Defining the Potential of Nanoscale Re‐Os Isotope Systematics Using Atom Probe Microscopy

Atom probe microscopy (APM) is a relatively new in situ tool for measuring isotope fractions from nanoscale volumes (< 0.01 μm³). We calculate the theoretical detectable difference of an isotope ratio measurement result from APM using counting statistics of a hypothetical data set to be ± 4δ or 0.4% (2s). However, challenges associated with APM measurements (e.g., peak ranging, hydride formation and isobaric interferences), result in larger uncertainties if not properly accounted for. We evaluate these factors for Re‐Os isotope ratio measurements by comparing APM and negative thermal ionisation mass spectrometry (N‐TIMS) measurement results of pure Os, pure Re, and two synthetic Re‐Os‐bearing alloys from Schwander et al. (2015, Meteoritics and Planetary Science, 50, 893) [the original metal alloy (HSE) and alloys produced by heating HSE within silicate liquid (SYN)]. From this, we propose a current best practice for APM Re‐Os isotope ratio measurements. Using this refined approach, mean APM and N‐TIMS ¹⁸⁷Os/¹⁸⁹Os measurement results agree within 0.05% and 2s (pure Os), 0.6–2% and 2s (SYN) and 5–10% (HSE). The good agreement of N‐TIMS and APM ¹⁸⁷Os/¹⁸⁹Os measurements confirms that APM can extract robust isotope ratios. Therefore, this approach permits nanoscale isotope measurements of Os‐bearing alloys using the Re‐Os geochronometer that could not be measured by conventional measurement principles.     

Textural and micro-analytical insights into mafic–felsic interactions during the Oruanui eruption,Taupo

Quenched juvenile mafic inclusions (enclaves) are an occasional but informative component in the deposits of large felsic eruptions. Typically, the groundmasses of these inclusions rapidly crystallize as the mafic magma is chilled against a more voluminous, cooler felsic host, providing a physical and chemical record of the nature and timing of mafic–felsic interactions. We examine mafic inclusions of two compositional lineages (tholeiitic and calc-alkaline) from deposits of the 25.4 ka Oruanui eruption (Taupo, New Zealand). 2-D quantitative textural data from analysis of back-scattered electron images reveal a marked diversity in the groundmass textures of the inclusions, including median crystal sizes (amphibole: 14–45 µm; plagioclase: 21–75 µm) and aspect ratios (amphibole: 1.7–4.2; plagioclase: 2.1–4.0), area number densities (amphibole: 122–2660 mm^?2; plagioclase: 117–2990 mm^?2), area fractions (?) of minerals (?_plag?=?23–45%, ?_amph?=?0–28%, ?_cpx?=?0–6%, ?_oxides?=?0.6–5.5%), and the relative abundance of plagioclase and amphibole (?_plag/?_amph?=?1.0–4.6). Textural parameters vary more significantly within, rather than between, the two compositional lineages, and in some cases show marked variations across individual clasts, implying that each inclusion’s cooling history, rather than bulk composition, was the dominant control on textural development. Groundmass mineral compositions are also diverse both within and between inclusions (e.g. plagioclase from An_34–92, with typical intra-clast variability of ~?20 mol%), and do not correlate with bulk chemistry. Diverse groundmass textures and mineral and glass chemistries are inferred to reflect complex interplay of a range of factors including the degree and rate of undercooling, bulk composition, water content and, possibly, intensive variables. Our data are inconsistent with breakup of a crystallizing ponded mafic layer at the base of the Oruanui melt-dominant body, instead implying that each inclusion partially crystallized as a discrete body with a unique cooling history. Extensive ingestion of mush-derived macro-crystals suggests that mechanical breakup of mafic feeder dikes occurred within a transition zone between the mush and melt-dominant magma body. In this zone, the mush lacked yield strength, as has been inferred from field studies of narrow (meters to few tens of meters) mush-melt transition zones preserved in composite intrusions. Evidence for plastic deformation of inclusions during eruption and the abundance of fresh residual glass in inclusions from all eruptive phases suggest that the inclusions formed syn-eruptively, and must have been formed recurrently at multiple stages throughout the eruption.     

