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Cosmic ray exposure dating with in situ produced cosmogenic 3He: Results from young Hawaiian lava flows
Earth and Planetary Science Letters, 97 (1990) 177-189 Elsevier
177
[KT]
Cosmic ray exposure dating with in situ produced cosmogenic 3He: results from young Hawaiian lava flows Mark D. Kurz
1, Debra
Colodner ~, Thomas W. Trull ~, Richard B. Moore 2 and Keran O'Brien 3
i Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.) z U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 (U.S.A.) ~ Department of Physics and Astronomy, Northern A rizona University, Flagstaff A Z 86011-6010 (U.S.A.)
Received April 18, 1989; revised version received November 9, 1989 In an effort to determine the in situ production rate of spallation-produced cosmogenic 3He, and evaluate its use as a surface exposure chronometer, we have measured cosmogenic helium contents in a suite of Hawaiian radiocarbon-dated lava flows. The lava flows, ranging in age from 600 to 13,000 years, were collected from Hualalai and Mauna Loa volcanoes on the island of Hawaii. Because cosmic ray surface-exposure dating requires the complete absence of erosion or soil cover, these lava flows were selected specifically for this purpose. The 3He production rate, measured within olivine phenocrysts, was found to vary significantly, ranging from 47 to 150 atoms g I yr-~ (normalized to sea level). Although there is considerable scatter in the data, the samples younger than 10,000 years are well-preserved and exposed, and the production rate variations are therefore not related to erosion or soil cover. Data averaged over the past 2000 years indicate a sea-level 3He production rate of 125 + 30 atoms g-1 yr-1, which agrees well with previous estimates. The longer record suggests a minimum in sea level normalized 3He production rate between 2000 and 7000 years (55 _+ 15 atoms g-1 yr-1), as compared to samples younger than 2000 years (125 _+ 30 atoms g-1 yr-i), and those between 7000 and 10,000 years (127 + 19 atoms g 1 yr-1). The minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength, as indicated by archeomagnetic data. However, the production rate variations (a factor of 2.3 + 0.8) are poorly determined due to the large uncertainties in the youngest samples and questions of surface preservation for the older samples. Calculations'using the atmospheric production model of O'Brien (1979) [35], and the method of Lal and Peters (1967) [11], predict smaller production rate variations for similar variation in dipole moment (a factor of 1.15-1.65). Because the production rate variations, archeomagnetic data, and theoretical estimates are not well determined at present, the relationship between dipole moment and production rate will require further study. Precise determination of the production rate is an important uncertainty in the surface-exposure technique, but the data demonstrate that it is feasible to date samples as young as 600 years of age providing that there has been no erosion or soil cover. Therefore, the technique will have important applications for volcanology, glacial geology, geomorphology and archaeology.
1. Introduction The presence of in situ produced cosmogenic 3 H e in t e r r e s t r i a l s u r f a c e r o c k s is n o w well d o c u mented [1-5]. Within the top two meters of the e a r t h ' s s u r f a c e , c o s m o g e n i c 3 H e is d o m i n a n t l y produced by spallation reactions between cosmic ray neutrons and the major elements of the rock [1,2,5]. A l t h o u g h i g n e o u s r o c k s c o n t a i n m a g m a t i c ( i n h e r i t e d ) h e l i u m , c o s m o g e n i c h e l i u m c a n b e distinguished from the inherited helium by crushing a n d s t e p h e a t i n g [1,2]. I f t h e p r o d u c t i o n r a t e o f cosmogenic 3He can be determined, then measure0012-821X/90/$03.50
© 1990 Elsevier Science Publishers B.V.
m e n t o f its a c c u m u l a t i o n will b e c o m e a v a l u a b l e geochronological tool. Surface exposure dating w i t h i n s i t u p r o d u c e d c o s m o g e n i c n u c l i d e s is n o t a n e w i d e a [6,7], b u t t h e r e h a v e b e e n few a t t e m p t s t o e x p e r i m e n t a l l y d e t e r m i n e p r o d u c t i o n r a t e s in rocks, or to evaluate the precision and limitations of the technique. Consequently, there have been few geological applications. C o s m o g e n i c 3 H e is i d e a l f o r e x p o s u r e - a g e d a t i n g b e c a u s e it is s t a b l e a n d h a s t h e h i g h e s t p r o d u c tion rate of any cosmogenic nuclide, approxim a t e l y 100 a t o m s g 1 y r - I a t s e a level [2]. H o w ever, c o m m o n g e o l o g i c f a c t o r s s u c h as e r o s i o n a n d
178
soil cover may lower apparent exposure ages. In order to test the precision, accuracy and age range for which exposure-age dating is feasible, data must therefore be obtained from well-dated samples that have never been eroded or shielded from cosmic rays. Hawaiian lava flows were selected for this purpose because radiocarbon dates are abundant [8] and flow morphology can be used to evaluate surface preservation. In order to constrain the production rate, and test the exposureage dating technique, this paper presents measurements of cosmogenic 3He in a suite of radiocarbon-dated Hawaiian lava flows ranging in age from 600 to 14000 years. The available radiocarbon dates were obtained from trees and treeroots buried by the lava flows and preserved in volcanic ash or soil layers [8-10]. The basalt samples analyzed here were collected from Mauna Loa and Hualalai volcanoes; they were selected primarily on the basis of the radiocarbon geochronology [8-10], the presence of olivine phenocrysts, and the likelihood of obtaining undisturbed surfaces. The results demonstrate that exposureage dating is feasible in this age range, and illustrate the limitations and uncertainties inherent to the technique. The present-day production rate of any cosmogenic nuclide varies significantly with latitude, longitude, and altitude because cosmic rays are deflected by the earth's magnetic field, and are attenuated by the atmosphere [11,12]. Attempts to estimate production rates have primarily relied on spallation rates recorded in photographic emulsions (commonly referred to as "star" production rates) or measurements in materials exposed on the earth's surface [11,12]. Measurements of cosmogenic nuclides in natural materials have demonstrated that estimated production rates often have large uncertainties [13]. In addition, because the earth's magnetic dipole moment has varied with time [14], there may be secular variations in production rate. These effects have been documented to some extent, particularly with respect to calibration of the ~4C dating technique (e.g. [11,15]). However, if exposure-age dating is to be a quantitative technique, the production rates must be precisely determined, and the effects of these parameters on in situ production rates must be known.
M.D. KURZ ETAL.
2. Methods 2.1. Calculation of cosmogenic helium content Previous studies have shown that olivine is an ideal phase for measuring cosmogenic helium [1,2], and because olivine is common in Hawaiian basalts, only samples with olivine phenocrysts were analyzed. Inherited (magmatic) helium in olivine phenocrysts is held primarily within melt and fluid inclusions [16], and the isotopic composition of the inherited helium can readily be determined by crushing in vacuum. In contrast to crushing, heating in vacuo releases a combination of cosmogenic helium and inherited helium [1,2]. The isotopic composition of the cosmogenic helium is enriched in 3He by roughly 104 ( 3 H e / 4 H e of 0.1 as compared to 10 5 for mantle-derived rocks), which allows the two components to be distinguished. The experimental procedure therefore requires that olivine mineral separates from each sample be analyzed twice. First the olivine grains are crushed in vacuo and most of the inherited helium is released and its isotopic composition is measured. The olivine powder that remains is then loaded into a furnace and melted in vacuo. The amount of cosmogenic 3He in the sample, referred to hereafter as 3Hec, can then be calculated from: 3He~. = 3 He m _ 3 Hei. The subscripts c, m and i refer to cosmogenic, melted fraction and inherited helium respectively. Because the amount of cosmogenic 4He is insignificant with respect to inherited 4He (4Hei>>> 4Hec), 3Hei can be calculated from: 3He i = 4 He m [3He/4He] i where the [3He/4He]i is determined by the crushing experiments. The assumptions have been verified by profiles within single lava flows [2]. The mass spectrometry and sample handling procedures are described elsewhere [2,17]. Because production rate is a function of depth in the rock and elevation [2,5,11], the samples were oriented with respect to vertical in the field and the elevation of the site established using topographic maps and verified with an altimeter. The helium results were normalized to sea level (i.e. for altitude and depth in rock) using a simple exponential: 3He c (sea level) =3 Hec ( X ) * exp [( X - 1030)/L]
COSMIC RAY EXPOSURE DATING
W I T H IN S I T U P R O D U C E D
COSMOGENIC
where X is the altitude in g cm 2 (1079 is the value at sea level) and L is the atmospheric attenuation length for the cosmic radiation (L assumed 160 g cm -2 [2,18]). The olivine phenocrysts were separated from a known depth interval (typically 1-4 cm depth) and the results were normalized to the surface, also using this simple exponential, resulting in corrections of only several percent in 3Hec values. The altitude corrections are significant; a sample at 3 km elevation will have a production rate 8 times higher than a sample at sea level. Similarly, if a lava flow has had one meter eroded off the top, the 3Hec observed will be roughly one-half the "true" surface value, demonstrating the potential importance of erosion. 2.2. Uncertainties in the method
Because surface preservation (i.e. well-known depth) is such an important aspect of the tech-
3HE
179
nique, every attempt was made to obtain samples from pnstin,~ lava flows. The sampling efforts were concentrated on the arid, leeward flanks of Hualalai and Mauna Loa volcanoes (Fig. 1) because the locally dry climates and low erosion rates improve the chances for preservation. The field work was based heavily on geological mapping of these volcanoes by Lipman, Lockwood and co-workers [19-21] and Moore et al. [10]. Surface preservation varies significantly with age, climate, topography, and human interference, and each sample was given a "quality rating": a numerical indication of confidence that the surface has remained undisturbed and that the age of the surface is well known (see Table 1). This is based primarily on the surface features of the lava flow, and the local geology. For the young lava flows, which are well exposed and uneroded, it was possible to collect samples for helium analysis from
Ka
08 Fig. 1. Location of the samples on the island of Hawaii. All samples were olivine-bearing lava flowserupted from Mauna Loa or Hualalai volcanoes. Further information on locations is given in Appendix 1.
180
near the site that charcoal was recovered, or to trace the flow to the collection site. This was not possible in all cases. Sample 44 and 49 (9679 and 14,067 years, respectively) were collected in different locations than the charcoal for radiocarbon, and the local geology was used to infer they are from the same lava flow. The geology is less constrained for the older lava flows and hence the age of these samples is less well known than the younger samples. In the case of sample 02, there is good evidence that the site was ash covered in the past (J. Lockwood, personal communication). These factors were also taken into account in giving the quality rating (i.e. resulting in ratings of 3 for samples 2, 44 and 49). In some instances this is subjective because of the difficulty in evaluating factors such as past soil cover and human impact. In other cases (rating of 1), there is virtually no chance that the lava flows have ever been covered or eroded and the sample was collected near the site that the charcoal for ~4C was collected. The 14C ages of Rubin et al. [8] were calibrated using the dendro-chronological time scale of Stuiver and co-workers [22,23], and hence are not subject to carbon reservoir effects, or to geomagnetic fluctuations. It is however important to note that although t4C dates are treated here as the absolute geochronometer against which we "calibrate" the 3He data, the 14C data must be viewed with some caution. The 14C data were obtained from charcoal beneath the lava flows [10], and there are a number of ways the charcoal could have been compromised. Circulation of groundwater and equilibration with " y o u n g " carbon from the hydrosphere could result in spuriously young ages [24]. Alternatively, plants growing in volcanic areas can respire volcanic CO2 which contains no ~4C, resulting in spuriously old ages [25]. However, because of the abundance of well preserved charcoal in ash layers beneath Hawaiian lava flows, and the relative youth of the lava flows, these are among the best dated lava flows available. The uncertainties in 14C ages, given in Table 1, are the lo counting errors given by Rubin et al. They caution that these errors do not "include variable factors such as contamination, isotopic fractionation, and other laboratory uncertainties". Unfortunately, it is impossible to evaluate which of the samples may have been affected by these
M.D. K U R Z E T AL.
