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ESR dating of quartz phenocrysts in some rhyolitic extrusive rocks using Al and Ti impurity centres
Quaternary Science Reviews 18 (1999) 1507}1514
ESR dating of quartz phenocrysts in some rhyolitic extrusive rocks using Al and Ti impurity centres Mark T. Wild, Brian J. Tabner*, Ray Macdonald Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK
Abstract ESR has been employed to determine the eruption date of rhyolitic rocks from three di!erent tectonic settings. Samples from the Olkaria complex, Kenya rift valley, yielded ages of 31$3 and 38$5 ka (148a) and 23$4 and 34$8 ka (570) from the Al and Ti centres, respectively. These compare with an inferred formation age of 65$12 ka (148a), obtained from U-series isochrons, and an eruption age of 5.7}9.7 ka (570), based on 14C and lake stands. Glass (melt) inclusions in sample 570 may have distorted the ESR age. The Youngest Toba Tu! (YTT), Sumatra, gave an average ESR eruption age of 81$17 ka which compares well with K/Ar and 40Ar/39Ar ages in the range 73}75 ka. However, there is evidence of thermal annealing in the Oldest (OTT) and Middle (MTT) Toba Tu!s by the more recent YTT eruption, with mean ages of ca. 349 ka and 326 ka, respectively, signi"cantly below the K/Ar age of 840$30 ka (OTT) and the 40Ar/39Ar age of 501$5 ka (MTT). A sample from the Battleship Rock Tu!, Jemez Volcanic Field, New Mexico, gave a mean ESR age of ca. 247 ka which contrasts markedly with a previously determined ESR age of 59$6 ka, but compares more favourably with previous K/Ar ages of 278$52, 130$70 and 180$70 ka. Subsequent geothermal activity and high ambient temperatures in this area may be responsible for these apparent discrepancies. ( 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron spin resonance (ESR) spectroscopy was "rst suggested as an alternative to thermoluminescence for dating in the Earth sciences in 1967 (Zeller et al., 1967) but it was some years before Ikeya (1975) investigated and dated a speleothem from the Akiyoshi Cave in Japan. Applications of ESR to materials of di!ering geological signi"cance have been adequately reviewed (see, for example, GruK n, 1989; Ikeya, 1994). The ESR method relies upon the sample acting as a dosimeter of itself and of its immediate surroundings, either since its formation or since the last occasion it was &reset'. Minerals such as quartz are suitable materials for ESR dating as they contain several impurity centres (such as Al, Ge and Ti) as well as a defect centre (E@). A review of these impurity centres and the information that can be obtained from their ESR spectra has been presented by Weil (1984). They are sensitive to arti"cial c-radiation as well as natural radiation. Application of the additive dose method (GruK n, 1989) allows the &accumulated' dose
* Corresponding author. Tel.: 01524-65201; fax: 01524 593985. E-mail address: [email protected] (B.J. Tabner)
responsible for the &natural' signal intensity to be determined. The natural signal intensity results from bombardment by a, b- and c-radiation from K, U and Th in the surrounding rock and from U and Th within the quartz. Chemical analysis, therefore, allows calculation of the annual dose. There are, however, a number of potential problems. For example, Toyoda and Ikeya (1991) and Buhay et al. (1992) have suggested that errors may be related to the stability of the defect centres. Thus, in a comparative study of the OHC with the Ge and Al centres, Shimokawa and Imai (1987) determined di!ering stabilities of the various centres to thermal annealing. The Al and Ti centres are thought to be reasonably stable. However, Buhay et al. (1992) point out that underestimations of age might occur because these centres have di!erent &blocking' temperatures, i.e., the temperature at which an accumulation of the defect centre signal can start. Apparent eruption ages di!ering signi"cantly from true ages might then result from volcanic rocks which have cooled relatively slowly, e.g. the central facies of thick ignimbrite sheets or lava domes. Little is known about the blocking temperatures of either the Al or Ti centres, although that for the former has been estimated at 50}603C (Shimokawa & Imai, 1987).
0277-3791/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 4 1 - 4
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Shimokawa et al., (1988) and Imai and Shimokawa (1988) have attributed di!erences between ESR ages and those determined by other methods to an incomplete knowledge of geological and other variables, such as water content of the rocks, cosmic dose, b attenuation and the precise value of the a attenuation factor.
2. Samples The analysed rocks were chosen for the following reasons. The Battleship Rock material provides a good opportunity to re-examine a unit already dated by ESR (Toyoda et al., 1995) and by "ssion track and K/Ar methods (Self & Wol!, 1988). The Kenyan samples were chosen to test the ESR method for very young volcanic rocks, in the age range ((20 ka) where a viable technique would be particularly useful. Furthermore, we selected a glassy and devitri"ed sample to test whether post-eruptive crystallization could a!ect ESR dates. The Toba Tu! samples were chosen for this study for three reasons: (i) They have been well dated by conventional techniques; (ii) The quartz phenocrysts are relatively abundant and large (up to 2 cm, mean diameter ca. 10 mm). They have been highly fractured (during or after eruption); (iii) The three members of the Toba Tu! give a range of ages which might help to establish the overall) range of applicability of the ESR method. 2.1. Battleship rock samples The Battleship Rock material is from the Jemez volcanic "eld, Rio Grande rift valley, New Mexico. Activity in the Jemez volcanic "eld started around 13 Ma ago (Gardner et al., 1986). Eruption of the Otowi and Tshirege members of the Bandelier Tu! accompanied formation of the Toledo and Valles calderas, at 1.50 and 1.13 Ma, respectively (Spell et al., 1990). Rhyolitic domes erupted from the ring fracture zone following the formation of the Valles caldera. The youngest of these postcaldera units consists of three members, one of which, the battleship rock member (BRM), consists of non-welded to densely welded ash #ow deposits. The age of the Battleship Rock Tu! has recently been determined by ESR (Toyoda et al., 1995) and reported as 59$6 ka. This date contrasts markedly with those reported by Go! et al., 1989 by K/Ar on sanidine of 278$52 ka and those of Miyaji et al. (1985) on zircon of between 130$70 and 180$70 ka. Previous age determinations of this unit are, therefore, confusing and we hoped our examination would help to clarify these apparent contradictions. Our sample comes from the moderately welded, noncrystalline facies of the BRM, from Battleship rock itself, (35352.1@N 106338.5@W) and thus slightly further north than sample BSR04 of Toyoda et al. (1995). Pumice
