In situ production of terrestrial cosmogenic helium and some applications to geochronology

Geochimica et Cosmochimica Acta Vol. 50, pp. 2855-2862 © Pergamon Journals Ltd. 1986. Printed in U.S.A.

0016-7037/86/$3.00

+ .00

LEITER In situ production of terrestrial cosmogenic helium and some applications to geochronology* MARKO. KURZ Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.

(Received June 30, 1986; accepted in revised form November 17, 1986) Abstract-The concentrations of cosmogenic 3He have been measured in a series of basaltic drill core samples from Hawaiian volcanoes Haleakala and Mauna Loa. The 3He concentration in the surface of a radiocarbon dated Mauna Loa flow (20,000 years) gives reasonable agreement with a theoretical production rate of 140 atoms g-Iyrl, and suggests that the uncertainty in this rate is roughly 10%. The results illustrate the feasibility of using 3He to measure exposure ages of young basaltic lava flows and for measuring erosion rates. Erosion rates calculated from the three Haleakala cores range from 7 to II meters/million years. The drill core data demonstrate that accurate depth control is crucial to the use and evaluation of cosmogenic helium. Depth profiles from several of the older cores display a non-exponential depth dependence of 3Hec below 170 g cm- 2, which is attributed to the contribution from 6Li(n, a)T, where the neutrons are from stopped muons. This has important implications for depth dependence of cosmogenic 3He because muons are weakly attenuated compared to the nucleonic component that produces spallation.

I. INTRODUCTION

COSMOGENIC NUCLIDES are of fundamental importance to many aspects of earth and planetary studies, with new applications being made possible by advances in accelerator mass spectrometry (REEDyet aI., 1983). The recent discovery of cosmogenic helium in terrestrial surface rocks adds a new and potentially useful nuclide to the list of terrestrial cosmogenic nuclides (KURZ et al., 1985; KURZ, 1986; CRAIG and POREDA, 1986). Unlike many other cosmogenic nuclides, 3He is stable, making it useful for measuring long time scale processes (i.e.• > I m.y.). In addition, the production rate for 3He is among the highest of cosmogenic nuclides and measurement by conventional mass spectrometry is quite sensitive (present detection limits are -5000 atoms). These characteristics make it ideal for application to measurement of erosion rates, exposure ages and ancient cosmic ray fluxes. Cosmogenic helium is also ofconsiderable importance to mantle geochemists, since it interferes with the measurement of magmatic helium in sub-aerially erupted igneous rocks. There are a number of important issues that must be addressed before the applications ofcosmogenic helium can be properly evaluated. First, terrestrial production rates are not precisely known. Although several authors have made theoretical estimates ofproduction rates (LAL and PETERS, 1967; YOKOYAMA et al.. 1977), which can be used to estimate 3He production (e.g.• KURZ, 1986), they have never been experimentally verified. As a corollary to this, the production mech-

• WHOI Contribution No. 6335.

anisms are only qualitatively understood, which means that the attenuation length, and hence depth dependence of the helium producing cosmic radiation, are also unknown. The goal ofthis study is to gain a better understanding of the 3He production rates and mechanisms, and to test its importance for measurements of erosion rates, exposure ages, and mantle helium in surficial rock samples. One ofthe major problems with the earlier study of cosmogenic 3He was that the samples were few in number, and came from unknown depth in a single lava flow. Because production of a cosmogenic nuclide by spallation decreases exponentially with depth, this can introduce quite large errors into calculations of erosion rates and exposure ages. In addition, as shown below, the depth dependence of production is an important means of evaluating production mechanisms. The strategy employed here was to obtain samples that are well located with respect to orientation and depth, within single lava flows, in order to minimize uncertainty regarding shielding depth. Cosmogenic 3He depth profiles were measured in drill cores from geologically well-constrained basaltic lava flows from Mauna Loa and Haleakala volcanoes in the Hawaiian Islands. The drill core from Mauna Loa was collected from a young pahoehoe flow 4C age 20,000 years; LIPMAN and SWENSON, 1984) that has retained its surface flow structures and therefore has experienced minimal erosion. These characteristics allow an independent estimate of production rates, and an evaluation of the feasibility of exposure age dating. The cores from Haleakala volcano were collected from lava flows that are significantly older (500,000 to 800,000 years), and have experienced some erosion. These

2855

e

2856

M.D.Kurz

samples, from several different altitudes, allow an evaluation of erosion rate calculations, and because of their greater age have significantly greater 3He concentrations. II. EXPERIMENTAL The samples discussed here were all collected using a commercially available (JKS Drilling, Inc., Model No. 10) water cooled, diamond-tip rock drill that produces I inch diameter drill cores. Locations and geological description of each drill core are given in the Appendix. Core 30 was collected from a radiocarbon dated flow of the Ka'u volcanic series exposed at the Southwest rift of Mauna Loa (LIPMAN and SWENSON, 1984). Cores 36, 40, and 33 were collected from exposures of Kula formation lavas on Haleakala volcano at sea level, 4500 feet and 9700 feet elevation, respectively. The Kula formation has been dated at 500,000 to 800,000 years in age (McDouGALL, 1964; NAUGHTON et al., 1980). The drill core depths are reported in terms oftotal shielding mass (units of g cm- 2) to normalize for density variations within and between cores (from 1.4 to 2.9 glee). The shielding mass was calculated in two different ways: by weighing the entire core and dividing by surface area, and also by measuring the density at depth intervals. In most cases the recovery was close to 100 percent and the two methods agree quite well and the uncertainty in the depth is roughly I em (- 2 glcm2). In the case of core 30 (Mauna Loa), there were several significant gaps, resulting in greater uncertainty with respect to depth (see Appendix). It is also important to note that individual lava flows can be quite non-uniform in three dimensions, containing voids and irregular flow structures, so that the local shielding mass used here is only an approximation. Sub-samples for helium analysis were taken from each core by cutting a 2 to 3 cm length interval, which introduces minimal uncertainty regarding the shielding mass above each sub-

