Apparent partial loss age spectra of Neoarchean hornblende (Murmansk Terrane, Kola Peninsula, Russia): the role of biotite inclusions revealed by 40Ar/39Ar laserprobe analysis

• Main Authors: De Jong, Koenraad, Wijbrans, Jan R.
• Other Authors: Institut des Sciences de la Terre d'Orléans (ISTO), Institut national des sciences de l'Univers (INSU - CNRS)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS), Department of Isotope Geochemistry, Faculty of Life and Earth Sciences, Vrije Universiteit, Studium
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2006
• Subjects: [SDU.STU.TE]Sciences of the Universe [physics]/Earth Sciences/Tectonics; Kola Peninsula; Murmansk; kola peninsula
• Online Access: https://hal-insu.archives-ouvertes.fr/hal-00109454; https://hal-insu.archives-ouvertes.fr/hal-00109454/document; https://hal-insu.archives-ouvertes.fr/hal-00109454/file/deJong-TER-669-HAL-1.pdf

Metamorphic hornblende frequently yields spectra with progressively increasing 40Ar/39Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid-state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K-contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step-heating of drilled biotite-free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step-heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite-free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step-heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane's cooling.