The document summarizes preliminary (U-Th)/He dating results of zircon and apatite samples from three impact structures: Manicouagan (Canada), Saint Martin (Canada), and Bosumtwi (Ghana). The (U-Th)/He ages are consistent with previously published ages for the structures. For Saint Martin, the (U-Th)/He ages suggest the structure is older than previously thought and may not be part of a hypothesized Late Triassic multiple impact event. The results indicate (U-Th)/He dating can provide reliable ages for dating impact structures.

1. Dating of Impact Events:
(U-Th)/He single crystal and in-situ
zircon ages of impact deposits
Jo-Anne Wartho, Matthijs C. van Soest,
Brian D. Monteleone and Kip V. Hodges
With special thanks to:
Christian Koeberl (University of Vienna)
Jahan Ramezani (MIT)
Martin Schmieder (University of Stuttgart)
Elmar Buchner (University of Stuttgart)
Uwe Reimold (Museum für Naturkunde, Berlin)
Ruth Bezys (Manitoba Geological Survey)

2. Heat production at impact structures
Chicxulub
3D simulation
(170 km diameter)
5,500°C
(Gisler et al., 2004)
7 seconds
42 seconds
• Immediate impact heating is influenced by:
i) The type, size, angle and speed (17-72 km/sec) of the
bolide.
ii) The target material (e.g., water, sediments, or crystalline 2 minutes
rocks).
• During impact peak temperatures may reach into the thousands of degrees and
generally reach well above 1000˚C degrees even at smaller impact sites.
• Post-impact hydrothermal systems may survive for 103-106 years (e.g., Daubar &
Kring, 2001) and have peak temperatures in the 200 to 800˚C range.

3. Why (U-Th)/He?
• Apatite, Titanite, Zircon, Magnetite, and Monazite
have low closure temperatures for He diffusion. Mineral He Tc (°C)
• Catastrophic but short lived high temperature Apatite 88 - 106
heating events are likely to completely reset the Titanite 200 - 223
(U-Th)/He system in seconds.
Zircon 203 - 229
• Other geochronometers such as K-Ar, Rb-Sr take Magnetite 237 - 260
from hours to days and longer to reset. Monazite 245 - 268
• U-Pb resets upon complete recrystalization, ie. He closure temperatures (Tc)
calculated assuming:
when large volumes of impact melt are formed. Cooling rates of 100 - 1000°C/
Ma & Grain diameters of 100
• Previous (U-Th)/He dating of the 35.5 Ma µm Spherical or cylindrical
Chesapeake Bay impact structure yielded non- geometries (as appropriate for
reset 122-148 Ma apatite ages (Harvey, 2004). the mineral)

4. (U-Th)/He dating at ASU
Manicouagan Zircon
• He analysis: an ASI Alphachron, with IR diode
laser, 3He Spike, and Quadrupole MS.
• U and Th analysis: a Thermo X series ICP-MS, 100 µm
with 230Th/235U Spike.
Lake St. Martin zircon
• Clear inclusion free, euhedral grains are used for
apatite and where possible for zircon.
100 µm
• α-ejection correction based on: 38.0
Farley et al. (1996) & Farley (2002) for apatite. Durango Apatite
Hourigan et al. (2005) for zircon. 36.0
34.0
• Lab results for age standards:
Age (Ma)
32.0
Durango apatite: 31.90 ± 0.16 Ma (2SE,
30.0
n = 176).
Fish Canyon Tuff zircon: 28.10 ± 0.74 Ma (2SE, 28.0
n = 25). 26.0
0.030 0.040 0.050 0.060
U/Th

5. Terrestrial Impact Structures
Manicouagan
Saint Martin
Bosumtwi
http://www.fas.org/irp/imint/docs/rst/Sect18/Sect18_1.html

6. Details of the three dated impact structures
Manicouagan Saint Martin Bosumtwi
Diameter (km) 100 40 10.5
214 ± 5 (2σ, Rb-Sr mineral 250-200 ± 25 (K-Ar whole rock, 1.15 ± 0.15 (K-Ar, tektite,
Ages (Ma)
isochron, Jahn et al., 1978) McCabe & Bannatyne, 1970) Gentner et al., 1969)
214 ± 1 (2σ, U-Pb zircon, 219 ± 32 (2σ, Rb-Sr mineral 0.88 ± 0.13 (FT tektite, Durrani
Hodych & Dunning, 1992) isochron, Reimold et al., 1990) & Khan, 1971)
215.56 ± 0.05 (2σ, U-Pb zircon, 1.1 ± 0.10 & 1.03 ± 0.22 (2σ, Ar-
Ramezani et al, 2005, pers. Ar tektite & FT impact glass,
comm. 2008) Koeberl et al., 1997)
Target rocks Grenville metamorphics & Archaean metamorphics & 2.1-2.2 Ga metasediments
Ordovician carbonates Devonian sediments & metavolcanics
Bolide type ? ? Chondrite/Iron?
(Koeberl, 1998)
Exposed Yes Poorly Yes
Drilled Yes Yes Yes
Material dated Melt sheet (zircon & Melt rock (zircon & apatite) Suevite (350-380 m),
apatite) outside crater (zircon)
FT = fission track