factors, but the counting error is clearly a lower limit. For sample 31 there are four replicate ~4C ages (890, 740, 740, and 910; see Appendix 1), all with a counting error of _+60 years, that yield a mean of 820 _+ 90 years. Although these four replicates are not enough to rigorously determine the errors, the standard deviation of the mean provides an objective means of scaling the counting errors. In order to provide a more conservative error estimate we therefore scale the lo values of Rubin et al. (given in Appendix 1) by 1.5 (i.e. the standard deviation/counting error). The data in Table 1 are therefore the calibrated ~4C ages with conservative error estimates; the raw data as reported by Rubin et al. [8] are given in Appendix 1. Replicate 14C ages for sample 04 were 2300 + 60 and 2440 _+ 70 years which are also consistent with the error estimates. For the two lava flows with replicate ~4C dates, we utilize the mean in Table 1. In order to test the validity of the various corrections and the assumption that the surfaces are undisturbed, replicate samples were analysed for helium in several of the lava flows. All five analyses from the 655 year old lava flow have sea-level-normalized 3He c contents and production rates that are within two standard deviations from the mean (7.68 _+ 1.9 × 10 4 atoms g - l , and 117 + 29 atoms g-~ yr-1); the standard deviation of the mean is similar in size to the calculated lo errors. There are other replicate analyses for lava flows of various ages (2271 years, samples 05 and T87-08; 5345 years, sample 48; 8035, sample 01; 8514 years, samples 07 and 08), all of which are internally consistent within 2 standard deviations. The younger samples have significantly larger uncertainties because of the small n u m b e r of spallation-produced 3He atoms. Consequently, the cosmogenic helium signal becomes harder to detect, as illustrated by the small difference between crushed (inherited) and melting results in Table 1. The calculation of uncertainties in Table 1 does not include all possible sources of error. For example, in correcting for inherited 3He, we assume that the inherited 3 H e / 4 H e ratio of the phenocrysts within a given lava flow is uniform. Problems with this assumption could arise if a given lava flow has several populations of phenocrysts with different 3 H e / 4 H e , or if there is a contribution of implanted radiogenic 4He from the uranium enriched matrix. Any atmospheric contamination
181
COSMIC RAY EXPOSUREDATING WITH IN SITU PRODUCED COSMO(3ENIC 3HE TABLE 1 Helium data and calculated production rates for Hawaiian lava flows Sample
4He (cm 3 STP g-1 ( × 1 0 -9)
(3He/aHe)m (3He/aHe)i _+1 s
_+1 s
603+150 655_+ 67 655_+ 67 655-+ 67 655_+ 67 655_+ 67 820-+ 90 2271_+ 90 2271+ 90 2271-+ 90 2411_+ 90 2772+120 3091_+105 4493-+105 5345_+120 5345_+120 7204_+150 8035_+345 8035_+345 8514_+105 8514_+105 9679_+300 9941_+195
28.02 0.72 1.65 4.43 2.05 0.717 0.669 2.01 1.36 2.36 1.00 14.22 0.973 9.15 8.29 23.20 27.96 0.993 0.462 0.870 1.37 4.28 1.01
9.3 +0.04 11.88 _+0.77 13.39 _+0.27 10.09 -+0.27 9.86 _+0.2 10.57 _+0.64 12.8 -+0.2 11.22 _+0.2 10.96 _+0.31 9.943-+0.35 10.91 _+0.4 10.41 +0.1 17.61 _+0.59 13.01 -+0.15 13.45 -+0.17 10.16 -+0.08 11.66 _+0.08 40.4 _+0.74 76.87 -+1.4 41.73 _+0.87 34.91 _+0.75 26.12 -+0.29 28.63 _+0.67
8.848_+0.05 8.5 _+0.07 8.57 _+0.12 8.53 -+0.06 8.57 _+0.06~ 8.57 -+0.06 8.78 -+0.28 9.18 _+0.13~ 9.18 -+0.13 8.9 -+0.28 8.411_+0.15 8.68 -+0.11 8.34 _+0.06 8.93 -+0.06 8.97 -+0.1~ 8.97 _+0.1 9.836_+0.08 9.08 _+0.08 9.17 -+0.27 8.76 +0.08 10.12 _+0.12 8.88 -+0.06 1 3 . 1 +0.1
10,435_+150 13,312-+225 14,067-+225
5.17 6.69 0.392
Calibrated
|4C age 3-1 s (years)
KS78-47" KS87-03 KS87-14 KS87-15 T87-4 T87-4 KS87-31 T87-8 T87-8 KS87-05 KS87-04 KS87-43" KS87-13 KS87-46" KS87-48" KS87-48" KS87-42 KS87-01C KS87-01B KS87-08 KS87-07 KS87-44" KS87-02 RM889490* KS87-51" KS87-49"
13.52 _+0.19 7.84 +0.06 15.14 _+0.2 9.03 +0.15 186.5 _+4.2 9.42 -+0.19
3Hec -+ 1 s 3Hec (cm3 STP g 1 production ×10 14) rate-+ 1 s (atoms g l y r 1) sea level
Altitude (ft)
Depth (cm)
Sample surface quality
1.75 _+0.60 0.336_+0.10 1.101_+0.09 0,956_+0.21 0.366_+0.08 0,199_+0.09 0.372_+0.07 0,568_+0.09 0.335-+0.09 0.341-+0.18 0.348+0.09 3.40 -+0.43 1.25 _+0.12 5.17 _+0.32 5.14 -+0.35 3.82 -+0.63 7.06 _+0.78 4.30 _+0.24 4.33 _+0.25 3.97 -+0.27 4.69 _+0.23 10.21 -+0.31 2.17 -+0.16
161.9_+68 139.2_+45 116.9_+15 97.1_+23 151.3_+36 82.0-+36 124.0_+28 67.7+10 39.9_+11 41.7_+22 36.6_+9 70.0_+9 63.2_+6 63.4_+4 54.4_+4 40.5_+7 71.8_+8 141.2+10 139.8-+10 131.7-+9 125.6_+6 57.4-+2 56.6_+4
7680 120 6550 6780 140 140 60 80 80 120 380 7600 2600 7820 7720 7720 6480 280 280 20 920 7890 280
2 2.5 2.5 2.5 2 2 2 2 2 4 2 2.5 2.7 3 3.5 3.5 5 3 2 3.5 3 3 2
1 1 1 1 1 1 1 1 1 1 1 1 2 1 2 2 2 2 3 2 2 3 3
4.06 -+0.20 5.66 -+0.32 9.60 _+0.85
96.1-+5 74.9-+4 182.7-+16
650 2000 150
4 2 2
1 3 3
All samples were olivine mineral separates (0.15-0.5 g). The subscript m denotes results for melting of powders in vacuum, while the subscript i denotes results for crushing of 1-2 mm grains (inherited magmatic helium). 3 H e / 4 H e ratios are reported relative to the atmospheric value (/~/Ratm; where R a t m = l . 3 8 4 × 1 0 6). 14C ages are taken from Rubin et al. [8] and calibrated using the dendrochronological time scale of Stuiver et al. [22,23]. Values of 3Hec given are calculated for the altitude and depth in the rock of the sub-sample (i.e. calculated directly from other parameters given), values for production rate are normalized to sea level (see text). 3Hei is calculated from crushing and melting data: 3He i =4Hem[3He/4He]i . In all samples, crushing and melting were performed on the same mineral separate, except those denoted by ~ , in which the crushing data from a different sample of the same flow was used. 4 He blanks over the course of this study were between 3 and 4 × 10 n cm 3 STP with atmospheric 3 H e / 4 H e ratios, and were reproducible to 2 × 10-12 cm 3. Techniques are described in [2,15]. An asterisk ( * ) denotes samples from Hualalai volcano, all others from Mauna Loa; locations of the lava flows can be found in [8,10,19-21] (see also Appendix 1). Quality rating is an indication of confidence in the sample quality/preservation of the surface: 1 = excellent, 2 = some doubt, 3 = significant doubt or geological evidence to indicate poor preservation.
o r p r o b l e m w i t h b l a n k c o r r e c t i o n s will a l s o c o n tribute an error. A contribution from radiogenic h e l i u m is u n l i k e l y b e c a u s e t h e s a m p l e s a r e s o y o u n g , a n d t h e r e is a t p r e s e n t n o e v i d e n c e f o r inhomogeneity. N e v e r t h e l e s s , it is i m p o r t a n t to n o t e t h a t i n s e v e r a l c a s e s it w a s n e c e s s a r y t o utilize the inherited 3He/4He ratio from different
analyses of the same sample (samples noted with # i n T a b l e 1). I n m o s t c a s e s , t h e s a m e m a t e r i a l was analyzed in crushing and melting. We also include no contribution tn the error from the altitude correction. Due to the youth of the samples, any subsidence or uplift should be minimal. The value of the attenuation coefficient
182
M.D. KURZ
is also a possible source of error, because different investigators have used slightly different values. Lal uses an attenuation coefficient of 150 g cm 2. The value of 160 g cm -2 is used here because there it was obtained by measurements of in situ production that seem most analagous to the present situation [2,18]. 3. Results and discussion
3.1. Relationship between 3He and time Sea-level normalized 3He~ correlates well with calibrated ~4C age (Fig. 2), suggesting that it will be a useful geochronometer. However, some complications are also obvious, most notably the scatter in Fig. 2. All of the likely geological processes will lower the observed amount o f 3Hec; natural erosion, anthropogenic erosion, soil cover, and ash cover will all yield lower amounts of 3He~ than expected. Diffusive helium losses would also lower the amount of 3He~, although available diffusion data suggest that cosmogenic 3He diffuses too slowly to be important. Extrapolation from step-heating results yield a 3He~ diffusion coefficient of 5 × 10 -23 cm2/s in olivine (at 20°C), which suggests that diffusive losses are insignificant for the present samples ([26], and in preparation). Virtually the only way to obtain high 3He~ values would involve a radiocarbon date that is
3.0 / | erosion or soil cover
[
--
2.O ? 1.0
.io
" i__ 5000
i
__
i
10000
J ~5000
Cal/braled 140 Age/yeors b.p.] Fig. 2, Measured concentrations of cosmogenic helium (3 Hec), corrected to sea level, plotted against calibrated radiocarbon age. The half-filled symbols indicate those samples for which the surfaces were suspect due to erosion, soil cover, or uncertain age information: quality rating of 3 in Table 1). The older samples show more scatter on the diagram, which is attributed to poorer surface preservation.
ET AL.
spuriously low. Based on this simple reasoning, and the field evidence, the greater scatter for the older samples in Fig. 2 is most easily explained by poorer preservation of the older surfaces. (Several of the samples falling below the trend are suspect for geological reasons--see rating scale, Table 1.) Therefore, we treat the 3He c data as minimum values for each age, and attribute low 3Hec values to one of the geological processes mentioned above. As mentioned earlier and in Appendix 1, some of the samples also have uncertainties in their ages related to field geology. The sample with the highest 3Hec content in Fig. 2 (sample 49) has a low quality rating partly for this reason. The collection site for sample 49 was inferred to be the same as that of the 12,950 year radiocarbon date based on the local geology. Because soil cover and erosion would lower the apparent exposure age, the high 3He content in this sample may indicate that this age is too young for the site where the sample was collected. Because the only way to obtain too great a 3Hec content is with a spuriously young age this is reasonable conclusion for this sample (note the quality ratings). The 3He c values and ~4C ages can also be used to obtain 3He production rates, i.e. dividing 3Hec by age. This yields a production rate integrated over the entire surface exposure age of the sample. As is shown in Fig. 3, the calculated (integrated) production rates vary considerably. In addition to geological effects on surface preservation, numerous factors can alter production rates: altitude, latitude, and cosmic ray flux variations due to solar wind or terrestrial magnetic field fluctuations. Due to the young age of the samples, it is extremely unlikely that altitudes or latitudes have been altered by subsidence or tectonics. All local geological factors will lower the production rate (i.e. by erosion or cover), and the markedly low production rates between 2000 and 7000 years B.P. could be attributed to erosion or soil cover. However, these samples are among the youngest and have the best preserved surfaces, making this unlikely. In addition, production rates are consistently lower in this time interval although the samples come from different flows, different altitudes and different volcanoes. Higher production rates are observed for the lava flows younger than 2000 and older than 7000
COSMIC RAY EXPOSURE DATING
WITH IN SITU PRODUCED
COSMOGENIC
250
~,
200-
o
150-
_= I~
100
• ~
•
50-
n
g •'r
o
I
0
I
I
I
] 5000
I
I
~
I
I 10000
F
I
I
I 15000
Calibrated 14C age (years b.p,)
Fig. 3. Production rate of cosmogenic 3He (in atoms g - 1 y r - 1) as a function of lava flow age. The production rate, obtained by dividing 3Hec by age, is integrated over the exposure age of the sample. The m i n i m u m in production rate between 2000 and 7000 years may be attributed to increased magnetic intensity (dipole moment; see Fig. 4). As in Fig. 2, the half-filled symbols indicate samples having relatively poor surface preservation; scatter for these older samples is attributed to variable surface preservation.
