Pore-scale multiphase flow experiments were developed to nondelems. Mortensen and coworkers used a quantitative light structively visualize water flow in a sample of porous material using X-ray microtomography. The samples were exposed to similar bound-transmission technique to investigate millimeter-scale ary conditions as in a previous investigation, which examined the phase displacement processes in two dimensions and effect of initial flow rate on observed dynamic effects in the measured suggested, based on their results, that the current definicapillary pressure-saturation curves; a significantly higher residual tions of unsaturated hydraulic properties are perhaps insaturation and higher capillary pressures were found when the sample adequate. was drained fast using a high air-phase pressure. Prior work applying The lack of a firm theoretical foundation of the relathe X-ray microtomography technique to pore-scale multiphase flow tionship between capillary pressure and saturation has problems has been of a mostly qualitative nature and no experiments been the focus of a vast body of work by Gray and have been presented in the existing literature where a truly quantita-Hassanizadeh over the years (e.g., Hassanizadeh and tive approach to investigating the multiphase flow process has been

The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr-Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from ten rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal-scale, the total suite of 87Sr/86Sr ratios are highly variable (0.7042–0.7065 ± 0.0004 average 2se), however, the majority (95%) of the crystals are internally homogeneous within error. At whole-crystal-scale (where better precision is obtained) 87Sr/86Sr ratios are much more homogeneous (0.70512–0.70543 ± 0.00001 average 2se) and overlap with their host whole rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole rock Pb ratios. The plagioclase and whole rock isotopic compositions indicate significant crustal assimilation (≥20%) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system.

thick lithospheric cap, with 3 He contributed from volatile-rich fluids We present elemental and isotopic (Sr-Nd-Pb-Hf-Os-He) data from elsewhere in the Icelandic plume. The exact nature of the recycled on primitive alkalic lavas from the Prinsen af Wales Bjerge, component is not yet resolved, although Hf isotope compositions rule East Greenland. Stratigraphical, compositional and 40 Ar-39 Ar data out any significant role for recycled pelagic sediment, and the low indicate that this inland alkalic activity was contemporaneous with 187 Os/ 188 Os limits the participation of recycled basaltic material the upper parts of the main tholeiitic plateau basalts and also postand argues instead for a contribution from the mantle section of the dated them. The alkalic rocks show a marked crustal influence, recycled slab. indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high 3 He/ 4 He isotope ratios (R/R A 12•4-18•5), sub-chondritic 187 Os/ 188 Os i (0•120-0•126), low Nd i (>+4) and Hf i KEY WORDS: alkalic lavas; flood basalts; high 3 He/ 4 He; East Greenland; (>+6) that plot below the 'Nd-Hf mantle array', and trace recycled lithosphere; Iceland plume element characteristics similar to HIMU ocean island basalt (OIB). The uncontaminated magma is inferred to have more radiogenic 206 Pb/ 204 Pb values (>19•2) than the plateau basalts and Icelandic basalts, and thus represents a possible 'enriched' component to

Decades of study on volcanic arcs have provided insight into the overarching processes that control magmatism, and how these processes manifest at individual volcanoes. However, the causes of ubiquitous and dramatic intra-arc variations in volcanic flux and composition remain largely unresolved. Investigating such arc-scale issues requires greater quantitative comparison of geophysical and geochemical data, linked through sets of common intensive variables. To work towards these goals, we use observed lava compositions to estimate the heat budget associated with Quaternary volcanism in the Cascades Arc and compare this to the heat required to produce the observed geophysical properties of the crust. Here we show that along-strike volcanic variability in the Quaternary Cascades Arc is primarily related to variations in the flux of basalt into the crust, rather than variations in their crustal storage history. This approach shows promise for studying other large-scale frontier geologic problems in volcanic arcs.

Major, trace, and isotopic compositions of plagioclase megacrysts from plagioclase ultraphyric basalts (PUB) from multiple mid‐ocean ridges were determined to characterize the liquid compositions from which they were derived. Large (up to 2 cm) plagioclase megacrysts exhibit homogeneous major element compositions, with variable degrees of trace element and isotopic heterogeneity. The pattern and degree of heterogeneity is itself variable between megacrysts, with some being homogenous and others exhibiting a wide range of trace element and isotopic zoning. Liquids calculated to be in equilibrium with the plagioclase megacrysts exhibit a range equivalent to lavas erupted from entire ridge segments. Many of the megacrysts may be considered to be cognate xenocrysts in that they are related to the same mantle material as the host lavas in which they were carried to the surface. The observed trace element patterns together with the isotopic diversity suggest a complex set of processes are active, that include mixing of distinct primitive melts, fractional crystallization, and diffusive reequilibration. The degree to which each process influences the megacrysts varies between individual crystals in each sample. Equally important is the range of composition we see within individual crystals. For example, Ba concentrations vary by as much as a factor of 5 but not systematically from core to rim.