Abstract. We determined 40Ar/39Ar ages of buddingtonite, occurring together with muscovite, with the laser-ablation method. This is the first attempt to date the NH4-feldspar buddingtonite, which is typical for
sedimentary–diagenetic environments of sediments, rich in organic matter, or in hydrothermal environments, associated with volcanic geyser systems. The sample is a hydrothermal breccia, coming from the Paleoproterozoic pegmatite
field of the Korosten Plutonic Complex, Volyn, Ukraine. A detailed
characterization by optical methods, electron microprobe analyses,
backscattered electron imaging, and IR analyses showed that the
buddingtonite consists of euhedral-appearing platy crystals of tens of
micrometers wide, 100 or more micrometers in length, which consist of
fine-grained fibers of ≤ 1 µm thickness. The crystals are
sector and growth zoned in terms of K–NH4–H3O content. The
content of K allows for an age determination with the 40Ar/39Ar method, as well as in the accompanying muscovite, intimately intergrown with
the buddingtonite. The determinations on muscovite yielded an age of 1491 ± 9 Ma, interpreted as the hydrothermal event forming the breccia.
However, buddingtonite apparent ages yielded a range of 563 ± 14 Ma
down to 383 ± 12 Ma, which are interpreted as reset ages due to Ar
loss of the fibrous buddingtonite crystals during later heating. We conclude that buddingtonite is suited for 40Ar/39Ar age determinations as a
supplementary method, together with other methods and minerals; however, it
requires a detailed mineralogical characterization, and the ages will likely represent minimum ages.