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Cortical networks exhibit complex stimulus-response patterns that are based on specific recurrent interactions between neurons. For example, the balance between excitatory and inhibitory currents has been identified as a central component of cortical computations. However, it remains unclear how the required synaptic connectivity can emergeindevelopingcircuitswheresynapsesbetweenexcitatoryandinhibitoryneurons are simultaneously plastic. Using theory and modeling, we propose that a wide range of cortical response properties can arise from a single plasticity paradigm that acts simultaneously at all excitatory and inhibitory connections--Hebbian learning that is stabilized by the synapse-type-specific competition for a limited supply of synaptic resources. In plastic recurrent circuits, this competition enables the formation and decorrelation of inhibition-balanced receptive fields. Networks develop an assembly structure with stronger synaptic connections between similarly tuned excitatory and inhibitory neurons and exhibit response normalization and orientation-specific centersurround suppression, reflecting the stimulus statistics during training. These results demonstrate how neurons can self-organize into functional networks and suggest an essential role for synapse-type-specific competitive learning in the development of cortical circuits. [ABSTRACT FROM AUTHOR]

Simple Summary: There has been an exponential rise in the availability of artificial intelligence systems in endoscopy in recent years. As a result, maintaining an informed understanding of the utility and efficacy of existing systems has become increasingly complex. This review aims to summarise the expanse of research in this area to guide proceduralists in making informed decisions regarding the use of artificial intelligence in colonoscopy. It focuses primarily on the application of artificial intelligence for the detection and characterisation of colorectal polyps in order to improve the efficacy of colorectal cancer screening and prevention. Colorectal cancer remains a leading cause of cancer-related morbidity and mortality worldwide, despite the widespread uptake of population surveillance strategies. This is in part due to the persistent development of 'interval colorectal cancers', where patients develop colorectal cancer despite appropriate surveillance intervals, implying pre-malignant polyps were not resected at a prior colonoscopy. Multiple techniques have been developed to improve the sensitivity and accuracy of lesion detection and characterisation in an effort to improve the efficacy of colorectal cancer screening, thereby reducing the incidence of interval colorectal cancers. This article presents a comprehensive review of the transformative role of artificial intelligence (AI), which has recently emerged as one such solution for improving the quality of screening and surveillance colonoscopy. Firstly, AI-driven algorithms demonstrate remarkable potential in addressing the challenge of overlooked polyps, particularly polyp subtypes infamous for escaping human detection because of their inconspicuous appearance. Secondly, AI empowers gastroenterologists without exhaustive training in advanced mucosal imaging to characterise polyps with accuracy similar to that of expert interventionalists, reducing the dependence on pathologic evaluation and guiding appropriate resection techniques or referrals for more complex resections. AI in colonoscopy holds the potential to advance the detection and characterisation of polyps, addressing current limitations and improving patient outcomes. The integration of AI technologies into routine colonoscopy represents a promising step towards more effective colorectal cancer screening and prevention. [ABSTRACT FROM AUTHOR]

Background: Colorectal adenocarcinoma is an important and preventable complication of inflammatory bowel disease (IBD). A previous case series suggested mental health issues and poor engagement in care as novel risk factors. Aims: To confirm the role of patient engagement in care in the development of neoplasia using a case‐control methodology. Methods: Patients in a single referral centre from 2007 to 2017 with colorectal adenocarcinoma, high‐grade dysplasia or multifocal low‐grade dysplasia were included as neoplasia cases. Each case was assigned up to three matched controls (matched for age, gender, underlying disease, IBD type and phenotype and disease duration). Novel and known risk factors were compared between groups. Results: Thirty‐two cases with 88 matched controls were included. Patients with neoplasia were more likely to have poor adherence to, or engagement with, care (odds ratio (OR) 4.79). They were also more likely to have chronic use of opioids (OR 3.86) and long‐term prednisolone (OR 2.97). Of note, no difference was found in measures of socioeconomic disadvantage, reflecting equitable access to healthcare in the public institution where the care was studied. As previously shown, patients with neoplasia had multiple markers of increased cumulative burden of inflammation, including more IBD‐related hospital admissions, elevated inflammatory markers and severe inflammation at colonoscopy. Conclusions: This study confirms poor adherence or engagement with care as a new risk factor for colorectal adenocarcinoma in patients with IBD; identifying a vulnerable group whom clinicians should endeavour to engage in order to avoid this catastrophic complication. [ABSTRACT FROM AUTHOR]

Microbial methane oxidation - or methanotrophy - is a key control of the global methane budget on Earth, and perhaps in other planetary systems. Here, we explore the potential role of mass-18 isotopologues of methane, expressed as Δ13CH 3 D and Δ12CH 2 D 2 values, in tracking both aerobic and anaerobic methanotrophy in nature. We examine two well documented methanotrophic environments: the Lake Pavin (France) water column, where methane degradation is dominated by aerobic methanotrophy (AeOM), and the Black Sea sediments (offshore Romania), dominated by anaerobic methanotrophy (AOM) coupled to sulfate-reduction. In both settings, lighter isotopologues are preferentially consumed, generating elevated 13CH 4 /12CH 4 , 12CH 3 D/12CH 4 , 13CH 3 D/12CH 4 and 12CH 2 D 2 /12CH 4 ratios. This results in increasing of δ13C and δD values in the residual methane for both settings, as observed commonly in systems dominated by methanotrophy. As a result, AeOM and AOM cannot be easily distinguished by the development of δ13C and δD. In contrast, the Δ13CH 3 D and Δ12CH 2 D 2 (departure from stochastic) values have opposite trajectories, with minimal decreases in the case of the AeOM-dominated system, but dramatic increases in the case of AOM, with Δ13CH 3 D and Δ12CH 2 D 2 reaching values as high as 15.7 ‰ and 76.6 ‰, respectively. This contrasting behavior of clumped isotopologues signatures illustrates fundamental distinction between the two processes and the way they segregate methane isotopologues. These data demonstrate that both AeOM and AOM have distinctive kinetic isotope effects in natural settings, consistent with preliminary laboratory work. In particular, we find that γ-values (which measure the deviation to the product of 'normal' bulk isotope fractionation factors) are close to unity in the case of AeOM (i.e. a negligible clumped isotope effect), but significantly below unity in the case of AOM (i.e. strong clumped isotope effect). In addition, our data also illustrate how AOM under low-sulfate conditions may promote methane isotopologue equilibration. Taken together, we suggest these data and apparent isotopologue fractionation factors extrapolated from these two environments may help refine the potential bio-signatures of methane affected by methanotrophy. [ABSTRACT FROM AUTHOR]