years. The mean 3Hec production rate in the time period 2000-7000 years is 55 + 15 atoms g t y r - t and the mean value for younger samples is 125 + 30 (for samples 7000-10,000 years old, with good quality rating, the mean is 127 + 19 atoms g-a yr a). Applying a standard t-test to these data suggests that the populations are distinct at greater than 96% confidence level. Unfortunately the best evidence for the difference in production rate comes from the youngest samples and those younger than 2000 years old have the largest uncertainties. With this uncertainty in mind, we conclude that the 3He production rate varied significantly within the last 10,000 years, unrelated to surface preservation. More data are clearly required to confirm this variation. 3.2. Possible causes of production rate variations The possibility that there are significant variations in production rate with time is a fundamental problem for the application of cosmogenic 3He to exposure-age dating. The consistency of the data presented here, and the good prese~ation of the younger lava flows, suggests that the variations for the last 10,000 years are not related to erosion or cover. With the caveat that the youngest samples have large uncertainties, some of the produc-
3HE
183
tion rate variations therefore may be related to changes in terrestrial cosmic ray flux due to solar or geomagnetic modulation. Significant variations in atmospheric ]4C content (of up to 10%) have been previously observed, and are attributed to both of these processes [15,27-31], in addition to reservoir effects related to changing amounts of carbon in the atmosphere and ocean [32-34]. Solar wind modulation of the cosmic ray flux is unlikely to have an effect on 3He production, because the solar variations occur on a shorter timescale than those observed in Fig. 2, and should be least important at low latitudes such as at Hawaii (20 o N). T h e 3 H ¢ c production discussed here differs from atmospheric 14C production in that 3He is produced in situ within the rock, whereas the ]4C is produced globally in the atmosphere. Because the atmosphere is well mixed and is a large reservoir of carbon, ]4C production rate variations are damped out, and are affected by exchanges between reservoirs. Thus, the in situ produced isotope should display a greater dependence on geomagnetic field. 3Hec also differs from ]4C because it accumulates in the rock, acting as an integrator of the production through time, which complicates the time dependence. One possible cause of the 3He: production rate variation is the strength of the terrestrial dipole. Because the cosmic radiation incident on the top of the atmosphere consists of charged particles, it is deflected by the earth's magnetic field, which has the largest effect near the equator and the least near the poles. One test of this hypothesis is to compare 3Hec production rate and dipole moment variations. Fig. 4a shows a recent compilation of the archaeomagnetic data by McEllinny and Senanayake [14] fit to a third-order polynomial (solid line). As mentioned earlier, t h e 3Hec acts as an integrator of the production rate and will be related to the inverse of the integrated dipole moment. Fig. 4b shows the integral of the curve in 4a divided by time, and inverted because cosmic ray flux is inversely proportional to dipole moment; the 3He¢ production rate variation over the same time scale is shown in Fig. 3. The minim u m in production rates shown in both Figs. 3 and 4b occurs over the same time interval, which suggests that dipole moment variations could cause some of the production rate variations.
184
M.D. K U R Z E T A L .
~
1.40
/
1.20
5\o t
1.oo o.80 0
2000
4000
6000
8000
2000
4000
6oo0
8o00
1.00
J'~ 0.90
% 0.80
0.70
0
Calibratedt4C Age (yearsb.p.) Fig. 4. (a) Dipole moment fluctuations (from archaeomagnetic measurements compiled by McElhinny and Senanayake [14]) normalized to the present value of the dipole moment; the points are 500 year averages. The curve is a third-order polynomial fit to the data points. (b) The inverse of the integral of the curve shown above, divided by time. The curve in part a is integrated because 3Hec accumulates in the rock over the entire time interval, and it is inverted because cosmic ray flux is diminished during times of strong dipole moment. Note that the minimum in production rate corresponds to the minimum observed in Fig. 3.
The dipole m o m e n t data shown in Fig. 4a are averages over 500-year intervals (in order to filter out the non-dipole variations and average the noisy data set); different attempts to represent the global dipole in this fashion differ with respect to size of the variations [14,30,34], but agree that there was a maximum in dipole m o m e n t approximately 2500 years before present. Local non-dipole components may also have significant effect on the sealevel production rates, which is another reason that the compilation in Fig. 4 [7] is only an approximation to the field intensity near Hawaii. Therefore, although the magnitude of the variations is poorly constrained, the shape of the curve in Fig. 4 is reasonably well defined. Unfortunately, even if the dipole m o m e n t variations were well defined, there are few means of evaluating the magnitude of the effect that the fluctuations would have on in situ production rates. One of the few published calculations is the model of O'Brien [35] which calculates production
in the atmosphere from atmospheric transport equations. In order to evaluate the possible effect of geomagnetic field intensity on 3He production rate (Fig. 4), the model of O'Brien [35] was applied to the latitude of Hawaii (19.75 ° geomagnetic latitude, cut-off rigidity 13 GV). The model uses the straight-ahead approximation to the stationary Boltzman equation to calculate cosmic ray propagation in the atmosphere, and spallation yields are calculated from cross section data [35]. Previous use of this method yielded good agreement with atmospheric a4C inventories, but were not applied to in situ production. The calculations yielded a sea-level star production at Hawaii of 1.34 × 10 5 stars g-1 s 1. Using the yield values of Lal and Peters [11] for tritium and 3He (0.14 and 0.12 per spallation event, respectively) gives a present-day 3He c production rate of 110 atoms g 1 yr 1, which is in excellent agreement with values determined from the lava flows less than 2000 years in age (125_+30 atoms g 1 yr-1). These production rates are in reasonable agreement with estimates using the methods of Lal and Peters (105 atoms g 1 yr-1 [11]), and Y o k o y a m a et al. (101 atoms g - ] y r - 1 [12]). The only previous attempt to directly measure 3Hec production rate, using a single 28,000 year old Hawaiian lava flow, yielded a value of 97 atoms g 1 yr 1 [2,36]. In order to estimate the effect of magnetic field on production, the calculations were performed at various vertical cutoff rigidities, which is analogous to changing the magnetic field. The results of these calculations at sea level and at 3 km altitude are shown in Fig. 5. Changes in dipole m o m e n t have a smaller effect on production at sea level, because a greater proportion of the sea-level production is derived from high-energy primaries that are unaffected by magnetic field (i.e. energies >> 13 GV). However, even at 3 km altitude, the predicted effect on production is significantly less than the 3He production rate variations shown in Fig. 3. The model predicts a variation in production rate of 1.25 at sea level and 1.35 at 3 km altitude for dipole m o m e n t variation between 0.5 and 1.5 the present value. The predicted variation for the samples in figures 2 and 3 would be even less due to the integrating effect of total exposure, 1.15 and 1.24 at sea level and 3 k m altitude respectively. A n o t h e r method of estimating the effect of geomagnetism on production rate is to use ob-
COSMIC RAY EXPOSURE DATING WITH IN S1TU PRODUCED COSMOGENIC ~HE 14
t~
10
os
[ 05
i 10
15
J 20
Normalized Dipole Moment (M/Mp)
Fig. 5. Variation in p r o d u c t i o n rate predicted by the model calculations of O'Brien [35]. Both the field a n d p r o d u c t i o n rates are normalized to the rates o b t a i n e d for the present field (1.0), to show the steeper d e p e n d e n c e of p r o d u c t i o n rate on dipole m o m e n t at higher altitude. The star p r o d u c t i o n rates (in air) o b t a i n e d for the present field (rigidity 13 GV) were 1.34 × 1 0 - 5 at sea level (1033 g c m 2) and 2.25 × 10 - 4 stars g 1 s-1 at 3 k m elevation (700 g cm 2).
served rates of star production (i.e. spallation) rates in photographic emulsions. Lal and Peters [11] have used this method to predict production rates of various cosmogenic nuclides. Based on star production rates, Lal ([11,37], and personal communication) estimates variation by a factor of 1.8 in production rate (at 3 km altitude) for the same variation in dipole moment (0.5-1.5 the present value), which would result in an integrated variation in 3Hec production of 1.65. The data in Table 1 yield a 3He production rate variation by a factor of 2.3_+ 0.8, if samples younger than 2000 years are compared to those 2000-7000 years in age. The fact that the variations predicted by both the atmospheric propagation model [35], and empirical star production data [11] are significantly lower than those observed, suggests that dipole moment variations may not account for all the 3He production rate variations described here. However, given the large uncertainty in both the measurements and the theoretical calculations, a significant fraction of the production rate variation shown in Fig. 3 could have been produced by variations in dipole moment. As mentioned above, an additional factor is the possibility that non-dipole components could have resulted in locally larger geomagnetic
185
fluctuations. For example, Coe et al. [38] measured paleD-intensities in Hawaiian lava flows significantly higher than predicted (particularly between 2000 and 5000 years) which they attributed to the non-dipole field. This suggests that the local intensity variations may have been significantly larger than is shown in Fig. 4a. However, this hypothesis is difficult to evaluate without further data. Quantification of the relationship between production rates and dipole moment will require additional data from dated surface rocks, and will be important to exposure-age dating because a correction will be required. It is important to note that geomagnetic effects are latitude dependent, being most important near the equator, and insignificant at latitudes greater than 50 ° . Therefore, this will be unimportant for high latitude studies. If a relationship between 3Hec production rate and dipole moment can be established, then helium measurements could provide a new constraint on the postulated variations in the earth's magnetic field intensity. This use of in situ produced cosmogenie nuclides was previously suggested by Lal et al. [37], and holds promise for resolving some existing controversy regarding the dipole moment. For example, if the 3He production rate variations are related to dipole moment, this can be used to distinguish between carbon reservoir effects and dipole moment effects on atmospheric 14C inventories. However, given the uncertainties in both production rates, and theoretical estimates of geomagnetic effect on production rates, this application will require considerable effort in the future.
4. Conclusions
This study demonstrates that helium exposureage dating is feasible in extremely young samples, as young as 600 years at sea level. At higher elevations, significantly younger ages could be determined. The data from young radiocarbon-dated lava flows illustrate that one important uncertainty with the technique is the necessity of constant exposure to cosmic rays, and hence the complete absence of cover or erosion. Therefore, in cases where surface preservation cannot be evaluated, the technique will simply yield minimum exposure ages, which may still be useful.
186
However, where geological information can be used to evaluate preservation and exposure, the technique will be an extremely useful geochronometer, particularly where other dating methods are not possible. Therefore it should have important applications in glacial geology, geomorphology, and volcanology. The main goal of this study was to precisely determine the 3Hec production rate. The data suggest a present-day sea-level production rate in olivine of 125 _+ 30 atoms g-~ yr 1 (as deduced from the lava flows less than 2000 years in age). Because of the number of determinations, and the quality of the surfaces, this value should supercede the previous estimates [2,36]. Because the dominant helium producing reaction is neutron induced spallation on the major elements of the rock, which in this case are atoms of O, Si, Mg, and Fe, this production rate should be weakly composition dependent, and will be useful for other silicate minerals. However, the large uncertainties in the production rate determinations demonstrates the need for additional studies of this kind. The temporal record deduced from dated lava flows displays a surprising 3Hec production rate variability over the last 10,000 years. The production rate determined from samples 2000-7000 years of age is significantly lower than obtained from those less than 2000 years, and those older than 7000 years. Because the lava flows were selected to minimize problems of surface exposure, and the younger samples had well preserved surfaces, we believe that the variations in production rate are not entirely due to erosion or soil cover. The minimum in production rate between 2000 and 7000 years B.P. corresponds to a maximum in past magnetic dipole moment and may be partly explained by geomagnetic modulation of cosmic ray flux. However, the observed variation in 3He production rate, a factor of 2.3 +_ 0.8, is poorly determined due to the large uncertainties in the helium measurements in very young samples, and the problem of variable surface preservation for the older samples. Two methods of estimating the effect of dipole moment on production rates, predict a variation by a factor of 1.15-1.65 in 3He production rate. Although this is significantly smaller than that obtained from the helium measurements, these calculations demon-
M.D. K U R Z E T A L .
strate that the relationship between dipole moment and production rates should be considered in cosmic ray exposure age dating at low latitudes. Further data, from different sample types and geographic locations, will be required to test the relationship between dipole moment and production rates, as will further efforts to refine theoretical estimates. Perhaps most importantly, the data reported here demonstrate some of the problems that must be overcome in utilizing cosmic ray exposure-age dating. Because geological information can be used to evaluate surface preservation, the primary obstacle to quantitative use of helium exposure-age dating is the uncertainty in ~Hec production rates, and the possible variation in production rates. If the temporal production rate variations are related to geomagnetic variations, then any low latitude use of this technique will require calibration, much like the radiocarbon calibration with dendrochronology. At high latitudes ( > 55°), the earth's magnetic field has little effect on cosmic rays, and dipole moment variations will have a negligible effect on production. Therefore, the uncertainty in production rates may be smaller for cosmic ray exposure dating of high latitude samples.
