High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K

Morgan, Leah E.; Ramos, D. P. Santiago; Davidheiser‐Kroll, Brett; Faithfull, John; Lloyd, Nicholas S.; Ellam, Rob M.; Higgins, John A.

doi:10.1039/c7ja00257b

Cited by 97 publications

(125 citation statements)

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“…This large depletion in 41 K is hard to explain by any known processes. The largest variation of K isotopes seen among terrestrial natural samples is only ~1‰, and this is due to low-temperature processes such as hydrothermal alteration and clay formation (Parendo et al, 2017;Morgan et al, 2018;Santiago Ramos et al, 2018). Such a large K isotopic fractionations has not yet been observed among carbonaceous and ordinary chondrites or differentiated lunar, martian and eucrite meteorites (Bloom et al, 2018;Tian et al, 2018;Ku and Jacobsen, 2019).…”

Section: The Origin Of the K Isotopic Fractionation In Aubritesmentioning

confidence: 99%

Potassium isotopic compositions of enstatite meteorites

Zhao

Lodders

Bloom

et al. 2019

Meteorit & Planetary Scien

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Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass-independent and mass-dependent isotopic fractionations. Due to the analytical challenges to obtain high-precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high-precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from −2.34‰ to −0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the Bulk Silicate Earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (−0.47 ±0.57‰) is indistinguishable from the BSE value (−0.48 ±0.03‰), thus further corroborating the isotopic similarity between Earth' building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent-body processing. Our sample of the main-group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ 41 K=−2.34 ±0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K-bearing sulfide mineral is predicted to be enriched in 39 K during equilibrium exchange with silicates.

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Section: The Origin Of the K Isotopic Fractionation In Aubritesmentioning

confidence: 99%

Potassium isotopic compositions of enstatite meteorites

Zhao

Lodders

Bloom

et al. 2019

Meteorit & Planetary Scien

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“…At that time, no variations between differentiated and undifferentiated meteorites were observed due to the relatively large analytical uncertainties using Secondary Ion Mass Spectrometry (SIMS). Since 2016, high-precision K isotope analytical methods using Multi-Collector Inductively-Coupled-Plasma Mass-Spectrometry (MC-ICP-MS) (Wang and Jacobsen, 2016a;Li et al, 2016;Morgan et al, 2018;Hu et al, 2018) revealed resolvable variations in K isotopic lab oven for two days through a two-step protocol (first in concentrated HF/HNO3 mixture and then in aqua regia). Fully digested samples were dried down and refluxed with 0.7 N HNO3.…”

Section: Introductionmentioning

confidence: 99%

Potassium isotope compositions of carbonaceous and ordinary chondrites: Implications on the origin of volatile depletion in the early solar system

Bloom¹,

Lodders²,

Chen³

et al. 2020

Geochimica et Cosmochimica Acta

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2020) Potassium isotope compositions of carbonaceous and ordinary chondrites: Implications on the origin of volatile depletion in the early solar system. Geochimica et Cosmochimica Acta, in press. ABSTRACT Among solar system materials there are variable degrees of depletion in moderately volatile elements (MVEs, such as Na, K, Rb, Cu, and Zn) relative to the proto-solar composition. Whether these depletions are due to nebular and/or parent-body (asteroidal or planetary) processes is still under debate. In order to help decipher the MVE abundances in early solar system materials, we conducted a systematic study of highprecision K stable isotopic compositions of a suite of whole-rock samples of wellcharacterized carbonaceous and ordinary chondrites. We analyzed 16 carbonaceous chondrites (CM1-2, CO3, CV3, CR2, CK4-5 and CH3) and 28 ordinary chondrites covering petrological types 3 to 6 and chemical groups H, L, and LL. We observed significant K isotope (δ 41 K) variations (−1.54 to 0.70 ‰) among the carbonaceous and ordinary chondrites. In general, the two major chondrite groups are distinct: The K isotope compositions of carbonaceous chondrites are largely higher than the Bulk Silicate Earth (BSE) value, whereas ordinary chondrites show K isotope compositions that are typically lower than the BSE value. Neither carbonaceous nor ordinary chondrites show clear/resolvable correlations between K isotopes and chemical groups, petrological types, shock levels, cosmic-ray exposure ages, fall/find occurrence, or terrestrial weathering. Importantly, the lack of a clear trend between K isotopes and K content among chondrites indicates that the K isotope fractionations were decoupled from the relative elemental K depletions, which is inconsistent with a single-stage partial vaporization or condensation process to account for these MVE depletion patterns among chondrites. The range of K isotope variations in the carbonaceous chondrites in this study is consistent with a fourcomponent (chondrule, refractory inclusion, matrix and water) mixing model that is able to explain the bulk elemental and isotopic compositions of the main carbonaceous chondrite groups, but requires a fractionation in K isotopic compositions in chondrules.We propose that the major control of the isotopic compositions of group averages is condensation and/or vaporization in pre-accretional (nebular) environments that is preserved in the compositional variation of chondrules. Parent-body processes, such as aqueous alteration, thermal metamorphism, and metasomatism, can mobilize K and affect the K isotopes in individual samples. In the case of the ordinary chondrites, the full range of K isotopic variations can only be explained by the combined effects of the size and relative abundances of chondrules, parent-body aqueous and thermal alteration, and possible sampling bias.compositions between different planetary materials (e.g., Earth and Moon) that were subsequently used to better understand the accretion mechanism of different planetary bodies.

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“…Potassium has two stable isotopes ( 39 K, 93.358% and 41 K, 6.730%) that differ by 5% in mass, making K isotopes useful tracers of silicate weathering. Although K isotopes have historically been difficult to measure with high precision, recent analytical advances now permit measurements of K isotopes at the scale of sub-permil precision [1][2][3][4][5]. The K isotopic composition of modern seawater is an important reference point to consider when using K isotopes to trace silicate weathering records in deep time; however, it is not yet well-constrained.…”

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confidence: 99%

“…The K isotopic composition of seawater also plays a pivotal role in understanding the global K budget, where seawater constitutes the isotopically heaviest terrestrial K reservoir at~0.6‰ higher than common igneous rocks [1][2][3][4][5][6][7][8]. This significant isotopic difference between seawater and silicate rocks requires either input of isotopically heavy K from rivers [9] and oceanic ridge hydrothermal fluids, or preferential removal of isotopically-light K to altered oceanic crust and marine sediments [10], or a combination of both.…”

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confidence: 99%

“…Alternatively, this difference may suggest that K isotopic distribution in global oceans is not in steady state. Published data on seawater are still scarce and differ by 0.14‰ [1][2][3][4]6]. A more systematic study on global seawater samples is thus needed to assess the homogeneity of K isotope distribution in the oceans, which will also advance understanding of the fluxes and processes related to the oceanic K budget and global K cycling.…”

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