fragments in the rock are slightly #attened and the rock has a crude foliation. The matrix is brownish, a result of iron oxides disseminated in the sanidine, with smaller amounts of plagioclase, hornblende and Fe}Ti oxides. Quartz phenocrysts are 0.25}5 mm in size (mean diameter ca. 2.5 mm) and were slightly fractured, presumably during eruption. The Battleship rock rhyolite is metaluminous, mildly potassic (Na O/K O"0.95) and 2 2 moderately calcic (CaO"1.63%). 2.2. Olkaria samples The Olkaria (Naivasha) volcanic complex occupies some 500 km2 near Lake Naivasha in the south-central Kenyan rift valley. The complex consists dominantly of rhyolitic domes, lava #ows and fall and #ow pyroclastic deposits, erupted from at least 80 small centres, possibly over the past 20 ka or so (Macdonald et al., 1987; Clarke et al., 1990). Sample 148a is from the crystalline interior of the Mulla pumiceous dome on the western edge of the complex. It is a member of the oldest group of rhyolites in the complex, Group 1 of Macdonald et al. (1987). The eruptive age of Group 1 is rather poorly constrained by a "ssion track age of 17 ka Bailey and Macdonald, unpublished data). Using the U-series disequilibrium method, Black (1994) obtained a whole rock isochron of 65$12 ka for Group 1, which may represent the time of formation of the relevant magmas. The phenocryst assemblage in 148a is quartz (1.8% modally), sanidine (3.2%), quartz-sanidine intergrowths (0.7 D%), titanomagnetite (0.1%) and zircon (0.1%). The rounded quartz phenocrysts are up to 2 mm across (mean diameter 1.0 mm) and contain rare glass (melt) inclusions and small crystals of alkali feldspar. The crystalline matrix of the rock is spherulitic, the texture representing relatively slow cooling within the lava dome after extrusion. Compositionally 148a is a high-silica, mildly peralkaline (comenditic) rhyolite, with relatively low abundances of CaO and FeOH. Sample 570 is from the glassy carapace of a lava #ow within a younger eruptive sequence at the Gorge farm centre and appears to have erupted between 9.7 and 5.7 ka ago on the basis of 14C dates and Lake Naivasha high- and low-stands (Clarke et al., 1990). Black et al., (1997) presented a U-series isochron for this rock which gave an age of 14.6$2.2 ka. This is thought to date the time of phenocryst crystallisation; the residence time of the magma, de"ned as the internal isochron age minus the eruptive age, is thus in the range 3}11 ka. The phenocryst assemblage in 570 is quartz#sanidine# fayalitic olivine#titanomagnetite#amphibole# aenigmatite. The matrix is non-hydrated glass. Quartz phenocrysts are (1.5 mm across (mean diameter 0.77 mm), rounded through resorption, and enclose very rare glass (melt) inclusions. The rock is a
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
moderately peralkaline (comenditic) rhyolite, low in Al O and CaO but relatively Fe-rich. 2 3 2.3. Toba Tuw Samples Measuring 100]300 km, the Toba caldera in northern Sumatra, Indonesia, is the largest known Quaternary caldera (Fig. 1). The complex abuts the Sumatran Fault,
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which is a result of oblique subduction of the Indian}Australian plate under the Sundaland (Southeast Asian) plate along the Sunda Trench. The Sundaland plate is composed of continental lithosphere (Chesner & Rose, 1991). Four ash #ow tu!s have been erupted from the system in the past 1.2 Ma (Chesner & Rose, 1991), of which the last three are collectively known as the Toba tu!s.
Fig. 1. Tectonic setting and sample location maps of the Toba Caldera Complex. The recently updomed areas along the western lake shore are associated with hydrothermal activity. After Chesner and Rose (1991).
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The Oldest Toba Tu! (OTT) appears to have been erupted from the SE part of the present depression, has an estimated volume of 500 km3 (Knight et al., 1986) and is '300 m thick (Chesner & Rose, 1991). The Middle Toba Tu! (MTT) averages 150 m in thickness with an estimated volume of 60 km3 (Chesner & Rose, 1991). The eruptive vent was probably in a similar location to that of the OTT. It overlays the Harranggoal Dacite Tu! and is, itself, overlain by the Youngest Toba Tu! (YTT). The present Toba caldera was associated with eruption of the YTT which has an estimated volume of 2500}3000 km3, making it one of the largest single eruptions in geological history (Chesner & Rose, 1991). The OTT has been previously dated by K/Ar at 0.84$0.03 Ma (Diehl et al., 1987) and the MTT has a previously determined sanidine 40Ar/39Ar age of 0.501$0.005 Ma (Chesner et al., 1991). The age of the YTT has been previously determined employing both K}Ar (73.5$3 and 74.9$12 ka (Ninkovitch et al., 1978) and 40Ar/39Ar [which yielded a weighted mean of 73$4 ka (Chesner et al., 1991)]. The Oldest and Middle Toba Tu! samples were supplied by Dr. C.A. Chesner [Eastern Illinois University (EIU)]. The OTT sample (T27) is from inside the caldera on the Uluan block and has been overlain by the YTT eruption. It is a densely welded vitrophyre and has a brownish-grey to light-grey colour (Chesner & Rose, 1991). It contains phenocrysts of quartz, plagioclase, sanidine, amphibole, Fe}Ti oxides, zircon, allanite, and rare orthopyroxene or fayalite (Chesner, 1998). The MTT sample (T8) is from the caldera walls in the Harranggoal section at the northern end of Lake Toba and is a densely welded vitrophyre. We had six samples of the YTT, three supplied by Dr. C. A. Chesner (T36A3, T57A1 and T94A2) and three by Dr. S. Blake [The Open University; (OU)] (TT9, TT11 and TT78). The deposit is largely unwelded and contains two types of pumice; (i) a crystal-rich variety (30}40 wt% modally) with biotite phenocrysts and SiO between 68 and 72 wt% crystals, 2 and (ii) a variety with 15}20 wt% crystals, poorer in biotite phenocrysts and with SiO between 73 and 76 2 wt%. There is a compositional continuum between the two types (Chesner, 1998). T36A3 was collected at Prapat, near lake level, from the same locality dated by K}Ar at 74 ka (Ninkovich et al ., 1978). T57A1 was collected high on the southern caldera wall, directly above a visible contact with the OTT. T94A2 is from a circular embayment in the caldera wall near Bakara. TT9 and TT11 are from the extensive deposit of #uvially reworked YTT pumice on the outwash of the Asahan River near Porsea (beneath &R' of Porsea in Fig. 1). TT78 was collected near the base of the YTT outcrop in a roadcut near the top of the cli! near Muara.
3. Sample preparation The original rock samples were crushed in a mechanical crusher and passed through a magnetic separator. The larger quartz phenocrysts were hand picked from the remaining material, crushed and those with grain size, after crushing, of 150}212 lm (found to give the most reproducible ESR spectra) were treated with hydrochloric acid, to remove any organic material and carbonates, and then with 40% hydro#uoric acid for 1 h to remove glass, surface particles and the outermost 30}40 lm of each grain to eliminate defects formed by external a-emitters. They were also washed with concentrated nitric acid to remove any residual iron salts. For most of the samples there was su$cient quartz to obtain 10 aliquots, each of 200 mg, for the dating experiments. However, only a limited amount was available for one of the Toba Tu! samples (TT9). Here, in order to extend the number of data points, a second irradiation of each aliquot was undertaken after recording the ESR spectrum following a "rst irradiation.
