Hello Matt et al.
Having been singled out as partly responsible for popularizing the term petrochronology, let me add some thoughts to the growing number of postings, pro and con. To start with, I admit that I was unaware of the Geology paper by Fraser et al. (1997) when I wrote the 2009 abstract (MAPT Edinburgh) you cite. However, the term has become everyday lingo in my group at University of Bern, before and since then, and several of my PhD students and postdocs have contributed their twist to what the term means though till Matt's note we have been unaware of any perils related to undergarments. In short, the term petrochronology has undergone an evolution.
Matt surmises that the term is redundant because (he says) it is not substantially different from geochronology. However, geochronology encompasses a wide range of approaches and techniques, thus more specific terms are useful when a subset of these is implied. And indeed several such terms are well established, e.g. thermochronology, surface dating, etc. Where Matt is on the mark, is that various directions and concepts are now being termed petrochronology, with no consensus of the precise meaning. Of course, nobody is in a position to mandate how such terms are to be used. What I can offer, for what it's worth, is some reflections on how we use and understand it.
Linguists teach us that meaning depends on context, so let's start there. These days, many things are in a state of flux in Greece (from where I am writing), but the root of the term "petrochronology" appears undisputed: Etymologically, it refers to how the age (or time) of a rock is determined.
Geologically, it's trickier: It is pretty clear what is meant by the age of a basalt, but what is the age of a metabasalt? It is ill defined what stage of the evolution an "eclogite age" refers to. Now structural geologists have long been identifying sequences of deformation phases, and as metamorphic petrologists working in polydeformed terrains, we continually struggle to associate mineral reactions and growth zones with deformation phases. It is in this context that we coined the term "petrochronology", referring to a rather specific APPROACH. Let me try to summarize what we mean by giving a rough recipe or work plan for an individual sample (thin section):
1. Identify one (or more) specific stage(s) of the metamorphic (and structural) evolution in the sample using textural criteria (overgrowth, inclusions, fabric alignment);
2. Document the phase relations (major + minor elements by EMP, trace elements by LA-ICP-MS, or by a combined method, e.g. Kylander et al. 2013);
3. Attempt to relate one (or more) specific growth zone of a suitably robust chronometer (e.g. Pb/U or Pb/Th) to each stage;
4. Use thermobarometry to constrain PT-conditions of local equilibria
5. Where 1
4 was successful, we have a well defined context for a metamorphic age, hence we use microdating techniques (SHRIMP, LA-ICP-MS, SIMMS) to analyze the isotopic ratios in each suitably large growth zone (in situ where possible, else in carefully separated and individually documented mineral grains).
[More detail below, as P.S.]
The CRUX of petrochronology, as we currently use the term, is thus that petrological (and structural) scrutiny is used on compositionally promising rock samples to ASK SPECIFIC QUESTIONS, which then are tackled using geochronological techniques.
If this seems like laborious, hard work, you are right. It is one of the reasons "petrochronology" to us is not "just" geochronology. The advert for Session T157 at the Denver GSA meeting (Frontiers in Petrochronology) appears to refer to something at least close in meaning, although the emphasis in the description is more on progress in geochronological techniques and rather little on the petrological meaning of mineral age data.
Language in science evolves as our understanding of phenomena and processes does, and terminology questions are perhaps only a marginal consequence of progress made.
Cheers, allow me to go for a swim now, so nobody can twist my you know whats
Martin
P.S. Remarks on the above recipe:
Stage 3 is often tricky, need to verify coexistence using TE data (e.g. for Kd-checks).
Our main work horses in mineral chronometry have been monazite, allanite, and zircon where possible in combination. Presently titanite and rutile are in our focus as well.
Initially we paid too little attention to the structural setting of each sample, but over the past years we have been working exclusively on oriented samples (and sections), in order to relate fabric characteristics of local assemblages (΅m to mm) to observed deformation fabrics in hand specimens (mm to dm) and meso- to megascopic observations (dm to tens of km). This allows for a much more complete integration with tectonics.
On 19.06.2013, at 20:10, Matthew Kohn wrote:
> Hey, not to twist anyone's undergarments in a bunch, but what's with this term "Petrochronology"?
>
> The earliest reference I can find to it is from 1969 in anthropology, in regards to a Yoruba (Nigeria) river god cult as a way of dating duration and intensity of devotion. Basically the number of rocks in a pot indicates how long (and devoted) you've been.
>
> In geology die Fraser et al. (1997) first propose it? Martin Engi certainly popularized it in a 2009 abstract.
>
> What I don't get is that "Petrochronology" literally means dating of stones. How is that substantively different from "Geochronology," which means the dating of Earth (materials)?
>
> I think John's session description below and Martin's 2009 abstract ultimately propose that petrochronology is different from geochronology by virtue of petrologic and geochemical context - textures, trace element chemistry, etc. But who would _ever_ employ, say, metamorphic geochronology _without_ that context? That is, metamorphic (and igneous) geochronology IS (or should be) petrochronology anyway.
>
> I know the term is catching on, but it really does seem redundant to me.
>
> Matt
>
>
> On Jun 19, 2013, at 8:10 AM, John Cottle wrote:
>
>> We invite contributions to session T157 and participation in our short-course at the upcoming 125th Anniversary GSA Annual meeting, which takes place in Denver, October 27-30, 2013
>>
>> Session T157: Frontiers in Petrochronology
>>
>> Conveners: John Cottle, Andrew Kylander-Clark
>>
>> Petrochronology is the interpretation of isotopic dates in the light of complementary geochemical/textural information gleaned from the same mineral(s). Continued advancements in geochronologic measurement techniques have increased the precision and accuracy with which individual dates can be measured. Geologic "events," once considered as having occurred at a single point in time, are now commonly recognized as a series of processes that occurred over a period of time, whether it be a few kyr or several Myr. Thus, as geochronologic precision and accuracy increase, so must the accompanying textural and chemical information that allows one to match a measured date to a specific geologic process, whether it be melt crystallization, or responses to changes in pressure, temperature, and/or fluid/melt infiltration, with or without accompanying deformation. This session aims to both highlight the array of emerging analytical techniques, as well as the application of these methods to provide insight into a range of tectonic and geochemical processes.
>>
>> Invited Speakers:
>>
>> - Julia Baldwin (University of Montana)
>> - Nigel Kelly (Colorado School of Mines)
>> - Mark Schmitz (Boise State)
>>
>> Abstract deadline is 6th August 2013 at 23:59 (Pacific Time)
>>
>> We look forward to seeing you in Denver!
>>
>> John and Andrew
>>
>> --
>>
>>
>> *************************************
>> John Cottle
>> Associate Professor
>> Department of Earth Science
>> & Earth Research Institute
>> University of California, Santa Barbara CA 93106-9630
>> Tel: +1 805-893-7315
>> Email: [log in to unmask]
>> Web: http://www.geol.ucsb.edu/faculty/cottle
>
> ****************************************************
> Dr. Matthew J. Kohn, Professor
> Department of Geosciences
> Boise State University
> 1910 University Dr.; MS1535
> Boise, ID 83725-1535
> [log in to unmask]
> http://earth.boisestate.edu/mattkohn/
> phone: (208)-426-2757 fax: (208)-426-4061
> ****************************************************
____________________________________________________
Prof. Martin ENGI tel +41 (0)31 - 631 84 97
Institute of Geological Sciences fax +41 (0)31 - 631 48 43
University of Bern
Baltzerstrasse 1, 3
CH-3012 Bern, Switzerland http://www.geo.unibe.ch/
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