Carbonate and silicate intercomparison materials for cosmogenic 36Cl measurements

Two natural mineral separates, labeled CoCal-N and CoFsp-N, have been prepared to serve as intercomparison material (ICM) for in situ-produced cosmogenic Cl-36 and natural chlorine (Cl-nat) analysis. The sample CoCal-N is derived from calcite crystals in a Namibian lag deposit, while the sample CoFsp-N is derived from a single crystal of alkali-feldspar from a Namibian pegmatite. The sample preparation took place at the University of Cologne and a rotating splitter was used to obtain homogeneous splits of both ICMs. Forty-five measurements of CoCal-N (between 1 and 16 per facility) and forty-four measurements of CoFsp-N (between 2 and 20 per facility) have been undertaken by ten target preparation laboratories measured by seven different AMS facilities. The internal laboratory scatter of the Cl-36 concentrations indicates no overdispersion for half of the laboratories and 3.9 to 7.3% (1 sigma) overdispersion for the others. We show that the CoCal-N and CoFsp-N splits are homogeneous regarding their Cl-36 and Cl-nat concentrations. The grand average (average calculated from the average of each laboratory) yields initial consensus Cl-36 concentrations of (3.74 +/- 0.10) x 10(6) at Cl-36/g (CoCal-N) and (2.93 +/- 0.07) x 10(6) at Cl-36/g (CoFsp-N) at 95% confidence intervals. The coefficient of variation is 5.1% and 4.2% for CoCal-N and CoFsp-N, respectively. The Cl-nat concentration corresponds to the lower and intermediate range of typical rock samples with (0.73 +/- 0.18) mu/g in CoCal-N and (73.9 +/- 6.8) mu/g in CoFsp-N. We discuss the most relevant points of the sample preparation and measurement and the chlorine concentration calculation to further approach inter-laboratory comparability. We propose to use continuous measurements of the ICMs to provide a valuable quality control for future determination of Cl-36 and Cl-nat concentrations.