4. ESR measurements ESR spectra were recorded at 77 K on a Varian E3 spectrometer operating with a magnetic "eld modulation of 100 kHz with an amplitude of 0.1 mT and a microwave power of 5 mW. Occasionally, slightly higher modulation amplitudes were employed (up to 0.63 mT). The latter spectra were found to give more reproducible peak intensities for total accumulated dose (D ) determinaE tions but are associated with a slight loss of spectral resolution. A typical ESR spectrum of the quartz phenocrysts extracted from the Gorge Farm (570) site is illustrated in Fig. 2. The Al centre shows well resolved hyper"ne structure consisting of a packet of 16 lines from low "eld (line no. 1, g"2.017) to high "eld (line no. 16, g"1.991). Previous workers (for example, Yokoyama et al., 1985) have taken the peak-to-peak height from the top of line 1 to the bottom of line 16 as a measure of spectral intensity and we have followed the same procedure. The overall height of the alkali metal cation compensated 1 : 1 : 1 : 1 quartet centred at g"1.932 was used for the Ti centre. This spectrum is typical of those obtained from all of the sites although the relative intensities of the main features di!ered from site to site. A 60Co source (Salford University) was used to irradiate the samples at a dose rate of ca. 0.45 kGy h~1 for a total dose of between 1.19 and 2.40 kGy (varying with sample). The exact dose received by each aliquot was determined by an alanine dosimeter [0.25 g of L-alanine (MTM Research Chemicals)] placed immediately adjacent to each aliquot. The resulting spectral intensities
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
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Fig. 2. ESR spectra of the quartz grains extracted from the lava from the Gorge Farm site recorded at 77 K with a modulation amplitude of 0.1 mT following a c-irradiation dose of 0.377 kGy. The g-values of the main features are indicated along with lines 1 and 16 (associated with the Al impurity centre).
were then compared with those of a dose/ESR intensity calibration obtained from alanine samples irradiated at the National Physical Laboratory (Teddington) covering the range of 0.497}12.00 kGy.
The annual dose, D, for each sample was determined from the U, Th, K concentrations of the samples (Table 2), using the formula given for quartz grains by GruK n (1989). Within this formula, corrections were applied to allow for b-attenuation and a-e$ciency (set at 0.1).
5. Chemical analysis The K O, U and Th concentrations are summarised in 2 Table 1. 6. Determination of accumulated and annual doses The D values were determined by the additive dose E method in which the spectral intensity is plotted against arti"cial dose and the D estimated by extrapolation to E zero spectral intensity. The D values (and their associated errors) were deterE mined from these plots employing the FIT-SIM program (GruK n, unpublished) which uses a simple saturating function for the best-"t and analytical procedures for the estimation of errors (Brumby, 1992).
7. Battleship Rock Dates obtained for the Battleship Rock Tu! are 234$39 ka from the Al centre and 243$58 ka from the Ti centre (Table 2). It is interesting to note that these dates are much older than the ESR determined dates reported recently by Toyoda et al. (1995), namely, 59$6 ka. They are, however, more in line with those reported by Go! et al. (1989) by K/Ar on sanidine of 278$52 ka and those of Miyaji et al. (1985) on zircon of between 130$70 and 180$70 ka. There are signi"cant discrepancies between these various dates. We anticipated that our ESR dates would agree with those obtained by Toyoda et al. (1995) but this is clearly not the case. The possibility that post eruption
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Table 1 Concentrations of (whole rock) radioactive elements in the samples K O (%) 2 Battleship Rock Olkaria Volcanics 148a 570 (quartz)! Toba Tu!s OTT}T27 MTT}8 YTT}T36A3 (quartz) }T57A1 (quartz) }T94A2 }TT9 }TT11 }TT78
U (ppm) Th (ppm)
Reference
4.0
5.4
19.1
4.52 4.45
9.9 32.17 7.11
52.1 109.6 28.86
2 2,3
4.98 4.29 4.63
4.63 5.08 4.63 0.20 2.66 0.2 5.08 5.8 3.75 3.0
23.03 23.6 23.03 0.96 14.63 0.79 23.6 31.9 23.6 21.0
4,5 4,5 5,6 5 5,6 5, 5,6 7 7 7
3.64 4.68 4.89 4.33 3.03
1
!The high U and Th concentrations arise from glass inclusions within the quartz grains. References 1 } Toyoda and Ikeya (1994); 2 } Macdonald et al. (1987); 3 } Black et al., (1997); 4 } Chesner and Rose (1991); 5 } S Black (unpublished data); 6 } Chesner (1998); 7 } Jones (1993).
magmas), and together the dates may indicate that the Group 1 rocks were formed and erupted within the period 65}35 ka ago. The ages obtained for 570 are 23$4 ka and 34$8 ka from the Al and Ti centres respectively (see Table 2). These ages are somewhat higher than the 5.7}9.7 ka eruptive age based on 14C and Lake Naivasha high- and low-stands (Clarke et al., 1990) and the U-series internal isochron age of 14.6$2.2 ka presented by Black et al. (1997). There is a paradox here, in that the U-series age is interpreted as re#ecting the age of phenocryst crystallisation, whilst the ESR age is expected to re#ect a posteruptive age, when the temperature of the lava decreases below the resetting temperature. However, one problem in the ESR dating of 570 arises from the glass melt inclusions within the quartz phenocrysts. In particular the a- and b-attenuation factors required for the internal dose under these circumstances are unclear and their overestimation within the calculation would lead to an overestimation of the eruption age.
9. Toba Tu4, Sumatra thermal activity in this region may be important is discussed in more detail below.
8. Olkaria volcanic complex, Kenya The ages obtained for 148a are 31$3 ka and 38$5 ka from the Al and Ti centres, respectively (Table 2). These ages are reasonably close to the U-series whole-rock isochron for Group 1 rocks (65$12 ka; Black, 1994, which may represent the time of formation of the relevant
The ESR ages, summarised in Table 2, give (Al & Ti) average ages of 0.349 and 0.326 Ma for the OTT (T27) and MTT (T8) samples respectively. These ages are within experimental error of one another and would appear to be signi"cant underestimates of previously determined eruptive ages of 0.84 Ma (K/Ar; Diehl et al., 1987) and 0.501 Ma (40Ar/39Ar; Chesner et al., 1991). The possibility of partial thermal resetting of these samples is discussed further below. Youngest Toba Tu!. The average ages from the EIU and OU samples from the Al centre are 75$21 and
Table 2 ESR dates from Al and Ti impurity centres for the various samples Al-centre D(mGy/a) Battleship Rock Olkaria Volcanics: 148a 570 Toba Tu!s OTT}T27 MTT}T8 YTT}T36A3 }T57A1 }T94A2 }TT9 }TT11 }TT78
3.81!
D (Gy) E
Ti-centre Age(ka)
D (Gy) E
Age(ka)
893$132
234$39
927$201
243$58
9.08 22.45
281$24 517$75
31$3 23$4
344$43 759$170
38$5 34$8
3.52 3.39 3.41 2.35 3.51 4.14 3.22 2.56
1052$113 1098$68 181$9 245$62 242$33 356$50 294$52 176$15
299$35 324$22 53$3 104$17 69$10 86$13 91$18 69$7
1405$299 1109$199 189$28 239$56 295$44 420$70"
399$93 327$65 56$97 102$26 84$14 101$19
206$30
81$13
!Toyoda and Ikeya (1994) give a value of 3.72 due to the use of a marginally di!erent average grain diameter in the calculation. "Data too scattered to provide a reliable D E
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
82$9 ka, respectively, and from the Ti centre are 81$19 and 91$10 ka, respectively. The average age from both centres for all of the samples is 81$17 ka. We consider these various average ages obtained by ESR to be in good agreement with those obtained employing other techniques. For example, K}Ar dates are 73.5$3 and 74.9$12 ka (Ninkovitch et al., 1978) whilst 40Ar/39Ar yields a weighted mean of 73$4 ka (Chesner et al., 1991).
