Back in August I wrote about an extremely important paper by John Wheeler of Liverpool University called “Dramatic effects of stress on metamorphic reactions”. This uses a theoretical approach to show that differential stress (squashing rocks) is a very important control on metamorphic reactions. If true, this would imply that many estimates of depth of metamorphic conditions (that ignore the squashing) are wrong. Maybe eclogites don’t form at great depth after all.
Counterblast
Conceived as a provocative paper, it’s no surprise to find a “Comment” on it in the latest edition of Geology. Written by Raymond Fletcher of Penn State, it aims to “show
that Wheeler’s claims do not have a sound basis” by constructing a more complete mathematical model “for metamorphic reaction and pressure solution” (the two processes that Wheeler’s original paper wound together).
For both our sakes, I’m not going to get into the detail of the mathematics (all papers are open-source, so you can read it yourselves). It isn’t massively complicated maths – single lines of algebra only – but what matters here is the assumptions and simplifications made and whether they are valid.
Fletcher’s comment picks on one aspect of the original paper – that in the section quantified the effects of differential stress, it focussed on a single way in which atoms can rearrange themselves called incongruent pressure solution. Fletcher’s set of equations are a more complete model that shows that Wheeler’s results are merely a ‘special case’ leading to ‘contrived outcomes’.
A spirited defence
‘Comments’ on papers are often followed by a ‘reply’ from the original author. As here where the ‘comment’ is negative, they are effectively a form of public combat.
Wheeler is uniformly polite and positive. He starts by thanking Fletcher for his stimulating Comment and listing the ways in which they agree. Then this:
“But it is inappropriate to say that I am wrong, first because his model is not of the incongruent pressure solution (IPS) pathway, second because it actually contains
confirmation of some of my claims, and thirdly because it is extremely
restricted in scope.”
He then proceeds to show that Fletcher’s model doesn’t just model a single pathway and the he lists the assumptions made by Fletcher and demolishes each one. For this audience member, Wheeler starts to win the battle by the depth of context he brings to the discussion.
For each assumption he refers to existing research into real world complications. These include: defining 3-D stress as a single term in an equation is complicated – taking the simple average of the 3 dimensions is not correct; fluid pressure may control reactions, not stress; the topology of grains is important and the one chosen by Fletcher extremely unrealistic; diffusion of atoms is often a limiting factor in metamorphism; porphyroblasts often grown in specific shapes – ‘interfacial’ kinetics may also be important.
For this (slightly biased) reader the knock-out blow was the fact that 3 times the research into these complications is his own allowing Wheeler to write that Fletcher “may well have rediscovered the sorts of problems described above (Ford and Wheeler, 2004) but by ignoring these he reduces the value of his assertion that…” .
All good replies to comments look to the future:
“In summary, Fletcher’s model is too restricted in scope to undermine my conclusions: we agree that a more general model is required. I challenge him and other interested readers (including myself) to construct such a model, which would be of great benefit to understanding how metamorphism and deformation interact.”
Science in action and in the open
I remind you again of the important implications of Wheeler’s paper – existing estimates of metamorphic conditions – used to build tectonic models – are suspect. To quote another article in Geology discussing it “the potential inaccuracy of depth estimates based on minerals would question current paradigms in geology“.
Wheeler’s original paper was a huge challenge to metamorphic petrology. It has withstood the first attempt to refute it. This is science in action, in open source papers for all to view. I hope you’ll read the papers yourself and we can follow the unfolding story together.
Hi Metageologist!
I’d really like to know who you are. Send me a note. It’s difficult to get something across in one page. Indeed, I did not expect many readers, like you, or, for that matter, Wheeler, to get the point. The point is, that one sets up a model according to well-known rules, such that a closed system of equations are obtained. These equations do not include the answer desired. Then you have to accept what you get. That’s what I did, but Wheeler did not. His set of relations is not closed, except by prescribing the answer desired. It is not legal to specify a “reaction pathway.” That is, in essence, specifying the answer. One specifies, instead, the rules that apply on all interfaces in the domain, as on transport & reaction. Then, the desired “reaction pathway” may emerge as one of a wide range of possible results, and, associated with this emergence, the conditions for it to do so will be obtained.
dear metageologist,
dear ray,
one of my team members showed me this discussion. first, i would like to thank ray for his comment and very clear explanation here. i fully agree.
maybe not everything which apparently looks dramatic is at the end that dramatic in nature. nature usually does not follow our suggestions on which pathways to take. more to this topic, you (metageologist) may check our review paper where we reviewed the non-hydrostatic thermodynamics approaches. we show that they have never been proven by any experiment. as an example we took wheeler’s method to show how weak predictive power it has when applied on qtz/coe experiments. the reference is: Tajcmanová L.,Vrijmoed J, Moulas E. (2015). Grain-scale Pressure Variations in Metamorphic Rocks: Implications for the Interpretation of Petrographic Observations. Lithos. 216-217, 338-351.
all the best
lucie