In Iceland Cosmogenic 3He production rates from Holocene lava flows
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Earth and Planetary Science Letters 246 (2006) 251 – 264
www.elsevier.com/locate/epsl
Cosmogenic 3 He production rates from Holocene lava flows
in Iceland
J.M. Licciardi a,⁎, M.D. Kurz b , J.M. Curtice b
a
Department of Earth Sciences, University of New Hampshire, Durham, NH 03824, USA
b
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Received 3 September 2005; received in revised form 11 March 2006; accepted 12 March 2006
Available online 25 April 2006
Editor: K. Farley
Abstract
We measured cosmogenic 3He production rates in olivine phenocrysts from four radiocarbon-dated postglacial basaltic lava
flows in the Western Volcanic Zone of Iceland. These measurements provide important new calibrations of cosmogenic nuclide
production rates near sea level at high latitudes. Calibration sites from Lambahraun (4040 ± 250 cal yr BP; n = 4), Leitahraun
(5210 ± 110 cal yr BP; n = 5), Búrfellshraun (8060 ± 120cal yr BP; n = 6), and Þingvallahraun (10,330 ± 80 cal yr BP; n = 4) yield a
mean production rate of 132 ± 5 atoms 3He g− 1 yr− 1 (±1σ; normalized to sea level at high latitudes with the standard atmosphere).
Mean production rates from the four flows agree within uncertainty, indicating that these calibrations establish a reproducible
local 3He production rate that will significantly increase the accuracy of exposure dating in Iceland. The 3He production rate in
Iceland is ∼ 17% higher than the mean of normalized Holocene values previously determined in the western USA. The high
production rates in Iceland are attributed to the influence of persistent low atmospheric pressure over Iceland (the Icelandic Low)
through the Holocene, which yielded higher cosmic ray fluxes. The Icelandic calibrations thus support previous suggestions that
cosmogenic isotope production rates should be adjusted for regional variations in long-term atmospheric pressure. By extending
the latitudinal extent of previous calibration studies, these results are also useful for evaluating scaling models.
© 2006 Elsevier B.V. All rights reserved.
Keywords: cosmogenic nuclides; helium; production rates; Icelandic Low; surface exposure ages
1. Introduction broad applicability to geomorphic problems have
contributed to wide acceptance of exposure dating
In the past decade, surface exposure dating using techniques [4].
terrestrial cosmogenic nuclides has become a powerful The accuracy of surface exposure dating is generally
geochronological technique [1-3]. Improvements in the limited by geologic factors, analytical uncertainties, and
knowledge of cosmogenic nuclide production rates, incomplete knowledge of isotope production rates, with
increasing precision of isotopic measurements, and the largest uncertainty commonly associated with the
production rates. Calculation of an exposure age for any
⁎ Corresponding author. Tel.: +1 603 862 3135; fax: +1 603 862 given sample site on the Earth's surface involves two
2649. main types of uncertainty directly linked with produc-
E-mail address: joe.licciardi@unh.edu (J.M. Licciardi). tion rates. First, there are uncertainties in production
0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2006.03.016252 J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264
rates measured at geological calibration sites and the Holocene that is notably higher than normalized values
possible temporal variations over the exposure durations previously determined in the western USA [10,11]. The
of the calibration surfaces. Second, there are uncertain- relatively high 3 He production rate in Iceland is
ties related to the altitudinal and latitudinal scaling of the attributed to the influence of persistent low atmospheric
production rates from their calibration sites to the dated pressure (the Icelandic Low) during the Holocene. This
surface. The combined uncertainty in scaled production interpretation is consistent with the suggestion by Stone
rates can be as high as 10–20% in some cases but is not [12] that scaled production rates of cosmogenic nuclides
well known [3,5]. One approach for minimizing these should be adjusted for regional anomalies in long-term
uncertainties is to base exposure ages on production atmospheric pressure.
rates determined at a calibration site with similar
altitude, geographic location, and exposure time to the 2. Samples
dated surface. In practice, however, it is often difficult to
locate suitable calibration surfaces. 2.1. Sample collection, calibration sites, and age
The primary motivation for this study was to control
determine a locally calibrated cosmogenic nuclide
production rate in Iceland, with the broad goal of Samples were chiseled from the surfaces of four well-
increasing the accuracy of exposure dating. The focus is dated postglacial basalt flows in the Western Volcanic
on cosmogenic 3He, which is most commonly measured Zone of Iceland, a rift zone that represents a 170-km-long
in olivine because of the helium-retentive properties of subaerial exposure of the Mid-Atlantic Ridge system
this mineral [6,7]. Iceland's surficial geology consists (Fig. 1) [13,14]. All four flows (Lambahraun, Leitah-
largely of olivine-bearing basaltic lithologies, hence 3He raun, Búrfellshraun, and Þingvallahraun; hraun = lava in
has wide utility for exposure dating in Iceland. Aside Icelandic) are dated by multiple radiocarbon ages (Table
from 3He (and 21Ne, which can also be measured in 1), contain visible olivine phenocrysts, and exhibit
olivine), currently only 36Cl can be used as an exposure surficial characteristics such as pahoehoe ropes,
dating tool for Icelandic basalt lithologies. Other squeeze-up features, and tumuli (Fig. 2) that suggest
cosmogenic nuclides have been measured in olivine preservation of the original flow morphology. The field
(10Be and 14C) although their use in this mineral phase strategy was designed to minimize potential problems
is not yet routine [8,9]. with erosion and past burial (e.g., [11]). We targeted
The data presented in this paper establish a surfaces with minimal erosion, as evidenced by well-
reproducible 3He production rate in Iceland during the preserved pahoehoe ropes on surfaces concordant with
Fig. 1. Location of sampled lava flows in the Western Volcanic Zone of Iceland. Dark stippled regions represent postglacial basalt flows. Black
regions indicate flows used for calibration of cosmogenic 3He production rates. Inset map shows study location in Iceland. Geologic mapping from
Jóhannesson and Sæmundsson [13], Sinton et al. [14], and references therein.J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 253
Table 1
Radiocarbon age control on Iceland calibration lava flows
Lava flow Collection site Sample ID Material Age Age Source
(14C yr BP) (cal yr BP)
Lambahraun Hnífagil IT-276B (AMS) Charcoal at base of flow 3550 ± 50 3860 ± 40 Sinton et al. [14]
Lambahraun Þórólfsfell IT-206C (AMS) Carbonized sticks in ash 3830 ± 40 4220 ± 70 Sinton et al. [14]
at base of flow
Mean cal yr age ± standard deviation→ 4040 ± 250
Leitahraun Elliðaá, Reykjavík Not reported Charcoal 5300 ± 340 6060 ± 350 Hospers [20];
Áskelsson [21]
Leitahraun Elliðaá, Reykjavík U-632 Charcoal at base of flow 4630 ± 90 5380 ± 100 Jónsson [18]
Leitahraun Hlíðardalsskóli in Ölfus U-523 Charcoal at base of flow 4530 ± 100 5180 ± 140 Kjartansson [17]
Leitahraun west of Hjalli in Ölfus Not reported Charcoal at base of flow 4575 ± 75 5120 ± 70 Jónsson [19]
Leitahraun Elliðaá, Reykjavík AAR-6436 (AMS) Wood (birch) at base of flow 4540 ± 50 5150 ± 40 Guðmundsson
(pers. comm.)
