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1. Uncertainty and sensitivity analysis for spatial and spectral processing of Pb isotopes in zircon by atom probe tomography

   https://doi.org/10.1002/9781119227250.ch16  

   Blum, TB; Reinhard, DA; Ty, YC; Prosa, J; Larson, DJ; Valley, JW (January 2017, Geography monograph series)

   Moser, DE; Corfu, F; Darling, J; Reddy, SM; Tait, K (Ed.)

   Measuring 207Pb/206Pb ratios by atom probe tomography (APT) has provided new insight into the nanoscale behavior of trace components in zircon, and their relationship to time, temperature and structure. Here, we analyze three APT data sets for a 3.77 Ga zircon from the Beartooth Mountains, USA, and apply systematic ranging approaches to understand the spatial and spectral uncertainties inherent in 207Pb/206Pb analysis by APT. This zircon possesses two, 100% concordant U-Pb analyses by secondary ion mass spectrometry (SIMS), indicative of closed U-Pb systematics on the micron scale since crystallization. APT data sets contain sub-spherical Pb-rich (>0.25% atomic) domains with diameter <15 nm. Broadly consistent Pb-rich regions are defined in applying six different permutations of the two most common cluster identification algorithms. Measured 207Pb/206Pb ratios within Pb-rich domains vary between 0.794±0.15 (±2σ) and 0.715±0.052 depending on the ranging approach, cluster definition protocol and number of clusters interrogated. For the bulk APT data sets, 207Pb/206Pb = 0.353±0.18; this is indistinguishable from the bulk 207Pb/206Pb ratio by SIMS (0.367±0.0037), and statistically distinct from the 207Pb/206Pb ratio within clusters. Bulk and clustered 207Pb/206Pb ratios are consistent with Pb clustering at ~2.8 Ga, during protracted metamorphism and magmatism in the Beartooth Mountains. 10.1002/9781119227250.ch16 

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2. Metasediments from the lower crust reveal the history of the Picuris orogeny, southwest USA

   https://doi.org/10.1130/B36836.1  

   Ringwood, Mary F.; Rudnick, Roberta L.; Kylander-Clark, Andrew R.C. (April 2023, Geological Society of America Bulletin)

   Petrologic and geochronologic data for metapelitic lower crustal xenoliths from New Mexico (USA) and Chihuahua (Mexico) states provide evidence for both a magmatic and collisional component to the enigmatic Mesoproterozoic Picuris orogeny. These garnet-sillimanite-bearing metapelites are found within the southern Rio Grande rift at Kilbourne Hole and Potrillo Maar in southern New Mexico and northern Chihuahua. Geothermobarometry and rutile with Quaternary U-Pb dates indicate equilibration in the local lower crust, which is actively undergoing ultra-high temperature (UHT) metamorphism (Cipar et al., 2020). The samples contain older detrital zircons dating back to the Paleoproterozoic, marking their deposition at the surface. Coupled zircon U-Pb dates and trace-element ratios (e.g., Gd/Yb) show a clear transition from oscillatory-zoned, low-Gd/Yb detrital magmatic zircon to featureless, high-Gd/Yb metamorphic zircon between 1500 and 1400 Ma, marking the transition from subduction to collision during this period. Metamorphic zircon and monazite grew in two major intervals. The first, between ca. 1450 and 1350 Ma, documents the journey of the sediments to depth within the orogen and provides evidence of extended Mesoproterozoic metamorphism in the region. The second corresponds with UHT metamorphism that commenced at ca. 32 Ma and is associated with the Rio Grande rift. Whereas nearly all garnets are homogeneous in both major and trace elements, a single garnet from one sample has a core defined by abundant quartz and acicular sillimanite inclusions. The core and rim of this garnet is homogeneous in major and most trace elements, but the rim is enriched in the slowest diffusing elements, Zr and Hf, which likely indicates rim growth at higher temperatures. We interpret the garnet core to have grown at the time of emplacement of the sediments into the lower crust. Because this occurred in the sillimanite stability field and because the metamorphic zircon and monazite all have negative Eu anomalies, indicating their equilibration with feldspar (stable at depths of <45 km), we conclude that the sediments were not emplaced via subduction and/or relamination of forearc sediments, but were instead metamorphosed under warmer, shallower conditions in an orogenic setting. Collectively, the data point to a collisional orogen during the inferred timing of the Picuris orogeny. These samples may therefore define the location of the Picuris suture zone, a key feature of this orogenic event. 

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3. Evidence for Oceans pre-4300 Ma Confirmed by Preserved Igneous Compositions in Hadean Zircon

   https://doi.org/10.2138/am-2023-9180  

   Cameron, Emilia M.; Blum, Tyler B.; Cavosie, Aaron J.; Kitajima, Kouki; Nasdala, Lutz; Orland, Ian J.; Bonamici, Chloe E.; Valley, John W. (March 2024, American Mineralogist)

   Abstract Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ18O in many of these zircons (6.0 to 7.5‰ are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and moon formation, and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation damaged, discordant and/or hydrous domains, and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted to pristine and not altered, and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data, and presents the first comprehensive evaluation of δ18O, OH/O, CL imaging, U-Pb concordance and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. In situ Raman data for band broadening correlated with δ18O analyses document low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff) are uniformly less than the first percolation point (dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2×1015 α-decays/mg, Ewing et al., 2003) and most zircons have Deff<1×1015 α-decays/mg. Modeling of representative alpha-recoil damage and annealing histories indicates that most zircons in this study have remained below the Deff of the first percolation point throughout their history. The δ18O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of pristine Hadean zircons between δ18O and OH/O, Deff, age, or U-Pb age-concordance. These carefully documented Hadean-age zircons possess low amounts of radiation damage in domains sampled by δ18O analysis, are water-poor. The mildly elevated δ18O values are a primary-magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma. 

