Bekaert David V.

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Last Name
Bekaert
First Name
David V.
ORCID
0000-0002-1062-6221

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Now showing 1 - 11 of 11
  • Article
    The vanadium isotope composition of Mars: Implications for planetary differentiation in the early solar system
    (European Association of Geochemistry, 2020-09-30) Nielsen, Sune G. ; Bekaert, David V. ; Magna, Tomas ; Mezger, Klaus ; Auro, Maureen E.
    The V isotope composition of martian meteorites reveals that Bulk Silicate Mars (BSM) is characterised by δ51V = −1.026 ± 0.029 ‰ (2 s.e.) and is thus ∼0.06 ‰ heavier than chondrites and ∼0.17 ‰ lighter than Bulk Silicate Earth (BSE). Based on the invariant V isotope compositions of all chondrite groups, the heavier V isotope compositions of BSE and BSM relative to chondrites are unlikely to originate from mass independent isotope effects or evaporation/condensation processes in the early Solar System. These differences are best accounted for by mass dependent fractionation during core formation. Assuming that bulk Earth and Mars both have a chondritic V isotopic compostion, mass balance considerations reveal V isotope fractionation factors Δ51Vcore-mantle as substantial as −0.6 ‰ for both planets. This suggests that V isotope systematics in terrestrial and extraterrestrial rocks potentially constitutes a powerful new tracer of planetary differentiation processes accross the Solar System.
  • Article
    High precision noble gas measurements of hydrothermal quartz reveal variable loss rate of Xe from the Archean atmosphere
    (Elsevier, 2022-05-09) Broadley, Michael W. ; Byrne, David J. ; Ardoin, Lisa ; Almayrac, Matthieu ; Bekaert, David V. ; Marty, Bernard
    Determining the composition of the Archean atmosphere and oceans is vital to understanding the environmental conditions that existed on the surface of the early Earth. The analysis of atmospheric remnants in fluid inclusions trapped in Archean-aged samples has shown that the Xe isotopic signature of the Archean atmosphere progressively evolved via mass-dependent fractionation, arriving at a modern atmospheric composition around the Archean-Proterozoic transition. The mechanisms driving this evolution are however not well constrained, and it is not yet clear whether the evolution proceeded continuously or via episodic bursts. Providing further constraints on the evolution of Xe in the Archean atmosphere is hampered by the limited amounts of atmospheric gas trapped within fluid inclusions during mineral formation, which impacts the precision at which the Archean atmosphere can be determined. Here, we develop a new crush-and-accumulate extraction technique that enables the heavy noble gases (Ar, Kr and Xe) released from crushing large quantities of hydrothermal quartz to be accumulated and analysed to a higher precision than was previously possible. Using this new technique, we re-evaluate the composition of atmospheric gases trapped within fluid inclusions of 3.3 Ga quartz samples from Barberton, South Africa. We find that the Xe isotopic signature is fractionated by +10.3 ± 1.0‰u−1 (2 SE) relative to modern atmosphere, which is within uncertainty of, but slightly lower than, the previous determination of 12.9 ± 2.4‰u−1 for this sample (Avice et al., 2017). We show for the first time that the Kr/Xe ratio measured within Archean quartz samples is enriched in Xe compared to the modern atmosphere, demonstrating that the atmosphere has lost Xe since the Archean. This further reinforces the proposal of atmospheric escape as the primary mechanism for Earth's Xe loss. We further show that the atmospheric Kr/Xe and Xe isotope fractionation recorded in the Barberton quartz at 3.3 Ga is incompatible with a model describing atmospheric loss at a continuous rate under a constant fractionation factor. This gives credence to numerical models of hydrodynamic escape, which suggest that Xe was lost from the Archean atmosphere in episodic bursts rather than at a constant rate. Refining the evolution curve of atmospheric Xe isotopes using the new technique presented here has the potential to shed light on discrete atmospheric events that punctuated the evolution of the Archean Earth and accompanied the evolution of life.
