Neodymium Isotope Geochemistry of a Subterranean Estuary

Rare earth elements (REE) and Nd isotope compositions of surface and groundwaters from the Indian River Lagoon in Florida were measured to investigate the influence of submarine groundwater discharge (SGD) on these parameters in coastal waters. The Nd flux of the terrestrial component of SGD is around 0.7±0.03 μmol Nd/day per m of shoreline across the nearshore seepage face of the subterranean estuary. This translates to a terrestrial SGD Nd flux of 4±0.2 mmol/day for the entire 5,880 m long shoreline of the studied portion of the lagoon. The Nd flux from bioirrigation across the nearshore seepage face is 1±0.05 μmol Nd/day per m of shoreline, or 6±0.3 mmol/day for the entire shoreline. The combination of these two SGD fluxes is the same as the local, effective river water flux of Nd to the lagoon of 12.7±5.3 mmol/day. Using a similar approach, the marine-sourced SGD flux of Nd is 31.4±1.6 μmol Nd/day per m of shoreline, or 184±9.2 mmol/day for the investigated portion of the lagoon, which is 45 times higher than the terrestrial SGD Nd flux. Terrestrial-sourced SGD has an εNd(0) value of −5±0.42, which is similar to carbonate rocks (i.e., Ocala Limestone) from the Upper Floridan Aquifer (−5.6), but more radiogenic than the recirculated marine SGD, for which εNd(0) is −7±0.24. Marine SGD has a Nd isotope composition that is identical to the εNd(0) of Fe(III) oxide/oxyhydroxide coated sands of the surficial aquifer (−7.15±0.24 and −6.98±0.36). These secondary Fe(III) oxides/oxyhydroxides formed during subaerial weathering when sea level was substantially lower during the last glacial maximum. Subsequent flooding of these surficial sands by rising sea level followed by reductive dissolution of the Fe(III) oxide/oxyhydroxide coatings can explain the Nd isotope composition of the marine SGD component. Surficial waters of the Indian River Lagoon have an εNd(0) of −6.47±0.32, and are a mixture of terrestrial and marine SGD components, as well as the local rivers (−8.63 and −8.14). Nonetheless, the chief Nd source is marine SGD that has reacted with Fe(III) oxide/oxyhydroxide coatings on the surficial aquifer sands of the subterranean estuary.

The Ludicovian of the Raahe–Ladoga Zone of the Fennoscandian Shield (Isotope-Geochemical Composition and Geodynamic Nature)

—In the northern Ladoga area, the age of the Sortavala Group rocks in the southeast of the Raahe–Ladoga zone of junction of the epi-Archean Fenno-Karelian Craton and the Paleoproterozoic Svecofennian province, their relationship with dome granitoids, the age of the provenances, and the time of metamorphic processes were estimated. The study was focused on the Nd isotope composition of rocks, the geochemical and isotope-geochronological parameters of zircon from the granite-gneisses of the Kirjavalakhti dome, the basal graywackes of the lower unit and the trachytes of the middle unit of the Sortavala Group, and the plagio- and diorite-porphyry dikes cutting the volcanosedimentary units of this group. The new isotope-geochemical data show a Neoarchean age of the granitoids of the Kirjavalakhti dome (2695 ± 13 Ma) and their juvenile nature (εNd(T) = +1.5). The granitoids underwent tectonometamorphic transformations (rheomorphism) in the Paleoproterozoic (Sumian) (2.50–2.45 Ga), which are recorded in the U–Th–Pb isotope system of the rims of the ancient cores of zircon crystals. The volcanosedimentary complex of the Sortavala Group formed on the heterogeneous polychronous (3.10–2.46 Ga) continental crust of the epi-Archean Fenno-Karelian Craton. With regard to the errors in determination of the age of clastic zircon, the minimum concordant U–Th–Pb ages of 1940–1990 Ma of detrital zircon from volcanomictic graywackes of the Pitkyaranta Formation can be taken as the upper age bound of terrigenous rocks, which agrees with the maximum age of the Sortavala Group rocks estimated from the U–Th–Pb (SIMS) age of 1922 ± 11 Ma of the Tervaoya diorites (Matrenichev et al., 2006). According to the proposed new tectonic model, the accumulation of the volcanosedimentary complex of the Sortavala Group, its metamorphism, erosion, and overlapping by the Ladoga Group turbidites had already occurred in the pericratonic part of the epi-Archean Fenno-Karelian Craton by the time of the Svecofennian continent–island arc collision, subduction, and formation of bimodal volcanoplutonic complexes of the young Pyhäsalmi island arcs and felsic volcanics of the Savo schist belt (1920–1890 Ma).

