Late metal-silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling

The short-lived $^{182}$Hf-$^{182}$W decay system is a powerful chronometer for constraining the timing of metal-silicate separation and core formation in planetesimals and planets. Neutron capture effects on W isotopes, however, significantly hamper the application of this tool. In order to correct for neutron capture effects, Pt isotopes have emerged as a reliable in-situ neutron dosimeter. This study applies this method to IAB iron meteorites, in order to constrain the timing of metal segregation on the IAB parent body. The $\epsilon^{182}$W values obtained for the IAB iron meteorites range from -3.61 $\pm$ 0.10 to -2.73 $\pm$ 0.09. Correlating $\epsilon^{\mathrm{i}}$Pt with $^{182}$W data yields a pre-neutron capture $^{182}$W of -2.90 $\pm$ 0.06. This corresponds to a metal-silicate separation age of 6.0 $\pm$ 0.8 Ma after CAI for the IAB parent body, and is interpreted to represent a body-wide melting event. Later, between 10 and 14 Ma after CAI, an impact led to a catastrophic break-up and subsequent reassembly of the parent body. Thermal models of the interior evolution that are consistent with these estimates suggest that the IAB parent body underwent metal-silicate separation as a result of internal heating by short-lived radionuclides and accreted at around 1.4 $\pm$ 0.1 Ma after CAIs with a radius of greater than 60 km.Comment: 11 pages, 8 figures, 2 tables; open access article under the CC BY-NC-ND license (see http://creativecommons.org/licenses/by-nc-nd/4.0/

Early Solar System chronology and isotopic linkage of non-carbonaceous chondrules, clasts and achondrites

Chondrules are ubiquitous igneous spherules, the main constituents of ordinary chondrites, and are considered to be critical building blocks of planetesimals, yet their age and formation mechanism(s) remain debated. Differences between ages determined from the long-lived Pb-Pb and the short-lived 26Al-26Mg chronometers have been attributed to the heterogeneous distribution of 26Al in the solar nebula, a radionuclide that was responsible for early differentiation of planetesimals. To evaluate this hypothesis, the 26Al-26Mg and the 207Pb-206Pb isotopic compositions were both measured by multi-collector ICP-MS in individual chondrules and igneous clasts in unequilibrated ordinary chondrites, and three achondrites. A subset of the chondrules were also analyzed by in situ 26Al-26Mg secondary ionization mass spectrometry (SIMS), to compare with their bulk Mg model ages and Pb-Pb ages, and for 54Cr isotopic anomalies. A comparison between bulk Mg model ages and Pb-Pb ages in the same chondrules suggests that the absolute ages for CAIs may be too young, and that Pb-Pb ages date precursor formation rather than time of crystallization. Precursors may have been previous generations of chondrules that were recycled by transient shock waves in the protoplanetary disk. Chronological results of EC 002 date its crystallization at 4566 Ma which is the oldest evidence of planetary crust formation. Analysis of two angrites NWA 10463 and NWA 8535 indicate a more diverse suite of processes that shaped the evolution of the angrite parent body and differentiation of early formed planetesimals. All together, the presented results support a homogeneous distribution of 26Al in the solar nebula

Fitting Thermal Evolution Models to the Chronological Record of Erg Chech 002 and Modeling the Ejection Conditions of the Meteorite

The history of accretion and differentiation processes in the planetesimals is provided by various groups of meteorites. Sampling different parent body layers, they reveal the circumstances of the metal–silicate segregation and the internal structures of the protoplanets. The ungrouped achondrite Erg Chech 002 (EC 002) added to the suite of samples from primitive igneous crusts. Here we present models that utilize thermochronological data for EC 002 and fit the accretion time and size of its parent body to these data. The U-corrected Pb–Pb pyroxene, Pb–Pb phosphate, and Ar–Ar ages used imply a best-fit planetesimal with a radius of 20–30 km that formed at 0.1 Ma after calcium-aluminum-rich inclusions. Its interior melted early and differentiated by 0.5 Ma, allowing core and mantle formation with a transient lower mantle magma ocean and a melt fraction of <25% at the meteorite layering depth. EC 002 formed from this melt at a depth of 0.8 km in a partially differentiated region covered by an undifferentiated crust. By simulating collisions with impactors of different sizes and velocities, we analyzed the minimum ejection conditions of EC 002 from its original parent body and the surface composition of the impact site. The magma ocean region distinct from the layering depth of EC 002 implies that it was not involved in the EC 002 genesis. Our models estimate closure temperatures for the Al–Mg ages as 1030–1200 K. A fast parent body cooling attributes the late Ar–Ar age to a local reheating by another, late impact

DIRAS3-Derived Peptide Inhibits Autophagy in Ovarian Cancer Cells by Binding to Beclin1

Autophagy can protect cancer cells from acute starvation and enhance resistance to chemotherapy. Previously, we reported that autophagy plays a critical role in the survival of dormant, drug resistant ovarian cancer cells using human xenograft models and correlated the up-regulation of autophagy and DIRAS3 expression in clinical samples obtained during &#8220;second look&#8222; operations. DIRAS3 is an imprinted tumor suppressor gene that encodes a 26 kD GTPase with homology to RAS that inhibits cancer cell proliferation and motility. Re-expression of DIRAS3 in ovarian cancer xenografts also induces dormancy and autophagy. DIRAS3 can bind to Beclin1 forming the Autophagy Initiation Complex that triggers autophagosome formation. Both the N-terminus of DIRAS3 (residues 15&#8211;33) and the switch II region of DIRAS3 (residues 93&#8211;107) interact directly with BECN1. We have identified an autophagy-inhibiting peptide based on the switch II region of DIRAS3 linked to Tat peptide that is taken up by ovarian cancer cells, binds Beclin1 and inhibits starvation-induced DIRAS3-mediated autophagy

Increased subcortical neural activity among HIV+ individuals during a lexical retrieval task

BACKGROUND: Deficits in lexical retrieval, present in approximately 40% of HIV+ patients, are thought to reflect disruptions to frontal-striatal functions and may worsen with immunosuppression. Coupling frontal-striatal tasks such as lexical retrieval with functional neuroimaging may help delineate the pathophysiologic mechanisms underlying HIV-associated neurological dysfunction. OBJECTIVE: We examined whether HIV infection confers brain functional changes during lexical access and retrieval. It was expected that HIV+ individuals would demonstrate greater brain activity in frontal-subcortical regions despite only minimal differences between groups on neuropsychological testing. Within the HIV+ sample, we examined associations between indices of immunosuppression (recent and nadir CD4+ count) and task-related signal change in frontostriatal structures. METHOD: 16 HIV+ participants and 12 HIV− controls underwent fMRI while engaged in phonemic/letter and semantic fluency tasks. Participants also completed standardized measures of verbal fluency. RESULTS: HIV status groups performed similarly on phonemic and semantic fluency tasks prior to being scanned. fMRI results demonstrated activation differences during the phonemic fluency task as a function of HIV status, with HIV+ individuals demonstrating significantly greater activation in BG structures than HIV− individuals. There were no significant differences in frontal brain activation between HIV status groups during the phonemic fluency task, nor were there significant brain activation differences during the semantic fluency task. Within the HIV+ group, current CD4+ count, though not nadir, was positively correlated with increased activity in the inferior frontal gyrus and basal ganglia. CONCLUSION: During phonemic fluency performance, HIV+ patients recruit subcortical structures to a greater degree than HIV− controls despite similar task performances suggesting that fMRI may be sensitive to neurocompromise before overt cognitive declines can be detected. Among HIV+ individuals, reduced activity in the frontal-subcortical structures was associated with lower CD4+ count