sample but requires analysis of quite small phenocryst populations. The sub-samples were crushed and the 1-2 mm olivines and clinopyroxenes separated by handpicking. The helium concentrations and isotopic compositions were then measured in these phenocrysts by gas mass spectrometry. The gas extraction line and methodology used is described elsewhere (KURZ et al., 1986). The blank for crushing and heating was identical and ¥aried from 3 to 4 X 10- 11 eeSTP 4He over the course of this study. The 3He blank is approximately 8lOX 10- 17 cc STP, but is less well determined (±20%) because it is close to the detection limit. A previous study has shown that most of the cosmogenic helium is contained in the solid phases, whereas the magmatic inherited component is selectively released by crushing in vacuum (KURZ, 1986). Therefore, the combination ofcrushing and step heating can be used to effectively separate the inherited magmatic helium from the cosmogenic helium. If the 3HetHe ratio ofthe inherited component is known, the amount of cosmogenic helium can be readily calculated, because cosmogenic 4He is insignificant compared to the inherited 4He: and where subscripts c, t, and i refer to cosmogenic, total, and inherited helium, respectively. The eHetHeh can be determined in a subset of the samples, by crushing in vacuum (because drill cores were collected from single lava flows), and the concentration of cosmogenic 3He can be determined by a single heating step. If 3He., is greater than 3He;, the uncertainty using this method is roughly I per cent for the 3Hec concentration (see Table 1). In the case of the younger, 14C dated lava flow from Mauna Loa (core 30), the error becomes larger, and it was necessary to crush the samples in vacuum before heating.

Table 1: Helium data for Hawaiian Drill Cores

SAMPLE CORE 30 KTE85-30-1-7 KTE85-30-1-7 KTE85-30-3-19 KTE85-30-3-19 KTE85-30-4-41 KTE85-30-4-41 KTE85-30-7-98 KTE85-30-8-116 KTE85-30-9-131 KTE85-30-9-139 CORE 33 KTE85-33-1-1 HA6

KTE85-33-2-24 KTE85-33-4-49 KTE85-33-6-74 KTE85-33-9-91 KTE85-33-9-100 KTE85-33-9-100

HAS CORE 34 KTE85-34-1-1 KTE85-34-1-11 KTE85-34-2-23 CORE 36 KTE85-36-1-2 KTE85-36-2-28 KTE85-36-3-43 KTE85-36-5-69 CORE 40 KTE85-40-1-5 KTE85-40-2-28 KTE85-40-55

WEIGHT

TOTAL DEPTH 'He/ 4 He 4 He (TOTAL) 'He (TOTAL) 'He (COSMOGENIC) Phase (G/CM'L)_~(R",/,-"R=a-=tm",)~_~(=c=cS=T=P~/g~r=am~) _ _~(~c=c=ST=P~/~gr=am~)_ _~(~c=c=ST=P~/~g=ram~)_ _=An=a=l~y=ze=d 3.84x10'· +0.02 1.89x10'· :;:0.01 2.00xl0· a :;:0.01 1.89xlO-· +0.01 3.20x10· a +0.02 2.44xl0-· +0.01 5.66xlO-· +0.03 2.37x10-· +0.01 6.96xl0- 1 °+o.04 9.03x10··o±o.05

1.90xlO· 13 +O.02 1.41xl0' 13 +0 .03 5.98x10· 13 +O.06 1.35xl0- 13 +O.03 9.03x10-' '+0.06 1. 24xlO - 13+0 • 02 1.80xl0- 13 +O.03 9.18x10' 14 :;:0.33 5.11x10· 14 +o.58 4.05xlO-· 4 ±0.14

9.22x10- 14 +O.3 9.27xl0- 14 +O.3 8.72xl0· 14 +o.9 8.69xl0" 4 :;:0.3 8.64xlO- 14 +1.2 6.17xlO-' 4 :;:0.2 4.23xlO-· 4 :;:0.4 3.41xlO-' 4 +O.3 3.41xl0-' 4 :;:0.6 1.85xl0·· 4 ±0.1

1.34xl0- a 2.14xl0'· 2.53x10- a 2.52x10- a 3.04x10· a 1.47xlO- a 2.47xlO- a 1.25x10· a 2.43x10-·

+0.01 +0.01 +0.01 +0.01 :;:0.02 +0.01 :;:0.01 +0.01 ±0.01

2.91xl0· 12 +0.02 2.33xl0-' ':;:0.05 2.29x10-"+0.02 1. 72x1O' "+0.01 1.35xl0-":;:0.01 1.lOxlO- 1 '+0.01 1. 11x10' "+0.01 9.43x10- 13 +O.07 6.64x10- 13 ±0.16

2. 76xlO-' '+0.02 2.30xl0· 12 +0.05 2.01xl0- 1 ':;:0.02 1.43xl0- 12 +O.01 1.01x10· 12 :;:0.01 9.35xlO- 13 +O.08 8.30x10- 13 :;:0.1 8.01x10· 13 :;:0.08 6.37xl0- 13 ±0.16

97.0+ 0.5 64.9+ 0.2 7O.1± 0.2

2.62x10- a +0.01 3.70x10- a +0.02 2.93xl0· a ±0.02

3.52xl0- 1 '+0.03 3.33xlO-":;:0.02 2.84xl0- 1 '±0.02

3.22x10-"+0.03 2. 91x10-"+0. 02 2.51x10-"±0.02

3 3

5 72 113 176

48.0+ 40.3:;: 35.7+ 23.9±

8.81x10-· 7.21x10-· 6.37x10-· 8.04x10-·

+0.04 +0.04 +0.03 ±0.04

5.85x10- 13 +O.06 4.02x1O- 1 ':;:0.04 3.15x10- 13 :;:0.03 2.65x10- 1 '±0.03

4.85x10- 13 +O.06 3.20xI0' 13 +O.04 2.42x10- 13 +O.04 1. 75xI0- 13 ±0.04

3

8 69 137

30.9+ 0.3 57.3+ 0.3 42.3± 0.3

2.55x10- a +0.01 7.80xl0-· :;:0.04 7.74x10-· ±0.04

1.09x10-"+0.01 6.34xl0- 13 :;:0.05 4.53xlO- 13 ±0.04

8.01x10"'+0.14 5.43xl0-"+0.05 3.65xl0- 13 ±0.05

3

35.8+ 53.9+ 21.6+ 51.7+ 20.4+ 36.7+ 23.0+ 28.0:;: 53.0:;: 32.4±

0.4 1.0 0.2 1.0 0.1 0.6 0.4 1.0 6.0 1.1

0.22780 0.05621 0.14426 0.13583 0.17111 0.10858 0.12746 0.11073 0.12660 0.08940

14 14 46 46 87 120 144 163

0.08770 0.08855 0.11650 0.13170 0.12740 0.10866 0.10940 0.10857 0.18439

1 28 51 111 169 207 229 229 366

157.3+ 0.7 784.6+15.7 65.5+ 0.3 49.3+ 0.2 32.2+ 0.1 54.2+ 0.2 32.5+ 0.2 54.6+ 0.3 197.5± 4.5

0.09322 0.09674 0.12155

2

23 48

0.09961 0.15229 0.12431 0.13535 0.07642 0.11548 0.07159

6 6

0.4 0.3 0.3 0.2

2 1* 1 2 1 2 1

1* 2 2

3 4 3 3

3 3 3 1 4

3

3

3 3 1 1

Code for phase analyzed: 1) Olivine grains 1-2 rom in size; 2) olivine powder remaining after vacuum crushing (<150 microns); 3) clinopyroxene grains 1-2 Jm1I in size; 4) clinopyroxene powder. An asterisk (*) denotes samples in which inclusion-free grains were selected.

2857

Terrestrial cosmogenic He III. RESULTS AND DISCUSSION

§

A. Importance to studies ofmagmatic helium The results from the drill cores are reported in Table 1 and summarized in Fig. 1. In each case, there is a strong depth dependence in 3Hec , which is quite conclusive evidence for the cosmogenic origin of the extremely high 3HefHe ratios in these samples (KURZ, 1986). The importance of this effect to mantle studies in subaerial lavas is demonstrated by the plot of total 3HefHe as a function ofdepth in clinopyroxenes from core 33 (Fig. 2). The 3He/4He ratio decreases from 157 times atmospheric (XRa) at the top, to 24 X Ra at the bottom, ofthe flow (1.5 meters). The inherited helium has a 3HefHe ratio of 8 X Ra (see Table I, and also KURZ, 1986), and is not even approached at the bottom of the core. Note also that the helium released by crushing samples from the top and bottom of the core were indistinguishable from 8 X Ra (see Table 2), which supports the hypothesis that crushing in vacuo selectively releases the inherited helium (KURZ, 1986). The data in Fig. 2 demonstrate that the isotopic composition of the helium released by total fusion extractions, in old lava flows (at high altitude), can be dominated by Cosmogenic helium. The total 3HefHe measured in any mineral separate will depend on the mixing ratio between inherited helium and cosmogenic helium within the crystal. For example, in a basaltic rock, olivines have lower inherited 4He concentrations (see Tables I and 2), and if exposed to cosmogenic helium production will have higher 3He/4He than coexisting clinopyroxenes. The 3HefHe ratio does not vary smoothly with depth in Fig. 2 because the 4He concentration also varies sig-

40 ·36

+

~

•

100

g "

'"

" ~

~

~ ~

2-

•

.'

0 i;'

• • •

200

300

c::l

400

!

40

80 120 'He/'He (R/Ra1mJ

160

FIG. 2. Total 3HetHe in clinopyroxenes from Core 33. These data are obtained by total fusion of 1-2 mm grains, and do not include any corrections for inherited or cosmogenic helium. Therefore the 3HetHe variability reflects the changing proportion ofcosmogenic 3He with shielding depth, and variations in inherited helium concentration with inclusion abundance (see text).

nificantly. The crushing experiments demonstrate that most of the 4He (i.e. inherited helium) is released by crushing, and is therefore contained in melt inclusions. The ratio 4He (crushed)fHe (powder) for core 33 is roughly 10, and the 4He variability is probably related to variability in inclusion abundance. The fact that 3He., displays a smooth depth profile supports this, and demonstrates that the correction for inherited 3He in the calculation of 3Hec is valid.

