Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics
Heumann, Arnd; Davies, Gareth R; Elliott, Tim (2002) Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics. Geochimica et Cosmochimica Acta, vol.66, no.10, pp.1821-1837, May 2002
| Authors | Heumann, Arnd; Davies, Gareth R; Elliott, Tim |
|---|---|
| Year | 2002 |
| Title | Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics |
| Source | Geochimica et Cosmochimica Acta, vol.66, no.10, pp.1821-1837, May 2002 |
| Publisher | Pergamon, Oxford, International III |
| ISSN | 0016-7037 |
| GeoRef | 2002-044612 |
| Keywords | absolute age; accessory minerals; acidic magmas; allanite; California; Cenozoic; crystallization; dates; Deer Mountain; domes; epidote group; feldspar group; fractional crystallization; framework silicates; glasses; igneous rocks; isochrons; Long Valley Caldera; magma chambers; magmas; magmatic differentiation; Mammoth Mountain; Mono County California; nesosilicates; orthosilicates; phenocrysts; Pleistocene; Quaternary; Rb/Sr; rhyolites; silicates; sorosilicates; thermal history; United States; uranium disequilibrium; volcanic glass; volcanic rocks; zircon |
| Notes | NSG Publ. No. 20011005 |
| DOI | 10.1016/S0016-7037(01)00883-3 |
Abstract
In this study, we present super 87 Rb/ super 86 Sr and super 230 Th/ super 238 U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. Both isotope systems indicate complex magma evolution with preeruptive mineral crystallization and magma fractionation, followed by extended storage in a silicic magma reservoir. Glass analyses yield a Rb-Sr isochron of 257+ or -39 ka, which can be explained by a feldspar-fractionation event approximately 150 ky before eruption. Individual feldspar-glass pairs confirm this age result. A mineral super 230 Th- super 238 U isochron in a low-silica rhyolite from the Deer Mountain Dome defines an age of 236+ or -1 ka, but the glass and whole rock do not lie on the isochron. U-Th fractionation of the rocks is controlled by the accessory minerals zircon and probably allanite, which crystallized at 250+ or -3 ka and 187+ or -9 ka, respectively. All major mineral phases contain accessory mineral phases; therefore, the mineral isochron represents a mixture of zircon and allanite populations. A precision of + or -1 ka for the mixing array implies that the minor phases must have crystallized within this timescale. Longer periods of crystal growth would cause the mixing array to be less well defined. U-series data from other low- and high-silica rhyolites indicate younger accessory mineral crystallization events at approximately 200 and 140 ka, probably related to the thermal evolution of the magma reservoir. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. We interpret the indistinguishable age results from both isotope systems approximately 250 ka to record the fractionation of small magma batches by filter pressing from a much larger underlying magma volume, followed by physical isolation and extended storage at the top of the magma reservoir for up to 150 ky.