Implications of grain size evolution on the seismic structure of the oceanic upper mantle

Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 282 (2009): 178-189, doi:10.1016/j.epsl.2009.03.014.We construct a 1-D steady-state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion-dislocation creep rheology. Grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth. Applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth, seafloor age, and mantle water content. The resulting grain size structure is used to predict shear wave velocity (VS) and seismic quality factor (Q). For a plate age of 60 Myr and an olivine water content of 1000 H/106Si, we find that grain size reaches a minimum of ~15 mm at ~150 km depth and then increases to ~20–30 mm at a depth of 400 km. This grain size structure produces a good fit to the low seismic shear wave velocity zone (LVZ) in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep. Further it predicts a viscosity of ~1019 Pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle, consistent with geophysical observations. We predict larger grain sizes than proposed in recent studies, in which the LVZ was explained by a dry mantle and a minimum grain size of 1 mm. However, we show that for a 1 mm grain size, diffusion creep is the dominant deformation mechanism above 100– 200 km depth, inconsistent with abundant observations of seismic anisotropy from surface wave studies. We therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle. Our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of ~1 mm.Funding for this research was provided by NSF Grants EAR-06-52707 and EAR-07-38880

The clinical and economic impact of alternative staging strategies for adenocarcinoma of the pancreas

Several innovative imaging modalities, including endoscopic ultrasound, have increased the number of available preoperative staging methods in patients with adenocarcinoma of the pancreas. Our goal was to estimate the clinical outcomes and cost-effectiveness of alternative staging strategies for pancreatic adenocarcinoma. METHODS : Decision analysis was used to simulate alternative staging strategies. Cost inputs were based on Medicare reimbursements; clinical inputs were obtained from the available literature. Model endpoints of interest were cost per curative resection and appropriateness of treatment allocation based on pathological stage. RESULTS : Endoscopic ultrasound followed by laparoscopy yielded the lowest cost per curative resection ($37,600) and minimized the number of unnecessary surgical explorations (5.4 per 100 patients staged). Requiring angiographic confirmation when endoscopic ultrasound demonstrated an unresectable tumor yielded an intermediate cost-effectiveness ratio and virtually eliminated the risk of overstaging. Laparoscopy alone maximized the resection rate, but each additional resection would cost approximately$2 million relative to a strategy employing both endoscopic ultrasound and angiography. CONCLUSIONS : Staging strategies incorporating endoscopic ultrasound may improve treatment allocation and are cost-effective relative to angiography-based strategies. A staging protocol that does not incorporate an imaging modality to detect vascular invasion dramatically increases the cost per additional curative resection compared with more comprehensive staging protocols.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74623/1/j.1572-0241.2000.02191.x.pd

The electrical structure of the central Pacific upper mantle constrained by the NoMelt experiment

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 1115–1132, doi:10.1002/2014GC005709.The NoMelt experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2-D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 to 0.5°C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3°C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory data set of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid-ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result.Funding for the NoMELT experiment was provided by the National Science Foundation through the following grant numbers: OCE-0927172, OCE-0928270, OCE-1459649, and OCE-0928663.2015-10-1

Azimuthal seismic anisotropy of 70-ma Pacific-plate upper mantle.

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 124(2), (2019):1889-1909, doi:10.1029/2018JB016451.Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2‐D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70‐Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2‐D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.We thank the Captain and crew of the R/V Marcus G. Langseth and the engineers and technicians from the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution, who provided the instruments through the National Science Foundation's Ocean Bottom Seismograph Instrument Pool (OBSIP). The professionalism and expertise of these individuals were key to the success of this experiment. We also thank Donna Blackman, Tom Brocher, Philip Skemer, and an anonymous reviewer for their thoughtful comments which greatly improved this paper. The OBS data described here are archived at the IRIS Data Management Center (http://www.iris.edu) under network code ZA 2011–2013. The travel time picks are archived in the Marine‐Geo Digital Library (http://www.marine‐geo.org/library/) with the DOI 10.1594/IEDA/324643. This work was supported by NSF grant OCE‐0928663 to D. Lizarralde, J. Collins, and R. Evans; NSF grant OCE‐0927172 to G. Hirth; NSF grant OCE‐0928270 to J. Gaherty; and an NSF Graduate Research Fellowship to H. Mark.2019-07-2

