If Eclogites are the highest grade metamorphic rock, and most part of the mantle covers Eclogite facies P-T condition, then why we can't call those "mantle-peridotite" rocks as Eclogites?
Don't mix up P-T conditions (metamorphic facies) and rock definitions. Eclogite is defined as a HP metamorphic rock of mafic composition, which primarily (>3/4) consists of garnet and omphacite in the absence of plagioclase. This is very different to a mantle peridotite, which is ultramafic and commonly shows high modal contents of olivine.
I agree with Jan. Don't mix a metamorphic rock name (eclogite) with a metamorphic rock facies (i.e., a rough interval of T-P in which a protolith with a basaltic composition is transformed in an eclogitic rock).
Geophysics says that real eclogitic rocks can form no more than 10% of the upper mantle (likely
In addition to the earlier comments, I believe the problem lies in the introductory statement "Eclogites are the highest grade metamorphic rock" which is, albeit correct, an old-fashioned approach. The eclogite facies was (long ago) not proposed for describing or discussing the mantle.
Nowadays we would talk about UHP conditions, which, as pointed out before, are conditions and not rocks.
Rather, why classifying mantle peridotites as igneous rocks?
For Bernardo's last question: peridotites are classified as igneous rocks because IUGS decided so in 1989 and 2002.
In the new version of the IUGS book a comment on such a paradox (i.e. to classify as igneous a rock with a metamorphic texture) will be added. BTW none would accept to move mantle rocks exclusively in the metamorphic container.
Michele
Good point Bernado! I always avoid calling it igneous as this doesn't makes much sense to me. On the other hand, as Michele says, calling it a metamorphic rock is similarly incorrect. In addition, we often have metasomatism. I like to just call it a mantle rock and I think it is not necessary to add igneous or metamorphic.....
Apparently a semantic debate, very interesting though! As long as rock's microstructure reflects solid-state processes (dislocation/diffusion creep) rather than crystallization from a melt, I'd call them metamorphic, as i usually do for peridotites. Most of the uppermost mantle fits this definition. Besides, there is no evidence for the mantle having experienced any melt-dominated stage since the Archean, so mantle rocks share most characteristics of metamorphic rocks, but their peculiar ultra-mafic compostion.
Dear ALL: the eclogite present in the mantle is of crustal origin, it represents ancient oceanic (mafic) crust deeply subducted into the mantle. Due to tectonic (coliisions) or magmatic processes (mantle nodules) some portions of this heavy and dense rock are brought to the surface, the rest founders into the mantle due to its high density (3,5 g/cm3)). Therefore, there is not a continuos "eclogitic layer" below the MOHO, but chunks of eclogite coming from the disruption of the thin mafic veneer in subducted slabs, and these are like raisins in a pudding! Moreover, a major junkyard of eclogite and sunken slabs is present on top of the core, in the D" zone, in the lower mantle: the double LLVZ, but even those zones are not completely made of eclogite, of course, but of overpressed perovskite-like structures, and other exotic structures which are being revealed in hyperbaric experiments with the diamond anvil press and laser beam bormardment. Regards, Sebastian.
@ Sebastian:
I believe that only a part (~30-60%?) of mantle eclogite can be ascribed to subducted basaltic lithologies. The rest is represented by delaminated formerly gabbroic lower continental crust and frozen basaltic melts not able to reach the surface. A basaltic melt crystallizing at pressures >1-1.5 GPa cannot form plagiocalse (the essential mineral to define a rock as a basalt), but rather crystallyzes garnet and omphacitic pyroxene, I.e., the typical paragenesis of eclogites.
A second comment: I do not understand why eclogites (with a density not higher than 3.8 g/cm3) can founder below the base of the Transition Zone, within a bridgmanitic mantle characterized by a density >5 g/cm3.
Cheers,
Michele
Dear All: very good observations those of Michele Lustrino! Indeed, some part of the eclogites are subducted oceanic crust, and the rest foundered mafic (granulitic?) crust or stagnant basaltic melts under the continental crust which were unable to reach the surface. The proportions of each eclogite source is really unknown. As if the eclogites are able to sink in a perovskite-structured dense lower mantle... well that could also happen, as when the eclogite paragenesis is exposed to ultrahigh pressures some other yet unknown phase transitions can happen to the omphacite and the garnet, which may attain multiple-oxide ilmenite or even perovskite structures as well, thus increasing the density of the former eclogites above 5 g/cm3. But, as far as I know, no studies have been made about those deep phase transformations of eclogites. Regards, Sebastián.
Eclogite is a type of metamorphic rock that forms at very high pressures and temperatures, typically at depths of around 50-100 kilometers in the Earth's mantle. It is composed of minerals that are stable under these high pressure and temperature conditions, such as garnet and omphacite.
It is not possible for the entire mantle or even a significant part of the mantle to be called eclogite, because the conditions required for the formation of eclogite are only found at relatively shallow depths within the mantle. The majority of the mantle is composed of much deeper layers of rock that are subjected to much higher pressures and temperatures, and these conditions are not conducive to the formation of eclogite.
Additionally, eclogite is typically formed through the metamorphism of pre-existing rocks, such as oceanic crust or continental crust, rather than through the solidification of magma or lava. As a result, eclogite is not found throughout the mantle, but rather in specific regions where it has been formed through metamorphism.
Eclogite is a type of metamorphic rock that forms under very specific conditions of high pressure and temperature, which are only found at relatively shallow depths within the Earth's mantle. As a result, it is not possible for the entire mantle or even a significant part of the mantle to be composed of eclogite.
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