Acknowledgements We thank Karl Turekian for his editorial patience and wisdom in dealing with this paper. M.D.K. wishes to thank Bill Jenkins for numerous helpful conversations, Dempsey Lott for his assistance with the vacuum equipment, and Armine Gulesserian for assistance with sample preparation. T.W.T. and D.C. gratefully acknowledge support from the Education office of the W H O I / M I T Joint Program in Oceanography, and an NSF fellowship respectively. We thank D. Sampson, Frank Trusdell, Peter Lipman, and Jack Lockwood for help with the sampling, and P. Lipman, E. Brook, and J. Quick for comments on the manuscript. We thank D. Lal for providing unpublished calculations, and for numerous stimulating conversations. The manuscript also benefitted from the labors of several anonymous reviewers. This work supported by NSF EAR8610611 and EAR88-3783 and NASA N A G 9-69. This is a W H O I Contribution no. 7257.
COSMIC RAY EXPOSURE DATING WITH IN SITU PRODUCED COSMOGENIC 3HE
Appendix 1-Descriptions of lava flows sampled *KS87-47. Pahoehoe lava flow with original surfaces, just east of Malekule, elevation 7680 ft (19°42'N, 155°52'40"W), Kailua quadrangle), Hualalai volcano. KS87-03, 14, 15 and T87-4. Aa lava flow, originating at southwest rift of Mauna Loa volcano (Pohina flow). Radiocarbon date is 640 _+45, USGS ~W4025. Sampled at several locations. 03: elevation 120 ft ( 1 9 ° 0 7 ' 1 8 " N , 155°32'W, Naalehu quadrangle). 14: elevation 6550 ft ( 1 9 ° 1 4 ' 5 5 " N , 1 5 5 ° 3 8 ' 2 0 " W , Puu o Keokeo). 15: elevation 6780 ft ( 1 9 ° 1 5 ' 3 4 " N , 155°38'14"W, Alika Cone). T87-4: elevation 140 ft ( 1 9 ° 0 7 ' 2 2 " N , 155°31'56"W, Naalehu quadrangle). KS87-31. Pahoehoe lava flow, with glassy flow textures, originating at south west rift of Mauna Loa volcano (named Kipuka Nene Flow by Lipman and Swenson). Sampled near Highway 11 at elevation 60 ft ( 1 9 ° 0 7 ' 1 0 " N , 155°32'03"W, Naalehu quadrangle). Radiocarbon dates: 890 _+60, USGS :~4137; 740 _+60, USGS :~W4156; 740 _+60, USGS ~W4012, 910 ± 60, USGS :~W4231. The mean of these determinations 820 -+ 90 is used here as the date for this lava flow. KS87-05, T87~8. Picritic lava flow originating at south west rift of Mauna Loa Volcano, containing 5-10% olivine phenocrysts, named Kaalaiki flow. Sampled near highway 11. Radiocarbon age is 2180 -+60; USGS ~W4015. 05: elevation 120 ft (19 ° 0 6 ' 2 7 " N , 155 ° 32'35"W, Naalehu quadrangle). T87-8: elevation 80 ft ( 1 9 ° 0 6 ' 2 3 " N , 155°32'21"W, Naalehu quadrangle). KS87-04. Eastern lobe of aa lava flow originating at south west rift of Mauna Loa Volcano, named Ninole flow by Lipman and Swenson. Elevation 380 ft, radiocarbon dates are: 2300 -+60, USGS #W4008; and 2440 -+70, USGS ~W4142. The mean of these two determinations is used here. Punaluu quadrangle. *KS87-43. Aa flow, with well preserved flow surface and sparse olivine, near summit of Hualalai volcano, (19°42'N, 155°52'W, Hualalai quadrangle). Elevation is 7600 ft, radiocarbon age is 2670 -+ 80; USGS ~W5076. KS87-13. Picritic aa lava flow, at elevation 2600 ft, Mauna Loa volcano (19 ° 20' N, 155 o 24' 27" W, Wood Valley quadrangle). Radiocarbon age is 2940 -+ 70 (USGS :~ W3841). * KS87-46. Eruption spatter, collected just northwest of Hainoa Crater, Hualalai volcano at 7820 ft elevation (19°41'52"N,
* Asterisk denotes samples from Hualalai volcano. All others from Mauna Loa volcano.
187
155 ° 52'18"W, Hualalai quadrangle). Radiocarbon age is 3990 -+70, USGS :~W5132. *KS87-48. Aa lava flow, collected just southwest of Malekule, with 5-7% olivine, elevation is 7720 ft (19°41'47"N, 155 o 53' 03" W, Kailua quadrangle). Radiocarbon date is 4720 -+80; USGS :~W4378. *KS87-42. Aa lava channel, just east of Waiaha stream gulley, olivine and plagioclase-phyric basalt, elevation 6480 ft ( 1 9 ° 4 0 ' 3 0 " N , 155°51'45"W, Hualalai quadrangle). Radiocarbon date is 6360-+ 100; USGS ~W5297. KS87-01. Olivine-rich lava flow in Hilo Municipal gulf course. Radiocarbon age is 7230_+230, USGS :~W5599. Sampled in two different locations, one north of stream bed (B) and one south of stream bed (C). KS87-07. Awawa Kahao lava flow at Puu Maemae, from ridge just south of wind farm, near South Point ( 1 8 ° 5 9 ' 3 0 " N , 155°40'10"W, Ka Lae quadrangle), at 920 ft elevation. The lava flow is poorly exposed, in a sparsely settled area. Radiocarbon age is 7750-+70; USGS ~W4351. KS87-08. Same lava flow as # 7 (based on Lipman and Swenson map), from pahoehoe surface at east end of flow, at Hanalua Bay, elevation 20 ft ( 1 8 ° 5 2 ' 2 2 " N , 155°40'03"W, Ka Lae quadrangle). Well preserved, and nicely exposed pahoehoe surface. Radiocarbon age is 7750+70; USGS ¢~W4351. *KS87-44. Massive olivine-rich spatter from rim of unnamed crater just north of Hainoa Crater, at 7890 ft elevation (19°41'46"N, 1 5 5 ° 5 2 ' 1 0 " W , Hualalai quadrangle). This sample was not actually radiocarbon dated, but was inferred to be of same age as dated flow (age 8770_+200; USGS :~W5299) having similar major element chemistry. Radiocarbon date is from northeast edge of same cone. KS87-02. Puu Hoakalei Picrite, sampled from 3 m high lava ridge in Hilo, just north of intersection of Ainola and Malaai roads. Radiocarbon age is 9020_+130. Mauna Loa volcano. This sample has a poor quality rating due to proximity to housing, and the likelihood that volcanic ash covered the area before human settlement (J. Lockwood, personal communication). *KS87-49. Olivine-rich basalt near intersection of highway 11 with 190, on east side of road. Elevation 150 ft ( 1 9 ° 3 8 ' 5 5 " N . 155 ° 59'48" W, Kailua quadrangle), radiocarbon age is 12,950 -+150. This sample is given a poor quality rating due to proximity to populated area, and due to the fact that sample was not collected near charcoal sample for radiocarbon dating, and is therefore inferred to be from same lava flow. *KS87-51. Olivine-rich aa from ridge near the western edge of Puu Anahulu trachyte flow from Puu Waa Waa (19°48'N. 1 5 5 ° 5 0 ' 3 0 " W , Puu anahulu quadrangle). 12,320+150, USGS ~W4365. Elevation 2000 ft.
188 *RM88-9490. Olivine-rich pahoehoe spine at Ahinahena, near the western edge of the Puu Anahulu trachyte flow from Puu Waawaa. 9490_+ 100; USGS ~W-4371. Elevation 650 ft. Good original surface.
References 1 M.D. Kurz, Cosmogenic helium in a terrestrial igneous rock, Nature 320, 435-439, 1986. 2 M.D. Kurz, In situ production of terrestrial cosmogenic helium and some applications to geochronology, Geochim. Cosmochim. Acta 50, 2855-2862, 1986. 3 H. Craig and R. Poreda, Cosmogenic 3He in terrestrial rocks: the summit lavas of Maui, Proc. Natl. Acad. Sci. 85, 970-974, 1986. 4 D. Porcelli, J.O.H. Stone and R.K. O'Nions, Enhanced 3 H e / 4 H e ratios and cosmogenic helium in ultramafic xenoliths, Chem. Geol. Isot. Geosci. Sect. 66, 89-98, 1987. 5 D. Lal, Production of 3 He in terrestrial rocks, Chem. Geol. Isot. Geosci. Sect. 66~ 89-98, 1987. 6 R. Davis and O.A. Schaeffer, Chlorine-36 in nature, Ann. N.Y. Acad. Sci. 62, 105-122, 1955. 7 F.M. Phillips, B.D. Leavy, N.O. Jannik, D. Elmore and P.W. Kubik, The accumulation of cosmogenic Chlorine-36 in rocks: a method for surface exposure dating, Science 231, 41-43, 1986. 8 M. Rubin, L.K. Gargulinski and J.P. McGeehin, Hawaiian Radiocarbon dates, U.S. Geol. Surv., Prof. Pap. 1350, 213 242, 1987. 9 J.P. Lockwood and P.W. Lipman, Recovery of charcoal for ~4C dating from beneath lava-lessons from Hawaii, J. Volcanol. Geotherm. Res. 4, 1979. 10 R.B. Moore, D.A. Clague, M. Rubin and W.A. Bohrson, Hualalai Volcano: a preliminary summary of geologic, petrologic, and geophysical data, U.S. Geol. Surv. Prof. Pap. 1350, 571-585, 1987. 11 D. Lal and B. Peters, Cosmic-ray produced radioactivity on the earth, in: Handbuch der Physik X L V I / 2 , S. Flugge, ed., pp. 551-612, Springer-Verlag, Berlin, 1967. 12 Y. Yokoyama, J.-L. Reyss and F. Guichard, Production of radionuclides by cosmic rays at mountain altitudes, Earth Planet. Sci. Lett. 36, 44-50, 1977. 13 J. Klein, R. Giegengack, R. Middleton, P. Sharma, J.R. Underwood and R.A. Weeks, Revealing histories of exposure using in situ produced 26A and 1°Be in Libyan desert glass, Radiocarbon 28, 547-555, 1986. 14 M.W. McElhinny and W.E. Senanayake, Variations in the geomagnetic dipole 1: the past 50,000 years, J. Geomagn. Geoelectr. 34, 39-51, 1982. 15 P.E. Damon, J.C. Lerman and A. Long, Temporal fluctuations of atmospheric 14C: causal factors and implications, Annu. Rev. Earth Planet. Sci. 6, 457-494, 1978. 16 M.D. Kurz, W.J. Jenkins and S.R. Hart, Helium isotopic systematics of oceanic islands and mantle heterogeneity, Nature 297, 43-47, 1982. 17 M.D. Kurz, J.J. Gurney, W.J. Jenkins and D.E. Lott, Helium isotopic variability within single diamonds from the Orapa kimberlite pipe, Earth Planet Sci. Lett. 86, 57-68, 1987.
