More Like this

1. Highly siderophile element evidence for mantle plume involvement during opening of the Atlantic Ocean

   https://doi.org/10.1016/j.epsl.2024.118768  

   Day, James MD; Woodland, Sarah J; Nutt, Kimberley L; Stroncik, Nicole; Larsen, Lotte M; Trumbull, Robert B; Pearson, D Graham (September 2024, Earth and Planetary Science Letters)

   Osmium isotope and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for picrites and basalts from the ∼132 Ma Etendeka large igneous province (LIP) and the ∼60 Ma North Atlantic Igneous Province (NAIP). Picrite dykes of the Etendeka LIP have HSE abundances and 187Os/188Os (0.1276 to 0.1323; γOsi = -0.5 to +3.1) consistent with derivation from high-degree partial melting (>20 %) of a peridotite source with chondritic to modestly supra-chondritic long-term Re/Os. High-3He/4He NAIP picrites from West Greenland represent large-degree partial melts with similarly elevated HSE abundances and 187Os/188Os (0.1273 to 0.1332; γOsi = -0.2 to +3.9). Broadly chondritic Os isotope ratios have also been reported for the ∼132 Ma Paraná LIP and the ∼201 Ma Central Atlantic Magmatic Province (CAMP). Consequently, LIP associated with Atlantic Ocean opening derive, at least in part, from partial melting of peridotite mantle distinct from the depleted mantle associated with mid-ocean ridge basalt volcanism. Modern locations with high-3He/4He (>25RA) include ocean island basalts (OIB) from Ofu (Samoa), Loihi (Hawaii) and Fernandina (Galapagos) in the Pacific Ocean, and from Iceland, which is considered the modern manifestation of NAIP magmatism. Unlike Etendeka and NAIP picrites, these modern OIB have Sr-Nd-Pb-Os isotopes consistent with contributions of recycled oceanic or continental crust. The lower degree of partial melting responsible for modern high-3He/4He OIB gives higher proportions of fusible recycled crustal components to the magmas, with radiogenic 187Os/188Os and low-3He/4He. The high-3He/4He, incompatible trace element-depleted mantle component in both LIP and OIB therefore also has long-term chondritic Re/Os, which is consistent with an early-formed reservoir that experienced late accretion. Atlantic LIP (CAMP; Paraná-Etendeka; NAIP) provide geochemical evidence for a prominent role for mantle plume contributions during continental break-up and formation of the Atlantic Ocean, a feature hitherto unrecognized in other ocean basin-forming events. 

   more » « less
2. Mantle melting in regions of thick continental lithosphere: Examples from Late Cretaceous and younger volcanic rocks, Southern Rocky Mountains, Colorado (USA)

   https://doi.org/10.1130/Ges02749.1  

   Farmer, G Lang; Morgan, Leah; Cosca, Michael; Mize, James; Bailley, Treasure; Turner, Kenzie; Mercer, Cameron; Ellison, Eric; Bell, Aaron (September 2024, Geosphere)

   Abstract Major- and trace-element data together with Nd and Sr isotopic compositions and 40Ar/39Ar age determinations were obtained for Late Cretaceous and younger volcanic rocks from north-central Colorado, USA, in the Southern Rocky Mountains to assess the sources of mantle-derived melts in a region underlain by thick (≥150 km) continental lithosphere. Trachybasalt to trachyandesite lava flows and volcanic cobbles of the Upper Cretaceous Windy Gap Volcanic Member of the Middle Park Formation have low εNd(t) values from −3.4 to −13, 87Sr/86Sr(t) from ~0.705 to ~0.707, high large ion lithophile element/high field strength element ratios, and low Ta/Th (≤0.2) values. These characteristics are consistent with the production of mafic melts during the Late Cretaceous to early Cenozoic Laramide orogeny through flux melting of asthenosphere above shallowly subducting and dehydrating oceanic lithosphere of the Farallon plate, followed by the interaction of these melts with preexisting, low εNd(t), continental lithospheric mantle during ascent. This scenario requires that asthenospheric melting occurred beneath continental lithosphere as thick as 200 km, in accordance with mantle xenoliths entrained in localized Devonian-age kimberlites. Such depths are consistent with the abundances of heavy rare earth elements (Yb, Sc) in the Laramide volcanic rocks, which require parental melts derived from garnet-bearing mantle source rocks. New 40Ar/39Ar ages from the Rabbit Ears and Elkhead Mountains volcanic fields confirm that mafic magmatism was reestablished in this region ca. 28 Ma after a hiatus of over 30 m.y. and that the locus of volcanism migrated to the west through time. These rocks have εNd(t) and 87Sr/86Sr(t) values equivalent to their older counterparts (−3.5 to −13 and 0.7038–0.7060, respectively), but they have higher average chondrite-normalized La/Yb values (~22 vs. ~10), and, for the Rabbit Ears volcanic field, higher and more variable Ta/Th values (0.29–0.43). The latter are general characteristics of all other post–40 Ma volcanic rocks in north-central Colorado for which literature data are available. Transitions from low to intermediate Ta/Th mafic volcanism occurred diachronously across southwest North America and are interpreted to have been a consequence of melting of continental lithospheric mantle previously metasomatized by aqueous fluids derived from the underthrusted Farallon plate. Melting occurred as remnants of the Farallon plate were removed and the continental lithospheric mantle was conductively heated by upwelling asthenosphere. A similar model can be applied to post–40 Ma magmatism in north-central Colorado, with periodic, east to west, removal of stranded remnants of the Farallon plate from the base of the continental lithospheric mantle accounting for the production, and western migration, of volcanism. The estimated depth of the lithosphere-asthenosphere boundary in north-central Colorado (~150 km) indicates that the lithosphere remains too thick to allow widespread melting of upwelling asthenosphere even after lithospheric thinning in the Cenozoic. The preservation of thick continental lithospheric mantle may account for the absence of oceanic-island basalt–like basaltic volcanism (high Ta/Th values of ~1 and εNd[t] > 0), in contrast to areas of southwest North America that experienced larger-magnitude extension and lithosphere thinning, where oceanic-island basalt–like late Cenozoic basalts are common. 

