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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)

   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. Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

   https://doi.org/10.1029/2022JB026265  

   Banik, Tenley J. ; Coble, Matthew A. ; Miller, Calvin F. ; Fisher, Christopher M. ; Jordan, Brennan T. ; Vervoort, Jeffrey D. ; Carley, Tamara L. ( June 2023 , Journal of Geophysical Research: Solid Earth)

   Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ¹⁸O∼+2 to +4‰ and indicate involvement of a low‐¹⁸O component in their source magmas. Hrafnsfjörður zircon Hf (mean sampleε_Hf∼ +15.3–16.0) and whole rock Hf and Nd (ε_Hf = +14.5 to +15;ε_Nd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sampleε_Hf∼ +11.8–13; whole rockε_Hf = +12.8 to +15.1;ε_Nd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (^208/204Pb = 37.95–37.96;^206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (^208/204Pb = 38.34–38.48;^206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions ofε_Hf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

    

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3. Eruption Rates, Tempo, and Stratigraphy of Paleocene Flood Basalts on Baffin Island, Canada

   https://doi.org/10.1029/2021GC010172  

   Biasi, Joseph ; Asimow, Paul ; Horton, Forrest ; Boyes, Xenia ( September 2022 , Geochemistry, Geophysics, Geosystems)

   High‐temperature melting in mantle plumes produces voluminous eruptions that are often temporally coincident with mass extinctions. Paleocene Baffin Island lavas—products of early Iceland mantle plume activity—are exceptionally well characterized geochemically but have poorly constrained stratigraphy, geochronology, and eruptive tempos. To provide better geologic context, we measured seven stratigraphic sections of the volcanic deposits and collected paleomagnetic data from 38 sites in the lavas and underlying Cretaceous sediments (Quqaluit Fm.). The average paleomagnetic pole from this study does not overlap with the expected pole for a stable North American locality at 60 Ma, yet the data have sufficient dispersion to average out secular variation. After ruling out other possibilities, we find that the picrites were probably erupted during a polarity transition, over less than 5 kyr. If so, the average eruption interval was ∼67 years per flow for the thickest sequence of exposed lavas. We also calculate that the flood basalts had a minimum total volume of ∼176 km³(excluding submerged lavas in Baffin Bay). This implies a minimum eruption rate of ∼0.035 km³ yr⁻¹, which is similar to rates found in West Greenland lavas but less than rates found in larger flood basalts. Despite this, the Baffin and West Greenland lavas temporally correlate with the “End C27n event” (a period of ∼2°C global warming) and may be its underlying cause.

    

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4. The multiple depleted mantle components in the Hawaiian-Emperor chain

   https://doi.org/10.1016/j.chemgeo.2019.119324  

   Harrison, L ; Weis, D ; Garcia, M.O. ( January 2020 , Chemical geology)

   null (Ed.)

