The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin

Abstract

Subduction zones represent the interface between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between Earth reservoirs by plate tectonics. Helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. In contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. Further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. Here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ) of the Andean Convergent Margin (ACM). Data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. He isotopes from the CVZ backarc range from 0.1 to 2.6 RA (n = 23), with the highest values in the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt. Atmosphere-corrected He isotopes from the SVZ range from 0.7 to 5.0 RA (n = 19). Taken together, these data reveal a clear southeastward increase in 3He/4He, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 ± 1 RA), approaching the mean He isotope value for arc gases of (5.4 ± 1.9 RA). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of 4He) to the helium budget. The crustal thickness in the CVZ (up to 70 km) is significantly larger than in the SVZ, where it is just ∼40 km. We suggest 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 ACM. We also report carbon isotopes from (n = 11) sites in the CVZ, where δ13C varies between −15.3‰ and −1.2‰ [vs. Vienna Pee Dee Belemnite (VPDB)] and CO2/3He values that vary by over two orders of magnitude (6.9 × 108–1.7 × 1011). In the SVZ, carbon isotope ratios are also reported from (n = 13) sites and vary between −17.2‰ and −4.1‰. CO2/3He values vary by over four orders of magnitude (4.7 × 107–1.7 × 1012). Low δ13C and CO2/3He values are consistent with CO2 removal (e.g., calcite precipitation and gas dissolution) in shallow hydrothermal systems. Carbon isotope fractionation modeling suggests that calcite precipitation occurs at temperatures coincident with the upper temperature limit for life (122°C), suggesting that biology may play a role in C-He systematics of arc-related volcanic fluid and gas emissions.

Ultrahigh-precision noble gas isotope analyses reveal pervasive subsurface fractionation in hydrothermal systems

“…Only Chilean arc samples show δ 13 C values of <−5‰, in the range of −8.6 to −11.4‰. This result is consistent with independent δ 13 C data acquired from gas vials that were collected along with the Giggenbach bottles analyzed in this study [δ 13 CO 2 = −8.72 and −8.64‰ for Campanario and Baño Morales, respectively ( 27 )]. δ 15 N values vary from −9.24‰ (Forrest Spring) to 0.13‰ (Crater Hills) for Yellowstone, from −3.32 to −0.86‰ for Iceland (Krýsuvík), and from −1.94‰ (Baño Morales) to 4.17‰ (Campanario) for Chile (typical uncertainty of ±0.8‰; fig.…”

“…S3), potentially tracking a contribution from recycled organic sediments (δ 13 C ~ −25‰ and δ 15 N ~ +6‰) ( 4 , 38 , 39 , 48 ). Here, we cannot exclude the possibility that light δ 13 CO 2 values in the Southern Volcanic Zone (SVZ)—at least partly—reflect secondary processes associated with calcite precipitation in shallow hydrothermal systems ( 7 , 27 ). Likewise, we cannot exclude the possibility that some of the carbon and noble gases collected in Chilean volcanic emissions originate from organic sediments in the continental crust (rather than from recycling of slab materials).…”

“…In this study, “dry” (volcanic) gas samples were collected from hydrothermal systems in Chile [Baño Morales and Campanario springs, SVZ ( 27 )], Iceland [Krýsuvík and Reykjanes (Gunnuhver) hot springs ( 40 )], Germany [E-VQ and E-SQ wells in Eifel ( 17 )], Costa Rica [Praxair well #17 and Poás Volcano ( 7 )], and United States [across the Yellowstone National Park (Washbun Spring, Forrest Spring, Crater Hills, Mud Volcano) ( 33 ) and Salton Sea Geothermal System (Davis-Schrimpf mud volcanoes) ( 50 )]. At each sampling location, we collected copper tubes and “Giggenbach” bottles.…”

“…The He and nitrogen isotope compositions of copper tube volcanic gas samples collected in this study were measured separately using the Noblesse Nu mass spectrometer in the Barry Lab at WHOI. For He isotope measurements ( 27 ), copper tube samples were connected to a noble gas extraction line using an O-ring connection, and ~5 cm 3 of gas was expanded into the cleanup line. The pressure was measured using a capacitance manometer, and then a small aliquot of gas was expanded into the cleanup portion of the line.…”

Magmatic Carbon and Helium in Springs Reveals the Vitality of a Dormant Volcano, Taranaki, New Zealand

“…The air corrected 3 He/ 4 He (R C /R A ) is also consistent with a magmatic contribution to Arawhata spring (5.13 R A ). It is common for magmatic helium to be observed at large distances from active volcanoes (Barry et al., 2013, 2019, 2021, 2022). This signature differs from sedimentary basin fluids in the Taranaki region that have air corrected 3 He/ 4 He values of 0.17–1.4 R A and δ 13 C‐CO 2 from −8 to −16.6‰, Giggenbach, 1997; Hilton et al., 2002).…”