Ryan D. Mills

The plutonic record of a silicic ignimbrite from the Latir volcanic field, New Mexico

Zircon U-Pb geochronologic data for plutonic rocks in the Latir volcanic field, New Mexico, demonstrate that the rocks are dominated by plutons that post-date ignimbrite eruption. Only zircon from the ring dike of the Questa caldera yields the same age (25.64 ± 0.08 Ma) as zircon from the caldera-forming Amalia Tuff (25.52 ± 0.06 Ma). The post-caldera Rio Hondo pluton was assembled incrementally over at least 400 ka. The magma accumulation rate for the exposed portion of the Rio Hondo pluton is estimated to be 0.0003 km3 a−1, comparable to rates for other plutons, and too slow to support accumulation of large eruptible magma volumes. Extrapolation of the accumulation rate for the Rio Hondo pluton over the history of the Latir volcanic field yields an estimated volume of plutonic rocks comparable to the calculated volume under the field as determined by geophysical studies. We propose that the bulk of the plutonic rocks beneath the volcanic center accumulated during periods of low volcanic effusivity. Furthermore, because the oldest portion of the Rio Hondo pluton is the granitic cap exposed beneath a gently dipping roof contact, the roof granite cannot be a silicic liquid fractionated from the deeper (younger) portions of the pluton. Instead, our data suggest that the compositional heterogeneity of the Rio Hondo pluton is inherited from lower crustal sources. We suggest that if magma fluxes are high enough, zoned ignimbrites can be formed by evolution of the melt compositions generated at the source with little or no shallow crustal differentiation.

Zircon U‐Pb geochronology of the Mount Givens Granodiorite: Implications for the genesis of large volumes of eruptible magma

The Mount Givens Granodiorite, a large pluton in the central Sierra Nevada batholith, California, is similar in area to zoned intrusive suites yet is comparatively chemically and texturally homogenous. New zircon U-Pb geochronology indicates that the pluton was constructed over at least 7 Ma from 97.92 ± 0.06 Ma to 90.87 ± 0.05 Ma. Combining the new geochronology with the exposed volume of the pluton yields an estimated magma flux of <0.001 km3/a. The geochronologic data are at odds with the previously speculated links between plutons such as the Mount Givens Granodiorite and large-volume homogeneous ignimbrites (often termed monotonous intermediates). Existing data indicate that large plutons accumulate at rates of ≤0.001 km3/a, 1–2 orders of magnitude less than fluxes calculated for dated monotonous intermediates. If monotonous intermediates are remobilized, erupted plutons accumulated at rates comparable to dated examples, they should preserve a record of zircon growth of up to 10 Ma. Alternatively, the long history of zircon growth recorded in plutons may be erased during the processes of reheating and remobilization that precede supervolcano eruption. However, zircon dissolution modeling, based on hypothetical temperature-time histories for preeruptive monotonous intermediates, indicates that rejuvenation events would not sufficiently dissolve zircon. We suggest that eruptions of monotonous intermediates occur during high magmatic flux events, leaving little behind in the intrusive rock record, whereas low fluxes favor pluton accumulation.

Experimental evidence for crystal coarsening and fabric development during temperature cycling

Oscillating temperature dramatically speeds up recrystallization of a magma analog consisting of ammonium thiocyanate and ammonium chloride crystals in liquid. Linear growth rates increase by a factor of 10 if temperature is oscillated up and down a few degrees (e.g., 47 ± 3 °C) relative to nominally static conditions. Crystals pulse in size during thermal cycling; over the course of hundreds of cycles larger crystals grow and smaller crystals shrink, dramatically skewing the crystal size distribution. Crystal dissolution and growth in a pulsing thermal gradient produces a pronounced fabric, with crystals of ammonium thiocyanate aligned subparallel to the direction of heat flow. Alignment occurs via selective dissolution and growth of crystals of diverse orientations inherited from the starting material. These results have important implications for understanding how crystals grow and for interpreting texture in igneous rocks. Temperature cycling is likely common in magmatic systems and needs to be considered when analyzing chemical zoning of igneous crystals and rock textures.

The Volcanic-Plutonic Connection

One way to frame the debate about the relationships between volcanic and plutonic rocks is this: are plutons samples of magma that passed through the crust, or residues left behind by extraction of erupted liquids? In the former case plutons are compositionally equivalent to cogenetic volcanic rocks, barring biases introduced by passing through the crustal filter; in the latter they are cumulates, having lost liquid to eruption. These hypotheses make specific predictions about trace-element variations, which we test using global geochemical databases for circum-Pacific convergent margins and western North America. Volcanic rocks are far more abundant in these datasets than plutonic rocks and are biased to more mafic compositions. After subsampling the volcanic dataset to match the plutonic dataset, we find little evidence for significant loss of liquid from plutons. Rather, plutonic and volcanic trace-element patterns are generally indistinguishable. Where distinctions do occur, they are backwards; for example, a higher proportion of plutonic rocks has low Eu, Zr, and Ba, features of fractionated liquids, than volcanic rocks. These observations support the hypothesis that liquids fractionated from crystal-rich magmas are of small volume and are relatively immobile (e.g., aplites). These conclusions, derived from bulk-rock geochemistry, are supported by U-Pb zircon geochronology and field and textural observation. These data support the view that plutonic rocks are texturally modified samples of the same magmas that erupt. Partial melting provides an alternative to crystal fractionation for the origin of high-silica volcanic rocks.

Interpreting two-dimensional cuts through broken geologic objects: Fractal and non-fractal size distributions

Numerical simulations and physical fragmentation experiments confirm the theoretical prediction that the fractal dimension of a two-dimensional (2-D) cut through a set of three-dimensional objects with fractal dimension D is approximately equal to D – 1. This leads to a size distribution in two-dimensional cuts that is skewed strongly toward larger objects compared to the three-dimensional distribution. Three-dimensional shape (aspect ratio) does not significantly affect the resulting 2-D size distribution except for highly nonequant objects, such as prolate ellipsoids with aspect ratios of 10 or more. In contrast, fragmentation of an object by breakage along persistent fractures results in a non-fractal distribution of sizes and far fewer small objects than predicted by fractal statistics. Powdering a rock by extensive crushing also results in non-fractal size distributions because particles are reduced to sizes on the order of 1 μm, a comminution limit below which further brittle fracture is difficult. Natural examples of fragmental objects observed in two-dimensional cuts, such as crushed rocks, breccias, and xenoliths, are generally consistent with a three-dimensional fractal dimension near 2.5 over one or two orders of magnitude in size. However, a limestone breccia from Death Valley exhibits a nonfractal size distribution consistent with fragmentation of a strongly jointed rock. Mafic enclaves in Yosemite National Park have a restricted size range of about one order of magnitude and a three-dimensional fractal dimension of ∼3.1, consistent with other enclave swarms. The restricted size range of enclaves may reflect the apertures of mafic dikes that fed them.