Development of the Long Valley rhyolitic magma system: strontium and neodymium isotope evidence from glasses and individual phenocrysts

TLDR: In this paper, the evolution of a magma system from initiation at 2.1 Ma to eruption at 0.76 Ma is described in the context of pre-caldera high-silica rhyolites and the voluminous, zoned rhyolitic Bishop Tuff.

About: This article is published in Geochimica et Cosmochimica Acta.The article was published on 1998-11-01 and is currently open access. It has received 75 citations till now. The article focuses on the topics: Magma chamber & Phenocryst.

Figures

Table 2. Strontium and neodymium isotope data of minerals from young lavas

Table 3. Strontium and neodymium isotope data of minerals from the Bishop Tuff

Fig. 3. Rb-Sr isotope diagrams for glasses and whole rocks from young Glass Mountain Lavas. Nine outer lavas define an isochron of 1.1516 0.010 Ma with an initial ratio of 0.70606 1. Six inner lavas define an isochron of 1.0916 0.034 Ma with a distinctly different initial ratio of 0.70576 1.

Fig. 2. Diagram summarizing the Nd isotope composition of lavas and minerals from Long Valley rhyolitic magmas (data sources: Halliday et al., 1984, 1989; Davies et al., 1994). Note the distinct Nd isotope compositional gap between the young and old Glass Mountain lavas. The earliest Bishop Tuff contains feldspars with Nd isotope compositions similar to both old and young Glass Mountain Lavas demonstrating a mixed provenance for minerals within the earliest Bishop Tuff.

Fig. 6. Strontium isotope evolution diagram for early Bishop Tuff. Motif as in Fig. 5. Shaded area encompasses plagioclases and hashed areas sanidines. Sanidine and plagioclase rim ages are within error of the eruption age of the Bishop Tuff. Some sanidine and plagioclase grains have never been in isotopic equilibrium with the host glass and yield ages significantly younger than the eruption age of the Bishop Tuff.

Fig. 1. Sketch map of Glass Mountain modified from Bailey (1989) and Metz and Bailey (1995). Prefix Y and O for younger (post-1.2 Ma) and older (pre-1.2 Ma) Glass Mountain lavas, respectively (individual flow units distinguished by letters). K-Ar ages from Metz and Mahood (1985). Unmarked area on map are tuffs of Glass Mountain and Bishop Tuff plus caldera lake sediments and Recent surficial deposits. Younger outer lavas which lie on Rb-Sr isochron of age 1.156 0.01 Ma represented by a vertical lined pattern. Younger inner lavas which lie on Rb-Sr isochron of age 1.096 0.03 Ma represented by a horizontal lined pattern. Older inner lavas which lie on Rb-Sr isochron of age 2.0476 0.013 Ma represented by a dotted pattern. Older outer lavas which lie on Rb-Sr isochron of age 1.8946 0.016 Ma represented by a dashed pattern.

Frequently asked questions

Q1. What have the authors contributed in "Development of the long valley rhyolitic magma system: strontium and neodymium isotope evidence from glasses and individual phenocrysts" ?

In this paper, the authors used the Nd isotope compositions of sanidine and plagioclase from the younger Glass Mountain lavas and late erupted Bishop Tuff to provide evidence of magma residence times of up to 360 kyr. 

Q2. How long did the glass mountain lavas remain molten?

Glass Mountain lavas appear to have formed from at least four magma batches which were periodically tapped but remained molten for .105 years (Halliday et al., 1989; Davies et al., 1994). 

Q3. What is the way to distinguish feldspars from xenocrys?

Consideration of Sr diffusion within feldspars potentially provides a method for distinguishing between an origin of the feldspar as phenocrysts that have extended residence times within the host magma in contrast to derivation as xenocrysts from a disrupted crystallized rind. 

Q4. How many sanidine grains would exchange with the host magma in 1.3 Ma?

In addition, Sr diffusion calculations demonstrate that sanidine grains studied from the Bishop Tuff would exchange ;25% of their total Sr with the host magma in 1.3 Ma. Plagioclases would exchange closer to 40% of their Sr in the same period of time. 

Q5. How long will the feldspars be altered by diffusional processes?

As a consequence of at least one population of the feldspars being derived from magmas comparable to the OGML, i.e., higher Rb contents, the Rb/Sr ratios of these feldspars will be significantly altered by diffusional processes if the “phenocrysts” have resided in a magma for over 1 Myr. 

Q6. What is the reason for the preservation of Rb-Sr isochrons in the Glass?

The preservation of four regional Rb-Sr isochrons in the Glass Mountain lavas establishes that the chemical fractionation process(es) that produced these extreme magma compositions occurred rapidly. 

Q7. What is the reason for the pre-eruptive ages of the Glass Mountain lavas?

Recently Knesel and Davidson (1997) have argued that the partial assimilation of magma chamber wall rocks could be responsible for the pre-eruptive ages of the Glass Mountain lavas. 

Q8. What is the way to compare the feldspars with the source rocks?

the combined Sr-Nd isotope data from the cores of the feldspars can be used to make comparisons with possible source rocks but cannot provide absolute age information. 

Q9. What is the recent debate about the rate of magma production, storage, and possible?

Most recently, a debate about the rate of magma production, storage, and possible modification of large silicic magma bodies was initiated principally as a consequence of studies on the Long Valley system (Halliday et al., 1989; Sparks et al., 1991; Mahood, 1991; Lu et al., 1992; Hervig and Dunbar, 1992; Davies et al., 1994; Duffield et al., 1995).