B. Production rates and use ofhelium for exposure age dating The most critical parameter in evaluating the importance of cosmogenic helium to any geological situation is the production rate. YOKOYAMA et al. (1977) calculated in situ spallation production rates for tritium (T) in surface rocks, which decays to 3He; using their calculated rate, and assuming that direct 3He production rates are similar (KRUGER and HEYMANN, 1968), we previously estimated a production rate of 1100 atoms g-Iyr at the summit of Haleakala. This assumes a difference in nucleon intensity of --.8 between Mont Blanc (62°N geomagnetic latitude) and Hawaii (38°N geomagnetic latitude) which was taken from the measurements of ROSE et al. (1956). However, the sea level neutron measurements of ROSE et al. were performed in 1954 during solar minimum, so this value may be slightly high. POMERANTZ and AGRAWAL (1962) made

30 10"4 '---'-_L.--'-_L.--'-_L.--'----'

o

100

200

300

Table 2.

Crushing Analyses (Inherited helium)

400

Depth Below Surface (g cm-2)

FIG. I. 3Hec as a function of depth for all the cores (Table I). The youngest flow (Core 30) has the lowest 3Hec , and among the other three, 3Hec increases with altitude. Note that below 170 g cm-2, the depth dependence deviates significantly from a simple exponential for both cores 33 and 36. The total depth of400 gcm -2 corresponds to approximately 1.5 meters depth in the rock.

Sample

30-1-7 30-3-19 30-4-41 30-9-131 HA-6 HA-5

36-5-69

Phase

Weight

01. 01. 01. 01. CPX

.2775 .1489 .1193 .1428 .1813 .1965 .0732

CPX CPX

"He ccSTP/gram

2.946 1.213 1.085 8.799 2.350 2.16 4.157

x 10" x 10-' x 10-' x 10-' x 10-' x 10-' x 10-'

'He/"He (R/Ra)

18.5 18.5 18.3

.2 .2 .3

17.8

.2

8.2 8.0 8.8

.1 .1 .5

M.D.Kurz

2858

neutron meaSurements during solar maximum (19581959) that yield a value of .6 for the difference between the two locations. Therefore we will adopt a value of .7 to average out the sun-spot modulation. (Note that CRAIG and POREDA, 1986, used a somewhat lower value of .42, taken directly from YOKOYAMA et al., 1977.) Using an apparent atmospheric attenuation length of 160 g cm- 2 , the production rate at sea level in Hawaii is therefore approximately 130 atoms g-Iyr-I. Another independent means of estimating the production rate is given by tAL and PETERS (1967), using spallation production rates observed in photographic emulsions. From their estimates ofthe production rates versus latitude, and the 3He and T produced per spallation event, a total 3He production rate is -140 atoms g-Iyr-I, which is in reasonable agreement. As is shown in Table 3, spallation is the dominant mechanism at shallow depths. The only other significant contribution is from the 6Li(n, a)T reaction, which is dependent on Li abundance. Therefore, it is reasonable to adopt 130140 atoms g-Iyr-I as a minimum rate. Using the measured 3Hec values in the top of core 30 (10.1 ± .1 X 10- 14 ccSTP/gram), and the radiocarbon age (20,150 ± 800 years), the integrated production rate in this lava flow is 135 ± 5 atoms g-Iyr-I, in quite good agreement with the theoretical rates. Although the lava flow is well preserved enough to ignore erosion, this is also a minimum production rate because the lava flow may have been covered with soil during some of its existence on the surface. A soil cover of only 5 cm would decrease the production rate by 5 atoms g-Iyr-I (assuming a density of 1. 7 glee and attenuation length of 160 gfcm 2). In light of the local geology this is quite possible; the soil cover roughly 100 meters inland from the sample locality is approximately 50 cm. The measured minimum production rate is in reasonable agreement with the minimum theoretical estimates, and suggests that the 3He production rate is at least 135 atoms g-Iyr-I. Therefore, 3He production at

Table 3.

Production mechanisms for 'He in rocks Rate* (atoms g-'yr- 1 )

1. Spallation*

130-140

2. "Li {n,a)T + 'He n from cosmic ray secondaries n from muons

3. Muon Capture 'Li {~-,a)T + 'He 'Li (~- ,3n) 'He 'Li (~- ,2n) 'He

4. Photonuclear Processes X (y,T) Y X (y, 'He) Y

7** 1.7** <1.5

X

10-'

<.227

* Rates. are estimated for sea level at geomagnetic latitude of Hawaii (see text). ** Assuming Li content of 2 ppm in major element matrix of clinopyroxene.

