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

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

40Ar/39Ar geochronology of Neogene phreatomagmatic volcanism 3 in the western Pannonian Basin, Hungary

Uncorrected proofNeogene alkaline basaltic volcanic fields in the western Pannonian Basin, Hungary, including the Bakony–Balaton Highland and the Little Hungarian Plain volcanic fields are the erosional remnants of clusters of small-volume, possibly monogenetic volcanoes. Moderately to strongly eroded maars, tuff rings, scoria cones, and associated lava flows span an age range of ca. 6 Myr as previously determined by the K/Ar method. High resolution 40Ar/39Ar plateau ages on 18 samples have been obtained to determine the age range for the western Pannonian Basin Neogene intracontinental volcanic province. The new 40Ar/39Ar age determinations confirm the previously obtained K/Ar ages in the sense that no systematic biases were found between the two data sets. However, our study also serves to illustrate the inherent advantages of the 40Ar/39Ar technique: greater analytical precision, and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field. Periods of increased activity with multiple eruptions occurred at ca. 7.95 Ma, 4.10 Ma, 3.80 Ma and 3.00 Ma. These new results more precisely date remnants of lava lakes or flows that define geomorphological marker horizons, for which the age is significant for interpreting the erosion history of the landscape. The results also demonstrate that during short periods of more intense activity not only were new centers formed but pre-existing centers were rejuvenated

Apparent partial loss age spectra of Neoarchaean hornblende (Kola Peninsula, Russia): the role of included biotite shown by Ar/Ar laserprobe analysis

Metamorphic hornblende frequently yields spectra with progressively increasing Ar/Ar 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. In order 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 such micro-sampled biotite-free hornblende discs yielded flat age, Ca/K and Cl/K ratio spectra. In contrast, furnace step-heated hornblende separates from the same samples produced spectra with progressively increasing apparent ages and Ca/K ratios. Biotite-free samples yielded flat age and ratio spectra by both laser and furnace analysis. So, apparent loss spectra result from degassing of included much younger biotite before its hornblende host during laboratory step-heatin

Device for obtaining 3d biopsy

A biopsy device for taking a 3D biopsy may comprise an outer sleeve, a hollow main shaft, a biopsy tube and a tube shaft. The hollow main shaft may have a distal end portion with a sideward facing notch, and the main shaft may be adapted to be accommodated within the outer sleeve. The biopsy tube may be provided for receiving cut and thus isolated tissue. A proximal end of the biopsy tube may be releasably attachable to a distal end of the tube shaft so that the biopsy tube is movable together with the tube shaft within the hollow main shaft between a proximal position in which the biopsy tube is not located in the notch, and a distal position in which the biopsy tube is located in the notch

40Ar/39Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of NW Iberia

The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by 40Ar/39Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures. Transitional levels between HP–HT and IP upper units in the O´ rdenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating. Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Forna´s extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP– HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (sensu stricto) and Early Variscan convergence, as well as an older, Early Palaeozoic cycle