Background: Emergency general surgery is an emerging public health issue globally, with substantial healthcare burden. Interhospital transfer of critically unwell surgical patients has been the mainstay of bridging gaps in surgical coverage in regional and rural locations, despite evidence of greater morbidity and mortality. Delays in transfer invariably occurs and compounds the situation. Our aim was to examine the factors influencing interhospital transfer delays in emergency general surgical patients. Methods: A systematic search of PubMED and EmBase, was performed by two researchers from 2020 to 23rd Feb 2021, for English articles related to interhospital transfer delays in emergency general surgical patients, with an age of >16. Articles were critically appraised and data were extracted into a pre‐specified data extraction form. No data was suitable for statistical analysis and a narrative synthesis was performed instead. Results: Six relevant articles were identified from the search. All studies were retrospective cohort studies with moderate to high risk of bias. Lack of consultant surgeon input, after hours transfer, need for intensive care bed and poor transfer documentation may have a role in interhospital transfer delays. Patients with public health insurance, multiple comorbidities and non‐emergency medical conditions experience longer transfer request time and may be at risk of precipitating interhospital transfer delays. Transfer delays are seen in transfers over longer distances. Conclusion: There is a paucity of knowledge on what and how factors influence interhospital transfer delays in emergency general surgical patients. Well‐designed prospective cohort studies are required to bridge this knowledge gap. [ABSTRACT FROM AUTHOR]

Background: Interhospital transfers in Australia facilitate access to acute surgical services, however transfer delays can occur. The aims of this study were to examine Australian mortality audit data on acute surgical patients who were transferred after presenting with a surgical emergency, and to identify modifiable predictors of transfer delay. Methods: Surgical admissions between 1 January 2001 and 18 August 2020 were retrospectively extracted from the Australian and New Zealand Audit of Surgical Mortality database. Relevant factors and themes of interest were collated. Results were presented as odds ratios (OR) and 95% confidence intervals (CI), with statistical significance defined as P <0.05. Results: After exclusion, a final 8270 cases were analysed. Non‐modifiable predictors identified were female gender (OR 1.34, 95% CI 1.05–1.70, P = 0.0184), comorbidities (OR 1.50, 95% CI 1.40–161, P <0.0001) and major non‐trauma non‐vascular specialty (OR 1.54 to 7.77, depending on specialty, P < 0.05). Modifiable predictors were inadequate clinical assessment (OR 49.48, 95% CI 32.91–74.38, P <0.0001), poor communication (OR 6.62, 95% CI 3.70–11.85, P <0.0001) and multiple transfers (OR 6.30, OR 95% 4.31–9.21, P <0.0001). Age, lack of bed and after‐hours transfer did not predict transfer delays. Metropolitan transfers was protective against transfer delays (OR 0.64, 95% CI 0.47–0.86, P = 0.0035). Conclusion: In the view of the receiving surgeon or assessor, all transfer delays potentially contributed to patient deaths, and may have been preventable. Strategies directed at modifiable factors could minimize delays. Increased surgical services in non‐metropolitan regions could reduce need for transfer. Prospective data is required to examine if the same predictors are observed in surgical patients who survive. [ABSTRACT FROM AUTHOR]

Eosinophilic esophagitis (EoE) is a leading cause of food bolus impaction in children and adults. The mechanism of dysphagia in EoE, particularly non-obstructive dysphagia, remains incompletely understood. While fibrostenotic processes appear to be critical in the development of dysphagia, somatosensory dysfunction and dysmotility also contribute. This review considers potential mechanisms of dysphagia and evaluates the utility of current and future treatment strategies in this context. [ABSTRACT FROM AUTHOR]

The trapped melt fractional crystallization model for the IIIAB iron meteorites put forward by J. T. Wasson two decades prior is revisited. The basic precepts upon which the model was based remain true, and the model can be implemented using Ir and Au solid/liquid distribution coefficients that are broadly consistent with experimental data. For this reason, the difference between the Wasson model and some more recent trapped melt models lies mainly with inferences about the S concentrations of the core of the IIIAB iron meteorite parent body. For the Wasson model, S bulk concentrations of about 2 wt% are implied. For the more recent model, much greater concentrations of between about 12–15 wt% are indicated. The two different trapped melt models profoundly influence the interpretation of high δ57Fe values relative to chondrites in the IIIAB irons. The Wasson model suggests that there should be more variations in δ57Fe than are observed among these meteorites, while the more recent trapped melt model relies on the crystallization of FeS from the trapped melt to raise the δ57Fe of the latter, thus minimizing the variability. The interpretation of Fe isotope ratios in the IIIAB meteorites therefore depends critically on the S concentration of the parent body core. [ABSTRACT FROM AUTHOR]

Background: The Lyell McEwin Hospital entered into a public–private collaborative agreement in 2019 in order to access Da Vinci® Xi Surgical Systems (Intuitive Surgical, Sunnyvale, CA, USA) in private hospitals. This study aimed to examine the costs associated with usage of robot surgical systems under the agreement, and the potential for acquisition in the public hospital. Methods: Retrospective data on robotic-assisted and equivalent operations performed between 1 May 2019 and 30 April 2020 were collected and formed subsequent model inputs. Cost data were from hospital records and the local Da Vinci® Xi distributor. Clinical workflow of operations was simulated with a decision-analytic model, with output being costs incurred. The model’s base case scenario assumed 5% of cases were robotically assisted. Results: A total of 35 robotic-assisted, 101 laparoscopic and 34 open operations were performed. Patients were predominantly male and overweight with multiple comorbidities. Length of stay and duration of operation were associated with increased costs (p < 0.001, r² 0.72). In the base case scenario, there was a cost of AU$26 424 per patient, with an open conversion rate of 11%. Increasing robotic-assisted case percentage resulted in reduced cost, open conversion rates and length of stay. Extrapolation of cost model data indicated if 50% were robotic-assisted cases, then the initial capital investment (assumed at AU$4 000 000) could be recovered in 10 years. Conclusion: Our model highlights potential advantages of performing greater numbers of robotic-assisted operations in a collaborative environment. Cost-effective analysis with prospective data could evaluate if these results are translatable and potentially support acquisition of robotic systems in the public sector. [ABSTRACT FROM AUTHOR]

This study applies a combined isotope and doubly-substituted isotopologue ('clumped') methane approach to samples collected over a 9-year long-term experiment at the Kidd Creek scientific observatory located 2.4 and 2.9 km depth below surface, combined with previously published data from 2.1 km below surface. The observatory is located in a fractured rock system within Kidd Creek Mine in Timmins, Ontario, Canada, situated within a 2.7 Ga Volcanogenic Massive Sulphide (VMS) deposit on the Canadian Shield. Isotope and isotopologue methane data suggest a temporal variation in the various sources of methane within the fracture fluids system between 2.1 and 2.9 km below surface. Predominantly abiogenic methane is identified in samples collected from the deepest level of the mine (2.9 km). Comparing new data from the 2.4 km level with previous data from 2.1 km suggests addition of a small component of microbially-generated methane to the fracture water systems at 2.4 km. The temporal evolution of the methane isotopologue signatures suggest an additional process is occurring within these waters. Specifically, methane in samples from 2.4 km (and some from 2.9 km) approach low-temperature thermodynamic equilibrium in clumped isotopologue space, which is not consistent with kinetically-controlled methane production (either microbial or abiogenic). Anaerobic Oxidation of Methane (AOM) during microbial methanotrophy is shown to be the most likely process to drive such re-equilibration via isotopic bond re-ordering. This study provides an unprecedented high-resolution temporal record over more than a decade for methane in a deep subsurface crystalline environment and demonstrates the advantages of clumped isotopologue studies to identify multiple processes controlling the methane cycle in these systems including both abiotic and biotic methane production and methanotrophy. [ABSTRACT FROM AUTHOR]

