Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

TLDR: In this article, the authors present 87 Rb/86 Sr and 230 Th/ 238 U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system.

About: This article is published in Geochimica et Cosmochimica Acta.The article was published on 2002-05-15 and is currently open access. It has received 52 citations till now. The article focuses on the topics: Magma & Isochron.

Figures

Fig. 4. Inverse isochron diagrams of incremental-heating and laser-fusion analyses of sanidines. Calculated ages in 103 yr. PA plateau age.

Fig. 8. 230Th-238U isochrons for whole rocks, glasses, and minerals from postcaldera rhyolites of the Long Valley system erupted between 110 and 150 ky. (a1, b1) Enlarged areas of diagrams (a2) and (b2), which also include the zircons with their much higher U/Th ratios. Dashed lines equilines. Lines for age reference were calculated with a least-square linear regression technique (York, 1969). Error bars are all smaller than symbol size.

Fig. 10. Schematic model of the Long Valley magma system at 200 ka. The major low-silica magma volume is well mixed and represents an open-system magma reservoir, periodically fed from below (Heumann and Davies, 1997; Heumann, 1999). Filter pressing, driven by the exsolution of gas, forces highly differentiated interstitial liquids from the dominant magma volume, forming HSR. Simultaneously, the exsolution of gas inflates cupolas in the upper reaches of the reservoir, where the high-silica melts accumulate and are stored for up to 150 ky before eruption.

Table 1. Rb-Sr and Pb isotope data for whole rocks, glass, and mineral separates.a

Fig. 6. (232Th/230Th) vs. (238U/232Th) activity ratio diagram for whole rocks and selected glasses of Long Valley postcaldera rhyolites. Symbols are larger than error bars. Note that glasses have consistently lower (230Th/238U) than the respective whole rocks, which plot to the left of the equiline for LSR and to the right for HSR.

Fig. 7. Effect of accessory mineral fractionation of allanite and zircon on the U/Th ratio of melt. Star starting compositions of the melt. Calculations are based on Rayleigh fractionation law assuming U and Th partition coefficients deduced from isotope dilution analyses, microprobe analyses (allanite), and U/Th ratios of allanites from the literature (Barth et al., 1994). To change the (238U/232Th) of a melt to the same extent requires 10 times more zircon than allanite fractionation. This demonstrates the dominating role of allanite over zircon in fractionating Th from U in the residual melt.

Frequently asked questions

Q1. What are the contributions in "Crystallization history of rhyolites at long valley, california, inferred from combined u-series and rb-sr isotope systematics" ?

In this study, the authors present Rb/Sr and Th/U isotope analyses of glasses and phenocrysts from postcaldera rhyolites erupted between 150 to 100 ka from the Long Valley magmatic system. The authors interpret the indistinguishable age results from both isotope systems ( 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. These crystallization events are, however, only documented by the accessory minerals and had no further influence on bulk magma compositions. 

Q2. What is the composition of the Inyo Craters?

The Inyo Craters form a north–south chain of recent domes, which are the youngest expression of rhyolitic volcanism in the Long Valley region. 

Q3. What is the likely cause of the U-Th relationships?

The authors conclude that the linear U-Th relationships, as shown by minerals from the postcaldera rhyolites, are most likely a consequence of a mixing process involving accessory minerals that control U-Th fractionation. 

Q4. How much would the solution of zircon be dissolved in ten years?

Solution kinetics suggest that 50- m zircon grains would be totally dissolved in tens of years if temperature were raised by as little as 50°C (Harrison and Watson, 1984). 

Q5. What is the composition of the postcaldera rhyolites?

The postcaldera rhyolites have compositions that define a large and continuous compositional range from low to high-silica rhyolite (72 to 77%). 

Q6. What is the ion microprobe used to determine the ages of zircons?

The largest zircons from the low-silica samples (IC-24, IC-42) in their study overlap a proportion of the Reid et al. (1997) data with crystallization ages 200 ka, whereas the zircons from the HSR are comparable to a group of younger zircon grains, identified by the ion microprobe (140 to 150 ky). 

Q7. What is the explanation for the U-Th isotope systematics of samples ?

Petrographic observations establish that there are several generations of minor phases (Fig. 3), and two or more generations of zircon fractionation can readily explain the U-Th isotope systematics of these rocks. 

Q8. What is the key to understanding the magmatic processes that generated the rhyolites?

This process explains the generation of vapor-saturated, crystal-poor rhyolitic magmas because it accounts for the aphyric nature of many rhyolites, for the rapid production rates of rhyolites, and for the large inferred degrees of total fractional crystallization (low Sr contents; Halliday et al., 1991). 

Q9. What is the difference between sanidine and plagioclase?

Both sanidine and plagioclase have lower 87Sr/86Sr ratios than host glasses ( 87Sr/86Sr 20 to 100), as would be expected for a low Rb/Sr phase. 

Q10. What is the explanation for the disequilibrium data presented in this study?

The U-Th disequilibrium data presented in this study are best explained by a semiquantitative model, with crystallization of two accessory mineral phases at different times. 

Q11. What is the simplest scenario to explain the glass plotting below the mineral trend?

The simplest scenario to explain the glass plotting below the mineral trend is that the two accessory phases crystallized at different times, and in this instance, only the mineral–glass age of the last formed mineral provides true age information. 

Q12. What is the common consequence of a crystallization-dominated differentiation of magmas?

Such a process of retrograde or secondary boiling is a common consequence of crystallization-dominated differentiation of magmas (Sisson and Bacon, 1999).