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Apatite fission-track geochrono-thermometer to 60°C: Projected length studies
Chemical Geology (Isotope Geoscience Section), 72 (1988) 145-153 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
145
APATITE FISSION-TRACK GEOCHRONO-THERMOMETER TO 60°C: PROJECTED LENGTH STUDIES G.A. WAGNER Max-Planck-Institut fL.r Kernphysik, D-6900 Heidelberg (Federal Republic of Germany) (Received April 27, 1987; accepted for publication November 16, 1987)
Abstract Wagner, G.A., 1988. Apatite fission-track geochrono-thermometer to 60°C: Projected length studies. Chem. Geol. (Isot. Geosci. Sect.), 72: 145-153. A new approach for deciphering the low-temperature (i.e. < 150 °C) path of steadily cooling rocks is presented. It is based on projected length measurements of spontaneous and induced fission tracks in apatite. The method is corroborated for the grain populatio~ technique. It makes possible to distinguish different cooling patterns such as constant, decelerated, or accelerated rates as well as to date the cooling to ~ 60 °C - in addition to the conventional ~ 100°C cooling age. The method is applied to various rock samples from Central Europe (Oberpfalz, Odenwald, Schwarzwald, Bergell) and to the Fish Canyon Tuff, Colorado, U.S.A. Independent geological evidence supports this approach.
1. Introduction
Two decades ago, the early attempts to date apatite from crystalline basement rocks by means of fission tracks at first produced bewildering results: The ages turned out to be consistently younger than any known major geological event such as rock formation or metamorphism. A pilot study in the Hercynian basement of south Germany - combined with track annealing experiments - indicated that the apatite fission-track clock gives the time when the basement cooled to ~ 100°C during slow uplift (G.A. Wagner, 1968). Similar conclusions as to the low retention temperature of the apatite fission-track system were reached independently by Naeser and Fatal (1969) in their experimental annealing studies. This early work initiated the development of unique tech0168-9622/88/$03.50
niques to decipher the tectonic and thermal history of rocks. These techniques, which may appropriately be called "fission-track tectonics" and "fission-track geothermics", rely on the apatite fission-track system as a sensitive "geochrono-thermometer" around 100 °C. Although this effective retention temperature of fission tracks in apatite was originally derived by extrapolating experiments, it was later essentially confirmed by direct observations of the age vs. temperature profile in deep drill holes (Naeser and Forbes, 1976; Gleadow and Duddy, 1981; Hammerschmidt et al., 1984). The actual value of the effective retention temperature depends to some extent on the cooling rate (G.A. Wagner and Reimer, 1972) and the apatite chemistry (Green et al., 1985). An important development is the application of the apatite fission-track system also to sedimentary basins
© 1988 Elsevier Science Publishers B.V.
146
in order to study their thermo-tectonic evolution and, thus, their hydrocarbon potential ( Gleadow et al., 1983 ). Altogether, there are three different sources of information through which fission tracks in apatite contribute to the reconstruction of the thermo-tectonic history of rocks: (a) The measured fission-track age itself, which simply may be a cooling age to ~ 100 ° C (G.A. Wagner, 1968) or, depending on the geological history, may reveal the age and intensity of thermal overprinting (Miller and Wagner, 1979). Studies on single apatite grains with varying chemistry can increase the information ( Gleadow and Duddy, 1981 ). Provided the geothermal paleo-gradient is known, also tectonic interpretations are possible (G.A. Wagner et al., 1977). (b) The age vs. altitude or - as in case of boreholes - the age vs. depth profiles may give information on the uplift rate of crystalline rock units (G.A. Wagner and Reimer, 1972), the burial history of sedimentary basins ( Gleadow et al., 1983 ) and conceivably on geothermal palaeo-gradients (Briggs et al., 1981). (c) Length measurements on fission tracks can be used as a diagnostic tool for the type of thermal history such as fast or slow steady cooling or even more complex reheating ( Gleadow et al., 1986). In this paper, a further, more quantitative step towards the deciphering of the thermo-tectonic history is proposed which to some extent resumes previously ideas for apatite (G.A. Wagner and Storzer, 1975 ). This new approach is based: (1) on the assumption that the spontaneous track density used for dating is composed of two fractions, one produced within the partial stability zone ( / / i n Fig. 1) and the other in the full stability zone (III in Fig. 1 ) ; and ( 2 ) on the possibility of separating these two fractions by means of track length criteria. For simplicity, only'steady cooling cases are considered such as A or B in Fig. 1 using the grain population dating technique. Furthermore, chemical variations among the grains are not yet taken
tlm~
today
~/r~
z .~ ~
~
,
t
y
_B
....
I~ k
I
~
o
,,ogog ,, o
C. ~,.'..,,d o~,,~,,p~
~
_
e
i ,,~,~b,z,t~..-.on.
Fig. 1. Models of accumulating spontaneous fission tracks with respect to cooling type and track fading (after G.A. Wagner, 1972 ).
into account. It is stressed that exactly the same tracks are used for age counting as well as for length measurement. 2. P r o j e c t e d t r a c k l e n g t h s as t h e r m a l indicator
In previous studies the lengths of fission tracks in apatite were measured either on tracks intersected by and projected on a polished internal face ("projected track length") or on horizontal tracks confined within the crystal and intersected by host tracks, cracks or cleavage planes ("confined track length"). The advantage of using the "projected tracks" is their common occurrence, allowing fast and precise measurement with image analysing systems, that is exactly in the same operation with track counting, and, last but not least, one can measure the same tracks as one counts for an age determination (Van den haute, 1986). On the other hand, the advocates of the "confined tracks" rightly state that the length distribution of the "projected tracks" strongly distorts the true track-length distribution (e.g., Laslett et al., 1982). However, it must not be overlooked that even the "confined track" length distribution is biased and that these tracks do not truly represent those on which the age calculation is based. Therefore, the present study relies exclusively on "projected tracks" observed in randomly oriented apatite grains.