M.D. KURZ ETAL. 18 Y. Nakamura, H. Mabuchi and H. Hamaguchi, Beryllium-7 production from oxygen by atmospheric cosmic rays, Geochem. J. 6, 43 47, 1972. 19 P.W. Lipman, Rates of volcanic activity along the southwest rift of Mauna Loa Volcano, Bull. Volcanol. 45, 703-725, 1980. 20 P.W. Lipman and A. Swenson, Generalized geological map of the southwest rift of Mauna Loa, U.S. Geol. Surv. Map 1-1323, 1984. 21 J.P. Lockwood and P.W. Lipman, Holocene eruptive history of Mauna Loa volcano, U.S. Geol. Surv. Prof. Pap. 1350, 509-535, 1987. 22 M. Stuiver and P.J. Reimer, A computer program for radiocarbon age calibration, Radiocarbon 28, 1022-1030, 1986. 23 M. Stuiver, B. Kromer, B. Becker and C.W. Ferguson, Radiocarbon age calibration back to 13,300 years B.P. and the 14C age matching of the German oak and U.S. bristlecone pine chronologies, Radiocarbon 28, 969-979, 1986. 24 M.A. Kuntz, E.C. Spiker, M. Rubin, D.E. Champion and R.H. Lefebvre, Radiocarbon studies of latest Pleistocene and Holocene lava flows of the Snake River Plain, Idaho, Quat. Res. 25, 163-176, 1986. 25 M. Rubin, J.P. Lockwood and I. Friedman, Effects of volcanic emanations on carbon-isotope content of modern plants near Kilauea Volcano, U.s. Geol. Surv., Prof. Pap. 1350, 209-212, 1987. 26 T.W. Trull, Diffusion of helium isotopes in silicate glasses and minerals: implications for petrogenesis and geochronology, Ph.D. Thesis. M.1.T./Woods Hole Oceanographic Institution, 1989. 27 H.E. Suess, Secular variations of cosmogenic 14C on earth: their discovery and interpretation, Radiocarbon 28, 259-265, 1986. 28 R.E. Lingenfelter and R. Ramaty, Astrophysical and geophysical variations in 14C production, in: Radiocarbon Variations and Absolute Chronology, (I.U. Olsson, ed., Proc. 12th Nobel Symposium, John Wiley and Sons, New York, N.Y., 1970. 29 D. Lal and V.S. Venkatavaradan, Analysis of the causes of ~4C variations in the atmosphere, in: Radiocarbon Variations and Absolute Chronology (I.U. Olsson, ed., Proc. 12th Nobel Symposium, John Wiley and Sons, New York, N.Y., 1970. 30 V. Bucha, Influence of the Earth's magnetic field on radiocarbon dating, in: Radiocarbon Variations and Absolute Chronology, I.U. Olsson, ed., Proc. 12th Nobel Symposium, John Wiley and Sons, New York, N.Y., 1970. 31 W. Elsasser, E.P. Ney and J.R. Winckler, Cosmic ray intensity and geomagnetism, Nature 178, 1226-1227, 1956. 32 J. Beer, M. Andree, H. Oeschger, U. Siegenthaler, B. Bonani, H. Hofmann, M. Morenzoni, M. Nessi, M. Suter, W. Wolfli, R. Finkel and C. Langway, The Camp Century 10Be record: implications for long-term variations of the geomagnetic dipole moment, Nucl. Instrum. Methods B5, 380-384, 1984. 33 D. Lal, Carbon cycle variations during the past 50,000 years, in: The Carbon Cycle and Atmospheric C O 2 : N a t u ral Variations Archean to Present, Am. Geophys. Union, Geophys. Monogr. 32, 221-233, 1985.
COSMIC RAY EXPOSURE DATING WITH IN SITU PRODUCED COSMOGENIC 3HE 34 C.E. Barton, R.T. Merrill and M. Barbetti, Intensity of the earth's magnetic field over the last 10,000 years, Phys. Earth Planet. Inter. 20, 96-110, 1979. 35 K. O'Brien, Secular variations in the production of cosmogenic isotopes in the earth's atmosphere, J. Geophys. Res. 84, 423-431, 1979. 36 M.D. Kurz, Correction to: In situ production of terrestrial cosmogenic helium and some applications to geochronology, Geochim. Cosmochim. Acta 51, 1019, 1987.
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37 D. Lal, J.R. Arnold and K. Nishiizumi, Geophysical records of a tree: new application for studying geomagnetic field and solar activity changes during the past 10,000 years, Meteoritics 20, 403-414, 1985. 38 R.S. Coe, S. G r o m m 6 and E.A. Mankinen, Geomagnetic paleointensities from radiocarbon dated lava flows on Hawaii and the question of the Pacific nondipole low, J. Geophys. Res. 83, 1750-1756, 1978.
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[KT]
Cosmic ray exposure dating with in situ produced cosmogenic 3He: results from young Hawaiian lava flows Mark D. Kurz
1, Debra
Colodner ~, Thomas W. Trull ~, Richard B. Moore 2 and Keran O'Brien 3
i Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.) z U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 (U.S.A.) ~ Department of Physics and Astronomy, Northern A rizona University, Flagstaff A Z 86011-6010 (U.S.A.)
Received April 18, 1989; revised version received November 9, 1989 In an effort to determine the in situ production rate of spallation-produced cosmogenic 3He, and evaluate its use as a surface exposure chronometer, we have measured cosmogenic helium contents in a suite of Hawaiian radiocarbon-dated lava flows. The lava flows, ranging in age from 600 to 13,000 years, were collected from Hualalai and Mauna Loa volcanoes on the island of Hawaii. Because cosmic ray surface-exposure dating requires the complete absence of erosion or soil cover, these lava flows were selected specifically for this purpose. The 3He production rate, measured within olivine phenocrysts, was found to vary significantly, ranging from 47 to 150 atoms g I yr-~ (normalized to sea level). Although there is considerable scatter in the data, the samples younger than 10,000 years are well-preserved and exposed, and the production rate variations are therefore not related to erosion or soil cover. Data averaged over the past 2000 years indicate a sea-level 3He production rate of 125 + 30 atoms g-1 yr-1, which agrees well with previous estimates. The longer record suggests a minimum in sea level normalized 3He production rate between 2000 and 7000 years (55 _+ 15 atoms g-1 yr-1), as compared to samples younger than 2000 years (125 _+ 30 atoms g-1 yr-i), and those between 7000 and 10,000 years (127 + 19 atoms g 1 yr-1). The minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength, as indicated by archeomagnetic data. However, the production rate variations (a factor of 2.3 + 0.8) are poorly determined due to the large uncertainties in the youngest samples and questions of surface preservation for the older samples. Calculations'using the atmospheric production model of O'Brien (1979) [35], and the method of Lal and Peters (1967) [11], predict smaller production rate variations for similar variation in dipole moment (a factor of 1.15-1.65). Because the production rate variations, archeomagnetic data, and theoretical estimates are not well determined at present, the relationship between dipole moment and production rate will require further study. Precise determination of the production rate is an important uncertainty in the surface-exposure technique, but the data demonstrate that it is feasible to date samples as young as 600 years of age providing that there has been no erosion or soil cover. Therefore, the technique will have important applications for volcanology, glacial geology, geomorphology and archaeology.
1. Introduction The presence of in situ produced cosmogenic 3 H e in t e r r e s t r i a l s u r f a c e r o c k s is n o w well d o c u mented [1-5]. Within the top two meters of the e a r t h ' s s u r f a c e , c o s m o g e n i c 3 H e is d o m i n a n t l y produced by spallation reactions between cosmic ray neutrons and the major elements of the rock [1,2,5]. A l t h o u g h i g n e o u s r o c k s c o n t a i n m a g m a t i c ( i n h e r i t e d ) h e l i u m , c o s m o g e n i c h e l i u m c a n b e distinguished from the inherited helium by crushing a n d s t e p h e a t i n g [1,2]. I f t h e p r o d u c t i o n r a t e o f cosmogenic 3He can be determined, then measure0012-821X/90/$03.50
© 1990 Elsevier Science Publishers B.V.
m e n t o f its a c c u m u l a t i o n will b e c o m e a v a l u a b l e geochronological tool. Surface exposure dating w i t h i n s i t u p r o d u c e d c o s m o g e n i c n u c l i d e s is n o t a n e w i d e a [6,7], b u t t h e r e h a v e b e e n few a t t e m p t s t o e x p e r i m e n t a l l y d e t e r m i n e p r o d u c t i o n r a t e s in rocks, or to evaluate the precision and limitations of the technique. Consequently, there have been few geological applications. C o s m o g e n i c 3 H e is i d e a l f o r e x p o s u r e - a g e d a t i n g b e c a u s e it is s t a b l e a n d h a s t h e h i g h e s t p r o d u c tion rate of any cosmogenic nuclide, approxim a t e l y 100 a t o m s g 1 y r - I a t s e a level [2]. H o w ever, c o m m o n g e o l o g i c f a c t o r s s u c h as e r o s i o n a n d
178
soil cover may lower apparent exposure ages. In order to test the precision, accuracy and age range for which exposure-age dating is feasible, data must therefore be obtained from well-dated samples that have never been eroded or shielded from cosmic rays. Hawaiian lava flows were selected for this purpose because radiocarbon dates are abundant [8] and flow morphology can be used to evaluate surface preservation. In order to constrain the production rate, and test the exposureage dating technique, this paper presents measurements of cosmogenic 3He in a suite of radiocarbon-dated Hawaiian lava flows ranging in age from 600 to 14000 years. The available radiocarbon dates were obtained from trees and treeroots buried by the lava flows and preserved in volcanic ash or soil layers [8-10]. The basalt samples analyzed here were collected from Mauna Loa and Hualalai volcanoes; they were selected primarily on the basis of the radiocarbon geochronology [8-10], the presence of olivine phenocrysts, and the likelihood of obtaining undisturbed surfaces. The results demonstrate that exposureage dating is feasible in this age range, and illustrate the limitations and uncertainties inherent to the technique. The present-day production rate of any cosmogenic nuclide varies significantly with latitude, longitude, and altitude because cosmic rays are deflected by the earth's magnetic field, and are attenuated by the atmosphere [11,12]. Attempts to estimate production rates have primarily relied on spallation rates recorded in photographic emulsions (commonly referred to as "star" production rates) or measurements in materials exposed on the earth's surface [11,12]. Measurements of cosmogenic nuclides in natural materials have demonstrated that estimated production rates often have large uncertainties [13]. In addition, because the earth's magnetic dipole moment has varied with time [14], there may be secular variations in production rate. These effects have been documented to some extent, particularly with respect to calibration of the ~4C dating technique (e.g. [11,15]). However, if exposure-age dating is to be a quantitative technique, the production rates must be precisely determined, and the effects of these parameters on in situ production rates must be known.
M.D. KURZ ETAL.
2. Methods 2.1. Calculation of cosmogenic helium content Previous studies have shown that olivine is an ideal phase for measuring cosmogenic helium [1,2], and because olivine is common in Hawaiian basalts, only samples with olivine phenocrysts were analyzed. Inherited (magmatic) helium in olivine phenocrysts is held primarily within melt and fluid inclusions [16], and the isotopic composition of the inherited helium can readily be determined by crushing in vacuum. In contrast to crushing, heating in vacuo releases a combination of cosmogenic helium and inherited helium [1,2]. The isotopic composition of the cosmogenic helium is enriched in 3He by roughly 104 ( 3 H e / 4 H e of 0.1 as compared to 10 5 for mantle-derived rocks), which allows the two components to be distinguished. The experimental procedure therefore requires that olivine mineral separates from each sample be analyzed twice. First the olivine grains are crushed in vacuo and most of the inherited helium is released and its isotopic composition is measured. The olivine powder that remains is then loaded into a furnace and melted in vacuo. The amount of cosmogenic 3He in the sample, referred to hereafter as 3Hec, can then be calculated from: 3He~. = 3 He m _ 3 Hei. The subscripts c, m and i refer to cosmogenic, melted fraction and inherited helium respectively. Because the amount of cosmogenic 4He is insignificant with respect to inherited 4He (4Hei>>> 4Hec), 3Hei can be calculated from: 3He i = 4 He m [3He/4He] i where the [3He/4He]i is determined by the crushing experiments. The assumptions have been verified by profiles within single lava flows [2]. The mass spectrometry and sample handling procedures are described elsewhere [2,17]. Because production rate is a function of depth in the rock and elevation [2,5,11], the samples were oriented with respect to vertical in the field and the elevation of the site established using topographic maps and verified with an altimeter. The helium results were normalized to sea level (i.e. for altitude and depth in rock) using a simple exponential: 3He c (sea level) =3 Hec ( X ) * exp [( X - 1030)/L]
COSMIC RAY EXPOSURE DATING
W I T H IN S I T U P R O D U C E D
COSMOGENIC
where X is the altitude in g cm 2 (1079 is the value at sea level) and L is the atmospheric attenuation length for the cosmic radiation (L assumed 160 g cm -2 [2,18]). The olivine phenocrysts were separated from a known depth interval (typically 1-4 cm depth) and the results were normalized to the surface, also using this simple exponential, resulting in corrections of only several percent in 3Hec values. The altitude corrections are significant; a sample at 3 km elevation will have a production rate 8 times higher than a sample at sea level. Similarly, if a lava flow has had one meter eroded off the top, the 3Hec observed will be roughly one-half the "true" surface value, demonstrating the potential importance of erosion. 2.2. Uncertainties in the method
Because surface preservation (i.e. well-known depth) is such an important aspect of the tech-
3HE
179
nique, every attempt was made to obtain samples from pnstin,~ lava flows. The sampling efforts were concentrated on the arid, leeward flanks of Hualalai and Mauna Loa volcanoes (Fig. 1) because the locally dry climates and low erosion rates improve the chances for preservation. The field work was based heavily on geological mapping of these volcanoes by Lipman, Lockwood and co-workers [19-21] and Moore et al. [10]. Surface preservation varies significantly with age, climate, topography, and human interference, and each sample was given a "quality rating": a numerical indication of confidence that the surface has remained undisturbed and that the age of the surface is well known (see Table 1). This is based primarily on the surface features of the lava flow, and the local geology. For the young lava flows, which are well exposed and uneroded, it was possible to collect samples for helium analysis from
Ka
08 Fig. 1. Location of the samples on the island of Hawaii. All samples were olivine-bearing lava flowserupted from Mauna Loa or Hualalai volcanoes. Further information on locations is given in Appendix 1.
180
near the site that charcoal was recovered, or to trace the flow to the collection site. This was not possible in all cases. Sample 44 and 49 (9679 and 14,067 years, respectively) were collected in different locations than the charcoal for radiocarbon, and the local geology was used to infer they are from the same lava flow. The geology is less constrained for the older lava flows and hence the age of these samples is less well known than the younger samples. In the case of sample 02, there is good evidence that the site was ash covered in the past (J. Lockwood, personal communication). These factors were also taken into account in giving the quality rating (i.e. resulting in ratings of 3 for samples 2, 44 and 49). In some instances this is subjective because of the difficulty in evaluating factors such as past soil cover and human impact. In other cases (rating of 1), there is virtually no chance that the lava flows have ever been covered or eroded and the sample was collected near the site that the charcoal for ~4C was collected. The 14C ages of Rubin et al. [8] were calibrated using the dendro-chronological time scale of Stuiver and co-workers [22,23], and hence are not subject to carbon reservoir effects, or to geomagnetic fluctuations. It is however important to note that although t4C dates are treated here as the absolute geochronometer against which we "calibrate" the 3He data, the 14C data must be viewed with some caution. The 14C data were obtained from charcoal beneath the lava flows [10], and there are a number of ways the charcoal could have been compromised. Circulation of groundwater and equilibration with " y o u n g " carbon from the hydrosphere could result in spuriously young ages [24]. Alternatively, plants growing in volcanic areas can respire volcanic CO2 which contains no ~4C, resulting in spuriously old ages [25]. However, because of the abundance of well preserved charcoal in ash layers beneath Hawaiian lava flows, and the relative youth of the lava flows, these are among the best dated lava flows available. The uncertainties in 14C ages, given in Table 1, are the lo counting errors given by Rubin et al. They caution that these errors do not "include variable factors such as contamination, isotopic fractionation, and other laboratory uncertainties". Unfortunately, it is impossible to evaluate which of the samples may have been affected by these
M.D. K U R Z E T AL.
factors, but the counting error is clearly a lower limit. For sample 31 there are four replicate ~4C ages (890, 740, 740, and 910; see Appendix 1), all with a counting error of _+60 years, that yield a mean of 820 _+ 90 years. Although these four replicates are not enough to rigorously determine the errors, the standard deviation of the mean provides an objective means of scaling the counting errors. In order to provide a more conservative error estimate we therefore scale the lo values of Rubin et al. (given in Appendix 1) by 1.5 (i.e. the standard deviation/counting error). The data in Table 1 are therefore the calibrated ~4C ages with conservative error estimates; the raw data as reported by Rubin et al. [8] are given in Appendix 1. Replicate 14C ages for sample 04 were 2300 + 60 and 2440 _+ 70 years which are also consistent with the error estimates. For the two lava flows with replicate ~4C dates, we utilize the mean in Table 1. In order to test the validity of the various corrections and the assumption that the surfaces are undisturbed, replicate samples were analysed for helium in several of the lava flows. All five analyses from the 655 year old lava flow have sea-level-normalized 3He c contents and production rates that are within two standard deviations from the mean (7.68 _+ 1.9 × 10 4 atoms g - l , and 117 + 29 atoms g-~ yr-1); the standard deviation of the mean is similar in size to the calculated lo errors. There are other replicate analyses for lava flows of various ages (2271 years, samples 05 and T87-08; 5345 years, sample 48; 8035, sample 01; 8514 years, samples 07 and 08), all of which are internally consistent within 2 standard deviations. The younger samples have significantly larger uncertainties because of the small n u m b e r of spallation-produced 3He atoms. Consequently, the cosmogenic helium signal becomes harder to detect, as illustrated by the small difference between crushed (inherited) and melting results in Table 1. The calculation of uncertainties in Table 1 does not include all possible sources of error. For example, in correcting for inherited 3He, we assume that the inherited 3 H e / 4 H e ratio of the phenocrysts within a given lava flow is uniform. Problems with this assumption could arise if a given lava flow has several populations of phenocrysts with different 3 H e / 4 H e , or if there is a contribution of implanted radiogenic 4He from the uranium enriched matrix. Any atmospheric contamination
181
COSMIC RAY EXPOSUREDATING WITH IN SITU PRODUCED COSMO(3ENIC 3HE TABLE 1 Helium data and calculated production rates for Hawaiian lava flows Sample
4He (cm 3 STP g-1 ( × 1 0 -9)
(3He/aHe)m (3He/aHe)i _+1 s
_+1 s
603+150 655_+ 67 655_+ 67 655-+ 67 655_+ 67 655_+ 67 820-+ 90 2271_+ 90 2271+ 90 2271-+ 90 2411_+ 90 2772+120 3091_+105 4493-+105 5345_+120 5345_+120 7204_+150 8035_+345 8035_+345 8514_+105 8514_+105 9679_+300 9941_+195
28.02 0.72 1.65 4.43 2.05 0.717 0.669 2.01 1.36 2.36 1.00 14.22 0.973 9.15 8.29 23.20 27.96 0.993 0.462 0.870 1.37 4.28 1.01
9.3 +0.04 11.88 _+0.77 13.39 _+0.27 10.09 -+0.27 9.86 _+0.2 10.57 _+0.64 12.8 -+0.2 11.22 _+0.2 10.96 _+0.31 9.943-+0.35 10.91 _+0.4 10.41 +0.1 17.61 _+0.59 13.01 -+0.15 13.45 -+0.17 10.16 -+0.08 11.66 _+0.08 40.4 _+0.74 76.87 -+1.4 41.73 _+0.87 34.91 _+0.75 26.12 -+0.29 28.63 _+0.67
8.848_+0.05 8.5 _+0.07 8.57 _+0.12 8.53 -+0.06 8.57 _+0.06~ 8.57 -+0.06 8.78 -+0.28 9.18 _+0.13~ 9.18 -+0.13 8.9 -+0.28 8.411_+0.15 8.68 -+0.11 8.34 _+0.06 8.93 -+0.06 8.97 -+0.1~ 8.97 _+0.1 9.836_+0.08 9.08 _+0.08 9.17 -+0.27 8.76 +0.08 10.12 _+0.12 8.88 -+0.06 1 3 . 1 +0.1
10,435_+150 13,312-+225 14,067-+225
5.17 6.69 0.392
Calibrated
|4C age 3-1 s (years)
KS78-47" KS87-03 KS87-14 KS87-15 T87-4 T87-4 KS87-31 T87-8 T87-8 KS87-05 KS87-04 KS87-43" KS87-13 KS87-46" KS87-48" KS87-48" KS87-42 KS87-01C KS87-01B KS87-08 KS87-07 KS87-44" KS87-02 RM889490* KS87-51" KS87-49"
13.52 _+0.19 7.84 +0.06 15.14 _+0.2 9.03 +0.15 186.5 _+4.2 9.42 -+0.19
3Hec -+ 1 s 3Hec (cm3 STP g 1 production ×10 14) rate-+ 1 s (atoms g l y r 1) sea level
Altitude (ft)
Depth (cm)
Sample surface quality
1.75 _+0.60 0.336_+0.10 1.101_+0.09 0,956_+0.21 0.366_+0.08 0,199_+0.09 0.372_+0.07 0,568_+0.09 0.335-+0.09 0.341-+0.18 0.348+0.09 3.40 -+0.43 1.25 _+0.12 5.17 _+0.32 5.14 -+0.35 3.82 -+0.63 7.06 _+0.78 4.30 _+0.24 4.33 _+0.25 3.97 -+0.27 4.69 _+0.23 10.21 -+0.31 2.17 -+0.16
161.9_+68 139.2_+45 116.9_+15 97.1_+23 151.3_+36 82.0-+36 124.0_+28 67.7+10 39.9_+11 41.7_+22 36.6_+9 70.0_+9 63.2_+6 63.4_+4 54.4_+4 40.5_+7 71.8_+8 141.2+10 139.8-+10 131.7-+9 125.6_+6 57.4-+2 56.6_+4
7680 120 6550 6780 140 140 60 80 80 120 380 7600 2600 7820 7720 7720 6480 280 280 20 920 7890 280
2 2.5 2.5 2.5 2 2 2 2 2 4 2 2.5 2.7 3 3.5 3.5 5 3 2 3.5 3 3 2
1 1 1 1 1 1 1 1 1 1 1 1 2 1 2 2 2 2 3 2 2 3 3
4.06 -+0.20 5.66 -+0.32 9.60 _+0.85
96.1-+5 74.9-+4 182.7-+16
650 2000 150
4 2 2
1 3 3
All samples were olivine mineral separates (0.15-0.5 g). The subscript m denotes results for melting of powders in vacuum, while the subscript i denotes results for crushing of 1-2 mm grains (inherited magmatic helium). 3 H e / 4 H e ratios are reported relative to the atmospheric value (/~/Ratm; where R a t m = l . 3 8 4 × 1 0 6). 14C ages are taken from Rubin et al. [8] and calibrated using the dendrochronological time scale of Stuiver et al. [22,23]. Values of 3Hec given are calculated for the altitude and depth in the rock of the sub-sample (i.e. calculated directly from other parameters given), values for production rate are normalized to sea level (see text). 3Hei is calculated from crushing and melting data: 3He i =4Hem[3He/4He]i . In all samples, crushing and melting were performed on the same mineral separate, except those denoted by ~ , in which the crushing data from a different sample of the same flow was used. 4 He blanks over the course of this study were between 3 and 4 × 10 n cm 3 STP with atmospheric 3 H e / 4 H e ratios, and were reproducible to 2 × 10-12 cm 3. Techniques are described in [2,15]. An asterisk ( * ) denotes samples from Hualalai volcano, all others from Mauna Loa; locations of the lava flows can be found in [8,10,19-21] (see also Appendix 1). Quality rating is an indication of confidence in the sample quality/preservation of the surface: 1 = excellent, 2 = some doubt, 3 = significant doubt or geological evidence to indicate poor preservation.
o r p r o b l e m w i t h b l a n k c o r r e c t i o n s will a l s o c o n tribute an error. A contribution from radiogenic h e l i u m is u n l i k e l y b e c a u s e t h e s a m p l e s a r e s o y o u n g , a n d t h e r e is a t p r e s e n t n o e v i d e n c e f o r inhomogeneity. N e v e r t h e l e s s , it is i m p o r t a n t to n o t e t h a t i n s e v e r a l c a s e s it w a s n e c e s s a r y t o utilize the inherited 3He/4He ratio from different
analyses of the same sample (samples noted with # i n T a b l e 1). I n m o s t c a s e s , t h e s a m e m a t e r i a l was analyzed in crushing and melting. We also include no contribution tn the error from the altitude correction. Due to the youth of the samples, any subsidence or uplift should be minimal. The value of the attenuation coefficient
182
M.D. KURZ
is also a possible source of error, because different investigators have used slightly different values. Lal uses an attenuation coefficient of 150 g cm 2. The value of 160 g cm -2 is used here because there it was obtained by measurements of in situ production that seem most analagous to the present situation [2,18]. 3. Results and discussion
3.1. Relationship between 3He and time Sea-level normalized 3He~ correlates well with calibrated ~4C age (Fig. 2), suggesting that it will be a useful geochronometer. However, some complications are also obvious, most notably the scatter in Fig. 2. All of the likely geological processes will lower the observed amount o f 3Hec; natural erosion, anthropogenic erosion, soil cover, and ash cover will all yield lower amounts of 3He~ than expected. Diffusive helium losses would also lower the amount of 3He~, although available diffusion data suggest that cosmogenic 3He diffuses too slowly to be important. Extrapolation from step-heating results yield a 3He~ diffusion coefficient of 5 × 10 -23 cm2/s in olivine (at 20°C), which suggests that diffusive losses are insignificant for the present samples ([26], and in preparation). Virtually the only way to obtain high 3He~ values would involve a radiocarbon date that is
3.0 / | erosion or soil cover
[
--
2.O ? 1.0
.io
" i__ 5000
i
__
i
10000
J ~5000
Cal/braled 140 Age/yeors b.p.] Fig. 2, Measured concentrations of cosmogenic helium (3 Hec), corrected to sea level, plotted against calibrated radiocarbon age. The half-filled symbols indicate those samples for which the surfaces were suspect due to erosion, soil cover, or uncertain age information: quality rating of 3 in Table 1). The older samples show more scatter on the diagram, which is attributed to poorer surface preservation.
