LOWENSTERN, Jacob B., CLYNNE, Michael A., DUFFIELD, W.A. and SMITH, J.G.; U.S. Geological Survey, 345 Middlefield Road, Mail Stop 910, Menlo Park, CA 94025 (email@example.com)
Blocks of anorthoclase- and clinopyroxene-bearing micorgaphic granophyre (click for photo) were ejected with rhyolitic pumice during a late-Pleistocene eruption at the Alid volcanic center, Eritrea. The blocks are chemically, isotopically, and mineralogically similar to the host rhyolite. They appear to have crystallized at the top of a shallow magma body beneath Alid, a structural dome created as intruding granitic magma deformed and uplifted overlying lavas, marine sediments, and Precambrian basement rocks (see Duffield et al., this volume).
Temperature of the granitic magma, by whole-rock Zr geothermometry, was 875±25°C. Melt inclusions (n=21) in euhedral quartz phenocrysts from the pumice contain an average of 2.6 wt.% H2O, 3800 ppm Cl, and 1300 to 5200 ppm F, (analyses by FTIR spectroscopy and EPMA). Six inclusions analyzed for CO2 contained between 30 and 60 ppm. Saturation pressures calculated for individual inclusions require crystallization at depths (corresponding to lithostatic pressure) ranging from 1.4 to 3.4 km. Similar quartz grains in the granophyric blocks contain primary, 2-phase, vapor-rich fluid inclusions, consistent with vapor saturation during growth of these igneous phenocrysts. Granophyric intergrowths of quartz and feldspar in the groundmass contain abundant, primary, vapor-rich fluid inclusions, 1 to 10 µm in diameter. The presence of such inclusions requires that growth of the granophyric texture from the silicate melt was accompanied by degassing (2nd boiling).
Though crystallization apparently forced degassing, the converse also likely occurred. Earlier eruptions from the rhyolite magma chamber would have caused pressure drops that would induce degassing of the already vapor-saturated melt. If the magma resided at 2-km-depth, eruptive decompression from lithostatic to hydrostatic conditions would cause a drop from 2.6 to 1.8 wt.% dissolved H2O, causing compositional undercooling equivalent to 75°C. Micrographic and granophyric textures are consistent with such undercooling, and will be most common in shallow intrusive settings where pressure remains high enough to retain ~1-1.5 wt.% H2O. In contrast, greater depressurization, degassing, and undercooling would create glasses, fine-grained felsites, and crude granophyres, without the well-crystallized micrographic intergrowths observed at Alid.
granophyre, Alid, volatiles, degassing, Eritrea
Abstract from the January 1997 meeting of the IAVCEI, the International Association for Volcanology and Chemistry of the Earth's Interior, Puerto Vallarta, Mexico.
Lowenstern, Jacob B., Clynne, Michael A., and Bullen, Thomas D., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 (firstname.lastname@example.org, email@example.com, firstname.lastname@example.org)
We have begun a study of porphyritic pyroxene rhyolite, erupted as pyroclastic flow, and its intrusive equivalent, ejected during the same eruption as blocks of miarolitic granite with spectacular granophyric groundmass. These Pleistocene rocks are from the Alid volcanic center, a structural dome in the northern Danakil Rift, 100 km S. of Massawa, Eritrea, in the Afar Triangle of NE Africa.
The elliptical 5x7-km mountain consists of submarine and subaerial basaltic lavas and siltstones unconformably overlying Precambrian schists. The complete sequence was domed by magmatic intrusion as much as >1 km above the rift floor. Later, rhyolitic tephra (containing granite inclusions) was erupted from the summit region, presumably from the intrusion that caused deformation. Boiling-temperature fumaroles on Alid's summit and N. flanks hint that hot rock is still present at shallow depths.
Granite blocks (g) and pumice (p) have similar major-element and isotopic compositions (87Sr/86Sr: g=0.70468 vs. p=0.70441; 206Pb/204Pb: g=19.213 vs. p=19.214; 207Pb/204Pb: g=15.616 vs. p=15.618; 208Pb/204Pb: g=38.871 vs. p=38.879). Both have phenocrysts of normally zoned anorthoclase, ferroaugite, magnetite, minor quartz, zircon, sulfide and apatite. In the granite, pyroxene has been partially converted (supra-solidus) to Fe-rich biotite.
The granite's granophyric groundmass consists of intergrown quartz and potassium feldspar (~Or45), the latter consisting of plumose splays that nucleated directly from pre-existing phenocrysts. Feldspar compositions in the groundmass are slightly more Or-rich than the phenocrystic feldspar, which ranges from An10Or15Ab75 to about An1Or44Ab55. K/Na of the groundmass feldspar equals K/Na in pumiceous glass of the host pumice, hinting that the granophyric minerals grew by closed-system crystallization of the rhyolitic melt. Rapid growth of the granophyric texture may have been initiated during quench/degassing episodes.