Genesis and Nd Isotope Composition of Ferromanganese Deposits of the Sea of Okhotsk and the Kuril Island Arc

—The Nd isotope composition of ferromanganese deposits (FMD) from the central part of the Sea of Okhotsk and the Kuril island arc has been studied. The results showed that diagenetic samples from the Deryugin Basin have a heterogeneous Nd isotope composition. The positive εNd values here might be both due to the input of a terrigenous impurity and due to diffuse endogenetic element supply. The FMD samples from the Sonne underwater ridge show a εNd value specific to seawater (–3.2). The εNd value of hydrogenetic FMD from Volcano 7.14 is –3.4, which corresponds to the εNd value of the Pacific water. The FMD samples from Volcano 5.5 are characterized by εNd = –2.0. The higher εNd value might be due to a moderate dilution of the hydrothermal fluid by seawater and might also indicate the presence of volcanic fragments in the FMD samples. The highest εNd value (+4.4) has been established for volcaniclastic sandstone saturated with Fe and Mn hydroxides. It points to a mixing of volcanomictic and hydrothermal materials.

Nd isotope composition of the Ediacaran and earliest Cambrian phosphorite nodules and Fe sulphide from the East European Platform

An enhanced concentration of phosphorus has been found at the stratigraphic level of the disappearance of Ediacaran taxa in two areas, the Cis-Dniester region and the Moscow syneclise, on the East European Platform (EEP). The isotope composition of neodymium was determined in Fe sulphide and phosphorite in the same beds. Measured εNd(t) values in diagenetic phosphate nodules are similar to those in iron sulphide from the same layer. During the Ediacaran − Early Cambrian, accumulation of radiogenic Nd in the epeiric basins on the EEP increased progressively from −17.9 and −19.4 in pyrite from the sequence bottom to −7.9 and −8.5 in the Early Cambrian pyrite of the central part of the EEP. The Ediacaran phosphate nodules show εNd(t) ranging from −12.9 to −15.0, while that in the Early Cambrian nodules is typically c. −9.0. These data indicate the secular change in Nd isotope composition of the water reservoir on the EEP from Ediacaran to Cambrian.

Constraining Mantle Source Conditions at Iceland and Adjoining Ridges Using Markov Chain Monte Carlo Inversion

<p>Basalts are generated by adiabatic decompression melting of the upper mantle, and thus provide spatial and temporal records of the thermal, compositional, and dynamical conditions of their source regions. Uniquely constraining these factors through the lens of melting is challenging given the complexity of the melting process. To limit the <em>a priori</em> assumptions typically required for forward modeling of mantle melting, and to assess the robustness of the modeling results, we combine a Markov chain Monte Carlo sampling method with the forward melting model REEBOX PRO [1] simulating adiabatic decompression melting of lithologically heterogeneous mantle. Using this method, we invert for mantle potential temperature (Tp), lithologic trace element and isotopic composition and abundance, and melt productivity together with a robust evaluation of the uncertainty in these system properties. We have applied this new methodology to constrain melting beneath the Reykjanes Peninsula (RP) of Iceland [2] and here extend the approach to Iceland’s Northern Volcanic Zone (NVZ). We consider melting of a heterogeneous mantle source involving depleted peridotite and pyroxenite lithologies, e.g., KG1, MIX1G and G2 pyroxenites. Best-fit model sources for Iceland basalts contain more than 90% depleted peridotite and less than 10% pyroxenite with Tp ~125-200 °C above ambient mantle. The trace element and Pb and Nd isotope composition of the depleted source beneath the Reykjanes Peninsula is similar to DMM [3], whereas depleted mantle for the NVZ is isotopically distinct and more trace element enriched. Conversely, inverted pyroxenite trace element compositions are similar for RP and NVZ and are more enriched than previously inferred, despite marked differences in their Pb and Nd isotope composition. We use these new constraints on the Iceland source to investigate their relative importance in basalt genesis along the adjoining Reykjanes and Kolbeinsey Ridges. We find that the proportion of pyroxenite diminishes southward along Reykjanes Ridge and is seemingly absent to the north along the Kolbeinsey Ridge. Moreover, abundances of inverted RP and NVZ depleted mantle also diminish away from Iceland and give way to a common depleted source for the North Atlantic. These findings further illuminate the along-strike variability in source composition along the North Atlantic ridge system influenced by the Iceland melting anomaly, while reconciling geochemical, geophysical and petrologic constraints required to rigorously test plume vs. non-plume models.</p><p>[1] Brown & Lesher (2016); G^3, v. 17, p. 3929-2968</p><p><span>[2] Brown et al. (2020); EPSL, v. 532, 116007</span></p><p>[3] Workman and Hart (2005); EPSL, v.231, p. 53-72</p>

Glacial–interglacial Nd isotope variability of North Atlantic Deep Water modulated by North American ice sheet

The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember.