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Partitioning behavior of Sc, Ti, V, Mn, Sr, Y, Zr, Nb, Ba, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb, Hf, and Pb between dacitic silicate melt and clinopyroxene, orthopyroxene, and plagioclase has been determined based on laser ablation-inductively coupled plasma mass spectrometric (LA-ICPMS) analysis of melt inclusions and the immediately adjacent host mineral. Samples from the 1988 eruption of White Island, New Zealand were selected because petrographic evidence suggests that all three mineral phases are in equilibrium with each other and with the melt inclusions. All three phenocryst types are found as mineral inclusions within each of the other phases, and mineral inclusions often coexist with melt inclusions in growth-zone assemblages. Compositions of melt inclusions do not vary between the different host minerals, suggesting that boundary layer processes did not affect compositions of melt inclusions and that post-trapping modifications have not occurred.Partition coefficients were calculated from the host and melt inclusion compositions and results were compared to published values. All trace elements examined in this study except Sr are incompatible in plagioclase, and all measured trace elements except for Mn are incompatible in orthopyroxene. In clinopyroxene, Sc, V, and Mn are compatible, and Y, Ti, HREE, and the MREE are only slightly incompatible. Most partition coefficients overlap the wide range of values reported in the literature, but the White Island data are consistently at the lower end of the range in published values. Results from the literature obtained using modern microanalytical techniques such as secondary ion mass spectrometry (SIMS) or proton induced X-ray emission spectroscopy (PIXE) also fall at the lower end of the published values, whereas partition coefficients determined from bulk analysis of glass and crystals separated from volcanic rocks typically extend to higher values. Rapid crystal growth-rates, crystal zonation, or the presence of accessory mineral inclusions in phenocrysts likely accounts for the wide range and generally higher partition coefficients obtained using bulk sampling techniques. The results for 3+ cations from this study are consistent with theoretical predictions based on a lattice strain model for site occupancy. The results also confirm that the melt inclusion-mineral (MIM) technique is a reliable method for determining partition coefficients, as long as the melt inclusions have not experienced post-entrapment reequilibration.

The whole Earth geohydrologic cycle describes the occurrence and movement of water from the clouds to the core. Reservoirs that comprise the conventional hydrologic cycle defi ne the exosphere, whereas those reservoirs that are part of the solid Earth represent the geosphere. Exosphere reservoirs thus include the atmosphere, the oceans, surface water, glaciers and polar ice, the biosphere, and groundwater. Continental crust, oceanic crust, upper mantle, transition zone, lower mantle and the core make up the geosphere. The exosphere and geosphere are linked through the active plate tectonic processes of subduction and volcanism. While the storage capacities of reservoirs in the geosphere have been reasonably well constrained by experimental and observational studies, much uncertainty exists concerning the actual amount of water held in the geosphere. Assuming that the amount of water in the upper mantle, transition zone, and lower mantle represents only 10%, 10%, and 50% of their storage capacities, respectively, the total amount of water in the Earth's mantle (1.2 × 10 21 kg) is comparable to the amount of water held in the world's oceans (1.37 × 10 21 kg). Fluxes between reservoirs in the geohydrologic cycle vary by ~7 orders of magnitude, and range from 4.25 × 10 17 kg/yr between the oceans and atmosphere, to 5 × 10 10 kg/yr between the lower mantle and transition zone. Residence times for water in the various reservoirs of the geohydrologic cycle also show wide variation, and range from 2.6 × 10 −2 yr (~10 days) for water in the atmosphere, to 6.6 × 10 9 yr for water in the transition zone.

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Abundant geochemical and petrological data are available for the 1980–1986 and 2004–2006 eruptive periods and some of the units of the most recent Spirit Lake stage at Mount St. Helens. However, with the exception of bulk rock data, few geochemical data are available for the earlier eruptive units, including the Ape Canyon, Cougar, and Swift Creek stages. Here, the early magmatic history of Mount St. Helens has been constrained based on chemical zonation of phenocrysts from the different eruptive units and compositions of melt inclusions determined by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICPMS). Mount St. Helens has erupted adakitic composition magma over its entire 300 000 year history, beyond the most recent Spirit Lake stage as previously identified. The observed geochemical variations are consistent with fractional crystallization and do not require assimilation‐fractional crystallization to produce the observed trends. Similarly, large‐scale mixing of compositionally different magmas is not required. The source of the intermediate to felsic magmas at Mount St. Helens appears to be metamorphosed underplated arc gabbro and geochemical trends suggest that the contribution of underplated material to Mount St. Helens magma has decreased in the modern period.

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