Title: Passive degassing of lithospheric volatiles recorded in shallow young groundwater
Abstract The development of life on Earth has been enabled by its volatile-rich surface. The volatile budget of Earth’s surface is controlled by the balance between ingassing (for example, via subduction) and outgassing (for example, through magmatic and tectonic processes). Although volatiles within Earth’s interior are relatively depleted compared to CI chondrites, the total amount of volatiles within Earth is still substantial due to its vast size. However, the relative extent of diffuse degassing from Earth’s interior, not directly related to volcanism, is not well constrained. Here we use dissolved helium and high-precision argon isotopes combined with radiocarbon of dissolved inorganic carbon in groundwater from the Columbia Plateau Regional Aquifer (Washington and Idaho, USA). We identify mantle and crustal volatile sources and quantify their fluxes to the surface. Excess helium and argon in the groundwater indicate a mixture of sub-continental lithospheric mantle and crustal sources, suggesting that passive degassing of the sub-continental lithospheric mantle may be an important, yet previously unrecognized, outgassing process. This finding that considerable outgassing may occur even in volcanically quiescent parts of the crust is essential for quantifying the long-term global volatile mass balance. more »« less
The continental crust is a major geochemical reservoir, the evolution of which has shaped the surface environment of Earth. In this study, we present a new model of coupled crust-mantle-atmosphere evolution to constrain the growth of continental crust with atmospheric 40 Ar/ 36 Ar. Our model is the first to combine argon degassing with the thermal evolution of Earth in a self-consistent manner and to incorporate the effect of crustal recycling and reworking using the distributions of crustal formation and surface ages. Our results suggest that the history of argon degassing favors rapid crustal growth during the early Earth. The mass of continental crust, highly enriched in potassium, is estimated to have already reached >80% of the present-day level during the early Archean. The presence of such potassium-rich, likely felsic, crust has important implications for tectonics, surface environment, and the regime of mantle convection in the early Earth.
Barry, P. H.; De Moor, J. M.; Chiodi, A.; Aguilera, F.; Hudak, M. R.; Bekaert, D. V.; Turner, S. J.; Curtice, J.; Seltzer, A. M.; Jessen, G. L.; et al
(, Frontiers in Earth Science)
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 R A ( 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 R A ( n = 19). Taken together, these data reveal a clear southeastward increase in 3 He/ 4 He, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 ± 1 R A ), approaching the mean He isotope value for arc gases of (5.4 ± 1.9 R A ). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of 4 He) 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 δ 13 C varies between −15.3‰ and −1.2‰ [vs. Vienna Pee Dee Belemnite (VPDB)] and CO 2 / 3 He values that vary by over two orders of magnitude (6.9 × 10 8 –1.7 × 10 11 ). In the SVZ, carbon isotope ratios are also reported from ( n = 13) sites and vary between −17.2‰ and −4.1‰. CO 2 / 3 He values vary by over four orders of magnitude (4.7 × 10 7 –1.7 × 10 12 ). Low δ 13 C and CO 2 / 3 He values are consistent with CO 2 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.
Lopez, Taryn; Fischer, Tobias P.; Plank, Terry; Malinverno, Alberto; Rizzo, Andrea L.; Rasmussen, Daniel J.; Cottrell, Elizabeth; Werner, Cynthia; Kern, Christoph; Bergfeld, Deborah; et al
(, Science Advances)
Subduction transports volatiles between Earth’s mantle, crust, and atmosphere, ultimately creating a habitable Earth. We use isotopes to track carbon from subduction to outgassing along the Aleutian-Alaska Arc. We find substantial along-strike variations in the isotopic composition of volcanic gases, explained by different recycling efficiencies of subducting carbon to the atmosphere via arc volcanism and modulated by subduction character. Fast and cool subduction facilitates recycling of ~43 to 61% sediment-derived organic carbon to the atmosphere through degassing of central Aleutian volcanoes, while slow and warm subduction favors forearc sediment removal, leading to recycling of ~6 to 9% altered oceanic crust carbon to the atmosphere through degassing of western Aleutian volcanoes. These results indicate that less carbon is returned to the deep mantle than previously thought and that subducting organic carbon is not a reliable atmospheric carbon sink over subduction time scales.
