Other
Scientific paper
Sep 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..94..274h&link_type=abstract
Earth and Planetary Science Letters, Volume 94, Issue 3-4, p. 274-290.
Other
56
Scientific paper
The high-silica rhyolite lavas of Glass Mountain, California, provide a detailed record of the evolution of the Long Valley magmatic system during the 1.4 m.y. prior to the catastrophic eruption of the voluminous zoned rhyolitic Bishop Tuff at 0.73 Ma. The older lavas of Glass Mountain are extremely evolved, and were erupted from 2.1 to 1.2 Ma with 87Sr/86Sr of 0.707-0.739 and ɛNd of -3 to -4, whereas the younger lavas are slightly less evolved and were erupted between 1.2 and 0.79 Ma with 87Sr/86Sr of 0.706-0.707 and ɛNd close to -1, essentially identical to the Sr and Nd isotopic compositions of the Bishop Tuff. Neither the older nor younger lavas display a relationship between the isotopic compositions of Sr and Nd. The Pb isotopic compositions are effectively uniform at 206Pb/204Pb = 19.13-19.17. A group of older lavas that outcrop in a northwest-trending band near the topographic rim of the caldera collectively define an apparent Rb-Sr isochron age (t) of 2.09 +/- 0.06 Ma with an intercept of 87Sr/86Sr = 0.7060 +/- 3, where analyzed older lavas outcropping outboard of this band define an isochron of 1.90 +/- 0.02 Ma (87Sr/86Sr)t = 0.7063 +/- 4) These isochron ages are identical to the K-Ar ages for oldest erupted rhyolites in the two regions, respectively. The younger lavas away from the caldera define an apparent Rb-Sr isochron age (t) of 1.14 +/- 0.08 Ma with the same (87Sr/86Sr)t as the older lavas. The agreement between the 87Sr/86Sr ratios of the rhyolite magmas when emplaced in the Long Valley magmatic system (0.706) and that of basaltic lavas in the region (0.7059-0.7062) suggests that the Sr isotopic compositions of these Sr-poor rhyolites may be totally dominated by Sr from mantle-derived components.
K-Ar ages and stratigraphic relations for both the older and younger lavas seem to rule out the possibility that the Rb-Sr isochron ages record the times of eruption. The very low Sr concentrations of all the lavas (down to 0.1 ppm) cannot be produced simply by crustal fusion; the extremely high and variable Rb/Sr ratios (up to 2000) and low Sr concentrations must be due to extensive fractional crystallization. We interpret the isochrons as dating this process. To produce the observed volume of highly evolved rhyolite requires differentiation of a sizeable body of magma; thus the older lavas may have been derived from an evolved roof zone of a differentiated magma chamber that was present in the Long Valley region as early as 2.1 Ma. The similarity of the isotopic compositions of the younger Glass Mountain lavas and the Bishop Tuff, as well as the isochronous relationship of most of the younger lavas, indicate that the chamber containing the magma later to erupt as the younger lavas of Glass Mountain and the Bishop Tuff was already formed, isotopically homogenized, and had developed an evolved roof zone by about 1.1 Ma. There is no evidence for significant assimilation of crustal rocks or input of mantle-derived magma into the upper reaches of the system after this time.
The preservation of isochronous relationships between lavas (and their constituent glasses) implies that most of the crystallization of the magma (i.e., that which affected Rb/Sr ratios) took place in discrete dateable events. The fact that the three isochrons have the same (87Sr/86Sr)t requires that the magmas did not exist for long as highly evolved liquids prior to the differentiation events that established the range of Rb/Sr ratios and the isochrons, as such magmas would increase in 87Sr/86Sr by 0.0002-0.005 per 100,000 years (depending on Rb/Sr ratio) due to in-situ decay. Comparing the isochron ages of the Glass Mountain lavas with times of their eruption yields residence times of silicic magma as great as 0.7 m.y. To prevent the rhyolitic magma from cooling and crystallizing during these intervals, heat must have been supplied to the system, presumably in the form of new additions of magma. This magma must not have reached the upper regions of the magma reservoir; otherwise the isochrons would have been destroyed by mixing in these Sr-poor rocks.
Our interpretation of the isotopic data for Glass Mountain is at variance with recent models that invoke repeated melting of the deep crust rather than episodic tapping of magma chambers as mechanisms for producing repeated rhyolitic eruptions spanning periods of 105-106 years. The evidence for Long Valley is rather that major differentiation accompanies fresh inputs of magma into the system and that magma is maintained in stable zones in the chamber for hundreds of thousands of years.
Davidson Jon P.
Dempster Tim J.
Halliday Alex N.
Holden Pete
Mahood Gail A.
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