Positioning the titanite fission-track partial annealing zone

Chemical Geology 149 Ž1998. 117–125

Positioning the titanite fission-track partial annealing zone D.A. Coyle ) , G.A. Wagner Max-Planck-Institut fur ¨ Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany Received 28 April 1997; accepted 16 April 1998

Abstract As part of the KTB ŽKontinentale Tiefbohrung. programme in Germany, titanite Žsphene. fission-track data were acquired for samples to a depth of 9000 m, where the host rock temperature is ; 2658C. At this depth, age and confined track length information suggest that the partial annealing zone for the fission tracks in titanite was not reached. By reconstructing the thermal history and ‘backstacking’ of an uplifted partial annealing zone, it is suggested that the in situ partial annealing zone is likely to be found in the region 265–3108C. This value is more precise than previous estimates of the titanite partial annealing zone, and it is also higher than previous estimates. Microprobe analysis indicates that the chemical composition of the titanites is rather homogeneous throughout the whole drilled section. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Titanite fission-track; Partial annealing zone; KTB

1. Introduction In 1994, the KTB reached a final depth of 9101 m with an estimated bottom-hole temperature of ; 2658C and average thermal gradient of 27.58C kmy1 ŽBurkhardt et al., 1989.. As such, it offers the unique opportunity to evaluate the in situ behaviour of thermochronometers thought to have closure temperatures below this temperature, most importantly, fission tracks in zircon and titanite Žs sphene, CaTiSiO5 .. With the exception of that of fission tracks in apatite, the closure temperatures of all other radiometric clocks are estimates, based either upon extrapolations from laboratory experiments, or interpolations between the estimated closure temperatures of other systems ŽWagner and Van den haute, 1992..

)

Corresponding author.

The high in situ temperatures reached in the KTB borehole, together with the fact that the site was thermally stable at timescales of geologic interest, means that new, measured, constraints can be placed upon the closure temperatures of radiometric dating systems that are, or were thought to be, unstable within the range of temperatures observed in the KTB.

2. The KTB project The KTB drill-hole is situated on the western margin of the Bohemian Massif ŽHejl et al., 1997; Wagner et al., 1997.. There are in fact two drill holes, the initial pilot hole ŽVorbohrung, VB., completely cored to a depth of 4001 m; and the main hole ŽHauptbohrung, HB., only 200 m from the VB,

0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 0 4 1 - 2

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D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

drilled to a final depth of 9101 m, only partially cored. The area surrounding the KTB consists of three main tectono-metamorphic units: the Saxothuringikum, the Moldanubikum, and the Zone of Erbendorf–Vohenstrauss ŽZEV. in which the KTB is located. Each of them is characterized by its own unique metamorphic history. During the Variscan orogeny, the Moldanubikum was thrusted northward onto the Saxothuringikum. After their collision, both units were affected by a regional hightemperaturerlow-pressure metamorphism at about 320 Ma. The ZEV complex—not affected by the latter metamorphic event—underwent a mediumpressure metamorphism in the Devonian. This complex covers the Saxothuringian–Moldanubian suture and is predominantly composed of metapelites and amphibolites. After the low-pressure metamorphism post-tectonic granites intruded the region between 320 and 290 Ma. The Falkenberg granite which is situated a few kilometers east of the KTB drill site has a crystallization age of 311 " 8 Ma ŽWendt et al., 1986.. About 5 km west of the KTB location, the crystalline basement of the Bohemian Massif is separated from the South German sedimentary basin by the Franconian Line. This NW–SE striking fault system dips 45–508 towards the NE, underneath the basement. It appears as seismic reflector SE1 and intersects the KTB drill hole as a cataclastic zone at ; 7000 m depth. The Permian to Cretaceous sediments west of the Franconian Line are between 1000 and 3000 m thick. Since the latest Cretaceous, the sediments have undergone denudation. The regional distribution and polymict character of coarse-sized clastic sediments reveals strong uplift and denudation of the western Bohemian Massif during the Early Permian, the Triassic and the Upper Cretaceous ŽSchroder, 1987.. Miocene basaltic extrusions ¨ and eruption channels are very common in this area. Through its entire depth, the KTB-HB penetrates only rocks of the ZEV, of which the amphibolites contain abundant titanite and apatite.