147 The apatites were uniformly etched for 50 s in 5% nitric acid at 20°C and were measured in transmitted light (magnification 100 X 1.25 X 25) and oil immersion. At these etching conditions, the vast majority of the tracks is fully etched according to "stage b" ef Laslett et al. (1984) without developing large crystallographic etch pits at the intersec~ion point with the polished face. The oil immersion facilitates the measurement of the length of the etched track channel only. Some difference in the etchability due to chemistry and crystallographic orientation of the apatke grains is not critical since for each sample the spontaneous fission tracks are always compared with the induced ones. In this study it is not the true length distribution of the fission tracks but the difference between the spontaneou,~ and induced "projected length" distribution,,~ which is utilized for palaeogeothermic diagnosis. For the present study, 27 apatite samples were selected, namely 21 from the ,,~outh German Hercynian basement (Fig. 2), 5 from the Alps ( G.A. Wagner et al., 1977, 1979) and 2 from the Fish Canyon Tuff, Colorado, U.S.A. (Naeser et al., 1981). Their measured fission-track ages (tin) are listed in Table I. The age determination is based on the grain population technique, independent neutron dosimetry (Au and Co monitors, moldavite glass) and ~r-value of 8.46.10 -17 a -1. The length measurements were carried out on ~ 1000 spontaneous as well as induced fission tracks in at least 50 apatite grains. The projected track lengths range from 1 up to 18 #m. An example of a length distribution is given in Fig. 3. Also the mean values ~ and ~ for the spontaneous and induced length distributions, respectively, were calculated. When considering all samples, the average value for li amounts to 6.0_+0.3 (1G) #m. Although the mean induced track length li depends - for fixed etching conditions - to some extent o11 the apatite chemistry, its major uncertainty is due to inevitable bias in sampling short tracks. The mean spontaneous track length ~ is smaller than li
lOO~-n J - ~
I Hercynianbasement of southGermany • samplelocations
Fig. 2. Outcrop of Hercynian basement in south Germany with samplelocations. for all samples. This is easily understood since the spontaneous track population usually contains a considerable fraction of partially annealed tracks which are characterized by reduced lengths (Fig. 4). The ratio ls/li is given in Table I although it has no direct relevance in the present concept. More important is the shape of the length distribution (Fig. 3). Of special interest are the tracks with length of > 10/lm. They are significantly less frequent among the spontaneous tracks than among the induced ones. In order to define shape parameters for the length distributions, each track population ( = 100% ) is grouped according to track length in group a ( > 0-5/~m), group b ( >5-10/~m) and group c ( > 1 0 z m ) . On the average, c~ (i.e. the fraction > 10 ]~m) and b~ (i.e. the fraction 5-10 ~tm) of induced fission tracks amount to 15.0 + 2.3% and 41.8 + 2.6%, respectively. The variation of the corresponding fractions cs and bs of spontaneous fission tracks among the samples is larger. As shape parameters of the length distribution one may select the ratios (cs/bs)/(ci/bi) o r cJc~; in the following discussion only the latter one is used
148 TABLE I Fission-track apatite ages as derived from track counting and projected length measurement tm(*1) ( ~ 100°C) (Ma)
Cs/Ci
tf (.2) ( ~ 60°C) (Ma)
CJ4
~,,14
75.3 68.3 124.7 195.3 57.9 56.1 49.3 64.1 65.7 58.1 58.8 61.3 152.9 125,3 68.4 86.8
0.39 0.43 0.33 0.23 0.60 0.46 0.37 0.61 0.56 0.48 0.50 0.57 0.61 0.30 0.76 0.75
29 29 41 44 35 26 18 39 37 28 29 35 93 37 52 65
0.83 0.76 0.77 0.74 0.90 0.88 0.89 0.93 0.89 0.88 0.92 0.93 0.85 0.81 0.91 0.94
0.72 0.58 0.71 0.66 0.75 0.78 0.83 0.82 0.74 0.77 0.84 0.84 0.62 0.72 0.59 0.77
88.2 71.7
0.69 0.40
61 28
0.94 0.86
0.81 0.77
Steinbiih122 Heimbach 51 Elbenschwand 55
55.0 70.3 38.1
0.48 0.31 0.49
27 22 19
0.87 0.80 0.77
0.75 0.71 0.54
Val Bondsca BM2 Bagni del Masino BM7 Simplon KAW 160 Pedrinate KAW 1004 Pedrinate KAW 1005
16.8 14.0 2.5 24.1 25.9
0.78 0.51 0.63 0.53 0.47
13.1 7.2 1.6 12.8 12.3
0.94 0.89 0.93 0.92 0.92
0.73 0.77 0.81 0.83 0.85
28.1 26.7
0.96 0.92
26.9 24.7
0.99 0.98
0.84 0.68
(a) Oberpfalz: Weisse Marter 72 Rosall 73 Theisseil 74 Michldorf 76 P~illersreuth 80 Napfberg 84 Phaben-Brand 85 Reuth-Drahthammer 88 Windisch-Eschenbach 90 Iglersreuth 96 Silberh~itte 98 Flossenbilrg 99 Kainzelmilhle 101 Leuchtenberg 102 TrSstau 107 Weissenstein 108 (b ) Odenwald: BSllstein 0D4 Stiftsmfihle 0D6 ( c) Schwarzwald:
(d) Alps:
(e) Fish Canyon Tuff: 70 L 126 FC 3
*IThe measured fission-track apatite ages tm were taken from G.A. Wagner et al. (1987) for (a) and (c), G.A. Wagner and Storzer (1975) for (b), G.A. Wagner et al. (1979) for (d). The l a error is ~ +_67o. *2The l a error for the 60 ° C cooling age tf is ~ _+157o.
s i n c e i t is m o r e s e n s i t i v e a n d b e t t e r r e p r o d u c i b l e . T h e p r e c i s i o n w i t h w h i c h t h i s r a t i o is d e termined depends on the number of tracks
c o u n t e d a n d is ~ +_ 1 5 % , i f 1000 s p o n t a n e o u s and induced tracks each are considered. Among the investigated samples the ratio cJci varies
149 I
i
15 ~'~ 10
~~~':~L_ [- ii !:-~ i
"~
I
o
~ induced
spontan.
~
5
i
Hd SP Apatite 90 Windisch-Eschenbach 5% HN03,20°C, 50 sec; Oil each 1000 tracRs cs(>10#rn) = O56 ci (>10#rn) " ~- =0.89
10 15 Track Length (IJ,m)
Fig. 3. Length histogram for projected ~;racks in an apatite from Windisch-Eschenbach (Oberpfal z).
nearly by a factor of 5 (Table ::). Obviously if spontaneous and induced fission tracks have similar length distributions, as one could expect for fast cooling such as b p e A in Fig. 1, this ratio should be close or equal to unity. The higher the fraction of partially annealed, i.e. shorter tracks, as one would expect for samples with relatively long residence time in the partial stability zone H (type B in Fig. 1), the smaller this ratio becomes. In other words, the shape parameter cJci reflects the relative abundance of tracks produced in the full stability zone. Consequently, it is a sensitive diagnostic tool for deciphering the cooling evolution below 150°C. From geologically known cases (e.g., 100 .,oti~ r ,
,
..jr-
o
,
o
~5C
0
0
I
I
,
I
I
50
]
$
i
I
100
p/po Fig. 4. Laboratory annealingof freshly induced fission tracks reveals that the mean projected length l/lo of tracks is progressively reduced as the track density P/Po decreases.