Leitahraun Hlíðardalsskóli in Ölfus IT-401 (AMS) Charcoal at base of flow 4550 ± 65 5120 ± 70 Sinton et al. [14]
Leitahraun Elliðaá, Reykjavík IT-332 (AMS) Charcoal at base of flow 4580 ± 50 5300 ± 30 Sinton et al. [14]
Mean cal yr age ± standard deviation→ 5210 ± 110
Búrfellshraun Gálgaklettar, K1758 Birch stump on 5850 ± 110 6660 ± 130 Kjartansson [22]
Hafnarfjörður top of lava
Búrfellshraun Gálgaklettar, K1756 Peat beneath lava 7240 ± 130 8060 ± 120 Kjartansson [22]
Hafnarfjörður base
Búrfellshraun Gálgaklettar, K1757 Humin below 8740 ± 140 9730 ± 190 Kjartansson [22]
Hafnarfjörður base of lava
Preferred cal yr age ± standard deviation→ 8060 ± 120
Þingvallahraun Sog outlet from W-1912 Carbonized moss 9130 ± 260 10,260 ± 350 Kjartansson [24]
Þingvallavatn below lava
Þingvallahraun Sog outlet from IT-413 (AMS) Carbonized moss 9190 ± 65 10,330 ± 80 Sinton et al. [14]
Þingvallavatn below lava
Preferred cal yr age ± standard deviation→ 10,330 ± 80
To derive a single calibrated age for Lambahraun and Leitahraun, the individual 14C ages associated with each flow were first calibrated using the
CALIB 5.0.2 calibration program [51], and the means and standard deviations were then calculated from the calibrated (cal yr) ages. For
Búrfellshraun and Þingvallahraun, one calibrated age was chosen as the best representation and therefore preferred age of each flow (see text for
details). In the case of multiple calendar year intercepts, the calibrated age range with the greatest relative area under the probability distribution was
selected.
the uppermost level of the flow top. Sampling was beneath the base of the lava at two separate sites [14]
restricted to large tumuli and other features that stand provide a mean age of 4040 ± 250cal yr BP for the
well above the main flow surface (1–6 m relief; Fig. 2A) eruption (Table 1). Basalt samples were collected from
in order to reduce the likelihood of soil, volcanic ash, four separate calibration sites in the lava fields south of
moss, and snow cover. Surrounding topography at all the Hlöðufell table mountain, each near the top of large
sites was evaluated for shielding effects using a tumuli surfaced with ropey pahoehoe.
clinometer. Altitudes and latitudes of sample sites were Leitahraun represents lavas of the Leitin lava shield,
obtained from a hand-held GPS unit and from topo- which erupted from a vent on the eastern flank of
graphic maps. Samples were collected from multiple Bláfjöll, a mountain ridge ∼ 25 km southeast of Reykja-
sites (n = 4–6) on each lava flow to enable identification vík (Fig. 1) [15]. Leitahraun directly overlies lavas
of outliers in the data (e.g., [11]). Calibration sites were (Elliðaárhraun) that flowed northwest along the
preferentially selected from portions of the lava flows Elliðaá valley toward the present-day location of
directly associated with their constraining radiocarbon Reykjavík. Field relations suggest that Elliðaárhraun
ages. erupted from a vent in the vicinity of the Leitin shield
Lambahraun is a young low-angle lava shield in the summit [16]. Six radiocarbon ages from material
area south of Langjökull (Fig. 1). Two recently obtained directly beneath the bases of Leitahraun and Elliðaárh-
AMS radiocarbon ages on charcoal found directly raun overlap within 1σ uncertainty (Table 1),254 J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 Fig. 2. Photographs of flow features and surfaces sampled. (A) Tumuli on Leitahraun, standing 4–6m above the main flow surface. (B) Pahoehoe ropes on Þingvallahraun. indicating that these two flows are essentially identical (Fig. 2A), and are distributed within 5 km to the in age and represent eruptive phases of the monoge- southeast of the vent. netic Leitin lava shield [14]. Three of the radiocarbon Búrfellshraun issued from a vent at Búrfell, a small ages are recently obtained AMS ages on wood and crater-topped hill ∼13 km south of Reykjavík, and charcoal ([14], H.J. Guðmundsson, pers. comm.), and flowed northwest to the sea (Fig. 1). The best age for another three ages are conventional radiocarbon ages Búrfellshraun was considered by Kjartansson [22] to be obtained from charcoal [17-19]. For calibration 7240 ± 130 14C yr BP (8060 ± 120cal yr BP), obtained purposes, we use the mean of the six radiocarbon from charred peat directly beneath the lava base exposed ages to derive an age of 5210 ± 110 cal yr BP for along the sea-cliff flow margin; this age is interpreted to Leitahraun. The first published age of Elliðaárhraun, directly constrain the actual emplacement of the lava. 5300 ± 340 14C yr BP [20,21], is considered to be an Two other radiocarbon ages reported by Kjartansson old outlier (Table 1) but is within uncertainty of the [22] provide bracketing ages for Búrfellshraun that are mean. All five calibration sites on Leitahraun are consistent with the preferred age of the flow (Table 1). located on large tumuli surfaces with ropey pahoehoe One of the six calibration sites on Búrfellshraun (BÚR-
J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 255
1) is a squeeze-up spine of lava located 4km north of the [6,11]), which demonstrates that crushing does not
vent, and the remaining five sites are tumuli on the lava release cosmogenic helium in this procedure.
