Stelton et al 2015 U238Th230 dating of Yellowstone

Abstract

• Study focuses on the physical nature of the magma reservoir and mechanisms of rhyolite generation at Yellowstone caldera.

• Utilizes zircon and sanidine crystals from three rhyolites erupted between 170–70 ka.

• Presents 238U–230Th crystallization ages and trace-element compositions from crystals to understand the magma history.

• Findings suggest a long-lived magma reservoir (150–250 kyr) sourced the Central Plateau Member rhyolites, showing connections to older magmatic episodes.

Introduction

• Yellowstone Plateau volcanic field is a large-volume and long-lived silicic magmatic system.

• Has produced significant caldera-forming eruptions and numerous intra-caldera eruptions.

• The study aims to clarify the physical state of the magma reservoir and the mechanisms of eruptible magma generation.

Geological Background

Post-caldera Eruptive Activity at Yellowstone

• Yellowstone has experienced major caldera-forming eruptions followed by explosive episodes of rhyolitic volcanism.

• The most recent major eruption (Lava Creek Tuff) occurred around 639 ka.

• The Central Plateau Member (CPM) produced 18 lava flows and two ignimbrites between 170 to 70 ka.

  • Typical crystallinity: 5–20% with sanidine, quartz, and clinopyroxene as major mineral phases.

  • Oldest magma flows contain evidence of evolving magma reservoirs with changing geochemical signatures.

Methods Overview

• Characterization of zircon and sanidine included age and trace-element analysis using various dating methods (U-series, 40Ar/39Ar).

• Employed cathodoluminescence imaging to assess the texture and zoning patterns of crystals.

Sampling

• New and previously studied samples from CPM rhyolites including PPF, WYF, and GPF were analyzed for major element concentrations, Ba concentrations, and isotopes.

Results

Zircon Crystal Data

• Zircon interiors generally represent antecrystic origins from older episodic magmatism; surfaces crystallized shortly before or during eruption.

• Evidence shows that eruptible rhyolites are formed by extracting melts from a long-lived crystal-rich mush.

Sanidine Data

• Sanidine separates from each rhyolite are mainly autocrystic, indicating equilibrium with host melts.

• The rarity of antecrystic sanidine relative to zircons signifies selective processes in magma generation.

Timing and Mechanisms

• Crystallization ages suggest rapid generation of melt with little time in a liquid state before eruption.

• Large-volume eruptions can occur within 1 kyr of magma residence.

• Smaller volumes (e.g., GPF at 0.5 km3) may linger longer in the reservoir, providing insight into the dynamics of volcanism.

Discussion

Physical State of Magma Reservoir

• Long-lived crystal mush acts as a dominant storage state where eruptible magma bodies are ephemeral features.

• Predictions imply that if a liquid magma body is identified, it suggests the system is primed for eruptions.

Comparisons with Other Systems

• Insights gained from Yellowstone's crustal storage dynamics offer broader implications for understanding silicic magmatism globally.

Conclusions

1. The CPM rhyolites' generation is linked to a long-lived crystal mush with connections to earlier magmatic activity.

2. Zircon interiors often demonstrate inherited characteristics from earlier eruptive episodes, indicating recycling within the magma system.

3. The sanidine population is markedly less influenced by older sources, emphasizing unique crystallization histories.

4. Eruptible magmas demonstrate short residence times (<1 kyr) and lack significant reheating or remelting before eruption.

5. The crystal mush serves as a primary condition for the magmatic state, contrasting with models suggesting shallow melt reservoirs.

Acknowledgments

• Thanks to reviewers and contributors for insight and assistance in research methodologies.