10. Thermal stabilities of the samples In principle, the ESR technique is well suited to the dating of volcanic eruptions since the eruption temperature, commonly exceeding 8003C, is amply su$cient to reset the ESR signals to zero. To test this we raised the temperature of representative aliquots of both Kenyan samples and the Battleship Rock sample to 4003C. In all cases the ESR spectrum completely disappeared almost immediately. Shimokawa et al., (1988) reported similar observations when samples of a sand bed baked by the lava of Ueno basalt and of a weathered basalt baked by the Ikenotaira basalt lava were raised to 3503C. The Al and Ti centres are, therefore, completely zeroed at eruption. Accumulation of the ESR signals will commence once the blocking temperature has been achieved. Shimokawa and Imai (1987) have estimated this to be 50}603C for the Al centre, for example. Riehle (1973) has estimated that a 40 m thick ignimbrite will cool from about 7503C to about 503C in less than 1000 yr. This period is almost insigni"cant compared to the age of the samples reported here. It is very signi"cant, however, if a subsequent eruption overlies the sample of interest as thermal resetting is more than likely. Shimokawa and Imai (1987) have shown that the Al centre will be reset in less than 1 yr if maintained above 1003C and in 1,000-10,000 years if maintained at 50}603C, by subsequent geothermal activity. Obviously, activity at lower temperatures or for shorter periods will result in some partial resetting. Partial thermal resetting of the samples is the likely explanation for the underestimation of the ages of the OTT and MTT eruptions since both have been overlain by the massive YTT eruption. The similarity between the ages of these two samples is probably no more than coincidence. Partial thermal resetting could be a problem with BRM samples where our age is signi"cantly higher (ca. 240 ka) compared to those obtained by ESR previously [59$6 ka (Toyoda et al., 1995)] but is in better agreement with those obtained employing other techniques such as K/Ar (278$52 ka; Go! et al., 1989) and "ssion-track (180$70 and 130$70; Miyaji et al., 1985). The potential for thermal resetting of BRM samples is high. Not only do Toyoda and Ikeya (1994) report that ambient summer temperatures can peak above 503C but
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it is also clear that there has been some subsequent geothermal activity in the area. The present abnormally high geothermal gradients at the Fenton Hill site appear to be of recent origin occurring during the last few tens of thousands of years (Harrison et al., 1986). Clearly this could have a!ected some samples more than others. Although our sample is from the same general locality there is textural (and probably compositional) variation in the rocks and, consequently, they could have di!erent cooling histories. Paradoxically, our sample has been oxidised after eruption, yet the ESR age is roughly similar to the eruptive age determined by other techniques.
11. Summary We have obtained ESR ages on quartz phenocrysts in extrusive volcanic rocks ranging from 23 to 399 ka. It is quite possible that volcanic rocks as young as 5}10 ka could be dated by this method. We are, however, concerned about the maximum age range as our experiments indicate that thermal annealing of samples presents a serious problem. Evidence suggests that samples can be thermally reset completely if maintained at 1003C for less than 1 y or at 50}603C for between 1 and 10 ka (Shimokawa & Imai, 1987). Consequently, high ambient temperatures, subsequent geothermal activity or the overlaying of samples by material from subsequent eruptions are all likely to create problems.
Acknowledgements We would like to thank Drs. Eddie Rhodes (Royal Holloway, University of London) and Christophe Falgueres (Institut de Paleontologie Humaine, Paris) for helpful discussions concerning dating procedures and Dr. Rainer GruK n (Australian National University, Canberra) for allowing us to use copies of his programmes FIT-SIM and AGE. We would also like to thank Mr. David Barraclough for his expert technical assistance with the 60Co facility at the University of Salford and Dr. Stuart Black (Lancaster University) for helpful discussions and for analysis of some of the samples. We especially thank Drs. Craig Chesner (Eastern Illinois University) and Steve Blake (The Open University) for generously making available samples of various Toba Tu!s. Finally, we thank Lancaster University for the award of a University Research Studentship (to M.T.W).
References Black, S. (1994). U}Th disequilibria systematics of the Olkaria complex, Gregory Rift Valley, Kenya. Ph. D. Thesis, University of Lancaster.
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Black, S., Macdonald, R., & Kelly, M. R. (1997). Crustal origin for peralkaline rhyolites from Kenya: Evidence from U-Series disequilibria and Th-isotopes. Journal of Petrology, 38, 277}297. Brumby, S. (1992). Regression-analysis of ESR/TL dose-response data. Nuclear Tracks and Radiation Measurements, 20, 595}599. Buhay, W. M., Cli!ord, P. M., & Schwarcz, H. P. (1992). ESR dating of the Rotoiti Breccia in the Taupo volcanic zone, New Zealand. Quaternary Science Reviews, 11, 267}271. Chesner, C. A. (1998). Petrogenesis of the Toba tu!s, Sumatra, Indonesia. Journal of Petrology, 39, 397}438. Chesner, C. A., & Rose, W. I. (1991). Stratigraphy of the Toba tu!s and the evolution of the Toba caldera complex, Sumatra, Indonesia. Bulletin of Volcanology, 53, 343}356. Chesner, C. A., Rose, W. I., Deino, A., Drake, R., & Westgate, J. A. (1991). Eruptive history of Earth's largest Quaternary caldera (Toba, Indonesia) Clari"ed. Geology, 10, 200}203. Clarke, M. C. G., Woodhall, D. G., Allen, D., & Darling, G. (1990). Geological, volcanological and hydrological controls on the occurrence of geothermal activity in the area surrounding Lake Naivasha, Kenya. Nairobi: Ministry of Energy Report. Diehl, J. F., Onstott, T. C., Chesner, C. A., & Knight, M. D. (1987). No short reversals of Brunhes age recorded in the Toba tu!s, North Sumatra, Indonesia. Geophysical Research Letters, 14, 753}756. Gardner, J. N., Go!, F., Garcia, S., & Hagan, R. C. (1986). Stratigraphic relations and lithologic variations in the Jemez volcanic "eld, New Mexico. Journal of Geophysical Research, 91, 1763}1778. Go!, F., Gardner, J. N., Baldridge, W. S., Hulen, J. B., Nielson, D. L., Vaniman, D., Heiken, G., Dungan, M. A., & Broxton, D. (1989). Volcanic and hydrothermal evolution of Pleistocene Valles caldera and Jemez volcanic "eld. Memoir, New Mexico Bureau of Mines and Mineral Resources, 46, 381}434. GruK n, R. (1989). Electron spin resonance (ESR) dating. Quaternary International, 1, 65}109. Harrison, T. M., Morgan, P., & Blackwell, D. D. (1986). Constraints on the age of heating at the Fenton Hill site, Valles caldera, New Mexico. Journal of Geophysical Research, 91, 1899}1908. Ikeya, M. (1975). Dating a stalactite by electron paramagnetic resonance. Nature, 255, 48}50. Ikeya, M. (1994). ESR (EPR) dating based on natural radiation e!ects. Nuclear Geophysics, 8, 201}224. Imai, N., & Shimokawa, K. (1988). ESR dating of Quaternary tephra from Mt. Osore-zan using Al and Ti centres in quartz. Quaternary Science Reviews, 7, 523}527. Jones, S. (1993). Mechanisms of large silicic magma chamber zonation: The Youngest Toba Tuw, Sumatra pp 277. Ph. D. Thesis, The Open University.