The bipolar outflow and disk of the brown dwarf ISO217

We show that the very young brown dwarf candidate ISO217 (M6.25) is driving an intrinsically asymmetric bipolar outflow with a stronger and slightly faster red-shifted component based on spectro-astrometry of forbidden [SII] emission lines observed in UVES/VLT spectra taken in 2009. ISO217 is only one of a handful of brown dwarfs and VLMS (M5-M8) for which an outflow has been detected and that show that the T Tauri phase continues at the substellar limit. We measure a spatial extension of the outflow of +/-190mas (+/-30AU) and velocities of +/-40-50kms/s. We show that the velocity asymmetry between both lobes is variable on timescales of a few years and that the strong asymmetry of a factor of 2 found in 2007 might be smaller than originally anticipated when using a more realistic stellar rest-velocity. We also detect forbidden [FeII]7155 emission, for which we propose as potential origin the hot inner regions of the outflow. To understand the ISO217 system, we determine the disk properties based on radiative transfer modeling of the SED. This disk model agrees very well with Herschel/PACS data at 70mu. We find that the disk is flared and intermediately inclined (~45deg). The total disk mass (4e-6 Msun) is small compared to the accretion and outflow rate of ISO217 (~1e-10 Msun/yr). We propose that this discrepancy can be explained by either a higher disk mass than inferred from the model (strong undetected grain growth) and/or by an on average lower accretion and outflow rate than the determined values. We show that a disk inclination significantly exceeding 45deg, as suggested from Halpha modeling and from both lobes of the outflow being visible, is inconsistent with the SED data. Thus, despite its intermediate inclination angle, the disk of this brown dwarf does not appear to obscure the red outflow component, which is very rarely seen for T Tauri objects (only one other case).Comment: Accepted for publication at A&A; minor changes (language editing

Grain boundary energies and cohesive strength as a function of geometry

Cohesive laws are stress-strain curves used in finite element calculations to describe the debonding of interfaces such as grain boundaries. It would be convenient to describe grain boundary cohesive laws as a function of the parameters needed to describe the grain boundary geometry; two parameters in 2D and 5 parameters in 3D. However, we find that the cohesive law is not a smooth function of these parameters. In fact, it is discontinuous at geometries for which the two grains have repeat distances that are rational with respect to one another. Using atomistic simulations, we extract grain boundary energies and cohesive laws of grain boundary fracture in 2D with a Lennard-Jones potential for all possible geometries which can be simulated within periodic boundary conditions with a maximum box size. We introduce a model where grain boundaries are represented as high symmetry boundaries decorated by extra dislocations. Using it, we develop a functional form for the symmetric grain boundary energies, which have cusps at all high symmetry angles. We also find the asymptotic form of the fracture toughness near the discontinuities at high symmetry grain boundaries using our dislocation decoration model.Comment: 12 pages, 19 figures, changed titl

Dislocation Kinks in Copper: Widths, Barriers, Effective Masses, and Quantum Tunneling

We calculate the widths, migration barriers, effective masses, and quantum tunneling rates of kinks and jogs in extended screw dislocations in copper, using an effective medium theory interatomic potential. The energy barriers and effective masses for moving a unit jog one lattice constant are close to typical atomic energies and masses: tunneling will be rare. The energy barriers and effective masses for the motion of kinks are unexpectedly small due to the spreading of the kinks over a large number of atoms. The effective masses of the kinks are so small that quantum fluctuations will be important. We discuss implications for quantum creep, kink--based tunneling centers, and Kondo resonances

In vivo expansion of functionally integrated GABAergic interneurons by targeted increase in neural progenitors.

A central hypothesis for brain evolution is that it might occur via expansion of progenitor cells and subsequent lineage-dependent formation of neural circuits. Here, we report in vivo amplification and functional integration of lineage-specific circuitry in Drosophila Levels of the cell fate determinant Prospero were attenuated in specific brain lineages within a range that expanded not only progenitors but also neuronal progeny, without tumor formation. Resulting supernumerary neural stem cells underwent normal functional transitions, progressed through the temporal patterning cascade, and generated progeny with molecular signatures matching source lineages. Fully differentiated supernumerary gamma-amino butyric acid (GABA)-ergic interneurons formed functional connections in the central complex of the adult brain, as revealed by in vivo calcium imaging and open-field behavioral analysis. Our results show that quantitative control of a single transcription factor is sufficient to tune neuron numbers and clonal circuitry, and provide molecular insight into a likely mechanism of brain evolution