[IGPP Everyone] [EPSS Everyone] Geocheminar, Monday noon in Slichter 3853

[T. Mark Harrison]

*A comparison of oxygen fugacities of S-type granites across the
Archean-Proterozoic boundary*

Claire E. Bucholz (Caltech)

The partial pressure of oxygen in the atmosphere, and therefore mechanisms
of weathering/alteration and redox conditions during sediment deposition
and diagenesis, varied over Earth’s history. It might therefore be expected
that this would be expressed as secular variations in igneous rocks that
contain sediments as source components or contaminants. The largest
recognized increase in the partial pressure of atmospheric oxygen occurred
circa 2.4 Ga during what is known as the “Great Oxygenation Event” (GOE).
The GOE left clear imprints of increased pO2 on the sedimentary rock
record, including the appearance of fluvial and nearshore red beds and
retention of iron in paleosols, the disappearance of readily oxidized
detrital minerals such as pyrite and uraninite from clastic sedimentary
rocks, and the appearance of evaporative sulfate mineral deposits. Since
they are generated by melting involving such sediments, S-type granites
derived from sedimentary rocks deposited on either side of the GOE might
have significantly different pO2, mirroring such an abrupt change in the
redox state of Fe and S their sedimentary sources. However, it is also
possible, despite variations in atmospheric pO2 , the bulk redox state of
sedimentary source material depends strongly on the details of its
depositional environment and the presence or absence of organic carbon. In
this talk, I will develop constraints on the s during crystallization of
Archean and Proterozoic S-type granites to determine whether changes in
atmospheric pO 2 levels across the GOE are recorded in granites from the
continental crust. To isolate the effects of sediment melting or
incorporation into igneous rocks and to provide the most direct link
between igneous rocks and changing surface conditions, I studied a
geographically distributed suite of North American Archean and Proterozoic
S-type granites. To place constraints on the s of the S-type granites, I
compare Fe(T) /Fe(T) +Mg ratios in biotite and their host rocks, which when
examined together and compared among similar rock types, are highly
sensitive to the during crystallization. I will discuss our results in
light of changes in the sedimentary redox budgets for C, Fe, and S across
the GOE.

-- 
Mark Harrison
Distinguished Professor
Department of Earth, Planetary and Space Sciences
University of California, Los Angeles
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