3. Titanite annealing To be useful as a chronometer, the stability of a radiometric system under steady-state conditions

must be known. For fission tracks, it has been demonstrated that the primary effect in the geological environment that will significantly affect the tracks is temperature ŽFleischer et al., 1975; Wagner and Van den haute, 1992.. Fission-track zero-ages of titanite have not been observed, where the samples have been in a stable thermal regime for at least the past several millions of years. Because fission tracks in titanite have not been observed annealing in situ, the temperature stability of the fission tracks has had to be estimated, in one of two ways: through comparisons with radiometric ages derived from other systems, and by extrapolation from laboratory heating experiments. For this discussion, it is assumed that qualitatively the kinetics of fission-track annealing in titanite resemble those of apatite. Most important is the assumption that there is a zone of partial stability, within which the fission tracks decrease in length with increasing temperature. From numerous in situ observations, including the KTB, this ‘partial annealing zone’ ŽPAZ. in apatite is observed to span ; 608C, with the onset of annealing at ; 608C and complete annealing of the tracks at ; 1208C ŽGleadow and Duddy, 1981; Wagner et al., 1994.. Extrapolating experimental titanite annealing data suggests temperatures of 340–4308C for a 50% reduction in track density Žcompiled by Wagner and Van den haute, 1992.. If the titanite PAZ were of the same magnitude as that of apatite, this would suggest that fission tracks in titanite begin to anneal at temperatures of not less than 3108C. Such high values have been discounted because they are significantly higher than the estimated closure temperatures for other radiometric dating systems, notably RbrSr and KrAr of biotite. Samples dated with these techniques give ages that are always equal to or greater than titanite fission-track ages Že.g., Kohn et al., 1993., and thus, the titanite fission-track system is required to have a lower closure temperature. There is, of course, also uncertainty inherent in extrapolating kinetics-data by many orders of magnitude from laboratory timescales Ž10 2 –10 5 s. to geologic ones Ž10 13 s.. In this regard, interpolating between the temperatures of systems with well-known closure temperatures seems the most effective way of deducing the position of the titanite PAZ. Several studies estimate closure temperatures for fission tracks in titanite to

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125 Table 1 Geologic estimates of the closure temperature of the titanite fission-track system Temperature Ž8C.

References

260"20 240"40 250"50 F 300 275"25

Gleadow and Lovering, 1978 Harrison et al., 1979 Gleadow and Brooks, 1979 Fitzgerald and Gleadow, 1988 Kohn et al., 1993

119

used to determine if a sample is sufficiently etched was that the terminations of confined tracks must be distinctly observable. No minimum etch pit diameter was used as criterion. The external detector technique was employed for measuring the induced track densities. Irradiations were carried out at the Risø reactor ŽRoskilde, Denmark. in the graphite reflector facility with its highly thermalized neutrons. All the samples were measured at 2500-fold magnification with dry objectives and a combination of transmitted and incident light. The lengths of confined tracks were digitally measured using a drawing tuberdigitizing tablet in the Stanford configuration. All ages are calibrated against standards of known age, applying the ‘zeta’ calibration ŽHurford, 1990.. As age standards, the Fish Canyon Tuff ŽFCT. and the Mt. Dromedary ŽMTD. titanites were used and the Corning glass CN1 was used for comparing the thermal neutron fluences ŽTable 2..

be in the region of 2508C ŽTable 1.. The striking feature of past estimates of the closure temperature is that most of them span a very large temperature range up to 1008C. It is worth mentioning that these estimates of a nominal closure temperature are not the same as estimates of the position and magnitude of the partial annealing zone.