7. Preliminary (U-Th)/He zircon results
Manicouagan Bosumtwi
Manicouagan Saint Martin Bosumtwi
Average age (Ma, ± 2SE) 231 ± 10
213.6 ± 4.6 0.921 ± 0.034
All grains (n=5)
(n=9) (n=8)
Average age (Ma, ± 2SE) - -
235.2 ± 6.2
Excluding some grains
(n=4)
Histogram plots Saint Martin
Gaussian plots
Ludwig (2001)

8. Preliminary (U-Th)/He apatite results
Manicouagan Saint Martin
Manicouagan Saint Martin
Individual grain ages 205.9 ± 6.5, 192.8 ± 5.6 240.3 ± 8.6, 238.2 ± 6.6,
(Ma, ± 2σ) 176.9 ± 7.8, 166.3 ± 7.1 232.0 ± 6.3, 225.3 ± 6.1, 222 ±
16, 177.9 ± 4.9, 177 ± 12
162.0 ± 5.3
Average age (Ma, ± 2SE) 168.4 ± 8.9 (n=3) 231.5 ± 7.2 (n=5)
Excluding some grains 177.2 ± 1.4 (n=2)
Histogram plots
Gaussian plots
Ludwig (2001)

9. Terrestrial Impact Structures
Manicouagan (100)
215.56 ± 0.05 Ma
214 ± 5 Ma
214 ± 1 Ma
213.6 ± 4.6 Ma
Saint Martin (40)
250-200 ± 25 Ma
219 ± 32 Ma
235.2 ± 6.2 Ma
231.5 ± 7.2 Ma
Bosumtwi (10.5)
1.15 ± 0.15 Ma
1.1 ± 0.1 Ma
1.03 ± 0.22 Ma
0.88 ± 0.13 Ma
0.921 ± 0.034 Ma
Name of crater (diameter, km)
Previously determined ages (2σ)
New (U-Th)/He zircon ages (2SE)
http://www.fas.org/irp/imint/docs/rst/Sect18/Sect18_1.html New (U-Th)/He apatite ages (2SE)
Buchner et al., 2003; Durrani & Khan, 1971; Gentnet et al., 1969; Hodych & Dunning, 1992; Jahn et al., 1978;Koeberl et al., 1997;
McCabe & Bannatyne, 1970; Reimold et al., 1990; Ramezani et al., 2005; Staudacher et al., 1982.

10. Conclusions 1
• Preliminary (U-Th)/He zircon dates from the Manicouagan, Saint Martin and
Bosumtwi impact structures overlap with previously published U-Pb, Rb-Sr, Ar-Ar,
and fission track dates.
• Preliminary (U-Th)/He apatite dates from impact structures show more complexity,
but the St. Martin apatites yield dates comparable with the (U-Th)/He zircon dates.
• (U-Th)/He is a viable technique for the dating of impact structures.
• The technique is especially valuable for dating smaller impact craters, where
thermal/shock effects are less marked. (U-Th)/He dating of phosphates has already
been proven as a means of dating shock metamorphism in meteorites (e.g., Min et
al., 2003, 2004, 2005).

11. Conclusions 2
• The Saint Martin (U-Th)/He zircon age (235.2 ± 6.2 Ma) and apatite age
(231.5 ± 7.2 Ma) suggests that the impact structure is older than previously
thought. Therefore, the Saint Martin impact structure may not belong to the Late
Triassic multiple impact event (Spray et al, 1998).
Triassic Jurassic
6
Norian/Carnian Norian-Rhaetian/Hettangian
Rochechouart
5
France
Manicouagan
4
Canada
Saint Martin
3
Canada
2
Obolon
Ukraine
Red Wing
1
USA
Spray et al, 1998
0
260 250 240 230 220 210 200 190 180 170 160 150
Time (Ma)
Mataitis, 1980; Gerhard et al., 1982; Reimold et al., 1990; Hodych & Dunning, 1992; Gradstein et al., 1994; Kelley & Spray, 1997;
Masaitis, 1999; Valter et al., 2000; Ramezani et al., 2005; Gradstein et al., 2008.

12. Conclusions 2
• The Saint Martin (U-Th)/He zircon age (235.2 ± 6.2 Ma) and apatite age
(231.5 ± 7.2 Ma) suggests that the impact structure is older than previously
thought. Therefore, the Saint Martin impact structure may not belong to the Late
Triassic multiple impact event (Spray et al, 1998).
Triassic Jurassic
6
Norian/Carnian Rhaetian/Hettingian
Rochechouart
5
France
Manicouagan
4
Canada
Apatite
Zircon Saint Martin
3
Canada
2
Obolon
Ukraine
Red Wing
1
USA
0
260 250 240 230 220 210 200 190 180 170 160 150
Time (Ma)
Mataitis, 1980; Gerhard et al., 1982; Reimold et al., 1990; Hodych & Dunning, 1992; Gradstein et al., 1994; Kelley & Spray, 1997;
Masaitis, 1999; Valter et al., 2000; Ramezani et al., 2005; Gradstein et al., 2008.