fields adjacent to the flow margin at the modern-day The quantity of inherited 3He is calculated using the
coastline (Fig. 1). measured 3He/4He ratio of the helium released by
Þingvallahraun is an early postglacial lava flow crushing, and the total 4He in the sample:
adjacent to the northern and eastern shores of Þingval-
lavatn, the largest lake in Iceland, ∼ 35km inland and to
3
Heinherited ¼ ð3 He=4 HeÞcrushing 4Hetotal
the west of Reykjavík (Figs. 1 and 2B) [14].
The quantity of cosmogenic 3He is calculated as
Þingvallahraun was previously included in lavas
follows:
designated as Eldborgir by Sæmundsson [23], but was
distinguished by Sinton et al. [14] as a separate eruptive 3
Hecosmogenic ¼ 3 Hetotal − 3 Heinherited
unit. For the 3He production rate calibrations, we
sampled four tumuli sites on the Þingvallahraun lava All measured cosmogenic 3He concentrations (Table
fields directly associated with the age control for this 2) were normalized to the flow surface using a
unit. Two radiocarbon ages were obtained on carbonized correction that incorporates measured sample thickness
moss found beneath the lava base exposed along the and density, an attenuation coefficient of 170 g cm− 2,
north bank of the Sog river outlet from Þingvallavatn, and the known dependence of isotope production with
including a conventional age by Kjartansson [24] and an depth [6]. The surface-normalized production rate
AMS age by Sinton et al. [14]. The two ages overlap values are 1–2% higher than the uncorrected data.
within 1σ uncertainty (Table 1); the more recently
obtained AMS age of 9190 ± 65 14C yr BP (10,330 3. Results
± 80 cal yr BP) is adopted as the best age of
Þingvallahraun at the calibrations sites. 3.1. Analytical and geologic sources of uncertainty
2.2. Measurement of 3He Each individual production rate determination is
reported in Table 2 with a 1σ error that incorporates
Basalt samples from each calibration site were propagation of mass spectrometer measurement uncer-
sawed to a measured thickness (from 2 to 4 cm), and tainties. Reproducibility of individual measurements
the dry bulk density of the sawed slabs was measured was evaluated with duplicate analyses of olivine
(from 1.8 to 2.4 g cm− 3, reflecting the vesicular nature separates from the same calibration site. All three
of the flow tops). Olivine phenocrysts were separated duplicate pairs overlap within 1σ error, indicating
from samples by crushing, sieving, magnetic separa- high reproducibility of measurements and suggesting
tion and hand picking. The helium concentrations and that the error estimates are conservative. A mean
isotopic compositions were measured with a 90° production rate, incorporating 14C flow age uncertain-
magnetic sector mass spectrometer at the Woods ty, was calculated from the population of measure-
Hole Oceanographic Institution following previously ments for each flow. Uncertainties in flow ages
described methodology (see [6,11,25-27] for procedur- contribute very little error to the mean production
al details, reproducibility, and blanks). The olivine rates. Sample thickness corrections are minor and
phenocrysts contain 3He and 4He with both cosmo- contribute negligible error to the production rates [3].
genic and inherited (magmatic) sources. Phenocrysts No sites had shielding of > 10°, hence no corrections
measuring 1–0.355 mm (typically 100–300mg) were for shielding were necessary. Error estimates do not
first crushed in a vacuum, and the resulting powder include uncertainty in the scaling methods used to
was then melted in a resistance furnace. Crushing normalize production rate values to sea level at high
selectively releases the inherited component of helium, latitudes (SLHL), but scaling uncertainties are likely
which is held primarily within melt and fluid to be minor at the calibration sites (see Section 3.5).
inclusions. Melting of the resulting olivine powder No corrections for geomagnetic modulation of pro-
releases the helium held in the crystal matrix of the duction rates are necessary because production at the
phenocrysts, which contains the majority of the high latitudes of the Iceland field localities is
cosmogenic 3 He. Many paired measurements of insensitive to changes in the geomagnetic field
crushed and uncrushed olivine grains from the same [3,28] (see Section 3.6).
