Philip M. Piccoli

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 between fO₂ and 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 that fO₂ in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenous fO₂ (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 high fO₂ based on statistical analysis. Elevated fO₂ in OIB samples of these components is associated with higher whole‐rock CaO/Al₂O₃ and 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 the fO₂ among EM1‐, EM2‐, and HIMU‐type OIB, geochemical indices of lithospheric recycling, such as Sr‐Nd‐Pb‐Os isotopic systems, generally do not correlate with fO₂.

Mantle peridotites from three 3 × 3-meter grids sampled at kilometer distances from one another in the ca. 497 Ma Leka Ophiolite Complex (LOC), Norway, are examined to investigate the chemical and isotopic nature of oceanic mantle domains at the centimeter to kilometer scale. The lithology of each grid locality is predominantly harzburgite, but includes layers and lenses of dunite and pyroxenite. Major and lithophile trace element compositions indicate a history of prior melting at pressures at or slightly below the garnet stability field. The common presence of orthopyroxenite veins likely reflects infiltration of silicic melts associated with supra-subduction zone processes. Osmium isotopes and highly siderophile element (HSE) abundance data for centimeter-scale sampling of traverses from the pyroxenites into the harzburgites reveal that the formation of the veins had little effect on Os isotopic compositions, and Os, Ir, Ru and Re abundances in the harzburgites. Adjacent to one of the orthopyroxenite veins studied, however, Pt and Pd abundances appear to have been strongly modified by interactions with vein-forming melts or fluids at distances of as much as 4–6 cm from the pyroxenite-harzburgite contact. Leka harzburgites have initial γOs values (% deviation from a chondritic reference) that range from −4.7 to +2.2 (6.9% variation), with individual uncertainties of ±0.2 units. Averaged initial Os isotopic compositions for harzburgites from the three grid sites separated by as much as 6 km, by contrast, differ by only a maximum of 2.6%. Isotopic heterogeneity on the centimeter to meter scale is, therefore, larger than kilometer-scale heterogeneity, indicating that at least some of the Os isotopic heterogeneity commonly observed globally among mantle peridotites is the result of processes that acted on a local scale. The general uniformity of these isotopic compositions among the three grid sites suggests that the portion of the oceanic mantle sampled by the LOC was homogenous at the kilometer scale with respect to the long-term Re/Os ratio. The long-term projected Re/Os for LOC harzburgites is similar to the average required for modern abyssal peridotites. This observation strengthens previous interpretations, based largely on data for abyssal peridotites, that state the Os isotopic evolution of oceanic mantle is consistent with a long-term ¹⁸⁷Re/¹⁸⁸Os of ∼0.38. The present ∼3 to 4% difference between the Os isotopic composition of the modern oceanic mantle and estimates for primitive mantle suggests that at least ∼6% of the mass of the oceanic mantle has been removed from it in the form of Re-enriched, mafic oceanic crust. Despite the recycling of this crust back into the mantle, most of it has evidently not been mixed back into accessible portions of the upper oceanic peridotite mantle. Compared to composition estimates for the primitive mantle, the median HSE compositions for the three grid sites are moderately to strongly depleted in Pd and Re, consistent with the corresponding lithophile element evidence for 20–30% melt depletion. As with initial γOs values, most harzburgites from a given grid are characterized by greater variations in absolute and relative HSE abundances than the differences between the median abundances of the three grid sampling locales. This observation indicates that as with Os isotopes, the HSE abundance heterogeneity among the harzburgites most strongly reflects centimeter- to meter-scale melting and remobilization effects. Except for Ru, median HSE abundances for grid harzburgites are similar to median abundances for abyssal peridotites. The ∼30% lower median Ru/Ir in the LOC compared to the median ratio for abyssal peridotites suggests that the abundance of Ru in the oceanic mantle may be more variable than generally thought.

Spinel-structured solids were studied to understand if fast Li⁺ ion conduction can be achieved with Li occupying multiple crystallographic sites of the structure to form a “Li-stuffed” spinel, and if the concept is applicable to prepare a high mixed electronic-ionic conductive, electrochemically active solid solution of the Li⁺ stuffed spinel with spinel-structured Li-ion battery electrodes. This could enable a single-phase fully solid electrode eliminating multi-phase interface incompatibility and impedance commonly observed in multi-phase solid electrolyte–cathode composites. Materials of composition Li_1.25M(III)_0.25TiO₄, M(III) = Cr or Al were prepared through solid-state methods. The room-temperature bulk Li⁺-ion conductivity is 1.63 × 10⁻⁴ S cm⁻¹ for the composition Li_1.25Cr_0.25Ti_1.5O₄. Addition of Li₃BO₃ (LBO) increases ionic and electronic conductivity reaching a bulk Li⁺ ion conductivity averaging 6.8 × 10⁻⁴ S cm⁻¹, a total Li-ion conductivity averaging 4.2 × 10⁻⁴ S cm⁻¹, and electronic conductivity averaging 3.8 × 10⁻⁴ S cm⁻¹ for the composition Li_1.25Cr_0.25Ti_1.5O₄ with 1 wt. % LBO. An electrochemically active solid solution of Li_1.25Cr_0.25Mn_1.5O₄ and LiNi_0.5Mn_1.5O₄ was prepared. This work proves that Li-stuffed spinels can achieve fast Li-ion conduction and that the concept is potentially useful to enable a single-phase fully solid electrode without interphase impedance.