4. Experimental 5. Data

Titanites from the amphibolite cuttings recovered from the depths between ranging 510–8700 m in the KTB-HB were analyzed with the fission-track method. The titanite separates Ž100–200 m m. were obtained using conventional heavy-liquid and magnetic separating procedures. The mineral grains were mounted in epoxy and afterwards ground and polished to expose internal surfaces. The tracks were revealed by etching in an acid mixture of 1 part conc. HF, 2 parts conc. HNO 3 , 3 parts conc. HCl and 6 parts H 2 O at room temperature ŽNaeser and McKee, 1970.. Etching times varied for different samples from 20 to 30 min. The primary criterion

Twenty-two titanite samples have been analyzed from the KTB-HB. The fission-track densities and ages are given in Table 3. The results of confined track length measurements are summarized in Table 4. The titanite fission-track ages are shown plotted as a function of sample depth in Fig. 1, along with typical radial plots of single-grain ages. The radial plots and the x 2 values in Table 3, with the exceptions of samples HC7368, HC8700 and HC9000, are consistent with the assumption that the titanite grains in each sample are derived from a single population.

Table 2 Zeta values used for calibration of fission-track ages Ž rs s density w10 6 cmy2 x, Ns s number of spontaneous tracks; r i s density w10 6 cmy2 x, Ni s number of induced tracks in external detector; rd s density w10 6 cmy2 x, Nd s number of induced tracks in dosimeter; zeta value in wa cm2 x; x 2 value in w%x. Sample

Grains

rs

Ns

ri

Ni

x2

rd

Nd

zeta

FCT1 FCT2 FCT3 MTD1 MTD2

20 20 20 10 11

6.922 7.508 6.923 55.53 52.14

1231 1279 1277 2099 1235

15.36 15.78 16.27 31.46 30.78

2732 2688 3002 1189 729

39 9.3 70 0.1 16

0.942 0.942 0.992 0.942 0.942

4747 4747 4652 4747 4747

128.4 " 4.8 124.4 " 4.2 132.0 " 4.9 119.9 " 7.6 124.7 " 6.1

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

120

Table 3 Titanite fission-track data and ages for samples from the KTB-HB Sample

Depth wmx

Grains

rs

Ns

ri

Ni

x2

rd

Nd

Age " 1 s wMax

HC510b HDSP234 HDSP237 HC2650 HDSP238 HDSP241b HF4350 HF4500 HO10-B9 HC5573 HO27-D2 HO29 HC7368 HC7454 HC7688b HC7850 HC8250 HC8402 HC8550 HC8700 HC8970 HC9000

510 1391 2470 2650 2700 3900 4350 4500 4821 5573 6357 6540 7368 7454 7688 7850 8250 8402 8550 8700 8970 9000

13 12 13 16 14 13 13 13 11 15 10 8 18 17 20 5 20 10 10 20 7 10

2.76 2.76 4.03 3.35 3.74 1.98 2.03 1.47 1.42 0.489 1.15 1.10 1.53 1.47 1.82 2.56 1.96 1.28 1.11 1.96 1.93 3.00

1279 836 1623 2174 1777 786 1052 993 240 466 548 152 1025 762 1071 255 1635 538 452 1241 366 662

0.698 0.663 1.111 0.847 0.948 0.500 0.676 0.407 0.644 0.280 0.701 0.579 0.431 0.560 0.729 0.782 0.697 0.553 0.500 1.027 0.776 1.38

323 201 448 549 450 198 351 275 109 267 335 80 289 290 430 78 582 232 204 649 147 305

88 18 9.5 13 12 51 6.7 38 23 12 86 55 0.29 29 39 64 5.3 12 7.1 0 43 2.2

0.992 0.942 0.942 0.992 0.942 0.942 0.954 0.954 0.942 0.992 0.954 0.942 0.992 0.992 0.992 0.954 0.954 0.954 0.954 0.954 0.865 0.865

