TY - JOUR
T1 - Elevation dependence of cosmogenic 36Cl production in Hawaiian lava flows
AU - Desilets, Darin
AU - Zreda, Marek
N1 - Funding Information:
We thank Frank Trusdell and David Sharrod of the USGS Hawaiian Volcano Observatory for sharing their knowledge and experience with us. This material is based upon the work supported by the National Science Foundation under grants EAR-0001191, EAR-0126209 and ATM-0081403 and by Packard Fellowship in Science and Engineering 95-1832.
PY - 2006/6/30
Y1 - 2006/6/30
N2 - We measured an elevation profile of cosmogenic 36Cl in two well-preserved lava flows on Mauna Kea, Hawaii (19.8° N, 155.5° W) in order to directly constrain the elevation dependence of cosmogenic nuclide production rates. The flows are vertically-extensive hawaiites erupted at 40.1 ± 0.6 and 62.2 ± 1.0 ka from point-vents on the upper flanks of Mauna Kea. The average paleo cutoff rigidity (a measure of geomagnetic shielding of cosmic rays) for these flows is 11 GV and their paleo-elevation range is 2100-3700 m. Production of 36Cl is dominated by neutron reactions, with the high-energy 39K(n,x) and 40Ca(n,x) mechanisms accounting for nearly half of the 36Cl production and the low-energy reaction 35Cl(n,γ) responsible for the remaining half. Production by negative muons is small at the elevations of our samples, accounting for less than 2% of the total production in the lowest elevation samples. The elevation dependence of 36Cl production measured in these lava flows is described by an effective attenuation length of 138 ± 5 g cm- 2. This result is close to the value of 140 g cm- 2 determined from neutron monitor surveys of high-energy nucleon fluxes, but significantly below the value of 149 g cm- 2 determined from measurements of low-energy neutrons. The predicted atmospheric attenuation length for these lava flows, incorporating both high- and low-energy mechanisms, is 144 g cm- 2. The good agreement between the 36Cl elevation profile and cosmic-ray surveys validates the use of neutron flux measurements to scale 36Cl production rates when production by muons is negligible.
AB - We measured an elevation profile of cosmogenic 36Cl in two well-preserved lava flows on Mauna Kea, Hawaii (19.8° N, 155.5° W) in order to directly constrain the elevation dependence of cosmogenic nuclide production rates. The flows are vertically-extensive hawaiites erupted at 40.1 ± 0.6 and 62.2 ± 1.0 ka from point-vents on the upper flanks of Mauna Kea. The average paleo cutoff rigidity (a measure of geomagnetic shielding of cosmic rays) for these flows is 11 GV and their paleo-elevation range is 2100-3700 m. Production of 36Cl is dominated by neutron reactions, with the high-energy 39K(n,x) and 40Ca(n,x) mechanisms accounting for nearly half of the 36Cl production and the low-energy reaction 35Cl(n,γ) responsible for the remaining half. Production by negative muons is small at the elevations of our samples, accounting for less than 2% of the total production in the lowest elevation samples. The elevation dependence of 36Cl production measured in these lava flows is described by an effective attenuation length of 138 ± 5 g cm- 2. This result is close to the value of 140 g cm- 2 determined from neutron monitor surveys of high-energy nucleon fluxes, but significantly below the value of 149 g cm- 2 determined from measurements of low-energy neutrons. The predicted atmospheric attenuation length for these lava flows, incorporating both high- and low-energy mechanisms, is 144 g cm- 2. The good agreement between the 36Cl elevation profile and cosmic-ray surveys validates the use of neutron flux measurements to scale 36Cl production rates when production by muons is negligible.
KW - Mauna Kea
KW - attenuation length
KW - cosmic rays
KW - cosmogenic nuclides
KW - scaling factors
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U2 - 10.1016/j.epsl.2006.03.050
DO - 10.1016/j.epsl.2006.03.050
M3 - Article
AN - SCOPUS:33744947807
SN - 0012-821X
VL - 246
SP - 277
EP - 287
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
IS - 3-4
ER -