sea level is roughly a factor of 2 greater than 26AI, and a factor of 4 greater than 1000e (KLEIN et aI., 1985). The data from core 30 also illustrates the potential of cosmogenic helium for measuring exposure ages. Based on known production rates, the calculated exposure age of this sample is within error of the radiocarbon age. Although the time-integrated soil cover presents a slight uncertainty for Core 30, and the production rate is somewhat uncertain, the method could readily be extended to younger samples. Based on present measurement capabilities, arid using the 4He concentrations in olivine from core 30, exposure ages could be measured on lava flows as young as 250 years at sea level (i.e. 3Hec == 2 X 10- 15 cc, or 10 times the present detection limit). Cosmogenic helium may therefore provide a method of dating young basaltic lava flows where radiocarbon dating is not possible. Another example of the usefulness of 3He for dating exposure surfaces is given by data from cores 33 and 34, as illustrated in Fig. 3. These 2 cores were taken from 2 different surfaces on the top of a single lava block. The top surface sampled by core 33 is younger, based on the extent of weathering, and also appears to have been exposed by removal ofa single block (rather than by gradual erosion; see Fig. 3). If this surface was exposed at zero time (i.e. several tens ofyears ago), the 3He concentration at the top of 33 and at the bottom of core 34 would be identical. The difference that is obServed (2 ± .2 X 10- 13 cc 3He STP g-I), is related to the exposure age of the younger surface. Using a production rate of 920 atoms g-Iyr-I (140 atoms g-Iyr-I adjusted to this. altitude) the difference in 3Hec is equivalent to 4950 ± 500 years. The dating of such young surfaces may become a powerful tool in glacial geology, geomorphology, and archaeology. A potential uncertainty in these applications is postformation loss of cosmogenic helium from minerals. If the stopping length of the spallation products (i.e. helium and tritium) is longer than the grain size ofthe mineral, one might exPect significant losses by diffusion through radiation damage regions. Because the grain size of all the samples analyzed is roughly 2 mm and the stopping length of nucleonic spallation products in nuclear emulsions is roughly 50 microns (ROSSI, 1952), this effect should be minimal in the present case. The agreement between theoretical production rates and the value calculated for core 30 suggests that such losses are insignificant, at least on the time scale of 20,000 years. In a detailed inventory ofa single sample, KURZ (1986) found that large olivine and clinopyroxene crystals contained equal amounts of cosmogenic helium (see also Core 33), but that the fine-grained matrix had experienced significant losses. This also supports the idea that losses are likely to be related to radiation damage, and hence grain size. y olume diffusion is unlikely to be an important loss mechanism, even for the older samples, based on the diffusion data ofHART (1984). Additionally, one would not expect clinopyroxene and olivine to have experienced equal losses, which would be required to explain

2859

Terrestrial cosmogenic He Cosmogenic 3He (cc STP g-1JxfO- f2

1.5

2.0

2.5

3.0

......

0

~

10

E:

~

0

.c.

3He

c

20

--

\

~ 10

~ ....

~

20

~

30

&l

40

~

50

~

~

~

30

CORE 34

'" /

I CORE 33

/

/

/

......

E:

~ ~ I<)

Cl> \.)

~ ....

~ ~

~

~

,..

..<:::

~

~

FIG. 3. Variation of 3Hecas a function ofdepth below surface for Cores 33 (top only) and 34. As shown by the sketch in the bottom right, these two cores were taken from two different surfaces on the same boulder. The difference in 3Hec at equivalent levels in the boulder (~3Hec) is attributed to the exposure age of the younger surface (Core 33) which is equivalent to 4850 ± 500 years.

equal 3Hec concentrations. Although these arguments are not conclusive, available information suggests that post-formation losses are insignificant for large grainsize samples. C. Depth dependence oFHe and production mechanisms

The nucleonic component ofcosmic rays is strongly attenuated in condensed matter, and its intensity decreases with depth (x) by exp(-x/I), with an apparent attenuation length I of approximately 160 g cm -2. The simple exponential decrease is an approximation that ignores the zenith angle dependence of cosmic rays, but is valid for the short depth intervals discussed here (YOKOYAMA et aI., 1973). In accordance with this, the 3Hec from the top 170 gfcm2 of the four cores decreases exponentially with depth. The best fit values of I ( I 70 ± 2, 165 ± 7, and 164 ± 3 g/cm 2 for cores 33,36, and 40, respectively), are within the range of attenuation lengths for the nucleonic component of cosmic rays (ROSSI, 1952; MABUCHI et al., 1971; NAKAMURA et al., 1972). Cores 30 and 34 (110 ± 10 and 185 ± 10 gfcm 2) are subject to greater uncertainty. However, for the two cases where there is data for depths below 170 g cm- 2 (33 and 36), the 3Hec values are significantly higher than would be predicted by simple exponential depth dependence. In the case of Core 33, the deviations are consistently high, and are outside of uncertainty with respect to depth or 3He (i.e. >20"). Several explanations are possible for this effect: I. 3He Production by the secondary nucleonic particles from spallation. 2. Higher production at depth due to a change in production mechanism (e.g. 6Li(n, a)T). 3. Geometry dependent production such as variation with zenith angle, or a reorientation oflava blocks.

3He production by secondaries will alter the constant I and increase the production at greater depths. However, the attentuation length I ofthe secondaries should be similar to, or less than, the attenuation length of the primaries (because the secondaries must have lower energy). The 3Hec excesses cannot be explained by this effect because the production is too high at depths greater than 170 g cm- 2• Using a least squares fitting routine for two exponentials (e.g. YOKOYAMA et al., 1973), with a primary lof 150 g cm- 2 , the data can only be fit if the secondary I is approximately 1000 g cm- 2 , which is roughly the attenuation length for muons. The lack of an increase over the first few cen· timeters, as is sometimes found in lunar soils (e.g. YOKOYAMA et al., 1973), is probably absent here because the energy of the spallation generating radiation is much lower, producing lower energy secondaries, and also because solar cosmic rays are unimportant. Geometry effects also seem unlikely, because several of the samples in Fig. 3 were collected from the side of the lava block, while the core was taken from the center; both samples fallon the curve, which suggests that this effect is unimportant. The size of the lava blocks, the evidence of flow structures on the bottom ofthe blocks, and variability ofvesicularity with depth also suggest that they have not changed orientation since eruption. The preferred explanation for the increased production at depth is that there is a change in production mechanism. Spallation is clearly the dominant mechanism (see Table 2), but at greater depths in the lava, the nucleonic component is greatly decreased, and muons become the dominant cosmic ray particles. Nucleii that capture muons have a high probability of emitting neutrons (CHARALAMBUS, 1971), which in turn can produce 3He via 6Li(n, a)T. As is shown in Fig. 4, the sum of an exponential and the muon depth dependence:

gives a reasonable fit to the data for core 33, where K is a constant and IL-(X) is the depth dependence of the muon stopping rate. The muon stopping rate as a function ofdepth was derived from experimentally determined values at 20 and 200 g cm- 2 (BARTON and SLADE, 1965; HAMPEL and KIRSTEN, 1975). The con· stant K is related to Li content:

