I wonder if this is even possible to know....
Do geophysicists and geologists have even a 'theoretical' idea about how deep Earth's churning of its layers goes?
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1$\begingroup$ en.wikipedia.org/wiki/Mantle_convection, en.wikipedia.org/wiki/Volcanism, en.wikipedia.org/wiki/Magmatism as a start. These are two poor questions that can't have good answers without prior knowledge. $\endgroup$– user29219Oct 4 at 14:52
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$\begingroup$ The deepest we've dug is the en.wikipedia.org/wiki/Kola_Superdeep_Borehole, which is 12.2km deep, or 0.42% of the way to the outer core. (Note that this doesn't actually answer any of your questions) $\endgroup$– Mooing DuckOct 4 at 15:11
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2$\begingroup$ I've found some rock that originated in the outer core, possibly even the inner core $\endgroup$– ValorumOct 4 at 15:16
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Diamond is stable at high temperatures only under very high pressures. Mineral inclusions in a diamond, and the rocks in which a diamond are embedded, can offer further clues as to the depth from which the diamond came.
The Wikipedia article on diamond claims "A smaller fraction of diamonds (about 150 have been studied) come from depths of 330–660 km, a region that includes the transition zone. They formed in eclogite but are distinguished from diamonds of shallower origin by inclusions of majorite (a form of garnet with excess silicon). A similar proportion of diamonds comes from the lower mantle at depths between 660 and 800 km. [20]" (my emphasis)
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As far as I know the best estimate is "upper mantle" from mantle xenoliths but that's not terribly specific. I am not sure if knowing more than that is possible right now.
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$\begingroup$ The term is used for a wide range of magmatic compositions, but often times with magma that came a long way through continental crust. A mid ocean ridge basalt otoh regularly originates in the upper mantle. So there is so much more to it. $\endgroup$– user29219Oct 4 at 15:05