mineral separates yielded cosmogenic 3He concentra- Results exhibit some scatter in cosmogenic 3He
tions that agree within 2σ analytical uncertainty (e.g., surface concentrations among the population of256
Table 2
Helium data for Iceland lava flows
3
Sample Oliv. Alt. Lat. Lon. He/4He 3
He/4He 4
He 3
Hec 3
He#c Lal Prod. rate Stone Prod. rate
comp. (km) (°N) (°W) (R/Ra, crush) (R/Ra, melt) (10− 10 cm3 g− 1) (105 at g− 1) (105 at g− 1) scaling (at g− 1 yr− 1, scaling (at g− 1 yr− 1,
±1σ) ±1σ)
Lambahraun (4040 ± 250cal yr BP)
J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264
IC02-16-19084 Fo82 ± 0.1 0.459 64.3843 20.5549 13.6 ± 0.8 73.6 ± 3.9 3.4 ± 0.1 7.5 ± 0.6 7.7 ± 0.6 1.586 120 ± 9 1.683 113 ± 9
IC02-17-17338 Fo82 ± 0.2 0.457 64.3836 20.5564 13.4 ± 3.0 112 ± 9 2.0 ± 0.1 7.3 ± 0.9 7.4 ± 0.9 1.583 116 ± 14 1.680 109 ± 13
IC02-19-19323 Fo82 ± 0.6 0.447 64.3644 20.5531 12.7 ± 2.9 51.4 ± 1.5 5.9 ± 0.1 8.5 ± 0.7 8.6 ± 0.7 1.569 135 ± 12 1.665 127 ± 11
IC02-20-16373 Fo82 ± 1.3 0.446 64.3629 20.5514 13.7 ± 0.9 213 ± 21 1.1 ± 0.1 8.1 ± 1.3 8.2 ± 1.3 1.567 130 ± 21 1.663 123 ± 19
Means → 125 ± 12 118 ± 9
Leitahraun (5210 ± 110 cal yr BP)
LEIT-1-1067 Fo80 ± 0.5 0.243 63.9726 21.4674 13.8 ± 0.4 26.6 ± 0.8 16.8 ± 0.2 8.0 ± 0.6 8.2 ± 0.6 1.289 122 ± 9 1.374 114 ± 8
LEIT-2-1374 Fo81 ± 0.2 0.247 63.9738 21.4671 13.9 ± 0.8 48.9 ± 1.3 7.0 ± 0.2 9.1 ± 0.5 9.3 ± 0.5 1.294 137 ± 7 1.379 129 ± 7
LEIT-3-1191 Fo81 ± 0.5 0.273 63.9765 21.4783 15.3 ± 0.5 25.7 ± 0.5 22.9 ± 0.2 8.8 ± 0.6 8.9 ± 0.6 1.328 129 ± 9 1.414 121 ± 9
LEIT-4-1131 Fo81 ± 0.2 0.289 63.9822 21.4773 14.2 ± 1.6 56.6 ± 2.4 6.0 ± 0.2 9.5 ± 0.8 9.6 ± 0.8 1.349 136 ± 11 1.436 128 ± 10
LEIT-5-1315 Fo81 ± 0.2 0.277 63.9839 21.4727 13.2 ± 1.1 28.1 ± 0.5 18.7 ± 0.2 10.4 ± 0.8 10.5 ± 0.8 1.333 151 ± 12 1.420 142 ± 11
LEIT-5-1129 Fo81 ± 0.2 0.277 63.9838 21.4727 15.8 ± 1.5 43.4 ± 1.1 8.8 ± 0.2 9.0 ± 0.7 9.2 ± 0.7 1.333 132 ± 10 1.420 124 ± 10
Means → 133 ± 8 125 ± 8
Búrfellshraun (8060 ± 120cal yr BP)
BÚR-1-2410 Fo89 ± 1.6 0.096 64.0592 21.8744 14.0 ± 0.2 96.0 ± 1.5 4.1 ± 0.1 12.5 ± 0.4 12.6 ± 0.4 1.109 141 ± 4 1.187 132 ± 4
BÚR-2-2401 Fo85 ± 1.5 0.030 64.0892 21.9679 12.9 ± 0.2 66.1 ± 1.0 5.6 ± 0.1 11.1 ± 0.3 11.3 ± 0.3 1.033 136 ± 4 1.108 127 ± 3
BÚR-3-2713 Fo86 ± 2.2 0.022 64.0896 21.9690 12.5 ± 0.6 25.0 ± 0.3 26.6 ± 0.2 12.3 ± 0.7 12.6 ± 0.7 1.024 153 ± 9 1.098 142 ± 8
BÚR-3-2780 Fo86 ± 2.2 0.022 64.0896 21.9690 11.4 ± 0.5 378 ± 22 0.8 ± 0.1 11.5 ± 1.2 11.7 ± 1.2 1.024 142 ± 15 1.098 132 ± 14
BÚR-4-2585 Fo87 ± 1.3 0.026 64.0917 21.9704 13.5 ± 0.3 156 ± 3 2.1 ± 0.1 11.2 ± 0.5 11.4 ± 0.5 1.029 137 ± 6 1.103 128 ± 6
BÚR-4-2854 Fo87 ± 1.3 0.026 64.0917 21.9704 14.7 ± 0.1 36.2 ± 0.6 13.5 ± 0.1 10.8 ± 0.3 11.0 ± 0.3 1.029 132 ± 4 1.103 123 ± 4
BÚR-5-2732 Fo87 ± 1.7 0.028 64.0935 21.9685 13.1 ± 0.4 116 ± 2 2.6 ± 0.1 9.7 ± 0.4 9.9 ± 0.4 1.031 120 ± 5 1.105 112 ± 5
BÚR-6-2792 Fo87 ± 1.9 0.027 64.0871 21.9689 13.5 ± 0.2 63.9 ± 1.1 5.6 ± 0.1 10.5 ± 0.3 10.7 ± 0.3 1.030 129 ± 4 1.104 120 ± 3
Means → 135 ± 10 126 ± 9
Þingvallahraun (10,330 ± 80cal yr BP)
IC02-1-18787 Fo85 ± 0.1 0.131 64.1645 21.0369 14.5 ± 0.3 38.5 ± 0.8 16.9 ± 0.1 15.1 ± 0.6 15.3 ± 0.6 1.150 129 ± 5 1.230 121 ± 5
IC02-7-10543 Fo83 ± 0.2 0.122 64.1649 21.0368 11.0 ± 1.2 82.8 ± 3.1 4.3 ± 0.2 11.5 ± 0.8 11.7 ± 0.8 1.140 99 ± 7 1.218 93 ± 6
IC02-10-25131 Fo85 ± 0.4 0.121 64.1562 21.0480 15.1 ± 0.3 62.6 ± 1.7 8.7 ± 0.1 15.3 ± 0.6 15.5 ± 0.6 1.138 132 ± 5 1.217 123 ± 5
IC02-11-17430 Fo85 ± 0.1 0.120 64.1561 21.0480 14.5 ± 0.4 183 ± 8 2.7 ± 0.1 16.9 ± 1.2 17.1 ± 1.2 1.137 146 ± 10 1.216 136 ± 9
Means → 136 ± 9 127 ± 9
Grand means → 132 ± 5 124 ± 4
Notes to Table 2:J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 257 Fig. 3. Cosmogenic 3He production rates determined from calibration lava flows. Solid black diamonds are rates normalized to SLHL using Table 2 in Lal [5] with standard atmosphere. Solid gray circles are rates normalized to SLHL using Stone [12] with site-specific mean SLP incorporated (1005.8mbar). Open symbols indicate one outlier not included in means from Þingvallahraun. Error bars represent 1σ analytical uncertainty only, and do not include errors due to scaling and other uncertainties. calibration sites on individual lava flows (Fig. 3), with a surface degradation would result in higher 3 He degree of scatter similar to that found in previous 3He production rate values. Snow cover is assumed to be calibration studies involving multiple calibration sites on insignificant at the Iceland calibration sites, owing to monogenetic landforms [11]. Most scatter is within the the relatively mild mean temperatures of the prevailing range of analytical error, given that individual measure- maritime climate in southwest Iceland (∼ 0 °C during ments generally agree within 2σ uncertainty for each the winter months and ∼ 5°C for the year; climate data lava flow. The only notable exception is one measure- available from the Icelandic Meteorological Office at ment from Þingvallahraun (sample IC02-7), which is http://www.vedur.is), and no corrections were made. considered an outlier and was excluded from the mean Among the flows investigated (and disregarding the production rate derived from Þingvallahraun. It is outlier from Þingvallahraun), measurements from unclear whether this outlier can be explained by a Lambahraun exhibit the greatest degree of scatter complex exposure history, perhaps by human excava- (Fig. 3). The Lambahraun calibration sites are tions, or if geological factors play a role in its somewhat higher in elevation (446–459 m) and further anomalously low value. inland than the other calibration sites, and are more Aside from analytical error, some scatter may also removed from the moderating