This document provides an overview of 40Ar/39Ar geochronology methods, challenges, and applications. It discusses: - Justifications for using 40Ar/39Ar to date mafic igneous rocks given suitable minerals and sample size requirements. - Methodological approaches including sample preparation, irradiation, mass spectrometry, and testing results using standards. - Difficulties posed by alteration and approaches to overcome them such as comparing untreated and acid-treated aliquots. - Successful studies dating mafic magmatism in Australia, Brazil, and altered volcanic rocks both onshore and offshore. - Current projects and developments including improving dating of hydrothermally altered samples.

1. 40Ar/39Ar geochronology of mafic
magmatism: problems, solutions,
and applications
Paulo M. Vasconcelos – The University of Queensland
paulo@earth.uq.edu.au
Isabela O. Carmo – Petrobras S.A.
icarmo@petrobras.com.br
Contributions: David Thiede,Allan Gomes, Karine Carvas, Tracey Crossingham,
Ben Cohen, Kurt Knesel, Zorano Souza, Umberto Cordani, Leila Marques, Emanuel
Jardim de Sá, Marcos Nascimento, João Marinho de Morais Neto.

2. Roadmap:
• brief introduction to UQ-AGES and TANG3O
• justify why 40Ar/39Ar geochronology is useful, necessary, or sometimes the only method
• illustrate the methodological approaches in 40Ar/39Ar
• show some of the difficulties
• show steps to overcome some of the difficulties
• illustrate some successful studies in Australia
• illustrate some successful studies in NE Brazil
• illustrate some successful studies in southeasthern Brazil
• illustrate some advantages in dating altered volcanic rocks (on shore and offshore)
• current projects
• developments

3. Thermochronology And Noble Gas Geochemistry
and Geochronology Organization

7. Ultra-high
Vacuum Sample
Chamber
Silica
Window

11. UQ
EXTRACTION
LINE
FEATURES:
• 3 LASER INLET LINES
• 1 RESISTANCE FURNACE
INLET LINE
• 2 SAES-C50 GETTERS
• TRIODE ION PUMP
• 2 VARIAN 70L TURBO
PUMPS
• 2 DRY SCROLL PUMPS
• PHOTON MACHINE 970
nm DIODE LASER
• PHOTON MACHINE CO2
LASER
• HEINE-TYPE RF
• 2 BECKER CELLS
DRY
SCROLL
PUMPS
TURIN/USG
S COLD
FINGER
HEINE
RF

15. Approach and Methods for Dating Volcanic Rocks
 40Ar/39Ar step-heating method:
- Sample crushing

16.  40Ar/39Ar step-heating method:
- Sample crushing
- Acid treatment (HCl and HNO3):
• Untreated aliquots
• Acid-cleaned aliquots
Approach and Methods

17. std
std
std
std
std
std
 Irradiation
Approach and Methods

18. Test of the reliability of the 40Ar/39Ar
results
• Analysis of an international standard
with known age:
GA1550 biotite,
age 98.8 ±
0.5Ma (McDougall
& Harrison 1999)

19.  Irradiation at OSU Reactor
http://radiationcenter.oregonstate.edu
Approach and Methods

20. Noble Gas Mass Spectrometry:
• Single collector
• Peak Hopping
Approach and Methods

21. Why date mafic igneous rock
by 40Ar/39Ar geochronology?
• Mineralogy:
- mafic magmas are often devoid or poor in U-rich minerals suitable for U-Pb
geochronology (zircon, baddeleyite)
- zircons may occur as xenocrysts in volcanic sequences
- rich in various minerals (feldspars, biotite, amphibole) datable by 40Ar/39Ar
- both phenocrysts and matrix datable
- datable by whole rock analysis
• Mass:
- need several kgs to extract zircon or baddeleyite
- only small fragments (< 2-3 mm) needed for 40Ar/39Ar geochronology
• Process:
- 40Ar/39Ar method provides information on timing of extrusion/cooling
- 40Ar/39Ar method also provides information on susequent re-heating and/or
alteration
• Intercomparison:
- date all samples by the same method in a magmatic province
Disadvantages: K-minerals susceptible to alteration and diffusional losses

