| Full text | |
| Author(s): |
Hueck, Mathias
;
Wemmer, Klaus
;
Ksienzyk, Anna K.
;
Kuehn, Rebecca
;
Vogel, Nadia
Total Authors: 5
|
| Document type: | Journal article |
| Source: | EARTH-SCIENCE REVIEWS; v. 232, p. 18-pg., 2022-09-01. |
| Abstract | |
K-Ar and Ar/Ar illite geochronology are powerful and versatile tools that can be used to provide age constraints on diverse geological processes such as diagenesis, hydrothermalism, and low-temperature metamorphism and deformation in the brittle and ductile-brittle regimes. However, the interpretation of illite Ar dates can be difficult, especially since results from grain-size fractionated samples are typically grain-size dependent, with the finest fractions yielding the youngest dates. This is usually the result of a mixed isotopic signal, caused by a physical mixture between different populations, by partial isotopic and/or chemical resetting, or by a recurrent crystallization history. In order to extract meaningful ages from such datasets, two main lines of interpretation exist: assuming that end fractions (i.e., the finest and coarsest materials of a sample) constitute limit ages, or extrapolating "pure" authigenic and inherited ages using quantifiable mineralogical parameters (such as polytype composition). The two strategies are tested using a well-constrained case study from very low-grade metamorphic samples from the Rhenish Massif (Germany). Six fractionated samples from a continuous outcrop produce internally consistent K-Ar ages that can be interpreted in terms of limit ages and are supported by illite "crystallinity" and polytype composition. Nonetheless, extrapolation of dates from clastic metasedimentary rocks into "pure" end-member ages (illite age analyses - IAA) result in geologically meaningless results that could only be recognized as such because of the analysis of intercalated detritus-free metatuffs. The reason for this is that some of the fundamental premises for the extrapolation of illite ages are not satisfied. These premises are: authigenic and inherited illite populations must be homogeneous in their mineralogical compositions; authigenic illite must have been crystallized during a single near-instantaneous event; and both authigenic and inherited populations must have the same K content. The case study demonstrates the importance of interrogating the limitations and the premises of any treatment of geochronological data in order to avoid over-interpretation. Both strategies for interpreting illite dates are complementary and none is intrinsically better than the other. Because of this complexity, it is important to validate and complement illite argon geochronology by acquiring large datasets and performing additional analyses. (AU) | |
| FAPESP's process: | 19/06838-2 - Pan-African shear zones in the Kaoko Belt (Namibia) and links across Southwestern Gondwana: Neoproterozoic evolution and Phanerozoic reactivation |
| Grantee: | Mathias Hueck |
| Support Opportunities: | Scholarships in Brazil - Post-Doctoral |