Talk:Peridotite

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Peridot

What if any, is the relationship between peridot and peridotite? Are the names a coincidence?

I do not know for sure but would think that it has something to do with the high amount of olivine in both aforementioned substances.

Peridot, the gemstone is in fact, magnesium rich forsterite olivine. Fayalite, the iron olivine, is usually less green and transparent.Rolinator 12:14, 28 December 2005 (UTC)

Yes. Peridot is a gem variety of olivine, while peridotite is a rock composed chiefly of olivine. Peridot, being an olivine, is a mineral. Peridotite, being a rock, is a composition of minerals but composed chiefly of peridot or another olivine mineral. --Valich 03:39, 31 October 2006 (UTC)

Geology Project

I have added the Geology template as this article has a lot of geology information that hard to ignore. Solarapex 03:32, 11 May 2007 (UTC)

Potential for Carbon Storage

It has been recently discovered that Peridotite can turn CO2 in the air into harmless compounds like calcite at an astonishing rate. since this rock could potentially become a major topic of interest to environmental efforts, it might be a good idea to mention that. source: http://cleantechnica.com/2008/11/10/scientists-discover-rock-that-can-absorb-carbon-dioxide-emissions-directly-from-the-air/ —Preceding unsigned comment added by 69.132.41.57 (talk) 01:50, 11 November 2008 (UTC)

Igneous or metamorphic rock

On 25 May 2016, User:Obradow changed the first sentence of this article from "Peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene" to "Peridotite is a dense, coarse-grained igneous rock or metamorphic rock, consisting mostly of the minerals olivine and pyroxene". This assertion about peridotite being a metamorphic rock is unsourced and, I believe, not the current general geological consensus about peridotite. Therefore, I have reverted User:Obradow's edit and I invite discussion on this talk page.

I think the consensus within geology is that peridotite is an igneous rock formed by igneous processes, although it may subsequently be altered/tectonised/metamorphosed to become metaperidotite (or meta-peridotite), a metamorphic rock. I think when some geologists refer to metaperidotites as peridotites they are being inaccurate. (They also do this with other metamorphosed igneous rocks. For example, I think all, or almost all, komatiites are actually metakomatiites but they are often described as komatiites. Similarly, metamorphosed basalts are really metabasalts but they are often described as basalts). I suggest that this shorthand habit of some geologists of inaccurately using igneous rocks names for both the unaltered igneous protolith and the metamophosed rock can confuse anybody who is not an igneous petrologist and this should be avoided on Wikipedia. I welcome comments about this. GeoWriter (talk) 20:27, 27 May 2016 (UTC)

Most peridotites are also tectonites and have been subject to dynamic metamorphism, but they were at some point rocks that crystallised from a liquid and are, therefore in my view, igneous. This article could usefully have something about the deformation state of most peridotites and the implications of the observed fabrics (mantle flow, seismic anisotropy). Mikenorton (talk) 21:45, 27 May 2016 (UTC)

Hi GeoWriter, thanks for initiating discussion.→In Anthony Hall book, Igneous Petrology (1987) there is following paragraph: " Ultramafic rocks are of several types. Some have crystallized directly from an ultramafic magma (for example komatiites), but most have not crystallized from magma of their own composition. Large plutonic bodies of ultramafic occur in following asociations:1. as layers in layered basic complexes, e.g Stillwater, Bushveld;2 as ultramafic stocks of "appinitic" or "Alaskan" type, e.g Appin , Duke Island; 3 as components of basic alkaline complexes,e.g. Fen, Palbora;4 as ultramafic intrusions showing definitive as evidence of intrusion as ultramafic magma, e.g. Norde Bumanndsfjord; and 5 the majority of peridotites and serpenetinites are distributed throughout orogenic belts, and do not show such clear evidence of their magmatic parentage."→ the texture of upper mantle peridoties, we can sample of earth surface, is metamorphic→ mantle melting is metamorphic proccess. This is just short answer in lack of time. I will write more as soon as possible.Mantle peridoties are special rocks.I will look up IUGS classification. My intention is not to confuse but to clarify. Cheers! Obradow (talk) 23:08, 27 May 2016 (UTC)