22. 40Ar/39Ar Geochronology
High Precision-High Accuracy Geochronological Method for:
• Extrusive Igneous Rocks and Minerals
• Rapidly Cooled Intrusive Igneous Rocks and Minerals
• Rapidly Cooled Metamorphic Rocks and Minerals
• Rapidly Cooled Hydrothermal Rocks and Minerals
• Chemical Sediments
• Diagenetic Minerals
• Weathering Minerals
40Ar/39Ar Geochronology

23. Closure temperature (Tc) is the temperature at which a system (rock, mineral, etc.)
becomes closed to gains and losses of parent and daughter isotopes for a given isotopic
system, assuming an stable and monotonic
cooling history.
For the K-Ar system,
closure temperature is given by:
Tc 
E
{Rln[A(D0 /a2
)]}
Tc = closure temperature
Ea = activation energy for molecular diffusion
(in kJ/mol or kcal/mol)
R = Universal Gaas Constant (8.31441 J/mol/K)
A = geometric factor
D0 = frequency factor (in cm2/s),
a = radius of diffusion domain
T = dT/dt = cooling rate
{[RTc
2
]/[E(dT /dt)]}

25. A plutonic example: Granito do Cabo
From M. Nascimento (2005)

27. 40Ar/39Ar Thermochronometry
Application of 40Ar/39Ar method to any K-bearing
earth material that formed above and slowly cooled
below its closure temperature.
Thermochronometric method used to study:
• Slowly cooled intrusive igneous rocks and minerals
• Slowly cooled metamorphic rocks and minerals
• Slowly cooled hydrothermal phases
• Sediments with complex thermal histories
• Any rock or mineral that underwent re-heating after
its formation

29. Tapajós Gold Deposits

30. Jones et al. (2016)
Major study sites
 Large Number of Samples per Site

31. Glass House Mts, E QLD hot spot, AustraliaPhoto by Isabela O Carmo (2001-2002)
Eastern Australian (Cenozoic) Volcansim

33. Eastern Australian (Cenozoic) Volcansim
• Leucitites
• Seamounts
• Central volcanoes
• Lava fields

34. Wellman & McDougall,
Tectonophysics (1974)
One of the earliest
geochronological documentations
of a hotspot trace

35. 3 Decades on…
(where do we go from here?)
Compilation in:
Vasconcelos, Knesel, Cohen & Heim, AJES (2008)

36. & Why?
Provides a powerful tool for testing & informing plate
reconstructions & for unraveling tectonic events

37. Working hypothesis:
This collision should have caused a
change in the motion of the
Australian plate
&
this change should be recorded in
hotspot tracks

38. • 15 volcanic centres
spanning c. 800km
• > 300 mineral grains
& groundmass
fragments from ~ 100
samples by laser
incremental heating
Our 40Ar/39Ar experiment
Cohen, Vasconcelos & Knesel, AJES (2007)
Knesel, Cohen, Vasconcelos, & Thiede, Nature (2008) and …..
many more…

39. Mineral step-heating
trachyte plug (Beerburrum, GHM)
basaltic lava (Maleny)

40. 25
Westward deflection + reduced
migration rate
provide strong evidence for
first contact between the OJP & Melanesian arc at 26 Ma.

41. Slower plate velocity explains
why SE Qld area is anomalous
The volcanoes erupted between 26-23 Ma are
anomalous:
 larger
 overlap
 have more crustal melts
 Slower plate velocity enabled greater time over hotspot
for volcano construction & to melt crust
Knesel, Cohen, Vasconcelos, Thiede, Nature 2008
SRTM data: www2.jpl.nasa.gov/srtm
Tweed Digital Elevation Model
Many of the volcanoes are
quite big – the largest is
Tweed, which is ~100 km
across

43. Crosshingham et al. (2017)

45. Crosshingham et al. (2017)

47. Crosshingham et al. (Lithos, 2018)

49. Crosshingham et al. (J. Petrol., in press/2018)

50. Crosshingham et al. (J. Petrol., in press/2018)

53. Crosshingham et al. (J. Petrol., in press/2018)

54. Jones et al. (2016)
Major study sites
 Large Number of Samples per Site

58. Perlingeiro et al. (JVGR, 2013)

60. Souza et al. (JSAES, 2013)

61. Souza et al. (JSAES, 2013)

62. Souza et al. (JSAES, 2013)

63. ALINHAMENTO
MACAU –
QUEIMADAS (AMQ)
• GEOCRONOLOGIA 40Ar/39Ar
DO PROJETO CRONOBORO
Souza et al. (2004)
Jardim de Sá et al. (2005)