   Oceanic island basalts are targeted for geochemical study because they provide a direct window into mantle composition and a wealth of information on the dynamics and timescales associated with Earth mixing. Previous studies mainly focused on the shield volcanic stage of oceanic islands and the more fusible, enriched mantle components that are easily distinguished in those basalts. Mantle depleted compositions are typically more difficult to resolve unless large amounts of this material participated in mantle melting (e.g., mid-ocean ridges), or unique processes allow for their compositions to be erupted undiluted, such as very small degrees of melting of a source with minimal fusible enriched components (e.g., rejuvenated basalts) or as xenoliths (e.g., abyssal peridotites). Mantle depleted components, defined here as material with low time-integrated Rb/Sr (low 87Sr/86Sr) and high time-integrated Sm/Nd and Lu/Hf ratios (high 143Nd/144Nd and 176Hf/177Hf) relative to primitive mantle, derive from a potentially very large volume reservoir (up to 80% of the mantle), and therefore need adequate characterization in order estimate the composition of the Earth and mantle-derived melts. This review focuses on mantle depleted compositions in oceanic island basalts using the Hawaiian-Emperor chain as a case study. The Hawaiian-Emperor chain is the ∼6000 km long geological record of the deeply sourced Hawaiian mantle plume, active for>81 Myr. Hawaiian volcanism evolves through four volcanic stages as a volcano traverses the Hawaiian plume: alkalic preshield, tholeiitic shield (80–90% volcano volume), alkalic postshield (∼1%), and silica undersaturated rejuvenated (< 0.1%). We report Pb-Sr-Nd-Hf isotope compositions and trace element concentrations of three rejuvenated Northwest Hawaiian Ridge basalts and compare them to an exhaustive compiled dataset of basalts from the Hawaiian Islands to the Emperor Seamounts. The Northwest Hawaiian Ridge (NWHR) includes 51 volcanoes spanning ∼42 m.y. between the bend in the Hawaiian-Emperor chain and the Hawaiian Islands where there is no high-precision isotopic data published on the rejuvenated-stage over ∼47% of the chain. NWHR and Hawaiian Island rejuvenated basalts are geochemically similar, indicating a consistent source for rejuvenated volcanism over ∼12.5 million years. In contrast, shield-stage basalts from the oldest Emperor Seamounts are more depleted in isotopic composition (i.e., higher 176Hf/177Hf, and 143Nd/144Nd with lower 87Sr/86Sr and 208Pb*/206Pb*) and trace element concentrations (i.e., much lower concentrations of highly incompatible elements) than all other depleted Hawaiian basalts younger than the bend, including NWHR rejuvenated basalts. The strongly depleted source for the oldest Emperor Seamounts (> 70 Ma) was likely related to interaction with the Kula-Pacific-Izanagi mid-ocean ridge spreading system active near the Hawaiian plume in the Late Cretaceous. In contrast, the incompatible trace element ratios of NWHR rejuvenated basalts require a distinct source in the Hawaiian mantle plume that was imprinted by ancient (> 1 Ga) partial melting, likely ancient recycled oceanic lithosphere. This review of the geochemistry of Hawaiian depleted components documents the need for the sampling of multiple distinctive depleted compositions, each preferentially melted during specific periods of Hawaiian plume activity. This suggests that the composition of depleted components can evolve during the lifetime of the mantle plume, as observed for enriched components in the Hawaiian mantle plume. Changes in the composition of depleted components are dominantly controlled by the upper mantle tectonic configurations at the time of eruption (i.e., proximity to a mid-ocean ridge), as this effect overwhelms the signal imparted by potentially sampling different lower mantle components through time. 

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5. Near-zero 33S and 36S anomalies in Pitcairn basalts suggest Proterozoic sediments in the EM-1 mantle plume

   https://doi.org/10.1016/j.epsl.2022.117422  

   Labidi, Jabrane ; Dottin, James W. ; Clog, Matthieu ; Hemond, Christophe ; Cartigny, Pierre ( April 2022 , Earth and planetary science letters)

   Volcanic rocks erupted among Pitcairn seamounts sample a mantle plume that exhibits an extreme Enriched Mantle-1 signature. The origin of this peculiar mantle endmember remains contentious, and could involve the recycling of marine sediments of Archean or Proterozoic ages, delaminated units from the lower continental crust, or metasomatized peridotites from a lithospheric mantle. Here, we report the sulfur multi-isotopic signature (32S, 33S, 34S, 36S) of 15 fresh submarine basaltic glasses from three Pitcairn seamounts. We observe evidence for magmatic degassing of sulfur from melts erupted ∼2,000 meters below seawater level (mbsl). Sulfur concentrations are correlated with eruption depth, and range between 1300 ppm S (collected ∼ 2,500 mbsl) and 600 ppm S (∼2,000 mbsl). The δ34S values can be accounted for under equilibrium isotope fractionation during degassing, with αgas-melt between 1.0020 and 1.0001 and starting δ34S values between −0.9‰ and +0.6‰. The δ34S estimates are similar or higher than MORB signatures, suggesting the contribution of recycled sulfur with a ∼ 1‰ 34S enrichment compared to the Pacific upper mantle. The Δ33S and Δ36S signatures average at +0.024±0.007‰ and +0.02±0.07‰ vs. CDT, respectively (all 1σ). Only Δ33S is statistically different from MORB, by +0.02‰. The Δ33S enrichment is invariant across degassing and sulfide segregation. We suggest it reflects a mantle source enrichment rather than a high-temperature fractionation of S in the basalts. Despite the small magnitude of the 33S-36S variations, our data require a substantial amount of recycled sulfur overwhelm the Pitcairn mantle source. We show that models involving metasomatized peridotites, lower crust units, or Archean sediments, may be viable, but are restricted to narrow sets of circumstances. Instead, scenarios involving the contribution of Proterozoic marine sediments appear to be the most parsimonious explanation for the EM-1 signature at Pitcairn. 

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