Kevin W. Burton

Ferrihydrite is a ubiquitous scavenger in aqueous environments that removes large amounts of dissolved metals thus it is pivotal to global geochemical cycles. Previous work found contradictory directions of stable isotope fractionation amongst light rare earth elements (REE) between aqueous solutions and ferrihydrite. Here, the isotope fractionation of 146Nd/144Nd on ferrihydrite has been revisited using a double spike that corrects for mass fractionation during sample processing. Surprisingly, contrary to previous observations, Nd on ferrihydrite was found to be enriched in lighter isotopes compared to Nd in aqueous solution by 103lnαSol-Liq = -0.113 ± 0.030. We also report the first data set of stable δ146/144Nd isotopes in seawater samples which are isotopically heavier than Fe-Mn (hydr)oxides by 0.357 ‰ on average. This is consistent with the scavenging of lighter Nd isotopes by ferrihydrite seen in the adsorption experiments. The fractionation is, however, much larger than observed in the experiments. This can be explained by carbonate complexes being the predominant aqueous complex of Nd in seawater which are isotopically heavier than free Nd by 0.107 ‰. However, additional data on natural samples is required to elucidate heterogeneities in seawater and Fe-Mn oxides in marine sediments. The enrichment of lighter Nd in the surface complex can be best explained by stiffer bonds in the aqueous complex which is demonstrated by a lower Debye-Waller factor (σ2). This study lays the foundation for understanding the geochemical cycling of Nd and other REE between Earth’s crust and hydrosphere using stable isotope signatures.

Carbon cycling between surface and mantle reservoirs is pivotal in fostering habitability of Earth. A critical yet poorly constrained parameter is whether crustal carbon can “survive” devolatilization processes that accompany slab subduction and therefore influence deep carbon budgets. Carbonatites provide a key record to address this important topic. Here, we present high-precision potassium isotope data for a large set of carbonatite samples from both continental and oceanic settings, spanning from 2 billion years ago to the present. Modeling suggests that the heavy potassium isotopic compositions of carbonatites are inherited from their mantle sources, rather than resulting from magmatic and postmagmatic processes. Our results demonstrate a strong link between the subduction of oceanic crust and the recycling of carbonates into the mantle sources of carbonatites. These findings support the hypothesis that subduction of carbonate-bearing altered oceanic crust has been a critical mechanism for transferring carbon into the deep Earth through time.

Rhenium (Re) is a redox-sensitive element. Recent advances in the precision of measurement of the stable isotopic composition of Re (δ187Re) allow exploration of its potential as a proxy for paleoredox and/or chemical weathering [1]. However, as yet, there have been few studies reporting the geochemical cycling of Re and stable Re isotopes in the modern environments [2] [3], and processes that regulate the Re isotope behavior in hydrothermal systems remain unexplored.Here we present results of the analysis of Re concentration and δ187Re (relative to NIST3143) for water samples collected from hydrothermal and groundwater systems in Iceland. We show that Re in basalt-hosted boiled hydrothermal fluids from Hellisheidi, Nesjavellir, Reykjanes and Svartsengi sites is isotopically heavier (δ187Re = –0.01 to +0.32‰) than Re in Icelandic basalts (δ187Re = ~–0.32‰). The direction of fractionation holds regardless of types of fluid reservoir (meteoric vs. seawater), and is consistent with precipitation of isotopically light sulfides in the hydrothermal system and/or kinetic fractionation of Re during degassing. By contrast, Re in cold (< 10°C) groundwaters collected from the Mývatn area is isotopically indistinguishable from host basalt. Natural hot spring waters exhibit variable δ187Re values (–0.28 to +0.26‰), likely reflecting mixing between hydrothermal and groundwater endmembers. The relatively isotopically heavy δ187Re from hydrothermal sources has the potential to modify the oceanic budget, which has implications for the isotope mass balance of Re.[1] Dellinger et al. (2020) JAAS, 35, 377. [2] Dickson et al. (2020) GCA, 287, 221-228. [3] Dellinger et al. (2021) EPSL, 573, 117131.

Serpentinites are an important sink for both inorganic and organic carbon, and their behavior during subduction is thought to play a fundamental role in the global cycling of carbon. Here we show that fluid-derived veins are preserved within the Zermatt-Saas ultra-high pressure serpentinites providing key evidence for carbonate mobility during serpentinite devolatilisation. We show through the O, C, and Sr isotope analyses of vein minerals and the host serpentinites that about 90% of the meta-serpentinite inorganic carbon is remobilized during slab devolatilisation. In contrast, graphite-like carbonaceous compounds remain trapped within the host rock as inclusions within metamorphic olivine while the bulk elemental and isotope composition of organic carbon remains relatively unchanged during the subduction process. This shows a decoupling behavior of carbon during serpentinite dehydration in subduction zones. This process will therefore facilitate the transfer of inorganic carbon to the mantle wedge and the preferential slab sequestration of organic carbon en route to the deep mantle.

CAIs, the oldest solids in the Solar System, probably formed further away from the Sun than previously thought.

The trace element and isotopic compositions of mid-ocean ridge basalts (MORB) provide an important cornerstone for all studies seeking to understand mantle evolution. Globally there is a significant over-enrichment in the incompatible trace element concentrations of MORB relative to levels which should be generated by fractional crystallization. Thermal and geochemical constraints suggest that MORB require generation in open system magma chambers. However, the petrology of lower oceanic crustal rocks suggests instead that these enrichments maybe formed through reactive porous flow (RPF). Stable isotope compositions are process dependent and therefore provide an excellent mechanism to compare these contrasting models. This study presents the first neodymium (Nd) stable isotope compositions of Indian MORB and well characterized gabbroic rocks from the lower oceanic crust sampled at the Southwest Indian Ridge (SWIR) (Hole 735B). Indian MORB is extremely homogenous with a mean δ¹⁴⁶Nd of −0.025 ± 0.005‰ which is identical to the composition of Pacific MORB. Despite significant variability in the source composition of MORB globally (i.e., ¹⁴³Nd/¹⁴⁴Nd) their indistinguishable δ¹⁴⁶Nd compositions suggests δ¹⁴⁶Nd was homogenized through a consistent process (i.e., repeated melt addition in the open-system magma chambers across the global ridge network). In stark contrast, oceanic gabbros have δ¹⁴⁶Nd ranging from −0.026 to −0.127‰, doubling the natural variability in Nd stable isotopes observed in terrestrial rocks. Clinopyroxene separates possess variable δ¹⁴⁶Nd but are isotopically heavier than the gabbroic whole rocks at the same major element compositions. These large variations in δ¹⁴⁶Nd cannot be generated solely by the fractionation or accumulation of magmatic minerals. Hole 735B preserves widespread evidence of RPF which could induce kinetic isotope fractionation during crystal growth. However, the maximum kinetic isotope fractionations that can be generated in clinopyroxene are only ca. 0.02‰, therefore several cycles of dissolution and reprecipitation of isotopic signatures at grain boundaries are required to explain the range of δ¹⁴⁶Nd observed in the gabbros. The large disconnect between the average composition of the oceanic crust (δ¹⁴⁶Nd = −0.067‰) and MORB, combined with limited evidence of melt extraction to the upper crust at Hole 735B, led to the conclusion that melts involved in RPF have not contributed in a substantial way to the Nd isotope composition of erupted MORB.

Geochemical Perspectives Letters is a new journal of the European Association of Geochemistry.