Background: Appendicectomy is a safe surgical procedure with minimal risks. Low mortality rates (0.04–0.24%) have been reported from a number of countries. Appendicectomy mortality rates have not been reported in Australia in recent years. The aim of the study was to conduct an appendicectomy mortality audit, to identify clinical management issues and to calculate the mortality rate of appendicectomy in Australia. Methods: This study analysed data from a peer review of appendicectomy deaths in Australian hospitals of all states and territories, from 2006 to 2017. Additional procedural data were obtained from the Australian Institute of Health and Welfare for the calculation of mortality rates. Results: The final analysis included 82 patients of whom 62 were elderly (age >65 years) with 47 males and 35 females. Two or more comorbidities were present in 68 patients. Open operation was performed in 62 patients, with complicated appendicitis found in 50 cases. The majority of management issues were non‐surgical. Of the 16 surgical management issues identified, 11 issues occurred post‐operatively in the elderly, which were potentially preventable. The overall mortality was 0.02% with 0.01% in the young (age <65 years) and 0.20% in the elderly. Conclusion: Australia's appendicectomy mortality rate was very low and comparable to international figures. Surgical management of appendicectomy was satisfactory in general. However, post‐operative care in the elderly was found to be poor at times, with room for improvement. Further research is needed whether improvements in post‐operative care could alter the clinical course to death in the elderly. [ABSTRACT FROM AUTHOR]

Oxygen fugacity is a measure of rock oxidation that influences planetary structure and evolution. Most rocky bodies in the Solar System formed at oxygen fugacities approximately five orders of magnitude higher than a hydrogen-rich gas of solar composition. It is unclear whether this oxidation of rocks in the Solar System is typical among other planetary systems. We exploit the elemental abundances observed in six white dwarfs polluted by the accretion of rocky bodies to determine the fraction of oxidized iron in those extrasolar rocky bodies and therefore their oxygen fugacities. The results are consistent with the oxygen fugacities of Earth, Mars, and typical asteroids in the Solar System, suggesting that at least some rocky exoplanets are geophysically and geochemically similar to Earth. [ABSTRACT FROM AUTHOR]

Abstract Stable isotope compositions of methane (δ13C and δD) and of short-chain alkanes are commonly used to trace the origin and fate of carbon in the continental crust. In continental sedimentary systems, methane is typically produced through thermogenic cracking of organic matter and/or through microbial methanogenesis. However, secondary processes such as mixing, migration or biodegradation can alter the original isotopic and composition of the gas, making the identification and the quantification of primary sources challenging. The recently resolved methane 'clumped' isotopologues Δ13CH 3 D and Δ12CH 2 D 2 are unique indicators of whether methane is at thermodynamic isotopic equilibrium or not, thereby providing insights into formation temperatures and/or into kinetic processes controlling methane generation processes, including microbial methanogenesis. In this study, we report the first systematic use of methane Δ13CH 3 D and Δ12CH 2 D 2 in the context of continental sedimentary basins. We investigated sedimentary formations from the Southwest Ontario and Michigan Basins, where the presence of both microbial and thermogenic methane was previously proposed. Methane from the Silurian strata coexist with highly saline brines, and clumped isotopologues exhibit large offsets from thermodynamic equilibrium, with Δ12CH 2 D 2 values as low as −23‰. Together with conventional δ13C and δD values, the variability in Δ13CH 3 D and Δ12CH 2 D 2 to first order reflects a mixing relationship between near-equilibrated thermogenic methane similar to gases from deeper Cambrian and Middle Ordovician units, and a source characterized by a substantial departure from equilibrium that could be associated with microbial methanogenesis. In contrast, methane from the Devonian-age Antrim Shale, associated with less saline porewaters, reveals Δ13CH 3 D and Δ12CH 2 D 2 values that are approaching low temperature thermodynamic equilibrium. While microbial methanogenesis remains an important contributor to the methane budget in the Antrim Shale, it is suggested that Anaerobic Oxidation of Methane (AOM) could contribute to reprocessing methane isotopologues, yielding Δ13CH 3 D and Δ12CH 2 D 2 signatures approaching thermodynamic equilibrium. [ABSTRACT FROM AUTHOR]

Thermal stability is an essential parameter for coil design. Previous measurements in an MgB2 solenoid have revealed a change in trend in the minimum quench energy (MQE) dependence on j= I/ Ic showing lower values than expected for operative current close to the critical current (j ∼ 0.90). The obtained MQE is comparable to the one predicted by analytical models in a single tape (1D) being of the order of 1–10 mJ, which indicates that propagation across winding and layers (3D) is not taking place. This reduction of MQE is not obtained in the effective continuum model, because the different turns and layers are not reproduced. Obtaining 1D propagation depends on the ratio between the thermal conductivity of the conductor and insulation and on the cooling conditions at the boundary layers. A numerical model of a superconducting solenoid with full representation of the winding is used to study MQE as a function of j (0.6–0.99). The quench is triggered applying heat pulses of 40 ms in a region of 4 mm of the outer layer of the coil. The effect of changing the cooling boundary conditions of the coil from adiabatic to isothermal is also investigated. [ABSTRACT FROM PUBLISHER]

Background: Anti‐tumour necrosis factor alpha (anti‐TNF) agents are a highly effective treatment for inflammatory bowel disease (IBD). Skin lesions, including psoriasiform, eczematous and lupoid eruptions, may paradoxically result from anti‐TNF use and cause significant morbidity leading to discontinuation of therapy. There are no consensus guidelines on the management of these lesions. Aims: This systematic review considers the existing evidence regarding cutaneous complications of anti‐TNF therapy in IBD and the development of an algorithm for management. Methods: A systematic review was performed by searching Medline (Pubmed) and Embase for articles published from inception to January 2021. The following search terms were used 'anti‐tumour necrosis factor alpha', 'infliximab', 'adalimumab', 'certolizumab', 'golimumab', 'inflammatory bowel disease', 'Crohn disease', 'Ulcerative colitis', 'psoriasis', 'psoriasiform', 'dermatitis', 'lupus', 'skin lesion' and 'skin rash'. Reference lists of relevant studies were reviewed to identify additional suitable studies. Results: Thirty‐four studies were included in the review. Eczema can generally be managed with topical agents and the anti‐TNF can be continued, while the development of lupus requires immediate cessation of the anti‐TNF and consideration of alternative immunomodulators. Management of psoriasis and psoriasiform lesions may follow a step‐wise algorithm where topical treatments will be trialled in less severe cases, with recourse to an alternative anti‐TNF or a switch to an alternative class of biological agent. Conclusion: Assessment of anti‐TNF skin lesions should be performed in conjunction with a dermatologist and rheumatologist in complex cases. High‐quality prospective studies are needed to clarify the validity of these algorithms in the future. [ABSTRACT FROM AUTHOR]