Analysis of stratigraphical sequences at Cocina Cave (Spain) using rare earth elements geochemistry

This study investigates the stratigraphical sequence of Cocina Cave (Spain) employing and testing for the first time the capability of rare earth elements as markers of human activities in caves. Located in Dos Aguas (Valencian Community, Spain), Cocina Cave is characterized by the presence of several Holocene archaeological deposits from the final Mesolithic to the present day and is a pivotal site for understanding the socio-ecological dynamics of the last hunter-gatherer inhabitants of the Iberian Peninsula and the transition to pastoral and agricultural economies in the Western Mediterranean. However, the identification of strata from particular time-periods in the cave is often difficult due to the homogeneity of layers, the poor archaeological record in some strata and the presence of severe disturbance phenomena. The methodological approach of this study consisted of cross-referencing rare earth elements and other chemical markers with the archaeological stratigraphical interpretation, in an attempt to not only support the identification of the anthropic contribution to the formation of Cocina Cave strata, but also to characterize and confirm different natural and occupational episodes, particularly those associated with hunter-gatherer, early agriculturalist, and shepherd activities. Sediments were collected from different excavation areas and analysed for major elements, trace elements, rare earth elements (REE), soil organic matter (SOM) amounts and pH. Multivariate statistics were employed to group samples according to their elemental profile, and these were then compared to the archaeological temporal interpretation. The obtained results showed that REE amount and fractionation geochemical processes were regulated by carbonates, phosphates and pH. The use of REE as markers was particularly useful as their concentrations and their calculated ratios and anomaly distributions were demonstrated to be highly consistent with the archaeological stratigraphical interpretation.     

Recharge response and kinematics of an unusual earthflow in Liechtenstein

Landslides - Understanding landslide behavior over medium and long timescales is crucial for predicting landslide hazard and constructing accurate landscape evolution models. The behavior of...     

The role of Coriolis in sandbank formation due to a headland/island system

Tidal flow past a headland was modelled and the resulting sandbank formation due to bedload transport by eddy systems was examined with particular emphasis on the role of Coriolis. The headland configuration was extended to the more complex case of a headland with a detached island and the role of Coriolis in sandbank formation was again examined. In the headland case, the contribution of Coriolis resulted in the expected change in bed level at the centre of the residual eddies (positive contribution for the cyclonic eddy and negative change for the anticyclonic eddy). In the headland/island case, the role of Coriolis in the resulting bed level change was a function of eddy shedding from the island, defined by the island wake parameter. When eddy shedding occurred, the inclusion of Coriolis had a large magnitude effect on creating flood/ebb sandbank asymmetry. When eddies remained attached to the island, Coriolis had a minor role in the magnitude of this asymmetry.     

Distinguishing extensive and intensive properties for meaningful geocomputation and mapping

A most fundamental and far-reaching trait of geographic information is the distinction between extensive and intensive properties. In common understanding, originating in Physics and Chemistry, extensive properties increase with the size of their supporting objects, while intensive properties are independent of this size. It has long been recognized that the decision whether analytical and cartographic measures can be meaningfully applied depends on whether an attribute is considered intensive or extensive. For example, the choice of a map type as well as the application of basic geocomputational operations, such as spatial intersections, aggregations or algebraic operations such as sums and weighted averages, strongly depend on this semantic distinction. So far, however, the distinction can only be drawn in the head of an analyst. We still lack practical ways of automation for composing GIS workflows and to scale up mapping and geocomputation over many data sources, e.g. in statistical portals. In this article, we test a machine-learning model that is capable of labeling extensive/intensive region attributes with high accuracy based on simple characteristics extractable from geodata files. Furthermore, we propose an ontology pattern that captures central applicability constraints for automating data conversion and mapping using Semantic Web technology.     

Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO3 chondrites

The distribution of the short‐lived radionuclide ²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly ¹⁶O‐rich (Δ¹⁷O = ?25 to ?20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ¹⁷O of grossite and melilite range from ~ ?11 to ~0‰ and from ~ ?23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial ²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀ is seen, with four having (²⁶Al/²⁷Al)₀ ≤1.1 × 10^?5 and six having (²⁶Al/²⁷Al)₀ ≥3.7 × 10^?5. Five of the ²⁶Al‐rich CAIs have (²⁶Al/²⁷Al)₀ within error of 4.5 × 10^?5; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10^?5 given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the ²⁶Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially ¹⁶O‐rich (Δ¹⁷O ~?24‰) melilite and grossite and ¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic ²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.