Zircon ²³⁸U-²³⁰Th model age distribution histograms of zircon from the three eruptive units analyzed from the LVC: ∼27 ka dacite of Lassen Peak (top), ∼1.1 ka rhyodacite of Chaos Crags (top middle) and 1915 dacite of Lassen Peak (bottom middle).

<p>The bottom histogram compiles all the zircon ages from across all three units. Parts of the histograms in black represent the surface (rim) analyses of zircon, while white are polished interior analyses. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157.s001" target="_blank">Appendix S1</a> for full ages and errors.</p

Hf versus Eu/Eu* for LVC zircon, color-coded for Ti-in-Zr temperatures.

<p>At temperatures greater than approximately 730°C (Hf <9,500 ppm), Eu/Eu* varies across a much wider range (0.4–0.75). This represents the zircon growth during the down-temperature path after a rejuvenation event while those with Eu/Eu* between 0.2–0.4 and Hf >9,000 ppm represent growth in the crystallizing mush in baseline zircon storage conditions.</p

²³⁸U-²³⁰Th model ages versus selected trace element abundances or ratios.

<p>Bumpass sequence (350-190 ka; dark grey) and the Eagle Peak and Twin Lakes sequences (<90 ka; light grey; both from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Clynne3" target="_blank">[37]</a>) are marked on the figure, along with the eruptions from which the zircon were sampled (dashed lines). The eruptive hiatus (190-90 ka) is marked in white. From top to bottom, (a) Hf (ppm) of zircon surfaces and interiors; (B) Eu/Eu* of zircon surfaces and interiors; (c) Th/U of zircon surfaces and interiors.</p

Ti-in-zircon temperatures and ⁴⁸Ti (ppm) for each unit, surfaces and complete LVC zircon.

<p>Ti-in-zircon temperatures and ⁴⁸Ti (ppm) for each unit, surfaces and complete LVC zircon. All temperatures calculated using a(Si) = 1.0, a(Ti) = 0.6 with methods from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Ferry1" target="_blank">[46]</a>.</p><p>Ti-in-zircon temperatures and ⁴⁸Ti (ppm) for each unit, surfaces and complete LVC zircon.</p

Localized Rejuvenation of a Crystal Mush Recorded in Zircon Temporal and Compositional Variation at the Lassen Volcanic Center, Northern California

<div><p>Zircon ages and trace element compositions from recent silicic eruptions in the Lassen Volcanic Center (LVC) allow for an evaluation of the timing and conditions of rejuvenation (reheating and mobilization of crystals) within the LVC magmatic system. The LVC is the southernmost active Cascade volcano and, prior to the 1980 eruption of Mount St. Helens, was the site of the only eruption in the Cascade arc during the last century. The three most recent silicic eruptions from the LVC were very small to moderate-sized lava flows and domes of dacite (1915 and 27 ka eruptions of Lassen Peak) and rhyodacite (1.1 ka eruption of Chaos Crags). These eruptions produced mixed and mingled lavas that contain a diverse crystal cargo, including zircon. ²³⁸U-²³⁰Th model ages from interior and surface analyses of zircon reveal ages from ∼17 ka to secular equilibrium (>350 ka), with most zircon crystallizing during a period between ∼60–200 ka. These data support a model for localized rejuvenation of crystal mush beneath the LVC. This crystal mush evidently is the remnant of magmatism that ended ∼190 ka. Most zircon are thought to have been captured from “cold storage” in the crystal mush (670–725°C, Hf >10,000 ppm, Eu/Eu* 0.25–0.4) locally remobilized by intrusion of mafic magma. A smaller population of zircon (>730°C, Hf <10,000 ppm, Eu/Eu* >0.4) grew in, and are captured from, rejuvenation zones. These data suggest the dominant method to produce eruptible melt within the LVC is small-scale, local rejuvenation of the crystal mush accompanied by magma mixing and mingling. Based on zircon stability, the time required to heat, erupt and then cool to background conditions is relatively short, lasting a maximum of 10 s–1000 s years. Rejuvenation events in the LVC are ephemeral and permit eruption within an otherwise waning and cooling magmatic body.</p></div

Yb/Gd versus Th/U for LVC zircon.

<p>Crystal mush “cold” storage conditions are marked with a dashed box while the dashed line with the arrows marks the path to warmer conditions during rejuvenation events. Th/U should increase while Yb/Gd decreases as greater basaltic input heats the crystal mush <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Klemetti2" target="_blank">[41]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Barth1" target="_blank">[51]</a>.</p

Composition, age, volume and T_Zr (calculated using [49]) for selected lavas of the Lassen Volcanic Center.

<p>All compositional and age data from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Clynne1" target="_blank">[28]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Clynne3" target="_blank">[37]</a>.</p><p>* does not include dark bands within banded pumice that are not relevant for calculating the T_Zr for the 1915 dacite.</p>#<p>Range of temperatures derived from multiple compositions and number in parentheses is the selected value for the unit.</p>2<p>Glass analysis from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Quinn1" target="_blank">[48]</a>.</p><p>Composition, age, volume and T_Zr (calculated using <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157-Boehnke1" target="_blank">[49]</a>) for selected lavas of the Lassen Volcanic Center.</p

²³⁸U-²³⁰Th isochron diagram showing interior (dark) and surface (light) analyses from the three eruptive units analyzed from the LVC.

<p>Error bars shown are 1σ, with reference isochrons (10, 27, 100 k.y., along with the equiline). See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone.0113157.s001" target="_blank">Appendix S1</a> for full age and other compositional data.</p

Cathodoluminescent images of zircon analyzed via SHRIMP-RG in this study.

<p>Top panel shows zircon used in surface analysis, mounted in indium. The bottom panel shows two zircon used in polished interior analysis, mounted in epoxy. All ages are reported with 1σ errors.</p

U-Th model age versus Ti-in-zircon temperatures from all three eruptive units analyzed.

<p>A majority of zircon analyzed fall between 680–725°C, while a subpopulation is warmer, with a range between 730–820°C. The three eruptive sequences and three eruptions sampled are marked in the same fashion as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone-0113157-g005" target="_blank">Figure 5</a>. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113157#pone-0113157-g008" target="_blank"><i>Figure 8</i></a><i> Inset</i>. Histogram of Ti-in-zircon temperatures for the LVC with a probability density function for the data (red line), showing the main population centered around 700°C and a smaller peak of higher-T zircon above 735°C (small shoulder to right of main peak).</p