Elazar, Oded; Pearson, D Graham; Moynier, Frederick; Hu, Yan; Nielsen, Sune G; Middleton, Julien T; Auro, Maureen; Jaques, A Lynton; Shirey, Steven B
(, Journal of Internaltional KImberlite Conference Abstracts (JIKCA))
Mantle-derived, low-degree melts, such as kimberlites, carbonate-rich olivine lamproites (CROLS), and cratonic olivine lamproites, are the main carriers of diamonds. They are rare ultramafic, volatile-rich volcanic magmas, generally restricted to stable cratons, and are the deepest-sourced magmas erupted onto Earth’s surface. As hybrid magmas, their formation mechanism and mantle sources remain enigmatic and highly debated, especially the nature of the processes leading to their “enriched” isotopic signatures. The often extreme isotopic compositions of Sr, Nd, Pb, and Hf suggest that the mantle sources of these magmas vary between an ancient and geochemically depleted component and various enriched components. The enriched components could include crustal material recycled into the convective mantle or metasomatized lithospheric mantle. For the latter, discriminating between assimilation by sub-lithospheric magmas during the ascent or melting of element-enriched material from within the lithospheric mantle is paramount concerning petrogenesis. As the stable isotope composition of K, and Ba vary between surface and mantle reservoirs, they are well-suited tools for addressing the cause of different radiogenic isotopic signatures and to better constrain the mantle sources of these important magmas. Here, we use collision cell multi-collector inductively-coupled-plasma mass-spectrometry (MC-ICP-MS) and traditional MC-ICP-MS to conduct the first comprehensive whole-rock K and Ba stable isotope study on a wide range of low-degree mantle-derived melts. All the deep-seated, low-degree melts analyzed here show no correlation between melting/differentiation indices and δ41K and δ138Ba compositions, implying that any isotopic fractionation during melting or eruption was limited and that the different mantle and crustal reservoirs affecting these melts dominate their isotopic variability. Overall, kimberlites show limited δ41K and δ138Ba variability, with a median δ41K of -0.40 ± 0.06‰ (2SE) and δ138Ba of 0.00 ± 0.07‰ (2SE), within error relative to an estimated bulk silicate Earth [(BSE: δ41K= -0.42±0.07‰ (2SD) and δ138Ba=0.03±0.04‰ (2SD)], suggesting significant sublithospheric input. While the sample size is small (N=4), Canadian kimberlites from Lake De Gras display a bi-modal distribution with δ41K values slightly higher and lower relative to BSE, ascribed to crustal and lithospheric contamination. Like kimberlites, South African CROLS show limited K isotope variability with a median δ41K of -0.48 ± 0.02‰ (2SE). Their compositions are non-resolvable from two Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) xenoliths. The δ138Ba of the CROLS also shows limited variation with a median δ138Ba of 0.00 ± 0.07‰ (2SE), plotting within BSE estimations. Compared to the other low-degree mantle-derived melts, cratonic olivine/leucite-bearing lamproites from West Australia show a wide range in δ41K (-0.97‰ to +0.34‰) and δ138Ba (-0.30‰ to +0.27) values. The observed large K isotopic variation in cratonic lamproites is similar to that observed in post-collisional lamproites and is ascribed to sediment recycling. Argyle lamproites define robust correlations between potassium and barium elemental abundances, and their stable isotopes call for significant hydrothermal fluid-assisted leaching and isotopic fractionation.
Abstract The existence of a thin, weak asthenospheric layer beneath Earth’s lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.
@article{osti_10598275,
place = {Country unknown/Code not available},
title = {Passive degassing of lithospheric volatiles recorded in shallow young groundwater},
url = {https://par.nsf.gov/biblio/10598275},
DOI = {10.1038/s41561-025-01702-7},
abstractNote = {Abstract The development of life on Earth has been enabled by its volatile-rich surface. The volatile budget of Earth’s surface is controlled by the balance between ingassing (for example, via subduction) and outgassing (for example, through magmatic and tectonic processes). Although volatiles within Earth’s interior are relatively depleted compared to CI chondrites, the total amount of volatiles within Earth is still substantial due to its vast size. However, the relative extent of diffuse degassing from Earth’s interior, not directly related to volcanism, is not well constrained. Here we use dissolved helium and high-precision argon isotopes combined with radiocarbon of dissolved inorganic carbon in groundwater from the Columbia Plateau Regional Aquifer (Washington and Idaho, USA). We identify mantle and crustal volatile sources and quantify their fluxes to the surface. Excess helium and argon in the groundwater indicate a mixture of sub-continental lithospheric mantle and crustal sources, suggesting that passive degassing of the sub-continental lithospheric mantle may be an important, yet previously unrecognized, outgassing process. This finding that considerable outgassing may occur even in volcanically quiescent parts of the crust is essential for quantifying the long-term global volatile mass balance.},
journal = {Nature Geoscience},
volume = {18},
number = {6},
publisher = {Nature Publishing Group},
author = {Tyne, R_L and Broadley, M_W and Bekaert, D_V and Barry, P_H and Warr, O. and Langman, J_B and Musan, I. and Jenkins, W_J and Seltzer, A_M},
}
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