Shaul Hurwitz

Bubble nucleation in rhyolitic melts: experiments at high pressure, temperature, and water content

Abstract We report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release. Two rhyolitic obsidians, one containing less than 1% of microlites of Fe—Ti oxides and the other about 20% of various crystals were hydrated at 150 MPa and 780–850°C. After saturation was reached (5.3–5.5 wt% water), pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time, before the final, rapid quenching of the experiments. The results demonstrate the importance of heterogeneous nucleation. Microlites of Fe—Ti oxides are very efficient as sites for bubble nucleation. In their presence, modest nucleation was observed even after decompression by 6 –10 8 bubbles cm −3 ). In the absence of microlites, no nucleation occurred at ΔP MPa. At ΔP > 10 MPa, bubbles also nucleated on crystals of biotite, zircon and apatite. Modest nucleation (10 3 –10 5 cm −3 ) took place even in crystal-free samples, but it was still heterogeneous. When ΔP exceeded 80 MPa, nucleation in crystal-free samples became extensive (10 5 –10 7 cm −3 ). The lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous. Nucleation rates were controlled mainly by the availability of sites. Rates were faster than 10 6 cm −3 s −1 when microlites were present, and faster than 10 5 cm −3 s −1 in the absence of microlites at ΔP > 70 MPa. Narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop. The experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation. When a large number of efficient sites (e.g., Fe—Ti oxide) are present, bubble nucleation requires very little supersaturation, and to a good approximation, gas and magma are in equilibrium. In magmas that are crystal-free or contain crystals that are inefficient at nucleating bubbles, very high degrees of supersaturation are required in order to initiate nucleation. These two modes of exsolution may lead to contrasting styles of convection, pressure build up and eruption.

The tectonic framework of a complex pull-apart basin: Seismic reflection observations in the Sea of Galilee, Dead Sea transform

Abstract A multi-channel seismic reflection survey consisting of 20 lines with a total length of 180 km was conducted in the Sea of Galilee. The data provide new insights into the Pliocene–Quaternary evolution of the Kinarot–Beit–Shean pull-apart basin (KBSB) along the Dead Sea transform. Two distinct zones are defined beneath the lake: (1) a graben that underlies most of the lake, bounded by steep north–south longitudinal strike-slip faults and (2) shallow pre-rift units underlying the northwestern wider part of the lake. We suggest that before approximately 4 Ma, the KBSB grew due to northward movement of the Korazim Plateau and by crustal stretching along the rift axis. Since the Pliocene (∼4 Ma), lateral slip has been transferred from the southern segment of the basins western marginal fault to normal faults in the Galilee, and to the eastern margin of the Korazim Plateau by the newly formed, Almagor fault, which makes a restraining bend along the transform. N–S lithospheric stretching below the KBSB has diminished and the Korazim Plateau has changed from being a detached block to a compressional saddle. A phase of rapid subsidence, and formation of a half-graben structure in the northern part of the basin approximately 1 Ma ago was coeval with major deformation in areas adjacent to the KBSB, indicating major reorganization of the plate boundary in the region. Currently, most transform motions are probably taken up along a single fault on the eastern side of the KBSB, implying that the main trough under the Sea of Galilee is in a late stage of growth as a pull-apart.

Spatial and Temporal Characteristics of Saline Springs: Sea of Galilee, Israel

Spatial and temporal characteristics of the saline springs that emerge along the western shore of the Sea of Galilee (Lake Kinneret) are analyzed. Three groups of onshore springs (Tiberias, Fuliya, and Tabgha) and two groups of offshore springs (Barbutim and Maagan), contribute saline water to the lake with concentrations in the range of 300 to 18,000 mg Cl/L, depending on location and season. It is well accepted that water emerging from these springs is a mixture of two endmembers: deep-seated saline ground water and shallow, fresh circulating ground water. Temporal trends of discharge rates and of chloride (representing the deep saline aquifer) and nitrate (representing the shallow fresh water aquifer) concentrations within each group of springs are presented. Results show the proportions of the two water bodies while mixing are time dependent. Discharge and concentration peaks in Tabgha springs precede those in Fuliya and Tiberias springs by approximately two months. An analytical solution shows that in Tabgha, variations of these parameters are mainly controlled by recharge variations in the Galilee, and follow an exponential function. In Fuliya and Tiberias, variations of these parameters are mainly dependent on lake level, and follow a sine-cosine function. The different patterns are attributed to different hydraulic properties of the discharge area.