influence of the ocean. arise from geologic uncertainties such as erosion and Variable snow depths on the flow surface may past cover by snow and soil. The presence of well- therefore contribute to the scatter among the Lambah- preserved pahoehoe ropes at most calibration sites raun measurements (e.g., as in [11]). The mean suggests that surface erosion is minimal, and no production rate obtained from Lambahraun is lower corrections were applied. Incorporation of an erosion than those from the other three flows, but the correction to account for the possibility of unrecognized difference is not significant within measurement error. Notes to Table 2: Olivine compositions (expressed as forsterite content) are mean values and standard deviations derived from measurements on five individual phenocrysts from each sample location. For the microprobe analyses, the accelerating voltage and beam current used were 15kV and 10nA, respectively, typical counting times were 20–40s, and 1σ standard deviations of the counts were 0.5–1%. The data were reduced with the CITZAF program [52]. Helium measurement uncertainties are based on 0.5% uncertainty on the 4He peak and an error of 2 × 10− 12 (about 3%) on the blank. 3He/4He ratios are reported relative to the atmospheric value (R/Ra, where Ra = 1.384 × 10− 6). For all samples, crushing and melting were performed on the same mineral separate; some olivines had unusually low helium concentrations, which explains the large uncertainties and scatter in inherited (crush) 3He/4He ratios for individual flows. Cosmogenic 3Hec concentrations marked with “#” are normalized to the surface assuming an attenuation coefficient of 170g cm− 2 [6]. Production rates are normalized to SLHL using two alternative scaling schemes: (a) Table 2 in Lal [5], standard atmosphere; (b) Stone [12], site-specific mean SLP incorporated (1005.8mbar). Scaling factors are the ratio of production at sample location to production at SLHL. Uncertainties of individual production rate determinations include propagated 1σ analytical uncertainties only. Uncertainties of mean production rates are equivalent to the standard deviation of the mean, and incorporate uncertainty in calibrated radiocarbon ages of the lava flows.
258 J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264
3
3.2. Comparison to previous He production rate versus fayalite). In order to evaluate possible composi-
calibrations tional effects on production rate, we performed major
element analyses of olivine phenocrysts from each
Table 3 summarizes the currently available 3He calibration site with the JEOL JXA-733 Superprobe at
production rates measured in olivine, grouped by the MIT Electron Microprobe Facility. The nineteen
region. All production rates are normalized to SLHL calibration sites exhibit a narrow range of olivine
using various scaling methods. When normalized with compositions, from Fo80 to Fo89, and individual values
either the widely used Lal [5] scaling model or an are coherent within single flows (Table 2). Given the
alternative model by Dunai [29], mean production rates small compositional variation among the samples,
appear to vary considerably by region, with the highest compositional differences are unlikely to be an
rates in Patagonia and the lowest rates in the Canary important source of observed scatter in 3He production
Islands. It is important to note, however, that the rates reported here. Olivine phenocrysts used for the 3He
calibrations listed in Table 3 are derived from surfaces calibrations in the western USA [10,11,30] have
spanning a broad range of ages (from ∼ 550yr BP to compositions (Fo74 to Fo85) similar to those determined
∼ 281 ka). Because of the possibility for temporal here. Assuming that Lal's calculations for compositional
variations in production rates caused by geomagnetic dependence are correct, the maximum compositional
modulation, long-term air pressure variation through variation among samples (Fo74 to Fo89) corresponds to a
time over calibration sites, and other factors (discussed predicted production rate difference of only ∼5%, hence
in the following sections), a meaningful evaluation of the discrepancy between the Iceland production rates
regional variability in 3He production rates should be and values calibrated in the western USA cannot be
based on a comparison of calibrated rates determined attributed to compositional effects.
over similar time scales. With this in mind, we consider
the Iceland 3He production rates in the context of 3.4. Effect of postglacial isostatic rebound
previous calibrations reported for Holocene surfaces
only. Changing surface altitude caused by postglacial
Mean production rates derived from the four isostatic rebound of Iceland should have a small effect
Holocene lava flows in Iceland agree within their on the apparent 3He production rate at the calibration
respective standard deviations, and yield a grand mean sites. If a calibration site has rebounded, it follows
of 132 ± 5 atoms 3 He g − 1 yr − 1 (± 1σ; in olivine, that its initial altitude was lower in the past, and hence
normalized to SLHL with Lal scaling and the standard the actual production rate was lower during that time
atmosphere). The 3He production rate in Iceland is due to a greater shielding thickness of the atmosphere.
∼ 17% higher than the mean of normalized Holocene The time-integrated apparent production rate at the
values determined in the western USA. If the calibration rebounding calibration site, when normalized to sea
sites with large snow cover corrections (∼20%; Yapoah level, would be erroneously low if left unadjusted for
Crater, South Belknap) are excluded from the western this effect.