4652 4747 4747 4652 4747 4747 4808 4808 4747 4652 4808 4747 4652 4652 4652 4808 4808 4808 4808 4808 4358 4358

243 " 16 243 " 19 212 " 12 243 " 12 231 " 13 232 " 19 178 " 11 214 " 15 130 " 15 108 " 8.5 97.8 " 6.9 112 " 16 212 " 20 163 " 11 154 " 9.1 194 " 25 167 " 8.4 138 " 11 132 " 11 122 " 10 135 " 13 120 " 12

Ages are pooled ages, except for samples with x 2 F 5.0%, which are central ages Ž rs s density w10 6 cmy2 x, Ns s number of spontaneous tracks; r i s density w10 6 cmy2 x, Ni s number of induced tracks in external detector; rd s density w10 6 cmy2 x, Nd s number of induced tracks in dosimeter; x 2 value in w%x..

The radial plots of samples HC8700 and HC9000 could indicate that the titanite grains in these samples belong to two distinct age groups. The radial

plot of sample HC7368 shows a wider than expected dispersion of the single-grain ages, without indicating the presence of sub-populations of distinct ages.

Table 4 Mean confined track lengths for titanites of the KTB-HB Žfossil tracks. and of the standards Mt. Dromedary and Fish Canyon Žinduced tracks.

6. Interpretation

Sample

Number of tracks

Mean length w m mx

Standard deviation w m mx

HC510 HDSP237 HDSP238 HDSP241 HO29 HC7454 HC7688 HC8250 HC8700 HC8970 HC9000 Induced

14 51 48 9 1 13 21 3 12 32 55 100

11.3 12.1 12.5 12.5 14.7 10.6 11.2 11.9 11.1 10.5 10.4 12.8

2.6 2.2 0.9 1.1 y 2.5 0.9 0.6 1.5 1.1 1.5 0.8

The profile of apparent titanite fission-track ages vs. depth reveals two episodes of significant cooling of the KTB rocks. The first, as evidenced by the invariant ages between the surface and 4000 m, represents an episode of cooling as a result of denudation at ; 245 Ma ago, during the Triassic. Evidence that the cooling is closely coupled with denudation and with not other effects is provided by extensive Triassic alluvial fans which developed in the adjacent sedimentary basin, abutting the Franconian Line ŽSchroder, 1987.. That the ages are invari¨ ant over 4000 m is not evidence that a coherent 4000 m column of rock cooled through the titanite PAZ, rather that there was significant late- to post-Creta-

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

121

Fig. 1. Plot of apparent fission track age vs. depth for the KTB-HB. Radial plots of single grain age data for several samples are shown to the right. All radial plots use the log transformation, and are plotted to a common scale. Error bars are 1 s . The position of the Franconian Line ŽSE1 reflector. is shown. Note the uplifted fossil Partial Annealing Zone ŽPAZ. in the depth range ; 4000–5500 m.

ceous stacking of this portion of the KTB through reverse faulting to achieve the measured 4000 m ŽDuyster et al., 1995; Wagner et al., 1997.. A second major cooling event occurred in the Cretaceous, as evidenced by another section of invariant ages be-

tween ; 5500 and 7000 m. As with the Triassic cooling event, there is also stratigraphic evidence that significant denudation of the ZEV occurred at this time ŽSchroder, 1987.. This event, which is also ¨ revealed by the apatite fission-track data of the KTB-

122

Sample

CaO

TiO 2

V2 O 3

MnO

FeO

NiO

SiO 2

Al 2 O 3

MgO

Total

HC510 HF4500 HC8402 HC8700 HC9000 FCT MTD

28.97"0.22 29.10"0.32 29.04"0.22 29.11"0.19 29.47"0.23 27.62"0.24 27.69"0.37

38.26"0.41 37.74"0.72 37.71"0.54 38.03"0.46 38.08"0.54 38.61"0.81 37.93"0.45

0.12"0.07 0.05"0.03 0.02"0.03 0.04"0.04 0.05"0.05 0.00"0.00 0.00"0.00

0.04"0.04 0.04"0.03 0.03"0.01 0.06"0.02 0.02"0.02 0.23"0.03 0.22"0.02

0.24"0.05 0.54"0.12 0.49"0.20 0.27"0.09 0.34"0.11 1.87"0.14 2.40"0.18

0.02"0.02 0.03"0.01 0.03"0.02 0.02"0.02 0.01"0.01 0.01"0.01 0.01"0.01

30.49"0.36 29.91"0.52 30.45"0.55 30.31"0.46 30.53"0.27 29.76"0.44 29.53"0.63

1.32"0.27 1.45"0.37 1.49"1.5 1.62"0.23 1.47"0.23 1.37"0.07 1.54"0.16

0.01"0.01 0.04"0.03 0.01"0.01 0.01"0.01 0.01"0.01 0.05"0.01 0.07"0.01

99.46 98.86 99.26 99.46 99.97 99.44 99.32

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

Table 5 Microprobe analysis of titanites Žin wt.%, "1 s of 15 grains.

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

VB ŽCoyle et al., 1997., is probably linked to the onset of compression in the Alps to the south. The progressive decrease in ages between 4000 and 5500 m is interpreted to be a post-Triassic PAZ uplifted during the upper Cretaceous. This is evidence that there was little, if any, denudation of the KTB site between the Triassic and the Cretaceous. At 7000 m, there is a major discontinuity, where the ages jump from Cretaceous to Triassic again. This jump, across the SE1 reflector ŽFranconian Line., records the magnitude of thrusting across this fault, and indicates a relative vertical displacement of the adjacent crustal blocks by ; 3000 m. Such a simple interpretation does not exist for the data below 7000 m. However, for estimating the position of the PAZ, only the deepest samples in this section are of immediate importance. From these, the age, recording still the Cretaceous cooling event, and length data Ž11.1 m m mean length. at 8700 m and ; 2508C suggest that this sample has not yet intersected the PAZ. The steady decrease in ages below 7000 m does not represent a present-day PAZ because the deepest sample still has a relatively narrow length distribution that exhibits little shortening. Had this portion not been uplifted, and these samples are currently annealing, then the confined tracks at 8700 m would be expected to have significantly shorter lengths Žassuming annealing since ; 240 Ma ago. than that of induced fission tracks Ž12.8 m m.. Instead, we interpret the steadily decreasing ages below 8000 m to be a second uplifted PAZ, cooled at the same time as the one seen at 4000–5500 m. The section between 7000 and 8000 m has most likely been disturbed by faulting, and may represent yet another portion of the paleo-PAZ. The young component of age in the deepest sample is interpreted to date the time of cooling of the lower PAZ, and this is contemporaneous with the time of cooling of the upper fossil PAZ, evidenced by the section of similar Cretaceous ages between 5500 and 7000 m. The sample at 8700 m must have been at a position near the bottom of the PAZ, but still within it, for there to have been other grains whose ages were not completely overprinted. This effect is occasionally observed in apatite, and is usually attributed to compositional variations between grains ŽGreen et al., 1986.. That there are titanites of different genesis in the KTB amphibolites is known ŽKontny et al., 1993..

123

In order to investigate to which extent the age variation of the samples as well as the age dispersion of the single grains might reflect the variation in chemical composition among the samples and single grains, quantitative electron microprobe analysis was performed on five selected KTB titanite samples and both age standards. The results ŽTable 5. reveal that all the analyzed KTB samples exhibit a remarkable homogeneous composition. Also, the chemical variation among single grains from the same rock samples is rather small. Therefore, it seems unlikely that the chemical composition effects the observed variation and dispersion of the titanite fission-track ages. Consequently, the observed age dispersion in the samples HC7368, HC8700 and HC9000 remains presently an open problem. Furthermore, the KTB titanites differ chemically only slightly from the age standards by somewhat higher calcium and lower manganese and iron contents. This means that the thermal stability characteristics of fission tracks in titanite derived in this study may be applicable to a wider context than the KTB region.