ET AL.
spuriously low. Based on this simple reasoning, and the field evidence, the greater scatter for the older samples in Fig. 2 is most easily explained by poorer preservation of the older surfaces. (Several of the samples falling below the trend are suspect for geological reasons--see rating scale, Table 1.) Therefore, we treat the 3He c data as minimum values for each age, and attribute low 3Hec values to one of the geological processes mentioned above. As mentioned earlier and in Appendix 1, some of the samples also have uncertainties in their ages related to field geology. The sample with the highest 3Hec content in Fig. 2 (sample 49) has a low quality rating partly for this reason. The collection site for sample 49 was inferred to be the same as that of the 12,950 year radiocarbon date based on the local geology. Because soil cover and erosion would lower the apparent exposure age, the high 3He content in this sample may indicate that this age is too young for the site where the sample was collected. Because the only way to obtain too great a 3Hec content is with a spuriously young age this is reasonable conclusion for this sample (note the quality ratings). The 3He c values and ~4C ages can also be used to obtain 3He production rates, i.e. dividing 3Hec by age. This yields a production rate integrated over the entire surface exposure age of the sample. As is shown in Fig. 3, the calculated (integrated) production rates vary considerably. In addition to geological effects on surface preservation, numerous factors can alter production rates: altitude, latitude, and cosmic ray flux variations due to solar wind or terrestrial magnetic field fluctuations. Due to the young age of the samples, it is extremely unlikely that altitudes or latitudes have been altered by subsidence or tectonics. All local geological factors will lower the production rate (i.e. by erosion or cover), and the markedly low production rates between 2000 and 7000 years B.P. could be attributed to erosion or soil cover. However, these samples are among the youngest and have the best preserved surfaces, making this unlikely. In addition, production rates are consistently lower in this time interval although the samples come from different flows, different altitudes and different volcanoes. Higher production rates are observed for the lava flows younger than 2000 and older than 7000
COSMIC RAY EXPOSURE DATING
WITH IN SITU PRODUCED
COSMOGENIC
250
~,
200-
o
150-
_= I~
100
• ~
•
50-
n
g •'r
o
I
0
I
I
I
] 5000
I
I
~
I
I 10000
F
I
I
I 15000
Calibrated 14C age (years b.p,)
Fig. 3. Production rate of cosmogenic 3He (in atoms g - 1 y r - 1) as a function of lava flow age. The production rate, obtained by dividing 3Hec by age, is integrated over the exposure age of the sample. The m i n i m u m in production rate between 2000 and 7000 years may be attributed to increased magnetic intensity (dipole moment; see Fig. 4). As in Fig. 2, the half-filled symbols indicate samples having relatively poor surface preservation; scatter for these older samples is attributed to variable surface preservation.
years. The mean 3Hec production rate in the time period 2000-7000 years is 55 + 15 atoms g t y r - t and the mean value for younger samples is 125 + 30 (for samples 7000-10,000 years old, with good quality rating, the mean is 127 + 19 atoms g-a yr a). Applying a standard t-test to these data suggests that the populations are distinct at greater than 96% confidence level. Unfortunately the best evidence for the difference in production rate comes from the youngest samples and those younger than 2000 years old have the largest uncertainties. With this uncertainty in mind, we conclude that the 3He production rate varied significantly within the last 10,000 years, unrelated to surface preservation. More data are clearly required to confirm this variation. 3.2. Possible causes of production rate variations The possibility that there are significant variations in production rate with time is a fundamental problem for the application of cosmogenic 3He to exposure-age dating. The consistency of the data presented here, and the good prese~ation of the younger lava flows, suggests that the variations for the last 10,000 years are not related to erosion or cover. With the caveat that the youngest samples have large uncertainties, some of the produc-
3HE
183
tion rate variations therefore may be related to changes in terrestrial cosmic ray flux due to solar or geomagnetic modulation. Significant variations in atmospheric ]4C content (of up to 10%) have been previously observed, and are attributed to both of these processes [15,27-31], in addition to reservoir effects related to changing amounts of carbon in the atmosphere and ocean [32-34]. Solar wind modulation of the cosmic ray flux is unlikely to have an effect on 3He production, because the solar variations occur on a shorter timescale than those observed in Fig. 2, and should be least important at low latitudes such as at Hawaii (20 o N). T h e 3 H ¢ c production discussed here differs from atmospheric 14C production in that 3He is produced in situ within the rock, whereas the ]4C is produced globally in the atmosphere. Because the atmosphere is well mixed and is a large reservoir of carbon, ]4C production rate variations are damped out, and are affected by exchanges between reservoirs. Thus, the in situ produced isotope should display a greater dependence on geomagnetic field. 3Hec also differs from ]4C because it accumulates in the rock, acting as an integrator of the production through time, which complicates the time dependence. One possible cause of the 3He: production rate variation is the strength of the terrestrial dipole. Because the cosmic radiation incident on the top of the atmosphere consists of charged particles, it is deflected by the earth's magnetic field, which has the largest effect near the equator and the least near the poles. One test of this hypothesis is to compare 3Hec production rate and dipole moment variations. Fig. 4a shows a recent compilation of the archaeomagnetic data by McEllinny and Senanayake [14] fit to a third-order polynomial (solid line). As mentioned earlier, t h e 3Hec acts as an integrator of the production rate and will be related to the inverse of the integrated dipole moment. Fig. 4b shows the integral of the curve in 4a divided by time, and inverted because cosmic ray flux is inversely proportional to dipole moment; the 3He¢ production rate variation over the same time scale is shown in Fig. 3. The minim u m in production rates shown in both Figs. 3 and 4b occurs over the same time interval, which suggests that dipole moment variations could cause some of the production rate variations.
184
M.D. K U R Z E T A L .
~
1.40
/
1.20
5\o t
1.oo o.80 0
2000
4000
6000
8000
2000
4000
6oo0
8o00
1.00
J'~ 0.90
% 0.80
0.70
0
Calibratedt4C Age (yearsb.p.) Fig. 4. (a) Dipole moment fluctuations (from archaeomagnetic measurements compiled by McElhinny and Senanayake [14]) normalized to the present value of the dipole moment; the points are 500 year averages. The curve is a third-order polynomial fit to the data points. (b) The inverse of the integral of the curve shown above, divided by time. The curve in part a is integrated because 3Hec accumulates in the rock over the entire time interval, and it is inverted because cosmic ray flux is diminished during times of strong dipole moment. Note that the minimum in production rate corresponds to the minimum observed in Fig. 3.
The dipole m o m e n t data shown in Fig. 4a are averages over 500-year intervals (in order to filter out the non-dipole variations and average the noisy data set); different attempts to represent the global dipole in this fashion differ with respect to size of the variations [14,30,34], but agree that there was a maximum in dipole m o m e n t approximately 2500 years before present. Local non-dipole components may also have significant effect on the sealevel production rates, which is another reason that the compilation in Fig. 4 [7] is only an approximation to the field intensity near Hawaii. Therefore, although the magnitude of the variations is poorly constrained, the shape of the curve in Fig. 4 is reasonably well defined. Unfortunately, even if the dipole m o m e n t variations were well defined, there are few means of evaluating the magnitude of the effect that the fluctuations would have on in situ production rates. One of the few published calculations is the model of O'Brien [35] which calculates production
in the atmosphere from atmospheric transport equations. In order to evaluate the possible effect of geomagnetic field intensity on 3He production rate (Fig. 4), the model of O'Brien [35] was applied to the latitude of Hawaii (19.75 ° geomagnetic latitude, cut-off rigidity 13 GV). The model uses the straight-ahead approximation to the stationary Boltzman equation to calculate cosmic ray propagation in the atmosphere, and spallation yields are calculated from cross section data [35]. Previous use of this method yielded good agreement with atmospheric a4C inventories, but were not applied to in situ production. The calculations yielded a sea-level star production at Hawaii of 1.34 × 10 5 stars g-1 s 1. Using the yield values of Lal and Peters [11] for tritium and 3He (0.14 and 0.12 per spallation event, respectively) gives a present-day 3He c production rate of 110 atoms g 1 yr 1, which is in excellent agreement with values determined from the lava flows less than 2000 years in age (125_+30 atoms g 1 yr-1). These production rates are in reasonable agreement with estimates using the methods of Lal and Peters (105 atoms g 1 yr-1 [11]), and Y o k o y a m a et al. (101 atoms g - ] y r - 1 [12]). The only previous attempt to directly measure 3Hec production rate, using a single 28,000 year old Hawaiian lava flow, yielded a value of 97 atoms g 1 yr 1 [2,36]. In order to estimate the effect of magnetic field on production, the calculations were performed at various vertical cutoff rigidities, which is analogous to changing the magnetic field. The results of these calculations at sea level and at 3 km altitude are shown in Fig. 5. Changes in dipole m o m e n t have a smaller effect on production at sea level, because a greater proportion of the sea-level production is derived from high-energy primaries that are unaffected by magnetic field (i.e. energies >> 13 GV). However, even at 3 km altitude, the predicted effect on production is significantly less than the 3He production rate variations shown in Fig. 3. The model predicts a variation in production rate of 1.25 at sea level and 1.35 at 3 km altitude for dipole m o m e n t variation between 0.5 and 1.5 the present value. The predicted variation for the samples in figures 2 and 3 would be even less due to the integrating effect of total exposure, 1.15 and 1.24 at sea level and 3 k m altitude respectively. A n o t h e r method of estimating the effect of geomagnetism on production rate is to use ob-
COSMIC RAY EXPOSURE DATING WITH IN S1TU PRODUCED COSMOGENIC ~HE 14
t~
10
os
[ 05
i 10
15
J 20
Normalized Dipole Moment (M/Mp)
Fig. 5. Variation in p r o d u c t i o n rate predicted by the model calculations of O'Brien [35]. Both the field a n d p r o d u c t i o n rates are normalized to the rates o b t a i n e d for the present field (1.0), to show the steeper d e p e n d e n c e of p r o d u c t i o n rate on dipole m o m e n t at higher altitude. The star p r o d u c t i o n rates (in air) o b t a i n e d for the present field (rigidity 13 GV) were 1.34 × 1 0 - 5 at sea level (1033 g c m 2) and 2.25 × 10 - 4 stars g 1 s-1 at 3 k m elevation (700 g cm 2).
served rates of star production (i.e. spallation) rates in photographic emulsions. Lal and Peters [11] have used this method to predict production rates of various cosmogenic nuclides. Based on star production rates, Lal ([11,37], and personal communication) estimates variation by a factor of 1.8 in production rate (at 3 km altitude) for the same variation in dipole moment (0.5-1.5 the present value), which would result in an integrated variation in 3Hec production of 1.65. The data in Table 1 yield a 3He production rate variation by a factor of 2.3_+ 0.8, if samples younger than 2000 years are compared to those 2000-7000 years in age. The fact that the variations predicted by both the atmospheric propagation model [35], and empirical star production data [11] are significantly lower than those observed, suggests that dipole moment variations may not account for all the 3He production rate variations described here. However, given the large uncertainty in both the measurements and the theoretical calculations, a significant fraction of the production rate variation shown in Fig. 3 could have been produced by variations in dipole moment. As mentioned above, an additional factor is the possibility that non-dipole components could have resulted in locally larger geomagnetic
185
fluctuations. For example, Coe et al. [38] measured paleD-intensities in Hawaiian lava flows significantly higher than predicted (particularly between 2000 and 5000 years) which they attributed to the non-dipole field. This suggests that the local intensity variations may have been significantly larger than is shown in Fig. 4a. However, this hypothesis is difficult to evaluate without further data. Quantification of the relationship between production rates and dipole moment will require additional data from dated surface rocks, and will be important to exposure-age dating because a correction will be required. It is important to note that geomagnetic effects are latitude dependent, being most important near the equator, and insignificant at latitudes greater than 50 ° . Therefore, this will be unimportant for high latitude studies. If a relationship between 3Hec production rate and dipole moment can be established, then helium measurements could provide a new constraint on the postulated variations in the earth's magnetic field intensity. This use of in situ produced cosmogenie nuclides was previously suggested by Lal et al. [37], and holds promise for resolving some existing controversy regarding the dipole moment. For example, if the 3He production rate variations are related to dipole moment, this can be used to distinguish between carbon reservoir effects and dipole moment effects on atmospheric 14C inventories. However, given the uncertainties in both production rates, and theoretical estimates of geomagnetic effect on production rates, this application will require considerable effort in the future.