Sub-Neptune exoplanets are commonly hypothesized to consist of a silicate-rich magma ocean topped by a hydrogen-rich atmosphere. Previous work studying the outgassing of silicate material has demonstrated that such atmosphere–interior interactions can affect the atmosphere's overall structure and extent. However, these models only considered SiO in an atmosphere of hydrogen gas, without considering chemical reactions between them. Here, we couple calculations of the chemical equilibrium between H, Si, and O species with an atmospheric structure model. We find that substantial amounts of silane, SiH4, and water, H2O, are produced by the interaction between the silicate-rich interior and hydrogen-rich atmosphere. These species extend high into the atmosphere, though their abundance is greatest at the hottest, deepest regions. For example, for a 4 M⊕ planet with an equilibrium temperature of 1000 K, a base temperature of 5000 K, and a 0.1 M⊕ hydrogen envelope, silicon species and water can comprise 30 per cent of the atmosphere by number at the bottom of the atmosphere. Due to this abundance enhancement, we find that convection is inhibited at temperatures ≳2500 K. This temperature is lower, implying that the resultant non-convective region is thicker, than was found in previous models that did not account for atmospheric chemistry. Our findings show that significant endogenous water is produced by magma–hydrogen interactions alone, without the need to accrete ice-rich material. We discuss the observability of the signatures of atmosphere–interior interaction and directions for future work, including condensate lofting and more complex chemical networks. [ABSTRACT FROM AUTHOR]

We describe a unique and novel isotope ratio mass spectrometer (IRMS), the Panorama , developed explicitly for high-mass-resolution analysis of isotopologue ratios of gas samples. The double-focussing instrument routinely operates at a mass resolving power of 40,000 with a maximum useful MRP of ∼80,000. The instrument achieves this exceptional MRP for a multi-collector using a Matsuda ion optical design with an ESA radius of 1018 mm and a magnetic sector radius of 800 mm. Collectors comprise 9 Faraday cups and a single channel of ion counting each with continuously variable collector slits. First results demonstrate both accuracy and precision comparable to, and in some cases, surpassing, other gas-source multi-collector IRMS instruments for singly-substituted species. For example, accurate bulk D/H and 13 C/ 12 C for methane gas measured with CH 4 as the analyte are measured simultaneously with internal precision of 0.02–0.04‰ (1 std error) and ∼0.006‰ (1 se), respectively. Ion counting with continuous rebalancing of sample and standard gases permits high-precision measurements of rare, multiply-substituted isotopologues with relative abundances as small as ∼0.1 ppm. In the case of methane, both 13 CH 3 D/ 12 CH 4 and 12 CH 2 D 2 / 12 CH 4 ratios are measured with precision of ∼0.1‰ and ∼0.5‰, respectively. Accuracy of the multiply-substituted species measurements is demonstrated using isotope ratio mixing experiments. The ability to measure both Δ 13 CH 3 D and ΔCH 2 D 2 (‰ variations relative to the stochastic reference frame) provides heretofore unmatched capabilities to identify kinetic reaction pathways, isotope fractionation during transport, mixing, as well as temperatures of formation for methane gas. The high-resolution instrument can be used for a wide variety of applications. For example, it easily resolves 36 Ar + from 18 O 18 O + for oxygen bond-ordering studies. It also easily resolves 14 N 16 O+ from 15 N 15 N+ for measurements of the doubly-substituted N 2 species. [ABSTRACT FROM AUTHOR]

Earth and the Moon are shown here to have indistinguishable oxygen isotope ratios, with a difference in Δ'17O of -1 ± 5 parts per million (2 standard error). On the basis of these data and our new planet formation simulations that include a realistic model for primordial oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy, high-angular-momentum impact. The results indicate that the late veneer impactors had an average D'17O within approximately 1 per mil of the terrestrial value, limiting possible sources for this late addition of mass to the Earth-Moon system. [ABSTRACT FROM AUTHOR]

High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors. [ABSTRACT FROM AUTHOR]

Chondritic meteorites are made of primitive components that record the first steps of formation of solids in our Solar System. Chondrules are the major component of chondrites, yet little is known about their formation mechanisms and history within the solar protoplanetary disk (SPD). We use the reconstructed concentrations of short-lived 26Al in chondrules to constrain the timing of formation of their precursors in the SPD. High-precision bulk magnesium isotopic measurements of 14 chondrules from the Allende chondrite define a 26Al isochron with 26Al/27Al = 1.2(±0.2) × 10-5 for this subset of Allende chondrules. This can be considered to be the minimum bulk chondrule 26Al isochron because all chondrules analyzed so far with high precision (∼50 chondrules from CV and ordinary chondrites) have an inferred minimum bulk initial (26Al/27Al) ≥ 1.2 × 10-5. In addition, mineral 26Al isochrons determined on the same chondrules show that their formation (i.e., fusion of their precursors by energetic events) took place from 0 Myr to ∼2 Myr after the formation of their precursors, thus showing in some cases a clear decoupling in time between the two events. The finding of a minimum bulk chondrule 26Al isochron is used to constrain the astrophysical settings for chondrule formation. Either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at ∼1.5 My after the start of the Solar System or the transport of precursors from the condensation zone to potential storage sites stopped after 1.5 My, possibly due to a drop in the disk accretion rate. [ABSTRACT FROM AUTHOR]

• Isotope fractionation of melts is controlled by terminal gas velocities. • Theoretical analysis allows for extrapolation to a variety of astrophysical environments. • Isotope fractionation of pebbles feeding planet growth may be controlled by terminal velocity. • Chemical and isotopic compositions of planets may depend on pebble terminal velocities. We use new experiments and a theoretical analysis of the results to show that the isotopic fractionation associated with laser-heating aerodynamic levitation experiments is consistent with the velocity of flowing gas as the primary control on the fractionation. The new Fe and Mg isotope data are well explained where the gas is treated as a low-viscosity fluid that flows around the molten spheres with high Reynolds numbers and minimal drag. A relationship between the ratio of headwind velocity to thermal velocity and saturation is obtained on the basis of this analysis. The recognition that it is the ratio of flow velocity to thermal velocity that controls fractionation allows for extrapolation to other environments in which molten rock encounters gas with appreciable headwinds. In this way, in some circumstances, the degree of isotope fractionation attending evaporation is as much a velocimeter as it is a barometer. [ABSTRACT FROM AUTHOR]

