Talk:Earth's mantle

Chemical and mineral composition - contradiction?

I'm having some trouble understanding this article. One particular paragraph (which contains no citations) seems to be contradicted by information elsewhere in the article:

"The upper mantle is dominantly peridotite, composed primarily of variable proportions of the minerals olivine, clinopyroxene, orthopyroxene, and an aluminous phase. The aluminous phase is plagioclase in the uppermost mantle, then spinel, and then garnet below ~100 km. Gradually through the upper mantle, pyroxenes become less stable and transform into majoritic garnet."

According to the "olivine" and "pyroxene" articles, olivine has the formula (Mg, Fe)₂SiO₄, and pyroxene has the formula XY(Si,Al)₂O₆

Now, if you look at the table in the article's "Composition" section, the one headed "Composition of the Earth's upper mantle (depleted MORB)", you'll notice none of the compounds therein has a formula which includes either O₆ or O₄. The upper mantle, according to this table, is composed primarily of silicon dioxide and magnesium oxide.

Due to the high olivine content of mantle xenoliths, I would guess that the very uppermost parts of the upper mantle might indeed be mostly olivine and pyroxene, but that this is not true throughout the upper mantle. Am I correct? Either way, could somebody explain this apparent contradiction to me?

Thanks,

AstridRedfern (talk) 17:11, 9 February 2020 (UTC)

Geologists often give the chemical composition of rocks as a list of oxides e.g. a typical sample of dunite is 40% SiO₂, 49% MgO etc. This does tend to give the (usually misleading) impression that dunite is made of two oxide minerals, quartz (SiO₂) and periclase (MgO). But this is not actually the case because dunite is actually made of at least 90% olivine. (This apparent contradiction also occurs with chemical analysis of minerals too, which by definition cannot be made of other minerals. Olivine can be typically 40% SiO₂ ("quartz"), 49% MgO ("periclase"), but actually olivine is not made of quartz and periclase, it is by definition made entirely of olivine). For rocks, geologists try to overcome this apparent contradiction by representing this oxide-based chemical analysis as an idealised mineral composition instead, calculated using a set of mathematical and geochemical rules that convert the amounts of relatively simple oxide forms of the chemical analysis (O, O₂, O₃, O₄, O₅) into amounts of oxygen forms that are more typically found in the main rock-forming silicate minerals (e.g. O₄, O₆, O₈, O₁₀, O₁₈, O₂₂). As long as this calculation has been done correctly, I suppose it is two ways of expressing the same data. There is a Wikipedia article about this process, normative mineralogy, which may address some of the points that you have raised. GeoWriter (talk) 19:21, 9 February 2020 (UTC)

Partial melting of the mantle produces crust?!

Hello, I have limited knowledge of Geology but I believe that this affirmation is either wrong or misleading: "Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.[2]" The sentence implies that the melting (i.e. solid to liquid) produces a crust (solid) which may lead to misunderstanding. I suggest something in this line: "Solidification of igneous magma at mid-ocean ridges produces oceanic crust..."

Also, I've checked reference [2] and found no explanation to the sentence presented here.

Best regards, Coel Jo (talk) 01:49, 24 September 2020 (UTC)