64. Paraná Flood Basalts:
Rapid Extrusion Hypothesis
Supported by New 40Ar/39Ar Results
School of Earth Sciences
David S.Thiede &
Paulo M. Vasconcelos

65. The Controversy
• Duration of the volcanism in the Paraná continental
flood basalt province has two conflicting sets of 40Ar/39Ar
geochronology data.
• One set of results [Renne et al., 1992] indicates that the
Paraná flood volcanism began at 134±1 Ma (corrected for
the currently used value of 28.02±0.09 Ma for the Fish
Canyon fluence monitor) and lasted less than 1 Ma.
• Another set of results [Turner et al., 1994; Stewart et al.,
1996] indicates an interval of 10 Ma, beginning at ca. 138
Ma and ending at ca. 128 Ma.

66. BGC
Results

67. Open
University
Results

68. The Possible Causes
• each group of researchers collected and analyzed their own sample
suite and employed different 40Ar/39Ar methodologies.
• Renne et al. (1992):
- samples collected from four vertical Serra Geral sections;
- incremental-heating method on plagioclase and whole rock grains;
- geochronological information from plateau ages in % 39Ar release
spectra.
• Turner et al. (1992) and Stewart et al. (1994):
- sampled a broader region in Brazil, Paraguay, and SE Uruguay
- used a combination of laser and furnace stepped heating on single
crystals, whole rock grains, and in situ laser total fusion extraction
of Ar from various spots in their samples;
- derived age information by plotting the results from stepped
heating or spot analysis onto an 39Ar/40Ar vs. 36Ar/40Ar isotope
correlation diagram (inverse isochron method).

71. UV Pulsed Laser
50-200 µm
laser fusion pit
Laser Total-Fusion Spot Analysis:
• selected areas in the grain are instantaneous fused

72. Visible (blue-green) Continuous Ar Laser
1-2 mm
defocused laser beam
Laser Incremental-heating Analysis:
• the whole grain is incrementally and homogeneously heated

73. UQ-AGES Approach
• we re-dated, by the laser incremental-heating
method, three samples previously analyzed by
Turner et al., 1994 and Stewart et al., 1996.
• we dated three 1-2 mm total rock grains extracted
from the exact same hand specimens analyzed by
these authors (supplied by Marcel Regelous), which
represented the oldest [138.4±1.3 (1s) Ma, sample
PAR-1] and youngest [127.7±4.6 (1s) Ma, sample
DSM23; 129.4±1.3 (1s) Ma, sample DSM05A]
samples in the 10 Ma age range reported by Turner
et al. (1994) and Stewart et al. (1996).

76. Conclusions
• widespread age distribution obtained by Turner et al. (1994)
and Stewart et al. (1996) is an artifact of the methodology
used, and it does not record a true age spread.
• all nine grains analyzed in this study are within error from
each other and correspond, within error, to the ages obtained
by Renne et al. (1992) suggesting that the duration of the
Paraná volcanism was less than 2 Ma.
• geological models that assume that the Paraná flood basalt
volcanism extends for more than 2 Ma are not supported by
geochronological information currently in the literature.

79. 40
Ar*
39
ArK
=
40
Ar/39
Ar[ ]m
-295.5 36
Ar/39
Ar[ ]m
+ 295.5 36
Ar/37
Ar[ ]Ca
37
Ar/39
Ar[ ]m
1- 39
Ar/37
Ar[ ]Ca
37
Ar/39
Ar[ ]m
-
40
Ar
39
Ar
é
ë
ê
ù
û
ú
K

80. Non-treated aliquots Acid-treated aliquots
Paraná Flood Basalts:
New 40Ar/39Ar Results

81. Non-treated
aliquots
Acid-treated
aliquots
I) DSM-05A:
Non-treated aliquots Acid-treated aliquots

82. II) DSM-23 Non-treated aliquots Acid-treated aliquots

83. III) PAR-01 Non-treated aliquots Acid-treated aliquots

84.  Best age determination for each sample:
 All 40Ar/39Ar ages are within the main emplacement stage of
134.7 ± 1 Ma proposed by Renne (2015).
Sample
Thiede and
Vasconcelos (2010)
Present study
DSM-05A 134.2 ± 0.8 Ma 135.01 ± 0.35 Ma
DSM-23 134.6 ± 1.1 Ma 134.96 ± 0.36 Ma
PAR-01 134.8 ± 0.7 Ma 135.18 ± 0.33 Ma
• Untreated sample;
• 1 year after irradiation.
• Both untreated and acid-
treated samples;
• 6 months after irradiation.
Take Home Message: Baksi’s Concerns not Justified

