Abstract
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.
Highlights
IntroductionBuddingtonite, the monoclinic (space group P 21/m) ammonium feldspar (NH4AlSi3O8), occurs in two geological settings: in igneous rocks in hydrothermal environments, such as volcanic geyser fields (Erd et al, 1964), and in sediments rich in organic matter (OM), such as coal seams (Dai et al, 2018), oil shales (e.g., Loughnan et al, 1983; Patterson et al, 1988), phosphorites (Gulbrandsen, 1974), black shales (e.g., Harlov et al, 2001), and sandstones (Ramseyer et al, 1993)
No attempts have been made to date buddingtonite, and here we present our results from a sample with a hydrothermal buddingtonite– muscovite association, formed in the Paleoproterozoic Volyn pegmatite field, Ukraine (Fig. 1)
In combination with accompanying minerals such as muscovite in the reported case here, it allows for estimating the age framework of a sedimentary–diagenetic or a hydrothermal event
Summary
Buddingtonite, the monoclinic (space group P 21/m) ammonium feldspar (NH4AlSi3O8), occurs in two geological settings: in igneous rocks in hydrothermal environments, such as volcanic geyser fields (Erd et al, 1964), and in sediments rich in organic matter (OM), such as coal seams (Dai et al, 2018), oil shales (e.g., Loughnan et al, 1983; Patterson et al, 1988), phosphorites (Gulbrandsen, 1974), black shales (e.g., Harlov et al, 2001), and sandstones (Ramseyer et al, 1993). Buddingtonite is often characterized as a “nondescript and amorphous” appearing mineral (e.g., Pampeyan, 2010), mostly fine-grained or in micrometer-wide rims on K-feldspar. It is a trace component in pegmatitic feldspar (Solomon and Rossman, 1988) but forms continuous solid solution with K-feldspar (Pöter et al, 2007; Svenson et al, 2008), which might be formed either due to partial replacement of igneous or detrital K-feldspar or together with authigenic Kfeldspar. The K content in buddingtonite would allow applying the K–Ar decay system to date the formation of such buddingtonite– K-feldspar solid solutions, which would give valuable time information for a diagenetic or a hydrothermal event.
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