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\begin{document}

\title{The High-pO2 Method to Prevent U-Loss During Single-Aliquot Hematite
(U-Th)/He Measurement: Development, Implementation, and Automation}



\author[1]{Florian Hofmann}%
\affil[1]{University of Southern California}%


\vspace{-1em}



  
  \date{May 28, 2024}


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\begin{abstract}
Single-aliquot (U-Th)/He dating of iron-oxides requires less mass and
has a higher spatial resolution than the two-aliquot approach, and is,
therefore, a more reliable tool for quantifying the timescales of
weathering processes, fault activity, and the development of soils and
surfaces. Highly helium-retentive hematite samples must be heated to
1000-1100 \selectlanguage{ngerman}°C to completely degas He, but we show that U is lost
progressively during laser-heating starting at \textasciitilde{}900 °C,
with major U-loss at \textasciitilde{}980 °C and complete U-loss at
1050-1100 °C. This partial or complete loss of U leads to incorrect
(U-Th)/He ages that appear older than the true ages. We performed a
series of heating tests on hematite and goethite aliquots with
independently determined (U-Th)/He ages of 10-1761 Ma in which the
helium release was determined by isotope-dilution mass spectrometry and
phase change was monitored by infrared spectroscopy. As hematite is
heated, reduction of Fe3+ causes a phase change to magnetite and then to
elemental Fe. We correlate the onset of U-loss to the phase change from
hematite to magnetite. By raising the temperature of this phase
transition using a high oxygen partial pressure (pO2) in the sample
chamber during laser-heating, we show that the onset temperature of
U-loss is equally raised. Samples can therefore be safely degassed at
higher temperatures without any detectable U-loss. In our implementation
of this technique, the O2 in the sample chamber is being withdrawn from
a tank using a pipette and it is being released before and captured
after the degassing process on activated charcoal in a cold finger with
a movable LN2 Dewar flask. We describe the automation of this process
for routine degassing of hematite samples. We show that an average age
calculated on a reference hematite sample from replicate aliquots
(n=12), which were analyzed using this procedure, has a relative
uncertainty of 2\% (1\selectlanguage{greek}σ\selectlanguage{english}), and is within uncertainty of the previously
measured two-aliquot age. We suggest this high-pO2 degassing procedure
as a way to precisely and reproducibly determine single-aliquot hematite
and goethite (U-Th)/He ages.%
\end{abstract}%



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