Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

Introduction to ‘The 1997–8 El Niño Atlas of oceanographic conditions along the west coast of North America (23°N–50°N)’

Hydrographic data collected along the West Coast of North America between January 1997 and January 1999 have been compiled into a web-based Atlas of the 1997–8 El Niño event. This paper discusses the organization of the Atlas, describes the data that were incorporated into the Atlas and explains how vertical and horizontal distributions for the different properties were constructed. Examples of figures from the Atlas are shown.     

Metal tissue levels in Steller sea lion (Eumetopias jubatus) pups

The endangered Western population of the Steller sea lion declined for three decades for uncertain reasons. We present baseline data of metal concentrations in pups as a first step towards investigating the potential threat of developmental exposures to contaminants. Seven metals were investigated: arsenic, cadmium, silver, aluminum, mercury, lead and vanadium. Vanadium was detected in only a single blubber sample. Mercury appears to be the most toxicologically significant metal with concentrations in the liver well above the current action level for mercury in fish. The concentrations of aluminum, arsenic, silver, cadmium and lead were present in one-fourth to two-thirds of all samples and were at either comparable or below concentrations previously reported. Neither gender nor region had a significant effect on metal burdens. Future work should consider metal concentrations in juveniles and adults and toxicological studies need to be performed to begin to assess the toxicity of these metals.     

SPIRITUAL PILGRIMS AT MOUNT SHASTA,CALIFORNIA*

ABSTRACT. Even the casual visitor cannot fail to notice unusual activity on the slopes of Northern California's Mount Shasta. Prayer flags, altars, and crystals are found in the meadows; drumming, chanting, and meditation are commonplace. Non‐indigenous spiritual pilgrims have found Mount Shasta a sacred place. An amorphous group of spiritual seekers, these are sometimes referred to as “New Age” adherents or “Crystal People.” Within the Shasta‐Trinity National Forest, the situation of this sacred site exemplifies the difficulties of reconciling nonsecular claims to public lands with secular management mandates. Spiritual activism at Mount Shasta includes recently successful opposition to development of a Forest Service‐endorsed ski area. Using a questionnaire survey and interviews, we compare the characteristics, activities, and attitudes toward resource management of spiritual pilgrims and others who visit Mount Shasta's meadows. Conclusions are drawn about the environmental values and concerns of all visitors and of spiritual pilgrims in particular, including some that bear on pilgrim activities and ecological restoration efforts.     

A Prominent Apparition of Neptune's South Polar Feature

During the last week of June 2001, a bright apparition of Neptune's South Polar Feature (SPF) at 70°S was observed to develop and decay in less than 30 hours, displaying contrast of ∼2.5 at 619 nm. Assuming that the same SPF was observed on two consecutive rotations of Neptune, the feature moved eastward at 3.2±1.8° hr⁻¹ (130±80 m s⁻¹). The SPF made no obvious appearances during eight other Hubble Space Telescope (HST) observations of Neptune between July 2000 and June 2001, although there was a faint feature at 70°S in one image in October 2000. A prominent SPF was present in near-IR Keck Telescope images in August 2000. Bright SPFs are seen on ∼10% of the HST images of Neptune obtained since 1994, and a fainter SPF is visible on another ∼10%. An SPF bright enough to be visible at HST resolution was present around half the time during the last week of Voyager's approach to Neptune in August 1989, with one prominent brightening, suggesting that the SPF is less visible now than in 1989.     

Formation enthalpy of ThSiO4 and enthalpy of the thorite → huttonite phase transition

The standard enthalpy of formation of thorite and huttonite and the enthalpy of the phase transition between these polymorphs were determined using high-temperature oxide melt solution calorimetry and transposed temperature drop calorimetry. Standard enthalpies of formation of thorite and huttonite are reported for the first time and are −2117.6 ± 4.2 kJ/mol and −2110.9 ± 4.7 kJ/mol, respectively. Based on our measurements, thorite and huttonite are metastable relative to SiO₂ (quartz) and ThO₂ (thorianite) at standard conditions, but are presumably stabilized at high temperature by the entropy contribution. Based on the measured enthalpy of the thorite-huttonite phase transition of 6.7 ± 2.5 kJ/mol, a dP/dT slope for the transformation was calculated as −1.21 ± 0.45 MPa/K.     

FFT-enhanced IHS transform method for fusing high-resolution satellite images

Existing image fusion techniques such as the intensity–hue–saturation (IHS) transform and principal components analysis (PCA) methods may not be optimal for fusing the new generation commercial high-resolution satellite images such as Ikonos and QuickBird. One problem is color distortion in the fused image, which causes visual changes as well as spectral differences between the original and fused images. In this paper, a fast Fourier transform (FFT)-enhanced IHS method is developed for fusing new generation high-resolution satellite images. This method combines a standard IHS transform with FFT filtering of both the panchromatic image and the intensity component of the original multispectral image. Ikonos and QuickBird data are used to assess the FFT-enhanced IHS transform method. Experimental results indicate that the FFT-enhanced IHS transform method may improve upon the standard IHS transform and the PCA methods in preserving spectral and spatial information.     

The effects of thin layer sand renourishment on tidal marsh processes: Masonboro Island, North Carolina

The objective of this study was to determine if the placement of dredged material on sediment-starved back barrier marshes in southeastern North Carolina could offset submergence without negatively affecting function. Clean sediment was placed in thickness from 0 to 10 cm, on deteriorated and non-deteriorated marsh plots. Original stem densities were greater, in non-deteriorated plots (256 stems m⁻²) compared to deteriorated sites (149 stems m⁻²). By the second growing season (after sediment additions), stem densities in the deteriorated plots (308 stems m⁻²) approached levels in the non-deteriorated plots (336 stems m⁻²). Sediment additions to, both nos-deteriorated and deteriorated plots resulted in a higher redox potential with plots receiving the most sediment exhibiting the highest E_h values. In deteriorated plots, placement of dredged material had the greatest effect on plant density, but also affected soil oxidation-reduction potential and sediment deposition (or mobility). Following sediment placement, substrate texture and composition incrementally returned to prefill conditions due to a combination of bioturbation and sedimentation. Where infaunal differences occurred, they were generally less abundant in deteriorated plots, but responses to sediment addition were variable. Sediment addition had little effect on the non-deteriorated plots, suggesting that the disposal of certain types of dredged material in marshes may be useful to mitigate the effects of marsh degradation without adversely affecting non-deteriorating marsh.