4. Conclusions
This study demonstrates that helium exposureage dating is feasible in extremely young samples, as young as 600 years at sea level. At higher elevations, significantly younger ages could be determined. The data from young radiocarbon-dated lava flows illustrate that one important uncertainty with the technique is the necessity of constant exposure to cosmic rays, and hence the complete absence of cover or erosion. Therefore, in cases where surface preservation cannot be evaluated, the technique will simply yield minimum exposure ages, which may still be useful.
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However, where geological information can be used to evaluate preservation and exposure, the technique will be an extremely useful geochronometer, particularly where other dating methods are not possible. Therefore it should have important applications in glacial geology, geomorphology, and volcanology. The main goal of this study was to precisely determine the 3Hec production rate. The data suggest a present-day sea-level production rate in olivine of 125 _+ 30 atoms g-~ yr 1 (as deduced from the lava flows less than 2000 years in age). Because of the number of determinations, and the quality of the surfaces, this value should supercede the previous estimates [2,36]. Because the dominant helium producing reaction is neutron induced spallation on the major elements of the rock, which in this case are atoms of O, Si, Mg, and Fe, this production rate should be weakly composition dependent, and will be useful for other silicate minerals. However, the large uncertainties in the production rate determinations demonstrates the need for additional studies of this kind. The temporal record deduced from dated lava flows displays a surprising 3Hec production rate variability over the last 10,000 years. The production rate determined from samples 2000-7000 years of age is significantly lower than obtained from those less than 2000 years, and those older than 7000 years. Because the lava flows were selected to minimize problems of surface exposure, and the younger samples had well preserved surfaces, we believe that the variations in production rate are not entirely due to erosion or soil cover. The minimum in production rate between 2000 and 7000 years B.P. corresponds to a maximum in past magnetic dipole moment and may be partly explained by geomagnetic modulation of cosmic ray flux. However, the observed variation in 3He production rate, a factor of 2.3 +_ 0.8, is poorly determined due to the large uncertainties in the helium measurements in very young samples, and the problem of variable surface preservation for the older samples. Two methods of estimating the effect of dipole moment on production rates, predict a variation by a factor of 1.15-1.65 in 3He production rate. Although this is significantly smaller than that obtained from the helium measurements, these calculations demon-
M.D. K U R Z E T A L .
strate that the relationship between dipole moment and production rates should be considered in cosmic ray exposure age dating at low latitudes. Further data, from different sample types and geographic locations, will be required to test the relationship between dipole moment and production rates, as will further efforts to refine theoretical estimates. Perhaps most importantly, the data reported here demonstrate some of the problems that must be overcome in utilizing cosmic ray exposure-age dating. Because geological information can be used to evaluate surface preservation, the primary obstacle to quantitative use of helium exposure-age dating is the uncertainty in ~Hec production rates, and the possible variation in production rates. If the temporal production rate variations are related to geomagnetic variations, then any low latitude use of this technique will require calibration, much like the radiocarbon calibration with dendrochronology. At high latitudes ( > 55°), the earth's magnetic field has little effect on cosmic rays, and dipole moment variations will have a negligible effect on production. Therefore, the uncertainty in production rates may be smaller for cosmic ray exposure dating of high latitude samples.
Acknowledgements We thank Karl Turekian for his editorial patience and wisdom in dealing with this paper. M.D.K. wishes to thank Bill Jenkins for numerous helpful conversations, Dempsey Lott for his assistance with the vacuum equipment, and Armine Gulesserian for assistance with sample preparation. T.W.T. and D.C. gratefully acknowledge support from the Education office of the W H O I / M I T Joint Program in Oceanography, and an NSF fellowship respectively. We thank D. Sampson, Frank Trusdell, Peter Lipman, and Jack Lockwood for help with the sampling, and P. Lipman, E. Brook, and J. Quick for comments on the manuscript. We thank D. Lal for providing unpublished calculations, and for numerous stimulating conversations. The manuscript also benefitted from the labors of several anonymous reviewers. This work supported by NSF EAR8610611 and EAR88-3783 and NASA N A G 9-69. This is a W H O I Contribution no. 7257.
COSMIC RAY EXPOSURE DATING WITH IN SITU PRODUCED COSMOGENIC 3HE
Appendix 1-Descriptions of lava flows sampled *KS87-47. Pahoehoe lava flow with original surfaces, just east of Malekule, elevation 7680 ft (19°42'N, 155°52'40"W), Kailua quadrangle), Hualalai volcano. KS87-03, 14, 15 and T87-4. Aa lava flow, originating at southwest rift of Mauna Loa volcano (Pohina flow). Radiocarbon date is 640 _+45, USGS ~W4025. Sampled at several locations. 03: elevation 120 ft ( 1 9 ° 0 7 ' 1 8 " N , 155°32'W, Naalehu quadrangle). 14: elevation 6550 ft ( 1 9 ° 1 4 ' 5 5 " N , 1 5 5 ° 3 8 ' 2 0 " W , Puu o Keokeo). 15: elevation 6780 ft ( 1 9 ° 1 5 ' 3 4 " N , 155°38'14"W, Alika Cone). T87-4: elevation 140 ft ( 1 9 ° 0 7 ' 2 2 " N , 155°31'56"W, Naalehu quadrangle). KS87-31. Pahoehoe lava flow, with glassy flow textures, originating at south west rift of Mauna Loa volcano (named Kipuka Nene Flow by Lipman and Swenson). Sampled near Highway 11 at elevation 60 ft ( 1 9 ° 0 7 ' 1 0 " N , 155°32'03"W, Naalehu quadrangle). Radiocarbon dates: 890 _+60, USGS :~4137; 740 _+60, USGS :~W4156; 740 _+60, USGS ~W4012, 910 ± 60, USGS :~W4231. The mean of these determinations 820 -+ 90 is used here as the date for this lava flow. KS87-05, T87~8. Picritic lava flow originating at south west rift of Mauna Loa Volcano, containing 5-10% olivine phenocrysts, named Kaalaiki flow. Sampled near highway 11. Radiocarbon age is 2180 -+60; USGS ~W4015. 05: elevation 120 ft (19 ° 0 6 ' 2 7 " N , 155 ° 32'35"W, Naalehu quadrangle). T87-8: elevation 80 ft ( 1 9 ° 0 6 ' 2 3 " N , 155°32'21"W, Naalehu quadrangle). KS87-04. Eastern lobe of aa lava flow originating at south west rift of Mauna Loa Volcano, named Ninole flow by Lipman and Swenson. Elevation 380 ft, radiocarbon dates are: 2300 -+60, USGS #W4008; and 2440 -+70, USGS ~W4142. The mean of these two determinations is used here. Punaluu quadrangle. *KS87-43. Aa flow, with well preserved flow surface and sparse olivine, near summit of Hualalai volcano, (19°42'N, 155°52'W, Hualalai quadrangle). Elevation is 7600 ft, radiocarbon age is 2670 -+ 80; USGS ~W5076. KS87-13. Picritic aa lava flow, at elevation 2600 ft, Mauna Loa volcano (19 ° 20' N, 155 o 24' 27" W, Wood Valley quadrangle). Radiocarbon age is 2940 -+ 70 (USGS :~ W3841). * KS87-46. Eruption spatter, collected just northwest of Hainoa Crater, Hualalai volcano at 7820 ft elevation (19°41'52"N,
* Asterisk denotes samples from Hualalai volcano. All others from Mauna Loa volcano.
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155 ° 52'18"W, Hualalai quadrangle). Radiocarbon age is 3990 -+70, USGS :~W5132. *KS87-48. Aa lava flow, collected just southwest of Malekule, with 5-7% olivine, elevation is 7720 ft (19°41'47"N, 155 o 53' 03" W, Kailua quadrangle). Radiocarbon date is 4720 -+80; USGS :~W4378. *KS87-42. Aa lava channel, just east of Waiaha stream gulley, olivine and plagioclase-phyric basalt, elevation 6480 ft ( 1 9 ° 4 0 ' 3 0 " N , 155°51'45"W, Hualalai quadrangle). Radiocarbon date is 6360-+ 100; USGS ~W5297. KS87-01. Olivine-rich lava flow in Hilo Municipal gulf course. Radiocarbon age is 7230_+230, USGS :~W5599. Sampled in two different locations, one north of stream bed (B) and one south of stream bed (C). KS87-07. Awawa Kahao lava flow at Puu Maemae, from ridge just south of wind farm, near South Point ( 1 8 ° 5 9 ' 3 0 " N , 155°40'10"W, Ka Lae quadrangle), at 920 ft elevation. The lava flow is poorly exposed, in a sparsely settled area. Radiocarbon age is 7750-+70; USGS ~W4351. KS87-08. Same lava flow as # 7 (based on Lipman and Swenson map), from pahoehoe surface at east end of flow, at Hanalua Bay, elevation 20 ft ( 1 8 ° 5 2 ' 2 2 " N , 155°40'03"W, Ka Lae quadrangle). Well preserved, and nicely exposed pahoehoe surface. Radiocarbon age is 7750+70; USGS ¢~W4351. *KS87-44. Massive olivine-rich spatter from rim of unnamed crater just north of Hainoa Crater, at 7890 ft elevation (19°41'46"N, 1 5 5 ° 5 2 ' 1 0 " W , Hualalai quadrangle). This sample was not actually radiocarbon dated, but was inferred to be of same age as dated flow (age 8770_+200; USGS :~W5299) having similar major element chemistry. Radiocarbon date is from northeast edge of same cone. KS87-02. Puu Hoakalei Picrite, sampled from 3 m high lava ridge in Hilo, just north of intersection of Ainola and Malaai roads. Radiocarbon age is 9020_+130. Mauna Loa volcano. This sample has a poor quality rating due to proximity to housing, and the likelihood that volcanic ash covered the area before human settlement (J. Lockwood, personal communication). *KS87-49. Olivine-rich basalt near intersection of highway 11 with 190, on east side of road. Elevation 150 ft ( 1 9 ° 3 8 ' 5 5 " N . 155 ° 59'48" W, Kailua quadrangle), radiocarbon age is 12,950 -+150. This sample is given a poor quality rating due to proximity to populated area, and due to the fact that sample was not collected near charcoal sample for radiocarbon dating, and is therefore inferred to be from same lava flow. *KS87-51. Olivine-rich aa from ridge near the western edge of Puu Anahulu trachyte flow from Puu Waa Waa (19°48'N. 1 5 5 ° 5 0 ' 3 0 " W , Puu anahulu quadrangle). 12,320+150, USGS ~W4365. Elevation 2000 ft.
188 *RM88-9490. Olivine-rich pahoehoe spine at Ahinahena, near the western edge of the Puu Anahulu trachyte flow from Puu Waawaa. 9490_+ 100; USGS ~W-4371. Elevation 650 ft. Good original surface.
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