Knight, M. D., Walker, G. P. L., Ellwood, B. B., & Diehl, J. F. (1986). Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tu!s: Constraints on the sources and eruptive styles. Journal of Geophysical Research, 91, 10355}10382. Macdonald, R., Davies, G. R., Bliss, C. M., Leat, P. T., Bailey, D. K., & Smith, R. L. (1987). Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya Rift Valley. Journal of Petrology, 28, 979}1008. Miyaji, M., Izett, G. A., Naeser, C. W., Naeser, N. D., & Andreissen, P. A. M. (1985). Zircon "ssion-track ages on some volcanic rocks and pyroclastic #ow deposits on the Jemez Mountains, New Mexico. Bulletin of the Volcanological Society of Japan, 30, 90. Ninkovich, D., Shackleton, N. J., Abdel-Monem, A. A., Obradovich, J. D., & Izett, G. (1978). K}Ar age of the late Pleistocene eruption of Toba. North Sumatra. Nature, 276, 574}577. Riehle, J. R. (1973). Calculated compaction pro"les of rhyolitic ash #ows. Geological Association of America Bulletin, 84, 2193}2216. Self, S., & Wol!, J. A. (1988). The El Cajete Series, Valles caldera, New Mexico. Journal of Geophysical Research, 93, 6113}6127. Shimokawa, K., & Imai, N. (1987). Simultaneous determination of alteration and eruption ages of volcanic rocks by electron spin resonance. Geochimica et Cosmochimica Acta, 51, 115}119. Shimokawa, K., Imai, N., & Moriyama, A. (1988). ESR dating of volcanic and baked rocks. Quaternary Science Reviews, 7, 529}532. Spell, T. L., Harrison, T. M., & Wol!, J. A. (1990). 40Ar/39 Ar dating of the Bandelier Tu! and San Diego canyon ignimbrites, Jemez Mountains, New Mexico: Temporal constraints on magmatic evolution. Journal of Volcanology and Geothermal Research, 43, 175}193. Toyoda, S., & Ikeya, M. (1991). Thermal stabilities of paramagnetic defect and impurity centres in quartz: Basis for ESR dating of thermal history. Geochemical Journal, 25, 437}445. Toyoda, S., & Ikeya, M. (1994). ESR dating of quartz with stable component of impurity centres. Quaternary Geochronology (Quaternary Science Reviews), 13, 625}628. Toyoda, S., Go!, F., Ikeda, S., & Ikeya, M. (1995). ESR dating of quartz phenocrysts in the El Cajete and Battleship Rock Members of Valles rhyolite, Valles caldera, New Mexico. Journal of Volcanology and Geothermal Research, 67, 29}40. Weil, J. A. (1984). A review of electron spin spectroscopy and its application to the study of paramagnetic defects in crystalline quartz. Physics and Chemistry of Minerals, 10, 149}165. Yokoyama, Y., Falgueres, C., & Quaegebeur, J. P. (1985). ESR dating of quartz from Quaternary sediments: First attempt. Nuclear Tracks and radiation Measurements, Part D, 10, 921}928. Zeller, E. J., Levy, P. W. & Mattern, P. L. (1967). Geologic dating by electron spin resonance. Proceedings of the Symposium on Radioactive Dating and Low Level Counting I.A.E.A., Wien, (pp. 531}540).
ESR dating of quartz phenocrysts in some rhyolitic extrusive rocks using Al and Ti impurity centres Mark T. Wild, Brian J. Tabner*, Ray Macdonald Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK
Abstract ESR has been employed to determine the eruption date of rhyolitic rocks from three di!erent tectonic settings. Samples from the Olkaria complex, Kenya rift valley, yielded ages of 31$3 and 38$5 ka (148a) and 23$4 and 34$8 ka (570) from the Al and Ti centres, respectively. These compare with an inferred formation age of 65$12 ka (148a), obtained from U-series isochrons, and an eruption age of 5.7}9.7 ka (570), based on 14C and lake stands. Glass (melt) inclusions in sample 570 may have distorted the ESR age. The Youngest Toba Tu! (YTT), Sumatra, gave an average ESR eruption age of 81$17 ka which compares well with K/Ar and 40Ar/39Ar ages in the range 73}75 ka. However, there is evidence of thermal annealing in the Oldest (OTT) and Middle (MTT) Toba Tu!s by the more recent YTT eruption, with mean ages of ca. 349 ka and 326 ka, respectively, signi"cantly below the K/Ar age of 840$30 ka (OTT) and the 40Ar/39Ar age of 501$5 ka (MTT). A sample from the Battleship Rock Tu!, Jemez Volcanic Field, New Mexico, gave a mean ESR age of ca. 247 ka which contrasts markedly with a previously determined ESR age of 59$6 ka, but compares more favourably with previous K/Ar ages of 278$52, 130$70 and 180$70 ka. Subsequent geothermal activity and high ambient temperatures in this area may be responsible for these apparent discrepancies. ( 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Electron spin resonance (ESR) spectroscopy was "rst suggested as an alternative to thermoluminescence for dating in the Earth sciences in 1967 (Zeller et al., 1967) but it was some years before Ikeya (1975) investigated and dated a speleothem from the Akiyoshi Cave in Japan. Applications of ESR to materials of di!ering geological signi"cance have been adequately reviewed (see, for example, GruK n, 1989; Ikeya, 1994). The ESR method relies upon the sample acting as a dosimeter of itself and of its immediate surroundings, either since its formation or since the last occasion it was &reset'. Minerals such as quartz are suitable materials for ESR dating as they contain several impurity centres (such as Al, Ge and Ti) as well as a defect centre (E@). A review of these impurity centres and the information that can be obtained from their ESR spectra has been presented by Weil (1984). They are sensitive to arti"cial c-radiation as well as natural radiation. Application of the additive dose method (GruK n, 1989) allows the &accumulated' dose
* Corresponding author. Tel.: 01524-65201; fax: 01524 593985. E-mail address: [email protected] (B.J. Tabner)
responsible for the &natural' signal intensity to be determined. The natural signal intensity results from bombardment by a, b- and c-radiation from K, U and Th in the surrounding rock and from U and Th within the quartz. Chemical analysis, therefore, allows calculation of the annual dose. There are, however, a number of potential problems. For example, Toyoda and Ikeya (1991) and Buhay et al. (1992) have suggested that errors may be related to the stability of the defect centres. Thus, in a comparative study of the OHC with the Ge and Al centres, Shimokawa and Imai (1987) determined di!ering stabilities of the various centres to thermal annealing. The Al and Ti centres are thought to be reasonably stable. However, Buhay et al. (1992) point out that underestimations of age might occur because these centres have di!erent &blocking' temperatures, i.e., the temperature at which an accumulation of the defect centre signal can start. Apparent eruption ages di!ering signi"cantly from true ages might then result from volcanic rocks which have cooled relatively slowly, e.g. the central facies of thick ignimbrite sheets or lava domes. Little is known about the blocking temperatures of either the Al or Ti centres, although that for the former has been estimated at 50}603C (Shimokawa & Imai, 1987).
0277-3791/99/$ - see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 3 7 9 1 ( 9 9 ) 0 0 0 4 1 - 4
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Shimokawa et al., (1988) and Imai and Shimokawa (1988) have attributed di!erences between ESR ages and those determined by other methods to an incomplete knowledge of geological and other variables, such as water content of the rocks, cosmic dose, b attenuation and the precise value of the a attenuation factor.
2. Samples The analysed rocks were chosen for the following reasons. The Battleship Rock material provides a good opportunity to re-examine a unit already dated by ESR (Toyoda et al., 1995) and by "ssion track and K/Ar methods (Self & Wol!, 1988). The Kenyan samples were chosen to test the ESR method for very young volcanic rocks, in the age range ((20 ka) where a viable technique would be particularly useful. Furthermore, we selected a glassy and devitri"ed sample to test whether post-eruptive crystallization could a!ect ESR dates. The Toba Tu! samples were chosen for this study for three reasons: (i) They have been well dated by conventional techniques; (ii) The quartz phenocrysts are relatively abundant and large (up to 2 cm, mean diameter ca. 10 mm). They have been highly fractured (during or after eruption); (iii) The three members of the Toba Tu! give a range of ages which might help to establish the overall) range of applicability of the ESR method. 2.1. Battleship rock samples The Battleship Rock material is from the Jemez volcanic "eld, Rio Grande rift valley, New Mexico. Activity in the Jemez volcanic "eld started around 13 Ma ago (Gardner et al., 1986). Eruption of the Otowi and Tshirege members of the Bandelier Tu! accompanied formation of the Toledo and Valles calderas, at 1.50 and 1.13 Ma, respectively (Spell et al., 1990). Rhyolitic domes erupted from the ring fracture zone following the formation of the Valles caldera. The youngest of these postcaldera units consists of three members, one of which, the battleship rock member (BRM), consists of non-welded to densely welded ash #ow deposits. The age of the Battleship Rock Tu! has recently been determined by ESR (Toyoda et al., 1995) and reported as 59$6 ka. This date contrasts markedly with those reported by Go! et al., 1989 by K/Ar on sanidine of 278$52 ka and those of Miyaji et al. (1985) on zircon of between 130$70 and 180$70 ka. Previous age determinations of this unit are, therefore, confusing and we hoped our examination would help to clarify these apparent contradictions. Our sample comes from the moderately welded, noncrystalline facies of the BRM, from Battleship rock itself, (35352.1@N 106338.5@W) and thus slightly further north than sample BSR04 of Toyoda et al. (1995). Pumice
fragments in the rock are slightly #attened and the rock has a crude foliation. The matrix is brownish, a result of iron oxides disseminated in the sanidine, with smaller amounts of plagioclase, hornblende and Fe}Ti oxides. Quartz phenocrysts are 0.25}5 mm in size (mean diameter ca. 2.5 mm) and were slightly fractured, presumably during eruption. The Battleship rock rhyolite is metaluminous, mildly potassic (Na O/K O"0.95) and 2 2 moderately calcic (CaO"1.63%). 2.2. Olkaria samples The Olkaria (Naivasha) volcanic complex occupies some 500 km2 near Lake Naivasha in the south-central Kenyan rift valley. The complex consists dominantly of rhyolitic domes, lava #ows and fall and #ow pyroclastic deposits, erupted from at least 80 small centres, possibly over the past 20 ka or so (Macdonald et al., 1987; Clarke et al., 1990). Sample 148a is from the crystalline interior of the Mulla pumiceous dome on the western edge of the complex. It is a member of the oldest group of rhyolites in the complex, Group 1 of Macdonald et al. (1987). The eruptive age of Group 1 is rather poorly constrained by a "ssion track age of 17 ka Bailey and Macdonald, unpublished data). Using the U-series disequilibrium method, Black (1994) obtained a whole rock isochron of 65$12 ka for Group 1, which may represent the time of formation of the relevant magmas. The phenocryst assemblage in 148a is quartz (1.8% modally), sanidine (3.2%), quartz-sanidine intergrowths (0.7 D%), titanomagnetite (0.1%) and zircon (0.1%). The rounded quartz phenocrysts are up to 2 mm across (mean diameter 1.0 mm) and contain rare glass (melt) inclusions and small crystals of alkali feldspar. The crystalline matrix of the rock is spherulitic, the texture representing relatively slow cooling within the lava dome after extrusion. Compositionally 148a is a high-silica, mildly peralkaline (comenditic) rhyolite, with relatively low abundances of CaO and FeOH. Sample 570 is from the glassy carapace of a lava #ow within a younger eruptive sequence at the Gorge farm centre and appears to have erupted between 9.7 and 5.7 ka ago on the basis of 14C dates and Lake Naivasha high- and low-stands (Clarke et al., 1990). Black et al., (1997) presented a U-series isochron for this rock which gave an age of 14.6$2.2 ka. This is thought to date the time of phenocryst crystallisation; the residence time of the magma, de"ned as the internal isochron age minus the eruptive age, is thus in the range 3}11 ka. The phenocryst assemblage in 570 is quartz#sanidine# fayalitic olivine#titanomagnetite#amphibole# aenigmatite. The matrix is non-hydrated glass. Quartz phenocrysts are (1.5 mm across (mean diameter 0.77 mm), rounded through resorption, and enclose very rare glass (melt) inclusions. The rock is a
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
moderately peralkaline (comenditic) rhyolite, low in Al O and CaO but relatively Fe-rich. 2 3 2.3. Toba Tuw Samples Measuring 100]300 km, the Toba caldera in northern Sumatra, Indonesia, is the largest known Quaternary caldera (Fig. 1). The complex abuts the Sumatran Fault,
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which is a result of oblique subduction of the Indian}Australian plate under the Sundaland (Southeast Asian) plate along the Sunda Trench. The Sundaland plate is composed of continental lithosphere (Chesner & Rose, 1991). Four ash #ow tu!s have been erupted from the system in the past 1.2 Ma (Chesner & Rose, 1991), of which the last three are collectively known as the Toba tu!s.
Fig. 1. Tectonic setting and sample location maps of the Toba Caldera Complex. The recently updomed areas along the western lake shore are associated with hydrothermal activity. After Chesner and Rose (1991).
