Thermochronic recently blogged about a high profile paper that recently suggested that the partitioning coefficients for argon between mantle minerals and melts is greater than one (Watson et al. 2007). As it turns out, Heber et al. 2007, published earlier this year, suggests that the D values are actually between 10-3 and 10-4 for all noble gasses. The Heber et al. 2007 paper describes a very well-thought out and executed research program, reads like a detective novel, and is generally everything that a scientific paper should be.
They perform a standard partitioning coefficient experiment- slowly cool a melt to grow crystals, quench it, and measure the concentration of the trace element of choice in both melt and crystal to determine the ratio. The brilliant thing about this study is the depth of self-critizism and examination which they perform to identify and eliminate potential contaminants or non-equilibrium effects. This stands in stark contrast to the Watson et al. paper, where they pretty much assume that their results are right, and arrive at a value 10,000 to 10 million times higher than the Heber et al. paper.
I don’t want to ruin a gripping read, but the things that Heber et al. overcome include (but are not limited to): Room temperature diffusion of He out of the glass between experiment completion and analysis time, bubble formation, melt inclusions, very low concentrations. The interpretations of the paper are well supported by the data, but do include an explanation of how these numbers can explain “primordial” OIB noble gas ratios.
Although the noble gas partition coefficients are low (on the order of 10-4), they are still 2 orders of magnitude higher than the partitioning coefficients of K, Th, and U, which are on the order of 10-6 in a hartzburgitic system. Thus, although noble gasses are incompatable, they are not as incompatable as the radioactive sources for 4He and 40Ar. So old depleted mantle will have a high primordial/radiogenic gas signature.
The fact that the Heber et al. and Watson et al. partition coefficients for argon differ by up to 8 orders of magnitude is striking. But I find the Heber et al. result more compelling, because the authors present, and then test, a dizzying array of potential complications in a rigorous manner. In contrast, Watson et al. make statements like,
“More generally, we believe that the inconsistency of our results and those of Broadhurst et al. with other experimental studies is due to the different experimental protocols used.”
While I have great respect for Professor Watson and his impressive research record, I am not interested in his beliefs, unless they are backed up by experimental evidence. And he makes no attempt to reconcile his results with previous work, much less try to disprove his own experiments. So a paper that dissects the potential causes of research discrepancies and addresses them is more convincing to me than one that relies on untested dismissals. As a final note, Watson et al. do not reference Heber et al. Heber et al. 2007 was published on 15 February, and available online since December 2006, while Watson et al. was not submitted until the beginning of May 2007.
Veronika S. Heber, Richard A. Brooker, Simon P. Kelley, Bernard J. Wood. 2007 Crystal-melt partitioning of noble gases (helium, neon, argon, krypton, and xenon) for olivine and clinopyroxene. Geochimica et Cosmochimica Acta 71 1041-1061.
E. Bruce Watson, Jay B. Thomas, Daniele J. Cherniak 2007 40Ar retention in the terrestrial planets. Nature 449 299-304.
update: sociological speculations on these papers is here.