Abstract
Subduction zones are the interface between Earth’s interior (crust and
mantle) and exterior (atmosphere and oceans), where carbon and other
volatiles are actively cycled between Earth reservoirs by plate
tectonics. Helium is highly sensitive to mantle inputs and can be used
to deconvolute mantle and crustal volatile pathways in arcs. We report
He isotope and abundance data for 18 deeply-sourced gas seep samples in
the Central Volcanic Zone (CVZ) of Argentina and the Southern Volcanic
Zone (SVZ) of Chile. We use 4He/20Ne values to assess the extent of air
contributions, as well as He concentrations. Air-corrected He isotopes
from the CVZ range from 0.21 to 2.58 RA (n=7), with the highest value in
the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt.
4He/20Ne values range from 1.7 to 546 and He contents range from 1.0 to
31 x 106 cm3STP/cm3. Air-corrected He isotopes from the SVZ range from
1.27 to 5.03 RA (n=7), 4He/20Ne values range from 0.3 to 69 and He
contents range from 0.5 to 175 x 106 cm3STP/cm3). Taken together, these
data reveal a clear southeastward increase in 3He/4He, with the highest
values (in the SVZ) plotting below the nominal range of values
associated with pure upper mantle He (8 ± 1 RA1), but approaching the
mean He isotope value for arc gases of ~5.4 RA2.
Notably, the lowest values are found in the CVZ, suggesting more
significant crustal contributions to the He budget. The crustal
thickness in the CVZ is up to 70 km, significantly more than in the SVZ,
where it is just 35-45 km3. It thus appears that crustal thickness
exerts a primary control on the extent of fluid-crust interaction, as
helium and other volatiles rise through the upper plate in the Andean
Convergent Margin. These data agree well with the findings of several
previous studies4-14 conducted on the volatile geochemistry along the
Andean Convergent Margin, which suggest a much smaller mantle influence,
presumably associated with thicker crust masking the signal in the CVZ.
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[13] Inostroza et al., 2020 [14] Robidoux et al., 2020