How to identify rock when somebody gives you piece of stone? Just with naked eye. First thing you look is the texture, they are so clearly sedimentary, metamorphic or igneous. There are many instances where when you are not expert it could be difficult to say, for example it is not so clear sometimes if massive amphibolite, is igneous or methamorphic. Plutonic rocks and some high grade methamorphic rocks have granular texture, and it could be difficult to tell apart. Some high grade methamorphic rocks are partially melted, in the field we can find both of them intermingled at same outcrop.Peridotites are tricky as well. They are rocks made by diverse processes, some are typical igeneous rocks, volcanic rocks but also plutonic cumulates. The earth’s upper mantle is made of peridotites, mainly formed from by crystallization from magma ocean. This ocean existed when the whole earth was molten, at the same time or after the core separation. I need to find references, and because I am interested I will do so. Ok that is igneous process. But after that in long earth history, these rocks are changed, it is solid stuff moving around under high T and P, under the MOHO, it recrystallises, it is not always only tectonite…. Only sometimes these rocks come on earth surface.I was always more interested in processes of rock formation rather in simple classification.But lets consult classification. In the book which has subtitle classification, textures, microstructures and mineral preferred orientation on the page 37 is written: mantle rocks are clearly methamorphic in character (e.g. Nicolas and Violette, 1982). The mantle peridoties are indeed metamorphic rocks.Obradow (talk) 10:01, 28 May 2016 (UTC)

Sorry Mikenorton I just realized that you suggested that I wait with changes until discussion is ended, simultaneously I was making additional changes. Ok I will waitObradow (talk) 10:52, 28 May 2016 (UTC)

My understanding of the quote from Hall (1987), "the majority of peridotites and serpentinites are distributed throughout orogenic belts, and do not show such clear evidence of their magmatic parentage" is that he is referring to metaperidotites in the loose/lazy/imprecise shorthand way of some authors that I described earlier, not the original igneous peridotites. The process of transporting the igneous peridotites from deep within the mantle to outcrops at the surface of the Earth has transformed the igneous rock into metamorphic metaperidotite and the metamorphic processes during orogenic emplacement or ophiolite obduction etc. have obscured an originally igneous texture. Peridotite mantle xenoliths erupted in lava at volcanoes, e.g. in Hawaii and Lanzarote, clearly show igneous textures and mineralogy, I suppose because these peridotites where uplifted too quickly for metamorphic mineral composition change processes to act on the xenoliths, which would also have been protected from dynamic tectonisation texture change processes by being enclosed within protective molten magma. I think it is very significant that Hall wrote "do not show such clear evidence of their magmatic parentage" (my bold emphasis of "their"). To me, this strongly suggests that Hall acknowledges that peridotites are igneous rocks but they may then be altered to metaperidotite, consistent with the geological consensus. If Hall had instead written "do not show such clear evidence of magmatic parentage" (that is, if he omitted the word "their"), he would then have been describing a different scenario in which peridotites may be truly metamorphic, even in the mantle before uplift to the surface. GeoWriter (talk) 11:11, 28 May 2016 (UTC)

If partial melting were to be regarded as a metamorphic process rather than an igneous process, that would perhaps call into question the existence of all igneous rocks and we would have bigger issues in geology and Wikipedia than just the nature of peridotites. GeoWriter (talk) 11:16, 28 May 2016 (UTC)

Before entering in discussion further, here is direct undoubted quote : mantle rocks are clearly methamorphic in character (e.g. Nicolas and Violette, 1982) Obradow (talk) 11:35, 28 May 2016 (UTC)

Some definitions of peridotite:

Glossary of Geology, edited by Neuendorf et al., published by American Geological Institute (2005), page 482:

"a) in the IUGS classification "a plutonic rock with M equal to or greater than 90 and ol/(ol+opx+cpx) greater than 40.

b) a general term for a coarse-grained plutonic rock composed chiefly of olivine, with or without other mafic minerals such as pyroxenes, amphiboles, or micas, and containing little or no feldspar."

Igneous Rocks: A Classification and Glossary of Terms, 2nd edition, edited by Le Maitre (2002), page 126:

"a collective term for ultramafic rocks consisting essentially of olivine with pyroxene and/or amphibole. Now defined modally in the ultramafic rock classification".

