Full Article Title
The Somerset Dam layered mafic intrusion in southeast Queensland, Australia, has been conventionally dated as Late Triassic by the apparently successful application of radioisotopic dating techniques. Mineralogical, geochemical and isotopic evidence indicates that all of this gabbro intrusion’s cyclic units were derived coevally from the same parental basaltic magma, with an initial homogeneous isotopic mixture ideal for yielding concordant isochron ages. However, newly obtained K-Ar, Rb-Sr, Sm-Nd and Pb-Pb radioisotopic data from 15 whole-rock samples (representing all gabbro macrolayers in four of the intrusion’s cyclic units) yield discordant isochron “ages”, although the excellent-fitting 15-point K-Ar isochron suggests the resultant 174±8 Ma “age” (Middle Jurassic) should be regarded as the revised conventional age of the layered intrusion. Nevertheless, it is concluded that these discordances between the radioisotope systems are likely due to changes in their decay rates in the past, with the longer halflife beta-emitter 87Rb being accelerated more and thus yielding an older “age”. Furthermore, the Sr, Nd and Pb isotopes indicate the parental basaltic magma was derived from a depleted mantle source, while the large spread of Nd TDM “ages” suggests accelerated radioisotopic decay rates during the partial melting and magma ascent. It is concluded that the Somerset Dam layered mafic intrusion has inherited the radioisotopic signature of its mantle source, and so the conventional radioisotopic dating techniques do not provide its true age.
Gabbro, layered intrusion, Australia, potassium-argon, rubidium-strontium, samarium-neodymium, leadlead, radioisotopic dating, whole-rock model “ages”, whole-rock isochron “ages”, discordances, decay “constants”, accelerated decay, mantle source inheritance
DigitalCommons@Cedarville provides a publication platform for fully open access journals, which means that all articles are available on the Internet to all users immediately upon publication. However, the opinions and sentiments expressed by the authors of articles published in our journals do not necessarily indicate the endorsement or reflect the views of DigitalCommons@Cedarville, the Centennial Library, or Cedarville University and its employees. The authors are solely responsible for the content of their work. Please address questions to email@example.com.
Snelling, Andrew A.
"Whole-rock K-Ar Model and Isochron, and Rd-Sr and Pb-Pb Isochron, "Dating" of the Somerset Dam Layered Mafic Intrusion, Australia,"
Proceedings of the International Conference on Creationism: Vol. 5
, Article 20.
Available at: https://digitalcommons.cedarville.edu/icc_proceedings/vol5/iss1/20
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.