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An alternative isochron method for measuring cosmogenic ^3He in lava flows

Blard, P.-H. and Pik, R. (2008) An alternative isochron method for measuring cosmogenic ^3He in lava flows. Chemical Geology, 251 (1-4). pp. 20-32. ISSN 0009-2541. doi:10.1016/j.chemgeo.2008.02.004.

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This study evaluates an alternative isochron method designed to improve the precision and the accuracy of the measurement of cosmogenic 3He (3Hec) and the calibration of 3Hec production rates in lava flows. While the accuracy of the standard two-steps routine ((i) crushing, (ii) melting) may be affected by possible crushing-induced release of matrix-sited 3Hec, this new isochron technique does not involve the preliminary crushing step. Such advance is thus potentially able to improve the accuracy of 3Hec determination. Moreover this method has the advantage to yield cosmogenic 3He concentrations using only one type of extraction technique. Cosmogenic 3He isochrons can be built successfully measuring the 3He and 4He concentrations of several phenocrysts replicates fused in vacuo, provided that these aliquots have identical magmatic 3He/4He ratios, identical 3Hec concentrations and variable magmatic helium contents. When plotted in the 3He/4He vs 1/4He space, such a suite of data define a straight line whose intercept yield the magmatic (3He/4He)mag ratio, and whose slope is the cosmogenic 3He concentration. The frequent addition of radiogenic 4He (4Helow asterisk) may be easily accounted taking advantage of a property that is characteristic of non-eroded lava flows: the 3Hec/4Helow asterisk ratio is not time-dependent. This radiogenic correction of the isochron slope involves the R-factor, whose calculation requires that the U and Th abundances in the phenocrysts and the host lava be known, as well as the diameter and the shape of the analyzed minerals. This correction also requires that all the analyzed aliquots have identical radiogenic 4He concentration. The isochron method was tested on surface samples of three Quaternary basalt flows from Mount Etna and Mauna Loa (aged of 6 ±1, 8.2 ± 0.1 and 41 ± 3 ka). The obtained data set yielded statistically robust isochrons and cosmogenic 3He concentrations that are not significantly different from those obtained with the standard method. Importantly, for samples with high magmatic/cosmogenic 3He ratio, the isochron technique offers both better accuracy and precision than the standard two-steps method. This alternative technique should thus open new perspectives for the particularly important task of 3Hec production rate calibrations from lava flow surfaces.

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Additional Information: Copyright © 2008 Elsevier. Received 30 September 2007; revised 7 January 2008; accepted 5 February 2008. Editor: B. Bourdon. Available online 18 February 2008. This manuscript greatly benefited from comments by Pete Burnard and a very constructive review by Pete Reiners. Three anonymous referees and editor Bernard Bourdon are also thanked for their careful evaluation of the manuscript. Authors also sincerely thank Bernard Marty, Jérôme Lavé and Ken Farley for fruitful discussions. Laurent Zimmermann and Bouchaïb Tibari are acknowledged for their valuable help in the CRPG noble gases laboratory. This is CRPG contribution #1907. We are very grateful to Samuel Niedermann who pointed out the error in Eq. (4).
Errata:P.-H. Blard and R. Pik. Erratum to “An alternative isochron method for measuring cosmogenic 3He in lava flows” [Chemical Geology 251 (2008) 20–32]. Chemical Geology 257 (2008) 264
Subject Keywords:Cosmogenic; Helium-3; Helium-4; Isochron; R-factor
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Centre de Recherches Pétrographiques et Géochimiques1907
Issue or Number:1-4
Record Number:CaltechAUTHORS:BLAcg08
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13066
Deposited By: Archive Administrator
Deposited On:16 Jan 2009 20:22
Last Modified:08 Nov 2021 22:35

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