We report a temperature calibration for Δ 47 from bivalve carbonate that lies within error of theoretical predictions (Schauble et al., 2006; Guo et al., 2009). The temperature sensitivity of this calibration is lower than several different earlier calibrations determined using either inorganic calcite, corals, foraminiferans and coccoliths, or brachiopods and bivalved molluscs, but it agrees with more recent Δ 47 -temperature relationships determined from measurements of clumped-isotopes in mollusc and brachiopod shells. We demonstrate that mollusc shell temperature calibrations originating from different laboratories that have not been corrected for instrument backgrounds may differ by as much as ∼0.07‰ in Δ 47 over the ∼0 to 30 °C temperature range even where dΔ 47 /d T agree. Because recent calibrations for Δ 47 vs. T from several different laboratories agree for bivalved mollusc shells, yet differ from an early calibration for bivalved molluscs, we suggest it is unlikely that temperature-Δ 47 variability is attributable to phylum-specific vital effects, and instead conclude that differences in calibration slope between phyla and/or inorganic calcite are more easily explained by variability in measurements made in different laboratories. Discrepancies in both calibration slopes and/or intercepts indicate that Δ 47 values measured in natural materials may be more significantly influenced by instrument-specific effects, as well as effects from sample preparation and handling and purification of CO 2 than current techniques are able to correct for, and therefore, temperatures obtained by comparing measurements of Δ 47 to independently determined calcite calibrations may err by a far greater amount than acknowledged in previous studies. [ABSTRACT FROM AUTHOR]

RATIONALE Rapid development of the CO2 'clumped-isotope' temperature proxy (Δ47) has involved both resource- and labor-intensive analytical methods. We report strategies for measuring Δ47 on a conventional mass spectrometer, with the usual CO2 set of three Faraday collectors while reducing the time devoted to standardization with temperature-equilibrated CO2. METHODS We measured Δ47 in CO2 on a mass spectrometer using only three Faraday cups by 'multicollector peak hopping', directing 46CO2+ and 47CO2+ ion beams into the Faraday cups normally used for m/z 44 and 45. Δ47 was calculated from the previously measured δ13C and δ18O values and we then measured 47CO2+/46CO2+. We invoke a protocol for correcting ion-beam intensities for secondary electrons to address common problems with compositional non-linearity and both long- and short-term measurement stability. RESULTS Our method eliminates the need for six Faraday cups for measuring Δ47, a possible advantage for many laboratories. In addition, by using a correction based on removing the effects of secondary electrons, we remove the Δ47 dependence on δ47CO2. As a result, our methods are robust against mass spectrometer operating conditions. CONCLUSIONS Δ47 can be precisely measured on mass spectrometers currently employed for measuring δ13C and δ18O values without significant resource investment. Accounting for ion scatter during measurements of CO2 results in improved external precision and long-term stability, while significantly increasing the machine time available for sample analysis. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: We modeled the Δ17O of atmospheric O2 using 27 ordinary differential equations comprising a box model composed of the stratosphere, troposphere, geosphere, hydrosphere and biosphere. Results show that 57% of the deficit in 17O in O2 relative to a reference water fractionation line is the result of kinetic isotope fractionation attending the Dole effect, 33% balances the positive Δ17O of O(1D) in the stratosphere, and 10% is from evapotranspiration. The predicted Δ′17O O2 relative to waters is −0.410‰ as measured at the δ18O of air. The value for Δ′17O O2 varies at fixed δ18O with the concentration of atmospheric CO2, gross primary production, and net primary production as well as with reaction rates in the stratosphere. Our model prediction is consistent with our measurements of the oxygen isotopic composition of air O2 compared with rocks if rocks define a fractionation line with an intercept in δ′17O=103ln(δ17O/103 +1) vs. δ′18O=103ln(δ18O/103 +1) space less than SMOW but more positive than some recent measurements imply. The predicted Δ17O is less negative than that obtained from recent measurements of O2 directly against SMOW. Underestimation of Δ′17O O2 can only be ameliorated if the integrated (bulk) Δ′17O for stratospheric CO2 is significantly greater than measurements currently allow. Our results underscore the need for high-precision comparisons of the 17O/16O and 18O/16O ratios of atmospheric O2, VSMOW, and rocks. [Copyright &y& Elsevier]

Abstract: Apparent excesses in early-solar 26Al, 36Cl, 41Ca, and 60Fe disappear if one accounts for ejecta from massive-star winds concentrated into dense phases of the ISM in star-forming regions. The removal of apparent excesses is evident when wind yields from Wolf–Rayet stars are included in the plot of radionuclide abundances vs. mean life. The resulting trend indicates that the solar radionuclides were inherited from parental molecular clouds with a characteristic residence time of 108 yr. This residence time is of the same order as the present-day timescale for conversion of molecular cloud material into stars. The concentrations of these extinct isotopes in the early solar system need not signify injection from unusual proximal stellar sources, but instead are well explained by normal concentrations in average star-forming clouds. The results imply that the efficiency of capture is greater for stellar winds than for supernova ejecta proximal to star-forming regions. [Copyright &y& Elsevier]

Aerobic methane oxidation (AeOM) is an important biological sink of methane on Earth. Stable isotopes are critical tools in tracking the sources and sinks of Earth's surface methane budget. However, the major factors that influence the two multiply-substituted (clumped) isotope signatures of AeOM, Δ 13 CH 3 D and Δ 12 CH 2 D 2 , are not well known. Here we quantify the influence of kinetics as a function of temperature and different methane monooxygenase (MMO) enzymes (modulated by copper) on the isotopologue concentrations of residual methane by the obligate aerobic methanotroph, Methylococcus capsulatus (Bath). We observe deviations from traditional closed-system distillation (Rayleigh) fractionation during exponential growth at high oxidation rates. We model this as a reservoir effect controlled by the ratio of oxidation rate in the cells to transport rate of methane into the cells, where environmental temperature affects both rates. We also test whether clumped isotope fractionation values vary for the particulate versus soluble MMOs, but the results show minimal differences. We further determine that the back reaction (re-equilibration) of methane with medium water is unlikely. Together, the observations and model demonstrate that at low oxidation-to-transport ratios, the clumped isotope signatures follow canonical Rayleigh fractionation, whereas at high ratios, more positive Δ 12 CH 2 D 2 values result, deviating from simple Rayleigh-like trajectories. This study shows that the methane oxidation-to-transport ratio is a critical influence on clumped isotope signatures of AeOM that should be considered when interpreting the isotopic data of natural methane samples in both open and closed systems. [ABSTRACT FROM AUTHOR]