British Geological Survey Rock Classification Scheme http://www.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=PDT :

"Coarse-grained, av. between 2-16mm normal crystalline intrusive rock; CI M>90%; olivine >40%, pyroxene <60%, hornblende <60% of Ol+Px+Hb"

GeoWriter (talk) 12:05, 28 May 2016 (UTC)

We are making here glossary of terms, based on current knowledge. It might me not generally known but the life work of professor Nicolas is there. Time will come when glossary of terms and British geological survey get this. Hall indeed wrote this sentence unclear because it is not so clear. if you had a book and read the chapter maybe it would be more clear to you. But to clarify when we speak about metamorphic processes in upper mantle we are not talking about processes of metamorphism during emplacement although they happen as well. The nature is tricky and sometimes difficult to put in words, so indeed all liquids which form during partial melting will later form igneous rocks, in process of partial melting, which is metamorphic and igneous process. It is both. So it can lead to ironical conclusion that all igneous rocks are metamorphic. That is not the point. The point is that we have to distinguish between crustal and mantle processes. For me geology is not about names but about processes. Obradow (talk) 12:46, 28 May 2016 (UTC)

User:Obradow, in response to your comment, "We are making here glossary of terms, based on current knowledge. It might be not generally known but the life work of professor Nicolas is there. Time will come when glossary of terms and British geological survey get this.", may I remind you of a few things about Wikipedia. Wikipedia follows current consensus. Wikipedia does not aim to lead to a new consensus. The current consensus in geology is that peridotite is an ultramafic plutonic igneous rock (and metaperidotite is its metamorphosed equivalent). Earlier, you gave me the impression that the IUGS's definition of peridotite was worth finding, but now that I have quoted the IUGS definition here, it seems to have lost its importance to you. In future, through debate in academia not on Wikipedia, Nicolas, Violette, you and others may eventually prove to the geological community (e.g. IUGS, AGI and BGS etc.) that peridotite is a metamorphic rock but, currently, the metamorphic origin of peridotite (as opposed to metaperidotite derived from altered igneous peridotite) is a minority view that strongly conflicts with the current consensus. GeoWriter (talk) 16:46, 28 May 2016 (UTC)

No sorry truth is maybe different. I thank you for 10 years on wikipedia!!!! But this is talk page, there is no need for this tone. I am allowed to write here and ask questions and my give my opinion. Is wiki different from wikipedia? I think that guy who invented and made first wiki had something else in mind, it is place to find clues the place where ideas are born..... Metamorphic origin in peridotite community is not disputed. I was maybe not able to transfer in our communication, well the whole geologist community isn’t, and I think that is problem. So I am here to be part of your community. There will be sooner or later somebody who know more about that and this will be clarified by time. And I am here to stay. But there is no need for high tone it can stay that peridotite is coarse grained igneous rock but it is not. It needs better definition. I am trying to formulate that that is all. I know you are doing difficult job here and you dont have time to check all this. But I have. Btw please do not put me in any group I am just having an idea that things could be done differently. Cheers! — Preceding unsigned comment added by Obradow (talk • contribs) 17:45, 28 May 2016 (UTC) Obradow (talk) 17:50, 28 May 2016 (UTC)

The definitions of peridotite that I have quoted on this talk page are all more recent than at least the early work of Nicolas, but his opinion seems to have been ignored by these definitions. If peridotite is to be regarded as a metamorphic rock, even when in the mantle before any later metamorphism may occur during emplacement to the surface, then why is this metamorphic origin not included in the definitions that I have quoted on this talk page, which define peridotite as igneous only? Why has Nicolas been unable to persuade defining groups such as IUGS, AGI or BGS to accept peridotite as a metamorphic rock? GeoWriter (talk) 18:04, 28 May 2016 (UTC)

i don't know lets find outObradow (talk) 18:09, 28 May 2016 (UTC)

ok it seems that I was studying peridotites very long time without checking IUGS classification. GeoWriter has right in that. These rocks have structures which are metamorphic though, I really didnt want to make so much trouble about name itself. I will try to make changes on this wiki regarding this. Anyway I will make a sandbox in future when I am editing this wiki, so until soon then. Cheers!Obradow (talk) 13:24, 2 June 2016 (UTC)

I am sorry that I was rude, that I rushed so much and that I did not introduce myself with many things for example NPOV. I had good intentions but was very clumsy. I gave reliable sources though. I am happy to discuss further. I will answer all questions [GeoWriter] asked again, following the things I learned in last few days. Thank youObradow (talk) 12:33, 3 June 2016 (UTC)