K= (f) X (m) X (t) where f is the fraction of neutrons reacting with 6Li, m is the neutron multiplicity of muon capture, and t is the age of the rock. In order to calculate the magnitude of the neutron6Li contribution, we can assume an age of 500 to 800 kyr, a p.eutron multiplicity of 1.5 (CHARALAMBUS, 1971), and a value of 3 X 10- 3 (Li of 2 ppm for clinopyroxene and olivine) for f The value off assumes Li partitioning co-efficients of roughly .2 determined in olivine and clinopyroxene phenocrysts (KURZ and

2860

M.D.Kurz

CORE 33 HALEAKALA Orr--------------::",-.---,

400 L-.L...J....L.L=------.J._ _.l.-_---L._ _..I...-_-J

00

10

2.0

30

Cosmogenic sHe (cc STP grom-fj x 10 -11

FIG. 4. 3Hec as a function depth for Core 33. This diagram shows the quite large deviation from an exponential depth dependence (i.e. I = 165 g/cm2) below 170 g/cm2• The curve that yields a better fit to the experimental points is a combination of a simple exponential and the effect of muon produced 3He via 6Li(n, a)T (see text). The dashed line is the assumed depth dependen~ of muon stopping rate.

SHIMIZU, unpublished ion probe data), and total Li content of the rock is 8 ppm (based on data from the literature, see KURZ, 1986). The muon stopping rate measured by HAMPEL and KIRSTEN (1975) at 200 g cm- 2 is 295 muons g-lyr- I • Using these numbers, the expected excess 3He would be approximately 1.2 X 106 atoms 3He per gram, which is roughly a factor of two less than tlte observed amount (9 X 10- 14 cc or 2.4 X 106 atoms per gram). As discussed below, it is quite clear that the lava flow described by Fig. 4 has had at least several meters eroded off the top, so any point within Core 33 has existed at significantly greater shielding depth in the past. The available experimental points suggest a factor of two increase in the muon stopping rate over the first 200 g cm- 2, and that below roughly 700 g cm- 2 the muon stopping rate begins to decrease with depth (see HAMPEL and KIRsTEN, 1975). If the stopping rate increases between 200 and 700 g cm- 2 , which is quite consistent with the experimental data, the discrepancy could re;:tdily be explained by this effect. Therefore, based on muon stopping rates, the excess 3Hebelow 170 g cm- 2 could be generated by 6Li(n, alT. Because Core 33 was obtained from a lava block rather than a flow, it is impossible at this point to rule out conclusively geometry and/or zenith angle contributions to this excess. However, this hypothesis will be readily tested by obtaining data from deeper drill cores oflava flows. If the muon-6Li mechanism is responsible for the excess, 3He production will be observed at much greater depths (muons have an attenuation length of 1000 g cm-2).

D. Erosion rates derived from cosmogenic helium As discussed above, the relative amounts of 3Hec in the surface of the four cores is qualitatively correct,

because the youngest core has the least 3Hec and within the other three cores, there is a general increase with altitude. However, the three cores from the Kula formation of Haleakala have undoubtedly been affected by erosion, which will decrease the amount ofobserved 3He relative to the amount predicted by the production rate. This is demonstrated by the fact that core 36 is at least 25 times older than core 30, but has only 5 times as much 3He. In addition, the three older cores have probably been affected by erosion to different extents, because they come from different weathering environments. As has been discussed by several authors (LAL and PETERS, 1967; HAMPEL et al., 1975; LAL and ARNOLD, 1985; NISHIZUMI et al., 1986), the erosion rate can be determined by the concentration of a cosmogenic nuclide. Assuming production rates and mechanisms discussed above, the 3Hec at time t and depth X is:

where P is the production rate, Mis 1/1, X is the depth, land m are the constants described above, and ~-(X) describes the assumed depth dependence of muon stopping (linear for the upper 300 g/cm2). Integrating and substituting Xl = X o - Et, where X o is the original depth and E is the linear erosion rate:

P

C 3(XI , t) = ME exp(- M(XI + Et) X [exp(MEt) -1] +Imt(~-(xl»'

The last term (describing muon production) will be negligible for the near-surface samples, and given the uncertainty in this mechanism and in evaluating muon stopping rate as a function ofdepth, this term is igp.ored for the purposes ofcalculation. However, examination of this equation (and the muon stopping rate with depth; HAMPEL et al., 1975) shows that for rapid erosion rates, and greater depths, this term will become significant. The erosion rates obtained for the surface 3Hec in the three cores from Haleakala are shown in Table 4. The rates vary between 7 and 11 meters per million years, which is significantly lower than erosion rates

Table 4. Core

P.

33 34

920 920

Erosion Rates cm/yr

m/my

1.03x10-' 8.5 x10- 4

10. 8.5 11.0

36

140

7.7 xl0- 4

40

370

1.1 x10-'

7.7

p. is the assumed production rate at zero depth in the core, at the appropriate altitude (atoms g-lyr- 1 ).