  • Article
    Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion
    (National Academy of Sciences, 2020-06-08) Broadley, Michael W. ; Barry, Peter H. ; Bekaert, David V. ; Byrne, David J. ; Caracausi, Antonio ; Ballentine, Christopher J. ; Marty, Bernard
    Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of 129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.
  • Article
    Isotopic evidence for the formation of the moon in a canonical giant impact
    (Nature Research, 2021-03-22) Nielsen, Sune G. ; Bekaert, David V. ; Auro, Maureen E.
    Isotopic measurements of lunar and terrestrial rocks have revealed that, unlike any other body in the solar system, the Moon is indistinguishable from the Earth for nearly every isotopic system. This observation, however, contradicts predictions by the standard model for the origin of the Moon, the canonical giant impact. Here we show that the vanadium isotopic composition of the Moon is offset from that of the bulk silicate Earth by 0.18 ± 0.04 parts per thousand towards the chondritic value. This offset most likely results from isotope fractionation on proto-Earth during the main stage of terrestrial core formation (pre-giant impact), followed by a canonical giant impact where ~80% of the Moon originates from the impactor of chondritic composition. Our data refute the possibility of post-giant impact equilibration between the Earth and Moon, and implies that the impactor and proto-Earth mainly accreted from a common isotopic reservoir in the inner solar system.
  • Article
    Fossil records of early solar irradiation and cosmolocation of the CAI factory: a reappraisal
    (American Association for the Advancement of Science, 2021-09-29) Bekaert, David V. ; Auro, Maureen E. ; Shollenberger, Quinn R. ; Liu, Ming-Chang ; Marschall, Horst R. ; Burton, Kevin W. ; Jacobsen, Benjamin ; Brennecka, Gregory A. ; MacPherson, Glenn J. ; von Mutius, Richard ; Sarafian, Adam ; Nielsen, Sune G.
    Calcium-aluminum–rich inclusions (CAIs) in meteorites carry crucial information about the environmental conditions of the nascent Solar System prior to planet formation. Based on models of 50V–10Be co-production by in-situ irradiation, CAIs are considered to have formed within ~0.1 AU from the proto-Sun. Here, we present vanadium (V) and strontium (Sr) isotopic co-variations in fine- and coarse-grained CAIs and demonstrate that kinetic isotope effects during partial condensation and evaporation best explain V isotope anomalies previously attributed to solar particle irradiation. We also report initial excesses of 10Be and argue that CV CAIs possess essentially a homogeneous level of 10Be, inherited during their formation. Based on numerical modeling of 50V–10Be co-production by irradiation, we show that CAI formation during protoplanetary disk build-up likely occurred at greater heliocentric distances than previously considered, up to planet-forming regions (~1AU), where solar particle fluxes were sufficiently low to avoid substantial in-situ irradiation of CAIs.
  • Article
    Groundwater residence time estimates obscured by anthropogenic carbonate
    (American Association for the Advancement of Science, 2021-04-21) Seltzer, Alan M. ; Bekaert, David V. ; Barry, Peter H. ; Durkin, Kathryn E. ; Mace, Emily K. ; Aalseth, Craig E. ; Zappala, Jake C. ; Mueller, Peter ; Jurgens, Bryant C. ; Kulongoski, Justin T.
    Groundwater is an important source of drinking and irrigation water. Dating groundwater informs its vulnerability to contamination and aids in calibrating flow models. Here, we report measurements of multiple age tracers (14C, 3H, 39Ar, and 85Kr) and parameters relevant to dissolved inorganic carbon (DIC) from 17 wells in California’s San Joaquin Valley (SJV), an agricultural region that is heavily reliant on groundwater. We find evidence for a major mid-20th century shift in groundwater DIC input from mostly closed- to mostly open-system carbonate dissolution, which we suggest is driven by input of anthropogenic carbonate soil amendments. Crucially, enhanced open-system dissolution, in which DIC equilibrates with soil CO2, fundamentally affects the initial 14C activity of recently recharged groundwater. Conventional 14C dating of deeper SJV groundwater, assuming an open system, substantially overestimates residence time and thereby underestimates susceptibility to modern contamination. Because carbonate soil amendments are ubiquitous, other groundwater-reliant agricultural regions may be similarly affected.
