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Ra-partitioning between phlogopite and silicate melt and ^(226)Ra/Ba–^(230)Th/Ba isochrons

Fabbrizio, Alessandro and Schmidt, Max W. and Günther, Detlef and Eikenberg, Jost (2010) Ra-partitioning between phlogopite and silicate melt and ^(226)Ra/Ba–^(230)Th/Ba isochrons. Lithos, 114 (1-2). pp. 121-131. ISSN 0024-4937.

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In this study we experimentally determine phlogopite/melt partition coefficients of Ra and other trace elements in a lamproitic system. This work was achieved using an analytical technique (LA-ICP-MS) with low detection limits (~0.01 fg) permitting the measurement of the very low Ra concentrations feasible in experiments (~1 ppb). D_(Ra)^(phlogopite/melt) was determined to 2.28 ± 0.44 and 2.84 ± 0.47 in two experiments, the ratio D_(Ra)/D_(Ba) is around 1.6. The compatibility of Ra in phlogopite results from an ionic radius being close to the apex of the lattice strain parabola for earth alkalis in the large XII-coordinated interlayer site of phlogopite. A re-evaluation of D_(Ra) and D_(Ra)/D_(Ba) for magmatic minerals containing appreciable Ra, yields D_(Ra)^(mineral/melt) ranging from ~2.6 for phlogopite down to 2–3 • 10^(−5) for pyroxenes, and D_(Ra)/D_(Ba)^(mineral/melt) from ~4 for leucite to 2 • 10^(−2) for orthopyroxene. The influence of melt composition on D_(Ra)/D_(Ba) is less than 10%. All investigated minerals have different D_(Ra)/D_(Ba), strongly fractionating Ra from Ba. Thus, for magmatic systems, (^(226)Ra)/Ba in the various minerals is not constant, these minerals do not form a straight line in the (^(226)Ra)/Ba–(^(230)Th)/Ba system at the time of crystallization and thus, there is no (^(226)Ra)/Ba–(^(230)Th)/Ba isochron at t_0. 226Ra–230Th–Ba mineral dating is thus applicable only to model ages calculated from mineral–glass pairs with known D_(Ra).

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Additional Information:© 2009 Elsevier B.V. Received 1 January 2009; revised 3 August 2009; accepted 6 August 2009. Available online 21 August 2009. The authors thank Kathrin Hametner and Max Ruethi for their technical help. Georg Zellmer and an anonymous reviewer provided helpful comments.
Subject Keywords:Radium; Phlogopite; LA-ICP-MS; Partition coefficients; Lamproite; Isochron diagram
Issue or Number:1-2
Record Number:CaltechAUTHORS:20100304-095021967
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Official Citation:Alessandro Fabbrizio, Max W. Schmidt, Detlef Gunther, Jost Eikenberg, Ra-partitioning between phlogopite and silicate melt and 226Ra/Ba-230Th/Ba isochrons, Lithos, Volume 114, Issues 1-2, January 2010, Pages 121-131, ISSN 0024-4937, DOI: 10.1016/j.lithos.2009.08.004. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17658
Deposited By: Tony Diaz
Deposited On:12 Mar 2010 19:01
Last Modified:03 Oct 2019 01:31

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