Helium thermochronology methods are advancing rapidly as we improve (1) our understanding and measurement of He mobility in minerals and (2) our ability to robustly interpret He cooling ages as the timing of an event in Earth history and/or a rate of some Earth system process. My group at ISU pursues both avenues of research. In addition to the active projects described below, I am currently laying the foundation for projects in Idaho and surrounding parts of the Rocky Mountain west.
Applied ThermochronologyThermochronologic record of eruptions that fed the Columbia River flood basalt eruptions | with Leif Karlstrom
sampling the Maxwell Lake dike
Feeder dikes of the Miocene Columbia River flood basalts are spectacularly exposed in the Wallowa Mountains of eastern Oregon, where they were injected into a Cretaceous batholith. We use low-temperature thermochronology to document how long (in years) individual feeder dikes were active conduits. Check out our new paper demonstrating this method. Ongoing work is expanding this approach across the Wallowas. Leif just got a CAREER grant to fund this work!
Anthropogenic fire and the erosional response to the arrival of humans in Australia | with Eric Portenga
trackless zircon grains from core K6E are likely the result of resetting during paleowildfire
The arrival of humans and anthropogenic fire to Australia at ca. 50-65 ka may have caused the rapid, continent-scale changes in vegetation, landscape evolution, and megafauna abundance observed in the geologic record. However, the relative timing of these events—and their connections—remains controversial. Wildfire, like other short-duration high-T events, produces characteristic thermochronologic signatures in soils and outcrops. Together with Michael Bird (James Cook University) and Stuart Thomson (University of Arizona), we have identified a likely thermochronologic wildfire signature in sediment from a core spanning ca. 150 ka from a sinkhole in Australia’s Northern Territory. We are processing material from key parts of this core in order to simultaneously track the landscape’s fire history using thermochronology and its erosion history using paleo-erosion rates derived from cosmogenic radionuclide measurements. This work is funded by a small grant from the Quaternary Research Fund and a Purdue Rare Isotope MEasurement (PRIME) Lab seed grant, and we will use these data to apply for funding from NSF to expand and complete this study
Apatite thermochronology from central Colorado Plateau | with Pete Reiners, Stuart Thomson, Xavier Robert, and Kelin Whipple
sampling Mt. Hillers, Henry Mountains
(Not an active project). See papers below for our take (as of 2016-2019) on how to work with complex apatite (U-Th)/He data.
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Method DevelopmentThe monazite (U-Th)/He thermochronometer | with Nathan Niemi
results of cycled step-heating experiments on monazite
Monazite is a LREE phosphate, commonly used as a Th-Pb geochronometer, that exhibits significant compositional variability and very high (1-15 wt. %) Th, and therefore He, content. Single-crystal variability in monazite Th and LREE composition is hypothesized to result in a previously reported large range of He diffusion parameters and thereby effective closure temperatures (~180–290 ˚C for 10˚C/My cooling rate). Such variable diffusion kinetics are methodologically challenging, because a diffusion experiment needs to be performed on each crystal analyzed, but this variability makes this system a rich archive of thermal history information. See Rapid stepped-heating experimental method for routine monazite (U-Th-Sm)/He thermochronology.
When and where are low-temperature thermochronometers affected by crustal magmatism? | with Pete Reiners and Jean Braun
resetting aureoles above mid-crustal heat anomalies
This is an active interest of mine, and my current project regarding the thermochronologic record of magmatism is an applied project in the Wallowa Mountains (see description at left).
Our 2018 modeling paper on this topic: Toward robust interpretations of low-temperature thermochronology in magmatic terranes. Our 2019 paper The thermochronologic record of erosion and magmatism in the Canyonlands region of the Colorado Plateau addresses the challenge of possible regional variability in the geothermal gradient when interpreting cooling ages, even in regions not traditionally considered magmatic terranes. The effects of grungy, secondary, U-Th-rich phases on apatite He ages | with Devon Orme and Pete Reiners
GBPs on apatites from the Henry Mountains laccoliths
(Not an active project). External sources of U, Th, Sm and He are important considerations when confronted with dispersed apatite He datasets. Our paper provides a practical guide for identifying these effects.
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