Cohen, B.E.

Lee, M.R.

Mark, D.F.

Smith, C.L.

English

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Cohen, B.E., Mark, D.F., Lee, M.R., and Smith, C.L. (2015) Cooling of the 
H Chondrite Parent Body: Examination and Assessment of 40Ar/39Ar Age 
Data. In: 46th Lunar and Planetary Science Conference, The Woodlands, 
TX, USA, 16-20 Mar 2015. 
 
 
Copyright © 2015 The Authors 
 
 
http://eprints.gla.ac.uk/102431/  
 
 
 
 
 
Deposited on:  10 February 2015 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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COOLING OF THE H CHONDRITE PARENT BODY: EXAMINATION AND ASSESSMENT OF 
40Ar/39Ar AGE DATA.  B. E. Cohen1, D. F. Mark1, M. R. Lee2, and C. L. Smith3, 1Scottish Universities Environ-
mental Research Centre (SUERC), Rankine Avenue, East Kilbride, G75 0QF, UK (ben.cohen@glasgow.ac.uk), 
2School of Geographical and Earth Sciences, University of Glasgow, G12 8QQ, UK, 3Department of Earth Sciences, 
The Natural History Museum, London, SW7 5BD, UK. 
 
 
Introduction:  Due to the large original size of the 
H chondrite parent – estimated at ca. 100 km – its inte-
rior underwent significant heating and metamorphic 
recrystallization as a result of accretion and decay of 
short-lived radionuclides [1-3] (Fig. 1). In a bolide that 
was not substantially influenced by impacts or an ir-
regular distribution of heat sources, the interior por-
tions are expected to have witnessed higher degrees of 
heating and recrystallization (petrologic type 6, Fig. 1) 
than material found closer to the surface, which cooled 
substantially faster and underwent little or no recrystal-
lization (petrologic types 3 and 4, Fig. 1).  
 
 
 
Fig. 1:  (a) Onion-shell model for the cooling of the 
H chondrite parent body [1], showing the zonation 
from low to high petrologic type (3-6, respectively). 
(b) Schematic diagram showing cooling curves for 
petrologic types 3 to 6, and times recorded by various 
thermochonometers.  
 
