Mineralogy, Petrology, and Magmatic Conditions from the Fish Canyon Tuff, Central San Juan Volcanic Field, Colorado
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Abstract:
The Fish Canyon Tuff is one of the largest currently recognized ash-flow tuffs (> 3000 km3). It is a crystal-rich quartz latite containing about 40 per cent phenocrysts of plagioclase, sanidine, biotite, hornblende, quartz, magnetite, sphene, and ilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene, and hornblende. The consistency of mineralogy and whole rock chemistry confirms that the Fish Canyon tuff is remarkably homogeneous. Most chemical variations can be accounted for by phenocryst-matrix fractionation, probably due to glass winnowing during eruption and emplacement. The composition of the parent magma, corrected for such winnowing, is very similar to that of calc-alkaline batholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ± 30 °C for most of the outflow tuff. No evidence for a regular thermal gradient in the magma chamber could be detected. Two feldspar and Fe-Ti oxide equilibria indicate that the magma developed at depths of 25 to 30 km (about 9 kb pressure), and was erupted without time for phenocryst re-equilibration. The reconstructed composition of the liquid in equilibrium with the phenocrysts also suggests a deep source for this ash flow. A late, upper package of flow units have mineralogical characteristics which may reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO2 = — 11.5 ±0.3) but are similar to those obtained from other continental calc-alkaline ash-flow tuffs. The water fugacity is limited by calculations using biotite equilibria and experimental work relating to the stability of the phenocryst assemblage. Best estimates are that water fugacity was 2000 ± 1000 bars. The activities of sulphurous gases are estimated at fSO2 = 2 to 4 bars, fso2 = 150 to 200 bars, fH2S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous, granitic magma body of batholithic proportions. Calculations of its probable viscosity, density, and size indicate that the system should convect with any reasonable thermal gradient. Convective mixing may account for the homogeneity of the parent magma body.Keywords:
Phenocryst
Silicic
Magma chamber
Sanidine
Batholith
Hornblende
A study of amphiboles and associated minerals in samples of Soufrière Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 µm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe–Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 ± 10°C, but was variably heated before eruption. The heating occurred within ∼30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855°C at a PH2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860°C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830–860°C. The average ascent rate of magma during much of the 1995–2002 eruption has been >0·02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.
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Hornblende
Amphibole
Igneous differentiation
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Isotopic fingerprinting of individual mineral phases, complemented by crystal size data, provides a unique avenue for elucidating the details of evolutionary histories of crustal magma systems. Here we report the first measurements of Sr isotopic compositions of single crystals as a function of size and Sr isotopic profiles constructed through microdrill sampling of sanidine crystals from a high-silica rhyolite lava from the Taylor Creek Rhyolite, NM. Whole-rock 87Sr/86Sr increases monotonically with modal abundance of sanidine phenocrysts, suggesting Taylor Creek magma evolved through a coupled process of assimilation and crystallization. In contrast, sanidine phenocrysts do not show simple monotonic increases in 87Sr/86Sr as a function of crystal size and core-to-rim stratigraphy. Instead, 87Sr/86Sr ratios and Sr concentrations of individual sanidines increase with crystal size to a maximum at ∼4 mm and then decrease with further increase in size. Microsampling of two crystals greater than 4 mm in length showed core-to-rim increase then decrease in 87Sr/86Sr, whereas a single sanidine crystal less than 4 mm in length displayed a simple core-to-rim decrease in 87Sr/86Sr. Furthermore, in contrast to measured size distributions of crystals in volcanic rocks, which commonly decrease exponentially with increasing size, crystal size frequency histograms are bell shaped, with decreasing numbers of crystals in the smallest size class. All these results are consistent with a model involving continuous phenocryst nucleation and growth in a crustally contaminated magma into which a lower-87Sr/86Sr, lower-Sr magma was injected. In such a scenario, it is argued that curved crystal size distributions mirror variations in nucleation rate in response to changes in undercooling as the magma body evolved from assimilation- to recharge-dominated regimes.
