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1. Hot and Heterogenous High‐ ³ He/ ⁴ He Components: New Constraints From Proto‐Iceland Plume Lavas From Baffin Island

   https://doi.org/10.1029/2019GC008654  

   Willhite, Lori N.; Jackson, Matthew G.; Blichert‐Toft, Janne; Bindeman, Ilya; Kurz, Mark D.; Halldórsson, Sæmundur A.; Harðardóttir, Sunna; Gazel, Esteban; Price, Allison A.; Byerly, Benjamin L. (December 2019, Geochemistry, Geophysics, Geosystems)

   Abstract The Icelandic hotspot has erupted basaltic magma with the highest mantle‐derived³He/⁴He over a period spanning much of the Cenozoic, from the early‐Cenozoic Baffin Island‐West Greenland flood basalt province (49.8R_A), to mid‐Miocene lavas in northwest Iceland (40.2 to 47.5R_A), to Pleistocene lavas in Iceland's neovolcanic zone (34.3R_A). The Baffin Island lavas transited through and potentially assimilated variable amounts of Precambrian continental basement. We use geochemical indicators sensitive to continental crust assimilation (Nb/Th, Ce/Pb, MgO) to identify the least crustally contaminated lavas. Four lavas, identified as “least crustally contaminated,” have high MgO (>15 wt.%), and Nb/Th and Ce/Pb that fall within the mantle range (Nb/Th = 15.6 ± 2.6, Ce/Pb = 24.3 ± 4.3). These lavas have⁸⁷Sr/⁸⁶Sr = 0.703008–0.703021,¹⁴³Nd/¹⁴⁴Nd = 0.513094–0.513128,¹⁷⁶Hf/¹⁷⁷Hf = 0.283265–0.283284,²⁰⁶Pb/²⁰⁴Pb = 17.7560–17.9375,³He/⁴He up to 39.9R_A, and mantle‐like δ¹⁸O of 5.03–5.21‰. The radiogenic isotopic compositions of the least crustally contaminated lavas are more geochemically depleted than Iceland high‐³He/⁴He lavas, a shift that cannot be explained by continental crust assimilation in the Baffin suite. Thus, we argue for the presence oftwogeochemically distinct high‐³He/⁴He components within the Iceland plume. Additionally, the least crustally contaminated primary melts from Baffin Island‐West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than Siqueiros mid‐ocean ridge basalts (1300 to 1410 °C), which attests to a hot, buoyant plume origin for early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high‐³He/⁴He domain is dense, located in the deep mantle, and sampled by only the hottest plumes. 

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2. Wave Generation by Flare-accelerated Ions and Implications for ³ He Acceleration

   https://doi.org/10.3847/1538-4357/ad217f  

   Fitzmaurice, A.; Drake, J. F.; Swisdak, M. (March 2024, The Astrophysical Journal)

   Abstract The waves generated by high-energy proton and alpha particles streaming from solar flares into regions of colder plasma are explored using particle-in-cell simulations. Initial distribution functions for the protons and alphas consist of two populations: an energetic, streaming population represented by an anisotropic (T_∥>T_⊥), one-sided kappa function and a cold, Maxwellian background population. The anisotropies and nonzero heat fluxes of these distributions destabilize oblique waves with a range of frequencies below the proton cyclotron frequency. These waves scatter particles out of the tails of the initial distributions along constant-energy surfaces in the wave frame. Overlap of the nonlinear resonance widths allows particles to scatter into near-isotropic distributions by the end of the simulations. The dynamics of³He are explored using test particles. Their temperatures can increase by a factor of nearly 20. Propagation of such waves into regions above and below the flare site can lead to heating and transport of³He into the flare acceleration region. The amount of heated³He that will be driven into the flare site is proportional to the wave energy. Using values from our simulations, we show that the abundance of³He driven into the acceleration region should approach that of⁴He in the corona. Therefore, waves driven by energetic ions produced in flares are a strong candidate to drive the enhancements of³He observed in impulsive flares. 

