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1. The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

   https://doi.org/10.1029/2021GL094301  

   Moynier, Frédéric ; Jackson, Matthew G. ; Zhang, Ke ; Cai, Hongming ; Halldórsson, Sæmundur Ari ; Pik, Raphael ; Day, James M. D. ; Chen, Jiubin ( September 2021 , Geophysical Research Letters)

   The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ¹⁹⁹Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ¹⁹⁹Hg for terrestrial sub‐aerially‐derived materials and positive Δ¹⁹⁹Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐³He/⁴He lavas from Samoa and Iceland, providing an average of Δ¹⁹⁹Hg = 0.00 ± 0.10, Δ²⁰¹Hg = −0.02 ± 0.0.09,δ²⁰²Hg = −1.7 ± 1.2; 2SD,N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ¹⁹⁹Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ¹⁹⁹Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ¹⁹⁹Hg which correlate with⁸⁷Sr/⁸⁶Sr, consistent with variable proportions of continental (low Δ¹⁹⁹Hg and high⁸⁷Sr/⁸⁶Sr) and oceanic (high Δ¹⁹⁹Hg and low⁸⁷Sr/⁸⁶Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions.

    

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2. 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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3. 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)

   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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4. Expedition 391 Preliminary Report: Walvis Ridge Hotspot

   https://doi.org/10.14379/iodp.pr.391.2022  

   Sager, W. ; Hoernle, K. ; Höfig, T.W. ( May 2022 , Preliminary report)

   Hotspot tracks (quasilinear chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are simple chains of seamounts that are often compared to chains of pearls, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts, and areas of what appear to be randomly dispersed seamounts. The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. The older ridge is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamount (guyot) province, to gather igneous rocks to better understand the formation of track edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard outbreak of the coronavirus disease 2019 (COVID-19) virus, Expedition 391 proceeded to drill at four of the proposed sites: three sites on the eastern Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the island of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to the island of Gough). One hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The latter comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first hole was terminated because a bit jammed shortly after penetrating igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Paleocene to Late Cretaceous (Campanian). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement yielded 11 submarine lava units ranging from pillows to massive flows, which have compositions varying from tholeiitic basalt to basaltic andesite, the first occurrence of this composition recovered from the TGW track. These units are separated by seven sedimentary chalk units that range in thickness from 0.1 to 11.6 m, implying a long-term interplay of sedimentation and lava eruptions. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154 m thick sedimentary section, the bottom ~108 m of which is Maastrichtian–Campanian (possibly Santonian) chalk with vitric tephra layers. Igneous basement coring progressed only 39.1 m below the sediment-basalt contact, recovering three massive submarine tholeiite basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomagnetic data from Sites U1577 and U1576 indicate that their volcanic basements formed just before the end of the Cretaceous Normal Superchron and during Chron 33r, shortly afterward, respectively. Biostratigraphic and paleomagnetic data suggest an east–west age progression across Valdivia Bank, becoming younger westward. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk, Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine, and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging in thickness from 0.46 to 10.19 m. Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Abundant glass from pillow lava margins was recovered at Sites U1575, U1576, and U1578. Although the igneous penetration was only two-thirds of the planned amount, drilling during Expedition 391 obtained samples that clearly will lead to a deeper understanding of the evolution of the Tristan-Gough hotspot and its track. Relatively fresh basalts with good recovery will provide ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study. 

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5. Expedition 391 summary

   Sager, W. ; Hoernle, K. ; Höfig, T.W. ; Avery, A.J. ; Bhutani, R. ; Buchs, D.M. ; Carvallo, C.A. ; Class, C. ; Dai, Y. ; Dalla Valle, G. ; et al ( October 2023 , Proceedings of the International Ocean Discovery Program Expedition reports)

   Hotspot tracks (chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are often simple, nearly linear chains of seamounts, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts (most are flat-topped guyots). The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. Walvis Ridge, forming the older part of the track, is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamounts of the Guyot Province, to collect igneous rocks to better understand the formation of volcanic edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard Coronavirus (COVID-19) outbreak, Expedition 391 proceeded to drill at four of the proposed sites: three sites on Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track of the Guyot Province, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the islands of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to Gough Island). The first hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge just north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The sediments ranged in age from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; 72–78 Ma). The igneous basement comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first of these holes was terminated because a bit jammed shortly after entering the igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Late Pleistocene (~0.43–1.24 Ma) to Late Cretaceous (Campanian; ~79–81.38 Ma). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement recovered in Hole U1576B yielded 11 submarine lava units (total thickness = ~65 m). The flows range from pillows to massive flows with compositions varying from tholeiitic basalt to basaltic andesite, only the second occurrence of the latter composition recovered from the TGW track thus far. These units are separated by seven sedimentary chalk units that range 0.1–11.6 m in thickness, implying a long-term interplay of sedimentation and lava eruptions. These intercalated sediments revealed Upper Cretaceous (Campanian) ages of ~77–79 Ma for the upper two interbeds and ~79–81.38 Ma for the lower beds. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154.8 m thick sedimentary section ranging from the Paleocene (Thanetian; ~58.8 Ma) to Upper Cretaceous (Campanian; ~81.43–83.20 Ma). Igneous basement coring progressed only 39.1 m below the sediment/basalt contact, recovering three massive submarine tholeiitic basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomagnetic data from Sites U1577 and U1576 indicate that the former volcanic basement formed just before the end of the Cretaceous Normal Superchron and the latter during Chron 33r, shortly afterward. Biostratigraphic and paleomagnetic data suggest that Valdivia Bank becomes younger from east to west. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk (~55.64–63.5 Ma), Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine (some fresh), and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging 0.46–10.19 m in thickness. Investigations of toothpick samples from the intercalated sediments were examined, each revealing the same age range of ~63.5–64.81 Ma (lower Paleocene; Danian). Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Nearly 7 months after Expedition 391, JOIDES Resolution transited from Cape Town to the north Atlantic. During this transit (Expedition 397T), 7.9 days of ship time were used to drill two holes (U1584A and U1585A) at sites on the Gough and Tristan tracks that had been omitted because of COVID-19–related time loss on the earlier cruise. For both, coring was begun only a short distance above the igneous basement to save time. The 75.2 m thick section drilled in Hole U1584A contains two sedimentary units: clay-rich carbonate sediments overlie a pumice-dominated volcaniclastic deposit containing basalt fragments. Because the goal was to core basalt and the base of the volcaniclastic deposit was not imaged in the seismic profile, the hole was terminated early to save operation time for the next site. In Hole U1585A, coring penetrated a 273.5 m thick sediment section overlying an 81.2 m thick pile of massive basalt flows. The sediment section is divided into four units: The uppermost unit consists of nannofossil chalk; The two intermediate units contain alternating chalk and volcaniclastic sediments containing several breccia units; and The lowermost unit consists of volcanic breccia containing juvenile blocks, bombs, and accretionary lapilli. This thick sedimentary section documents a transition from shallow-water volcanism to open-ocean sedimentation as the seamount subsided. The thick underlying basalt section is made up of four sparsely to highly phyric massive flows, the thickest of which is >43 m thick. Samples of these units are mostly basalt with a few trachybasalts and one trachyandesite. Although the igneous penetration was less than planned, coring during Expeditions 391 and 397T obtained samples that clearly will lead to an improved understanding of the evolution of the TGW hotspot and its track. Reasonable recovery of fresh basalt in some holes provides ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study. 

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