  • Article
    High (3)He/(4)He in central Panama reveals a distal connection to the Galápagos plume
    (National Academy of Sciences, 2021-11-23) Bekaert, David V. ; Gazel, Esteban ; Turner, Stephen ; Behn, Mark D. ; de Moor, J. Maarten ; Zahirovic, Sabin ; Manea, Vlad C. ; Hoernle, Kaj A. ; Fischer, Tobias P. ; Hammerstrom, Alexander ; Seltzer, Alan M. ; Kulongoski, Justin T. ; Patel, Bina S. ; Schrenk, Matthew O. ; Halldórsson, Saemundur ; Nakagawa, Mayuko ; Ramírez, Carlos J. ; Krantz, John A. ; Yucel, Mustafa ; Ballentine, Christopher J. ; Giovannelli, Donato ; Lloyd, Karen G. ; Barry, Peter H.
    It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high 3He/4He (up to 8.9RA) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source 3He/4He >10.3RA (and potentially up to 26RA, similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1RA). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high 3He/4He values in central Panama are likely derived from the infiltration of a Galápagos plume–like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a “mantle wind” toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits.
  • Article
    The helium and carbon isotope characteristics of the Andean Convergent Margin
    (Frontiers Media, 2022-06-13) Barry, Peter H. ; de Moor, J. Maarten ; Chiodi, Agostina ; Aguilera, Felipe ; Hudak, Michael R. ; Bekaert, David V. ; Turner, Stephen ; Curtice, Joshua ; Seltzer, Alan M. ; Jessen, Gerdhard L. ; Osses, Esteban ; Blamey, Jenny M. ; Amenabar, Maximiliano J. ; Selci, Matteo ; Cascone, Martina ; Bastianoni, Alessia ; Nakagawa, Mayuko ; Filipovich, Rubén ; Bustos, Emilce ; Schrenk, Matthew O. ; Buongiorno, Joy ; Ramirez, Carlos J. ; Rogers, Timothy J. ; Lloyd, Karen G. ; Giovannelli, Donato
    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.
  • Article
    Possible discontinuous evolution of atmospheric xenon suggested by Archean barites
    (Elsevier, 2021-06-25) Almayrac, Matthieu ; Broadley, Michael W. ; Bekaert, David V. ; Hofmann, Axel ; Marty, Bernard
    The Earth's atmosphere has continually evolved since its formation through interactions with the mantle as well as through loss of volatile species to space. Atmospheric xenon isotopes show a unique and progressive evolution during the Archean that stopped around the Archean-Proterozoic transition. The Xe isotope composition of the early atmosphere has been previously documented through the analysis of fluid inclusions trapped within quartz and barite. Whether this evolution was continuous or not is unclear, requiring additional analyses of ancient samples, which may potentially retain remnants of the ancient atmosphere. Here we present new argon, krypton and xenon isotopic data from a suite of Archean and Proterozoic barites ranging in age from 3.5 to 1.8 Ga, with the goal of providing further insights in to the evolution of atmospheric Xe, whilst also outlining the potential complications that can arise when using barites as a record of past atmospheres. Xenon released by low temperature pyrolysis and crushing of two samples which presumably formed around 2.8 and 2.6 Ga show Xe isotope mass dependent fractionation (MDF) of 11‰.u−1 and 3.4‰.u−1, respectively, relative to modern atmosphere. If trapped Xe is contemporaneous with the respective formation age, the significant difference in the degree of fractionation between the two samples provides supporting evidence for a plateau in the MDF-Xe evolution between 3.3 Ga and 2.8 Ga, followed by a rapid evolution at 2.8–2.6 Ga. This sharp decrease in MDF-Xe degree suggests the potential for a discontinuous temporal evolution of atmospheric Xe isotopes, which could have far reaching implications regarding current physical models of the early evolution of the Earth's atmosphere.
