Relationship of magma storage, recharge, mixing, and eruption from U-series crystallization ages and diffusion timescales at Lassen Volcanic Center, CA

Abstract:

Understanding the physical conditions within magma reservoirs, the dynamic processes that occur within them, and how they relate to the scale and timing of volcanic eruptions is of significant importance to volcanology and our ability to quantify volcanic hazard potential. Methods such as U-series geochronology and mineral diffusion allow us to evaluate different aspects and timescales of magmatic processes and to constrain the thermal histories of magma bodies (e.g., [1]). We are using combined U-series ages and diffusion durations to examine the physical conditions of magma storage and subsequent remobilization at Lassen Volcanic Center (LVC), CA. Together these data will provide the percentage of time spent at temperatures where magmas are mobile and eruptible, the duration of magma storage, and lag time between eruption and mixing events. Mineral and outcrop-scale textures indicative of magma mixing processes have been well documented at the LVC. Major oxide mixing trends suggest similar mafic and felsic end-members for the Chaos Crags (1143 ± 23 ybp) and 1915 eruptions; however, the Chaos Crags magmas are far less mixed. Constraining the time lag between the initial establishment of a crystal-rich reservoir and later intrusions of recharge magmas and how that affects magma mixing is necessary to assess the relationship between mixing and eruption. These independent methods enable the characterization of magmatic thermal histories by constraining the total time represented in the crystal record from crystallization ages (²³⁸U-²³⁰Th and ²³⁰Th-²²⁶Ra disequilibria) of mineral populations, the durations spent at high temperature from diffusion (internal diffusion), and post-mixing timescales (rim diffusion). We will present preliminary results from the Chaos Crags and 1915 eruptions of the LVC. We will combine plagioclase (± biotite and hornblende) bulk mineral separate ²³⁸U-²³⁰Th and ²³⁰Th-²²⁶Ra crystal ages with mineral diffusion profiles to constrain the total time scale of crystal storage, the proportion of time spent above temperatures of rheological lock-up (~40-50% crystalline), and the temporal relationship between magma recharge and eruption.

[1] K.M. Cooper and A.J.R. Kent, Nature 506, 480-483.