Past medical history included morbid obesity, umbilical hernia and an Orbera Intragastric Balloon System (Apollo Endosurgery Inc., Austin, TX, USA) inserted 3 years prior. An abdominal computed tomography scan identified a radiodense structure positioned within a distal small bowel loop, with proximal small bowel obstruction (Fig. The Meckel's diverticulum is positioned to the left side of the photograph, with the deflated black intragastric balloon situated inside and extending out towards the right. gl To the authors' knowledge, this is the first report of a small bowel obstruction caused by intragastric balloon impacting inside a Meckel's diverticulum. [Extracted from the article]

Magnesium isotopes are potentially powerful tracers for high-temperature geochemical processes if relevant fractionation factors are known. However, experimental data for Mg isotope fractionation are lacking at high temperatures. We performed experiments at 600, 700, and 800°C and 1GPa to establish the equilibrium magnesium isotope partitioning between forsterite (Mg2SiO4) and magnesite (MgCO3) and between spinel (MgAl2O4) and magnesite, making use of the carbonate as an isotope exchange medium to overcome sluggish diffusion-limited magnesium isotope exchange between spinel and forsterite. Using the three-isotope method, the magnitudes of exchange between forsterite and magnesite, and between spinel and magnesite, were determined at the three temperatures for varying lengths of time, allowing equilibrium isotope partitioning to be established. Results are as follows: =0.04±0.04‰ at 800°C, 0.11±0.10‰ at 700°C, and 0.44±0.10‰ at 600°C; and =0.90±0.28‰ at 800°C, 1.10±0.27‰ at 700°C, and 1.73±0.38‰ at 600°C. From these experimentally determined equilibrium fractionation values, we derive the temperature-dependent equilibrium fractionation between spinel and forsterite by difference, yielding values of 0.86±0.29‰ at 800°C, 0.99±0.29‰ at 700°C, and 1.29±0.39‰ at 600°C. These data agree well with first-principles estimates of equilibrium magnesium isotope fractionation between spinel and forsterite at high temperatures. The data allow the calculation of an experimentally determined equation for the temperature dependence of 26Mg/24Mg fractionation between spinel and olivine: . This first high-T experimental calibration of Mg isotope fractionation of mantle minerals is consistent with expectations based on the crystal chemical environment of Mg in these phases. [ABSTRACT FROM AUTHOR]

Abstract: The geochemical implications of mass-dependent fractionation of stable Ni isotopes in natural materials are difficult to assess because the inter-mineral equilibrium fractionation factors for Ni are unknown. We report experimentally determined equilibrium fractionation of Ni isotopes between Ni-metal and Ni-talc from 500°C to 950°C and 8 to 13 kbar using the three-isotope method. While significant isotope exchange was observed between metal and talc, little or no exchange occurred between Ni-metal and Ni-oxide under similar conditions. These results demonstrate the importance of chemical potential differences between the initial and final phase assemblages in facilitating isotope exchange in the experiments. Metal had greater 62Ni/58Ni than talc in all experiments and the equilibrium temperature dependence may be described by the relation (±2se). Kinetic fractionation resulting from diffusion appeared to perturb isotopic equilibration at long time scales and/or high temperatures. These experiments place constraints on Ni isotope variations observed in natural systems. For example, Ni isotope fractionation between alloy and chondrule in the Ausson meteorite is larger than predicted for an equilibrium process at the canonical metamorphic temperatures of L5 chondrites. At 2500K, the results imply a difference of 0.04‰ between equilibrated bulk silicate Earth and chondrites, indicating that Ni isotopes are not likely to be useful as tracers of planetary differentiation, unless fractionation occurred by a Rayleigh distillation process involving a well-mixed silicate reservoir. [Copyright &y& Elsevier]

Abstract: S. Simon et al. incorrectly suggest that in earlier work we claimed there was no Ti3+ in Wark-Lovering rim pyroxenes. In neither the paper by nor the subsequent paper by did we assert that there was no Ti3+ in rim pyroxenes. Rather, we found that many pyroxenes have Ti3+ below detection while others have lower Ti3+/Ti4+ than is typical of CAI interiors, indicating rim formation in a relatively oxidizing environment. showed through exhaustive testing that the suggestion by that EMPA data in the paper by were flawed is incorrect. Here we consider each point raised in the comment by S. Simon et al. and reiterate that our electron microprobe data and the XANES data of agree and demonstrate a statistically significant (∼2σ) or greater difference between rim and interior pyroxene Ti3+/Ti4+. We show that the oxidation states of Ti in Wark-Lovering rim pyroxenes, the chemistry of rim pyroxenes, and the modal abundances of rim minerals are best explained by reaction between the CAI and gas that was orders of magnitude more oxidizing than the solar-like gas from which the CAIs originally formed. [Copyright &y& Elsevier]

The use of wind and react bi2212 wire for high field (>20 T) insert coils has been demonstrated to be a promising technology. While cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets there is little data for the high field application. The present work shows data from systematic quench measurements at 4.2 K on two 6-layer 300 mm high bi2212 solenoid coils at different currents I between 50% and 95% of IC in different background fields of 15, 18 and 20 T from a wide bore compact NbTi-Nb3Sn magnet developed by Oxford Instruments. The quench was induced by localized heat pulses (20–100 ms) and recorded with temporal-spatial resolved voltage taps and thermometers. By precise control of the power delivered in the heat pulse, the minimum quench energy MQE and the time constant for the quasi-stationary minimum propagation zone MPZ was obtained for each test condition. MQE was found to follow a scaling law of IC^2/I^4 in three different coils. The MPZ exhibited the expected anisotropy, extending predominately in the tangential direction and confined in a single winding layer. The propagation was also predominantly along the winding layer with a relatively slow velocity less than 50 cm/s, with an even lower radial propagation velocity of \sim3 mm/s. In the present study, the interplay between the HTS coils and with the LTS background field were also measured and the results are highly relevant to the design of 20 T plus high field magnets with integrated LTS and HTS coils. The results were analysed in the context of a wide current temperature range for current sharing in the bi2212 wire and the T^3 dependence of heat capacity at low temperatures. [ABSTRACT FROM AUTHOR]

Abstract: The difference between the precise MC-ICPMS analyses of bulk calcium–aluminum-rich inclusion (CAI) fragments (e.g., Jacobsen et al., 2008) and supra-canonical values obtained by micro-analytical techniques, e.g., laser ablation MC-ICPMS (Young et al., 2005) and SIMS (Taylor et al., 2005), at face value seems to be problematic and therefore leads many to dismiss claims of the solar system (26Al/27Al)0 greater than ~5×10−5 as spurious. Here we use mass balance calculations to quantify the importance of open system isotopic exchange during CAI evolution and show that in situ supra-canonical, in-situ canonical, and bulk canonical measurements can all exist for an individual CAI. The calculations describe mechanisms of isotopic exchange that may have occurred early (100''ska) and late (~1.5Ma) in the solar nebula and much later (>10''sMa) on parent body planetesimals. A range of possible modal mineralogies is modeled in order to populate the compositional space defined by in situ and bulk CAI measurements. In support of these simulated data we describe in situ measurements of 27Al/24Mg, 25Mg/24Mg, and 26Mg/24Mg obtained by LA-MC-ICPMS comprising core-to-rim traverses across three CV3 CAIs. The CAIs exhibit distinctive Mg isotopic zoning profiles and varying abundances of daughter products of the short-lived nuclide 26Al that are consistent with varying amounts of open system isotope exchange. [Copyright &y& Elsevier]