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3. Variational inference of effective range parameters for ³ He− ⁴ He scattering

   https://doi.org/10.1088/1361-6471/ad9296  

   Burnelis, Andrius; Kejzlar, Vojtech; Phillips, Daniel R (December 2024, Journal of Physics G: Nuclear and Particle Physics)

   Abstract We use two different methods, Monte Carlo sampling and variational inference (VI), to perform a Bayesian calibration of the effective-range parameters in³He–⁴He elastic scattering. The parameters are calibrated to data from a recent set of³He–⁴He elastic scattering differential cross section measurements. Analysis of these data forE_lab≤ 4.3 MeV yields a unimodal posterior for which both methods obtain the same structure. However, the effective-range expansion amplitude does not account for the 7/2⁻state of⁷Be so, even after calibration, the description of data at the upper end of this energy range is poor. The data up toE_lab = 2.6 MeV can be well described, but calibration to this lower-energy subset of the data yields a bimodal posterior. After adapting VI to treat such a multi-modal posterior we find good agreement between the VI results and those obtained with parallel-tempered Monte Carlo sampling. 

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4. Geochemical Constraints on the Origin of Primitive Potassic Lavas in the Eastern Virunga Volcanic Province

   https://doi.org/10.1029/2023GC010950  

   Pitcavage, E.; Furman, T.; Nelson, W. R.; Graham, D. W.; Shirey, S.; Kulyanyingi, P. K.; Barifaijo, E. (June 2023, Geochemistry, Geophysics, Geosystems)

   Abstract Young mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data from monogenetic cinder cones near Bufumbira, Uganda, in the Virunga Volcanic Field:⁸⁷Sr/⁸⁶Sr = 0.7059–0.7079,ε_Nd = −6.5 to −1.3,ε_Hf = −6.3 to +0.9,²⁰⁸Pb/²⁰⁴Pb = 40.1–40.7,²⁰⁷Pb/²⁰⁴Pb = 15.68–15.75, and²⁰⁶Pb/²⁰⁴Pb = 19.27–19.45. Olivine phenocrysts from the Bufumbira lavas have³He/⁴He = 6.0–7.4R_A. The isotopic data, in conjunction with major and trace element systematics, indicate that primitive Bufumbira magmas are derived from two different metasomatized lithospheric source domains. Melts generated by lower degrees of melting record greater contributions from ∼1 to 2 Ga isotopically enriched garnet‐amphibole‐phlogopite pyroxenite veins within the lithosphere. As melting progresses, these vein melts become increasingly diluted by melts that originate near the lithosphere/asthenosphere boundary, shifting the isotopic compositions toward the common lithospheric mantle (CLM) proposed by Furman and Graham (1999,https://doi.org/10.1016/s0024-4937(99)00031-6). This ∼450–500 Ma source domain appears to underlie all Western Rift volcanic provinces and is characterized by⁸⁷Sr/⁸⁶Sr ∼ 0.705,ε_Nd∼ 0,ε_Hf∼ +1 to +3,²⁰⁶Pb/²⁰⁴Pb ∼ 19.0–19.2,²⁰⁸Pb/²⁰⁴Pb ∼ 39.7, and³He/⁴He ∼ 7R_A. Basal portions of the dense subcontinental lithospheric mantle may become gravitationally unstable and founder into underlying warmer asthenosphere, exposing surfaces where melting of locally heterogeneous veins produces small‐volume, alkaline mafic melts. Mafic lavas from all Western Rift volcanic provinces record mixing between the CLM and locally variable metasomatized source domains, suggesting this style of melt generation is fundamental to the development of magma‐poor rifts. 

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5. New ²⁶ P(p, γ) ²⁷ S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

   https://doi.org/10.3847/1538-4357/accf9c  

   Hou, S. Q.; Liu, J. B.; Trueman, T. C. L.; Li, J. G.; Pignatari, M.; Bertulani, C. A.; Xu, X. X. (June 2023, The Astrophysical Journal)

   Abstract Accurate nuclear reaction rates for²⁶P(p,γ)²⁷S are pivotal for a comprehensive understanding of therp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of²⁶P(p,γ)²⁷S because of the lack of nuclear mass and energy level structure information for²⁷S. We reevaluate this reaction rate using the experimentally constrained²⁷S mass, together with the shell model predicted level structure. It is found that the²⁶P(p,γ)²⁷S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances atE= 1.104, 1.597, and 1.777 MeV above the proton threshold in²⁷S. The new rate is overall smaller than the other previous rates from the Hauser–Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new²⁷S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in therp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of²⁷S/²⁶P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path²⁶P(p,γ)²⁷S(β⁺,ν)²⁷P is not as important as previously thought for producing²⁷P. The adoption of the new reaction rates for²⁶P(p,γ)²⁷S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements. 

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