I want to apologize to user:GeoWriter for the way I answered his questions. I hope that I can provide now better answer after introducing myself more into problem. I think lot of confusion comes from my inability to answer to his questions properly. Indeed peridotite is igneous rock as classified by IUGS, and I agree that igneous rocks which experienced initial low grade metamorphism but essentially retain their igneous textures should not have prefix meta in order not to confuse readers on wikipedia as user:GeoWriter suggested. When I added that peridotite can be also metamorphic I had in mind something else. When we are dealing with plutonic cumulate peridotite it is clear that they are igneous rocks, however most mantle peridotites have metamorphic textures. These textures are first time described by Mercier and Nicolas, 1975. I think this is generally accepted, since the authors who are describing mantle peridotites are using their terminology (protogranular, porphyroclastic and equigranular) and there is no opposite opinion to my knowledge. What I just have recently learned is that there were no recommendations on metamorphic rock nomenclature equivalent to those from the IUGS subcommittee on igneous rocks. At least until 1995. Namely, David Shelley in his book Igneous and Metamorphic rocks under the microscope states that. Inspired by hierarchical flow chart system of identification for igneous rocks (Le Maitre, 1989), he proposed series of steps for metamorphic rock identification. He classified peridotite in the group of well crystallized and recrystallized rocks. Here is exact quote: “To many readers, peridotites may seem out of place here. Are they not igneous? Some are, but majority are thoroughly metamorphic, having undergone prolonged deformation and recrystallization. Their terminology is the same for metamorphic and igneous varieties”. I am not aware if he officially wrote to International Union of Geological Sciences and how these proposals are actually made or what happened since 1995. This is worth checking. But even before checking it I hope I could convince user:GeoWriter that my assertion about peridotite being also metamorphic rock is sourced. I am not aware that there were discussion about peridotite not being metamorphic and that consensus were made not to call the peridotite metamorphic rock or to call them metaperidotite. The IUGS classification and all other references user: GeoWriter provided are correct indeed because they are classifications and definitions of igneous rocks and peridotite is indeed igneous rock. There is no dispute about that. The question is could it be also metamorphic and then if there is consensus how to call them. Additionally, having in mind that partial melting is highest grade metamorphic process that crustal rocks can experience I thought that in that respect mantle partial melting residuum could be called metamorphic.user:GeoWriter has right that International Union of Geological Sciences should have a last word about this questions. I have just found 2007 recommendations by IUGS Subcommission on Systematics of Metamorphic rocks. I will look into it and get back as soon as possible. I am sorry that I haven’t looked into this before.Obradow (talk) 10:55, 16 June 2016 (UTC)

here are recommendations by IUGS Subcommission on Systematics of Metamorphic rocks which I found as external link on article about metamorphism(since I dont have access to original manuscript, I would appreciate if somebody could check original): "Use of ‘metamorphic/igneous’ ultramafic terms :Ultramafic rocks containing olivine, and/or pyroxene and/or hornblende such as peridotite, harzburgite, lherzolite, wehrlite, websterite, pyroxenite and hornblendite may be formed by either metamorphic or by igneous processes and therefore fall in the common ground between metamorphic and igneous terminology. The SCMR recommends that for these rocks the definitions, based on mineral content, as given by Le Maitre (1989, 2002) should be used. These definitions are adopted without any implication to the rock genesis. When garnet or other major or minor minerals are present they should be indicated by the appropriate prefix. If it is desirable to emphasise the metamorphic nature of one of these ultramafic rocks then this should be specifically stated. Alternatively, a structural root name may be given (e.g. pyroxene-olivine gneiss, ultramafic garnet-pyroxene granofels). Note: under the SCMR rules, terms such as metaperidotite imply that the protolith was a peridotite, and do not make any statement about the present mineral content or structure of the rock." I want to apologize once more for the way I was discussing on this talk page.Obradow (talk) 11:40, 16 June 2016 (UTC)

Critic of first paragraph

Peridotite is not a rock it is group of diverse rocks. It is certainly not always coarse grained. It can be igneous( both volcanic and plutonic) crustal rock or of mantle origin. So I suggest change : Peridotite is group of diverse rocks of ultramafic composition. In its composition is more similar to some meteorites then the rocks of earth crust, it is characterised with higher amount of Mg and lower Si than average continental crust and even oceanic crust. Volcanic rocks of these composition erupted only in early earth history. Obradow (talk) 11:11, 28 May 2016 (UTC)

Presumably you are referring to komatiites? I don't see them anywhere being regarded as peridotites any more than basalts are gabbros or rhyolites are granites. We should define peridotites as they are defined in the literature. If you have a source that gives an alternative definition, then it would be good to see that. Mikenorton (talk) 10:19, 29 May 2016 (UTC)

I apologize that was my very sloppy mistake. Thank you! It is huge disgrace I will live with. This is the example why classifications are needed. We should be clear what we are speaking about....Obradow (talk) 20:53, 29 May 2016 (UTC)