Thermoluminescence (TL) is a promising technique for rapid screening of the large numbers of Antarctic meteorites, permitting identification of interesting specimens that can then be studied in detail by other, more definite techniques. Specifically, TL permits determination of rough terrestrial age, identification of potential paired groups and location of specimens with unusual pre-fall histories. Meteorites with long terrestrial ages are particularly valuable for studying transport and weathering mechanisms. Pairing studies are possible because TL variations among meteorites are large compared to variations within individual objects, especially for natural TL. Available TL data for several L3 fragments, three of which were paired by other techniques, are presented as an example of the use of TL parameters in pairing studies. Additional TL measurements, specifically a blind test, are recommended to satisfactorily establish the reliability of this pairing property. The TL measurements also identify fragments with unusual pre-fall histories, such an near-Sun orbits

Sutton, S. R.

Walker, R. M.

English

NASA Technical Reports Server

104
THERMOLUMINESCENCE OF ANTARCTIC METEORITES: A RAPID
SCREENING TECHNIQUE FOR TERRESTRIAL AGE ESTIMATION, PAIRING
STUDIES, AND IDENTIFICATION OF SPECIMENS WITH UNUSUAL PRE-
FALL HISTORIES. S. R. Sutton and R. M. Walker, McDonnell Center for the Space Sci-
ences, Washington University, St. Louis, MO 63130.
ABSTRACT: Thermoluminescence (TL) is a promising technique for rapid screening of the
large numbers of Antarctic meteorites, permitting identification of interesting specimens that
can then be studied in detail by other, more definitive techniques. Specifically, TL permits
determination of rough terrestrial age, identification of potential paired groups and location of
specimens with unusual pre-fall histories. Meteorites with long terrestrial ages are particularly
valuable for studying transport and weathering mechanisms. Pairing studies are possible
because TL variations among meteorites are large compared to variations within individual
objects, especially for natural TL. Available TL data for several L3 fragments, three of which
have been paired by other techniques, are presented as an example of the use of TL parameters
in pairing studies. Additional TL measurements, specifically a blind test, are recommended to
satisfactorily establish the reliability of this pairing property. TL measurements also identify
fragments with unusual pre-fall histories, such as near-sun orbits.
INTRODUCTION: Thermoluminescence is a relatively rapid analytical technique. Although
the results may sometimes be ambiguous, permitting several interpretations, it is potentially
useful as a method for rapidly identifying interesting specimens that can then be examined by
other methods in a timely fashion. This paper discusses the use of TL in determination of ter-
restrial ages, pairing properties, and unusual pre-fall histories. Field observations raise immedi-
ate questions concerning certain specimens and it would be useful to have a rapid technique for
giving first order answers to these questions. For example, in the 1984-85 field season, frag-
ments of a distinctive meteorite were found scattered over a large area. The fact that some
fragments were found on white ice, not blue ice (as is usually the case), suggests that the frag-
ments were part of a shower that arrived on Earth quite recently. Corroboration of this
hypothesis (or not) could influence plans for the study of these fragments as well as affect think-
ing about concentration mechanisms.
TL MEASUREMENT: The thermoluminescence measurement consists of heating the sample
(typically 5 mg of powder) from room temperature to about 500° C and plotting the TL inten-
sity versus sample temperature, the so-called "glow curve." Two types of glow curves, natural
and artificial, are registered for each fragment. The natural glow curve is that measured for the
"as-received" material while the artificial glow curve is that measured after draining the natural
TL and irradiating the sample to some known dose (e.g., 10 rads with beta particles). In gen-
eral, natural TL provides information on the thermal and irradiation history of the object while
information on the phosphors and their abundance is derived from artificial TL. Two charac-
teristics of the glow curves lend themselves to these analyses, shape and intensity.
TERRESTRIAL AGE: Upon arrival on Earth, meteorites are shielded from cosmic irradia-
tion by the Earth's atmosphere and their natural TL decays. Attempts to use the extent of TL
decay for accurate terrestrial age determination [1,2] have been hindered by a lack of accurate
knowledge of (1) the TL levels present in individual fragments immediately after fall and (2) the
Antarctic storage temperature. It has been shown, however, by comparison with radiometrically-
determined terrestrial ages that TL measurements are capable of providing approximate terres-
trial ages. That is, specimens with high natural TL must have short terrestrial ages while
those with low natural TL are likely to have long terrestrial ages (a caveat in the latter case,
however, is that rare near-sun orbits can also reduce natural TL; see below). Melcher [l] used
the intensity of natural TL expressed as equivalent dose (ED) as a measure of terrestrial age.
The ED at a given glow curve temperature, the laboratory dose required to match the natural
https://ntrs.nasa.gov/search.jsp?R=19860019358 2020-03-20T14:43:18+00:00Z
105
TL intensity, is given by
Natural TL , ,
 nED = x Lab DoseArtificial TL
Eleven L and H chondrites from Antarctica gave ED values at 200° C in the glow curve ranging
from 0.13 to 22.7 krads. Estimated terrestrial ages ranged from < 100,000 to > 400,000 years
and were consistent with Cl ages. An order of magnitude difference in ED corresponds to
roughly an order of magnitude difference in age. In another terrestrial age study of Antarctic
meteorites, McKeever [2] examined the shape of the natural glow curve expressed as the ratio of
the low temperature intensity (LT) to the high temperature intensity (HT) for eight L and H
chondrites. TL terrestrial ages again were consistent with radiometric estimates.
PAIRING STUDIES: The TL emitted by a meteorite is a complex combination of many fac-
tors including phosphor TL characteristics and abundance, thermal history, irradiation history
and terrestrial age. Because these factors are likely to be unique for each meteorite, unpaired
meteorites are expected to have significantly different TL properties. Pairing studies using TL
are possible because observed meteorite-to-meteorite TL variations are generally large compared
to variations within single objects. The similarity of TL response from two objects would be
consistent with pairing while discrepancy would be evidence for non-pairing.
INTER-METEORITE TL VARIATIONS: (1) Natural TL - In Melcher's terrestrial age study
[1], the eleven L and H chondrites from Antarctica gave ED (200° C) covering a dynamic range
of about a factor of 200. A variation of about 200 was also found in LT/HT values in
McKeever's work [2]. (2) Artificial TL - The shape of the artificial glow curve in equilibrated
chondrites has been found to be very similar although significant variations are found for une-
quilibrated chondrites [1,3]. This characteristic is not expected to be a very sensitive indicator
of pairing. The variation in intensity of artificial glow curves, TL sensitivity, depends on petro-
logic type. Sears et al. [3] have found a variation of > 1,000 in TL sensitivity for type 3's while
types 4-6 vary by only a factor of about 10.
INTRA-METEORITE TL VARIATIONS: (1) Natural TL - Two effects are expected to be most
important in producing natural TL gradients within individual meteorites, attenuation of cosmic
radiation and thermal gradients during atmospheric entry. Simulation experiments suggest that
cosmic ray attenuation effects should be less than a factor of two over dimensions of several tens
of centimeters [4]. Natural TL gradients observed in 3 meteorites are consistent with this value
(Ucera- 30%/lOcm [5]; Plain view- 60%/lOcm [6]; St. Severin- 50%/30cm [4]. In most cases,
thermal decay of natural TL produced during atmospheric entry is significant only within a few
millimeters of the fusion crust [7,8] so that careful selection of samples for TL analysis can avoid
this effect. An exception, however, is Farmville (H4) which shows a factor of 10 variation in ED
(200° C) across a 20 cm slab possibly resulting from an oriented entry [9]. (2) Artificial TL -
Artificial TL variations result principally from phosphor abundance heteorogeniety but varia-
tions in phosphor TL characteristics can also be significant for unequilibrated specimens. The
range of TL sensitivity measurements on different fragments from the same meteorite is typi-
cally better than a factor of two [10]. For both artificial TL and natural TL measurements, the
reproducibility of 5 mg aliquots from powdered small chips (—100 mg) is typically better than ±
20%, i.e., small compared to the effects described above [l].
TL PAIRING DATA FOR SELECT ANTARCTIC L3 CHONDRITES: Although the current
TL data set for Antarctic meteorites is limited, data does exist for three L3 chondrites, ALIIA-
77015, 77140 and 77214, paired by other techniques (petrography, noble gases and radionuclides
[11]). Table 1 summarizes the TL data on these fragments and other unpaired L3's. The small
