Florian
Hofmann
Research Assistant Professor
Geophysical Institute, University of Alaska Fairbanks
RESEARCH
I am interested in finding novel applications for noble gas geochemistry and geochronology, including the (U‐Th)/He and 40Ar/39Ar methods, as well as stable (³He, ²¹Ne) and radioactive (10Be, 26Al, 36Cl) cosmogenic nuclides. I apply the tools of geochronology and geochemistry to studying the formation of soils and surfaces, measuring the timing and rates of volcanic processes and tectonic deformation, and reconstructing paleoclimate. I’m currently establishing a 40Ar/39Ar dating laboratory at the University of Alaska Fairbanks.
offset histories of large Strike-slip faults
I am applying thermochronometric systems with a large range of closure temperatures (U-Pb in zircon, titanite, and apatite, 40Ar/39Ar in K-feldspar and hornblende, and (U-Th)/He in zircon, apatite, and magnetite) to piercing points on large strike-slip faults such as the San Andreas Fault in California and the Denali fault in Alaska to reconstruct their exhumation history. I will use thermochronologic modeling to provide constraints on the offset history, displacement rates, and structural development of the fault systems.
Chronology of glacial deposits in the Alpine Foreland using cosmogenic nuclides
Together with students and faculty from the University of Munich, we are establishing an absolute chronology for the moraines and glaciofluvial deposits in the Bavarian Alpine Foreland that have been interpreted as a series of glacial periods (Würm, Riss, Mindel, Günz, Donau, Biber). In-situ cosmogenic 36Cl dating of glacial erratics and 10Be/26Al isochron burial dating of glaciofluvial terraces will facilitate an intercorrelation between different types of deposits, which have been interpreted to be the same age based on morphological characteristics. It will also allow the correlation of the effects of regional climatic variations with global climate.
microCT Pre-Screening of geochronologic samples
MicroCT scanning of samples can be used to detect inclusions in opaque mineral phases. These inclusions have the potential to alter the bulk ages measured by various methods (3He, (U-Th)/He, 40Ar/39Ar, etc.) by contributing parent and/or daughter nuclides. I have shown that inclusions contribute significant amounts of 3He In magnetite and that this contribution from mineral inclusions can be avoided by pre-screening samples with microCT and selecting only grains without inclusions for 3He measurement (Hofmann et al., 2021). This technique can also be used to identify the mineral phase, for example, to distinguish apatite and zircon (Cooperdock/Hofmann et al., 2022). I am currently working on extending this microCT pre-screening method to other mineral phases and geochronologic techniques to improve the precision and reliability of geochronologic ages.
Cosmogenic 3He dating of iron-oxide phases
Stable cosmogenic nuclides can be used to determine exposure ages of modern surfaces and they can give insights into how long buried materials have resided within about 2 m of a paleo-surface. I have determined >5 Ma of Miocene paleo-exposure of pisoliths in paleosols of the Bohnerz Formation in Central Europe (Hofmann et al., 2017). This method has also revealed extremely low surface erosion rates of limestone plateaux in Southern Germany over the last 10 Ma (Chapter 3 of my dissertation). I’m working on extending the method to fine-grained pedogenic iron-oxides in active soils and detrital magnetite. I have utilized this technique to date a soil at Whitewater Hill in order to determine slip rates of the Banning Strand of the San Andreas fault on San Gorgonio Pass (Chapter 5 of my dissertation).
(U-Th)/He dating of iron-oxides
I am applying (U-Th)/He dating of goethite and hematite to study iron-oxide dissolution-precipitation in soils and paleosols. Previous applications of this technique to date goethite precipitation in soils and paleosols include pisoliths of the Bohnerz Formation in Central Europe (Hofmann et al, 2017; Chapters 2 and 3 of my dissertation) and groundwater markers of the Upland Complex in Tennessee/Arkansas/Mississippi (Odom et al., 2020). A recently developed method makes it possible to also obtain accurate and precise single-aliquot ages of hematite (Hofmann et al., 2020). I am also working on strategies to make magnetite (U-Th)/He dating more feasible for geochronologic and thermochronologic applications.