Transient salt transport modeling of shallow brine beneath a freshwater lake, the Sea of Galilee, Israel

During a lake highstand phase in the late Pleistocene the former saline Lake Lisan covered the topographic depression of Kinarot Basin currently occupied by the freshwater lake, Sea of Galilee. It was hypothesized that during this period, the dense saline waters of Lake Lisan percolated into the sediment. The recession of the saline lake from the basin and the rapid formation of a freshwater lake triggered solute transport from the sediment into the lake. A one-dimensional numerical model of solute transport that considers sediment compaction was developed to simulate chloride transport from the sediment into the lake. Simulation results were compared with measured chloride concentration profiles in sediment cores. On the basis of a sensitivity analysis, results are in agreement with the hypothesis that Lake Lisan solutes are currently discharged into the Sea of Galilee. The calculated upward water velocity in the sediment ranges between 9

Transient groundwater-lake interactions in a continental rift: Sea of Galilee, Israel

The Sea of Galilee, located in the northern part of the Dead Sea rift, is currently an intermediate fresh-water lake. It is postulated that during a short highstand phase of former Lake Lisan in the late Pleistocene, saline water percolated into the subsurface. Since its recession from the Kinarot basin and the instantaneous formation of the fresh-water lake (the Sea of Galilee), the previously intruded brine has been flushed backward toward the lake. Numerical simulations solving the coupled equations of fluid flow and of solute and heat transport are applied to examine the feasibility of this hypothesis. A sensitivity analysis shows that the major parameters controlling basin hydrodynamics are lake-water salinity, aquifer permeability, and aquifer anisotropy. Results show that a highstand period of 3000 yr in Lake Lisan was sufficient for saline water to percolate deep into the subsurface. Because of different aquifer permeabilities on both sides of the rift, brine percolated into aquifers on the western margin, whereas percolation was negligible on the eastern side. In the simulation, after the occupation of the basin by the Sea of Galilee, the invading saline water was leached backward by a topographydriven flow. It is suggested that the percolating brine on the western side reacted with limestone at depth to form epigenetic dolomite at elevated temperatures. Therefore, groundwater discharging along the western shores of the Sea of Galilee has a higher calcium to magnesium ratio than groundwater on the eastern side.

Geophysical (Time Domain Electromagnetic Model) delineation of a shallow brine beneath a freshwater lake, the Sea of Galilee, Israel

The Sea of Galilee is a freshwater lake, into which saline water emerges through onshore and offshore springs and through flux from the lakes sediments. The novel surface marine modification of the time domain electromagnetic method was used to map the spatial distribution of brines in the sediments below the lake. Results indicate that electrical resistivities of 1.0 and 0.5 ohm-m are detected at depths of ;10 m below the lake bottom in most of the lake area, which are equivalent to ;11,000 and 22,000 mgCl/L, respectively. Relatively fresh groundwater was detected beneath most of the shoreline. Faulting controls the vertical interfaces between saline and fresh groundwaters. It is hypothesized that salt transport is dominated by molecular diffusion in the central part of the lake and by advection from regional aquifers in the margins.

Mapping saline groundwater beneath the Sea of Galilee and its vicinity using time domain electromagnetic (TDEM) geophysical technique

Goldman, M., Gvirtzman, H., and Hurwitz, S. 2004. Mapping saline groundwater beneath the Sea of Galilee and its vicinity using time domain electromagnetic (TDEM) geophysical technique. Isr. J. Earth Sci. 53: 187‐197.