Alpine samples KAW 160 and BM7) one derives cs/ci - 0.5-0.7 for constant cooling behaviour; smaller values represent acceleration and higher values deceleration as general trend of steady cooling. Just for comparison, the ratio lJli of the mean track lengths is, not unexpectedly, a rather blunt tool for palaeothermal diagnosis, as is easily noted from the values in Table I. So far, the ratio cJci has been discussed only as a qualitative indicator for the relative abundance Psm/P~ of the track density P~m produced in the full stability zone III (Fig. 1 ) compared to the total spontaneous fission-track density p~. However, it is desirable to quantify this fraction. Once this is achieved, one could calculate from Psm the age when the rock temperature dropped to the full stability zone for tracks in apatite, i.e. ~ 60 ° C. Ideally, after correcting the track population PsH which was formed within the partial stability zone H for track loss due to thermal fading, one could even calculate from pssr (corrected) + p ~ m t h e age when the fissiontrack accumulation in apatite started, i.e. 140 ° C. It can now be shown that the ratio cJci itself is practically equal to P~m/P~, i.e.:
P~III = ( cJci)ps The evidence for this is two-fold: (1) In laboratory annealing experiments the reduction C/Co of a given track density co of freshly induced fission tracks of > 10 #m was measured for various degrees of track fading. The results in Fig. 5 indicate that at a track density reduction P/Po = 0.9 the ratio c/co is already reduced to 0.2. This means that the long tracks of > 10 pm are reduced much faster than the total tracks. Thus, in a slowly cooling rock which is just emerging from the partial stability zone tracks of > 10 gm would constitute only a very minor fraction ( < 2% ) of the spontaneous tracks present at that moment. (2) A similar result was directly observed at the drill core of Urach III: apatites with 60 °C actual rock temperature contain practicallly no
150 1.0
I
partial stability zone. Because ~ is composed of the mean lengths ~zi and ~ m of both fractions PsIs/P~and p~m/p~ according to:
Apatite Tll
= (1-cUci) l r1+ (Cs/Ci)
O O
0.5
and since ~ m = ~ it follows that:
J
0.5
lJli = (1 -Cs/Ci) l~H/li +c~/c~ 1.0
P/P0
From this equation the reduction l~n/li of the mean length of the tracks produced in the partial stability zone can be calculated according to:
Fig. 5. Laboratory annealing of freshly induced fission tracks. The fraction c/co of tracks > 10 #m decreases faster than the total number P/Po.
tracks > 10 #m (M. Wagner, 1985). Therefore, almost all tracks of > 10 # m which are observed in apatites collected at ambient surface temperatures originate from the full stability zone. Based on this evidence, the fission-track age tf calculated from the track density P~m (replacing p~ in the usual age equation) is interpreted as the time the rock temperature cooled to the full stability zone of fission tracks in apatite, which corresponds to ~ 60 ° C. Clearly, detailed studies on drill-core apatites and chemical checks on the apatite composition may modify this conclusion somewhat. Also, the uplift rate influences the actual value of the full track retention temperature. But these problems are hardly different from those of the cooling temperature Corresponding to the measured age tin. For all the apatites investigated, the tf ages are listed in Table I. In this way two cooling ages, namely tm and tf, which correspond to ~ 100 o and ~ 60 ° C for steadily slowly cooling rocks, respectively, can be derived for each apatite sample in the same measuring operation. Before these results are geologically discussed, another interesting point shall be noted. The mean length ~ of the spontaneous tracks may implicitly contain some diagnostic information on the cooling behaviour within the
lsll/~ = ( Is/li - cJci) / (1 - c J c i ) For steady cooling at a linear rate one predicts from annealing experiments (Fig. 4) l~i~/l~- 0.75-0.80. Actually, in the apatites from the Urach III drill core at the present rock temperature of 65°C the value l~H/l~=0.79 was directly observed (M. Wagner, 1985). For acccelerated cooling within the partial stability zone one should expect l~Jli-values of <0.75. The Isx~/l~ ratios of the investigated samples range from 0.58 to 0.85.
3. Geological application The validity of the concept presented so far needs to be checked against geological evidence. This is done for the various regions from which the samples originate. The Fish Canyon Tuff apatite is widely used as an age standard in fission-track dating (Naeser et al., 1981; Hurford and Hammerschmidt, 1985). Because it occurs in volcanic tephra, it presumably cooled rather quickly. The fission tracks likely became stable in this apatite soon after the volcanic event which, in any case, is one prerequisite for its use as an age standard. In two apatite samples the cs/ci ratio was determined as 0.96 and 0.92. From track counting statistics the l a precision of these values is considered to be +8%, that is, they are not significantly different from unity. This is in accordance with the presumption of a very fast
151
cooling immediately after the tephra deposition such as type A in Fig. 1. These findings support the suitability of the Fish Canyon Tuff apatite as an age standard in fission-track dating. However, they do not exclude the possibility that some track fading may have occurred as, according to P. Van den haute (pers. commun., 1987), is indicated by a mean track length ratio in FCT-3 apatite, namely 0.9[4 for confined tracks, and 0.938 and 0.916 for projected tracks on prism and basal faces, respectively. In the present study, a length ratio of 9.993 and 0.975 for the two apatite samples (argitrary orientation) was observed. The important point in this context is that the cJci-value~ for two Fish Canyon Tuff samples are the largest so far found of all investigated apatite samples (Table I) and that their respective cooling ages to 100°C (28.1+0.9 and 26.7+0.8 Ma) and to 60°C (26.9 + 2.2 and 24.7 + 2.0 Ma) agree within their statistical error limits. This evidence supports the concept introduced here. Most of the apatites studied originate from the Hercynian crystalline basement of the Oberpfalz area in northeast Bavaria which was selected as the target area (near Windisch- Eschenbach) for the deep continental drilling project KTB ("Kontinentale T=ef-Bohrung") (see Fig. 2). The track length analyses of these apatites are listed in Table I. The most notable result is that samples from the same region, but not necessarily from the same r~ck type, possess similar measured fission-track ages (tin) as well as similar cJci ratios and, thus, similar t~ ages. Consequently, samples from defined regions show the similar cooling ages to 100 ° and 60 ° C. However, from region to region the cooling pattern changes considerably. This is illustrated in Fig. 6 where samples from each region are combined into one cooling curve. Without going into geological details, a few remarks seem appropriate here. Some of the cooling trends revealed by the track length analysis had been presumed in the past on the basis of geological and geomorphological considerations (SchrSder, 1976; Louis, 1984) such
350
300
250
i
t
'°L ~
Age (Ma) 200 150
i
100
50
0
(
60
~I00 1
2
3
45678
Fig. 6. Thermo-tectonic evolution of various regions of the Oberpfalz based on fission-track apatite analysis. Curves: 1 = No. 76; 2 = No. 101; 3 = Nos. 74 and 102; 4 = Nos. 72 and 73; 5=No. 108; 6=No. 107; 7 = N o s . 80, 88, 90, 96, 98 and 99; 8 = Nos. 84 and 85.
