Dating Fabrics

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Dating Fabrics
The Application of 40Ar/39Ar Geochronology
EURISPET 2008

• Marnie Forster

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APPLICATION
40Ar/39Ar Geochronology research in
Australia TANG3O (Thermochronology and Noble Gas,
Geochronology and Geochemistry Organisation)
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Versatility of argon chronology

• Timing of metamorphic events

• Dating deformation (not accessory minerals)

• Timing of shear zone movement

Photo Grant Dawson (PhD candidate Uni Qld)

• Recording complex thermal histories

• Recrystallisation

• Timing of fluid events

• Timing of a thermal pulse

• Timing of weathering profiles

• Timing of magmatism, kimberlites, volcanism

• Timing of fault movement (e.g. clays,
  pseudotachylites)

• Geospeedometry

• Timing of mineralisation

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MELBOURNE UNIVERSITY

• Mantle geochronology and geochemistry
• Mantle xenoliths
• Diamonds and inclusions
• Ore deposit geochronology and geochemistry
• Ar-Ar dating of potassic inclusions in sulphides
• Ar-Ar dating of kimberlites and related rocks
• Ar-Ar thermochronology
• e.g. Himalayas, Antarctica

David Phillips http//www.earthsci.unimelb.edu.au/
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MELBOURNE UNIVERSITY
David Phillips
List of Publications
Phillips, D. and Miller, J.M. 2005. Testing time
for the fools clock? Ar40/Ar39 dating of pyrite.
Geochimica etCosmochimica Acta, 69,
567. Phillips, D., Harris, M.W., Kiviets, G.B.
2004. Ar40/Ar39 analyses of clinopyroxene
inclusions in African diamonds Implications for
source ages of detrital diamonds. Geochimica et
Cosmochimica Acta, 68, 151-165. Kendrick, M.A.,
Mark, G. Phillips, D. 2007. Mid-crustal fluid
mixing in a Proterozoic Fe oxide-Cu-Au deposit,
Ernest Henry, Australia Evidence from Ar, Kr,
Xe, Cl, Br and I. Earth and Planetary Science
Letters 256, 328-343.
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UNIVERSITY of QUEENSLAND

• Principal research interests
• Supergene enrichment in ore deposits.
• Isotopic dating of weathering processes.
• Exploration geochemistry.
• Palaeoclimatology and landscape evolution.
• Application of K-Ar and 40Ar/39Ar and noble gas
  systematics to ore deposit genesis.
• Origin and geochemistry of gem deposits.

• PROJECTS
• Weathering Geochronology and Landscape Evolution
  Weathering and Nickel Laterite Evolution, New
  Caledonia
• Cenozoic and Mesozoic Magmatism and Volcanismin
  in SE Brazil
• Mechanisms and timing of silicification in
  Australia and the genesis of opal deposits

• gold mineralization under continental
  transtensional regime 40Ar/39Ar on the Jiaodong
  gold province, east China
• U-He, 40Ar/39Ar, and Re-Os as fingerprints of
  metal sources in orogenic gold deposits
• Hydrothermal vs. supergene origin of orebodies in
  banded iron formations in the Hamersley iron
  province
• Hydrothermal vs. supergene origin of orebodies in
  banded iron formations
• Supergene Enrichment in the Kalahari Manganese
  Fields, South Africa
• Timing of topaz and emerald mineralization in the
  Quadrilátero Ferrífero, Minas Gerais, Brazil
• Timing and mechanisms of gold mineralization in
  the Borborema province, NE Brazil
• 40Ar/39Ar geocronological constraints on
  postulated hominid fossil sites in Cueva
  Victoria, Spain
• Tracing possible sources for Chapada Diamantina
  diamonds and carbonados
• Timing of Pb-Zn mineralization in the Chapada
  Diamantina
• Timing and duration of gold mineralization under
  continental transtensional regime China