Abstract: Silicon has long been considered a possible light element in Earth''s core. If differences in 30Si/28Si ratios between metal (core) and silicate (mantle and crust) can be quantified, silicon isotopes may be used to constrain the amount of this element in the core, and in so doing elucidate the conditions that attended Earth''s differentiation. We investigate Si-isotope fractionation between metal and silicate in metal-rich enstatite meteorites as an analogue for Earth''s differentiation. We report here a 5 to 6‰ difference in the 30Si/28Si ratio between Si in metal and Si in silicate in the aubrites (enstatite achondrites) Mount Egerton and Norton County. The meteorites are believed to have derived from enstatite chondrites by melting and thermal metamorphism with final equilibration at 1200 and 1130±80K, respectively. Using the measured silicate–metal Si-isotope fractionation in these rocks we obtain a temperature dependence for fractionation of , in agreement with independent experimental and theoretical determinations. The measured silicate–metal fractionation suggests a ∼0.8‰ difference in the 30Si/28Si ratio between Earth''s core and mantle at P/T conditions relevant to core formation. Our results, based on thermodynamic calculations for Si solubility in iron-rich metal and the measured Si-isotopic silicate–metal fractionation, imply at least ∼6wt.% Si in the core (depending on the exact Δ 30SiBSE-chondrite value). The Si-isotope data also require that oxygen fugacity in the lower mantle increased during or after the process of core segregation by 1 to 2 log units. [Copyright &y& Elsevier]

Abstract: Variations in the stable isotope abundances of transition metals have been observed in the geologic record and trying to understand and reconstruct the physical/environmental conditions that produced these signatures is an area of active research. It is clear that changes in oxidation state lead to large fractionations of the stable isotopes of many transition metals such as iron, suggesting that transition metal stable isotope signatures could be used as a paleo-redox proxy. However, the factors contributing to these observed stable isotope variations are poorly understood. Here we investigate how the kinetics of iron redox electrochemistry generates isotope fractionation. Through a combination of electrodeposition experiments and modeling of electrochemical processes including mass-transport, we show that electron transfer reactions are the cause of a large isotope separation, while mass transport-limited supply of reactant to the electrode attenuates the observed isotopic fractionation. Furthermore, the stable isotope composition of electroplated transition metals can be tuned in the laboratory by controlling parameters such as solution chemistry, reaction overpotential, and solution convection. These methods are potentially useful for generating isotopically-marked metal surfaces for tracking and forensic purposes. In addition, our studies will help interpret stable isotope data in terms of identifying underlying electron transfer processes in laboratory and natural samples. [Copyright &y& Elsevier]

The prospect of medium/high field superconducting magnets using second generation (2G) HTS tapes is approaching reality with continued enhancement in the performance of these conductors. While the cryogenic stability and quench propagation are fundamental issues for the design and safe operation of superconducting magnets, there is insufficient understanding and experimental data for 2G HTS conductors, in particular for the high field scenario at low temperature (<77 K) where the current sharing regime is much larger than in low temperature superconductors. The present work includes a systematic characterization of the relevant thermal-electrical properties used for both qualitative discussion and numerical analysis. Direct measurements of one dimensional adiabatic quench initiation and propagation of a stabilizer-free 2G conductor have been carried out with spatial-temporal recording of temperature and voltage following the deposition of varying local heat pulses to the conductor at different temperatures between 30 K and 77 K carrying different transport currents. The minimum quench energy, and the heat generation in the minimum propagation zone (MPZ) have been obtained as a function of temperature and transport current. The results show quench features unique to HTS such as an increasing MPZ with transport current and higher quench energies at lower temperatures. The experimental results are discussed in the context of current sharing over a large temperature range. [ABSTRACT FROM AUTHOR]

Abstract: Heparin is a glycosaminoglycan well known for its anticoagulant properties. In addition, heparin possesses anti-inflammatory effects. Although the mechanisms responsible for the anticoagulant effects of heparin are well understood, those underlying its anti-inflammatory effects are not. This review presents some of the evidence from clinical and animal studies supporting an anti-inflammatory role for heparin and heparin-related derivatives. Potential mechanisms by which heparin can exert its anti-inflammatory effects are discussed. The clinical use of heparin as an anti-inflammatory agent has been held back by the fear of bleeding. Development of nonanticoagulant heparins or heparin derivatives should mitigate this concern. [Copyright &y& Elsevier]

Abstract: The iron isotopic fractionation factor between magnetite and fayalite was measured at high temperature in a piston–cylinder apparatus. For the first time, the three-isotope method [Matsuhisa, J., Goldsmith, J. R., and Clayton, R. N., 1978. Mechanisms of hydrothermal crystallisation of quartz at 250 °C and 15 kbar. Geochim. Cosmochim. Acta 42, 173–182.] has been used to determine equilibrium fractionation between two minerals directly. The experimentally-determined temperature-dependent iron isotope fractionation between fayalite and magnetite is described by 103ln α57 Mag–Fa ≅Δ57FeMag–Fa =0.30 (+/−0.024)×106 / T 2. The equation predicts measurable fractionation at magmatic temperatures. Our work bears on the magnitude of Fe isotope fractionation attending differentiation in magmatic systems and provides a new isotope thermometer for co-existing fayalite and magnetite. [Copyright &y& Elsevier]

Abstract: Optically thin surfaces of the solar circumstellar disk were likely sites for generating 16O isotope variability in the early Solar System. Astrochemical reaction network calculations predict that a robust feature of these photoactive horizons of the disk was conversion of CO gas to 16O-poor (high Δ17O) H2O ice on a timescale of 105 yr. Within several AU of the central star ultraviolet fluxes were too great for the oxygen isotopic effects of CO photodissociation to be sequestered in H2O, meaning that the CO self shielding oxygen isotopic effect was an outer disk phenomenon. Calculations depicting transport in the circumstellar disk suggest that CO photodissociation at disk surfaces triggered a wave of high-Δ17O H2O that passed from surface regions through the outer disk and into the rocky planet-forming region on a timescale of 105 to 106 yr. [Copyright &y& Elsevier]