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Magma chamber
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Three types of feldspar phenocrysts, mantled feldspars (anorthoclase cores and sanidine mantles), microperthitic feldspars and microscopically featureless sanidine are observed in a hand specimen of fayalite-hedenbergite trachyte from Oki-Dogo island in the Sea of Japan. Microscopically featureless sanidine phenocrysts of about Or46Ab49An5 were examined by X-ray method and a transmission electron microscope. Diffuse streaks in X-ray diffraction patterns suggest the existence of coherent cryptoperthites. TEM observations confirmed cryptoperthitic textures in the sanidine phenocrysts. Two types of cryptoperthites are observed: finer tweed (about 2-4 nm in periodicity) and coarser lamellar ones (around 8 nm). The former is attributed to a spinodal decomposition. The latter may be slightly coarsened from the former.
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Trachyte
Spinodal decomposition
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Six melt inclusions and their sanidine host crystals from the Bishop Tuff were analyzed by ion microprobe for the concentrations of Ba and Sr. Host crystals were analyzed 30 [mu]m away from the melt inclusions. The analyses yield an average sanidine/melt-inclusion partition coefficient for Ba of 28.4 [+-] 4.1 and for Sr of 11.35 [+-] 0.74. These values are significantly larger than those produced by analyses of mineral separates and matrix glasses, which largely fall between 4 and 7 for Ba and 1 and 4 for Sr. The new results are further confirmed by the concentration ratios of these two elements in the rims of the late Bishop sanidine phenocrysts and late Bishop matrix glasses. The new findings are considered closer to the real phenocryst/metal partition coefficients than those produced by bulk analyses of crystal separates and matrix glass, because ion microprobe analysis eliminates or at least greatly reduces the error introduced by the existence of chemical zonations and impurities. Melt inclusions are better representatives of the melt in equilibrium with the phenocrysts than the matrix glass, because matrix glass is subject to various chemical changes after formation of the phenocrysts. Modifications of chemical concentrations in melt inclusions by boundarymore » layer buildup during melt inclusion formation has been shown to be negligible for most elements. These new and considerably larger sanidine/melt partition coefficients for Ba and Sr shed new light on the debate over origin of high-silica rhyolites with extremely low concentrations of Ba and Sr. Using these new partition coefficients, only about 40% fractional crystallization is required to produce a rhyolitic magma with 2 ppm Ba from a magma that originally had 2,000 ppm Ba.« less
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Radiogenic nuclide
Sanidine
Alkali feldspar
Caldera
Igneous differentiation
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The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al2O3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer.
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Hornblende
Actinolite
Batholith
Amphibole
Greenschist
Paragenesis
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Phenocryst
Sanidine
Volcanology
Volcanic ash
Laser-induced breakdown spectroscopy
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Sanidine
Phenocryst
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Isotopic fingerprinting of individual mineral phases, complemented by crystal size data, provides a unique avenue for elucidating the details of evolutionary histories of crustal magma systems. Here we report the first measurements of Sr isotopic compositions of single crystals as a function of size and Sr isotopic profiles constructed through microdrill sampling of sanidine crystals from a high-silica rhyolite lava from the Taylor Creek Rhyolite, NM. Whole-rock 87Sr/86Sr increases monotonically with modal abundance of sanidine phenocrysts, suggesting Taylor Creek magma evolved through a coupled process of assimilation and crystallization. In contrast, sanidine phenocrysts do not show simple monotonic increases in 87Sr/86Sr as a function of crystal size and core-to-rim stratigraphy. Instead, 87Sr/86Sr ratios and Sr concentrations of individual sanidines increase with crystal size to a maximum at ∼4 mm and then decrease with further increase in size. Microsampling of two crystals greater than 4 mm in length showed core-to-rim increase then decrease in 87Sr/86Sr, whereas a single sanidine crystal less than 4 mm in length displayed a simple core-to-rim decrease in 87Sr/86Sr. Furthermore, in contrast to measured size distributions of crystals in volcanic rocks, which commonly decrease exponentially with increasing size, crystal size frequency histograms are bell shaped, with decreasing numbers of crystals in the smallest size class. All these results are consistent with a model involving continuous phenocryst nucleation and growth in a crustally contaminated magma into which a lower-87Sr/86Sr, lower-Sr magma was injected. In such a scenario, it is argued that curved crystal size distributions mirror variations in nucleation rate in response to changes in undercooling as the magma body evolved from assimilation- to recharge-dominated regimes.
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Silicic
Magma chamber
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