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UNIVERSITY of QUEENSLAND
List of Publications
Vasconcelos P. 40Ar/39Ar Geochronology of
Celadonite Insights into the Weathering History
of the Paraná Continental Flood Basalts. Geochim.
Cosmochim. Acta. Vasconcelos, P. and Conroy, M.
(2003) Geochronology of weathering and landscape
evolution, Dugald River, NW Queensland,
Australia. Geochimica et Cosmochimica Acta, 67
(13), 1-42. Carmo I.O. and Vasconcelos P.
40Ar/39Ar Geochronology Constraints on Late
Miocene Weathering Rates in Minas Gerais, Brazil.
Earth Planet. Sci. Lett. Feng, Y.X. and
Vasconcelos, P.M. (2001) Quaternary Continental
Weathering Geochronology by Laser-Heating
40Ar/39Ar Analysis of Supergene Cryptomelane.
Geology, 29(7)635-638. Li, J. W., Vasconcelos,
P. M., and Zhang, J. (2002) Behaviour of argon
gas release from manganese oxide minerals as
revealed by Ar-40/Ar-39 laser incremental heating
analysis. Chinese Science Bulletin, 47(18),
1502-1510.
Paulo Vasconcelos
http//www.earthsciences.uq.edu.au/Current/ArgonLa
b.html
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John de Laeter Centre of Mass Spectrometry Curtin
University

• Recent projects
• 40Ar/39Ar dating and isotopic geochemistry of
  large igneous provinces (CAMP, Karoo, Umkondo)
• 40Ar/39Ar speedy-step heating dating applied to
  structural inheritance of giant dike swarms,
• 40Ar/39Ar and U/Pb dating and isotopic
  geochemistry of subduction-related dike swarms,
• 40Ar/39Ar geochronology of impact craters, ocean
  island basalts (Hawaii, Tore Madeira rise) and
  shear zones (Argentina)

PLUS methological development including
investigations of standard calibrations, 39Ar and
37Ar recoil, inherited 40Ar, the K decay
constant and alteration
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List of Publications
Fred Jourdan
http//www.geology.curtin.edu.au/local/Staff_pages
/fred_jourdan.htm
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Australian National University RSES

• PRINCIPAL RESESARCH
• Timing Tectonic Processes
• Dating Deformation
• Dating Microstructures
• Geospeedometry
• Aegean structure and metamorphism

List of Publications
Marnie Forster
Forster M.A. Lister G.S. 2003. Cretaceous
metamorphic core complexes formed in the Otago
Schist, New Zealand. Australian Journal of Earth
Sciences, 50, 181-198. Forster, M.A. and
Lister, G.S. 2005. Several distinct
tectono-metamorphic slices in the Cycladic
eclogite-blueschist belt, Greece. Contributions
to Mineralogy and Petrology, 150,
523-545. Lister, G.S. Forster, M.A. and Rawling,
T.J. 2001. Episodicity during orogenesis. In
Miller, J.A., Holdsworth, R.E. Buick, I.S. and
Hand, M. (Eds). Continental Reactivation and
Reworking. Geological Society, London, Special
Publication, 184, 89-113. Forster, M.A.
Lister, G.S. 2004. The interpretation of apparent
age spectra produced in the Argon Partial
Retention Zone application of the method of
asymptotes and limits. Journal of Structural
Geology, 26, 287-305. Forster, M.A. and Lister,
G.S. 2008. Tectonic sequence diagrams and the
structural evolution of schists and gneisses in
multiply deformed terranes. Journal of the
Geological Society (London), Accepted 8th Feb
2008. Forster, M.A. Lister, G.S. 2008. Core
complex related extension of the Aegean
lithosphere initiated at the Eocene-Oligocene
transition. Journal of Geophysical Research-Solid
Earth, Accepted Jan 2008. Beltrando, M,,
Lister, G., Hermann, J., Forster, M., Compagnoni,
R. 2007. Deformation mode switches in the
Penninic units of the Urtier Valley (Western
Alps) evidence for a dynamic orogen. Journal of
Structural Geology, in press (Available On-line
Nov 2007).
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Age estimates for the many hominid and other
vertebrate fossils Single crystal laser fusion
40Ar-39Ar dating method on alkali feldspars From
rhyolitic tuffaceous beds found within the
sedimentary sequence of the Turkana Basin in East
Africa. These results are particularly
significant as they provide age estimates for the
many hominid and other vertebrate fossils
recovered from the sequences in the Omo-Turkana
Basin, so that individual fossils often can be
assigned an age to better than 50 or 100 thousand
years. The importance of these results is that
the evolutionary history of hominids is
constrained directly by age determinations rather
than by assumptions as to the evolutionary stage,
and also enables comparisons to be made
throughout a much wider area, including in areas
outside the Omo-Turkana Basin. In addition, some
of the tuffs also are recognized in deep sea
sedimentary cores from the Gulf of Aden and the
Arabian Sea, enabling correlations into the
marine sedimentary record to be made with
confidence.
Australian National University RSES
Ian McDougall
Outcrop of the Chari Tuff, dated at 1.383 0.028
Ma, Turkana Basin
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Australian National University RSES
List of Publications
McDougall, I., Brown, F. H. and Fleagle, J. G.
(2005) Stratigraphic placement and age of modern
humans from Kibish, Ethiopia. Nature, 433,
733-736. McDougall, I., and T. M. Harrison
(Eds.) (1999), Geochronology and Thermochronology
by the 40Ar/ 39Ar Method, 2nd ed., 269 pp.,
Oxford Univ. Press, New York. McDougall, I. and
Feibel, C. S. (2003) Numerical age control for
the Miocene-Pliocene succession at Lothagam, a
hominoid-bearing sequence in the northern Kenya
Rift. In Lothagam The Dawn of Humanity in
Eastern Africa (Leakey, M. G. and Harris, J.
M., eds), Columbia University Press, New York,
43-64. (Note Essentially reprinting of paper no.
235). Spell, T. L. and McDougall, I. (2003)
Characterization and calibration of 40Ar/39Ar
dating standards. Chemical Geology, 198,
189-211.
Ian McDougall
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Dating Deformation
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Evolution of an orogen
Photo Grant Dawson (PhD candidate Uni Qld)
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dating deformation is just part of the picture
Earth Processes
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Jeff LEE Exhumation of Gneiss domes
Jeff Lees Home Page, Central Washington
University web site
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Selected variety of techniques
STRUCTURAL METAMORPHIC GEOCHRONOLOGY
THERMOCHRONOLOGY
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structural mapping
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relative timing
Microstructural analysis
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PT thermobarometry
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ADDs the time component to the PT diagrams
U-Pb Zircon cores and metamorphic rims Argon
thermochronology K-feldspar modelling and white
mica analyses.
Lee, J. and Whitehouse, M.J., 2007, Onset of
mid-crustal extensional flow in southern Tibet
Evidence from U/Pb zircon ages, Geology, v. 35,
p. 45-48.
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Then ... Come up with a model and Tectonic
implications