USA mean, the offset between Iceland and western USA The only flow for which post-emplacement rebound
production rates widens to ∼ 22%. In contrast, the may be a concern is Þingvallahraun, which has an early
means of Holocene 3He production rates in Iceland and postglacial age of 9190 ± 65 14C yr BP (10,330 ± 80cal
Hawaii are indistinguishable within uncertainty, owing yr BP) and conceivably may have erupted from vents
primarily to the large variance of the Hawaii data set. that were exposed soon after the ice front had retreated
The large variance at Hawaii is partly due to from the Þingvellir area (located ∼ 50 km inland from
geomagnetic modulation at low latitudes [25]. In the the former ice margin position at Álftanes, and ∼ 35km
following sections, we therefore focus on the various inland from the closest present-day shorelines to the
factors that may contribute to the offset between the west and south). Total postglacial rebound in Iceland is
high 3 He production rates in Iceland relative to estimated to range from 100m near the present coastline
calibrations in the western USA. to > 500m toward the interior [31-33]. In coastal areas of
Iceland, most rebound was completed in about 1000 yr,
3.3. Effect of olivine composition on production rate i.e., by ∼ 9000 14C yr BP [31]. The anomalously fast
rebound is probably due to the low viscosity of the
Theoretical estimates of 3He production rates by Lal asthenosphere beneath the Iceland hotspot [31]. At the
[5] suggest that production may vary as a function of inland location of Þingvallahraun, where the ice cover
olivine composition (∼35% higher in pure forsterite would have been thicker and retreated later, theJ.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 259
Table 3
Cosmogenic 3He production rates in olivine by region
Sample/feature Age (cal yr BP) Lal 1991 Dunai 2000 Dunai 2001 Stone 2000
(at g− 1 yr− 1, ±1σ) (at g− 1 yr− 1, ±1σ) (at g− 1 yr− 1, ±1σ) (at g− 1 yr− 1, ±1σ)
Iceland
Lambahraun (4) 4040 ± 250 125 ± 12 131 ± 10 118 ± 9
Leitahraun (5) 5210 ± 110 133 ± 8 138 ± 9 125 ± 8
Búrfellshraun (6) 8060 ± 120 135 ± 10 136 ± 10 126 ± 9
Þingvallahraun (3) 10,330 ± 80 136 ± 9 138 ± 10 127 ± 9
Means → 132 ± 5 136 ± 5 124 ± 4
Western USA
Yapoah Crater⁎ (1) 2470 ± 740 105 ± 32 107 ± 32 108 ± 33
Yapoah Crater (5) 2470 ± 740 126 ± 31 127 ± 32 129 ± 32
South Belknap (5) 2760 ± 30 124 ± 16 125 ± 16 126 ± 16
Clear Lake (5) 2830 ± 50 111 ± 9 116 ± 9 105 ± 9 114 ± 9
Black Crater⁎ (1) 3240 ± 40 103 ± 8 107 ± 8 98 ± 17 105 ± 8
Lava Butte (2) 7070 ± 90 111 ± 4 115 ± 4 108 ± 10 114 ± 4
Giant Crater⁎ (1) 12,700 ± 60 95 ± 5 99 ± 5
Tabernacle Hill⁎ (3) 17,290 ± 260 121 ± 2 127 ± 3
Bonneville Flood⁎ (1) 17,470 ± 270 115 ± 4 118 ± 4
Means (all values) → 112 ± 10 116 ± 10 104 ± 7
Means (Holocene) → 113 ± 10 116 ± 9 104 ± 7 116 ± 10
Hawaii
KS87-47 (1) 550 ± 140 235 ± 100 252 ± 108 239 ± 102
KS87-03,14,15; T87-4 (4) 580 ± 20 184 ± 34 202 ± 40 188 ± 35
KS87-31 (1) 730 ± 60 202 ± 41 224 ± 46 240 ± 100 206 ± 42
T87-8; KS87-05 (2) 2200 ± 100 71 ± 23 79 ± 25 89 ± 27 72 ± 23
KS87-04 (1) 2410 ± 80 53 ± 14 59 ± 15 54 ± 14
KS87-43 (1) 2800 ± 70 91 ± 12 98 ± 13 101 ± 27 93 ± 12
KS87-13 (1) 3100 ± 110 89 ± 9 96 ± 10 96 ± 24 91 ± 9
KS87-46 (1) 4490 ± 80 83 ± 5 89 ± 6 94 ± 17 84 ± 5
KS87-48 (1) 5370 ± 40 61 ± 13 66 ± 14 62 ± 13
KS87-42 (1) 7290 ± 50 91 ± 10 98 ± 11 102 ± 24 92 ± 10
KS87-01C (1) 8030 ± 190 200 ± 12 222 ± 14 205 ± 13
KS87-07,08 (2) 8520 ± 70 183 ± 8 202 ± 9 187 ± 8
RM88-9490 (1) 10,740 ± 90 130 ± 7 143 ± 7 133 ± 7
KTE85-30-1-7 (1) 31,630 ± 2020 128 ± 8 143 ± 9 122 ± 21
Means (all values) → 129 ± 61 141 ± 67 121 ± 54
Means (Holocene) → 129 ± 63 141 ± 69 120 ± 59 131 ± 65
Canary Islands
TA-flow (3) 152 ± 26ka 98 ± 18 115 ± 22 99 ± 17
AFB-flow (4) 281 ± 19ka 99 ± 8 117 ± 9 99 ± 10
Means → 99 ± 10 116 ± 12 99 ± 10
Patagonia
Rio Pinturas (2) 68 ± 3ka 135 ± 7 142 ± 8
Cerro Volcán (5) 109 ± 3ka 145 ± 5 154 ± 5
Means → 140 ± 7 148 ± 8
Production rates are normalized to SLHL using four alternative scaling schemes: (a) Table 2 in Lal [5], standard atmosphere, no geomagnetic
corrections; (b) Dunai [29], standard atmosphere, no geomagnetic corrections; (c) Dunai [38], standard atmosphere, geomagnetic corrections
incorporated; (d) Stone [12], site-specific mean SLP incorporated, no geomagnetic corrections. Western USA data from Cerling and Craig ([10]; with
asterisks) and Licciardi et al. [11]; Hawaii data from Kurz [53] and Kurz et al. [25]; Canary Islands data from Dunai and Wijbrans [54]; Patagonia data
from Ackert et al. [44]. Italicized value in parentheses following sample/feature name indicates number of calibration sites used to calculate
production rate. Uncertainties of mean production rates are equivalent to the standard deviation of the mean, and incorporate uncertainty in calibrated
radiocarbon ages of the lava flows. In some cases where the mean is derived from a small population of values (e.g., 2–3 measurements), uncertainties
are quoted as the propagated analytical uncertainty contained in the mean, incorporating lava flow age uncertainties. Note that production rates in this
table from the western USA and Hawaii supersede values reported in a previous compilation by Licciardi et al. [11], as they are updated with newly
calibrated radiocarbon ages (using [55] for KTE85-30-1-7 and [51] for all other calibrations) and correct errors made in the prior calculations of
uncertainty in the means.260 J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264
magnitude of total rebound would presumably have available 3He production rates were normalized using
been somewhat greater than 100 m and may have been the alternative scaling schemes of Lal [5] and Dunai
largely complete sometime after 9000 14C yr BP as [29] (Table 3). The offset in production rates between
dictated by the ice retreat history. The glacial and Iceland and the western USA is found to be nearly
rebound histories specific to the Þingvellir region, equivalent for both scaling models, which argues
however, are poorly known [34-36]. against differences in altitudinal and latitudinal scaling
The rebound data compiled by Sigmundsson [31] can as an explanation for variation in 3He production rates
be used as a guide to the influence of rebound on between these two particular regions. These results are
production rates. As a first-order approximation, we the first near-sea-level high-latitude production rate
assume that the Þingvallahraun calibration sites expe- calibrations of any in situ cosmogenic nuclide, and they
rienced 120 m of rebound at a linear uplift rate for the significantly extend the geographic coverage in previ-
first 1000 yr following emplacement of the lava. ous calibration studies.