  • Article
    An evaluation of the C/N ratio of the mantle from natural CO2-rich gas analysis: Geochemical and cosmochemical implications
    (Elsevier, 2020-12-01) Marty, Bernard ; Almayrac, Matthieu ; Barry, Peter H. ; Bekaert, David V. ; Broadley, Michael W. ; Byrne, David J. ; Ballentine, Christopher J. ; Caracausi, Antonio
    The terrestrial carbon to nitrogen ratio is a key geochemical parameter that can provide information on the nature of Earth's precursors, accretion/differentiation processes of our planet, as well as on the volatile budget of Earth. In principle, this ratio can be determined from the analysis of volatile elements trapped in mantle-derived rocks like mid-ocean ridge basalts (MORB), corrected for fractional degassing during eruption. However, this correction is critical and previous attempts have adopted different approaches which led to contrasting C/N estimates for the bulk silicate Earth (BSE) (Marty and Zimmermann, 1999; Bergin et al., 2015). Here we consider the analysis of CO2-rich gases worldwide for which a mantle origin has been determined using noble gas isotopes in order to evaluate the C/N ratio of the mantle source regions. These gases experienced little fractionation due to degassing, as indicated by radiogenic 4He / 40Ar* values (where 4He and 40Ar* are produced by the decay of U+Th, and 40K isotopes, respectively) close to the mantle production/accumulation values. The C/N and C/3 He ratios of gases investigated here are within the range of values previously observed in oceanic basalts. They point to an elevated mantle C/N ratio (∼350-470, molar) higher than those of potential cosmochemical accretionary endmembers. For example, the BSE C/N and 36 Ar / N ratios (160-220 and 75 x 10-7, respectively) are higher than those of CM-CI chondrites but within the range of CV-CO groups. This similarity suggests that the Earth accreted from evolved planetary precursors depleted in volatile and moderately volatile elements. Hence the high C / N composition of the BSE may be an inherited feature rather than the result of terrestrial differentiation. The C / N and 36 Ar / N ratios of the surface (atmosphere plus crust) and of the mantle cannot be easily linked to any known chondritic composition. However, these compositions are consistent with early sequestration of carbon into the mantle (but not N and noble gases), permitting the establishment of clement temperatures at the surface of our planet.
  • Article
    Ultrahigh-precision noble gas isotope analyses reveal pervasive subsurface fractionation in hydrothermal systems
    (American Association for the Advancement of Science, 2023-03-16) Bekaert, David V. ; Barry, Peter H. ; Broadley, Michael W. ; Byrne, David J. ; Marty, Bernard ; Ramírez, Carlos J. ; de Moor, J Maarten ; Rodriguez, Alejandro ; Hudak, Michael R. ; Subhas, Adam V. ; Halldórsson, Saemundur A. ; Stefánsson, Andri ; Caracausi, Antonio ; Lloyd, Karen G. ; Giovannelli, Donato ; Seltzer, Alan M.
    Mantle-derived noble gases in volcanic gases are powerful tracers of terrestrial volatile evolution, as they contain mixtures of both primordial (from Earth's accretion) and secondary (e.g., radiogenic) isotope signals that characterize the composition of deep Earth. However, volcanic gases emitted through subaerial hydrothermal systems also contain contributions from shallow reservoirs (groundwater, crust, atmosphere). Deconvolving deep and shallow source signals is critical for robust interpretations of mantle-derived signals. Here, we use a novel dynamic mass spectrometry technique to measure argon, krypton, and xenon isotopes in volcanic gas with ultrahigh precision. Data from Iceland, Germany, United States (Yellowstone, Salton Sea), Costa Rica, and Chile show that subsurface isotope fractionation within hydrothermal systems is a globally pervasive and previously unrecognized process causing substantial nonradiogenic Ar-Kr-Xe isotope variations. Quantitatively accounting for this process is vital for accurately interpreting mantle-derived volatile (e.g., noble gas and nitrogen) signals, with profound implications for our understanding of terrestrial volatile evolution.