2861

Terrestrial cosmogenic He

using other methods (e.g. RUXTON and McDoUGALL, 1967; RICE, 1980). Presumably, this is because the rates reported here were measured away from river valleys. The dominant mechanism for erosion is generally believed to be mass wasting by water flow over a surface, which will ofcourse be fastest within stream beds. The erosion rates estimates given here are somewhat uncertain because the sample ages are not well-determined, and also because we have not accounted for volcano subsidence. Note that the rates given in Table 4 agree quite well with the values reported by CRAIG and POREDA (1986) for samples from the same area, but that the values used in calculating the rates are quite different. As mentioned above, CRAIG and PoREDA (1986) used a production rate that is roughly 40 per cent lower, but also measured 3He concentrations that are roughly a factor of 2.5 lower, so that the differences roughly cancel. Despite the uncertainties, the erosion rates are geologically consistent, with the core from the southern (dry) side of the island having the slowest rate. At this early stage in the use of helium to measure erosion rates, it is unclear whether the differences in Table 3 are significant. However, the data demonstrate the usefulness of the approach, and the importance of obtaining drill core samples. IV. CONCLUSIONS

I. The measured minimum production rate of cosmogenic 3He at 37° geomagnetic latitude is 135 ± 5 atoms g-lyear- I , as determined by data from the radiocarbon dated lava flow. This is in reasonable agreement with the theoretical production rate of 130-140 atoms g-lyr- I . 2. Based on the internal consistency of the drill core data, 3He losses are minimal on the time scales discussed here. 3. Based on the 3Hec-depth profiles, at depths greater than 170 g/cm2 muon produced neutrons contribute significant quantities of 3He via the reaction 6Li(n, alT. This suggests that cosmogenic helium will persist to much greater depths than would be expected from spallation reactions, but confirmation will require more data. 4. Cosmogenic 3He can be used to measure exposure ages as low as 250 years in basaltic lava flows, if the conditions are appropriate. 5. Cosmogenic 3He can also be effectively used to measure erosion rates as is demonstrated by the three Haleakala cores. The data presented here demonstrate the importance of collecting samples with accurate depth control (e.g. drill cores) for this purpose. The muon 6Li(n, a)T production mechanism suggests that this technique may also be applied to situations where erosion rates are more rapid.

Acknowledgements-The author wishes to thank W. J. Jenkins for his valuable comments and timely advice on this work; his pioneering efforts at lowering 3He detection limits have made this type ofstudy possible. Discussions with Mike Garcia

and Peter Lipman were critical to selecting the sampling sites. John Bullister, Tom Trull and Karl Turekian provided valuable discussion; Don Bogard, Frank Podosek and David Fisher made useful comments on the manuscript. Dempsey Lott and Lolita Surprenant provided expert assistance in the laboratory, and Molly Lumping typed the manuscript. The field work would not have been as successful without the assistance of Mike Garcia, Don Elthon and Tom Trull. The author also thanks Ron Nigata and the National Park Service for permission to sample in the Hawaii Volcanoes National Park. This research was partially supported by NSF grants OCE8516082, EAR86-10611 and NASA NAG-969.

Editorial handling: F. A. Podosek REFERENCES BARTON J. C. and SLADE M. (1965) The intensity of stopping pions at sea level and underground. Proc. Int. Con/. Cosmic Rays, London, vol. 2 1006. CRAIG H. and POREDA R. J. (1986) Cosmogenic 3He in terrestrial rocks: the summit lavas of Maui. Proc. Nat. Acad. Sci., 83, 1970-1974. CHARALAMBUS S. (1971) Nuclear transmutation by negative stopped muons and the activity induced by the cosmic-ray muons. Nuclear Physics A166, 145-161. HAMPEL W. and KIRSTEN T. (1975) Measurement of cosmic ray muon-induced radioisotopes S7Co, SHCo and 6OCo in nickel. Proc. of14th International Cosmic Ray Conference, Munich, 1895-1899. HAMPEL W., TAKAGI J., SAKAMOTO K. and TANAKA S. (1975) Measurement of muon-induced 26AI in terrestrial silicate rock. J. Geophys. Res. 80, 3757-3760. HART S. R. (1984) Helium diffusion in olivine. Earth Planet. Sci. Lett. 70, 297-302. KRUGER S. T. and HEYMANN D. (1968) Cosmic-ray produced hydrogen 3 and helium 3 in stony meteorites. J. Geophys. Res. 73,4784-4787. KLEIN J., GIEGENGACK R., MIDDLETON R., SHARMA P., UNDERWOOD J. R. and WEEKS R. A. (1985) Revealing histories of exposure using in situ produced 26AI and 10& in Libyan desert glass. Radiocarbon (in press). KURZ M. D. (1986) Cosmogenic helium in a terrestrial igneous rock. Nature 320, 435-439. KURZ M. D., O'BRIEN P., GARCIA M. and FREY F. A. (1985) Isotopic evolution of Haleakala volcano: Primodial, radiogenic, and cosmogenic helium. EOS 66, 1120. KURZ M. D., GURNEY J. J. and JENKINS W. J. (1986) Helium isotopic variability within single diamonds from the Orapa Kimberlite pipe. Earth Planet. Sci. Lett. (submitted). LAL D. and PETERS B. (1967) Cosmic-ray-produced radioactivity on the earth. In Handbuch der Physik XLVI/2 (ed. S. FLUGGE), pp. 551-612. Springer-Verlag, Berlin. LAL D. and ARNOLD J. R. (1985) Tracing quartz through the environment. Proc. Indian Acad. Sci. 94, 1-5. LIPMAN P. and SWENSON A. (1984) GeneraIized geologic map of the southwest rift of Mauna Loa. USGS Map 1-1323. MABUCHI H., GENSHO R., WADA Y. and HAMAGUCHI H. (1971) 32p induced by atmospheric cosmic rays in laboratory chemicals. Geoch. J. 4, 105-110. MACDoNALD G. A. (1979) Geologic map of Haleakala volcano national park. USGS Map 1-1088. McDoUGALL I. (1964) K-Ar ages from lavas ofthe Hawaiian Islands. Bull. Geol. Soc. Amer. 75, 107-128. NAKAMURA Y., MABUCHI H. and HAMAGUCHI H. (1972) 7Be production from oxygen by atmospheric cosmic rays. Geoch. J. 6,43-47. NAUGHTON J. J., MACDoNALD G. A. and GREENBERG V. A. (1980) Some additional K-Ar ages ofHawaiian rocks: The Maui complex. J. Volcano Geothermal Res. 7, 339345. NISHIIZUMI K., LAL D., KLEIN J., MIDDLETON R. and AR-