Abstract: Silicon isotope ratios of a typical CAI from the Leoville carbonaceous chondrite, obtained in situ by laser ablation MC-ICPMS, together with existing 25Mg/24Mg data, reveal a detailed picture of the astrophysical setting of CAI melting and subsequent heating. Models for the chemical and isotopic effects of evaporation of the molten CAI are used to produce a univariant relationship between P H2 and time during melting. The result shows that this CAI was molten for a cumulative time of no more than 70 days and probably less than 15 days depending on temperature. The object could have been molten for an integrated time of just a few hours if isotope ratio zoning was eliminated after melting by high subsolidus temperatures (e.g., >1300 K) for ∼500 yr. In all cases subsolidus heating sufficient to produce diffusion-limited isotope fractionation at the margin of the solidified CAI is required. These stable isotope data point to a two-stage history for this igneous CAI involving melting for a cumulative timescale of hours to months followed by subsolidus heating for years to hundreds of years. The thermobarometric history deduced from combining Si and Mg isotope ratio data implicates thermal processing in the disk, perhaps by passage through shockwaves, following melting. This study underscores the direct link between the meaning of stable isotope ratio zoning, or lack thereof, and the inferred astrophysical setting of melting and subsequent processing of CAIs. [Copyright &y& Elsevier]

Abstract: The magnesium isotopic composition of Calcium-, Aluminium-rich Inclusions (CAIs) and chondrules from the CBb chondrites HH237 and QUE94411 was measured using MC-ICPMS coupled with a laser ablation system. CAIs from CBb chondrites exhibit limited mass-dependent fractionation (δ 25Mg′ (DSM3) <1.3‰) and formed with undetectable 26Al (26Al/27Al <4.6×10−6). Petrographic observations suggest that CBb CAIs are igneous. The magnesium isotopic composition of CBb igneous CAIs contrast with that of CV3 igneous CAIs which are usually mass fractionated and formed with an elevated initial abundance of 26Al. We contend that the absence of 26Al in CAIs is due either to a late formation in the case of a stellar origin of 26Al, or to a lack of exposure to impulsive flares in the case of an irradiation origin of 26Al. In both cases, it implies that a protoplanetary disk was present ∼4563 Ma ago, when CBb chondrites agglomerated. Chondrules have δ 25Mg′ (DSM3) varying from −0.80 to 0.95‰. A rough negative correlation is observed between the δ 25Mg′ of chondrules and their 24Mg/27Al ratio. This correlation is attributed to evaporation rather than mixing. Contrary to CAIs, chondrules from CBb chondrites have a magnesium isotopic composition similar to that of CV3 chondrules. This last result is surprising as CBb chondrules are significantly different from CV3 chondrules in mineralogy and chemistry. If chondrules from CBb chondrites formed in an impact-related vapour plume as proposed by Krot et al. [A.N. Krot, Y. Amelin, P. Cassen and A. Meibom, Young chondrules in CB chondrites formed by a giant impact in the early Solar System, Nature 436 (2005) 989–992], our data show that physical conditions in the vapour plume were similar to those of the solar accretion disk at the time and location of the formation of CV chondrules. We note that the oxygen isotopic composition of CAIs is incompatible with their remelting in the putative impact vapour plume. Alternatively, it is possible that CBb chondrules formed in a protoplanetary disk as the differences between these and “normal” CV3 chondrules can also be explained in term of spatial and temporal variations of the protoplanetary disk. We show that their young Pb–Pb age is not an argument in favour of an impact origin as protoplanetary disks can last as long as 10 Myr around protostars. If CBb chondrites formed in the solar accretion disk, we speculate they might be the last formed chondrite group. Such a hypothesis might shed light on the unique properties of CBb chondrites. [Copyright &y& Elsevier]

Abstract: Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities (f O2) near that of the Iron–Wüstite (IW) f O2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f O2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for f O2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) f O2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f O2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3×105 yr of their formation in the solar nebula. [Copyright &y& Elsevier]

The canonical initial [sup 26]Al/[sup 27]Al ratio of 4.5 x 10[sup-5] has been a fiducial marker for the beginning of the solar system.Laser ablation and whole-rock multiple-collector inductively coupledplasma-source mass spectrometry magnesium isotope analyses ofcalcium-and aluminum-rich inclusions (CAIs) from CV3 meteoritesdemonstrate that some CAIs had initial [sup 26]Al/[sup 27]Al values atleast 25% greater than canonical and that the canonical initial[sup 26]Al/[sup 27]Al cannot mark the beginning of solar systemformation. Using rates of Mg diffusion in minerals, we find that thecanonical initial [sup 26]Al/[sup 27]Al is instead the culmination ofthousands of brief high-temperature events incurred by CAIs during a10[sup -5]-year residence time in the solar protoplanetary disk. [ABSTRACT FROM AUTHOR]

The critical Rayleigh number for convective flow of pore water through a permeable spherical body is derived. The result affords constraints on carbonaceous chondrite planetesimal sizes and rates of heat production. If the chemical compositions of carbonaceous chondrites are taken as evidence against flow of pore water, then the present calculations suggest that the parent bodies were less than ∼80 km in diameter. The maximum diameter for absence of flow depends in part on the high permeabilities required by estimates of high water/rock ratios in altered carbonaceous chondrites. Pore water would have flowed through parent bodies larger than ∼120 km in diameter with typical rock-like permeabilities. Convective flow and isochemical alteration are not compatible and our calculations can be used to identify the combinations of parent body size and heat generation consistent with one or the other. [Copyright &y& Elsevier]

The mass-dependent fractionation laws that describe the partitioning of isotopes are different for kinetic and equilibrium reactions. These laws are characterized by the exponent relating the fractionation factors for two isotope ratios such that α2/1 = α3/1β. The exponent β for equilibrium exchange is (1/m1 − 1/m2)/(1/m1 − 1/m3), where mi are the atomic masses and m1 < m2 < m3. For kinetic fractionation, the masses used to evaluate β depend upon the isotopic species in motion. Reduced masses apply for breaking bonds whereas molecular or atomic masses apply for transport processes. In each case the functional form of the kinetic β is ln(M1/M2)/ln(M1/M3), where Mi are the reduced, molecular, or atomic masses. New high-precision Mg isotope ratio data confirm that the distinct equilibrium and kinetic fractionation laws can be resolved for changes in isotope ratios of only 3‰ per amu. The variability in mass-dependent fractionation laws is sufficient to explain the negative Δ17O of tropospheric O2 relative to rocks and differences in Δ17O between carbonate, hydroxyl, and anhydrous silicate in Martian meteorites. (For simplicity, we use integer amu values for masses when evaluating β throughout this paper.) [Copyright &y& Elsevier]

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in situ in meteoritical materials show that there are mass-dependent variations in δ25Mg and δ26Mg up to 1.5 ‰ per amu in chondrules and 0.3‰ per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high δ25Mg and low Δ17O. The laser ablation analyses of the CAI show that high-precision determinations of both δ25Mg and δ26Mg can be used to discriminate features of the 26Al-26Mg isotope system that are relevant to chronology from those that result from element mobility. [Copyright &y& Elsevier]