transition zone (Earth)
{{short description|Part of the Earth's mantle}}
{{use dmy dates|date=July 2024}}
The transition zone is the part of Earth's mantle that is located between the lower and the upper mantle, most strictly between the seismic-discontinuity depths of about {{convert|410|to|660|km}}, but more broadly defined as the zone encompassing those discontinuities, i.e., between about {{convert|300|and|850|km}} depth.{{cite journal |last1=Goes |first1=Saskia |title=Compositional heterogeneity in the mantle transition zone |journal=Nature Reviews Earth & Environment |date=2022 |volume=3 |issue=8 |page=533-550 |doi=10.1038/s43017-022-00312-w|bibcode=2022NRvEE...3..533G |hdl=1721.1/148207 |hdl-access=free }} Earth's solid, rocky mantle, including the mantle transition zone (often abbreviated as MTZ), consists primarily of peridotite, an ultramafic igneous rock.
The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of {{convert|410|and|660|km}}. This is thought{{by whom?|date=July 2024}} to occur as a result of rearrangement of grains in olivine (which constitutes a large portion of peridotite) at a depth of {{convert|410|km}}, to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of {{convert|660|km}}, evidence suggests due to pressure changes ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen using body waves from earthquakes, which are converted, reflected or refracted at the boundary, and predicted from mineral physics, as the phase changes are temperature and density-dependent and hence depth dependent.
410 km discontinuity – phase transition
A peak is seen in seismological data at about {{convert|410|km}} as is predicted by the transition from α- to β-Mg2SiO4 (olivine to wadsleyite). From the Clapeyron slope, this change is predicted to occur at shallower depths in cold regions, such as where subducting slabs penetrate into the transition zone, and at greater depths in warmer regions, such as where mantle plumes pass through the transition zone.{{cite book |first=C. M. R. |last=Fowler |title=The Solid Earth: An Introduction to Global Geophysics |edition=2nd |publisher=Cambridge University Press |year=2005 |isbn=978-0-521-89307-7}}{{page needed|date=July 2024}} Therefore, the exact depth of the "410 km discontinuity" can vary.
660 km discontinuity – phase transition
The 660 km discontinuity appears in PP precursors (a wave which reflects off the discontinuity once) only in certain regions but is always apparent in SS precursors. It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths ({{convert|640|–|720|km|disp=or}}). The Clapeyron slope predicts a deeper discontinuity in cold regions and a shallower discontinuity in hot regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase.{{Cite journal |last1=Ito |first1=E |last2=Takahashi |first2=E |date=1989 |title=Postspinel transformations in the system Mg2SiO4–Fe2SiO4 and some geophysical implications |journal=Journal of Geophysical Research: Solid Earth |volume=94 |issue=B8 |pages=10637–10646 |bibcode=1989JGR....9410637I |doi=10.1029/jb094ib08p10637}} This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models. Cold downwelling material might pond on this transition.{{Cite journal |last1=Fukao |first1=Y. |last2=Obayashi |first2=M. |date=2013 |title=Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity |journal=Journal of Geophysical Research: Solid Earth |volume=118 |issue=11 |pages=5920–5938 |bibcode=2013JGRB..118.5920F |doi=10.1002/2013jb010466 |s2cid=129872709 |doi-access=free}}
Other discontinuities
There is another major phase transition predicted at {{convert|520|km}} for the transition of olivine (β to γ) and garnet in the pyrolite mantle.{{Cite journal |last1=Deuss |first1=Arwen |last2=Woodhouse |first2=John |date=2001-10-12 |title=Seismic Observations of Splitting of the Mid-Transition Zone Discontinuity in Earth's Mantle |journal=Science |language=en |volume=294 |issue=5541 |pages=354–357 |bibcode=2001Sci...294..354D |doi=10.1126/science.1063524 |issn=0036-8075 |pmid=11598296 |s2cid=28563140}} This one has only sporadically been observed in seismological data.{{Cite book |last=Egorkin |first=A. V. |title=Upper Mantle Heterogeneities from Active and Passive Seismology |date=1997-01-01 |publisher=Springer Netherlands |isbn=9789048149667 |editor-last=Fuchs |editor-first=Karl |series=NATO ASI Series |pages=51–61 |language=en |chapter=Evidence for 520-Km Discontinuity |doi=10.1007/978-94-015-8979-6_4}}
Other non-global phase transitions have been suggested at a range of depths.{{Cite book |last1=Khan |first1=Amir |url=https://books.google.com/books?id=iXC6CAAAQBAJ |title=The Earth's Heterogeneous Mantle: A Geophysical, Geodynamical, and Geochemical Perspective |last2=Deschamps |first2=Frédéric |date=2015-04-28 |publisher=Springer |isbn=9783319156279 |language=en}}