Incorporation of this vertical movement in the altitudinal
scaling of surface production results in a correction that 3.6. Influence of the Earth's geomagnetic field
increases the integrated normalized production rate at
each site by only 0.5–0.6%. If the 120m of rebound Production rates of in situ produced cosmogenic
occurred over a longer time interval, or if any residual nuclides vary significantly with latitude because the
rebound has continued to occur gradually following the cosmic ray flux is deflected by the Earth's geomagnetic
initial phase of rapid rebound, this increase would be field to varying degrees from equatorial to polar regions
somewhat higher. Given the very small magnitude of (e.g., [3,37]). Production rates may also vary through
rebound-related adjustment to 3He production rates at time due to secular variations in the intensity and
Þingvellir, as well as the observation that the adjustment orientation (inclination) of Earth's geomagnetic field
increases the offset between production rates in Iceland [25,28,38]. The sensitivity of production rates to
and the western USA, it is unlikely that the high 3He changes in geomagnetic field strength and inclination
production rate in Iceland can be explained by the is probably negligible at the high latitude of the Iceland
effects of isostatic rebound. calibration sites (∼ 64°N), whereas the predicted
sensitivity is measurably greater at the lower latitudes
3.5. Differences in altitudinal and latitudinal scaling of previously reported 3He calibration sites in the
models western USA, Hawaii, and the Canary Islands. More-
over, geomagnetic modulation of the cosmic ray flux
Production rates of in situ cosmogenic nuclides vary and attendant production rates becomes more significant
significantly with latitude and altitude because cosmic with increasing altitude [37,38]. For these reasons,
rays are deflected by the Earth's magnetic field and comparisons of 3He production rates measured at
attenuated by the atmosphere. The most commonly used calibration sites distributed over a broad range of
factors for altitude and latitude scaling are based on star latitudes and altitudes must consider the influence of
production rates in photographic emulsions combined geomagnetic field changes.
with cosmic ray neutron-monitor data from around the The cosmogenic 3He production rates reported here
world [5,37]. Several alternative scaling models have display no significant temporal variation over the
been proposed (e.g., [29,38,39]), but the most com- period from ∼ 4 to 10ka (Table 2), a time interval
monly accepted model remains that of Lal [5]. Lal when there were large fluctuations in the intensity of
estimated the overall uncertainty of his scaling factors to the Earth's dipole moment and in the position of the
be about 10%. geomagnetic pole [40-42]. This result is entirely
Due to the relatively small range of altitudes, the consistent with the theoretical prediction that time-
scaling factors required to normalize the production integrated production rates at high latitudes (> 60°) are
rates to SLHL are modest (1.024–1.583, using Table 2 insensitive to changes in the geomagnetic field
in Lal [5]). Scaling uncertainties are therefore relatively [28,37]. Calibrated cosmogenic 3He and 36Cl produc-
small for the SLHL-normalized results reported here, tion rates in the western USA exhibit very little
and we do not include scaling-related error in the temporal variation during the Holocene, indicating
values. Differences in scaling models must be consid- that production rates at mid-latitudes are only weakly
ered, however, when comparing the Iceland results to affected by geomagnetic modulation of the cosmic ray
normalized production rates calibrated elsewhere over a flux [11,43]. Kurz et al. [25] reported a large temporal
broad range of latitudes and altitudes. Accordingly, all variability in 3 He production rates derived fromJ.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264 261
calibration sites in Hawaii (∼ 19°N), which reflects a function of mean annual atmospheric pressure rather
more pronounced influence of geomagnetic field than altitude. The pressure-dependent scaling factors
strength on production rates at low latitudes. The allow derivation of corrections to spallogenic cosmo-
Hawaii results, together with theoretical considera- genic nuclide production rates around the world, based
tions, demonstrate that magnetic field effects may lead on the present-day global distribution of SLP. Due to the
to important differences in production rates between influence of the Icelandic Low, Stone's analysis predicts
high and low latitudes. that SLHL-normalized production rates in Iceland
Dunai [38] modeled the effect of secular variations should be ∼6–8% higher than values scaled using the
of the geomagnetic field on selected 3He production standard atmosphere (SLP = 1013.25 mbar). If the in-
rates from calibration sites in Hawaii, the western strumental-era global mean pressure field is representa-
USA, and the Canary Islands. The recalculated produc- tive of conditions through the Holocene, production
tion rates, adjusted for geomagnetic modulation and rates calibrated from Holocene-age surfaces in Iceland
normalized to SLHL, are reproduced in Table 3. Dunai's should exhibit a comparable positive deviation from
analysis suggests that accounting for the influence of normalized values calibrated from sites experiencing
the geomagnetic field widens the offset between Holo- a higher mean SLP, such as the western USA. The
cene 3He production rates in Iceland and adjusted new 3He data provide an important empirical test of this
values from lower latitude calibration sites. If this hypothesis.