86. 90%
7% 3%
Tholeiitic basalts
Tholeiitic andesites
Rhyolites, rhyodacites
and dacites

87. PhD Thesis by Allan Gomes
Collaboration with Otavio Licht

88. Challenges and Opportunities

89. Oreiro (2005/2006, BGP, 14(1):95-112)
Salt base
Fm. Cabiúnas (volcanics)

90. From

91. Challenges:
•sidewall core (small)

92. Challenge: intense hydrothermal alteration
Sphalerite
Hydrothermal
Carbonates
Pyrite and Chalcopyrite

93. Challenge: intense hydrothermal alteration

94. Altered Phenochrysts-Fresh Microlites
Fresh Phenochrysts-Altered Microlites

95. Pre- and post-salt
Geocronologia 40Ar/39Ar
resultados preliminares
> 300 samples dated so far

96. MBB-211
Idade de cristalização da lava basáltica
(fenocristal de plagioclásio)
MBB-211 (Plagioclase)
Fresh
Ca-plagioclase
phenocryst
Plagioclase phenocryst
Plateau age compatible with
isochron and probability ages.
113.4 ± 0.5 Ma
Fresh plagioclase
(bytownite/labrad
orite)
113.3 ± 0.4 Ma
Fresh matrix

97. MBB-25 (Plagioclase)
Sericitized
Ca-plagioclase

98. XP
EDS
map
KK, Na
Ca, Na
MBB-20 (WR)

99. 116.7 ± 0.4 Ma
115.7 ± 0.3 Ma
K/Ca > 0.3
MBB-20 (WR)

100. MBB-20 (WR)

101. Post-salt intrusion – 40Ar/39Ar Geochronology
89 ± 5 Ma
85.4 ± 0.7 Ma
78.4 ± 0.8 Ma
amphibole
biotite
plagioclase
gabbro

102. Diffusion
• Mechanism for Ar loss from a mineral over geologic
residence time
• Governed by the Arrhenius equation:
diffusion rate (D/a2) = (D0/a2) e-Ea/RT
Activation Energy (Ea)
higher = more retentive
Diffusion Constant (D0/a2)
lower = more retentive
• A measure of the ‘conductance’ of diffusion paths in a mineral
- Musset, 1969

104. • systematic variations in diffusive behavior with apparent closure
temperatures for 0.1–1 mm grains of 200–400 °C (assuming a 10 °C/Ma
cooling rate)
• there is no broadly applicable set of diffusion parameters that can be
utilized in feldspar thermal modeling
• sample-specific data are required
• considerable inaccuracies may exist in published thermal histories obtained
using multiple diffusion domain (MDD) models fit to Arrhenius plots for
exsolved alkali feldspar

105. Determine the most probable thermal history
40Ar/39Ar modelling of K-feldspar experiment on resistance furnace
Resistance furnace experiment
Calculate fractions of 39Ar released
Determine diffusion parameters
Plot/Interpret Arrhenius diagrams
Use diffusion coefficients and activation energy
(+ thermochronological constraints) to invert data
From Morais Neto

106. Becker
Cell

107. Becker
Cells
Pierce w/
266nm
UV laser
970nm
Diode laser
300-plus °C
Heat to
~700 °C
Bake to
120°C

108. Karine Carvas PhD
Collaboration with USP/IAG and Prof Leila Marques

109. Campo Karine 2018

110. Carvas et al. (VII SVAA, 2018)

111. Carvas et al. (VII SVAA, 2018)

112. Carvas et al. (VII SVAA, 2018)
Ca
Na
Albitization – Na metasomatism
“fresh” plagioclase

113. Carvas et al. (VII SVAA, 2018)

114. Carvas et al. (VII SVAA, 2018)
Ca
Na
Albitization – Na metasomatism

115. From

116. Take Home Message:
• South Atlantic offshore basins offer a
great new frontier of opportunities in the
refinement and application of 40Ar/39Ar
geochronology