Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

Abstract

Despite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.

Magma Reservoir Below Laguna del Maule Volcanic Field, Chile, Imaged With Surface‐Wave Tomography

“…Geologic, petrologic, geochronologic, and geomorphic findings at LdM have contributed to a model of a voluminous, crystal‐rich magma reservoir that has grown and persisted beneath the LdM lake basin for at least 60 kyr (Andersen et al, ; Andersen, Singer, et al, ; Hildreth et al, ; Singer et al, ). Much of this reservoir persisted in a near‐solidus state.…”

“…However, repeated intrusion of magma ascending from the deep crust during postglacial times has heated the reservoir from below. These additions of heat and volatiles to the crystal‐rich reservoir thermally buffered bodies of higher‐melt‐fraction mush (30–50% melt) and contributed to the segregation of discrete, modest‐volume, eruptible rhyolitic magma bodies (Andersen et al, , ). The residence time of rhyolite erupted during the late Holocene, derived from diffusion chronometry of trace element zoning in feldspars, favors a timescale of extraction to eruption of rhyolitic melt on the order of decades to centuries and suggests its in situ volumes did not substantially exceed the erupted volumes, <3 km 3 (Figure ; Andersen et al, ).…”

Storage and Evolution of Laguna del Maule Rhyolites: Insight From Volatile and Trace Element Contents in Melt Inclusions

“…Moreover, zircon crystallization rates and Ti‐in‐zircon temperatures indicate that hot zones and regions of cold storage may exist contemporaneously within the mush reservoir at LdM (Andersen et al, 2019). Persistent intrusion of recharge magma is hypothesized to fuel volcanism at LdM and incubate the hot zones creating localized mush bodies with extractable interstitial rhyolitic melt, from which caps of crystal‐poor rhyolite are accumulated and erupted (Andersen et al, 2019). LdM plagioclase diffusion timescales indicate short rhyolite residence times (decades to centuries) prior to eruption (Andersen et al, 2018).…”

Coexisting Discrete Bodies of Rhyolite and Punctuated Volcanism Characterize Yellowstone's Post‐Lava Creek Tuff Caldera Evolution

“…Covariation of Yb/Gd and Nd/Yb, which reflect the slopes of REE patterns for the zircons, is consistent with melts related by fractionation of major phases and accessory minerals in rhyolites. Yb/Gd in zircons is usually positively correlated with indices of differentiation such as Hf (Andersen et al, ; Barth & Wooden, ; Claiborne, Miller, & Wooden, ) and reflects the fractionation of pyroxenes or accessory minerals such as titanite that sequester middle REE (e.g., Andersen et al, ; Figure ). Titanite is absent in Yellowstone rhyolites, but clinopyroxene and orthopyroxene hosting inclusions of apatite, chevkinite, and zircon are common (Stelten et al, ; Vazquez et al, , ).…”

“…Accordingly, the covariation in Nd/Yb and Yb/Gd of these rhyolites can be explained primarily by relative fractionation of pyroxenes and chevkinite ± allanite. However, UBM rhyolites define an array with higher Nd/Yb than LCT zircons and a subset of CPM zircons (Figure ), suggesting a lesser role for fractionation of chevkinite as well as distinctive parental melts (e.g., Andersen et al, ). These characteristics could reflect standard crystal‐melt separation and/or residual mineralogy in a partially melted source.…”