The Moho as a magnetic boundary

Mantle derived rocks — peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow in Hawaii, as well as peridotite, dunite, and eclogite xenoliths from Roberts Victor, Dutoitspan, Kilbourne Hole, and San Carlos diatremes — are weakly magnetic at room temperature. Saturation magnetization values range from 0.013 emu/gm to <0.001 emu/gm equivalent to ∼0.01 to 0.001 wt % Fe3O4. A review of pertinent literature dealing with analysis of the minerals in mantle xenoliths provides evidence that metals and primary Fe3O4 are absent, and that complex Cr, Mg, Al, Fe spinels dominate the oxide mineralogy. These spinels would be non-magnetic at mantle temperatures. The crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. Curie depth estimates from aeromagnetic anomalies do not require a source of magnetization in the mantle. All of the available evidence supports the new magnetic results, indicating that the seismic Moho is a magnetic boundary. The source of magnetization is in the crust and the maximum Curie isotherm depends on the type of magnetic mineralogy and is located at depths which will vary with the regional geothermal gradient.

[1]  B. Lienert,et al.  A magnetic study of the serpentinization process at Burro Mountain, California , 1979 .

[2]  S. Smithson.  Modeling continental crust: Structural and chemical constraints , 1978 .

[3]  S. Haggerty.  Mineralogical contraints on Curie isotherms in deep crustal magnetic anomalies , 1978 .

[4]  D. Green,et al.  Laboratory Duplication of Phase Equilibria Observed in Natural Garnet Lherzolites , 1978, The Journal of Geology.

[5]  R. Stolt,et al.  AN ATTEMPT TO DEFINE THE CURIE POINT ISOTHERM IN NORTHERN AND CENTRAL ARIZONA , 1977 .

[6]  I. Pashkevich,et al.  Magnetic model for the earth's crust under the Ukrainian Shield , 1977 .

[7]  F. Wicks,et al.  Serpentine textures and serpentinization , 1977 .

[8]  J. Dawson.  Sub-cratonic crust and upper mantle models based on xenolith suites in kimberlite and nephelinitic diatremes , 1977, Journal of the Geological Society.

[9]  H. Meyer.  Mineralogy of the upper mantle: A review of the minerals in mantle xenoliths from kimberlite , 1977 .

[10]  A. Tripp,et al.  Curie depth determination from aeromagnetic spectra , 1977 .

[11]  D. Fountain.  The Ivrea—Verbano and Strona-Ceneri Zones, Northern Italy: A cross-section of the continental crust—New evidence from seismic velocities of rock samples , 1976 .

[12]  J. B. Moody.  Serpentinization: a review , 1976 .

[13]  Douglas J. Smith,et al.  Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau , 1976 .

[14]  A. Green.  Interpretation of Project MAGNET Aerornagnetic Profiles Across Africa , 1976 .

[15]  W. Maclean,et al.  The paragenesis of zoned chromite from central Manitoba , 1975 .

[16]  B. W. Evans,et al.  Chrome-spinel in progressive metamorphism—a preliminary analysis , 1975 .

[17]  A. Jessop,et al.  A Comparison of the Thermal Characters of Shields , 1975 .

[18]  N. Boctor,et al.  Sulfide-oxide minerals in eclogite from Stockdale Kimberlite, Kansas , 1975 .

[19]  O. Eckstrand.  The Dumont serpentinite; a model for control of nickeliferous opaque mineral assemblages by alteration reactions in ultramafic rocks , 1975 .

[20]  P. H. Nixon,et al.  Origins of the ultramafic nodules from some kimberlites of northern Lesotho and the Monastery Mine, South Africa , 1975 .

[21]  P. Roeder,et al.  The Crystallization of Spinel from Basaltic Liquid as a Function of Oxygen Fugacity , 1974, The Journal of Geology.

[22]  Robert B. Smith,et al.  Yellowstone Hot Spot New Magnetic and Seismic Evidence , 1974 .

[23]  M. G. Best.  Contrasting types of chromium-spinel peridotite xenoliths in basanitic lavas, western Grand Canyon, Arizona , 1974 .

[24]  S. Smithson,et al.  A continental crustal model and its geothermal implications , 1974 .

[25]  R. Meissner.  The ‘Moho’ as a transition zone , 1973 .

[26]  G. Czamanske,et al.  Sulfides in eclogite nodules from a kimberlite pipe, South Africa, with comments on violarite stoichiometry , 1973 .

[27]  R. Shuey,et al.  Aeromagnetics and the Transition Between the Colorado Plateau and Basin Range Provinces , 1973 .

[28]  N. Sobolev,et al.  Sulfidic formations in deep xenoliths from kimkerlite pipes in Yakutia , 1973 .

[29]  T. McGetchin,et al.  A crustal-upper-mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike , 1972 .

[30]  H. Taylor,et al.  Temperatures of serpentinization of ultramafic rocks based on O18/O16 fractionation between coexisting serpentine and magnetite , 1971 .

[31]  F. Gibb.  Crystal-liquid relationships in some ultrabasic dykes and their petrological significance , 1971 .

[32]  F. Gibb,et al.  The magmatic oxygen fugacity of an ultrabasic dyke , 1971 .

[33]  D. Hall.  Regional magnetic anomalies, magnetic units, and crustal structure in the Kenora District of Ontario , 1968 .

[34]  A. H. Saad.  Magnetic properties of ultramafic rocks from Red Mountain, California , 1968 .

[35]  J. Jaeger,et al.  HEAT FLOW AND SURFACE RADIOACTIVITY MEASUREMENTS IN THE PRECAMBRIAN SHIELD OF WESTERN AUSTRALIA. , 1968 .

[36]  T. Irvine.  CHROMIAN SPINEL AS A PETROGENETIC INDICATOR: PART 2. PETROLOGIC APPLICATIONS , 1967 .

[37]  A. Buddington,et al.  Iron-Titanium Oxide Minerals and Synthetic Equivalents , 1964 .

[38]  Y. Shimizu.  Magnetic viscosity of magnetite. , 1960 .