as the early and slow uplift of the Erbendorf-Vohenstrauss zone (lines 2 and 3 in Fig. 6), the rapid Late Cretaceous and Palaeogene uplift with subsequent slowing down of the Milnchberger Gneismasse and Fichtelgebirge (lines 5 and 6, respectively, in Fig. 6), and the most recent, strong uplift of the Steinwald (line 8 in Fig. 6). These trends seem to hold true also for the cooling within the partial stability zone, as is evident from the l~rJli ratios in Table I. For the two apatites from the Odenwald a distinctly different cooling pattern is revealed for the northern part in contrast to the sample from the Heidelberg Granite (Fig. 7). This demonstrates the predicted southward movement of the maximum uplift rate during the Late Cretaceous and Tertiary (G.A. Wagner, 1968). The three samples from the Schwarzwald show more or less uniform uplift during the Tertiary (Fig. 7). The fast, recent uplift is related to the break-up of the Rheingraben in the
100
75
Age (Ma) 50
25
0
"10
o =
60
i..--
10o Fig. 7. Thermo-tectonic evolution of Odenwald (Nos. OD4 and OD6) and Schwarzwald (Nos. 22, 51 and 55) Hercynian basement based on fission-track apatite analysis.
152 Age (Na)
30
10
20
-
3
2 o
o v m
10 ~o~
70
_I
E O
boulders do not represent a steady cooling evolution since track accumulation started in their apatites. Because projected track length studies cannot reveal a bimodal distribution, this case should be studied with confined track lengths too.
w
-2
4. Conclusions
110 BM2
BN7
KAW160
-3
Fig. 8. Thermal evolution ofBergell granodiorite (Nos. BM2 and BM7) and Molasse boulders (Nos. KAW 1004 and 1005) based on fission-track apatite analysis. One sample (KAW 160) from the Simplon area is included. Due to rapid uplift the cooling temperatures for t~ and tf here are assumed 10°C higher than for the Hercynian basement samples.
immediate neighbourhood. It is also evident from confined track length measurements on Schwarzwald apatites (Michalski, 1987 ). The length measurements on apatites from two Bergell granodiorites support the fast and uniform uplift (Fig. 8) as deduced by previous studies ( G.A. Wagner et al., 1979 ). The cooling temperatures are assumed to be ~ 10 °C higher, i.e. 110 ° and 70°C, due to fast uplift. It is interesting to note how the topographic elevation effect has an influence on the ratio cJci. For samples with higher elevation (sample BM2 from 2380 m vs. sample BM7 from 1060 m ), but from the same tectonic block, cJcl increases. The length measurements on apatites of two Bergell boulders buried in the Oligocene Molasse sediments gave surprising results. Previously, it was concluded that they reached the surface rather rapidly during the Late Oligocene, eroded, and, after transportation, were deposited in the Molasse sediments where they stayed cool until today (G.A. Wagner et al., 1979). Therefore, a cooling history close to type A (Fig. 1 ) was expected. However, the ratio cJci revealed tf ages around 12-13 Ma for both boulders. This makes re-heating of the Molasse sediments to ~ 60 ° C during Miocene due to burial likely, such as type C in Fig. 1. Obviously, these
For apatites with a steady cooling history below ~ 150°C, the projected track length measurements provide a sensitive diagnostic tool which allows us to distinguish constant from accelerated and decelerated cooling evolution. Furthermore, they allow us to calculate the cooling age to ~ 60 ° C. This method becomes possible since the same tracks are employed for age determination and length measurements. The additional experimental effort is minor because track counting and length measurements are done in one and the same operation. In order to gain optimum information from apatite fission-track analyses for deciphering tectonic and geothermic histories, it is recommended to use the projected track length technique for steady cooling rocks and the confined track length technique for thermally complex evolving rocks or, ideally, to apply both techniques together.
Acknowledgement
I should like to thank Dr. C.W. Naeser, Denver, for supplying the Fish Canyon Tuff apatite and I. Michalski, Heidelberg, for the Schwarzwald apatite samples. I appreciate Dr. P. Van den haute's critical remarks on the manuscript and help in arranging the neutron irradiation and dose calibration in the thermal column at the Thetis reactor, Gent, Belgium. The work was financially supported by the Deutsche Forschungsgemeinschaft.
153
References Briggs, N.D., Naeser, C.W. and McCulloh, T.H., 1981. Thermal history of sedimentary basins by fission track dating. Nucl. Tracks, 5: 235-237. Gleadow, A.J.W. and Duddy, I.R., 1981. A natural longterm track annealing experimen~ for apatite. Nucl. Tracks, 5: 169-174. Gleadow, A.J.W., Duddy, I.R. and Lovering, J.F., 1983. Fission track analysis: A new tool for the evaluation of thermal histories and hydrocarbc~n potential. APEA (Aust. Pet. Explor. Assoc.) J., 23: 93-102. Gleadow, A.J.W., Duddy, I.R., Green~ P.F., Laslett, G.M. and Lovering, J.F., 1986. Confined';rack lengths in apatites - A diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol., 94: 405-415. Green, P.F., Duddy, I.R., Gleadow, A.J.W., Tingate, P.R. and Laslett, G.M., 1985. Fission track annealing in apatite: Track length measurements and the form of the Arrhenius plot. Nucl. Tracks, 10: 323-328. Hammerschmidt, K., Wagner, G.A. and Wagner, M., 1984. Radiometric dating on research drill core Urach I I I : A contribution to its geothermal history. J. Geophys. Res., 54: 97-105. Hurford, A.J. and Hammerschmidt, K., 1985.4°Ar/Z~Arand K/Ar dating of the Bishop and Fish Canyon Tufts: Calibration ages for fission-track dating standards. Chem. Geol. (Isot. Geosci. Sect.), 58: 23-32. Laslett, G.M., Kendall, W.S., Gleadow, A.J.W. and Duddy, I.R., 1982. Bias in the measurement of fission-track length distributions. Nucl. Tracks, 6: 79-85. Laslett, G.M., Gleadow, A.J.W. and Duddy, I.R., 1984. The relationship between fission track length and track density in apatite. Nucl. Tracks, 9: 29-~8. Louis, H., 1984. Zur Reliefentwickl~ng der Oberpfalz. Borntr~iger, Berlin, 66 pp. Michalski, I., 1987. Apatit-Spaltspurdatierung des Grundgebirges von Schwarzwald und Vogesen: Die postvariscische Entwicklung. Dissertation, University of Heidelberg, Heidelberg, 25 pp. Miller, D.S. and Wagner, G.A., 1979. Age and intensity of thermal events by fission track analysis: The Ries im-
pact crater. Earth Planet. Sci. Lett., 43: 351-358. Naeser, C.W. and Faul, H., 1969. The fission track annealing in apatite and sphene. J. Geophys. Res., 74: 705-710. Naeser, C.W. and Forbes, R.B., 1976. Variation of fissiontrack ages with depth in two deep drill holes. Eos (Trans. Am. Geophys. Union), 57:353 (abstract). Naeser, C.W., Zimmermann, R.A. and Cebula, G.T., 1981. Fission track dating of apatite and zircon: An interlaboratory comparison. Nucl. Tracks, 5: 65-72. SchrSder, B., 1976. Saxonische Tektonik im Ostteil der siiddeutschen Scholle. Geol. Rundsch., 65: 34-54. Van den haute, P., 1986. Apatite fission-track dating applied to Precambrian terranes. In: S. Deutsch and A.W. Hofmann (Editors), Isotopes in Geology - Picciotto Volume. Chem. Geol., 57:155-165 (special issue), Wagner, G.A., 1968. Fission track dating of apatites. Earth Planet. Sci. Lett., 4: 411-415. Wagner, G.A., 1972. The geological interpretation of fission track ages. Trans. Am. Nucl. Soc., 15: 117. Wagner, G.A. and Reimer, G.M., 1972. Fission track tectonics: The tectonic interpretation of fission track apatite ages. Earth Planet. Sci. Lett., 14: 263-268. Wagner, G.A. and Storzer, D., 1975. Spaltspuren und ihre Bedeutung fiir die thermische Geschichte des Odenwaldes. Aufschluss, 27: 79-85. Wagner, G.A., Reimer, G.M. and J~iger, E., 1977. Cooling ages derived by apatite fission track, mica Rb-Sr and K-At dating: The uplift and cooling history of the Central Alps. Mere. Ist. Geol. Mineral. Univ. Padova, 30: 1-27. Wagner, G.A., Miller, D.S. and J~iger, E., 1979. Fission track ages on apatite of Bergell rocks from Central Alps and Bergell boulders in Oligocene sediments. Earth Planet. Sci. Lett., 45: 355-360. Wagner, G.A., Michalski, I. and Zaun, P., 1987. Apatite fission track dating of the Hercynian basement in South Germany. In: H. Behr (Editor), Exploration of the Deep Continental Crust. Springer, Heidelberg (in press). Wagner, M., 1985. Spaltspurendatierungen am Bohrkern Urach-III - Ein Beitrag zur W~irmegeschichte der geothermischen Anomalie Urach. Dissertation, University of Heidelberg, Heidelberg, 247 pp.
145
APATITE FISSION-TRACK GEOCHRONO-THERMOMETER TO 60°C: PROJECTED LENGTH STUDIES G.A. WAGNER Max-Planck-Institut fL.r Kernphysik, D-6900 Heidelberg (Federal Republic of Germany) (Received April 27, 1987; accepted for publication November 16, 1987)
Abstract Wagner, G.A., 1988. Apatite fission-track geochrono-thermometer to 60°C: Projected length studies. Chem. Geol. (Isot. Geosci. Sect.), 72: 145-153. A new approach for deciphering the low-temperature (i.e. < 150 °C) path of steadily cooling rocks is presented. It is based on projected length measurements of spontaneous and induced fission tracks in apatite. The method is corroborated for the grain populatio~ technique. It makes possible to distinguish different cooling patterns such as constant, decelerated, or accelerated rates as well as to date the cooling to ~ 60 °C - in addition to the conventional ~ 100°C cooling age. The method is applied to various rock samples from Central Europe (Oberpfalz, Odenwald, Schwarzwald, Bergell) and to the Fish Canyon Tuff, Colorado, U.S.A. Independent geological evidence supports this approach.
1. Introduction
Two decades ago, the early attempts to date apatite from crystalline basement rocks by means of fission tracks at first produced bewildering results: The ages turned out to be consistently younger than any known major geological event such as rock formation or metamorphism. A pilot study in the Hercynian basement of south Germany - combined with track annealing experiments - indicated that the apatite fission-track clock gives the time when the basement cooled to ~ 100°C during slow uplift (G.A. Wagner, 1968). Similar conclusions as to the low retention temperature of the apatite fission-track system were reached independently by Naeser and Fatal (1969) in their experimental annealing studies. This early work initiated the development of unique tech0168-9622/88/$03.50
niques to decipher the tectonic and thermal history of rocks. These techniques, which may appropriately be called "fission-track tectonics" and "fission-track geothermics", rely on the apatite fission-track system as a sensitive "geochrono-thermometer" around 100 °C. Although this effective retention temperature of fission tracks in apatite was originally derived by extrapolating experiments, it was later essentially confirmed by direct observations of the age vs. temperature profile in deep drill holes (Naeser and Forbes, 1976; Gleadow and Duddy, 1981; Hammerschmidt et al., 1984). The actual value of the effective retention temperature depends to some extent on the cooling rate (G.A. Wagner and Reimer, 1972) and the apatite chemistry (Green et al., 1985). An important development is the application of the apatite fission-track system also to sedimentary basins
© 1988 Elsevier Science Publishers B.V.
146
in order to study their thermo-tectonic evolution and, thus, their hydrocarbon potential ( Gleadow et al., 1983 ). Altogether, there are three different sources of information through which fission tracks in apatite contribute to the reconstruction of the thermo-tectonic history of rocks: (a) The measured fission-track age itself, which simply may be a cooling age to ~ 100 ° C (G.A. Wagner, 1968) or, depending on the geological history, may reveal the age and intensity of thermal overprinting (Miller and Wagner, 1979). Studies on single apatite grains with varying chemistry can increase the information ( Gleadow and Duddy, 1981 ). Provided the geothermal paleo-gradient is known, also tectonic interpretations are possible (G.A. Wagner et al., 1977). (b) The age vs. altitude or - as in case of boreholes - the age vs. depth profiles may give information on the uplift rate of crystalline rock units (G.A. Wagner and Reimer, 1972), the burial history of sedimentary basins ( Gleadow et al., 1983 ) and conceivably on geothermal palaeo-gradients (Briggs et al., 1981). (c) Length measurements on fission tracks can be used as a diagnostic tool for the type of thermal history such as fast or slow steady cooling or even more complex reheating ( Gleadow et al., 1986). In this paper, a further, more quantitative step towards the deciphering of the thermo-tectonic history is proposed which to some extent resumes previously ideas for apatite (G.A. Wagner and Storzer, 1975 ). This new approach is based: (1) on the assumption that the spontaneous track density used for dating is composed of two fractions, one produced within the partial stability zone ( / / i n Fig. 1) and the other in the full stability zone (III in Fig. 1 ) ; and ( 2 ) on the possibility of separating these two fractions by means of track length criteria. For simplicity, only'steady cooling cases are considered such as A or B in Fig. 1 using the grain population dating technique. Furthermore, chemical variations among the grains are not yet taken
tlm~
today
~/r~
z .~ ~
~
,
t
y
_B
....
I~ k
I
~
o
,,ogog ,, o
C. ~,.'..,,d o~,,~,,p~
~
_
e
i ,,~,~b,z,t~..-.on.
Fig. 1. Models of accumulating spontaneous fission tracks with respect to cooling type and track fading (after G.A. Wagner, 1972 ).
into account. It is stressed that exactly the same tracks are used for age counting as well as for length measurement. 2. P r o j e c t e d t r a c k l e n g t h s as t h e r m a l indicator
In previous studies the lengths of fission tracks in apatite were measured either on tracks intersected by and projected on a polished internal face ("projected track length") or on horizontal tracks confined within the crystal and intersected by host tracks, cracks or cleavage planes ("confined track length"). The advantage of using the "projected tracks" is their common occurrence, allowing fast and precise measurement with image analysing systems, that is exactly in the same operation with track counting, and, last but not least, one can measure the same tracks as one counts for an age determination (Van den haute, 1986). On the other hand, the advocates of the "confined tracks" rightly state that the length distribution of the "projected tracks" strongly distorts the true track-length distribution (e.g., Laslett et al., 1982). However, it must not be overlooked that even the "confined track" length distribution is biased and that these tracks do not truly represent those on which the age calculation is based. Therefore, the present study relies exclusively on "projected tracks" observed in randomly oriented apatite grains.
147 The apatites were uniformly etched for 50 s in 5% nitric acid at 20°C and were measured in transmitted light (magnification 100 X 1.25 X 25) and oil immersion. At these etching conditions, the vast majority of the tracks is fully etched according to "stage b" ef Laslett et al. (1984) without developing large crystallographic etch pits at the intersec~ion point with the polished face. The oil immersion facilitates the measurement of the length of the etched track channel only. Some difference in the etchability due to chemistry and crystallographic orientation of the apatke grains is not critical since for each sample the spontaneous fission tracks are always compared with the induced ones. In this study it is not the true length distribution of the fission tracks but the difference between the spontaneou,~ and induced "projected length" distribution,,~ which is utilized for palaeogeothermic diagnosis. For the present study, 27 apatite samples were selected, namely 21 from the ,,~outh German Hercynian basement (Fig. 2), 5 from the Alps ( G.A. Wagner et al., 1977, 1979) and 2 from the Fish Canyon Tuff, Colorado, U.S.A. (Naeser et al., 1981). Their measured fission-track ages (tin) are listed in Table I. The age determination is based on the grain population technique, independent neutron dosimetry (Au and Co monitors, moldavite glass) and ~r-value of 8.46.10 -17 a -1. The length measurements were carried out on ~ 1000 spontaneous as well as induced fission tracks in at least 50 apatite grains. The projected track lengths range from 1 up to 18 #m. An example of a length distribution is given in Fig. 3. Also the mean values ~ and ~ for the spontaneous and induced length distributions, respectively, were calculated. When considering all samples, the average value for li amounts to 6.0_+0.3 (1G) #m. Although the mean induced track length li depends - for fixed etching conditions - to some extent o11 the apatite chemistry, its major uncertainty is due to inevitable bias in sampling short tracks. The mean spontaneous track length ~ is smaller than li
lOO~-n J - ~
I Hercynianbasement of southGermany • samplelocations
Fig. 2. Outcrop of Hercynian basement in south Germany with samplelocations. for all samples. This is easily understood since the spontaneous track population usually contains a considerable fraction of partially annealed tracks which are characterized by reduced lengths (Fig. 4). The ratio ls/li is given in Table I although it has no direct relevance in the present concept. More important is the shape of the length distribution (Fig. 3). Of special interest are the tracks with length of > 10/lm. They are significantly less frequent among the spontaneous tracks than among the induced ones. In order to define shape parameters for the length distributions, each track population ( = 100% ) is grouped according to track length in group a ( > 0-5/~m), group b ( >5-10/~m) and group c ( > 1 0 z m ) . On the average, c~ (i.e. the fraction > 10 ]~m) and b~ (i.e. the fraction 5-10 ~tm) of induced fission tracks amount to 15.0 + 2.3% and 41.8 + 2.6%, respectively. The variation of the corresponding fractions cs and bs of spontaneous fission tracks among the samples is larger. As shape parameters of the length distribution one may select the ratios (cs/bs)/(ci/bi) o r cJc~; in the following discussion only the latter one is used
148 TABLE I Fission-track apatite ages as derived from track counting and projected length measurement tm(*1) ( ~ 100°C) (Ma)
Cs/Ci
tf (.2) ( ~ 60°C) (Ma)
CJ4
~,,14
75.3 68.3 124.7 195.3 57.9 56.1 49.3 64.1 65.7 58.1 58.8 61.3 152.9 125,3 68.4 86.8
0.39 0.43 0.33 0.23 0.60 0.46 0.37 0.61 0.56 0.48 0.50 0.57 0.61 0.30 0.76 0.75
29 29 41 44 35 26 18 39 37 28 29 35 93 37 52 65
0.83 0.76 0.77 0.74 0.90 0.88 0.89 0.93 0.89 0.88 0.92 0.93 0.85 0.81 0.91 0.94
0.72 0.58 0.71 0.66 0.75 0.78 0.83 0.82 0.74 0.77 0.84 0.84 0.62 0.72 0.59 0.77
88.2 71.7
0.69 0.40
61 28
0.94 0.86
0.81 0.77
Steinbiih122 Heimbach 51 Elbenschwand 55
55.0 70.3 38.1
0.48 0.31 0.49
27 22 19
0.87 0.80 0.77
0.75 0.71 0.54
Val Bondsca BM2 Bagni del Masino BM7 Simplon KAW 160 Pedrinate KAW 1004 Pedrinate KAW 1005
16.8 14.0 2.5 24.1 25.9
0.78 0.51 0.63 0.53 0.47
13.1 7.2 1.6 12.8 12.3
0.94 0.89 0.93 0.92 0.92
0.73 0.77 0.81 0.83 0.85
28.1 26.7
0.96 0.92
26.9 24.7
0.99 0.98
0.84 0.68
(a) Oberpfalz: Weisse Marter 72 Rosall 73 Theisseil 74 Michldorf 76 P~illersreuth 80 Napfberg 84 Phaben-Brand 85 Reuth-Drahthammer 88 Windisch-Eschenbach 90 Iglersreuth 96 Silberh~itte 98 Flossenbilrg 99 Kainzelmilhle 101 Leuchtenberg 102 TrSstau 107 Weissenstein 108 (b ) Odenwald: BSllstein 0D4 Stiftsmfihle 0D6 ( c) Schwarzwald:
(d) Alps:
(e) Fish Canyon Tuff: 70 L 126 FC 3
*IThe measured fission-track apatite ages tm were taken from G.A. Wagner et al. (1987) for (a) and (c), G.A. Wagner and Storzer (1975) for (b), G.A. Wagner et al. (1979) for (d). The l a error is ~ +_67o. *2The l a error for the 60 ° C cooling age tf is ~ _+157o.
s i n c e i t is m o r e s e n s i t i v e a n d b e t t e r r e p r o d u c i b l e . T h e p r e c i s i o n w i t h w h i c h t h i s r a t i o is d e termined depends on the number of tracks
c o u n t e d a n d is ~ +_ 1 5 % , i f 1000 s p o n t a n e o u s and induced tracks each are considered. Among the investigated samples the ratio cJci varies
149 I
i
15 ~'~ 10
~~~':~L_ [- ii !:-~ i
"~
I
o
~ induced
spontan.
~
5
i
Hd SP Apatite 90 Windisch-Eschenbach 5% HN03,20°C, 50 sec; Oil each 1000 tracRs cs(>10#rn) = O56 ci (>10#rn) " ~- =0.89
10 15 Track Length (IJ,m)
Fig. 3. Length histogram for projected ~;racks in an apatite from Windisch-Eschenbach (Oberpfal z).
nearly by a factor of 5 (Table ::). Obviously if spontaneous and induced fission tracks have similar length distributions, as one could expect for fast cooling such as b p e A in Fig. 1, this ratio should be close or equal to unity. The higher the fraction of partially annealed, i.e. shorter tracks, as one would expect for samples with relatively long residence time in the partial stability zone H (type B in Fig. 1), the smaller this ratio becomes. In other words, the shape parameter cJci reflects the relative abundance of tracks produced in the full stability zone. Consequently, it is a sensitive diagnostic tool for deciphering the cooling evolution below 150°C. From geologically known cases (e.g., 100 .,oti~ r ,
,
..jr-
o
,
o
~5C
0
0
I
I
,
I
I
50
]
$
i
I
100
p/po Fig. 4. Laboratory annealingof freshly induced fission tracks reveals that the mean projected length l/lo of tracks is progressively reduced as the track density P/Po decreases.
Alpine samples KAW 160 and BM7) one derives cs/ci - 0.5-0.7 for constant cooling behaviour; smaller values represent acceleration and higher values deceleration as general trend of steady cooling. Just for comparison, the ratio lJli of the mean track lengths is, not unexpectedly, a rather blunt tool for palaeothermal diagnosis, as is easily noted from the values in Table I. So far, the ratio cJci has been discussed only as a qualitative indicator for the relative abundance Psm/P~ of the track density P~m produced in the full stability zone III (Fig. 1 ) compared to the total spontaneous fission-track density p~. However, it is desirable to quantify this fraction. Once this is achieved, one could calculate from Psm the age when the rock temperature dropped to the full stability zone for tracks in apatite, i.e. ~ 60 ° C. Ideally, after correcting the track population PsH which was formed within the partial stability zone H for track loss due to thermal fading, one could even calculate from pssr (corrected) + p ~ m t h e age when the fissiontrack accumulation in apatite started, i.e. 140 ° C. It can now be shown that the ratio cJci itself is practically equal to P~m/P~, i.e.:
P~III = ( cJci)ps The evidence for this is two-fold: (1) In laboratory annealing experiments the reduction C/Co of a given track density co of freshly induced fission tracks of > 10 #m was measured for various degrees of track fading. The results in Fig. 5 indicate that at a track density reduction P/Po = 0.9 the ratio c/co is already reduced to 0.2. This means that the long tracks of > 10 pm are reduced much faster than the total tracks. Thus, in a slowly cooling rock which is just emerging from the partial stability zone tracks of > 10 gm would constitute only a very minor fraction ( < 2% ) of the spontaneous tracks present at that moment. (2) A similar result was directly observed at the drill core of Urach III: apatites with 60 °C actual rock temperature contain practicallly no
150 1.0
I
partial stability zone. Because ~ is composed of the mean lengths ~zi and ~ m of both fractions PsIs/P~and p~m/p~ according to:
Apatite Tll
= (1-cUci) l r1+ (Cs/Ci)
O O
0.5
and since ~ m = ~ it follows that:
J
0.5
lJli = (1 -Cs/Ci) l~H/li +c~/c~ 1.0
P/P0
From this equation the reduction l~n/li of the mean length of the tracks produced in the partial stability zone can be calculated according to:
Fig. 5. Laboratory annealing of freshly induced fission tracks. The fraction c/co of tracks > 10 #m decreases faster than the total number P/Po.
tracks > 10 #m (M. Wagner, 1985). Therefore, almost all tracks of > 10 # m which are observed in apatites collected at ambient surface temperatures originate from the full stability zone. Based on this evidence, the fission-track age tf calculated from the track density P~m (replacing p~ in the usual age equation) is interpreted as the time the rock temperature cooled to the full stability zone of fission tracks in apatite, which corresponds to ~ 60 ° C. Clearly, detailed studies on drill-core apatites and chemical checks on the apatite composition may modify this conclusion somewhat. Also, the uplift rate influences the actual value of the full track retention temperature. But these problems are hardly different from those of the cooling temperature Corresponding to the measured age tin. For all the apatites investigated, the tf ages are listed in Table I. In this way two cooling ages, namely tm and tf, which correspond to ~ 100 o and ~ 60 ° C for steadily slowly cooling rocks, respectively, can be derived for each apatite sample in the same measuring operation. Before these results are geologically discussed, another interesting point shall be noted. The mean length ~ of the spontaneous tracks may implicitly contain some diagnostic information on the cooling behaviour within the
lsll/~ = ( Is/li - cJci) / (1 - c J c i ) For steady cooling at a linear rate one predicts from annealing experiments (Fig. 4) l~i~/l~- 0.75-0.80. Actually, in the apatites from the Urach III drill core at the present rock temperature of 65°C the value l~H/l~=0.79 was directly observed (M. Wagner, 1985). For acccelerated cooling within the partial stability zone one should expect l~Jli-values of <0.75. The Isx~/l~ ratios of the investigated samples range from 0.58 to 0.85.