ED and TL sensitivity differences between 77140 and 77214 are consistent with the paired
identification for these two fragments. However, the TL sensitivity for 77015 is a factor of 2
greater, a greater difference than the measured reproducibility of replicate measurements. TL
sensitivity differences on this order led Sears et al. [12] to suggest that two separate falls are
106
represented, one containing three members including 77015 and another containing 5 members
including 77140 and 77214. The LT/HT values (natural TL glow curve shape parameter),
different by a factor of 3 for 77015 and 77214, are consistent with this interpretation. Table 1
also shows the clearly distinct TL data for two unpaired fragments, 77003 and 77278.
TABLE 1: TL Pairing Data for Several Antarctic L3 Chondrites
ED(200°C)a
Specimen (krads) LT/HT TL Sensitivity"
Paired:
ALHA-77015
ALHA-77140
ALHA-77214
Unpaired:
ALHA-77003
ALHA-77278
nd
4.95
6.5
22.7
3.2
1.35
nd
3.90
52.5
nd
0.1 5C
0.08e
0.078C
0.48e
0.25C
aMelcher [l]; ^McKeever [2]: °Sears, et al. [12]; relative to Djahala; eMelcher [9]; nd=not determined
UNUSUAL PRE-FALL HISTORIES: An important asset of meteorite TL measurements is
the ability to quickly identify meteorites with atypical thermal and/or irradiation histories. The
classic example is Malakal (L5) which has extremely reduced low temperature ED although its
high temperature ED is "normal" [l]. The extent of reduction suggests that the object suffered
solar heating in an orbit with a perihelion of 0.5-0.6 AU, consistent with its unusually high " Al
(79 ± 2 dpm/kg [13]). In another such example, the abnormally reduced low temperature ED's
of two lunar meteorites, ALHA-81005 and Yamato-791197, were interpreted as resulting from
heating during impact ejection and a subsequent space exposure which was sufficiently short to
prohibited significant TL reaccumulation. A maximum Earth transit time of only ~2,000 years
was indicated for both objects [14,15].
CONCLUSIONS: Natural and artificial thermoluminescence measurements on Antarctic
meteorite fragments provide valuable information on the histories of these objects. Meteorites
with short terrestrial ages can be easily identified as well as those likely to have very long terres-
trial ages. Pairing studies are also feasible. Natural TL, probably the more sensitive pairing
property, is observed to vary by a factor of 200 among individual Antarctic chondrite fragments
while variations within fragments are expected to be less than a factor of 2. Inter-meteorite
variation of artificial TL is less, only a factor of 10, but can be greater for particular meteorite
classes (e.g., the > 1,000-fold variation observed in type 3 unequilibrated chondrites). A sys-
tematic test program including a blind test is recommended to establish the reliability of TL
pairing more definitively than is currently possible with the limited available data. TL can also
pinpoint meteorites with unusual pre-fall histories, such as those with near-sun orbits or those
such as lunar meteorites which have been exposed in space for only a short time. Such informa-
tion would be useful in planning studies of particular meteorites by complementary (and more
time consuming) techniques.
REFERENCES: [1] Melcher, C. L. (1981) Geochim. Cosmoehim. Ada, 45, 615-626. [2] McKeever, S.
W. S. (1982) Earth Planet. Sei. Lett. 58, 419-429. [3] Sears, D. W., J. N. Grossman, C. L. Melcher, L. M.
Ross and A. A. Mills (1980) Nature 287, 791-795. [4] Lalou, C., D. Nordemann and J. Labeyrie (1970) C.
R. Aead. Sei. Paris, Ser. D, 270, 2401-2404. [5] Vaz, J. E. (1972) Meteoritics, 7, 77-86. [6] Vaz, J. E. and
D. W. Sears (1977) Meteoritics 12, 47-60. (7] Melcher, C. L. (1979) Meteoritics, 14, 309-316. [8] Sears, D.
W. (1975) Modern Geology, 5, 155-164. [9] Melcher, C. L. (1985) nrivate communication. [10] Sears, D.
W. G. and K. S. Weeks (1983) Lunar Planet. Sei. XIV, 682-683. [11] Scott, E. R. D. (1984) Proc. Ninth
Symposium on Antarctic Meteorites, in press. [12] Sears, D. W., J. N. Grossman and C. L. Melcher (1982)
Geochim. Cosmoehim. Acta 46, 2471-2481. [13] Cressy, P. J., Jr. and L. A. Rancitelli (1974) Earth Planet.
Sei. Lett. 22, 275-283. [14] Sutton, S. R. and G. C. Crozaz (1983) GRL 10, 809-812. [15] Sutton, S. R.
(1985) abstract 10th Symposium on Antarctic Meteorites, Tokyo.


Thermoluminescence of Antarctic meteorites: A rapid screening technique for terrestrial age estimation, pairing studies and identification of specimens with unusual prefall histories

Thermoluminescence of Antarctic meteorites: A rapid screening technique for terrestrial age estimation, pairing studies and identification of specimens with unusual prefall histories

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server