Helium isotope characteristics of Andean Convergent Margin geothermal fluids.

BARRY P.; BEKAERT D; SELTZER A.; CURTICE J; DE MOOR M.; JESSEN G.; GIOVANELLI D.; SCHRENK M; BUONGIORNO J.; AGOSTINA CHIODI; RAMÍREZ C; ROGERS T.; LLOYD K.

Congreso; AGU Fall-Meeting-2021; 2021

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 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.