Using a combination of Pb-Pb, 244Pu fission track, 
and 40Ar/39Ar chronology, Trieloff et al. [1] found that 
the cooling ages for all three chronometers formed a 
consistent pattern, with the H4 chondrites yielding 
comparatively old ages (ca. 4550-4560 Ma), with the 
H6 samples cooling up to 150 Ma later. This distribu-
tion of ages is consistent with a simple unperturbed 
cooling history for the H chondrite parent body, and 
has been termed the onion-shell model [1]. 
Other studies of the H chondrites have, however, 
indicated that the cooling of the parent body may not 
be this straightforward. In particular, metal-cooling 
rates from some stones indicate more rapid cooling 
than would be expected from the onion-shell model, 
which implies substantial impacts disturbed the early 
thermal history of at least part of the parent body [4].  
We have sought to better constrain and understand 
the cooling history of the H chondrite parent body by 
two approaches. Firstly, we are compiling results from 
previous 40Ar/39Ar chronology from these meteorites, 
and recalculating the ages based on the most current 
versions of the K decay constants and neutron fluence 
standards. Secondly, we are undertaking detailed high-
resolution step-heating 40Ar/39Ar analysis of selected 
stones. These aspects are discussed below. 
40Ar/39Ar age compilation and recalculation:  In 
the past decade there have been considerable advances 
in the precision and accuracy of the constants and 
standard values used for 40Ar/39Ar chronology, most 
notably the revision of the 40K decay constants [5] and 
the ages for neutron-fluence monitors [5; 6].  
These revisions are of critical importance for mete-
orite studies – at the age of the solar system, results 
reported using the decay constants of Steiger & Jäger 
[7] are ca. 30 Ma younger than using the revised values 
of Renne et al. [5]. As an example, using the decay 
constants of Steiger & Jäger [7], the H4 chondrite For-
est Vale has an 40Ar/39Ar age of 4522 ± 8 Ma [1], 
whereas using the revised decay constants [5] the cool-
ing age for this meteorite is 4552 ± 8 Ma [3]. This dif-
ference substantially effects interpretations of the cool-
ing history of the H chondrite parent body: using the 
‘old’ constants, Forest Vale cooled 43 Ma after the end 
of chondrule formation (4564.7 ± 0.3 Ma [8]) – but 
using the ‘new’ constants the cooling age for Forest 
Vale is only 13 Ma after chondrule formation. 
As has been noted by various workers [1; 3], this 
recalculation is crucial for comparing 40Ar/39Ar results 
with ages from other chronometers (Hf-W, U-Pb, Pb-
Pb, U-Th/He, etc.), as is undertaken when evaluating 
the cooling history of the chondrites (Fig. 1). In addi-
1876.pdf46th Lunar and Planetary Science Conference (2015)
tion, for such comparisons to occur, all sources of sys-
tematic and random uncertainty have to be incorpo-
rated into the age calculation, and data should be re-
ported at the 2 sigma confidence level. In order to ex-
amine the cooling history of the H chondrite parent 
body, we are therefore compiling a database of recal-
culated 40Ar/39Ar ages from the literature that exists for 
the H chondrites (e.g., [1; 9-13]). In addition to the 
recalculated ages, we are compiling information on the 
constants used, which will enable future recalculations 
in the event of further evaluation of decay constants 
and standards used in the 40Ar/39Ar technique. 
We note however that no recalculation is being un-
dertaken for the redetermination of Earth atmospheric 
40Ar/36Ar from 295.5 ± 0.5 [14] to 298.56 ± 0.31 [15; 
16]. This is because articles do not report sufficiently 
detailed analytical data to undertake this calculation 
(essentially the raw analytical data is required, which 
has historically not been feasible to publish) [17]. For-
tunately for highly-radiogenic samples, including me-
teorites, the effects of the modified Earth atmospheric 
40Ar/36Ar is negligible [17]. Nevertheless, with the 
advance of online data repositories, there is a clear 
need for publishing extensive analytical datasets [17], 
as the future will undoubtedly bring further improve-
ments to the precision and accuracy of constants and 
standards – and without full data reporting, any previ-
ously published data will become increasingly more 
incompatible with newly generated results. 
High-resolution 40Ar/39Ar chronology of the H 
chondrites:  Meteorites can contain diverse argon res-
ervoirs, including radiogenic 40Ar trapped in the mete-
orite since initial cooling, cosmogenic Ar, and ad-
sorbed Earth atmospheric argon. These reservoirs can 
be complicated by Ar loss during impact ejection, 
Earth atmospheric entry, terrestrial weathering, and 
39Ar and 37Ar recoil due to neutron irradiation prior to 
analysis. In an attempt to distinguish between the dif-
ferent reservoirs and characterize potential losses of 
Ar, we are undertaking high-resolution heating sched-
ules (>30-40 steps) on fifteen H chondrites. Chon-
drules and matrix are being analyzed separately in or-
der to characterize the Ar budgets in these materials. 
Samples for high-resolution 40Ar/39Ar were chosen 
by focusing on falls rather than finds, to minimize the 
amount of terrestrial weathering experienced by the 
meteorites. We also selected stones with a low shock 
stage [18] (less than S3, and ideally less than S2) to 
minimize the amount of resetting experienced by the 
meteorites, which could cause the Ar ages to reflect the 
impact history rather than primary cooling. 
Acknowledgements:  We thank the following in-
stitutions for providing material: British Natural Histo-
ry Museum, UK; Smithsonian Museum, USA; The 
Field Museum, Chicago, USA; Texas Christian Uni-
versity, USA; and The Hunterian Museum, Glasgow, 
UK. 
References:  [1] Trieloff M. et al. (2003) Nature, 422, 
502-506. [2] Kleine T. et al. (2008) EPSL, 270, 106-
118. [3] Henke S. et al. (2013) Icarus, 226, 212-228. 
[4] Wittmann A. et al. (2010) JGR, 115, E07009. [5] 
Renne P. R. et al. (2011) GCA, 75, 5097-5100. [6] 
Kuiper K. F. et al. (2008) Science, 320, 500-504. [7] 
Steiger R. H., and Jäger E. (1977) EPSL, 36, 359-362. 
[8] Connelly J. N. et al. (2012) Science, 338, 651-655. 
[9] Bogard D. D. et al. (1976) JGR, 81, 5664-5678. 
[10] Trieloff M. et al. (1994) Meteoritics, 29, 541–542. 
[11] Bogard D. D. et al. (2001) Meteoritics & Planet. 
Sci., 36, 107-122. [12] Folco L. et al. (2004) GCA, 68, 
2379-2397. [13] Swindle T. D. et al. (2009) 
Meteoritics & Planet. Sci., 44, 747-762. [14] Nier A. 
O. (1950) Physical Review, 77, 789-793. [15] Lee J.-Y. 
et al. (2006) GCA, 70, 4507-4512. [16] Mark D. F. et 
al. (2011) GCA, 75, 7494-7501. [17] Renne P. R. et al. 
(2009) Quat. Geochron., 4, 299-298. [18] Stöffler D. et 
al. (1991) GCA, 55, 3845-3867. 
 
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Cooling of the H Chondrite Parent Body: Examination and Assessment of 40Ar/39Ar Age Data

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Cooling of the H Chondrite Parent Body: Examination and Assessment of 40Ar/39Ar Age Data

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