7. The titanite PAZ There are two sources of data concerning the position of the titanite partial annealing zone ŽPAZ.. Firstly, the data from the deepest KTB samples tell us that the PAZ must begin at temperatures greater than or equal to ; 2658C. The titanite samples from 8700, 8970 and 9000 m Žcorresponding to ; 2658C. depth still show relatively narrow length distributions of fossil fission tracks that exhibit little shortening. Their fission-track ages still record the Cretaceous cooling event Ž; 125 Ma.. This suggests that these samples have not yet intersected the PAZ. Secondly, since the in situ PAZ has not yet been directly encountered by the KTB drill-hole, its position is estimated by ‘backstacking’ the uplifted fossil PAZ observed at 4–5.5 km ŽCoyle and Wagner, 1995.. This is possible because the Post-Carboniferous thermal history of the crustal slab, penetrated by KTB, is now reasonably well-known. Details of the thermal history reconstruction were presented elsewhere ŽWagner et al., 1997.. Very briefly, 300 Ma ago the KTB-rock-column of 9 km vertical extent still was confined to a layer of only 2 km thickness

124

D.A. Coyle, G.A. Wagnerr Chemical Geology 149 (1998) 117–125

at 10–12 km depth. This is concluded from the absence of metamorphic as well as geochronometric ŽK–Ar muscovite, amphibole. gradients with respect to the pre-Carboniferous record. Repeated events of reverse faulting led to crustal stacking so that this layer was thickened to the present amount. Major episodes of such fault movements occurred during the Triassic and the Cretaceous. In this T-t-reconstruction, the 4–5.5 km depth section of the KTB column became uplifted 250 Ma ago from a depth of complete fission-track annealing in titanite to a temperature zone Ž) 2658C. in which the tracks underwent partial annealing, i.e., the PAZ. Assuming the present geothermal gradient, this corresponds to an uplift from a depth below 10.5 km to a level below 9 km. The section stayed in this temperature zone for ; 150 Ma. During Cretaceous uplift, this section reached its present depth of 4–5.5 km where no further track annealing occurred. If the width of the uplifted fossil PAZ Ž1.5 km, corresponding to 458C. is assumed to be representative, then the position of the in situ PAZ, estimated this way, is from 265– 3108C. This is not a perfect solution, and there are several limitations to this: Ža. The top of the PAZ is not less than 2508C. Žb. If there is more section stacking above 4000 m than accounted for in our model, then the PAZ could be at higher temperatures. Žc. The bottom position of the uplifted PAZ is not well-constrained, and so the PAZ could be less than 458C wide, shifting the temperature of total annealing to lower values. The likelihood and extent of these effects occurring are unknown, and can potentially cancel each other out. Although as more data become available and the estimate becomes more precise, we think that the values of 265–3108C are unlikely to vary by more than 5–108C, at least for titanite of similar chemical composition as in this study. This represents a useful working estimate of the position of the titanite PAZ, and a measurable improvement over past estimates.

Acknowledgements The authors thank Joachim Janicke and Prof. Dr. Ahmed El Goresy Žboth MPI Heidelberg. for the microprobe analysis. Dr. Raymond Jonkheere, Dr.

Ulrich Glasmacher Žboth MPI Heidelberg. and Dr. Diane Seward ŽETH Zurich ¨ . suggested useful improvements to the manuscript. The manuscript was further improved by both referees, Dr. Paul Andriessen and Dr. Tony Hurford. The financial support from the Deutsche Forschungsgemeinschaft Žproject Wa287-8. is acknowledged.

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