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The Oldest Toba Tu! (OTT) appears to have been erupted from the SE part of the present depression, has an estimated volume of 500 km3 (Knight et al., 1986) and is '300 m thick (Chesner & Rose, 1991). The Middle Toba Tu! (MTT) averages 150 m in thickness with an estimated volume of 60 km3 (Chesner & Rose, 1991). The eruptive vent was probably in a similar location to that of the OTT. It overlays the Harranggoal Dacite Tu! and is, itself, overlain by the Youngest Toba Tu! (YTT). The present Toba caldera was associated with eruption of the YTT which has an estimated volume of 2500}3000 km3, making it one of the largest single eruptions in geological history (Chesner & Rose, 1991). The OTT has been previously dated by K/Ar at 0.84$0.03 Ma (Diehl et al., 1987) and the MTT has a previously determined sanidine 40Ar/39Ar age of 0.501$0.005 Ma (Chesner et al., 1991). The age of the YTT has been previously determined employing both K}Ar (73.5$3 and 74.9$12 ka (Ninkovitch et al., 1978) and 40Ar/39Ar [which yielded a weighted mean of 73$4 ka (Chesner et al., 1991)]. The Oldest and Middle Toba Tu! samples were supplied by Dr. C.A. Chesner [Eastern Illinois University (EIU)]. The OTT sample (T27) is from inside the caldera on the Uluan block and has been overlain by the YTT eruption. It is a densely welded vitrophyre and has a brownish-grey to light-grey colour (Chesner & Rose, 1991). It contains phenocrysts of quartz, plagioclase, sanidine, amphibole, Fe}Ti oxides, zircon, allanite, and rare orthopyroxene or fayalite (Chesner, 1998). The MTT sample (T8) is from the caldera walls in the Harranggoal section at the northern end of Lake Toba and is a densely welded vitrophyre. We had six samples of the YTT, three supplied by Dr. C. A. Chesner (T36A3, T57A1 and T94A2) and three by Dr. S. Blake [The Open University; (OU)] (TT9, TT11 and TT78). The deposit is largely unwelded and contains two types of pumice; (i) a crystal-rich variety (30}40 wt% modally) with biotite phenocrysts and SiO between 68 and 72 wt% crystals, 2 and (ii) a variety with 15}20 wt% crystals, poorer in biotite phenocrysts and with SiO between 73 and 76 2 wt%. There is a compositional continuum between the two types (Chesner, 1998). T36A3 was collected at Prapat, near lake level, from the same locality dated by K}Ar at 74 ka (Ninkovich et al ., 1978). T57A1 was collected high on the southern caldera wall, directly above a visible contact with the OTT. T94A2 is from a circular embayment in the caldera wall near Bakara. TT9 and TT11 are from the extensive deposit of #uvially reworked YTT pumice on the outwash of the Asahan River near Porsea (beneath &R' of Porsea in Fig. 1). TT78 was collected near the base of the YTT outcrop in a roadcut near the top of the cli! near Muara.
3. Sample preparation The original rock samples were crushed in a mechanical crusher and passed through a magnetic separator. The larger quartz phenocrysts were hand picked from the remaining material, crushed and those with grain size, after crushing, of 150}212 lm (found to give the most reproducible ESR spectra) were treated with hydrochloric acid, to remove any organic material and carbonates, and then with 40% hydro#uoric acid for 1 h to remove glass, surface particles and the outermost 30}40 lm of each grain to eliminate defects formed by external a-emitters. They were also washed with concentrated nitric acid to remove any residual iron salts. For most of the samples there was su$cient quartz to obtain 10 aliquots, each of 200 mg, for the dating experiments. However, only a limited amount was available for one of the Toba Tu! samples (TT9). Here, in order to extend the number of data points, a second irradiation of each aliquot was undertaken after recording the ESR spectrum following a "rst irradiation.
4. ESR measurements ESR spectra were recorded at 77 K on a Varian E3 spectrometer operating with a magnetic "eld modulation of 100 kHz with an amplitude of 0.1 mT and a microwave power of 5 mW. Occasionally, slightly higher modulation amplitudes were employed (up to 0.63 mT). The latter spectra were found to give more reproducible peak intensities for total accumulated dose (D ) determinaE tions but are associated with a slight loss of spectral resolution. A typical ESR spectrum of the quartz phenocrysts extracted from the Gorge Farm (570) site is illustrated in Fig. 2. The Al centre shows well resolved hyper"ne structure consisting of a packet of 16 lines from low "eld (line no. 1, g"2.017) to high "eld (line no. 16, g"1.991). Previous workers (for example, Yokoyama et al., 1985) have taken the peak-to-peak height from the top of line 1 to the bottom of line 16 as a measure of spectral intensity and we have followed the same procedure. The overall height of the alkali metal cation compensated 1 : 1 : 1 : 1 quartet centred at g"1.932 was used for the Ti centre. This spectrum is typical of those obtained from all of the sites although the relative intensities of the main features di!ered from site to site. A 60Co source (Salford University) was used to irradiate the samples at a dose rate of ca. 0.45 kGy h~1 for a total dose of between 1.19 and 2.40 kGy (varying with sample). The exact dose received by each aliquot was determined by an alanine dosimeter [0.25 g of L-alanine (MTM Research Chemicals)] placed immediately adjacent to each aliquot. The resulting spectral intensities
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
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Fig. 2. ESR spectra of the quartz grains extracted from the lava from the Gorge Farm site recorded at 77 K with a modulation amplitude of 0.1 mT following a c-irradiation dose of 0.377 kGy. The g-values of the main features are indicated along with lines 1 and 16 (associated with the Al impurity centre).
were then compared with those of a dose/ESR intensity calibration obtained from alanine samples irradiated at the National Physical Laboratory (Teddington) covering the range of 0.497}12.00 kGy.
The annual dose, D, for each sample was determined from the U, Th, K concentrations of the samples (Table 2), using the formula given for quartz grains by GruK n (1989). Within this formula, corrections were applied to allow for b-attenuation and a-e$ciency (set at 0.1).
5. Chemical analysis The K O, U and Th concentrations are summarised in 2 Table 1. 6. Determination of accumulated and annual doses The D values were determined by the additive dose E method in which the spectral intensity is plotted against arti"cial dose and the D estimated by extrapolation to E zero spectral intensity. The D values (and their associated errors) were deterE mined from these plots employing the FIT-SIM program (GruK n, unpublished) which uses a simple saturating function for the best-"t and analytical procedures for the estimation of errors (Brumby, 1992).
7. Battleship Rock Dates obtained for the Battleship Rock Tu! are 234$39 ka from the Al centre and 243$58 ka from the Ti centre (Table 2). It is interesting to note that these dates are much older than the ESR determined dates reported recently by Toyoda et al. (1995), namely, 59$6 ka. They are, however, more in line with those reported by Go! et al. (1989) by K/Ar on sanidine of 278$52 ka and those of Miyaji et al. (1985) on zircon of between 130$70 and 180$70 ka. There are signi"cant discrepancies between these various dates. We anticipated that our ESR dates would agree with those obtained by Toyoda et al. (1995) but this is clearly not the case. The possibility that post eruption
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Table 1 Concentrations of (whole rock) radioactive elements in the samples K O (%) 2 Battleship Rock Olkaria Volcanics 148a 570 (quartz)! Toba Tu!s OTT}T27 MTT}8 YTT}T36A3 (quartz) }T57A1 (quartz) }T94A2 }TT9 }TT11 }TT78
U (ppm) Th (ppm)
Reference
4.0
5.4
19.1
4.52 4.45
9.9 32.17 7.11
52.1 109.6 28.86
2 2,3
4.98 4.29 4.63
4.63 5.08 4.63 0.20 2.66 0.2 5.08 5.8 3.75 3.0
23.03 23.6 23.03 0.96 14.63 0.79 23.6 31.9 23.6 21.0
4,5 4,5 5,6 5 5,6 5, 5,6 7 7 7
3.64 4.68 4.89 4.33 3.03
1
!The high U and Th concentrations arise from glass inclusions within the quartz grains. References 1 } Toyoda and Ikeya (1994); 2 } Macdonald et al. (1987); 3 } Black et al., (1997); 4 } Chesner and Rose (1991); 5 } S Black (unpublished data); 6 } Chesner (1998); 7 } Jones (1993).
magmas), and together the dates may indicate that the Group 1 rocks were formed and erupted within the period 65}35 ka ago. The ages obtained for 570 are 23$4 ka and 34$8 ka from the Al and Ti centres respectively (see Table 2). These ages are somewhat higher than the 5.7}9.7 ka eruptive age based on 14C and Lake Naivasha high- and low-stands (Clarke et al., 1990) and the U-series internal isochron age of 14.6$2.2 ka presented by Black et al. (1997). There is a paradox here, in that the U-series age is interpreted as re#ecting the age of phenocryst crystallisation, whilst the ESR age is expected to re#ect a posteruptive age, when the temperature of the lava decreases below the resetting temperature. However, one problem in the ESR dating of 570 arises from the glass melt inclusions within the quartz phenocrysts. In particular the a- and b-attenuation factors required for the internal dose under these circumstances are unclear and their overestimation within the calculation would lead to an overestimation of the eruption age.
