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Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques

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Published/Copyright: April 1, 2015
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American Mineralogist
From the journal American Mineralogist Volume 93 Issue 5-6

Abstract

Precise dating of K-rich minerals in volcanic rocks via the 40Ar/39Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating 40Ar/39Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine 40Ar/39Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 μm thick K-depleted (<8% K2O) alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra. We propose that these altered zones promote open-system behavior. Phenomena such as loss of K or 40Ar*, 39ArK recoil into internal K-depleted phases, and 39ArK recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration. Due to the presence of these multiple potential pathways that promote the gain or loss of isotopes, such altered biotite crystals are unsuitable for high-resolution 40Ar/39Ar dating, comparisons with sanidine ages, or calibration of the geomagnetic polarity timescale.

Keywords: Biotite; sanidine; 40Ar/39Ar; recoil; Bridger Formation; Wagon Bed Formation; Wind River Formation; EPMA; tephra; tuff; volcanic ash
Received: 2006-11-22
Accepted: 2007-12-10
Published Online: 2015-4-1
Published in Print: 2008-5-1

© 2015 by Walter de Gruyter Berlin/Boston

Articles in the same Issue

  1. The crystal structure of vurroite, Pb20Sn2(Bi,As)22S54Cl6: OD-character, polytypism, twinning, and modular description
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  3. Dingdaohengite-(Ce) from the Bayan Obo REE-Nb-Fe Mine, China: Both a true polymorph of perrierite-(Ce) and a titanic analog at the C1 site of chevkinite subgroup
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  7. Oxide-melt solution calorimetry of selenides: Enthalpy of formation of zinc, cadmium, and lead selenide
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  22. Birchite, a new mineral from Broken Hill, New South Wales, Australia: Description and structure refinement
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https://doi.org/10.2138/am.2008.2517
Keywords for this article
Biotite; sanidine; 40Ar/39Ar; recoil; Bridger Formation; Wagon Bed Formation; Wind River Formation; EPMA; tephra; tuff; volcanic ash

Articles in the same Issue

  1. The crystal structure of vurroite, Pb20Sn2(Bi,As)22S54Cl6: OD-character, polytypism, twinning, and modular description
  2. Mineralogy of the Paso Robles soils on Mars
  3. Dingdaohengite-(Ce) from the Bayan Obo REE-Nb-Fe Mine, China: Both a true polymorph of perrierite-(Ce) and a titanic analog at the C1 site of chevkinite subgroup
  4. Thermal equation of state of CaGeO3 perovskite
  5. Quantitative absorbance spectroscopy with unpolarized light: Part I. Physical and mathematical development
  6. Quantitative absorbance spectroscopy with unpolarized light: Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra
  7. Oxide-melt solution calorimetry of selenides: Enthalpy of formation of zinc, cadmium, and lead selenide
  8. Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques
  9. Rationale for the existence of four- and eight-reversals in antigorite
  10. Structural relaxation around substitutional Cr3+ in pyrope garnet
  11. Metasomatic thorite and uraninite inclusions in xenotime and monazite from granitic pegmatites, Hidra anorthosite massif, southwestern Norway: Mechanics and fluid chemistry
  12. Interlayer potassium and its neighboring atoms in micas: Crystal-chemical modeling and XANES spectroscopy
  13. Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite
  14. Fracture toughness, hardness, and elastic modulus of kyanite investigated by a depth-sensing indentation technique
  15. Crystal chemistry of the natrojarosite-jarosite and natrojarosite-hydronium jarosite solid-solution series: A synthetic study with full Fe site occupancy
  16. A new experimental thin film approach to study mobility and partitioning of elements in grain boundaries: Fe-Mg exchange between olivines mediated by transport through an inert grain boundary
  17. Titanium as a cathodoluminescence activator in alkali feldspars
  18. Biogenic vs. abiogenic magnetite nanoparticles: A XMCD study
  19. Solubility of andradite, Ca3Fe2Si3O12, in a 10 mol% NaCl solution at 800 °C and 10 kbar: Implications for the metasomatic origin of grandite garnet in calc-silicate granulites
  20. A 3D reconstruction of plagioclase crystals in a synthetic basalt
  21. Rudashevskyite, the Fe-dominant analogue of sphalerite, a new mineral: Description and crystal structure
  22. Birchite, a new mineral from Broken Hill, New South Wales, Australia: Description and structure refinement
  23. Crystal structure of synthetic Al4B2O9: A member of the mullite family closely related to boralsilite
  24. Interaction of gypsum with As(V)-bearing aqueous solutions: Surface precipitation of guerinite, sainfeldite, and Ca2NaH(AsO4)2·6H2O, a synthetic arsenate
  25. Jahnsite-(NaFeMg), a new mineral from the Tip Top mine, Custer County, South Dakota: Description and crystal structure
  26. Evidence for anomalously large degree of polymerization in Mg2SiO4 glass and melt
  27. Theoretical infrared absorption coefficient of OH groups in minerals
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