3. Geological application The validity of the concept presented so far needs to be checked against geological evidence. This is done for the various regions from which the samples originate. The Fish Canyon Tuff apatite is widely used as an age standard in fission-track dating (Naeser et al., 1981; Hurford and Hammerschmidt, 1985). Because it occurs in volcanic tephra, it presumably cooled rather quickly. The fission tracks likely became stable in this apatite soon after the volcanic event which, in any case, is one prerequisite for its use as an age standard. In two apatite samples the cs/ci ratio was determined as 0.96 and 0.92. From track counting statistics the l a precision of these values is considered to be +8%, that is, they are not significantly different from unity. This is in accordance with the presumption of a very fast
151
cooling immediately after the tephra deposition such as type A in Fig. 1. These findings support the suitability of the Fish Canyon Tuff apatite as an age standard in fission-track dating. However, they do not exclude the possibility that some track fading may have occurred as, according to P. Van den haute (pers. commun., 1987), is indicated by a mean track length ratio in FCT-3 apatite, namely 0.9[4 for confined tracks, and 0.938 and 0.916 for projected tracks on prism and basal faces, respectively. In the present study, a length ratio of 9.993 and 0.975 for the two apatite samples (argitrary orientation) was observed. The important point in this context is that the cJci-value~ for two Fish Canyon Tuff samples are the largest so far found of all investigated apatite samples (Table I) and that their respective cooling ages to 100°C (28.1+0.9 and 26.7+0.8 Ma) and to 60°C (26.9 + 2.2 and 24.7 + 2.0 Ma) agree within their statistical error limits. This evidence supports the concept introduced here. Most of the apatites studied originate from the Hercynian crystalline basement of the Oberpfalz area in northeast Bavaria which was selected as the target area (near Windisch- Eschenbach) for the deep continental drilling project KTB ("Kontinentale T=ef-Bohrung") (see Fig. 2). The track length analyses of these apatites are listed in Table I. The most notable result is that samples from the same region, but not necessarily from the same r~ck type, possess similar measured fission-track ages (tin) as well as similar cJci ratios and, thus, similar t~ ages. Consequently, samples from defined regions show the similar cooling ages to 100 ° and 60 ° C. However, from region to region the cooling pattern changes considerably. This is illustrated in Fig. 6 where samples from each region are combined into one cooling curve. Without going into geological details, a few remarks seem appropriate here. Some of the cooling trends revealed by the track length analysis had been presumed in the past on the basis of geological and geomorphological considerations (SchrSder, 1976; Louis, 1984) such
350
300
250
i
t
'°L ~
Age (Ma) 200 150
i
100
50
0
(
60
~I00 1
2
3
45678
Fig. 6. Thermo-tectonic evolution of various regions of the Oberpfalz based on fission-track apatite analysis. Curves: 1 = No. 76; 2 = No. 101; 3 = Nos. 74 and 102; 4 = Nos. 72 and 73; 5=No. 108; 6=No. 107; 7 = N o s . 80, 88, 90, 96, 98 and 99; 8 = Nos. 84 and 85.
as the early and slow uplift of the Erbendorf-Vohenstrauss zone (lines 2 and 3 in Fig. 6), the rapid Late Cretaceous and Palaeogene uplift with subsequent slowing down of the Milnchberger Gneismasse and Fichtelgebirge (lines 5 and 6, respectively, in Fig. 6), and the most recent, strong uplift of the Steinwald (line 8 in Fig. 6). These trends seem to hold true also for the cooling within the partial stability zone, as is evident from the l~rJli ratios in Table I. For the two apatites from the Odenwald a distinctly different cooling pattern is revealed for the northern part in contrast to the sample from the Heidelberg Granite (Fig. 7). This demonstrates the predicted southward movement of the maximum uplift rate during the Late Cretaceous and Tertiary (G.A. Wagner, 1968). The three samples from the Schwarzwald show more or less uniform uplift during the Tertiary (Fig. 7). The fast, recent uplift is related to the break-up of the Rheingraben in the
100
75
Age (Ma) 50
25
0
"10
o =
60
i..--
10o Fig. 7. Thermo-tectonic evolution of Odenwald (Nos. OD4 and OD6) and Schwarzwald (Nos. 22, 51 and 55) Hercynian basement based on fission-track apatite analysis.
152 Age (Na)
30
10
20
-
3
2 o
o v m
10 ~o~
70
_I
E O
boulders do not represent a steady cooling evolution since track accumulation started in their apatites. Because projected track length studies cannot reveal a bimodal distribution, this case should be studied with confined track lengths too.
w
-2
4. Conclusions
110 BM2
BN7
KAW160
-3
Fig. 8. Thermal evolution ofBergell granodiorite (Nos. BM2 and BM7) and Molasse boulders (Nos. KAW 1004 and 1005) based on fission-track apatite analysis. One sample (KAW 160) from the Simplon area is included. Due to rapid uplift the cooling temperatures for t~ and tf here are assumed 10°C higher than for the Hercynian basement samples.
immediate neighbourhood. It is also evident from confined track length measurements on Schwarzwald apatites (Michalski, 1987 ). The length measurements on apatites from two Bergell granodiorites support the fast and uniform uplift (Fig. 8) as deduced by previous studies ( G.A. Wagner et al., 1979 ). The cooling temperatures are assumed to be ~ 10 °C higher, i.e. 110 ° and 70°C, due to fast uplift. It is interesting to note how the topographic elevation effect has an influence on the ratio cJci. For samples with higher elevation (sample BM2 from 2380 m vs. sample BM7 from 1060 m ), but from the same tectonic block, cJcl increases. The length measurements on apatites of two Bergell boulders buried in the Oligocene Molasse sediments gave surprising results. Previously, it was concluded that they reached the surface rather rapidly during the Late Oligocene, eroded, and, after transportation, were deposited in the Molasse sediments where they stayed cool until today (G.A. Wagner et al., 1979). Therefore, a cooling history close to type A (Fig. 1 ) was expected. However, the ratio cJci revealed tf ages around 12-13 Ma for both boulders. This makes re-heating of the Molasse sediments to ~ 60 ° C during Miocene due to burial likely, such as type C in Fig. 1. Obviously, these
For apatites with a steady cooling history below ~ 150°C, the projected track length measurements provide a sensitive diagnostic tool which allows us to distinguish constant from accelerated and decelerated cooling evolution. Furthermore, they allow us to calculate the cooling age to ~ 60 ° C. This method becomes possible since the same tracks are employed for age determination and length measurements. The additional experimental effort is minor because track counting and length measurements are done in one and the same operation. In order to gain optimum information from apatite fission-track analyses for deciphering tectonic and geothermic histories, it is recommended to use the projected track length technique for steady cooling rocks and the confined track length technique for thermally complex evolving rocks or, ideally, to apply both techniques together.
Acknowledgement
I should like to thank Dr. C.W. Naeser, Denver, for supplying the Fish Canyon Tuff apatite and I. Michalski, Heidelberg, for the Schwarzwald apatite samples. I appreciate Dr. P. Van den haute's critical remarks on the manuscript and help in arranging the neutron irradiation and dose calibration in the thermal column at the Thetis reactor, Gent, Belgium. The work was financially supported by the Deutsche Forschungsgemeinschaft.
153
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