One of the requirements for inferring mantle redox states via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry is to select mantle‐equilibrated zircon derived from melts that have not interacted with the hydrosphere nor assimilated exogenous materials. Traditional protocols for identifying mantle‐equilibrated zircon require a comprehensive examination of whole rock geochemistry in addition to zircon morphology, texture, and trace element and isotopic measurements. This study proposes a simple filter scheme for selecting mantle‐equilibrated zircon using trace element systematics alone (P, Ti, Y, Nb, rare earth elements [REE], Hf, Th, and U) to provide the geological context of zircon from three perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and (3) zircon saturation context. The filter scheme presented here, comprising of 13 criteria, is established based on detailed classifications of 2,173 zircon analyses from 30 independent references and is shown to distinguish between nonmagmatic zircon, magmatic zircon with significant inclusions and/or sourced from highly enriched source melts, and mantle‐equilibrated zircon. The filter scheme is validated with known, well‐characterized mantle‐equilibrated Jack Hills zircon (n = 53) and derives an average mantle redox state of (sd) circa 4,400 Ma, in agreement with previously published results. Further application of the filter scheme to zircon dataset (n = 76) from three metasedimentary rocks successfully reveals the mantle‐equilibrated zircon in the dataset, and implies a mantle redox state of (sd) circa 2,950 Ma. This filter scheme can be applied to studies of out‐of‐context detrital and/or xenocrystic zircon in the future.

In subduction‐related tectonic mélange, thermobarometry on individual blocks can in principle constrain the scale of lithological mixing along the subduction interface. Previous thermobarometric investigation of the tectonic amphibolite facies mélange unit in the Catalina Schist, Santa Catalina Island, California, USA, has suggested relatively limited mixing among blocks (≤12 km). Here we further investigate scales of mixing among metamorphically disparate (‘exotic’) blocks within epidote‐amphibolite and lawsonite‐blueschist facies mélange of the Catalina Schist using field and petrographic observations, Zr‐in‐rutile thermometry, and quartz‐in‐garnet elastic barometry. A new statistically based method is presented for calculating elastic barometry maximum pressures. The exotic blocks record peak metamorphic temperatures between 580 and 735°C and peak pressures between 1.16 and 1.65 GPa. Temperatures primarily fall in between those recorded by rocks in the amphibolite facies and epidote amphibolite facies units (643–735°C and 553–596°C respectively). The pressure estimates encompass those recorded by blocks from the amphibolite facies mélange (1.34–1.44 GPa), although the exotic blocks record a much larger range of pressures. The large range of recorded temperatures and pressures suggests that blocks within the epidote amphibolite unit were sourced from and mixed along a 20–30 km region of the subduction interface while an exotic block from the lawsonite blueschist facies unit appears to have been sourced from at least 70 km deeper than the unit it is hosted in. Metre‐ to kilometre‐scale variations in matrix mineral rheology likely control strain partitioning at the interface and permit differential transport of mélange blocks over variable length scales.

The use of indium in modern technologies has grown in recent decades, creating a growth in indium demand; thus, there is a need to constrain the spatial and temporal distribution of indium-bearing, granite-related deposits. Toward this end, a conceptual model and exploration vectors for the formation of granite-related indium deposits have been developed. The magmatic-hydrothermal system is modeled by consideration of crystal-melt and vapor-melt equilibria. The model calculates the efficiency of removal of indium from a melt into a volatile phase, which may serve as a component of an ore-forming fluid. The results of the model suggest that as the proportion of ferromagnesian minerals increases in the associated granites, the probability of indium ore formation decreases. Further, for a given modal proportion of ferromagnesian minerals, as the modal proportion of amphibole increases, the probability of indium ore formation decreases. Lastly, for a given modal proportion of biotite, as the magnesium content of the biotite increases (as would result from increasing oxidation of the magmatic system), the probability of indium ore formation decreases. Granites with the highest probability of being associated with indium ore formation will typically be part of A- or S-type igneous systems and will likely be highly fractionated (e.g., A-type topaz granites). I-type granites will generally have a lower potential of being associated with indium-bearing deposits. However, some I-type granites may be associated with indium-bearing deposits if the deposits contain granites (sensu stricto) or other related rocks (e.g., alaskites) that lack amphibole or other ferromagnesian phases.