Subducting tectonic plates carry water and other surficial components into Earth's interior. Previous studies suggest that serpentinized peridotite is a key part of deep recycling, but this geochemical pathway has not been directly traced. Here, we report Fe-Ni-rich metallic inclusions in sublithospheric diamonds from a depth of 360 to 750 km with isotopically heavy iron (delta Fe-56 = 0.79 to 0.90 parts per thousand) and unradiogenic osmium (Os-187/Os-188 = 0.111). These iron values lie outside the range of known mantle compositions or expected reaction products at depth. This signature represents subducted iron from magnetite and/or Fe-Ni alloys precipitated during serpentinization of oceanic peridotite, a lithology known to carry unradiogenic osmium inherited from prior convection and melt depletion. These diamond-hosted inclusions trace serpentinite subduction into the mantle transition zone. We propose that iron-rich phases from serpentinite contribute a labile heavy iron component to the heterogeneous convecting mantle eventually sampled by oceanic basalts.

Interpreting isotopic signatures documented in natural rocks requires knowledge of equilibrium isotopic fractionation factors. Here, we determine equilibrium Fe isotope fractionation factors between several common rock-forming minerals using a comparative approach involving three independent methods: (i) isotopic analyses of natural minerals from a metapelite from Mt. Moosilauke, New Hampshire, for which equilibration temperature and pressure are well constrained to be near the aluminosilicate triple point (T similar or equal to 500 degrees C, P similar or equal to 4 kbar), (ii) Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements of Fe force constants of minerals, and (iii) Density Functional Theory (DFT) ab initio calculations of Fe force constants of minerals.

Experiments show that the iron isotopic composition of iron meteorites can be explained by core crystallization, and suggest the presence of sulfur-rich core material that remains unsampled by meteorite collections.

As a transition metal that is moderately volatile at high temperatures, copper shows limited isotopic fractionation in terrestrial mantle-derived rocks but significant enrichment in its heavier isotope (up to 12.5 parts per thousand for Cu-65/Cu-63) in objects that experienced volatile loss during formation, such as tektites, trinitite glasses, and lunar rocks. Previous efforts to model the Cu isotope fractionation trend from measurements of delta Cu-65 in tektites found that the trend cannot be explained by the theoretical isotope fractionation factor (alpha) for free evaporation of Cu, making it necessary to experimentally study Cu isotope fractionation under conditions similar to tektite formation.

Chondrites display isotopic variations for moderately volatile elements, the origin of which is uncertain and could have involved evaporation/condensation processes in the protoplanetary disk, incomplete mixing of the products of stellar nucleosynthesis, or aqueous alteration on parent bodies. Here, we report high-precision K and Rb isotopic data of carbonaceous chondrites, providing new insights into the cause of these isotopic variations. We find that the K and Rb isotopic compositions of carbonaceous chondrites correlate with their abundance depletions, the fractions of matrix material, and previously measured Te and Zn isotopic compositions. These correlations are best explained by the variable contribution of chondrules that experienced incomplete condensation from a supersaturated medium. From the data, we calculate an average chondrule cooling rate of similar to 560 +/- 180 K/hour, which agrees with values constrained from chondrule textures and could be produced in shocks induced by nebular gravitational instability or motion of large planetesimals through the the nebula.

The Fe isotopic composition of twenty four glacial diamictite composites with depositional ages ranging from the Mesoarchean to the Palaeozoic serve as proxies of the average upper continental crust (UCC) and can be used to track how delta Fe-56 may have changed in the continental crust through time. The diamictites have elevated chemical index of alteration (CIA) values and other characteristics of weathered regoliths e.g., strong depletion in soluble elements such as Sr), which they inherited from their upper crustal source regions. The delta Fe-56 values in the diamictite composites range from -0.59 parts per thousand to +0.23 parts per thousand. Excluding three samples impacted by the incorporation of materials from Fe formations, the diamictites have an average delta Fe-56 of 0.12 +/- 0.13 parts per thousand (2 sigma), overlapping the recent estimated average delta Fe-56 of 0.09 +/- 0.03 %o (2 s.d.) in the upper continental crust (Dauphas et al., 2017, and references therein). There is no obvious correlation between delta Fe-56 of the glacial diamictites and the CIA. Our data suggest that the Fe isotope composition of the upper continental crust has been relatively constant throughout Earth history and that chemical weathering is not important in producing Fe isotope variations in the upper continental crust. Pre-Great Oxidation Event (GOE) anoxic weathering, when iron was soluble in its divalent state, did not generate different Fe isotopic signatures from the post-GOE oxidative weathering environment in the upper continental crust. Therefore, the large Fe isotopic fractionations observed in various marine sedimentary records are likely due to processes occurring in the oceans (e.g., biological activity) rather than abiotic redox reactions on the continents.

The variability of iron isotopes among rocky bodies in the inner Solar System provides a window onto the diversity of materials and mechanisms from which they formed. The magnitude of isotopic variation in mantle-derived rocks within a given body is similar to that between different planetary bodies. Isotopic signatures arising from primordial events, namely, evaporation/condensation, core formation and melting/crystallization, may be progressively diluted, modified, and redistributed over time by global recycling processes such as plate tectonics. Here, we assess the relative influence of these primordial mechanisms on the iron isotope compositions of igneous rocks and their implications for the structure and accretion histories of rocky planets.

Gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub-slab mantle to flow through the gaps and into the mantle wedge. This through-slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and/or dynamic uplift in the overriding plate, while the altered mantle flow patterns affect the trench evolution. Numerous geodynamic models have investigated the processes that form slab gaps, but few studies have examined the dynamics of slab gap-altered mantle flow, its effects on trench morphology and kinematics, or the controlling parameters on these processes. Here, laboratory subduction models with a pre-cut gap in a subducting silicone plate are used to explore how slab gap size, and slab gap depth influence the surrounding mantle flow field and trench dynamics. Results suggest that both the vertical extent and the depth of the top (trailing edge) of the slab gap are crucial parameters for modulating overall subduction dynamics. They show that a slab gap, which occurs near the surface and initially comprises 30% of the subducting plate width, can extend enough vertically in the slab to produce significant vertical flow through the gap. Changes to the trench geometry and kinematics are also evident in the models, such that double- and triple-arc geometries are formed during subduction of a shallow slab gap. All of these results are consistent with observations of slab gaps and their induced surface expressions, or the lack thereof, in Eastern Anatolia, East Java, Daly, and Argentina.