We present a study of the 3-dimensional (3D) propagation of electrical waves in the heart wall using Laminar Optical Tomography (LOT). Optical imaging contrast is provided by a voltage sensitive dye whose fluorescence reports changes in membrane potential. We examined the transmural propagation dynamics of electrical waves in the right ventricle of Langendorf perfused rat hearts, initiated either by endo-cardial or epi-cardial pacing. 3D images were acquired at an effective frame rate of 667Hz. We compare our experimental results to a mathematical model of electrical transmural propagation. We demonstrate that LOT can clearly resolve the direction of propagation of electrical waves within the cardiac wall, and that the dynamics observed agree well with the model of electrical propagation in rat ventricular tissue.
A B S T R A C TThe Ellenburger-San Saba aquifer discharges spring flows into the overlying Hamilton Creek bed in Burnet County, central Texas. The aquifer is susceptible to contamination from surface-water reservoirs because of the presence of dissolution cavities that are hydraulically connected to the reservoirs in some locations. There is concern that preferential groundwater seepage from reservoirs into the aquifer in these locations might ultimately degrade the quality of the springwater that enters Hamilton Creek. To investigate preferential groundwater seepage patterns and hydraulic connectivity between surface-water reservoirs and the Ellenburger-San Saba aquifer, geophysical reconnaissance surveys were completed between July 2017 and January 2018 to map dissolution cavities and locate preferential groundwater seepage within a specific region of the aquifer. Two-dimensional electric resistivity tomography and self-potential profiling were utilized, and a simplified, three-dimensional finite-element model of the field site was constructed to provide an interpretive aid. The self-potential data indicated the occurrence of preferential groundwater seepage through a porous seepage conduit that was imaged by the electric resistivity tomography data but did not indicate the occurrence of groundwater seepage through two fluid-filled dissolution cavities that were imaged by electric resistivity tomography data. Collectively, the surveying and modelling results demonstrate the efficacy of geoelectric methods for mapping the locations of dissolution cavities and preferential groundwater seepage in the electrically resistive karst terrane of the Ellenburger-San Saba aquifer.
The Spongtang ophiolite (Ladakh, NW India) constrains the nature of oceanic lithosphere before Indo-Asia collision and key stages in the development of the Himalayas. We report whole-rock
40
Ar/
39
Ar and
in situ
zircon
238
U–
206
Pb ages from its crustal and upper and lower mantle sequences. Major and trace elements from harzburgite minerals suggest that the ophiolite formed at a mid-ocean ridge-type spreading centre, whereas published spinel compositions from Spongtang dunites are consistent with a suprasubduction-zone setting. Rare earth element-in-two-pyroxene thermometry for the harzburgite yields 1058 ± 13°C whereas temperature from solvus-based two-pyroxene and olivine–spinel thermometry is lower (to 656°C). The distribution suggests that the mantle section of the ophiolite cooled at rates of 100° Ma
−1
or slower. Based on ages, major and trace element geochemistry, and geospeedometric estimates, we model the origin of the Spongtang ophiolite as forming within a mid-ocean ridge-type spreading centre with a spreading rate >2 cm a
−1
in the Neotethyan Ocean, possibly from the Late Triassic to Jurassic. By the Early Cretaceous, the ridge experienced increasing influence of subduction beneath the Spongtang oceanic lithosphere owing to a subduction polarity reversal. Based on
238
U–
206
Pb ages of the youngest Cenozoic zircon grain, latest obduction occurred between 64.3 ± 0.8 and 42.4 ± 0.5 Ma, in accordance with 56.7 ± 5.2 Ma whole-rock
40
Ar/
39
Ar ages.
Supplementary material:
Excel files with details of electron microprobe and inductively coupled plasma mass spectrometry (ICP-MS) analyses, argon isotopic whole-rock and secondary ion mass spectrometry (SIMS) analyses, and the
T
REE
calculations, including an inversion diagram showing regression through measured REE distributions in cpx and opx (from Liang
et al.
2013), are available at
https://doi.org/10.6084/m9.figshare.c.4261856
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