   more » « less
3. Chemical Stratigraphy of IODP Expedition 391 Site U1578, Guyot Province, Tristan-Gough-Walvis Ridge Hotspot Track with Links to Physical Properties Measurements

   Potter, Katherine E; Shervais, J W; Nelson, W R; White, D; Scholpp, J; Robertson, S; Widdowson, M; Buchs, D; Tshiningayamwe, M; Kubota, Y; et al (December 2023, American Geophysical Union)

   Walvis Ridge, a time-transgressive series of ridges, oceanic plateau, seamounts, guyots, and two active volcanic islands extending SW from the coast of Namibia, records the evolution of the Tristan-Gough-Walvis Ridge (TGW) hotspot and the opening of the South Atlantic since ~135 Ma. However, much of our current understanding of the interplay between geodynamic cycles, tectonism, and mantle plume generation along the TGW hotspot track is based upon a limited number of dredged rock samples. Here, we present preliminary whole rock major and trace element geochemistry and shipboard physical properties data from Site U1578, located on a Center track guyot in the Guyot Province. The 302 m of igneous section recovered from Site U1578 provides an extraordinary, > 1 Myr record of plume magmatism, submarine volcanism, and geochemical evolution. The chemical stratigraphy of core from Site U1578 provides important new perspectives on submarine volcanism, magma flux, and the transition between continental tholeiitic basalts of the Etendeka flood basalt province and alkaline lavas of the Guyot Province. Core from U1578 records the longest sequence of pillow, sheet, and massive lava flows in the TGW system. Eleven (of 12 total) lithologic flow units record shifts in major and trace element geochemistry and episodic cycles of recharge and fractional crystallization. Preliminary XRF and ICP-MS analyses indicate a dominantly pyroxenite source and document the shift between high TiO2 (>3.5 wt. %) to low TiO2 (<3.5 wt. %) alkaline basalts. Site U1578 core samples from the Guyot Province have lower Nb/Y and Zr/Nb compared to Walvis Ridge sites drilled closer to the African continent (Frio Ridge at Site U1575 and Valdivia Bank at Sites U1576 and U1577), coincident with a transition from plume-ridge interaction to intraplate magmatism with time. This shift resulted in a thicker lithospheric lid and thus deeper and lower degrees of melting, preferentially sampling the enriched plume component. Additionally, shipboard natural gamma radiation (NGR) and magnetic susceptibility (MS) measurements correlate well with observed lithologic characteristics and new ICP-MS and XRF analyses. A 100 m zone of high NGR values neatly overlaps high K2O, and olivine cumulate layers correlate to higher MS and higher concentrations of Cr and Ni. 

   more » « less
4. Basaltic Pulses and Lithospheric Thinning—Plio‐Pleistocene Magmatism and Rifting in the Turkana Depression (East African Rift System)

   https://doi.org/10.1029/2024JB029166  

   Cancel_Vazquez, Sahira M; Rooney, Tyrone O; Brown, Eric L; Bollinger, Andrew; Bastow, Ian D; Steiner, R Alex; Kappelman, John (August 2024, Journal of Geophysical Research: Solid Earth)

   Abstract The East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain. 

   more » « less
5. Geodynamic Evolution of the East African Superplume: Insights From Volcanism in the Western Turkana Basin

   https://doi.org/10.1029/2023JB026755  

   Cai, Yue; Mana, Sara; Cox, Stephen E.; Beck, Catherine C.; Feibel, Craig; Hanley, Jean; Liu, Tanzhuo; Bolge, Louise; Hemming, Sidney; Goldstein, Steven L. (September 2023, Journal of Geophysical Research: Solid Earth)

   Abstract There is a consensus that volcanism along the East African Rift System (EARS) is related to plume activities. However, because of our limited knowledge of the local lithospheric mantle, the dynamics of the plume are poorly constrained by magma chemistry. The Turkana Basin is one of the best places to study plume‐related volcanism because the lithospheric mantle there is unusually thin. New Ar‐Ar geochronology and geochemical data on lavas from western Turkana show that Eocene volcanics have relatively low²⁰⁶Pb/²⁰⁴Pb (<19.1) and high εNd (>3.78). Their relatively high Ba/Rb (35–78) ratios suggest contributions from the shallow lithospheric mantle. Oligo‐Miocene Turkana volcanics have HIMU‐ and EMI‐ type enriched mantle signatures with overall lower Ba/Rb ratios, which is consistent with partial melting of plume material. Pliocene and younger Turkana volcanics have low Ba/Rb and Sr‐Nd‐Pb isotope ratios that resemble those of Ethiopian volcanics with elevated³He/⁴He ratios. This temporal variation can be reconciled with a layered plume model where an outer layer of ancient recycled oceanic crust and sediment overlies more primitive lower mantle material. Beneath Ethiopia, the outer layer of the plume is either missing or punctured by the delamination of the thicker overlying lithospheric mantle atca.30 Ma, an event that would have facilitated the rapid upwelling of the inner portion of the plume and triggered the Ethiopian flood volcanism. The outer layer of the plume may be thicker in the southern EARS, which could explain the occurrence of young HIMU‐ and EMI‐type volcanics with primordial noble gas signatures. 

   more » « less