• no interpretation (or errors) in the data
  collection (?)

• interpretation does occur at the model stage

• only 2 deformation events D1, D2
• metamorphic events incorporated within D1, D2

• how to reduce this interpretation

... where is this leading ...
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USE Tectonic sequence diagrams
Otherwise ERRORS arise from how data is recorded,
leading to misinterpretation of what is being
dated
Forster Lister, 2008. Journal of Geological
Society (accepted)
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DATE DEFORMATION
AND TECTONIC SEQUENCES

• Dating deformation (not accessory minerals)

• Timing of metamorphic events

• Timing of shear zone movement

• Timing of a thermal pulse

• Geo-
• speedometry

Recording complex thermal and deformation
histories
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TECTONIC SEQUENCE DIAGRAMS
Relative timing of structural event sequences for
Otago Schist, New Zealand
An age verifies a sequences of relative
structural analysis and constrains the chronology
of events
Relative timing with structures and metamorphic
events

relative timing of events
dating on events
previous deformation scheme

F1?
F3?
F2
... apply dates to the exact event within a
sequence
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Complexities TSD for
Vanoise Western Alps
Greece HP belt Sifnos
Forster Lister, 2008. Journal of Geological
Society (accepted)
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Di Vincenzo et al 2006
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HOW best to date a sequence?A variety of
techniques and methods can be used to date a
single sequence
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To date a TSD
Lu-Hf
Lu-Hf
Sm-Nd
Sm-Nd
Re-Os
Which chronometers?
FT
Re-Os
FT
U-Pb
Ar/Ar
U-Pb
Ar/Ar
Sample Selection - structure/minerals
Rb/Sr
Ar/Ar
Rb/Sr
Rb/Sr
Which technique?
Biotite
Muscovite
Biotite
Muscovite
K-Feldspar
Which Grains?
Sphene
Zircon
Hornblende
Amphibole
K-Feldspar
Apatite
Monazite
Garnet
Phengite
Phengite
Allanite
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Rb/Sr

• A useful method for dating deformation
• Can be used in conjunction with Argon method
• or in situation where minerals are not K-rich or
  unsuitable for argon dating

Use e.g. white mica and calcite Temperatures
Rb/Sr system of white mica thermally stable to
amphibolite facies (gt550-600C). The high
temperatures required for potential thermally
induced resetting of the Rb-Sr system in phengite
ensures that phengite ages from greenschist to
amphibolite facies mylonites can, as a rule, be
regarded as deformation ages.
Glodny et al. 2005, CMP Kumerics et al. 2005,
JGSL Ring et al. 2007, GSL and Tectonics.
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DATING EPISODES OF DEFORMATION
Rb/Sr