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NOLD J. R. (1986) Production of loae and 26AI by cosmic rays in terrestrial quartz in situ. Nature 319, 134-136. POMERANTZ M. A. and AGRAWAL S. P. (1962) Spatial distribution of cosmic ray intensity and geomagnetic theory. Phil. Mag. 7, 1503. REEDY R. c., ARNOLD J. R. and LAL D. (1983) Cosmic-ray record in solar system matter. Ann. Rev. Nucl. Part. Sci. 33, 505-537. RICE R. J. (1980) Rates oferosion in the little Colorado Valley, Arizona. In Timescales in Geomorphology (eds. R. A. CuLLINGFORD et al.), pp. 317-332. Wiley & Sons. ROSE D. c., FENTON K. B., KATzMAN J. and SIMPSON J. A. (1956) Latitude effect ofthe cosmic ray nucleon and meson components at sea level. Can. J. Phys. 34, 968. ROSSI B. (1952) High Energy Particles. Prentice Hall, New Jersey. RUXTON B. P. and McDoUGALL I. (1967) Denudation rates in Northeast Papua. Amer. J. Sci. 265,545-561. YOKOYAMA Y., SATO J., REYSS J. L. and GUICHARD F. (1973) Variation of solar cosmic-ray flux deduced from 26Na_26Al data in lunar samples. Proc. Lunar Sci. Can! 4th, 22092227. YOKOYAMA Y., REYSS J. L. and GUICHARD F. (1977) Production of radionuclides by cosmic rays at mountain altitudes. Earth Planet. Sci. Lett. 36, 44-50. APPENDIX

Sample descriptions 1. Core 30: Two-meter thick pahoehoe lava flow with ropy flow structures preserved on the surface, on the west side of Mahana Bay, 18°56'19"N latitude, 155°39'59"W longitude. Radiocarbon collected from ash layer below the lava flow yields an age of20,150 ± 800 years (USGS W#4368; P. W. LIPMANN and J. P. LocKWOOD, pers. commun.) and places the lava in the Ka'u volcanics. The geology of this section of the south west rift zone of Mauna Loa is described by LIPMANN and

SWENSON (1984). The top 45 cm of the core is frothy basalt with 20-30% vesicles (.5 to I mm) and 4-7% green olivine crystals (1-2 mm); density is approximately 1.4 g/cm 3• Between 45 and 80 cm dePth, core quality was poor, with only fragments and rubble recovered. Between 80 cm and the bottom ofthe core (140 cm), the basalt was slightly more massive (density -2 g/cm3) with 10% vesicles (2-4 mm in size), and olivine in the same proportion as the upper section of the core. 2. Core 33: Ankaramite lava flow, sampled on the south side of White Hill, near Haleakala summit at 20 0 42'59"N latitude, 156°15'IO"W longitude, elevation 9720 feet. This lava flow is of Kula formation age (500,000 to 800,000 years; McDoUGALL, 1964; McDoNALD, 1979), and consists ofbroken up blocks I to 2 meters in size. The block that was drilled is one ofthe largest in size, and was inferred to be in its original orientation based on variation in vesicularity and the flow structure on the bottom of the block. The top 80 cm of the core is extremely vesicuIar, with 20-25% vesicles 7-8% olivine (1-4 mm) and 8-10% clinopyroxene (2-5 mm). Between 80 cm and the bottom ofthe block (150 cm), the density increases to 2.6 gJcm 3 due to a decrease in vesicularity (to 5-7%). Two samples were broken from the edge ofthe boulder (HA-5 and HA-6; see Table I). 3. Core 34: Drill core taken from the same boulder as Core 33, to a depth of 23 cm below a different surface. Based on the extent ofweathering,,and the geometry ofthe surface (see Fig. 3), this surface is inferred to be older than the surface drilled by Core 34. 4. Core 36: Kula formation ankaramite lava, 200 feet above sea level, on the south coast of Maui at 20 0 37'39"N latitude, 156° 12'58"W longitude. The lava consists of 2-4% vesicles, 4-5% clinopyroxene (1-3 mm), and 3-4% olivine (1-2 mm). A continuous section ofcore was recovered to a depth 72 cm. 5. Core 40: Kula formation ankaramite from road-cut on Highway 378, west side of Haleakala volcano; elevation 4480', 20°45'59"N latitude, 156° 17'35"W longitude. The lava consists of5-1O% vesicles (3-5 mm), 7-10% clinopyroxene (3-6 mm) and 2-3% olivine (1-3 mm).