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4. TRACE ELEMENTS AND INTERSTRIAL DISTANCES AS ENVIRONMENTAL AND ANTHROPOGENIC PROXIES IN THE ANTARCTIC SCALLOP, ADAMUSSIUM COLBECKI

   CRONIN, K.; WALKER, S.E.; GILLIKIN, D.; BOWSER, S. (July 2019, PaleoBios)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki may be a crucial paleoenvironmental proxy for coastal Antarctica. For example, two highly seasonal environmental parameters, glacial melt and productivity, were linked to trace elemental concentrations in a previous bulk shell analysis and a transect spanning ~ 3 months of juvenile growth. However, neither study examined seasonal variation in trace elements or tied variation to distances between small ridges (striae) on valve surfaces, which may also vary seasonally. Striae and interstrial growth between them are expressed as alternating narrow and wide groups (presumably winter and summer growth, respectively). If tied to trace elemental concentrations, striae could provide high-resolution sclerochronological proxies for seawater conditions. Here, we evaluate whether trace elements archived in A. colbecki striae can be used as seasonal indicators of glacial influence and nutrients over A. colbecki ontogeny. We examined trace elements from an adult and juvenile A. colbecki (shell height, 80.2 mm and 17.1 mm, respectively) collected live by divers from ~ 12 m water depth in Explorers Cove, western McMurdo Sound (2008 and 2016, respectively). Trace elements linked to glacial melt (Mg/Ca, Mn/Ca, Fe/Ca, and Pb/ Ca), metabolism (Mg/Ca), and productivity (Ba/Ca) were sampled with an LA-ICP-MS on each stria along the central growth axis of lower (right) valves from umbo to growing margin. Distances between sampled striae were measured along the central margin (FIJI). Interstrial distances (ISDs) and trace elements were compared using wavelet coherence analysis (Wavelet- Comp 1.1) and cross-correlation. Coherence and correlations that exceeded 95% significance are reported here. Coherence identifies areas of covariance between ISD and trace elements over ontogeny; cross-correlation describes the direction (±) of correlation between 113 NAPC 2019 PROGRAM & ABSTRACTS ISDs and trace elements where coherence exists. We expected trace elements that increase with glacial melt (Fe, Mn, Pb), productivity (Ba), and altered metabolism (Mg) to be coherent and correlate positively with ISD (highest concentrations at wide summer striae) throughout ontogeny. Preliminary results mostly do not conform to predictions. Though correlation remains consistently positive or negative under strong coherence, most elements are only coherent with ISD for short strial sequences (~ 8 striae) and only during adult growth. Of the elements associated with glacial melt, only Mn correlates positively with ISD and may be a potential proxy for seasonality. Other indicators of glacial melt (Pb/Ca, Fe/Ca) and productivity (Ba/Ca) correlate negatively with ISD. Mg/Ca correlates positively with ISD, indicating seasonal effects on metabolism. Ontogenetic variation in coherence urges cautious use of ISDs as proxies, but Pb/Ca (anthropogenic in Antarctica) is coherent with ISD throughout ontogeny; further analysis might illuminate seasonal effects of human activities on Antarctic ecosystems. 

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5. Micropillar compression of single-crystal single-phase (Co, Cu, Mg, Ni, Zn)O

   https://doi.org/10.1063/5.0211263  

   Norman, Jacob_E; Wang, Xin; Dupuy, Alexander_D; Schoenung, Julie_M (July 2024, Applied Physics Letters)

   Bulk, polycrystalline (Co, Cu, Mg, Ni, Zn)O was synthesized using solid-state sintering. Micropillars were prepared and mechanically deformed along three crystallographic orientations: (001), (101), and (111). Pillars (001) and (111) cracked, while Pillar (101) remained intact. Pillars (001) and (101) exhibited activated slip systems, confirmed by a large stress drop, and the presence of slip bands, respectively. Schmid factor (SF) analysis was performed to examine the effect of grain orientations on dislocation activity and slip behavior. SF values range from 0 to 0.5, with non-zero values indicating potential for slip. Six slip systems exist in the (Co, Cu, Mg, Ni, Zn)O rock salt crystal structure: 1/2⟨110⟩11¯0. For the (001) orientation, four slip systems are potentially active (SF = 0.5). For the (101) orientation, there are four potentially active slip systems (SF = 0.25). For the (111) orientation, no potentially active slips systems exist (SF = 0). Dislocation structures, which were observed post-compression via transmission electron microscopy, demonstrated variations in size, number, and distribution across the pillar, depending on micropillar orientation. Entangled dislocations created misorientation in Pillar (001), which led to the possible formation of subgrains, while singular dislocations were observed in Pillar (101), and a lack of dislocations was observed in Pillar (111). Zener–Stroh type dislocation entanglement-mediated cracking is the proposed cause of the transgranular-type cracks in Pillar (001). The possible subgrain formation, or lack of formation, respectively, caused intergranular-type cracks to additionally form in Pillar (001), while Pillar (111) only exhibited transgranular-type brittle fracture. In combination, these findings highlight the importance of dislocation activity, without the need for elevated temperature, and grain orientation in controlling the mechanical deformation response in single-phase (Co, Cu, Mg, Ni, Zn)O. 

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