• For example
• Slickensides
• Veins

ignores D1 ... Dn and just dates the deformation
Müller 2003
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DATING EPISODES OF DEFORMATION
Rb/Sr
Pressure shadow with episodic growth fringes and
Pull-aparts form during different stages of
movement
Kspars dated with Argon
Müller 2003
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THE ARGON SYSTEM
Simple Guidelines to Better Analysis
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STEP-HEATING FURNACE TECHNIQUE
Apparent Age Spectra

• Most samples will have more than one gas
  population - appropriate schedule
• Reduce contamination

Reduce mixing

• by sample preparation
• by melting wrap
• by cleaning furnace, line and MS
• by sample analyzing method - start cold

Reduce Contamination
Note Continuous beam LASER best for bulk fusion
analysis not STEP-HEATING
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SAMPLE PREPARATION
APRZ Metamorphic rocks commonly have multiple
gas populations not completely reset with
overprinting events (T and time
dependent) THEREFORE variation will occur within
grains and/or between grains in different
structures
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SAMPLE PREPARATION
Mineral Separation FOR STEP-HEATING FURNACE
TECHNIQUE

• DONT go to the trouble of microstructurally and
  microchemically analysing sample and then put the
  rock into the mineral separation lab and ask for
  the e.g. white micas from this rock
• reduce mixing as much as possible by cutting out
  structures that are to be dated

• dont need much sample
• work out how much
• double or triple for replications
• then get the best grains for that structure

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QUALITY APPARENT AGE SPECTRA
Reducing Contamination FOR STEP-HEATING FURNACE
TECHNIQUE

• Samples irradiated in aluminium foil, rewrap in
  tin for analysis in furnace
• Degas your furance, clean line (check all
  sections) and MS

• CONTAMINATION ON FIRST STEPS
• least retentive sites
• melt tin at 360C
• start T low to stop mixing of first steps

• REDUCE MIXING
• Know the chemical composition of grains so as to
  prepare appropriate schedule and stop large
  releases of gas
• e.g. muscovite cf phengite
• keep steps small - adjust your heating schedule

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THE ARGON SYSTEM
ARGON PARTIAL RETENTION ZONE
Recent Reviewer Comment Ar/Ar dating is a neat
technique however, one of the major problems is
that only one single mineral, usually white mica
or feldspar, is being dated from a certain
microstructure and therefore it can never be
fully proven whether this single mineral is in
equilibrium with the entire microstructure from
which it has been derived
EXAMPLE IOS SHEAR ZONE, all scales
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• EXAMPLE
• IOS SHEAR ZONE
• maps of microchemical data
• microstructural control - structures and
  overprinting is complex

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Apparent Age Spectra
STEP-HEATING

• suitable for diffusion experiments
• Ar released from least retentive sites first
  with lower temperatures
• most retentive sites with higher temperatures
• i.e.. for K-feldspars
• smallest domains least retentive (e.g. 2
  microns)
• biggest domains most retentive (e.g. 60 microns)

km-scale SZ
Age Ma
Heating event on Naxos
Multiple steps on same T
39Ar released
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Apparent Age Spectra
Dated the white mica that defined a mylonitic
fabric where a small-scale shear zone overprinted
a km-scale shear zone. 10 Ma between the time of
operation of the two distinct shear zones
Mixing steps
Age Ma
completely reset argon system from older shear
zone NOT ALWAYS THE CASE
Real Plateau with many steps
low Ar least retentive sites
First steps
39Ar released
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Interpretation of spectra
Di Vincenzo et al 2006
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• variation in retentive sites is not rim to core
  in phengites
• domains sizes can be very small on least
  retentive sites

Di Vincenzo et al 2006
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• Your decide
• Which technique to use
• Preferably duplicate

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Results
Microchemical Data with ages

• plots of appropriate microchemical with age data
  analysis for each of the structures

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GREECE
EXAMPLE WORK IN PROGRESS
TIME OF SHEAR ZONE OPERATION NOT INCLUDED YET ...
TSD FOR SIFNOS
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Combined age data with other tools
Paragenetic sequences
Pseudosections
e.g. work in progress Groppo et al
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PT PATHS
PT paths defined
exhumation character and rates
Tectonic interpretations
Tectonic models
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GEOSPEEDOMETRY
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Based on Diffusion characteristics of K-feldspar
models the duration of events
Modelling MacArgon, G. LISTER
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