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Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies

Anderson, Don L. (1987) Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies. Journal of Geophysical Research B, 92 (B13). pp. 13968-13980. ISSN 0148-0227. https://resolver.caltech.edu/CaltechAUTHORS:20140506-141430254

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Abstract

Pressure-induced solid-solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle. Lateral variations in temperature cause a similar effect; abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion. Temperature-induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure-induced phase changes. In cold slabs the equilibrium mineral assemblage contains high-density, high-velocity phases which are not stable in hotter mantle. In particular, the ilmenite form of MgSiO_3 and the γ-spinel form of Mg_2SiO_4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling. Recent arguments for slab penetration into the lower-mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature-induced phase changes. When these effects are taken into account, the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection. The seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection. Mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions, and this is due to the presence of a melt phase and lower-velocity, high-temperature phase assemblages. Deep, > 200 km, long-lived continental roots, differing in chemistry from “normal” mantle, are not required when isobaric phase changes are taken into account. High-velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high-temperature phase assemblages. The whole mantle convection, thick continental root, and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis. Phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition. The lack of correlation of the geoid with ridges, shields, heat flow, and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle, consistent with layered convection.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1029/JB092iB13p13968 DOIArticle
http://onlinelibrary.wiley.com/doi/10.1029/JB092iB13p13968/abstractPublisherArticle
http://dx.doi.org/10.1029/JB093iB03p02225ErrataCorrection
Additional Information:© 1987 by the American Geophysical Union. Received March 17, 1987; revised July 21, 1987; accepted August 6, 1987. I appreciate extensive conversations with Brad Hager and Michael Gurnis and their critical review of the manuscript. Discussions with Tom Jordan have also been helpful. I thank Brad Hager, Rob Clayton, Tom Jordan, and Bob Engdahl for preprints. This research was supported by National Science Foundation grant EAR-8509350 and NASA grant NSG-7610. Contribution 4444, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
Funders:
Funding AgencyGrant Number
NSFEAR-8509350
NASANSG-7610
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Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences4444
Issue or Number:B13
Record Number:CaltechAUTHORS:20140506-141430254
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20140506-141430254
Official Citation:Anderson, D. L. (1987), Thermally induced phase changes, lateral heterogeneity of the mantle, continental roots, and deep slab anomalies, J. Geophys. Res., 92(B13), 13968–13980, doi:10.1029/JB092iB13p13968
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45538
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:06 May 2014 22:48
Last Modified:03 Oct 2019 06:32

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