analysis is valid, the influence of the Earth's geomagne- The mean 3He production rate in Iceland (132 ±
tic field can be ruled out as an explanation for high 3He 5 atoms g− 1 yr− 1; ± 1σ) is ∼ 17% higher than the mean
production rates in Iceland. of Holocene values determined in the western USA
(113 ± 10 atoms g− 1 yr− 1; ± 1σ), when all rates are
3.7. Influence of the Icelandic Low normalized to SLHL using the standard atmosphere
and no geomagnetic corrections. The pressure-depen-
Surface air pressure represents a potentially signif- dent scaling formulation of Stone [12] enables reeval-
icant control on in situ cosmogenic nuclide production uation of 3He production rates in Iceland and the western
rates [4,12,44-46] because production rates are expected USA by incorporating normalized mean SLP at the
to be relatively high beneath stable low-pressure calibration sites. Using the instrumental-era mean SLP in
systems and low beneath regions of high pressure. Iceland of 1005.8 ± 2.1mbar (measured at Reykjavík),
This anticorrelation between mean annual pressure and and a mean SLP value of 1016.9 ± 0.8 mbar in the
production rates arises because lower atmospheric western USA (averaged from 15 stations in the vicinity
pressure corresponds to less attenuation of the cosmic of calibration sites; W.M.L. Spangler and R.L. Jenne,
ray flux [12]. World monthly surface station climatology, NCAR data
Prevailing atmospheric pressure in Iceland is domi- set ds570.0, 2004, available at http://dss.ucar.edu/
nated by the influence of the Icelandic Low, a semi- datasets/ds570.0), we obtain mean Holocene 3 He
permanent low pressure system centered over the North production rates of 124 ± 4 atoms g− 1 yr− 1 in Iceland
Atlantic Ocean that constitutes a principle component of and 116 ± 10 atoms g− 1 yr− 1 in the western USA. The
modern Northern Hemisphere atmospheric circulation pressure-adjusted production rates thus overlap within
[47]. Instrumental records of SLP in Iceland (measured 1σ uncertainty. By comparison, deviations from the
at Reykjavík) extend back to 1821 [48] (data available standard atmosphere sea-level temperature (15 °C) have
from The Virtual Center for Decadal Climate Variability a negligible impact on the production rates; for example,
at ftp://ftp.cru.uea.ac.uk/data/nao_ice.dat), and glacio- incorporating the mean annual temperature of ∼ 5 °C in
chemical proxy evidence has been used to reconstruct Reykjavík in the scaling lowers the mean 3He production
pressure variations of the Icelandic Low to 1400 yr ago rate in Iceland by only 0.5%.
[49]. The longer term pressure history of the Icelandic The pressure-dependent scaling of Stone [12] can
Low is poorly constrained, but it is likely that the be used to iteratively derive the SLP conditions
presence of late Pleistocene continental ice sheets required to fully reconcile normalized production
adjacent to the circum-North Atlantic region yielded rates in Iceland and the western USA. Calculations
significantly different atmospheric circulation patterns show that the 17% offset in production rates
(e.g., [50]). corresponds to a normalized SLP difference of
Stone [12] evaluated the influence of spatial varia- 19 mbar, hence if the instrumental-era mean SLP at
tions in surface air pressure on production rates by calibration sites in the western USA (∼ 1017 mbar) is
recasting the spallation rate scaling factors of Lal [5] as a assumed to have remained constant through the262 J.M. Licciardi et al. / Earth and Planetary Science Letters 246 (2006) 251–264
Holocene, this suggests a time-integrated mean SLP nuclide production rates and associated exposure ages
of ∼998 mbar in Iceland during the Holocene. This should be corrected for local surface air pressure
value is notably lower than the instrumental-era mean anomalies. Routine adoption of these corrections will
SLP in Reykjavík (∼ 1006mbar), but within the increase the accuracy of surface exposure ages. Second,
bounds of mean SLP at the center of the Icelandic these results support previous suggestions [44] that
Low suggested by the 1400-yr proxy record (∼997– cosmogenic nuclides can be used as a tool for
1001 mbar; [49]). An even lower mean Holocene SLP paleobarometry, such that long-term deviations in the
in Iceland and/or higher SLP in the western USA mean pressure field can be reconstructed from time-
would be required to reconcile normalized production integrated production rates.
rates in these regions if one accepts the larger offset
implied by the geomagnetic adjustments of Dunai Acknowledgements
[38] (Table 3).
These considerations demonstrate that surface air We thank Richard S. Williams for logistical support
pressure effects can account for much, if not all, of the and inspiration, John M. Sinton, Karl Grönvold, and
apparent offset between 3He production rates in Iceland Hjalti J. Guðmundsson for providing unpublished data
and the western USA. The results suggest that the and guidance, Hreggviður Norðdahl and John O.H.
Icelandic Low was a persistent feature through the Stone for discussions, Dave Lund, Kristy Dahl, and
Holocene, and that the Icelandic Low was more intense Kate Denoncourt for field assistance, Todd Belanger,
in the early Holocene compared to present. Production Bryn Dumas, Chelsea Corr, Cindy Van Dyke, Julie
rate uncertainties prevent determination of a high- Bryce, and Wally Bothner for help with sample
precision value for mean Holocene SLP in Iceland. preparation, Nilanjan Chatterjee for assistance at the
Nonetheless, low atmospheric pressure can satisfactorily MIT Electron Microprobe Facility, and John Gosse for a
explain the anomalously high 3He production rates in thoughtful review that improved the manuscript. This
Iceland. work was supported by National Science Foundation
ATM-0214059 to Licciardi and Kurz, and a Woods Hole
4. Conclusions Oceanographic Institution Postdoctoral Scholarship to
Licciardi.
Calibration measurements from Holocene lava flows
in Iceland establish a reproducible local cosmogenic
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