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1. Post-spreading Basalts from the Nanyue Seamount: Implications for the Involvement of Crustal- and Plume-Type Components in the Genesis of the South China Sea Mantle

   https://doi.org/10.3390/min9060378  

   Zheng, Hao; Zhong, Li-Feng; Kapsiotis, Argyrios; Cai, Guan-Qiang; Wan, Zhi-Feng; Xia, Bin (June 2019, Minerals)

   Fresh samples of basalts were collected by dredging from the Nanyue intraplate seamount in the Southwest sub-basin of the South China Sea (SCS). These are alkali basalts displaying right-sloping, chondrite-normalized rare earth element (REE) profiles. The investigated basalts are characterized by low Os content (60.37–85.13 ppt) and radiogenic 187Os/188Os ratios (~0.19 to 0.21). Furthermore, 40Ar/39Ar dating of the Nanyue basalts showed they formed during the Tortonian (~8.3 Ma) and, thus, are products of (Late Cenozoic) post-spreading volcanism. The Sr–Nd–Pb–Hf isotopic compositions of the Nanyue basalts indicate that their parental melts were derived from an upper mantle reservoir possessing the so-called Dupal isotopic anomaly. Semiquantitative isotopic modeling demonstrates that the isotopic compositions of the Nanyue basalts can be reproduced by mixing three components: the average Pacific midocean ridge basalt (MORB), the lower continental crust (LCC), and the average Hainan ocean island basalt (OIB). Our preferred hypothesis for the genesis of the Nanyue basalts is that their parental magmas were produced from an originally depleted mantle (DM) source that was much affected by the activity of the Hainan plume. Initially, the Hainan diapir caused a thermal perturbation in the upper mantle under the present-day Southwest sub-basin of the SCS that led to erosion of the overlying LCC. Eventually, the resultant suboceanic lithospheric mantle (SOLM) interacted with OIB-type components derived from the nearby Hainan plume. Collectively, these processes contributed crustal- and plume-type components to the upper mantle underlying the Southwest sub-basin of the SCS. This implies that the Dupal isotopic signature in the upper mantle beneath the SCS was an artifact of in situ geological processes rather than a feature inherited from a Southern Hemispheric, upper mantle source. 

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2. Oxygen Fugacity of Global Ocean Island Basalts

   https://doi.org/10.1029/2023GC011249  

   Willhite, Lori N.; Arevalo, Jr., Ricardo; Piccoli, Philip; Lassiter, John C.; Rand, Devin; Jackson, Matthew G.; Day, James M. D.; Nicklas, Robert W.; Locmelis, Marek; Ireland, Thomas J.; et al (January 2024, Geochemistry, Geophysics, Geosystems)

   Abstract Mantle plumes contain heterogenous chemical components and sample variable depths of the mantle, enabling glimpses into the compositional structure of Earth's interior. In this study, we evaluated ocean island basalts (OIB) from nine plume locations to provide a global and systematic assessment of the relationship betweenfO₂and He‐Sr‐Nd‐Pb‐W‐Os isotopic compositions. Ocean island basalts from the Pacific (Austral Islands, Hawaii, Mangaia, Samoa, Pitcairn), Atlantic (Azores, Canary Islands, St. Helena), and Indian Oceans (La Réunion) reveal thatfO₂in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenousfO₂(average = +0.5 ∆FMQ; 2SD = 1.5) relative to prior estimates of global mid‐ocean ridge basalts (MORB; average = −0.1 ∆FMQ; 2SD = 0.6), though many individual OIB overlap MORB. Specific mantle components, such as HIMU and enriched mantle 2 (EM2), defined by radiogenic Pb and Sr isotopic compositions compared to other OIB, respectively, have distinctly highfO₂based on statistical analysis. ElevatedfO₂in OIB samples of these components is associated with higher whole‐rock CaO/Al₂O₃and olivine CaO content, which may be linked to recycled carbonated oceanic crust. EM1‐type and geochemically depleted OIB are generally not as oxidized, possibly due to limited oxidizing potential of the recycled material in the enriched mantle 1 (EM1) component (e.g., sediment) or lack of recycled materials in geochemically depleted OIB. Despite systematic offset of thefO₂among EM1‐, EM2‐, and HIMU‐type OIB, geochemical indices of lithospheric recycling, such as Sr‐Nd‐Pb‐Os isotopic systems, generally do not correlate withfO₂. 

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3. Depletion ages and factors of MORB mantle sources

   https://doi.org/10.1016/j.epsl.2019.115926  

   Zhang, Youxue Gan (January 2020, Earth and planetary science letters)

   Sr-Nd-Hf-Pb isotopes show that the depleted MORB mantle (DMM) is not homogeneous. The heterogene-ity is attributed to different ages of depletion and/or various degrees of depletion for a given domain of DMM, as well as multiple depletion events, metasomatism, and mixing between DMM and other man-tle components. A mid-ocean ridge basalt, in principle, should contain information about the depletion history of its mantle sources. Here we develop a model to extract the model depletion age and the com-position of a MORB mantle source prior to MORB production using Sr-Nd isotopes or Sr-Hf isotopes in a MORB. The complexities of multiple depletion events, mixing, metasomatism, and enrichment are not addressed in this contribution. The model is based on two assumptions: (i) Isotope evolution in a MORB mantle follows a two-stage evolution model, the first stage in the primitive mantle from the beginning of the solar system to the time of mantle depletion at age Td, and the second stage in the depleted mantle from age Tdto the present day. That is, there is only one single depletion event. The depletion age and degree of depletion of a given mantle source are to be determined. (ii) The trace element composition of a depleted mantle source corresponding to the given MORB can be related to a reference DMM by a log-linear relation with the compatibility index CoI (Zhang, 2014). Applying the two assumptions to the available and large MORB database (Gale et al., 2013), we calculate the global distribution of sub-ridge mantle age and composition. The results show: (i) Mean or reference MORB mantle composition of Salters and Stracke (2004)is close to the average depleted MORB mantle composition, whereas that of Workman and Hart (2005)is significantly more depleted than the average depleted MORB mantle. (ii)Model ages for sub-ridge mantle depletion are mostly between 0.8 to 3.0Ga. (iii) There are large-scale patterns in depletion ages for sub-ridge mantle regions. For example, beneath Mid-Atlantic Ridge, mantle depletion ages are young (0.8 to 2.1 Ga) north of 30◦N, older (1.6 to 4.5 Ga) between 25◦N to 35◦S), and mixed (0.6-4.4 Ga) south of 35◦S. The Pacific sub-ridge mantle has a narrow range of model depletion ages of 1.6 to 3.0 Ga, with a mean of 2.3 Ga. Indian sub-ridge mantle has a younger mean depletion age of 1.7 Ga. These large-scale patterns reveal history of mantle depletion, mantle convection, and possible mixing between older and younger depleted mantles. 

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4. A subduction influence on ocean ridge basalts outside the Pacific subduction shield

   https://doi.org/10.1038/s41467-021-25027-2  

   Yang, A. Y.; Langmuir, C. H.; Cai, Y.; Michael, P.; Goldstein, S. L.; Chen, Z. (December 2021, Nature Communications)

   Abstract The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle. 

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5. 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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