HR: 1340h
AN: PP33B-1287    [Abstracts]
TI: In-Situ Cosmogenic ³⁶Cl Production Rate Calibration from Basaltic Flows of Mount Etna (Sicily, 38° N)
AU: * Schimmelpfennig, I
EM: schimmel@cerege.fr
AF: CEREGE - CNRS, Europole mediterraneen de l'Arbois, Aix en Provence, 13545, France
AU: Benedetti, L
EM: benedetti@cerege.fr
AF: CEREGE - CNRS, Europole mediterraneen de l'Arbois, Aix en Provence, 13545, France
AU: Pik, R
EM: rpik@crpg.cnrs-nancy.fr
AF: CRPG - CNRS, 15, rue Notre Dame les pauvres, Vandoeuvre-Lès-Nancy, 54501, France
AU: Burnard, P
EM: peteb@crpg.cnrs-nancy.fr
AF: CRPG - CNRS, 15, rue Notre Dame les pauvres, Vandoeuvre-Lès-Nancy, 54501, France
AU: Blard, P H
EM: blard@gps.caltech.edu
AF: CALTECH, MS 100-23, 1200 E. California Blvd, Pasadena, 91125, United States
AU: Bourles, D
EM: bourles@cerege.fr
AF: CEREGE - CNRS, Europole mediterraneen de l'Arbois, Aix en Provence, 13545, France
AB: One of the CRONUS-EU goals is to provide high quality calibration sites from independently dated surfaces. Several previous studies have been conducted on ³⁶Cl production rate calibration (e.g. Stone et al. 1996, Phillips et al. 2001), which, however, used different protocols and yielded ³⁶Cl production rates with up to 40% discrepancies. The objectives of this study are 1- to understand the source of these discrepancies and 2- to calibrate ³⁶Cl production rates from its target elements Ca and K. As a first step we focused on testing the chemical protocol by performing a sequential ³⁶Cl extraction experiment on whole rock grains and Ca-rich plagioclase from the same sample. The sample was collected at Mt. Etna on a pahoehoe flow, which has a K-Ar fossil exposure time of (10±3) kyr. Cosmogenic ³He was also precisely measured within cogenetic olivine phenocrysts of this sample (Blard et al. 2005) and yields an exposure time of (10.4±1.5) kyr. Both, total Cl and ³⁶Cl concentrations from the first dissolution steps are high, 5800 ppm (whole rock) and 450 ppm (plagioclase) Cl, and 10⁷ - 10⁶ atoms ³⁶Cl/g of rock dissolved. After about 20% dissolution of the plagioclase sample, Cl is almost completely removed (1-3ppm) and ³⁶Cl concentrations reach a plateau value of 2*10⁵ atoms/g of rock. Using the Stone et al. (1996) and Evans et al. (1997) ³⁶Cl production rates for the target elements Ca and K, respectively, this plateau concentration yields an exposure age which is in excellent agreement with K-Ar dating and cosmogenic ³He ages. On the contrary, in the whole rock sample total Cl concentrations remain high (>330ppm) resulting in a considerable ³⁶Cl production from capture of low-energy neutrons by ³⁵Cl, an additional and still not well-constrained ³⁶Cl production mechanism. The resulting exposure ages from the whole rock are 35-45% higher than the independent ³He ages. For ³⁶Cl production rate calibration from Ca, we will use separated Ca-rich plagioclase from various Mt. Etna lava flows of different elevation and independently determined ages between 400 yr and 41 kyr. To better constrain the ³⁶Cl production rate from K, separated sanidine (K-rich feldspar) from a 15 kyr old lava flow of volcano Payun-Matru (Argentina, 36° S) will be used. Stone J.O., et al. (1996), Geochim. Cosmochim. Acta 60 679-692; Phillips F.M., et al. (2001), Chem. Geol. 175 689-701; Blard P.H., et al. (2005), EPSL 236 613-631; Evans J.M. et al. (1997), Nucl. Instr. and Meth. in Phys. Res. B 123 334-340
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 4918 Cosmogenic isotopes (1150)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting