COSMOGENIC CHLORINE-36 FROM CALCIUM SPALLATION
J. Stone, G.L. Allan, L.K. Fifield and R.G. Cresswell
Geochimica et Cosmochimica Acta 60, 679-692 (1996)
Abstract
Calcium is a major target element for cosmogenic Cl-36
production. Consequently Cl-36 rapidly reaches detectable levels in minerals
such as calcite and Ca-feldspar exposed at the earth's surface. Spallation
of calcium isotopes typically accounts for 80 - 90% of Cl-36 production
in these minerals, with subsidiary contributions from negative muon capture
by Ca-40 and thermal neutron capture by Cl-35. To provide a basis for surface
exposure dating, we have calibrated cosmogenic Cl-36 production in Ca-feldspar
from the 17,300 year old Tabernacle Hill basalt. At an altitude of 1445
m and an effective geomagnetic latitude of 40.9 degrees the Ca spallation
rate is 152 ± 11 atoms / (g Ca) / yr. The corresponding
rate at sea-level and high latitude is estimated at 48.8 ± 3.4 atoms
/ (g Ca) / yr. The muon capture rate used to derive these values is
8.8 ± 2.2 atoms / (g Ca) / yr at the Tabernacle Hill site,
scaled from a value of atoms / (g Ca) / yr at sea-level and high latitude.
The Ca spallation rate determined in this study is in excellent agreement
with previous whole-rock calibration measurements at Tabernacle Hill, when
these are recalculated with respect to the absolute timescale.
The calibration of Cl-36 production from Ca underpins development of
an exposure dating technique for calcite. Due to its high Ca content, the
Cl-36 production rate in calcite is higher than in any other common rock-forming
mineral. Measurement of Cl-36 in calcite, with an accelerator mass spectrometric
detection limit of ca. 5 x 10^3 atoms per gram, allows dating of limestone
surfaces exposed for periods ranging from 10^2 - 10^6 years. Alternatively,
erosion rates from less than 1 to greater than 1000 micrometres per year
can be determined in the case of eroding karst surfaces. Though the Cl-36
production rate is lower in Ca-feldspar than in calcite, measurements on
this mineral will provide a useful means of dating young basalt lavas.
