Lily L. Claiborne

Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

Abstract Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ~35-40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15-30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (TTiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ~30-40 for cumulates and 18-30 for high-SiO2granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and TTiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

Zircon reveals protracted magma storage and recycling beneath Mount St. Helens

Current data and models for Mount St. Helens volcano (Washington, United States) suggest relatively rapid transport from magma genesis to eruption, with no evidence for protracted storage or recycling of magmas. However, we show here that complex zircon age populations extending back hundreds of thousands of years from eruption age indicate that magmas regularly stall in the crust, cool and crystallize beneath the volcano, and are then rejuvenated and incorporated by hotter, young magmas on their way to the surface. Estimated dissolution times suggest that entrained zircon generally resided in rejuvenating magmas for no more than about a century. Zircon elemental compositions refl ect the increasing infl uence of mafi c input into the system through time, recording growth from hotter, less evolved magmas tens of thousands of years prior to the appearance of mafi c magmas at the surface, or changes in whole-rock geochemistry and petrology, and providing a new, time-correlated record of this evolution independent of the eruption history. Zircon data thus reveal the history of the hidden, long-lived intrusive portion of the Mount St. Helens system, where melt and crystals are stored for as long as hundreds of thousands of years and interact with fresh infl uxes of magmas that traverse the intrusive reservoir before erupting.

Quantitative 3D petrography using X-ray tomography 3: Documenting accessory phases with differential absorption tomography

Accessory minerals preserve important records of the evolution of magmatic systems, but study of their textures and contact relations is hindered by the lack of suitable methods for characterization. We show here that differential absorption X-ray tomography can be used to yield three-dimensional maps of selected elements, particularly Zr and rare earth elements (REE), making it possible to qualitatively and quantitatively document the textures of zircon and REE minerals in situ and in three dimensions. We apply this method to pumice from the Peach Spring Tuff (Nevada, Arizona, California) and Mount St. Helens (Washington State) and present a few illustrative examples of the kinds of data that can be extracted using elemental and conventional tomographic data. Particularly when combined with compositional and age data, the ability to visualize and document accessory minerals in three dimensions and in textural context opens exciting new possibilities for the study of accessory minerals and the rocks that contain them.

GEOLOGY OF MEADOW CREEK BASIN, SOUTHERN BLACK MOUNTAINS, ARIZONA: RECORD OF POST-SUPERERUPTION VOLCANISM (REU PROJECT, 2016)

Meadow Creek basin (MCB; southern Black Mountains AZ), 4 km E of the caldera of the 18.8 Ma Peach Spring Tuff (PST) supererupPon (Ferguson et al 2013), reveals a 3 m.y record of the a^ermath of that erupPon. Building on previous work in the area (Ransome 1923, Thorson 1971, LiggeX & Childs 1982; Ferguson pers. com.), NSF-REU undergrads mapped a 12 km2 area within MCB and expanded their spaPal scope through remote sensing (Schwat et al, Helfrich et al, Thompson et al 2016). Students further invesPgated the sequence with opPcal petrography and analysis by XRF, SEM-EDS, and LA-ICPMS.

Teaching Radioactive Decay and Radiometric Dating: An Analog Activity Based on Fluid Dynamics

ABSTRACT We present a new laboratory activity for teaching radioactive decay by using hydrodynamic processes as an analog and an evaluation of its efficacy in the classroom. A fluid flowing from an upper beaker into a lower beaker (shampoo in this case) behaves mathematically identically to radioactive decay, mimicking the exponential decay process, dependent on the amount of fluid in the upper beaker (representing the amount of parent isotopes) and the size of the hole in the beaker (representing the decay constant). Students measure the fluid depth with time for several runs with varied conditions, then graph their results, create decay equations, manipulate these equations and use them to “date” another experiment. They then apply their new understanding to make predictions regarding complications involved in the decay process and its use in dating (such as daughter loss). Student quiz performance improved from before to after the activity, indicating improved student learning. Student comments and questions indicated deep understanding and a new curiosity about the process and its application.