicating that
plume's head presently underlies the central part of the archipelago. The plume-type 3He/4He ratios on
Terceira and Graciosa agree with geochemical and seismic evidence of a deep-rooted mantle plume feeding the
Azores hot spot. Our finding that high 3He/4He ratios correspond to low 3He concentrations and high (arctype)
CO2/3He values exclude a simple plume supply of 3He-rich primitive mantle. Instead, the simultaneity of
both elevated CO2/3He and 3He/4He ratios is best explained by a 3He-rich contribution from the lower mantle
diluted in a CO2-rich feeding plume that contains a recycled altered oceanic plate component. The alternative
possibility of an enhanced time-integrated 3He/(U+Th) ratio in the Azores plume due to a greater
compatibility of helium relative to U and Th during melting events is difficult to reconcile with the enriched
pattern of volcanic rocks from the central islands. In any case, the Azores plume should derive from a mantle
reservoir that could escape convective homogenization for a very long period of time, in agreement with subchondritic
osmium isotopic ratios in volcanic rocks from the central islands of the archipelago.
Description:
Published
Description:
70−80
Description:
1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
Description:
JCR Journal
Description:
reserved
Keywords:
hydrothemal fluids
;
helium isotopes
;
04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