9. Toba Tu4, Sumatra thermal activity in this region may be important is discussed in more detail below.
8. Olkaria volcanic complex, Kenya The ages obtained for 148a are 31$3 ka and 38$5 ka from the Al and Ti centres, respectively (Table 2). These ages are reasonably close to the U-series whole-rock isochron for Group 1 rocks (65$12 ka; Black, 1994, which may represent the time of formation of the relevant
The ESR ages, summarised in Table 2, give (Al & Ti) average ages of 0.349 and 0.326 Ma for the OTT (T27) and MTT (T8) samples respectively. These ages are within experimental error of one another and would appear to be signi"cant underestimates of previously determined eruptive ages of 0.84 Ma (K/Ar; Diehl et al., 1987) and 0.501 Ma (40Ar/39Ar; Chesner et al., 1991). The possibility of partial thermal resetting of these samples is discussed further below. Youngest Toba Tu!. The average ages from the EIU and OU samples from the Al centre are 75$21 and
Table 2 ESR dates from Al and Ti impurity centres for the various samples Al-centre D(mGy/a) Battleship Rock Olkaria Volcanics: 148a 570 Toba Tu!s OTT}T27 MTT}T8 YTT}T36A3 }T57A1 }T94A2 }TT9 }TT11 }TT78
3.81!
D (Gy) E
Ti-centre Age(ka)
D (Gy) E
Age(ka)
893$132
234$39
927$201
243$58
9.08 22.45
281$24 517$75
31$3 23$4
344$43 759$170
38$5 34$8
3.52 3.39 3.41 2.35 3.51 4.14 3.22 2.56
1052$113 1098$68 181$9 245$62 242$33 356$50 294$52 176$15
299$35 324$22 53$3 104$17 69$10 86$13 91$18 69$7
1405$299 1109$199 189$28 239$56 295$44 420$70"
399$93 327$65 56$97 102$26 84$14 101$19
206$30
81$13
!Toyoda and Ikeya (1994) give a value of 3.72 due to the use of a marginally di!erent average grain diameter in the calculation. "Data too scattered to provide a reliable D E
M.T. Wild et al. / Quaternary Science Reviews 18 (1999) 1507}1514
82$9 ka, respectively, and from the Ti centre are 81$19 and 91$10 ka, respectively. The average age from both centres for all of the samples is 81$17 ka. We consider these various average ages obtained by ESR to be in good agreement with those obtained employing other techniques. For example, K}Ar dates are 73.5$3 and 74.9$12 ka (Ninkovitch et al., 1978) whilst 40Ar/39Ar yields a weighted mean of 73$4 ka (Chesner et al., 1991).
10. Thermal stabilities of the samples In principle, the ESR technique is well suited to the dating of volcanic eruptions since the eruption temperature, commonly exceeding 8003C, is amply su$cient to reset the ESR signals to zero. To test this we raised the temperature of representative aliquots of both Kenyan samples and the Battleship Rock sample to 4003C. In all cases the ESR spectrum completely disappeared almost immediately. Shimokawa et al., (1988) reported similar observations when samples of a sand bed baked by the lava of Ueno basalt and of a weathered basalt baked by the Ikenotaira basalt lava were raised to 3503C. The Al and Ti centres are, therefore, completely zeroed at eruption. Accumulation of the ESR signals will commence once the blocking temperature has been achieved. Shimokawa and Imai (1987) have estimated this to be 50}603C for the Al centre, for example. Riehle (1973) has estimated that a 40 m thick ignimbrite will cool from about 7503C to about 503C in less than 1000 yr. This period is almost insigni"cant compared to the age of the samples reported here. It is very signi"cant, however, if a subsequent eruption overlies the sample of interest as thermal resetting is more than likely. Shimokawa and Imai (1987) have shown that the Al centre will be reset in less than 1 yr if maintained above 1003C and in 1,000-10,000 years if maintained at 50}603C, by subsequent geothermal activity. Obviously, activity at lower temperatures or for shorter periods will result in some partial resetting. Partial thermal resetting of the samples is the likely explanation for the underestimation of the ages of the OTT and MTT eruptions since both have been overlain by the massive YTT eruption. The similarity between the ages of these two samples is probably no more than coincidence. Partial thermal resetting could be a problem with BRM samples where our age is signi"cantly higher (ca. 240 ka) compared to those obtained by ESR previously [59$6 ka (Toyoda et al., 1995)] but is in better agreement with those obtained employing other techniques such as K/Ar (278$52 ka; Go! et al., 1989) and "ssion-track (180$70 and 130$70; Miyaji et al., 1985). The potential for thermal resetting of BRM samples is high. Not only do Toyoda and Ikeya (1994) report that ambient summer temperatures can peak above 503C but
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it is also clear that there has been some subsequent geothermal activity in the area. The present abnormally high geothermal gradients at the Fenton Hill site appear to be of recent origin occurring during the last few tens of thousands of years (Harrison et al., 1986). Clearly this could have a!ected some samples more than others. Although our sample is from the same general locality there is textural (and probably compositional) variation in the rocks and, consequently, they could have di!erent cooling histories. Paradoxically, our sample has been oxidised after eruption, yet the ESR age is roughly similar to the eruptive age determined by other techniques.
11. Summary We have obtained ESR ages on quartz phenocrysts in extrusive volcanic rocks ranging from 23 to 399 ka. It is quite possible that volcanic rocks as young as 5}10 ka could be dated by this method. We are, however, concerned about the maximum age range as our experiments indicate that thermal annealing of samples presents a serious problem. Evidence suggests that samples can be thermally reset completely if maintained at 1003C for less than 1 y or at 50}603C for between 1 and 10 ka (Shimokawa & Imai, 1987). Consequently, high ambient temperatures, subsequent geothermal activity or the overlaying of samples by material from subsequent eruptions are all likely to create problems.
Acknowledgements We would like to thank Drs. Eddie Rhodes (Royal Holloway, University of London) and Christophe Falgueres (Institut de Paleontologie Humaine, Paris) for helpful discussions concerning dating procedures and Dr. Rainer GruK n (Australian National University, Canberra) for allowing us to use copies of his programmes FIT-SIM and AGE. We would also like to thank Mr. David Barraclough for his expert technical assistance with the 60Co facility at the University of Salford and Dr. Stuart Black (Lancaster University) for helpful discussions and for analysis of some of the samples. We especially thank Drs. Craig Chesner (Eastern Illinois University) and Steve Blake (The Open University) for generously making available samples of various Toba Tu!s. Finally, we thank Lancaster University for the award of a University Research Studentship (to M.T.W).
References Black, S. (1994). U}Th disequilibria systematics of the Olkaria complex, Gregory Rift Valley, Kenya. Ph. D. Thesis, University of Lancaster.
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