Isotopic constraints on the production rates, crystallisation histories and residence times of pre-caldera silicic magmas, Long Valley, California

Details Isotopic constraints on the production rates, crystallisation histories and residence times of pre-caldera silicic magmas, Long Valley, California Isotopic constraints on the production rates, crystallisation histories and residence times of pre-caldera silicic magmas, Long Valley, California

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.125...17d&link_type=abstract

Earth and Planetary Science Letters (ISSN 0012-821X), vol. 125, no. 1-4, p. 17-37

Astronomy and Astrophysics

Astrophysics

19

Argon Isotopes, California, Crystal Growth, Crystallization, Geochronology, Lava, Magma, Minerals, Petrography, Rhyolite, Silicon Dioxide, Volcanoes, Volcanology, Feldspars, Glass, Quartz, Rubidium, Strontium

Scientific paper

Pre-caldera high-silica rhyolites of Glass Mountain, California erupted episodically from 2.1 Ma until the catastrophic eruption of the Bishop Tuff at 0.74 Ma. The lavas are extremely evolved, with Rb/Sr ratios between 128 to 3640, the latter being the highest recorded from a volcanic rock. Glass separates from pre-1.2 Ma lavas define two geographically controlled Rb-Sr isochrons. Lavas adjacent to the current caldera rim define an isochron age of 2.047 +/- 0.013 Ma with an initial ratio of 0.7063 +/- 2, and lavas more distant from the caldera define an isochron of 1.894 +/- 0.013 Ma with the same initial ratio. The isochrons are consistent with the magmas forming within 26 ka, which implies a minimum magma production rate of 0.75 x 10(exp -3) cu km/yr over this period. New Ar-40 - Ar-39 ages on sanidine and biotite have established that lavas defining each isochron were erupted over a long time interval, the isochron ages being up to 360 ka older than the youngest eruption age. Rb-Sr isotope data are reported for minerals from three lavas with eruption ages of 1.990 +/- 0.012, 1.866 +/- 0.014 and 1.686 +/- 0.011 Ma. Petrographically early apatite inclusions in biotite and biotite inclusions in feldspar and quartz have glass-mineral Rb-Sr ages that are indistinguishable from the relevant regional isochron. Sr diffusion in feldspar is slow at the magmatic temperatures inferred for Glass Mountain rhyolites (approximately 700 deg C) such that over 0.5 Ma the cores of large feldspars (greater than 1 mm) will retain greater than 99.9% of their original Sr. The cores of sanidine and plagioclase yield glass-mineral ages that are up to 300 ka older than eruption ages. Feldspar rim ages for two samples are indistinguishable from eruption ages. The rims of sanidines and plagioclases from the third sample are 110 and 280 ka older than the eruption age and 180 and 20 ka younger than the cores. These mineral age data probably reflect the combination of extended periods of mineral growth and partial isotopic exchange with the host liquid during protracted residence in a magma reservoir. However, the Ar and Sr isotopic data for biotite phenocrysts are consistent with the presence of a significant component that is recycled from earlier magmatic pulses.

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